Please note: this is the second article in a series of articles on a V8 MR2 engine swap
In my introduction, I left off at the point where I decided to change from the Audi V8 engine, over to another engine for my V8 MR2 swap. The reason I originally chose the Audi was because there was a very good chance it would fit without cutting the car chassis. The axle clearance issue, unfortunately, could not be known until I removed the oil cooler/filter from the side of the engine. It then became apparent that the engine block itself was in the way. The block could not be cut and re-welded as the oil pump and oil pump drive pulley were also in the way. I had to purchase the engine in order to find these things out. This is the nature of swaps like this, that no one has ever done before. Know anyone that wants a 1997 Audi ABZ 4.2L V8 with 70K miles?
I realized at that point that it might be easier to find a transverse V8 engine, even if it’s too long to fit, and go ahead and cut the chassis and weld it back up again. Welding some sheet metal is easier and far cheaper than trying to solve the problems created by using the Audi V8, which might be solvable, but only with a lot of very expensive custom machined parts. Even then, I still feel the axle clearance issue will still cause problems, as it would require a 3rd CV joint be added to the right axle, and some large permanent angles would be required, which are never a good idea with CV joints. The areas of the MR2 chassis that needed cutting were thin sheet metal. They are structural, but they are not heavy structural, so welding sheet metal back into the cut areas should not pose too much of a problem, plus, I had a plan to reinforce those cut areas. More on this later in this article.
I need to backtrack a bit, as I have not discussed the transaxle yet, which is another major component that needs to fit besides the engine. During the initial research into this project, I decided to check out what the Fiero guys were doing. The Fiero, made by GM and Pontiac from 1984 to 1988, was a mid-engine affordable car, which also used a front wheel drive train that was simply transposed to the rear of the car, just like all 3 generations of MR2. The engine sat slightly ahead of the driving axle, so when the front wheel drive train was moved to the rear of the car, it became mid engine. Note that the definition of a mid-engine car is one where the engine sits ahead or on top of the rear axle, just like the MR2. Note that a rear-engine car has the engine behind the rear axle, like a Porsche 911 or the original VW Beetle. The Fiero guys have been doing V8 swaps into the Fiero for many years now, since the early 1990’s. The car is wider than the MR2 mk2 (mark 2), so it’s easier to fit a V8. I contacted one of the oldest companies that did V8 Fiero conversions and found out that they were using a new GM 6 speed front wheel drive transaxle. This transaxle was the strongest transaxle available to the average person. It had the highest torque rating.
Transmissions and transaxles are rated by torque from the factory. Transmissions and transaxles are not rated by horsepower, as horsepower does not kill transmissions. Too much torque is what can kill a transmission. Think of it like this, when you turn a bolt with a wrench, you are applying torque to the bolt. Apply too much torque to an already tight bolt, and you can snap the head off. Transmissions are similar in that too much torque can break the gear teeth right off of a gear.
The torque rating given is also based on the weight of the intended vehicle. Torque rating means, how much torque from an engine can the transmission or transaxle handle, and still last a long time, usually 100,000 miles. The heavier the car, the lower the torque capacity of the transmission. Use the same transmission in a lighter car, and the torque rating will be higher. A heavier car generates more stress on the transmission components when the car is accelerating at its maximum, when the engine is generating maximum torque. This is always first gear, as that is the maximum torque multiplication given by the transmission to the engine during driving. The 1997 Audi V8 I tried to use was rated for 295 ft-lbs of torque at its maximum. The Northstar has a similar rating.
The new transaxle the Fiero guys were using was a new GM unit rated for almost 300 ft-lbs of torque, in a 3500 lb car. The biggest surprise is that this transaxle could be purchase NEW on eBay for only $475 SHIPPED!!! That was in 2007. Nowadays, you can get it for around $375. There is a story behind this, but basically, GM surplussed out the 2006 version of this transmission, as they decided to change it in 2007.
This transaxle could take the torque of a healthy V8 engine. If the car were lighter than 3500 lbs (Fieros are around 2700 lbs, and mk2 MR2’s are 2700 to 2950 lbs), the torque capacity of this transaxle would be higher than its factory intended application. The 6 speed was also shorter than the stock MR2 transaxles by 1.75 inches, allowing more room for a longer motor and adaptor plate (if one was needed).
The icing on the cake, and the part I did not know until 6 months after buying the Audi V8, was that the Cadillac Northstar 4.6L V8 bolted up to this transaxle, with the exception of one bolt, that could easily be accommodated by a fabricated small bracket or welding some additional aluminum to the side of the transmission bell housing flange on the block. No adaptor plate needed!
By switching to the Northstar, I could solve the adaptor plate issues, the starter location issue, and most importantly, the axle clearance. This is because the Northstar was used originally in transverse applications from 1992 or 1993 up to 2002. It’s still used transversely, but there is a longitudinal version and GM moved the starter location. Checking my notes from November of 2007, I found that I had written that the Northstar was 23.75 inches long at the critical dimension-this is the length of the engine from the crank pulley bolt head to the rear face of the bell housing flange on the block. At the time, I may have accidentally included the flex plate in the critical dimension, but it is really not part of the critical dimension. It turns out that the 23.75 inches measurement was INCORRECT!
Back in Nov 2007, I did not use a straightedge on the Northstar I looked at in the wrecking yard, just a tape measure. Also, at the time, I assumed an adaptor plate would be needed for the Northstar to mate to the GM 6 speed (which was wrong as I later found out, and discussed above). If you have ever measured an engine for length, you would know that it can be tricky to get an accurate measurement as the engine has a lot of width and parts in the way of measuring. In December of 2008 (a year later), I went back to the wreckers and re-measured the Northstar. This time, I was much more careful. I found that it was actually 23.0 inches long, but remember, no adaptor plate was needed (which would add around ½ inch to the whole assembly). This puts a different spin on the problem.
OK, so if this is a bit confusing, or hard to picture, I will simplify it here. The stock MR2 turbo motor is the Toyota/Yamaha 3S-GTE, and the stock transaxle is the Toyota E153. The 3S-GTE is 20 inches at the critical dimension. The E153 is 16 inches long. Total length of this drive train is 36 inches. However, there is a notch in the left side of the car that matches the E153 case perfectly, giving it a little additional clearance. It won’t match the GM 6 speed. The stock MR2 drive train has about ¾ inches of clearance at the crank pulley, and the same on the transaxle end.
Now, the Northstar is 23 inches, and the 6 speed is 14.25 inches, for a total of 37.25 inches or 1.25 inches more than the MR2 stock drive train. This would not fit, as the MR2 chassis notch at the transmission end does not match the 6 speed. Cutting of the chassis is required, but not very much cutting.
The prior attempts to put do a V8 MR2 swap in the Gen2 attempted to use a Toyota 1UZ-FE V8 which is 26 inches long, and the E153 which is 16 inches, for a total of 42 inches. This is 4 inches more than stock! 4 inches might not sound like much, but it’s a lot when you are fitting a drive train. To make things worse, an adaptor plate is needed to mate the 1UZ to the E153 transaxle. So, add another ½ inch.
This approach required complete removal of the MR2 subframes, which are only 2 or 3 inches wide in those areas on each side of the car. This would weaken the car quite a bit. Also, the right shock tower would probably hit the rear head of the 1UZ V8, so it would have to be cut a lot. I think the guys that did the prior attempts figured this out after they cut their MR2s, and they gave up.
In December of 2008, I went ahead and purchased a low mileage Cadillac Northstar. Fortunately, 1994 to 1999 were all the same, and I found out that they were the easiest to add a stand alone engine management solution to. The later 2000 and up versions are far more difficult. The 1993 version is also the same, except that the intake manifold was cast from metal (aluminum or magnesium), instead of plastic like the 1994-1999 versions.
I was able to mate the Northstar to my 6 speed that I had purchased in December of 2007, a year earlier. Finally! I made some progress! I then did a test fit into my 1991 MR2, and made the cuts to the chassis, so now the Northstar and the 6 speed fit in the car. I just recently mounted an alternator to the Northstar, and found a way to fit it much more tightly to the block, giving the engine additional room around it in the car. I am currently in the process of building the motor and transaxle mounts. Once that is complete, I can send the axles out to be shortened and re-splined.
Getting back to the cutting of the chassis that I promised at the beginning of this article: The areas of the chassis that I had to cut are part of a stamped sheet metal “pseudo” sub frame. It’s not really a sub frame as its stamped sheet metal that is spot welded to the body, but they are in the shape of traditional frame rails (i.e. rectangular cross section). I notched these “frame rails” on each side of the car to clear the Northstar crank pulley and the case on the GM 6 speed. The metal is thin gage sheet metal. I will re-weld these areas with new sheet metal to cover the holes, and I will probably put additional sheet metal doublers over these cut areas, overlapping the weld joints. This will ensure that the repairs will be stronger than original. This is the standard approach in the aircraft industry, when facing a similar issue.
In addition to the doublers, I am going to create a new rear suspension cross member from scratch, as the original cross member interferes with the GM 6 speed case, and the new axle locations. This new cross member will have extended sections that the original did not have, which will straddle the notched areas of the sub frames, further enhancing strength. I did not need to cut the right shock tower, but that is not completely certain yet. I did have to remove some metal that was attached to the outside of the shock tower. It did not add much in the way of structure, so I felt it was OK to do this.
The end results of these modifications will probably add weight to the car, and I will weigh the new parts to get an idea of how much added weight there will be, but I estimate it will be less than 20 lbs more than stock. The V8 should easily handle an extra 20 lbs of car!
Please stay tuned for the next in this series of articles.
Update: It turns out that the custom cross member is NOT needed! I found a way to use the stock MR2 cross member (with seme minorl modifications). This is great, and is a HUGE time/money saver. The MR2 crossmember is extremely important because the rear suspension and rear toe control rods attach to it, so attempting to duplicate it accurately, and with enough strength is extremely difficult and time consuming. Now that I found a way to re-use the original cross member, I have save huge time and money.