high compression 2.7RS engines?

[swisscheese]

In another thread, the question of increasing the compression of a 2.4S engine was discussed. Using 2.2S pistons seem to be the (relative) easy solution. What are the possibilities with an 2.7RS engine? Custom pistons? (as I'm not aware of standard Porsche pistons suitable for this). Who has experience? Any dyno results?
I've heard of custom pistons, doing about 10.5:1, offering 240/250hp. Not bad.. Thanks for the response!
Tom

[beginner]

The following are just my thoughts. They are not definitions by any means.


Andial sells hi compression 2.7 mahle set. If you’re going to install case-savers, 2.5L long-stroke 10.5:1 is a safer bet because its spigot size is only 94.25mm as opposed to 97mm of 2.7RS. 97mm is fine as long you don’t install case-savers (new case only! and using dilavar studs) otherwise there’d hardly be any material (soft mg) left in-between the stud hole and spigot hole.

FYI, 2.4 is really 2.3 as it’s only 2341cc whereas 2.5 is on the dot.

When used in conjunction with an RSR flywheel, it’ll ……

[swisscheese]

Thanks, but I already got 2.7RS pistons/cylinders, so would like to stick to (at least) the cylinders.

Then you don't recommend doing the high-compression thing?
thanks
Tom

[Bobboloo]

Compression is really octane/detonation issue.

It really comes down to the octane of gas you have available. In California many times the highest octane available at the pump is 91 CLC. So in this situation I think 9.8:1 is about max compression you can go without twin plugs and avoid detonation. If you go with twin plugs then 10.3:1 is attainable.

In the Netherlands I think you guys rate octane as RON. 96 RON is about the same as 92 CLC. So if you have 96 RON available then 9.8:1 would probably be a safe compression ratio. You could run 10.3:1 with twin plugs or even higher depending on the prep work done to prevent any heat risers in the combustion chamber that could start detonation. (ceramic coatings, polishing etc.)

[swisscheese]

Hi Bobby,
thanks. In Germany there's now RON 100 available at the gas station. Bloody expensive, but seems to be doing "wonders", according to some drivers (or is this the famous M&M's for medicine effect?). Anyway, have to see what to do. I only want to change the pistons, for a higher compression ratio. No further modifs.

Nobody has done this before?
Tom

[Mr9146]

What about removing barrel shims or decking the heads?

[sithot]

Originally posted by Mr9146
What about removing barrel shims or decking the heads?

Loose timing chains and other geometry issues I imagine. There is some material that can be sacrificed but the machine shop better know what they're doing.

Off Topic a bit but here goes:

We sometimes use thicker base gaskets on early BMW engines that run a higher compression to cut out knocking. I've never experienced this myself because I run racing fuel. The main reason for racing fuel is that it is very stable and can sit in a bike all winter and not gum things up. I do keep the carbs dry by running all of the fuel out but anything left in them has never been an issue.

The second reason I run racing fuel is the smell. I'm addicted to it. Nothing in the world like that sweet smell. Conversely, there is nothing like getting douched by a Top Fuel or Funny Car getting tuned before a run. That's nasty as hell. When the Firemans Masks/Breathers come out it's time to run for the hills. That will be your only warning.
Keep this in mind if you are ever in the pits of an NHRA event.

Tom

[silver911]

With the widespread avaliability of BP Ultimate and Shell Optimax, witha RON rating of 98 or 100 depending on the market, 10:1 seems to be a safe ratio.

Certainly the 2.4l cars, my 2.4E for example, could easily run 2.2E or even S pistons and retain tis 'good' nature and not risk detonation.

Not sure the 10.5:1 ratio, without twin polugs would be as 'safe'.

From memory RON ratings, at the pumps have a tolerance of +/- 1 and a bit. Does somebody know for sure?

So you could get a surprise at times.

[nick-moss]

I have a 74 Carrera 2.7 with Mahle 92mm 2.8L P/Cs giving a CR of 10.5:1 . The cases have been boat-tailed and the motor has been dynoed at 239 bhp. It prefers 100 octane leaded fuel (still available at certain outlets in the UK) but has been run extensively on normal unleaded (92? RON) without problems. A recent rebuild has revealed zero pre-ignition damage.

Not a cheap motor to replicate, but if you want excitement and a fabulous noise, it's the way to go.

ps. I have a set of 2.2S P/Cs in excellent condition if anyone wants to raise the CR of their 2,4S

[Mr9146]

Originally posted by sithot
Loose timing chains and other geometry issues I imagine. There is some material that can be sacrificed but the machine shop better know what they're doing.
Tom

I wasn't asking if it could be done, I was asking Cheese if he'd thought of it ... if the mechanic doing the work doesn't realize there're consequences to these types of modifications, it's probably time to look for a new mechanic.

Removing barrel shims is the easiest and cheapest way to up compression ... especially since he's already got Ps and Cs. The only question is whether he has any shims that he can remove.