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Jim Richards's Avatar
 
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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!
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Old 12-02-2003, 04:09 AM
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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.
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1973 911T
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"Dave, hit the brakes, but don't look like your htting the brakes...what? I DON'T KNOW, BRAKE CASUAL!!!" dtw's thoughts after nearly rear ending a SHP officer
Old 12-02-2003, 05:11 AM
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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.
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Old 12-02-2003, 05:13 AM
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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?
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1973 911T
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"Dave, hit the brakes, but don't look like your htting the brakes...what? I DON'T KNOW, BRAKE CASUAL!!!" dtw's thoughts after nearly rear ending a SHP officer
Old 12-02-2003, 05:33 AM
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Thanks guys!

Quote:
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.
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.

Quote:
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.
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.

Quote:
We have the RS sets on sale right now (standard RS compression)...
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.
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Old 12-02-2003, 05:51 AM
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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
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Old 12-02-2003, 01:56 PM
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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

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
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Last edited by J P Stein; 12-04-2003 at 12:48 PM..
Old 12-02-2003, 02:13 PM
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260 from a 2.7L engine?! that'll be really impressive. How wild are the ge80 cams when on the 2.7?
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"Dave, hit the brakes, but don't look like your htting the brakes...what? I DON'T KNOW, BRAKE CASUAL!!!" dtw's thoughts after nearly rear ending a SHP officer
Old 12-02-2003, 02:15 PM
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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
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Old 12-02-2003, 02:35 PM
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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.
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"Dave, hit the brakes, but don't look like your htting the brakes...what? I DON'T KNOW, BRAKE CASUAL!!!" dtw's thoughts after nearly rear ending a SHP officer
Old 12-02-2003, 02:44 PM
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Jim,
Not sure if you got my email from earlier but your PM box was full
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1973 911T
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"Dave, hit the brakes, but don't look like your htting the brakes...what? I DON'T KNOW, BRAKE CASUAL!!!" dtw's thoughts after nearly rear ending a SHP officer
Old 12-02-2003, 06:16 PM
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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.
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Old 12-02-2003, 06:26 PM
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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.
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"Dave, hit the brakes, but don't look like your htting the brakes...what? I DON'T KNOW, BRAKE CASUAL!!!" dtw's thoughts after nearly rear ending a SHP officer

Last edited by Tim Walsh; 12-02-2003 at 06:40 PM..
Old 12-02-2003, 06:34 PM
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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.

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-- Last Engine rebuild project, Now a coffee table.
-- New engine rebuild project, Alive and well.
-- '72 911 Martini RS, '69 911E Targa, a 2004 Cayenne S, and a Miata too... Looking for a Cayman S
Old 12-02-2003, 06:57 PM
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Woooo, that looks nice. What's your compression ratio?
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Last edited by Jim Richards; 12-03-2003 at 03:38 AM..
Old 12-02-2003, 07:05 PM
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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.
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-- Last Engine rebuild project, Now a coffee table.
-- New engine rebuild project, Alive and well.
-- '72 911 Martini RS, '69 911E Targa, a 2004 Cayenne S, and a Miata too... Looking for a Cayman S
Old 12-03-2003, 08:13 AM
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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
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Old 12-03-2003, 02:00 PM
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