Advice on oversizing pistons

[Jaybird12]

Please advise. I'm trying to upgrade my 2.7L CIS. I'm looking at some JE pistons with a higher compression ratio. (9.5:1) I have found 90mm ones but there are 91 and 92mm available. Using my stock cylinders and boring them out , what are the pros and cons with oversizing the pistons?

[Jaybird12]

bump please

[Targalid]

I am a little confused by your question. If the bores are 90 mm, how can you fit a 91 or 92 mm piston inside? You would need to either overbore the cylinders or change them for new oversize cylinders.

[Jaybird12]

Originally Posted by Targalid

I am a little confused by your question. If the bores are 90 mm, how can you fit a 91 or 92 mm piston inside? You would need to either overbore the cylinders or change them for new oversize cylinders.

Sorry. Edited the post. I would bore out my original cylinders. I just want to know if it's possible and what problems I could encounter. Is there enough material there to bore out to 92mm?

[Jaybird12]

bump

[lpsalsaman]

You should consider the age of the block and if you are not the original owner, have this engine been rebuilt previously? My brother had to move to a 3.0 engine from his original 2.7 since Porsche in Germany told him that his engine had signs of a previous rebuilts and it wasn't worth rebuilding it again. I'm guessing that the metal was too weak/fatigue due to age? Not sure, anyway just saying as 2.7's are infamous for having issues.

[Jaybird12]

Ok. Thanks. I'm not the original owner but almost certain this one has not been rebuilt.

[Charles Freeborn]

The 2.7 is pretty expensive to "take up a notch". Simply changing out to larger bore (2.8, etc) will not give you too much bump in output unless you do head flow work, dual plugs (to allow pump gas with higher compression) and chuck the CIS for carbs or MFI. Cams gotta go too.Then you'll want a header to let it breath out too. Then, as said, how's the bottom end? Don't forget improved oil cooling too. I ran a back of the envelope estimate a while back and it's upwards of $12 grand to do it right, and that's with me doing all the engine in and out and as much of the busy work as I can. You'll end up with a very interesting engine, but probably (much) more money than a 3.0 or 3.2 swap.
-C

[AlfonsoR]

Have you already checked th line bore on the case?

[Jaybird12]

Dealing with a pretty low mileage engine but expecting the worst. Plan on keeping the CIS. As per a Pelican Technical Article, have invested in 964 cams, looking for higher compression JE pistons, upgraded cam tensioners and redo the heads. Hopefully all will be well in the bottom end.

[Mark Henry]

I do agree the cheapest route is to do a 3.0-3.2 swap.

2.7 overbore can be done, but for extra go I'd get some cams, carb's and a bit of head work done before the overbore on the P&C's

[Steve@Rennsport]

You really want to stay with a 90mm bore on those things since there isn't enough material between the bore and the CE ring grooves. I've seen too many shattered cylinders from detonation that the owner never even heard.

[Charles Freeborn]

Pelican article is regarding the 3.0 engine. Completely different platform.
-C

[Jaybird12]

Originally Posted by Charles Freeborn

Pelican article is regarding the 3.0 engine. Completely different platform.
-C

Really? I thought it was for all cars with CIS. You're sure? That will change my plans for sure. I already bought and machined cams.

[AlfonsoR]

Originally Posted by Jaybird12

Dealing with a pretty low mileage engine but expecting the worst. Plan on keeping the CIS. As per a Pelican Technical Article, have invested in 964 cams, looking for higher compression JE pistons, upgraded cam tensioners and redo the heads. Hopefully all will be well in the bottom end.

I am no expert, but from what I have read, the mag cases tend to "move" when you unbolt them. I would check line bore and make sure that the cylinder bases are perpendicular to the mains.

[jpearson]

I'd also love to hear more about the 964 cam upgrade in 2.7 vs 3.0. My 2.7 is sitting on my garage floor while I contemplate the rebuild. I had also been considering the 964 cam swap based on the Pelican article.

[Charles Freeborn]

If this is the article you're referring to :
Pelican Technical Article: Extracting Power from CIS 911s...
read carefully and you'll see he's wokrking with a 77 Carrera 3.0. I am not expert at what the exact differences are between that and say my 74 2.7s, but what he's doing does not apply to my engine. Steve (Rennsport) knows the answers.
-C

[Charles Freeborn]

Good info here too:
2.7 engine upgrade
-C

[Jaybird12]

Thanks Charles. Ill investigate immediately.