2.7RS hi comp pistons

[philippeF]

Hi all
Here's a question for MFI addicts(?)

Shall i consider major mods if i decide to replace my stock 2.7RS pistons with JE hi comp during rebuilt ?
best regards
Philippe

[jluetjen]

Why? Just because you're in there or do you have some specific objective in mind?

[Bobboloo]

There's not much to improve on really. The ports, induction and exhaust are pretty much ideal for a 2.7RS in stock configuration. You could always go twin plug if you want to go with pistons in the 10.3:1 and up range.

[KobaltBlau]

well, you could go from the 8.5:1 to a 9.5:1 JE or something. probably not worth it, though.

phillipe, will you be using 'S' cams or do you have something bigger in mind?

[Mr Beau]

Salut Philippe,

Your options depend on how much power you want. It everything is in good condition and you just want a bit more zip, the pistons will provide that. If you're willing to buy cams and worry about adjusting the fueling to compensate, the sky is the limit. How much power do you want and how much do you want to spend??

[philippeF]

just in mind to get better engine response ?
i may be wrong (engine is stock) cams MFI, i'm looking in fact some easy improvement over stock configuration....

best regards
Philippe

[Mr Beau]

Salut Philippe,

Pistons are probably the easiest gain to be had with the low original compression ratio of 8.5:1. Combine that with a sport muffler, and you may have the gains you're looking for.

Is the engine being rebuilt currently? Where is it you're located again?

[Bobboloo]

Since the motor has MFI I would be cautious about changing the exhaust because MFI likes a little back pressure. Stock seems to work best in this situation.

[Mr Beau]

If anything MFI should be less sensitive to pulsations going on compared with carbs. And as such a popular modification (including by the factory), I have trouble believing there would be a problem...

[kstylianos]

Remember, Porsche's compression ratios are actually about .25-.50 points lower than advertised. So, the stock 8.5:1's are may be around 8.0:1. JE's CR's are calculated with a 1mm deck height. You WILL notice a good bump in torque with the JE's.

[KobaltBlau]

what is the reason for this discrepancy? is it really consistent across compression ratios from 6.5:1 to 11.3:1??

[Wayne 962]

Originally posted by kstylianos
Remember, Porsche's advertised compression ratios are actually about .25-.50 points lower then advertised. So, the stock 8.5:1's are probably around 8.0:1. JE's CR's are calculated with a 1mm deck height. You WILL notice a good bump in torque with the JE's.

Do you have any hard facts or data to back up this statement?

-Wayne

[kstylianos]

No hard facts or data, but this info can be corroborated if you talk to the right folks.

Walt Watson and Steve Weiner both mentioned this while talking to them about my specific engine build up. They did not specify the reason for the discrepancy, but said if you do the measurements (deck height from factory, piston and head volume, stroke and bore) the actual CR comes out a little less than stated. Wish I had a few stock engines to collect data with, so I'll take the word of those who do it on a daily basis.

JE states that their Porsche pistons CR's are calculated using a 1.0mm deck height. Verify with JE or EBS.

[911SCfanatic]

What's Henry's 2.7RS kit include to raise the hp from 210 to 240? That would give you some indication as to the areas that could be improved.

[Wayne 962]

Originally posted by kstylianos
Walt Watson and Steve Weiner both mentioned this while talking to them about my specific engine build up. They did not specify the reason for the discrepancy, but said if you do the measurements (deck height from factory, piston and head volume, stroke and bore) the actual CR comes out a little less than stated. Wish I had a few stock engines to collect data with, so I'll take the word of those who do it on a daily basis.

I know that there are one or two listed CR numbers in the spec books that are slightly off (like the ones for the 2.2 engine), but I am not aware of any systemic variation in all of the CR numbers.

-Wayne

[ChrisBennet]

Here are some compression numbers Kevin Roush (GAS Motorsports) posted years ago on Rennlist:
Factory (Real) Motor
9.5:1 (9.2) 3.2L
7.0:1 (6.5) 3.3 Turbo
11.3:1 (10.4) 3.6L

-Chris

[KobaltBlau]

Thanks Chris, anything on the 3.0s?

[ChrisBennet]

Originally posted by KobaltBlau
Thanks Chris, anything on the 3.0s?

Sorry, no. I checked my notes and I haven't CC any stock motors. I'll probably CC the 3.6 I'm working on this week.
-Chris

[KobaltBlau]

cool, thanks!

[kstylianos]

Good info there, Chris.

I have a later SC (1982) with a stock specified 9.3:1 CR. It was noted that stock CR's were seen as low as ~ 8.8:1.

You as the engine builder really have to do your homework. Do the necessary measurements and calculations for CR, inspect measure and clean all parts that come back from the machine shop. This is what defines a quality rebuild from one that has just been slapped together, taking things for granted.