2.7Rs vs. High-Compression 2.7RS
 

I'm gathering info for next winter's project - a 2.7RS upgrade for our 73E's engine. One question that's been on my mind is whether to build a 2.7RS ,or the high-compression 2.7RS noted in Wayne's engine picks in the rebuilding book. This plan will make use of my 2.4E's 7R case and I will retain the MFI with plans to either mod my current setup or obtain 2.4S MFI throttle bodies / stacks.

What are the pros/cons of the high-compression RS engine? Are the pistons available from both Mahle & JE? Are JE pistons as good as Mahles, given that this will be a street / DE application? What's the difference in cost? Will the high-compr. version provide noticable gains? Will it have similar longevity, assuming I avoid the money shift? Is there something else I should consider?

Thanks for your help!

Jim,
I do know that your throttle bodies can be moded to S specs (I'm having my T's bored to E's by my machine shop) but the stacks are a different problem. I don't belive they can reliably bored out since their plastic (any materials experts out there?)

I was consitering doing this but decided for money costs went with a hicompression E engine.

We have the RS sets on sale right now (standard RS compression)...

-Wayne

Cost is the main con. The easiest way to build a HC 2.7 is with J&Es. You can get any compression you want with tons of valve clearance. Using RS P/C and changing the deck height always seemed like a bad idea to me due to the reduced clearances and changed geometries. I think a set of 2.7 J&Es plus reprocessed cylinders are around $1800 from EBS. The high compression Mahles are a fortune right now due the exchange rates. I know Wayne sells the regular 2.7 RSs at a very good price.

I've heard people say there is no measurable difference in HP between the 8.5:1 and HC version of this engine, but I'm not sure I believe this myself. At a minimum, they have more meat under the curve, and feel a lot crisper. I think the consensus is to limit a street engine to 9.5:1.

Jim,
If I remember right can get sunocco 94 (95?) so maybe you can safely bump up your compression a touch higher than 9.5:1?

Thanks guys!

Any con's with using JE's? Do they hold up to wear as well as Mahles? I seem to remember reading in BA's book that only Mahle P/Cs are manufactured to tight enough tolerances to meet Porsche's very tight specs.

I'm also praying for a stronger dollar. :)

I would go this route if someone comes out and tells me that, unequivocally, the high-compression RS's performance is noticably better. Not at the top end, but through out the torque band.

Tim, we can get Sunoco Ultra (94 octane) here, but I have to drive 20 minutes to find a Sunoco station. BTW - I'm going to be down your way this weekend (we're visiting out son at Wake Forest). PM me your phone # and I'll try to stop by Sunday.

Hmmm, I'll check out your sale price Wayne and talk to my "CFO" to see if I can buy P/Cs now and squirrel them away for use next winter.

Here is a reference point: I just rebuilt my 2.7 with 10.5:1 aluminium ARIAS pistons (and GE-80 cams). Runs fine on pump gas. Made 228hp with (untuned and quite rich) carbs. Only downside is a standard starter motor couldn't crank it over when hot.

Cheers - Ryan

ps: it is drilled for twin plug but the distr is not finished
pps: the mfi set up is also sitting on the bench...hoping for 260ish hp when all finished

I have been running 9.7:1 Shastas (KE forgings) for about 2 years now.....on pump gas. The motor has never been on any sort of dyno and my tuning skills are...questionable, but it seems to run well. Thus far I haven't seen any aluminum comming out the exhaust pipe:D

This is the only place (PP BBSs) that I have ever heard increasing compression has no effect on engine power....torque and HP.
The only 911 guy I ever asked about this was Steve Weiner.
He seemed to believe that the compression increase (8.5 to 9.5 )
was worthwhile......with less (% wise) increase as one went up
from there (9.5 to 10.5 for instance). He seemed reluctant to put a number on it.....can't blame him there...see tuning above :rolleyes:

260 from a 2.7L engine?! that'll be really impressive. How wild are the ge80 cams when on the 2.7?

Tim

The cams don't seem that crazy. The engine idles well and pulls (even in high gears) very strongly from 2000 rpm. The problem i have with my (old) carbs is a huge flat spot from 3000-4000 rpm - the thing really bogs at full throttle in that range, but when you get past 4200, 4300 it just rockets.

The dyno a/f ratio showed a massive dump of fuel at this point which looks to be causing the bogging down. The carbs have now been sold and i'm hoping the mfi will give a much more drivable engine - but unfortunately don't know when i or my mechanic will get a chance to finish it.

Cheers - Ryan

ryan,
Hmm.. sounds like you've got a winner that's one of my dream engines but I'll be happy with my 180hp 2.4 for a long time. Let us know how it turns out.

Jim,
Not sure if you got my email from earlier but your PM box was full

Got it Tim. Thanks! So far it looks like my wife and I are going to NC this weekend. Hope we can catch up.

I vaguely recall a thread where Roland talked about the poor tolerences of other pistons / cylinders as compared to Mahles. I think he said you'd buy new aftermarket pistons and the tolerences would be immediately be to the point where Porsche considered them worn out. I need to do a little digging to see if I can find that thread.

Gosh I sure wish Roland was still on the Pelican board.

Sounds good Jim.

I remember BA talking about that in his book but he also mentioned that the problems have been fixed. I'm not sure what the current situation is, but I can tell you in a week or so. dtw just got a set of 84mm JE's in only a week ago. Knowing Speedwerk's reputation they're ok, BUT they were sent back at least once for wrong compression ratio domes.

I've been happy with my new set, but they only have 400 miles on them. I didn't see anything to make me question the quality, workmanship or materials.

http://forums.pelicanparts.com/uploa...1070419996.jpg

Woooo, that looks nice. What's your compression ratio?

My J&E pistons are 9.5:1 and 92mm. Would you believe I measured 4mm of minimum exhaust valve clearance with early S-Cams. 2.5mm on the intakes. The pockets are obviously cut for very aggressive cams.

jgp

i think you're right about the pockets - i have 10.5:1 pistons and, wih GE-80 cams, only have 0.8mm valve clearance. from memory, the GE-80's have about a 6mm lift.

the motor has an awesome top end but, somewhat suprisingly, is still quite tractable down low

cheers - ryan