2.7 RS engine CR increase?

[Brett@pcc]

A customer wants to use stock parts, but increase compression on a 2.7 clone motor.

He has heard that moving the wrist pin hole outward form the crank end -- effectively lengthening the rod -- gives approximately +1 (CR) per +1mm.

This is something we have no experience with, but it's clearly easier than cutting down the deck height, with its timing chain implications.

Anyone have experience with keeping stock pistons but boosting compression ratios?

Thanks!

[Chuck Miller]

Brett,

Andial had Mahle make them 90mm 2.7 RS piston sets at 9.5 and 10.5 ......... Yes - THEY ARE MAHLE... They are complet sets w/cylinders/rings/pins/and clips.... in the paper, in the red box.

Yes - THEY ARE EXPENSIVE !!!!!!!!!!

At the Andial open house I bought the 9.5 set for my RS rebuild... they were well over $2600.00 for the set.

Good luck,

Chuck

[911pcars]

The difference between 8.5 and 9.5 isn't that much. Here's something you can do without purchasing the special Mahle $et.

If you're rebuilding the engine anyway, you will fly cut the crankcase spigots (where each cylinder mounts) to true the parallalism to the crank axis. You can measure and trim the highest cylinder(s) to get the cylinder heights the same. You will flycut the cylinder head-to-cylinder gasket surface (evenly) to true the surface. I think about 1mm (0.040") total is all you want to do. Not sure what the new compression ratio will be. You might have to cc the combustion chambers and do the math to figure out the new squeeze factor.

Then remove an equal amount from the chain boxes to re-center the cams in the opening (there's a seal that must be centered to avoid oil leaks). If the timing chains are "too long", you can have some oversize idler sprocket wheels made (German Precision does it) to bring the idler arms back into the correct geometry and away from the chain box wall.

I did this to mine awhile back. I'll have to dig up some calcs to see what I recorded. Of course, this is assuming the CR is actually what Mahle says the pistons produce. However, Chuck's route is probably the easiest method to get up to 9.5:1.

Sherwood Lee
http://members.rennlist.org/911pcars

[Halm]

Sherwood, Assuming the barrels are still good, would it not be cheaper, easier and overall safer to buy custom JE pistons to get the desired CR? Speaking completely from ignorance here.

[911pcars]

I think if engine builders had their "druthers" they would pick Mahles, but thats MHO. Some owners want factory-type parts as well because they're proven components. JE pistons, due to their metallurgy, used to require more cylinder clearance than the Mahles which results in cold engine piston slapping; not sure about the current versions.

There is a cost difference, but that depends on the availability and condition of the Nikasil cylinders that should be used with JE's. You get new cylinders with the Mahle set. You need serviceable Nikasil cylinders if you go with JE's. That's the potential cost-saving route. However, if you have to replate, that's an add'l cost which bumps up the package price, then the net difference is only a few hundred $$ or none. With JE's, you can arrive at most any compression ratio you'd like, but unless you're going to twin-plug the engine, going beyond 9.5:1 becomes riskier with normally available pump gas. In any case, you'll want to do the above inspection and machining anyway to bring the engine to spec.

Sherwood

[rst73]

BUY THE BEST AND CRY ONCE!

All the best!

Roger Grago
PORSCHE Newport Beach
R Gruppe #27
73 RS-T
70 2.4MFI VW Bus

[Tinker]

Brett,

I have heard about this being done, but no personal experience. Why don't you give Ollies or Competition Engineering a call and get their thoughts and costs. Should be substantially less than a new set of pistons........as long as the pistons aren't worn out.

Let us know what you find out.

Tinker

[farleyd]

Please excuse me to introduce. I have brand new boxed Mahle RS 90mm p/cs 8.5:1 set w/circlips. pins, rings for sale. 1300US$.
Lemme know.

[Brett@pcc]

Thanks, Tinker, will do!

[larrym]

WHY? is this customer planning to run 100/110 race gas all the time?

i have a 2.7 RS 8.5 that just barely runs w/o pinging on hot days on street 91 .. recently added an msd timing retard device to save the motor (distributor curve by Rennsport & solex cams)

[911pcars]

Larrym,
Not to seque too far off track. I have basically the same engine as yours (E cams) and early S distributor and do not have any low speed detonation nor need any ignition retard. Could your initial timing/mech. advance combo be creating too much of a spark lead for the fuel octane you're feeding it?

Getting back to Brett's orig. question re: lengthening the rod to increase C.R., you may have to machine some material off of the piston crown to provide clearance with cylinder head. And depending on the cams used, you may also have to machine the crown to maintain adequate valve-to-piston clearance.

Sherwood

[larrym]

my experience is at 8.8 CR or above, street 91 0ctane doesn't do it. Retarding spark isnt a performance solution

[Halm]

Larry, is 91 octane the average of what you can get in your area? It's 92/93 here in NC. Would the extra points eliminate your problem at 8.8 CR?

[jluetjen]

Retarding spark isnt a performance solution

Larry; I used to think the same thing with my ITB race car. Unfortunately the car didn't peform that well (nor finish the race that I was in) with a hole in one of the pistons.

I suggest that you seriously consider backing off on your timing to save your motor until you can dyno the motor and come up with a good solution for the gas that you are running. I'm not sure if it is the CR alone that is causing your problems since I'm running 9.9:1 in my E on street gas. It runs best on Sunoco 94, but can run (just not hard) on Texico or Mobil 93.

The thing that I learned from my ITB experience is that you are limited by the stock curve, and so you'll need to back off on the initial timing from where you are now so that you avoid pingging at the most critical part of the distributor curve. You'll then need to recurve the distributor or use a mapped ignition to provide the advance needed for good power and throttle response given your situation. You didn't mention what year/model 911 you have, because the solution will most likely be different for each era car.

For example, lets say that your engine is pingging pretty badly at 4000 RPM under full load and WOT. But at you have found better throttle response at 3000 and below with more advance. Rather then just advancing the initial timing which advances the spark across the board, you'll need to find a way to advance the timing below 3000 RPM and retard it (relative to it's current position) at 4000 RPM. Really the only way to do this is with a mapped ignition like Porsche in the 944's and the Carrera 3.2 and later cars.

[Wayne 962]

Originally posted by larrym
i have a 2.7 RS 8.5 that just barely runs w/o pinging on hot days on street 91 .. recently added an msd timing retard device to save the motor (distributor curve by Rennsport & solex cams)

I would suspect that you have some problems other than the CR causing your car to ping. This combination should work okay on CA gas...

-Wayne