Isuzu Motors Limited commissioned Georgetto Giugiaro in 1978 to create the replacement for their sporty model 117 Coupe, which was also designed by Giugiaro in 1966. Giugiaro was presented with a rear wheel drive Isuzu Gemini (I-Mark) rolling pan and was given carte blanche to design the new sporty model Isuzu. In the late 1970's, Giugiaro had been experimenting with variations of a wedge shaped design, and had named several successive wedge designs for the aces of the four houses in a deck of cards (hearts, clubs, diamonds, and spades). These are throught to be the Audi Quatro, Delorean DMC, BMW M1, and the fourth and final of the series, the Ace of Clubs, was the prototype that would become the Isuzu Piazza and Impulse. Giugiaro had previously penned the original design of the VW Scirocco in 1974, though it was extremely primitive by comparison to the Aces cars, especially the Clubs car, which incorporated much more sophisticated styling cues including radiator grille with incorporated headlights, bonded windows, aircraft style flush mounted glass, limosine style doors, built-in painted bumpers, hidden body seams, cut of the hatch wrapping into the sides, exterior drip channels eliminated, most sharply raked winshield of any mass production vehicle at design time, aircraft style instrument cluster with adjustable pods, and digital instrumentation.
VW, redesigned the Scirocco in house, in 1982, three years after the Isuzu Piazza went into production.
The design for the Clubs car and the Isuzu Piazza and Impulse was never, at any time, submitted to VW, because it was commissioned by Isuzu. In addition, it was rear wheel drive and powered by an Isuzu engine, none of which would be of any benifit to VW. Automotive designers such as Giugiaro do not make submissions to automobile manufacturers that are not commissioned, and if that work is refused by the manufacturer, that work remains the property of that manufacturer, paid for by the commission. No rejected design is ever presented to another car manufacturer. Such an action would be considered theft and fraud and and the idea of Giugiaro or any other designer engaging in such unethical and illegal actions is totally absurd.
It is a documented fact that Ford, Chrysler, GM and other car manufacturers purchased first line production Isuzu Piazzas to reverse engineer (Car and Driver, March, 1982) and the information gained from this became a recipe book for the car design teams of each of these manufacturers. From the look of the VW's in house redesign of the Scirocco in 1982, it is undeniable that VW also purchased a Piazza and copied the design details in their own work.
Certainly, the original Scirocco and the Impulse are similarly designed cars, which would be expected as they were both designed by the same man. While it is true that they are related, comparison shows that this relationship is more that of comparing Neanderthal man to Modern Homo Sapien, the Scirocco being the Neanderthal.
VW Scirocco owners may feel better by spreading myths and claiming that the Impulse was the rejected Scirocco redesign, or that it was a copy of the Scirocco, but the facts as presented above prove these fabrications to be nothing more than what they are, false.

While the DOHC and SOHC 1.6 liter engines used in the Geo Storm and also the Isuzu Stylus both share the model number "4XE1", this designation has more to do with the displacement and time period they were used than with interchangability of parts. The DOHC and SOHC engines use different blocks, cranks, pistons, and almost every other engine internal. The heads of these engines are not interchangable. Even if someone did manage to get a DOHC head bolted to a SOHC block, the compression ratio would not be as high as the original DOHC engine, and because the SOHC pistons lack the four reliefs for the valves, the valves would certainly break off from contacting the top of the piston.

The Geo Storm was manufactured by Isuzu Motors Limited in at the primary assembly plant in Fujisawa, Japan. It was wholely and completely manufactured by Isuzu Motors Limited, with an Isuzu 1.6 or 1.8 liter engine and a Isuzu 76 mm transmission. Isuzu provided these vehicles to General Motors Corporation and they were rebadged as "Geo" brand cars, and sold by GM.
GM also purchased cars from Suzuki (Swift/Metro, Sidekick/Tracker) and Toyota (Corolla/Prizm), however, those cars share no parts with any other of the various models of cars rebadged by GM.

Origin of the myth:
Aftermarket replacement parts manufacturers incorrectly cross referenced Toyota Corolla and Geo Prizm engine internals claiming that these pieces fit Geo Storm and Isuzu engines, because both engines, which have close to the same displacement, share the same bore and stroke measurements. The error was made due to a total lack or research and comparison on the part of the partsx manufacturer' employees. The error remains in many cross reference lists, because sales of pistons, rods, and other engine internals for Isuzu engines is so low that many of the manufacturers have never been confronted by a customer servicing an Isuzu engine that their part does not in fact fit an Isuzu engine, or they have determined that it is more trouble to correct the error in their database than the amount of trouble to deal with one or two unhappy Isuzu customers.

Over the last four or more decades, General Motors Corporation has steadily purchased more and more of the controlling interest (stock) of Isuzu Motors Limited. For at least the last two decades, Isuzu Motors has opperated as the Japanese division of GM, in the same way Holden is GM Australia, Vauxhall is GM UK, and Opel is GM Germany.
GM has been making the decisions of what Isuzu makes and how Isuzu markets those products since before 1985. GM has never been known for making logical or even profitable decisions. In addition, GM has always stipulated that Isuzu never heavily market its own badged products in any markets that they could compete with GM's domestic or rebadged products (Geo).
In 1992, GM looked at its poor sales of its own small cars, the Cavalier, Sunbird, Beretta, and Corsica, and compared those sales nubers to their rebadged Isuzu car, the Geo Storm. The comparison was rather embarassing, as GM saw it could not sell it's own domestic product while their rebadged product outsold their own cars. Their solution was to eliminate the Storm and thereby kill Isuzu's car making program.
It appears that most of Isuzu's car designers defected to Subaru, and it is conspicuous that Subaru introduced their Imprezza model only a few years after GM killed Isuzu's car making program. It is also conspicuous that the Imprezza is dimensionally and proportionally nearly identical to the Impulse RS AWD car, and it is equally conspicuous that the two share such similarity in name.
The fact of the matter is that politics brought about the end of Isuzu cars. Other car manufacturers have faced similar situations that also had nothing to do with the quality of the cars themselves, and there is no reason to think that Isuzu not making cars is an indication that their cars were of poor quality than there is to believe that the Supra, RX-7, CRX, Prelude, Integra, Starion, 3000GT, 300ZX, Scirocco, Countach, F40, or 911 are poor quality because they are no longer made by Toyota, Mazda, honda, Mitsubishi, Nissan, VW, Lamborghini, Ferrari, and Porsche.

Dyno testing has proven that 2 1/4 inch diameter pipe is too small for 1.6 liter, four cylinder, naturally aspirated engines. 2 1/2 inch diameter pipe outperformed 2 1/4 inch diameter pipe across the entire range of engine speed. 2 1/4 inch pipe performed 1-2 HP less than 2 1/2 inch pipe from 2,000 RPM through 4,500 RPM, where the gap widenned to 2-3 HP up through peak horsepower at 6,500 RPM and widenned to over 4 HP at maximum engine speed.
2 1/4 inch diameter pipe simply is not large enough not tocause a restriction in exhaust flow costing power at all engine speeds, but costing the most power at mid range and upper engine speeds.
When considering that this testing was done on 1.6 liter engines, it is quite obvious that 2 1/4 inch pipe is even less appropriate for larger engines, such as 2.0, 2.2, and 2.4 liter displacement, because these larger engines have the ability to pump even more exhaust than the 1.6 liter engines.

AEM, Venom, STR, and other companies sell "racing" fuel rails for common, mundane brand cars. These fuel rails have a 1/2 inch diameter (.196 square inch cross sectional area), round center bore, and the manufacturers claim these fuel rails will flow enough fuel to support 500 HP.
The original equipment fuel rail on the 1990-93 Isuzu 1.6 and 1.8 liter engines (even the SOHC ones), is extruded aluminum with a 1/2 inch by 7/16 inch square bore (.219 square inch cross sectional area). This is larger than the socalled upgrade fuel rails sold for the common, mundane brand cars. Using these same manufacturers claims, the Stock Isuzu fuel rail, which is 12% larger than their racing fuel rails, would flow enough fuel to support 560 HP.
Why would anyone want to downgrade from the stock Isuzu fuel rail to something small, like the "racing" fuel rails for common, mundane cars? Why is this an upgrade for owners of those other cars? Because they come from the factory with rediculously small fuel rails that barely support the level of fuel flow required for their stock level of performance and can not keep up with the fuel demands of even the most basic aftermarket tuning parts. Meanwhile, Isuzu, as always, overdesigned their engines and there is no need to upgrade this part because it will take more power than most could build into the engine.
Some would point out that the only reason the owners of those common, mundane cars buy those parts is because they are shiny and are ornaments in the engine compartment. For those, I would suggest that instead of considering downgrading your own fuel rail with a similar item, get out some steel wool and polish the stock one.

While this idea may be rooted solidly in the fact that a high horsepower rear wheel drive car, that can be driven in a way to spin the drive tires halfway down the track, it simply is not true of comparatively low horsepower, front wheel drive cars.
We have done testing with both 20 inch and 22 inch diameter Mickey Thompson racing slicks on mildly built 1990-91 model Isuzu front wheel drive, 1.6 liter, DOHC powered cars with approximately 140-155 HP, and have found that racing slicks will add time to the quarter mile ET, and slow the car by three or so miles per hour in trap speed when compared to the identical car at the same track on the same day, on stock sized street tires (Yokahama Avid H4 Touring tires). Why is this? Because racing slicks, with their lack of tread and grippy rubber compound, cause a dramatic increase in friction, making it more difficult for the engine to push them across the pavement and accelerate once the car is moving. Racing slicks make a dramatic difference in 60 foot and even 330 foot times, as this is because even a unaltered, stock engined FWD car has the ability to spin the drive tires on takeoff and when shifting into second gear and pushing the gas pedal to the floor. The added friction of the slick tires keeps the drive wheels from spinning and the car will move off the line from a standing stop more quickly. However, that fraction of a second gain on takeoff is dwarfed by the increased friction of the tires and increased effort required by the engine to move those tires across the pavement, once the car is moving and there is no fear of the tire spinning.
Racing slicks do wonders for reducing wheel spin and getting the car off the line more quickly, but until the engine has significantly more power and is pushing the 200 HP mark, using racing slicks will actually cause an increase in quarter mile time, due to the increase in friction of pushing those tires across the pavement once the car is already moving.

The Isuzu 76 mm transmission, as specced out by Isuzu, is all but indestructible when compared to the range of front wheel drive transmissions used in production vehicles. Isuzu hot rodders have had no reported problems running over 450 HP and an equal amount of torque through these transmissions, the most common complaint being that they can not find (or rather refuse to buy) axles that will take this level of power. Lotus used this transmission in their Elan M100 and bought roughly 1 1/2 times more transmissions than engines, anticipating the same failure rate as the Renault transmissions they used in their Esprits, only to find that in the decade following introduction of the Elan M100, they did not experience a single transmission failure in all the Elans they sold, forcing them to sell off several thousand transmissions at under half what they purchased them from Isuzu for. This is much stronger than Mitsubishi, honda, and arguably stronger than anything Muncie/New Venture/Getrag ever built. However, when considering the use of these transmissions by General Motors in 1994-1999 models such as Cavalier and Sunfire/Sunbird, it must be taken into consideration that General Motors changed the metal tempering specifications on both the gear clusters as well as the differentail planetary gears. This was done by General Motors to save money, as the Isuzu specification, as with almost every aspect of Isuzu car building, was far in excess of the capabilities of the vehicles the transmission was to be used in. General Motors reduced the steel tempering so that the transmissions they contracted from Isuzu were built to withstand only the capability of the engines they were using in their vehicles, and GM's specifications were far below Isuzu's own specifications. GM also had the synchros redesigned from a single piece unit to a two piece unit which engages slightly more smoothly but wears out much more quickly. As a result, GM enthusiasts complain loudly about the quality of the Isuzu transmissions, wrongly blaming Isuzu, when they should be blaming General Motors for telling Isuzu to provide GM with transmissions that are not as strong as the transmissions that Isuzu used in their own vehicles.
The solution is for GM enthusiasts to either seek out and use the Japanese spec transmission internals (this will limit the availability of gear ratios, which are typically lower in the Isuzu cars) or to seek supplimental steel tempering for the GM spec transmission internals when they build their transmissions, to bring them up to the tempering specification that Isuzu used in their own cars (through heat treating and/or cryogenic treating).
With specific regard to the Muncie/Getrag/New Venture transmission found in 2000+ Cavaliers and Sunfires, these tranmsissions have been found to be extremely weak compared to the Isuzu 76 mm transmissions even when built to the lower GM specification, and especially when compared to these transmissions when built to Isuzu's specificaition. The Getrag/... uses "plastic" synchros which wear even more quickly (5,000 miles) than even the two piece GM redesign for the Isuzu transmission. What is meant by the word "plastic"? This word is not used to mean "made of pertoleum products", but rather meaning "capable of continuous and permanent change of shape". This term is applied to the Getrag/... synchros for several reasons. First because they are made from a composite metal material similar to GM's growing fastination with pressed and formed parts made of powdered metal with epoxy or cement type bonding agents. These materials have shown poor durability and break easily when compared to true metal pieces, be they molded or machined. Second, GM chose to use a fiber based liner on the synchros instead of the more traditional and more durable brass liners used in older, more established and better performing designs. This change was made to smooth shifting action, but sacrifices severely in lifespan of the material, which wears just as quickly as the fiber in a clutch disk, requiring frequent replacement. Finally, the Getrag/... design makes use of synchros that are too small in comparison to the gear clusters that they are responsible to control, and the lack of a larger synchronizer size results in rapid wear and quicker failure.
Owners of the 2000+ vehicles with these Getrag/... transmissionshave reported even more transmission failures than did owners of the 94-99 cars with the reduced spec Isuzu transmissions. Clearly GM has decided that they will not be putting any money into building the transmissions of these vehicles any stronger than they must and performance enthusiasts are left to seek additional stress releiving and tempering when building a transmission to match their engine enhancements with the Getrag/... just as they must with the GM reduced spec Isuzu transmissions.
The most recent generation of the Getrag/... transmission used in the J-Body cars has an additional serious limitation in that the major gear clusters are not available to purchase from GM for replacement in the event that they break. The manufacturing process of these transmissions involves custom machining of the individual transmission case for the proper clearance where the bearings meet the case itself. As a result, the case halves are matched sets to the gear assembly installed within. It is not possible to change the gear clusters with new ones because the case has been machined to accept only the original cluster. It is not possible to change a cluster with a cluster from another used transmission for the same reason. Attempting to service one of these transmissions risks either an interference fitting or a loose fitting of the bearings and failure of the transmission for that reason. GM has designated these transmissions to be "non-rebuildable" for this reason, and has published to the transmission service industry that if serious failure of major gear assemblies occurs, this latest generation of Getrag/... transmissions must be replaced with new and can not be rebuilt.

These engines are not the "same". The 4XE1 and 4XE1-T engines have different compression ratios, as determined by the different pistons and rods used in these different engines. The pistons lower the compression in the turbo engine and the rods are both larger and have oil sprayers to cool the pistons (not to mention the head which has different camshafts and valves to work with the turbocharging system). While bolting the exterior pieces of a turbocharged engine onto and engine built to be naturally aspirated may be tempting, it is the fast way to turn that engine into a four hole trash can full of jagged pieces of steel and aluminum when that engine blows up because of excessive detonation caused by the combination of too much compression and boost pressure, because the OEM control system is designed to work with an engine built to lower compression.
If using the turbo system from the Impulse RS is the desired result, transplanting the entire engine or pulling the engine out and replacing all of the internals with the turbo model internals from the dealership parts department.
This is the same situation for those desiring to bolt the turbocharging system from a 1987-89 I-mark or Spectrum Turbo onto a Geo Storm SOHC engine.

The ignition timing of the 90-93 model Isuzu cars, Geo Storm, and Asuna Sunfire, is electronically controlled by the computer, through the ignition module in the distributor. The factory setting of the timing, set by twisting the ignition coil, determines only the baseline or minimum ignition timing from which the computer advances the ignition timing according to the ignition curve, or retards the timing as needed to prevent detonation.
Under normal operating conditions, with the ignition timing properly set to factory specification, the computer will advance the ignition timing as far as 45 degrees above the factory setting and retard the timing to zero degrees before the factory setting. This allows the engine to run properly.
If the baseline setting is advanced beyond the factory setting, the computer will compensate for this incorrect setting by retarding the timing and it will not advance the ignition timing past 45 degrees plus the correct factory setting (45+10=55 degrees), and more importantly, it will not be able to retard the timing back to the proper baseline setting as needed.
This is a computer controlled ignition system, it is not a carbureted vehicle. The computer can not be fooled into advancing the timing more than the pre programmed curve will allow, and the only way to re curve the distributor or alter the ignition timing is to reprogram the computer or install a programmable computer.