73 E MFI engine. Can I install 2.2S pistons to raise compression and do some I&E port work and get a correctly running engine using the existing adjustment's with the stock pump? If yes, how much can I open the ports before I run into a problem?
73 E MFI engine. Can I install 2.2S pistons to raise compression and do some I&E port work and get a correctly running engine using the existing adjustment's with the stock pump? If yes, how much can I open the ports before I run into a problem?
Early 911S member #166
I have no problem with the theory that all men are created equal.
But after that moment you are on your own and nobody owes you a damn thing.
No problem at all. This is a common upgrade to get the CR up. Your piston clearance will be fine and you shouldn't do any port work. The port spec is tied to the cam, not the CR, which is all you'd affect by changing pistons. You shouldn't have to do any adjusting of the MFI pump either as those adjustments are again tied to cam specs not CR.
Kenik
- 1969 911S
- 1965/66 911
- S Reg #760
- RGruppe #389
I had froged E spec pistons made for my 72 2.4T bringing the CR up to 8.0:1 from 7.5:1 (couldn't see building a 7.5:1 motor...). The only thing to consider, which is why I didn't go higher, is the resulting cylinder pressure with the same cams. My engine with the 7.5:1 pistons blew an amazing 150 PSI, so going much higher would result in detonation, etc....
Leaf green 72T, SOLD!
Gone back to my MoPar roots!
Alex,
The mechanical CR with 911T cams is indeed limited by the cylinder pressure. Too high and you get the onset of detonation.
On the other hand, with 911S cams, you can use considerably higher CR because of the greater overlap of the cams.
It is perfectly reasonable to take an 8.5 CR 911S to 10:1 CR and higher (depending on fuel octane). Almost regardless of fuel, a 911T cannot withstand much over 9:1 CR without detonation.
Mark, a 2.4 911E can work just fine with 9.8:1 CR with sufficient octane. Twin plugs will help. I agree about the ports and MFI – leave that original. I have built street 2.8E engines at 10.3:1 with stock ports and MFI (yes, you can adjust MFI that far). Unbelievably good street performance but high octane requirements. I still have one I’m going to bring back to life with twin plugs this time.
Best,
Grady
Grady - sounds like we need the books more on the MFI pumps and systems than the trannies! Not scads of people understand them either. I'm getting set up to do functional low cost tbody rebuilds - and pump expertise is going the way of WW2 vets. (Sorry to steal the thread.)
Chip
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