Engine - Engine Internals and Racing Engine Build
The Isuzu engine in the Geo Storm in factory spec won awards for highest output per unit of displacement for 1990
and 1991, and is already tuned well beyond the typical engine of its time. The parts used in the engine feature
materials and processes that were reserved for racing engines and high performance factory engines of the 1990’s.
These features were later adopted as standard by most of the automotive industry some 15+ years later. Finding
aftermarket stock level rebuild parts, that are up to the quality of the original parts, can be a challenge. And
the high level of design and factory tune does not leave huge amounts of power to be gained through upgrading
something that the factory cheaped out on.
More importantly, realizing the performance gains made upgrading the engine internals is going to depend on being
able to fine tune the computer system and changing the fuel and spark curves to match the needs of the modified
engine. Gone are the days of swapping carb jets and having the distributor re-curved. All of these things are now
computer controlled, and the only way to control them is to upgrade to a computer system that will allow the user
to control them.
The OEM pistons used in the 4X naturally aspirated engines are cast, while the turbocharged engine used forged
pistons. For a non-turbo engine build, the factory cast piston is more than adequate. If the factory compression
ratio is to be used, the factory pistons are a good choice, though they are no longer available because they have
been discontinued. For racing classes which require stock engine specs, this can be an issue, and will require
ordering a custom made reproduction of the OEM piston.
For custom performance pistons, it is likely that forged will be the only choice, and using a piston that is
stronger than necessary is not a bad alternative.
Custom made pistons also offer advantages of newer, more exotic metal alloys, use of alternate rings and wrist
pins, gas porting, additional oil passages, and exotic coatings, all to enhance performance. Newer alloys and
alternate machining techniques can yield some weight reduction when compared to the original pistons. Custom
pistons also offer the possibility of changing the compression ratio to create more power, and changing the
design of the face of the piston to improve combustion efficiency.
JE Pistons, Crower, Arias, and several others have provided custom performance pistons in the past. None of
these have proven to be any better than the stock pistons.
Ross and Wiseco are the top custom piston makers offering pistons for the Isuzu 4X engines. Isuzuperformance
worked with both Ross and Wiseco to develop these parts. The Wiseco product stands above all others, featuring
an exclusive dome-profile design, matching the contour of the top of the piston to the shape of the combustion
chamber, and maximizing the combustion area in the face of the piston to increase cylinder pressures and push
power output to its limit. This piston design is available exclusively through Isuzuperformance.
The OEM piston rings used in Isuzu engines are not a standard size that is commonly available in the aftermarket.
This means that the grooves machined into the side of the piston is a different size to accommodate the OEM ring,
than aftermarket performance rings. And that the builder/owner must decide which rings to use before purchasing
pistons, and then stick with that decision for the use of those pistons.
The OEM rings are extremely strong, will hold up to whatever is thrown at them, and certainly are not a weak point
in the engine. The original manufacturer was Nippon Piston Ring, who still offer piston rings for these engines,
and have condensed all of the 4X series rings down to one part number, meaning that the specification of the
rings has been brought up above the level of the 1.6 turbocharged engine (and this is an upgrade for non-turbo
applications). One serious advantage is that many racing sanctioning bodies (SCCA, etc.) specifically ban Total
Seal brand and type piston rings, even in classes that allow extensive engine modifications. And the OEM piston
rings are allowed.
Aftermarket performance rings are not specifically made for the Isuzu engines, and require the use of a piston
with the piston ring grooves made specifically to accommodate the different sized rings. Total Seal brand and
type piston rings offer better sealing of the combustion chamber and small but measurable power increase. As
stated above, Total Seal brand and type rings are often specifically banned by racing sanctioning bodies
The OEM connecting rods have not proven to be a weak point in the 4X Isuzu engines. The weak point has shown to
be the connecting rod bolts (See the item directly below).
Isuzuperformance worked with Pauter to develop performance rods with a cross (+) shaped profile. This design has
proven stronger than H beam and I beam designs, with the added advantage that the shape passes more easily through
the oil puddle in the oil pan, with less loss of momentum.
For those requiring OEM reproduction connecting rods (for racing classes requiring stock spec rods), the only
option is the Wiseco/K1 custom made OEM reproductions, available exclusively from Isuzuperformance.
Connecting Rod Bolts
The second most common cause of engine failure in Geo Storms is throwing a rod through the front of the block
due to connecting rod bolt failure. The original connecting rod bolts were manufactured using a process called
cold-heading, and were not heat treated. The crude manufacturing process involves pounding a round piece of
mild steel into the shape of a bolt, and then threading the blank. This process maximizes the weak points and
fault lines within the bolt, but it is the cheapest and fastest way to make bolts.
These low strength bolts will either stretch, allowing the nut to loosen itself and back off while the engine is
running, or they will fracture. Either way, the rod comes apart from the crankshaft in a very un-graceful way,
usually banging the crank journal several times, then bouncing off and firing through the front of the engine
block, with the piston following its lemming-like jump to reach the road surface.
The long and short of the situation is that if there is ever an opportunity to replace the original connecting
rod bolts with stronger, aftermarket bolts, do so. That opportunity might be during a main bearing inspection
or replacement, when replacing the head gasket or piston rings, prior to installing a used engine, or while
rebuilding an engine. In any event, stronger connecting rod bolts are money well spent.
Be forewarned that the 4XE1 SOHC and DOHC engines use an 8 mm diameter connecting rod bolt, while the 4XF1 engine
uses a 10 mm diameter connecting rod bolt. These are not interchangeable, and require two distinctly different
ARP - Does Not Make Isuzu Connecting Rod Bolts
Justin Phillips, Specialty Products Coordinator for ARP Incorporated, was contacted regarding the availability
of connecting rod bolts for Isuzu engines, and was provided with mechanical drawings and a sample of the Isuzu
engine connecting rod bolts. Mr. Phillip's response was:
Robie the Robot
I have pulled and examined one of the rod bolts provided in the rod for something that may match up
dimensionally. Unfortunately, I was unable to come across anything with the proper press and length. We would
need to look into custom manufacturing...
According to ARP, they do not make any bolts for Isuzu engines, they do not have anything that is close
enough to substitute, and the only alternative would be custom made bolts, which they have never done for
Robie the Robot / Robie Blair claims to sell Isuzu engine connecting rod bolts made by
As has already been revealed directly above, ARP do not make and has never made any bolts for Isuzu engines,
ARP does not make a bolt that is similar enough to substitute into Isuzu engines, and ARP has never made a
custom production run of connecting rod bolts for Isuzu engines.
Robie the Robot lists a single size bolt which Mr. Blair claims to fit both of the 4XE1 and 4XF1 engines.
This is not possible because the 4XE1 uses a 8 mm bolt while the 4XF1 uses a 10 mm bolt. Perhaps, Mr. Blair does
not know the difference between the 4XE1 and 4XF1 connecting rod bolts, and he does not know what he is selling,
or what it fits. Or perhaps he has created a misleading product listing for a product he does not actually have,
in order to organize a group-buy for an that item he intends to have made after he collects enough money. In
such case, Mr. Blair will only discover that there are two different sizes of bolts after he has collected his
customer’s payments, when he attempts to placing the order with a bolt manufacturer. Or, more likely, Mr. Blair
will discover his error only after he receives customer complaints for his delivering the wrong sized bolts.
Either way, this should raise serious concerns about the product, and the company offering the product.
Further details have come to light regarding the connecting rod bolts being sold by RTR and Mr. Blair.
RTR's customers have reported that upon receiving their purchase from Mr. Blair, they find that the bolts are
too large to fit into the Isuzu engine connecting rods, and they are instructed to drill out the connecting
rods to accept the larger diameter bolts. Several people additionally stated that upon drilling out the
rod as instructed, the bolts spun in the hole, the connecting rod was no longer usable, and it had to be
discarded and replaced with another connecting rod. If these reports are true, then Mr. Blair has found an
oversized bolt and expected his customers to modify their engines to use this larger diameter bolt. This is
a very bad idea. The original set of connecting rod bolts are matched for weight within a fraction of a gram.
Any drilling or machining of the bolt will change the weight, and the rods would have to be weighed and balanced
after the being drilled, and modified to match the weight of the set. Additionally, the holes which accept the
connecting rod bolts are not simply drilled into the rod, but are rather bored to a precision diameter, using
a tool called a reamer, mounted to a machine called a vertical mill. The bolt hole must be a precise diameter
which can not be made using a drill bit. This explains the reports of spun bolts.
Isuzuperformance designed and manufactured a heavy duty connecting rod bolt specifically for the 4XE1 engines,
which exceeds the strength and tolerance specifications of even noted bolt manufacturer ARP.
The Isuzuperformance connecting rod bolts are billet turned from solid, round bars of 4340 alloy steel.
Then heat treated completely through the material to 47 Rockwell hardness. The bolt knurl is inspected and
verified to match the upper spec of the OEM bolt tolerance. And finally, the bolt shoulder
is precision ground for exact clearance, and the entire bolt polished in a vibratory tumbler.
The Isuzuperformance connecting rod bolts have become the standard for racing, performance, and even OEM spec
Isuzu engine rebuilds.
For the 4X series engines, Isuzu adopted, by 1990’s standards, a very esoteric manufacturing process for the
crankshaft. Isuzu was one of the first manufacturers to adopt the Tuffride surface hardening process. Tuffride
is a patented subcategory of Nitride coating. Tuffride applies an extremely thin, hard layer over the surface of
steel, which, in most cases, will outlast the vehicle. But, according to the instructions in the factory shop
manual, if the crankshaft journal surface is scratched in any way (such as bearing failure, spun bearing, etc.),
the thin coating is removed and the crankshaft must be discarded and replaced with a new crankshaft at a cost of
$661.20 (1999 retail price) or $1,105.79 (2011 retail price, if still available).
This also poses a serious problem for any engine rebuilding project, because every single engine parts supplier
offers only reconditioned crankshafts for Isuzu engines, which rely on regrinding the crankshaft journals as
part of the reconditioning process. Regrinding the journals removes all of the coating, leaving untreated metal
exposed to the wear and stress of the bearings, and leading to a quick engine failure.
There are three primary processes used in crankshaft hardening: induction hardening, Tuffriding, and nitriding.
This becomes vitally important whenever servicing the crankshaft of an Isuzu engine. Any crankshaft which has a
scratched journal surface, must be either discarded or re-treated to restore the hardened surface. Reground
crankshafts are out of the question, unless the hardening is restored. Nitriding costs $500+ per minimum charge
per run, meaning there is a flat minimum fee for the process until the quantity or weight of the parts being done
exceeds the limit. Running four crankshafts at a time (which is within the limit) costs $125 each plus shipping.
Remanufactured crankshafts are selling for $230 each, and there certainly is not enough margin in that price for
the crankshaft regrinder to cover the cost of nitriding the crankshaft after grinding. It's clear that they are
selling an untreated crankshaft with no hardening at all, whatsoever.
- Induction hardening is the most common method because it is the cheapest and easiest. It was the only method
prior to the 1990’s,. and remains the most common for low performance applications. Induction hardening involves
heating the metal to a very high temperature and then quench cooling it extremely quickly. The hardening reaches
a depth of .060-.080 inches, leaving enough depth that it can be reground, but the process creates stress within
the metal and possibly warpage across the part being hardened.
- Tuffriding is a trademarked name for the ferritic nitrocarburizing process. It is most commonly found in high
performance OEM engines, because it is more expensive and involves more hazardous chemicals. The crankshaft is
immersed in a very hot cyanide bath, nitrogen compounds are introduced, and those nitrogen compounds bond to the
surface of the crankshaft, forming an extremely durable .005 inch thick coating. The process causes very little
stress to the metal and very little risk of warpage, but the coating depth is so thin that the piece can not be
reground without recoating afterward.
- Nitriding is even more expensive, and normally found only in racing applications. Nitriding applies nitrogen
to the crankshaft at lower temperatures than Tuffriding, and for a longer duration of time. The hardening
penetrates the steel to a depth of .010-.030 inches. The lower temperatures incur even less stress to the steel
and less risk of warpage. And the thicker depth allows for some very minor grinding and polishing of the journals
without risk of damaging or losing the hardening.
In the event that the engine is to be serviced, the best course of action is to find a good, undamaged, and
un-ground, used crankshaft, which has the original Tuffride coating intact. If one can not be found, the next
best alternative is to have the original crankshaft serviced locally, and send it out to be nitrided.
Modifying the Original Crankshaft
The factory crankshaft is very good in its original form, arguably much better than most of the commonly available
aftermarket racing parts for other brands of engines. The Isuzu crankshaft can not be lightened much without
compromising the strength, and if the flywheel, pulleys, sprockets, and clutch are lightened, the rotating mass
is reduced to the point that it becomes very difficult avoid killing the engine while engaging the clutch. Knife
edging the crankshaft, or excessively lightening it, is a very expensive way to accomplish weight reduction that
would be easier and less expensively accomplished through bolt on aluminum pieces.
Custom Made Crankshaft
A custom made crankshaft is possible, though impractical. No forged aftermarket crankshafts were ever offered,
due to the tens of thousands of dollars involved in mold costs. Billet crankshafts have proven reliability and
durability, especially when coupled with nitride hardening. However, billet cutting a crankshaft requires a 6
inch diameter round bar of exotic alloy material that is about 2 foot long, with a cost for just the material of
over $3,000+. Machining the journals requires a lathe machine that is able to spin the large piece of metal off
center to locate the rod journals. This machinery is exclusive to companies that make Formula 1 and Nascar engine
parts, with understandably high labor costs to match the hundred thousand dollars it costs to build a Nascar
engine (or more for F1). And then the $500+ cost to nitride harden the crankshaft. A one-off custom made
crankshaft made to exceed the strength and durability of the OEM crankshaft could easily cost $20,000+.
A windage tray reduces turbulence and momentum loss caused by oil splashing as the crankshaft and rods rotate
through the oil pan. The power improvement is small, but measurable, and windage trays are standard fair for
high performance engines.
Isuzuperformance made windage trays for the 4X engines.
A crank scraper is a plastic squeegee that is mounted to the side of the block or oil pan, which wipes excess oil
off of the crankshaft as it rotates.
Though less common, they have been proven to improve power output.
Milling / Decking The Block To Change/Raise The Compression Ratio – A Bad Idea
It’s the age old question: “How much can I mill the head to boost the compression ratio?”. The answer is that
this is the wrong way to change the compression ratio, and it results in more problems than benefits.
The valve timing is set by the timing belt teeth, and the position of these teeth on the camshaft gears and
crankshaft sprocket. The belt tensioner pulley is on the intake side. Any change in the distance between the
crankshaft and the cam sprockets will change the orientation of the camshafts with respect to the crankshaft.
Reducing that distance, by milling the head, advances the cam timing forward. A change in deck height of .039
inches will advance the valve timing forward one degree. The typical old-school-hot-rod head milling job will
throw the valve timing off several degrees, which will hurt performance more than the negligible change in
compression ratio that will be achieved.
The head and block should be milled only as much as is absolutely necessary to provide a good, flat surface for
gasket sealing, and no more.
The valvetrain provides several significant opportunities for performance gain through improved flow and weight
reduction. Improved flow is self explanatory, better flow yields more efficiency and more available power.
Weight reduction opportunities include both rotating assemblies, as well as reciprocating assemblies. Weight
reduction of rotating assemblies has already been explained to show greater gains than weight reduction of
unmoving parts and items. Consider the fact that the engine must work even harder to move reciprocating items
(those which move back and forth or up and down), than rotating items.
Valvetrain weight reduction has the secondary benefit of allowing the engine to safely operate at higher speeds,
meaning that the weight of the valvetrain is the limiting factor for determining the engine’s redline. Extremely
lightweight valvetrain components are what allows racing engines to turn 12,000+ RPM.
Cylinder Head Porting
Done correctly, good cylinder head porting will uncork flow and yield huge amounts of power. Done incorrectly,
bad cylinder head porting will waste lots of money with no power improvement, or even destroy a very valuable
piece of the engine.
Cylinder head porting is part artwork and part science. The artwork comes in with many years of experience, while
the science comes in with the use of a flow bench to measure the results of the artistic effort. It can not be
stressed enough that a flow bench is required for a professional porting job, because each port in the head is
shaped slightly differently, and the porting work for each of those ports must be measured for flow and the flow
balanced across the head to match. The engine is only going to make as much power as the weakest cylinder or
port, and any imbalance in flow is going to cause stress to the engine as two or more cylinders fight with each
other to operate better or worse than their neighbor. In addition to this, there are physical limitations within
the structure of the head (such as thinner areas in the water jacket casting) which limit the amount of porting
that can be done.
The long and short of the situation is that porting is not something to pay an amateur to do, because the result
will be an amateur job.
Replika Maschinen has risen to the top of the industry for their reputation for good, professional porting work.
And they prove the axiom that you really do get what you pay for.
High Lift Camshaft, Camshaft Regrind
Increasing valve lift height and duration is a fairly straightforward way to increase flow through the head and
make more power. However, with efficient four cylinder engines, such as the Isuzu 4X engines, which start out
at a higher level of tune, small changes in lift and duration make big changes in the power band. It is important
to make sure to retain the low and mid range power, where these engines need it the most. Blindly following the
bigger-is-better mantra, and pushing the lift and duration to excess, will result in an engine that is gutless
and sacrifices huge amounts of power everywhere below 6,000 RPM, and theoretically makes a huge amount of power,
several thousand RPM above redline, where the factory computer and un-lightened valvetrain will not allow it to
run. This underscores the importance of being realistic.
The difference between the lift measurement of the Non-turbo Solid Lifter and Turbo and Hydraulic camshaft lobes
is .042 inches. That’s less than the thickness of a dime coin. So, very slight valve lift changes make very big
differences in how these engines run. Changes in lift and duration that may sound insignificant, will
dramatically change the power output and power band of the engine, especially when paired with adjustable cam
sprockets, to allow for fine tuning of valve overlap and advancing or retarding the valve events.
Engine builders also need to be concerned with the condition of the stock camshaft, if it is to be used unmodified.
Experience has shown that there is often a lack of consistency in the stock cam lobe lift height and profile.
Measuring the lobe diameter of undamaged, unmodified, like new appearance, original camshafts has revealed as
much as a .016 inch variation in measured lobe diameter. The engine will produce only as much power as the lowest,
most poorly flowing camshaft lobe will allow air to flow through the valve, so inconsistent cam lobe height and
profile is a serious concern for those seeking to maximize engine power. In such cases, it is suggested to have
the camshaft examined and have the lobes reground and/or hard welded and reground to restore the original cam
lobe profile to all of the camshaft lobes.
It is also important to select a reliable, qualified camshaft grinder. Crower was a popular choice many years
ago. However, their work proved to be inconsistent, many times showing power losses, and often requiring the
same camshaft be returned and reground multiple times to show any improvement at all. Their track record proved
that they could not replicate the same grind on two different customer’s camshafts, so if one customer received
a regrind that worked, another customer could not order the same profile and lift for their regrind. As a result
of this, Crower fell out of favor, and you won’t find any vendors offering Crower’s services.
Isuzuperformance worked extensively with Web Cams to develop good, consistent, repeatable cam grind profiles for
the Isuzu 4X engines. Isuzuperformance remains Web Cam’s exclusive dealer for Isuzu engine applications.
Isuzu 4X DOHC Camshafts: Four Different Camshafts, Four Different Camshaft Indexes.
There are four different camshafts (and cam sprockets) used across the range if the 4X DOHC engine family (six
if you include the Elan M100 Turbo and Non-Turbo). They bolt in and fit the same in both generations of cylinder
heads. However, the index angles are different. The set pins are not indexed to the cam lobe center line at
the same angle. (The angle between LCA and the set pin is not the same for each of these camshafts). And the
angle between the centerline of the cam sprocket tooth and the set pin center line is not the same between the
There is five (5) degrees difference across the range of four different camshafts and the four different camshaft
sprockets that they go with. This means that changing from the original camshaft that came with the engine, to
another camshaft in the 4X family, will result in as much as five degrees change in valve event timing, and mixing
and matching the camshafts can result in ten degrees change (five for intake, five more for exhaust).
It is popular to swap out camshafts between the engine models in the 4X engine family. But, this becomes an
exercise in blind, random changes to the valve timing of the engine, depending on the mixing and matching of the
camshafts and sprockets.
The word of caution is that anyone swapping around camshafts had best be able to measure the index angle of the
camshaft set pin, the index angle of the set pin hole in the cam sprocket, and they best be able to adjust the
timing to correct for the various index points of the pieces they are mixing.
Adjustable Cam Sprockets
Adjustable cam sprockets allow for fine tuning of the intake and exhaust valve event timing. These adjustments
move the power band up and down in the RPM range, and increase or broaden the power band, depending on the
adjustment made. This fine tuning can result in significant improvement to usable power, and provide for tailoring
the engine’s power band to the specific use of the vehicle and needed improvement for the driving and racing
situation (more low end power for acceleration, or more high end power for increased maximum speed).
Replacing the original steel gear with a gear made out of aluminum also provides an opportunity to drop
valvetrain weight by 6+ ounces, which improves power output and allows for higher RPM engine operation.
Modifying the original steel cam gear by cutting it and adding an adjustable mechanism will result in added
weight, parasitic power loss, and reduced high RPM operation range.
Care must be taken for quality design and manufacturing, because something that “wobbles just a little at idle
speed”, is going to be come a serious problem up at the 7,800+ RPM engine redline speed.
It should also be pointed out, as was mentioned above in the
camshafts, that there are 4+ different cam sprocket designs used with the Isuzu 4X family of engines.
The index point, or angle between the camshaft set pin and the cam sprocket set pin hole, is different for
each of these specific applications. And the angle between the set pin center line and the sprocket tooth
center line is different as well. The intake and exhaust are different for the same engine, and the intake
and exhaust are different between the different engines within the family. A cam sprocket made for one of
these four models of engines will have a different zero point than is correct for a different engine within
the family, or a different camshaft from a different engine within the family.
Isuzuperformance made three different sets of aluminum, adjustable cam sprockets, for the second generation 4XE1
Non-turbo engines, for the 4XE1 Turbo engines, and for the 4XF1 engines.
Jumping Cam Sprocket Teeth To Adjust Valve Timing – A Bad Idea
Changing the orientation of the cam sprocket teeth and cam belt teeth (or jumping a tooth) is often naively
suggested for “fine tuning” or correcting the valve timing. It is also a common source of power loss and fuel
economy loss when the timing belt is replaced and installed improperly.
Each cam sprocket tooth 7.8 degrees. Jumping the belt one tooth on the cam sprocket changes the timing 7.8
degrees. This is a very large change in valve timing. The point of fine tuning the valve events is to change
the timing by a degree or two, because very small changes in the timing will make very big changes in the
operation of the engine. A change of almost 8 degrees advance or retard will move well past the desired “fine
tuning” point, and cause power losses.
Lifters – Solid vs. Hydraulic
The topic of lifters seems to confuse many, because Isuzu switched back and forth between types of lifters over
the life of the Geo Storm and its Isuzu sister cars.
The first generation of the 4XE1 DOHC engine was introduced in 1987 (1989 USDM), with hydraulic lifters, and
rated at 135 PS JDM and 125 HP USDM. The second generation 4XE1 DOHC engine was introduced with the 1989 Geo
Storm and 1990 Isuzu sister cars, with solid lifters, and rated at 140 PS JDM and 130 HP USDM. Isuzu’s press
literature directly and completely attributed the engine’s power increase to the change from hydraulic lifters
to solid lifters, further commenting that the change reduced the weight of the valvetrain by 10%.
The later 4XF1 engine went back to hydraulic lifters. The displacement increased from 1.6 liter to 1.8 liter, a
12.5% increase, while power increased only 10 HP, or 7.7%. Engine redline was also reduced from 7,800 RPM to
7,000 RPM, due to increased valvetrain weight. Without the power loss due to the change from solid to hydraulic
lifters, the power increase would have more accurately reflected the displacement increase.
The history shows that the 4XF1 engine was developed with the intention of supplying it to Lotus, for a second
generation Elan M100. Lotus had specified hydraulic lifters for the 5,500 engines that they had ordered for the
Elan M100 in the late 1980’s. Lotus preferred low maintenance to maximum power output. And Lotus sought the
same for its second generation Elan. When that car did not see fruition, Isuzu decided to use the engine,
unchanged, in its US market cars, and in a special edition Piazza JT221.
The actual maintenance difference is negligible. As long as the valve lash is measured and adjusted every couple
years, the 4X engines run very smoothly and problem free with solid lifters. With typical neglected maintenance,
and no valve adjustment at all, these engines see 200,000 miles, without excessive noise and no valve related
issues at all.
Hydraulic lifters have two primary disadvantages. First, they are quite heavy. A solid lifter weighs 1.2
ounces, while a hydraulic lifter full of oil weighs about 2.8 ounces (1.8 oz. for the dry lifter, and 1.0 oz.
for the oil it holds). Reducing weight in the valvetrain yields more power improvement than anywhere else, with
the additional benefit of increasing the usable maximum engine speed (engine redline). Secondly, the same
mechanism that adjusts out to take up the lash gap when the valve is at rest, also compresses slightly when the
cam lobe pushes the valve open, and this results in loss of lift at the valve. For this reason, camshafts made
for use with hydraulic lifters have taller lobes, to make up for the lift that is lost by the compression of the
hydraulic lifter mechanism.
If maximizing engine power output is the goal, hydraulic lifters should not be used.
Shim-Under-Bucket Solid Lifters
The factory solid lifters are shim-on-bucket design. This means that the shim to adjust the valve lash is set
into a shallow depression on top of the lifter bucket, and the cam lobe actually rides on the shim. This works
well with the factory camshafts. But, with the use of reground or higher lift camshafts, the increased lobe
height can cause problems with shim-on-bucket lifters. At higher engine speeds, and/or with stronger valve
springs, the taller lobe can rip the shim out of the lifter bucket, and fire it through the valve cover, right
out of the engine.
The remedy for this is to use shim-under-bucket style lifters, as are commonly used on high RPM motorcycle
engines. With this type of lifter, the shim takes the form of a lash-cap, which is placed between the lifter
bucket and the valve stem, while the cam lobe rides against the body of the lifter bucket. Moving the shim away
from the rotating cam lobe eliminates the possibility of firing the shim through the valve cover.
Another consideration is for possible weight reduction by using a lighter weight material for the lifter bucket,
such as titanium. Lifter weight can be cut in half or more. However, many racing classes specifically prohibit
titanium valvetrain components.
Stiffer Valve Springs
For engines using high lift camshafts, engines that are run at high RPM, or forced induction engines, valve
float can become an issue. This happens when the valve does not completely close between valve events. The
problem can be overcome by installing stronger valve springs. The drawback is that the stronger vale springs
require more work from the engine to compress, so it is desirable to use a valve spring that is just stiff
enough to keep the valve closed, and no stiffer.
The stock valves of the Turbocharged and Naturally Aspirated 4XE1 DOHC engines measure 42 pounds per inch spring
rate. This spring is more than adequate for 160 HP and 7-8 PSI of boost at 7,200 RPM with the taller turbo
engine camshafts. These springs should be more than adequate to handle quite a bit more valve lift at 8,000+
RPM in a naturally aspirated engine.
Some upgrade valve spring sets make use of double springs, with an inner and outer spring combined to make up
the stiffer spring rate. This is a proven and safe method to increase spring rate without sacrificing the
compressed spring coil stack height, which would be taller if a single spring with a larger wire diameter is
used. However, double springs will require that the lower spring seat and upper spring retainer be replaced,
to provide a smooth and flat surface for ends of the the smaller, inner spring to rest against.
Isuzuperformance pioneered uprated valve spring and retainer sets for the Isuzu 4X engines, offering springs
rated at 58, 70, and 100 lb./in ratings.
The 4X engines used in the Geo Storm came with solid steel valves. They are quite durable, but heavy, weighing
1.6 ounces for the intake valve and 1.5 ounces for the exhaust valve.
Custom made valves offer a range of possibilities, including exotic metal alloys, hollow valve shafts,
narrow/undercut shaft valves, and sodium filled valve shafts.
Another lesser known option with custom made valves is using flat face valves to raise the compression ratio.
The factory valves have a small concave dish in their face. Machining custom valves with a flat face fills in
this dished area, and raises the compression ratio slightly.
- Stainless steel offers greater durability, but the same weight as steel.
- Hollow and undercut valves offer weight reduction.
- Undercut valves also offer greater flow.
- Sodium filled valve stems, such as used in the turbocharged 4XE1 engine of the Impulse RS Turbo AWD, provide
better cooling for extreme heat conditions.
- Exotic alloys, especially titanium, offer significant weight reduction. Titanium valves reduce valvetrain
reciprocating weight by more than 12.4 ounces. However, many racing classes specifically ban the use of titanium
Custom valves from Ferrea have shown to be the best choice for the Isuzu 4X engines.
Increasing valve diameter (and valve seal diameter) is a good way to increase airflow through the engine or
balance flow if the valves are sized incorrectly from the factory. The traditional rule-of-thumb is that
naturally aspirated engines make the most power when the exhaust valve diameter is 75% of the intake valve
diameter. This is because it is relatively easy to push hot exhaust gasses out of the cylinder, while
comparatively difficult to pull fresh air into the cylinder. The rule-of-thumb for turbocharged, supercharged,
and nitrous injected engines, is that these make the most power when the exhaust valve diameter is 90%
of the intake valve diameter. This is because the fresh air is being forced into the cylinder by an impeller
(or augmented by an oxygen rich additive). It just so happens that the valve size ratio for the DOHC engines
is 90.4%, and is optimized for turbocharged applications.
For a full race naturally aspirated engine, it might be desirable to increase the intake valve size. And for
a full race turbo engine, it might be desirable to increase the size of both valves while keeping the same
size ratio between them. However, the 4XE1 and 4XF1 DOHC engine combustion chambers are rather tightly packed,
and there is not a lot of space available for larger diameter valves. Further concern should be made to take
into consideration for increased valve overlap, when using adjustable cam sprockets. In such cases, valve
clearance will need to be measured at set points with both valves partially open, to simulate the clearance
during valve overlap.
After some investigation, it may become very apparent that the factory valve sizes are in fact optimized and
Titanium Valve Spring Retainers
The OEM valve retainers in the 4X engines are steel, and weigh .6 ounces each. Titanium retainers weigh less
than half that, and cut the weight of the reciprocating mass in the valvetrain by over 4.8 ounces.
However, many racing classes specifically ban the use of titanium valvertrain components.
The factory head gasket, when properly installed, is more than strong enough for high performance naturally
aspirated and turbocharged use. Under these conditions, head gaskets are normally only a problem with excessive
detonation due to improper turbo tuning, low quality engine management (inadequate standalone ECU), or a failed
Most rebuilds typically use an OEM or OEM replacement headgasket.
Regarding OEM head gaskets:
There are four different head gasket patterns for the four different 4X series engines.
The SOHC and DOHC engines have a different head gasket pattern, because the blocks and heads of these engines
is NOT the same. The head gasket is reasonably easily identified by the slanted or diagonal oil passage
at the corner (instead of the vertically oriented passage found on the DOHC engines).
The three different DOHC engines use different head gaskets with differently sized and shaped water passages
To control the coolant flow through the engine, and provide appropriate cooling to match the heat produced by
the three different engines. The head gasket must be matched to the engine to prevent overheating or excessive
wear. Using the small water passage head gasket on a Turbocharged engine will result in overheating, blown
head gaskets, and typically warps the head. Using the large water passage on the Non-turbo engine will not
allow the engine to reach proper operating temperature, and the engine will experience excessive bearing
wear due to running cold with cold oil.
There have been two preferred OEM replacement head gaskets. Both were made by the manufacturer which supplied
the gaskets to Isuzu and bear all the markings of the OEM parts. But the two different companies are selling
two different patterns for two different engines. The Felpro PN 9685PT1 is the pattern for the TURBOCHARGED
4XE1(W) engine used in the Impulse RS AWD and Elan M100 Turbo. The Victor Reinze PN 5911 is the 4XE1 DOHC NON
TURBO gasket pattern which is correct for 1989-1991 Geo Storm GSi (as well as Isuzu Stylus XS and Impulse XS).
It appears that the Victor Reinze gasket has been discontinued, and it will take some investigation to see
if there is another match among the lesser known brands. Coreteco PN 20702 and DNJ PN HG36 look promising.
ITM PN 0941935 would need to be verified more closely.
For the 4XF1 1.8 liter DOHC engine, the head gasket appears to only be offered as part of the DNJ brand gasket
set PN FGS3027. The other alternative would be to obtain the 4XE1 DOHC Non-turbo gasket, use the original
gasket as a pattern to trace the larger water passages onto the smaller gasket, and carefully enlarge the water
passages to match using a Dremel tool and a grinding bit.
The head gaskets made by the OEM parts supplier (often repackaged as aftermarket OEM replacement parts sold
through auto parts stores) are identified by the number of tabs in the end of the gasket.
- 4XE1 DOHC Non-turbo - no tab; Victor Reinze PN 5911
- 4XE1 DOHC Turbo - 1 tab; Felpro PN 9685PT1
- 4XE1 SOHC - 2 tabs
- 4XF1 1.8 liter - 4 tabs, in DNJ gasket set PN FGS3027
For extreme compression and extreme forced induction levels, a high performance head gasket may be required.
offered a range of copper head gaskets in various thicknesses. Copper does not compress and
is basically impervious to damage from excessive cylinder pressures and even engine detonation. But copper
gaskets and difficult install and difficult to get to seal.
Multi Layer Steel (MLS) head gaskets are available from at least four different sources. Two Japanese companies,
and an Australian company. MLS gaskets use three or more layers of steel, with embossed
patterns in the various layers, and a coating on the surface of the steel, such as Viton fluoroelastomer,
to increase sealing potential. The thickness of the steel sheets can be varied to change or correct the
One word of caution: The Australian MLS head gasket has received numerous complaints.
This gasket is offered in one pattern, and even the maker does not know which of the four head gasket patterns
it matches. The water and oil passages of the 4XE1 DOHC Non-turbo, 4XE1 DOHC Turbo, and 4XF1 engines are
different, and the head gasket is used to control the water and oil flow through the engine. Using the wrong
pattern will result in either overheating or excessive bearing wear due to failure to reach the correct operating
temperature. And the 4XE1 SOHC has a completely different bolt pattern, water passage pattern, oil passage pattern,
and exterior shape from the DOHC engines.
Other complaints include leaks and failure to seal when installed. It seems the gasket lacks sufficient embossing
around the cylinder bores to prevent leakage between the cylinders, the water jacket and the oil passages. There
is an additional question as to the quality of the sealant coating on the metal.
Head Bolts – A Waste Of Money
The factory original head bolts are grade 12.9, 12 mm diameter, socket head cap screws, with a tensile strength
around 172,000 PSI. There has never been a documented case of failure of an original head bolt. There are people
who offer, and take money for heavy duty head bolts/studs for the 4X engines, but money spent replacing the
original head bolts is money wasted.
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