Automatic Transmission 968 Conversion

Stuart, I've got a few fast cars including a 366 Whp 944 Turbo 3.0 liter, a 1.8T powered VW Corrado and a 94 Corvette ZR1. I'm just looking for a relatively easy conversion that will be reasonably cheap, fast and reliable. Its a cabriolet so I'm not all that interested in absolute handling. I have a 944 race car for that. With that being said, is the LT1 or LS1 conversion smpler? I really appreciate everyones help.

LT1 Pros:
-Cheap
-Readily available

Cons:
-Cooling sucks (reversed cooled POS...probably spend a lot of time trying to keep it from overheating)
-Optispark sucks
-Sealing sucks (leaky, like a SBC)
-Heavier

If you're looking for a budget swap, i wouldn't do the LT1. The 5.3 is just as cheap, just as available, and doesn't have the issues above.

The 5.3L is the truck engine I have in my 2005 GMC, It has proven to be very reliable. What kind of accessories do I need to fit it into a 944? What is the best source vehicle?

i believe xshop has a truck motor in his conversion he should be able to shed light on the accesories on that setup and you might also consider the escalde motor also which is very simular to the ls3.the truck 5.3 is very comon motor in salvage yards.the ls3 is all alum. and would be lighter than truck motor but 5.3 probaly more readily available.

I did some checking on car-part.com and the aluminum L33 engines are only a little more expensive than the cast iron 5.3s. I see a plan emerging here. I know from reading other posts that the intake has to be changed to an LS1, I guess that the f body accessories are needed also, but would like someone to confirm that.

Unless you use some of xschop's parts, you will need the f body brackets and water pump. Be careful when piecing this stuff together, you may spend the same buying pieces as you would if you can find a cheap LS1!

Thanks for the advice. I won't decide until I get the 968 and evaluate it. Then, depending on what's available at the time, I'll decide on LS1 or L33. Because there appear to be many more Low mileage L33s available, that may still be the way to go.

I think the real challenge will be getting the Tiptronic’s TCM  (computer) to work with the GM’s PCM. Has anyone done this? Tom Egan may know.

I’m guessing you’d need to find a transmission genius that is an expert on the GM automatic’s as well as programming the ZF model 4HP18FL transmission. The computers will need to 'talk' to each other.

Or use a beefed up 3 speed 944 automatic, using the 944 kick-down cable to the throttle pedal.

Though it sure would be nice to have a locking torque converter.

I'm not sure if the 968 Automatic "talks" to the DME or just uses some of the same signals, TPS, RPM etc.

Does anyone know?

I can answer any questions you might have on the GM automatic trans. and its electronics. The ZF units are controlled by the tcm , that gets some information from the dme. I have rebuilt a lot of ZF units. As far as getting the GM pcm to work with the 968 TCM, wow that might be a challenge but I suppose any thing is possible. LS1 INTO A 944S2 WHAT FUN IT IS. Good Luck

im sure it could be done.also kwik performance has accessory drive setups for the ls motors.an ls 9 would be a neat motor to install.

I checked a 968 schematic online and there are at least 5 wire connections from the 968’s DME to the Tip’s TCM in which the function as well as the signaling type & direction need to be identified (since the DME would be replaced with the PCM from the LS engine) and may need to be duplicated.

Image 1 shows from DME pin 17 labeled “Fuel Injector 1”, goes to both that fuel injector and also to (Image 2) T46 pin 8 BK/GR which continues to (Image 3) pin 21 WT/GN of the TCM labeled “Load Signal”.

Next, image 1 shows from DME pin 42 labeled “Switch Lifting”, goes to (Image 2) T46 pin 2 BN which continues to some sort of shifter park/neutral control unit, but not to the TCM.

Next, image 1 shows from DME pin 52 labeled “Throttle Valve Signal”, goes to (Image 2) T46 pin 1 WT/BL which continues to (Image 3) pin 11 WT/BL of the TCM also labeled “Throttle Valve Signal”.

Next, image 1 shows from DME pin 51 labeled “Input Transmission Control”, goes to (Image 2) T46 pin 6 OR/YL which continues to (Image 3) pin 32 OR/YL of the TCM labeled “Engine Ignition Adjustment”.

Next, (oops, missed in image 1) from DME pin 6 labeled “Rotational Speed Control”, goes to (Image 2) T46 pin 7 BK/WT which continues to (Image 3) pin 3 BK/WT of the TCM labeled “Ignition Frequency”.

I guess once you get the car you could put a scope to each of these and see what’s what…unless someone has this info in a tiptronic training manual.


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It may be easier to Trick the Porsche TCM by leaving the DME wired into the car, but not actually controlling anything. It looks like the only real inputs it uses are TPS and RPM, so you would need to add those sensors to the GM engine so the DME recieves correct information. The TPS would be an easy one to add, you could possibly add it right to the throttle pedal. the RPM wouldnt be as easy but definatly doable, the guy on LS1Tech thats swapping a ls1 into a 996 turbo is using a crank pully mounted rotor with the proper tooth count to give the 996 DME accurate RPM input.

There are some tach rpm converters that can convert both signal levels and 8 cylinder to 4 cylinder signals. This may work for the RPM signal to the TCM. I don't really want to leave the DME in the car, since I will probably use it in the 968 powered race car.

I would also like to confirm what changes have to be made to the accessory mounting if I buy an F body engine. I will be running A/C and Power Steering. I know this has probably been covered, but in January I'll start buying the engine and parts for the conversion and don't want to end up spending extra because I made the wrong parts decision. My basic decision will be between the LS1 and LS3 and if its the LS1, F body or not. If a decent F body LS1 is available at a reasonable price does that solve all the clearance issues in the front of the engine?

F-body accessories will work fine and will give you adequate clearance; I have run them on all of my conversions. On my current build I tilted the top radiator forward for more room in front of the engine. An ls1 will be a lot less $$ than a LS3 but the LS3 will have a good bit more power and newer ECU. The LS3 will also have a DBW throttle body and that will give you better hood clearance since it does not have IAC motor on top of the TB. A 6.0 GTO engine would also be a good option and should ba a good bit less that a LS3 but give you the DBW throttle and the correct accessory drive. The CTS-V accessories sit closer to the engine than F-body and you can buy them complete from GM for around $750 which is a good deal. F-body accessories will not line up with the LS3 water pump but you can space it out to line up with the other pulleys. If you need a spare 968 automatic transmission and computer I have a good one I would like to sell cheap!

Thanks for the information. My 968 still hasn't been shipped. The shipping company promises they'll pick it up next week. I plan on getting it running with the 968 engine to check out the Automatic and other systems in the car. I will be traveling for the holidays, but expect to get going on the project in January.

On the 968 schematic diagram, Basically you are going to have to simulate any and all inputs going into the TCM. This is just a matter of finding out what the electrical values of the signals are with the original set up. Power and grounds will allready be connected to the tcm,along with inhibitor switch(PNDL),vss. The load signal pin 8 (t46)tcm is the input for controlling the trans. line pressure. Pin 42 dme signal only. no concern. Pin 11(t46)TPS tcm check original value, the GM signal will be .5 volt -4.5 volts @ WOT. Pin 32(t46)tcm should be a 12v signal from the ing. switch. Pin 3 (t46)tcm ignition frequency sounds like a crank signal. not real sure what this is used for. All your speed sensors on european cars are hall effect sensors. Gm uses a different type of speed sensor with a totally different signal. At the end of the day after the conversion is done you will have to find a way to scan the TCM and deal with any faults that might be there. Keep us posted sounds like a fun project.

Some where in this thread I used LS3 in place of L33. My real choice is probably between the LS1 and the L33. Sorry for the confusion.

L33 is a great engine they have the same heads as the LS6.

I've been checking prices and that is probably the way I will go.

My 968 arrived. Now the fun begins.

gd do you have any pics?

Here is a picture. I have started the parts accumulation process. So far I have bought a Quicktime bellhousing and a C5 pressure plate and flywheel. I am looking at LS1, LM7 and L33 engine options.

The next decision is whether to buy a drop out with harness, ECU and accessories or just the engine. I assume if I buy a drop out LS1, I can use just about everything.

For the LM7 or L33 options I can get a custom harness for $300, an ECU for $100, an LS1 intake, injectors, fuel rail for $150, a MAF and Throttle body (guess $150) and then I need F body accessory brackets (guess $200) That adds up to $900 plus the cost of the engine ($500 for a Low mileage LM7)

This puts me in at around $1400 not too bad. The pricing of drop out F body LS1 engines is about $2000 to $3000.

I would appreciate opinions on which engine to choose.

Drive by Wire vs Cable operated throttle bodies?

Other than horsepower are there any significant differences between early and late F body LS1s?

I assume that some of the ECUs will have to be re-flashed to eliminate VIN number dependencies and Catalytic converters.



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Go with a complete take out LS1 I just bought a 2001 with 53K complete for $2150.00 delivered. You will get all the right accessories with the LS1 vs the other engines so you won't spend time hunting parts. To go DBW you would have to move up to an LS2 and they are a lot more $$ than an LS1. I have built many fo these conversions and what adds up fast is the little items like the correct factory bolts, sensors, fittings and so on buying a complete engine with everything you need in the long run will be a money and time saver. Good luck!

I agree with 968ls1 about getting a complete engine... My LS1 has nickle and dimed me to death... My advice (and biggest regret) is getting the LS1, and not a LQ4 or LQ9... I could have a complete engine for what i have in just my shortblock and the basic rebuild kit... The difference in weight is very little like 60 LBS, you can easily loose that with other mods. And the extra power will be worth it, with just a cam upgrade you could be near 450 at the wheels...

Oh, you can do DBW on a LS1, you can use the corvette pedal and TAC module with either the early truck Tbody, or the LS1 vette TBODY... Dont do what i did and use the later truck stuff, the TBODY is tooooooo Tall. The early truck throttle body is just the corvette TBODY with the motor on the opposite side. I can get you pics on how I mounted the pedal sensor to the factory pedal setup.

HTH
Mark