I need a kind soul to advise me please... Piston Choice.

[M491Cabriolet]

OK guys... I have asked this question several times, but the typical answers have been something like, "drive one of each and decide for yourself!"

So, as a premise, I don't know anyone with Porsche longhood cars around me and have access only to my own car.

The premise and questions are... I have a 2.4 911T MFI engine I am rebuilding. I have a 911E MFI intake setup and new E cams. I have a set of 2.4 911S pistons and cylinders and a set of 2.2 911E pistons and cylinders.

I am at the point in my rebuild process that I have to decide which P/C set to use and I REALLY need some good advice from a good hearted soul with experience and wisdom. If I go with the E P/C set, my compression will be higher than the S P/C set. But I'm wondering about a few things.

First, for a fast street car, will the higher compression (9.6:1?) 2.2 911E pistons with a long stroke make a noticeable difference in the feel I get from the car?

Second, will the compression ratio from the 2.2 911E pistons mean having to use only the highest grade of gasoline?

Third, is the 2.4 911S setup with its stck 8.5:1 compression ratio inferior to the 2.2 911E setup on a long-stroke crank?

Which would YOU choose?


PLEEEEASE help!!! Thanks.

[Superman]

First, there are guys here who have forgotten more about these issues than I will ever learn. I know nothing.

There are guys who know what will happen when you try to use this piston with these heads on such-and-such a crankshaft. Some pistons will not work with certain cams and heads because the valve-to-piston clearance is not there. Or maybe you'll end up with a different CR than you think. You have asked a very technical question, but deserve an answer. There are people who know. I hope they get you the information quickly.

FWIW, higher compression ratios make a significant difference.

Also, I run premium in all the engines I care about. Regardless of your final decision here, you should use premium. Some folks may disagree, but unless you are running a stock engine with known CR and using stock ignition timing.......you should use premium. I strongly suggest ignition timing the way the experts do it. Try 35 degrees full advance. If it works, great. If not, back it off until it does work. Using this method, you will get noticeably increased engine performance, but you will be only a couple of degrees away from detonation. Use premium as a protection.

[M491Cabriolet]

Thanks. I agree about the gasoline. I just don't want to have to use racing fuel. The engine, which I never had the pleasure to hear running, is a 2.4 911T was upgraded to 2.4 911S pistons and cylinders. The engine has Carrara chain tensioners installed and sat for about 20 years. I have not had the cams measured, but I would assume they were modified as well (I can't guarantee that, though). I had John Dougherty make me a set of 911E profile cams with MFI drive, which I will use. I guess the only question here is should I use the 2.2 911E pistons or the 2.4 911S pistons, whose compression ratio was 8.5:1? Any advice--any advice at all--would be GREATLY appreciated! Please just keep in mind that the only car I can drive is my own. I don't know any early 911 owners. I would not feel comfortable flogging someone else's car anyway.

[BK911]

Mic both sets of pistons and cylinders and use whichever is in the best shape.

Or go with the 2.2Es for increased CR and more power.

Or just flip a coin and get it over with. Either will make an awesome engine and put a big smile on your face.

[M491Cabriolet]

Thanks. Without any strong advice, I may just do that. All things being equal with the condition of the pistons, I'd hate to be at this point and not make the BEST choice and just make an uninformed 50/50 choice. Even if both configurations would be good, one must certainly be better, I would think.

[FenderBender]

I am clueless, I would check the valve to piston clearence on the 2.2 Es and run them.
I was told with 10:1 compression and under you should not need twin plugs.
I am planning on rebuilding my 2.4T in the near future.

I hope this helps your decision.

[Flieger]

Use the higher compression 2.2 pistons. Then you will never be left regretting going soft and wondering what it would be like with the hotter setup.

[M491Cabriolet]

Thanks for that, but it causes me to ask a touchy question. I never drove either a 2.2 911S or a 2.4 911S. The 2.4 had an 8.5:1 compression ratio, while the 2.2 (I believe) had a much higher compression ratio. So, did the 2.4 911S totally "suck" compared to the earlier 2.2 911S? I think the 2.4 made up for the loss in compression through its increased capacity as offered by the longer stroke.

So there are two possibilities... the 2.4 911S pistons on the longer stroke paired with 911E cams to offer a more streetable torque band, making it as good as the 2.4 911S--only more user friendly --or-- the 2.2 911S pistons on the longer stroke, offering even higher compression than the 2.2 911S on the short stroke crank, but paired with the 911E cams, making it more street/user friendly. The only problem would be whether the compression ratio of this setup would require twin plugs or racing fuel, which I do not want to mess around with. If that would not be a problem, then maybe this setup would make the car a bit of a rocket.

By the way, my heads are 2.4 911S MFI heads with the larger ports, which I bought from Tom Butler.

What to do? What to do? I just don't know. SURELY, someone out there has come across this decision to make before me... Wayne Dempsey, do you have any advixe for me? Thanks!

[javadog]

If I was going to spend the money to rebuild an early 911 engine, I'd build a 2.7RS, maybe with a little higher compression. I'd sell the parts you have and use the money to buy a new set of Mahle pistons and cylinders. You may spend a little more money in the long run but you'll have the greatest 911 engine ever made.

I have a 2.2 with S pistons and E cams (using a 2L crank and heads) in a 914/6 and it's a really boring engine. That's not exactly what you are building, but I can tell you it is miles away from how an RS feels.

Good luck,
JR

[bkreigsr]

I rebuilt my 2.4 with the long crank and a set of factory (908) Mahle 86mm p/c to make one durable 2.5. Elgin 'mod s' cams, carillo rods, rsr clutch pkg, etc. etc.

final compression came out to 10.37:1, which was no problem when I was buying 94 octane on the east coast. also, the motor never complained about only having 6 plugs.
getting a suitable gas on the west coast is a different story.

there was some head and case work involved in the upgrade.

Bill K

[arrivederci]

Originally Posted by javadog

If I was going to spend the money to rebuild an early 911 engine, I'd build a 2.7RS, maybe with a little higher compression. I'd sell the parts you have and use the money to buy a new set of Mahle pistons and cylinders. You may spend a little more money in the long run but you'll have the greatest 911 engine ever made.

I have a 2.2 with S pistons and E cams (using a 2L crank and heads) in a 914/6 and it's a really boring engine. That's not exactly what you are building, but I can tell you it is miles away from how an RS feels.

Good luck,
JR

This is the best advice IMO. I currently have a 2.4T motor with 2.2S pistons in my '72. It is fun and torquey but with T cams and T MFI it doesn't match the top end power of a 2.4S motor. I'm still debating what exactly I'll build (high-compression 2.7RS, 2.8, etc), but I'll likely end up with a twin plug setup to be safe - we generally only get 91 octane in Colorado. In the grand scheme of things when looking at all the rebuild costs including rebuilding MFI, rebuilding/boring stacks, buying P's and C's, etc, the added expense of twin plugs won't be that much more.

[javadog]

Or, you could just use the 8.5:1 RS pistons and never worry about octane at all...

JR

[Flieger]

I have a 2.7 MFI with S cams and 9.5:1 compression. I run the best gas I can get which is not much: 91 E10. The motor is single plug and doesn't complain about anything, even with how I push it.

You will never regret having a more responsive, more powerful, more fun engine.

[berettafan]

don't have the experience to help in your decision but do have a '70T w/ a 2.4, webers and SUPPOSEDLY 2.2S pistons and cams. a recent spin by a fellow who has a lot of years in 911's suggests the cams are not 'S' however.

whatever it has my car is pretty fun to drive and definitely does not drop off at 4500 or wherever it is T's are supposed to lose breathing ability.

just the same look me up if you get to the beach (Ocean City) and you'll then know somebody else with a longhood!


oh, and when i need to rebuild i'd lean pretty hard towards 2.7RS spec. haven't driven such a car but hope to at some point.

[arrivederci]

Originally Posted by berettafan

don't have the experience to help in your decision but do have a '70T w/ a 2.4, webers and SUPPOSEDLY 2.2S pistons and cams. a recent spin by a fellow who has a lot of years in 911's suggests the cams are not 'S' however.

whatever it has my car is pretty fun to drive and definitely does not drop off at 4500 or wherever it is T's are supposed to lose breathing ability.

just the same look me up if you get to the beach (Ocean City) and you'll then know somebody else with a longhood!


oh, and when i need to rebuild i'd lean pretty hard towards 2.7RS spec. haven't driven such a car but hope to at some point.

The T cam drop-off doesn't really become noticeable until you hit 5.5K. Then it starts to really fall flat on its face by 6K, whereas an S cam is just peaking at 5.5K and still pulling strong at 6K. Mike Piera has a great comparison of torque curves on different early motors:

http://www.analogman.com/911/911eng.htm

Here is my dyno sheet - again it is a 2.4T motor with 2.2S pistons. Notice the hp remains flat from 5.5K - 6K, but falls off from 6K-6.5K. BTW, those are wheel hp/tq numbers (uncorrected).

http://members.rennlist.com/jpinkert/pepe%20dyno%20run.jpg

[Bobboloo]

All things being equal, as to the condition of the two sets, do you want more or less compression? I think that's pretty easy to answer. You want more which is the 2.2E P+C set. Either set will run on pump gas. The 2.4S set, however, will run on regular and unless this car is your daily driver why would you even consider that?

All things are not equal especially when it comes to used P+C's. No two sets are alike. That's why this point is mute until you have them measured. Once they are measured then you can choose the best set.

Are both sets Biral? If they are then you can even mix a match the best cylinders as long as the are from the same height group.

[Scott Clarke]

Go with the E pistons if they are in equal or better shape than the Ss. Porsche CRs tend to be lower than advertised. With a 84mm bore, 9.6:1 is not too much. This is the only free performance you may ever get!