Thread: 3.0 -> 3.?
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Carrera3.5L Carrera3.5L is offline
Doesn't want/need a 3.6L
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Join Date: May 2004
Posts: 1,635
Originally posted by rdane
I am trying to decide how to rebuild a 3.2 motronic at the moment myself.

Gotta think the 3.5 long stroke would be a great engine as well.
Andial's moster 3.7 conversion is worth a look too. Bigger is always better
And soon we'll have dyno info for a 3.5L Motronic as well and can determine if the extra expense (3.4 to 3.5) is really worth it.

I have the piston drawing to modify the 100mm piston generally used for 74.4mm cranks to make 3.5L into 3.6L (both normally aspirated and turbo version) using the 76.4mm 964 crank. Unfortunately, I don't have a drawing to do the same for 98's to make a long stroke 3.5L using the 76.4mm 964 crank.

Logically, you may assume that the Andial kit would just use bigger p's and c's and the 964 crank to make 3.7L but that isn't the case. The bore does increase to 102mm but the crank used is the 3.2L 74.4mm version, not the 964.

3.2>3.4 is 98mm P's and C's (no case machine work)
3.2>3.5 is 100mm P's and C's (with case machine work)
3.2>3.6 is modified 100mm P's and C's, 964 crank (with case machine work)
3.2>3.7 is 102mm P's and C's, 3.2L crank, aftermarket rods (with case machining)

The old saying is there is no substitute for cubic inches generally holds true, however the bigger the motor the more other factors come into play like twin-plugging and thermal limits. Bear in mind that 3.5L and 3.6L versions will have the case spigots opened up to 105mm from 103mm and this has been done for almost 20 years. For 3.7L, the case spigots have to be opened up even more (to 107mm). I don't know how many 3.7L conversions have been done, but it will be interesting to see if there are any long term reliability issues with a motor this big.

The 3.7L is attractive because it retains the 3.2L crank (lulling people into believing they can perhaps save some money), but I gotta believe that the 102mm piston and cylinders are REAL expensive and probably cost as much as 100's and a good used 3.6L crank would (or pretty close).

You will also need to twin-plug most of these bigger motor combos so that needs to be factored in as well. It would be real easy to spend 10K-12K just by purchasing the building block components and machine work and you haven't even gotten any of the normal rebuild parts or considered labor.

Here is an image of a 3.2L case that has been opened up from 103mm to 105mm for use of 100mm pistons and cylinders:

The price of machine work to do this is relatively insignificant in the big picture but for a 3.7L you will need to go 2mm larger. Will this be safe? Only time will tell I guess but I would think if Andial is the one developing this conversion then it probably is. They are usually very, very conservative with their offerings and claims and run prototype motors in Arnold's car for a good length of time.

I know Arnold put a good many miles on the very first 3.0L to 3.6L conversion that was done in the late '80's before the parts kit was released for sale.

1988 Carrera w/ 3.5L Twin-Plug

2008 Cayman S (coming soon)
Old 10-17-2004, 08:54 AM
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