Engine - Engine Swaps
Engine Swap to Another Isuzu Engine
4X Engine Series - I4
The chassis and frame of the 1990-93 Geo Storm (and Isuzu sister cars) is identical, the only significant
differences between the various models and trim levels being the engine and transmission installed. This makes
engine swaps within the engine family [4XE1 SOHC, 4XE1 DOHC, 4XE1 Turbo (DOHC), and 4XF1 (1.8 liter DOHC)],
simply a matter of removal, installation, and changing over the peripheral pieces to match the engine being
used. In all cases, the best strategy to ensure success is to follow simple steps:
6VD1 - V6
It may be tempting to gather parts from a list, but this will invariably involve the discovery of a part
overlooked or missing, and a project that should take a day or two can drag on into years while tracking down
the missing pieces from far-flung corners of the continent.
- Obtain a shop manual or manuals for both the recipient vehicle and engine being replaced, and the donor
vehicle and engine being transplanted.
- Obtain a parts catalog(s) that will allow for comparison of what parts are interchangeable and what parts
- Place the two complete vehicles next to each other and work directly from the donor vehicle into the recipient
vehicle. Remove one part at a time from the donor vehicle, and install each part directly into the recipient
vehicle exactly as it was removed from the donor.
This same strategy applies to swapping a 4XE1 DOHC or 4XF1 engine into a Storm Base Model, swapping a 4XF1 engine
into a 1989-1991 Storm GSi, or swapping an Impulse RS AWD 4XE1 Turbo engine into any Storm.
It is nearly unknown that the Isuzu 6VD1 V6 engine was a 1996 Championship Winning DTM engine.
In the early 1990's, Opel (GM Germany) was having problems with their narrow angle V6 engine in the Calibra
DTM race cars. They eventually decided to use the Opel Montery (Isuzu Trooper) 3.2 liter all aluminum 6VD1
engine, because the wide 75 degree angle and more durably block structure could handle higher engine speeds
better than their own engines. Cosworth destroked the 6VD1 to 2.5 liters, built custom heads and individual
throttle body induction system, and dry sump lubrication system. According to the most conservative power
specs, the engine made 457 HP at 12,000 RPM. The Japanese sources cite 485 HP at 12,300 RPM. The race
shops who bought the remaining engines, after DTM changed their engine displacement rules, state that the
true specs are 540 HP at 15,000 RPM.
The Isuzu V6 would make a very good power plant for a race car, if built correctly. The engine length would
not be a problem, but the wide angle of the V may require modification of the firewall.
SOHC-To-DOHC Head Swap – A Bad Idea
The SOHC 1.6 liter engine is rated at 90-95 HP, and the DOHC 1.6 liter engine at 130. Changing from one to the
other is an attractive upgrade.
There is a long held misconception that the difference between the 4XE1 SOHC and 4XE1 DOHC engines is confined to
the head and number of valves therein. And that the SOHC engine can be easily converted to a DOHC engine simply
by changing the head. This is simply not true.
The misunderstanding is rooted in the model code designation shared by both engines: “4XE1”. But the fact is that
Isuzu assigns model codes to engines based on displacement, not based on relationship or shared design or parts.
All of the 1.6 liter gasoline engines made by Isuzu in the late 1980’s and early 1990’s share the same model code
4XE1, because that designation was assigned to all of Isuzu’s 1.6 liter displacement gasoline engines, regardless
of their actual relationship to each other. The fact is that the 4XE1 SOHC and 4XE1 DOHC are not sister engines,
and one was not developed by changing the number of valves in the other.
The 4XE1 DOHC engine traces its origin back to the COA-II concept car which debuted in 1985. The design went
back to the drawing board, and incorporated many design elements of the G161W engines from the 117 Coupe and
Bellett GT-R. It was developed from the start as a turbocharged engine, which could also be detuned and offered
as a very potent naturally aspirated engine.
The 4XE1 SOHC engine is a direct descendant of the 1.5 liter 4XC1 engine, developed specifically and exclusively
for the North American market, because General Motors did not believe that US car buyers would accept the second
generation 1.5 liter 4XC1 engine that Isuzu had developed for its base model Gemini JT151 Sedan and Coupe. They
took the second generation 4XC1 engine, over bored the block to increase the displacement to 1.6 liters, and
renamed the new engine “4XE1”.
The 4XE1 SOHC and 4XE1 DOHC engines do not share the same engine block, crankshaft, or pistons, among many other
smaller differences. The water and oil passages between the heads and block are not the same pattern, hence the
fact that these two engines use different head gaskets. If the heads and blocks are switched, these passages do
not line up. And all of this ignored the different wiring harness, fuel injectors, and ECU, which are not shared
between the two engines.
If someone were to take a SOHC engine, remove the head, disassemble the block, swap in the DOHC crankshaft and
pistons and various other smaller bits that are different, modify the block and head so that the oil and water
passages line up, and custom make a head gasket to fit between the two disparate pieces, and change over the
wiring and electronics, then the expected power output would be 10+ HP below that of the 4XE1 DOHC engine. And
the labor and money expended to do such a project would far exceed simply taking a complete 4XE1 DOHC engine and
swapping the complete engine as a unit.
Engine Swap to Non-Isuzu Engine
Many who have been unable to tap the potential of the Isuzu engine, have pondered the idea of transplanting in an
engine from a different manufacturer. The response to this might best be left to a quote from Colin Spooner,
Lotus Design Director, who explained that once Lotus was acquired by GM, their choice for engine in the Elan M100
was an easy one:
It turned out that Isuzu had the greatest potential. … … We came to an agreement very quickly that; they had
a new generation of 16 valve 1.6-litre engines for front drive. … … The proposal sold itself. Isuzu were in the
very early stages of their work, but prototype engines were running on the bench and fairly accurate predictions
of weight and performance could be made --- enough to be sure that they would be very competitive three or four
years down the road. The normally aspirated (NA) version (4XE1) would be lighter, more compact, and more powerful
than the equivalent Toyota unit (4AGE), while the turbo version (4XE1-T) was considerably better. The only
serious alternative was Opel’s 16-valve 2-litre (ECOTEC). It was at a similar stage of development, but did not
fit the bill as ideally as the Isuzu. (We) weren’t sure of its character --- with less free-revving, low inertia
feel --- would suite a Lotus…
Toyota’s 4AGE and GM’s Ecotec are both popular four cylinder engines, and popular for use in engine swap
projects. However, both are, in Lotus’ opinion, inferior to the 4XE1. Who are we to argue with Lotus?
“Lotus Elan”; Mark Hughes; Osprey Automotive, Osprey Publishing; Page 61.
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