Lowering Compression
 

Hi Guys:

Why the compression in Porsches can't be lowered by using copper head spacers or shims under the cylinders like the VW engines? Is it because of the lenght of the chains and cylinder head studs?

Thanks,

Francisco Soto
Puerto Rico

Francisco,
I don't know if they are made for Porsches but in my old Chevy blower motor we used dished pistons.

Lar

Francisco,
The performance reason is that the cylinder-to-head minimal clearance provides what is known as “squish.” This causes almost all of the air-fuel mixture to be forced into the combustion chamber at TDC compression.

This has important advantages:
1. It causes more turbulence in the combustion chamber, allowing better and more complete combustion.
2. It prevents small volumes of combustible gasses from being available for detonating.

If you just add spacers under the cylinder you reduce the CR but you also reduce/eliminate squish, with bad results. If, for some reason, you need lower CR, get different pistons but keep the squish.

Lar,
Yes, some Porsche Turbos have flat and dished pistons. They still have “squish” around the perimeter.

Best,
Grady
gradyclay@hotmail.com

I've always thought that you could use an additional copper cylinder base gasket with no disadvantages. However, this would only lower your compression ratio but a few tenths.
Clint

U can use a 0.5mm head gasket and a 0.5mm base shim without upsetting cam chain geometry at all.

It's what Protomotive/Prototech do with their Stage 2 (&3) Turbo-systems to drop from 9.5:1 C/R, to 8.8:1 C/R, for a bit more boost and a better cylinder -> head seal.

WydRyd:

What did you do to your AC condenser? Did you relocate it under the car?
Thanks,

Francisco Soto

Francisco, I *will* be installing a "sub floor-pan" static condensor soon. I'm just awaiting the shipment to come in from the USA.

Why do u ask?

Are u thinking of a similar upgrade?

Hi Merv:

I think I am going Juan Ruiz route and install the OEM condenser under the car. He did it and looks good. About how it works, don't know yet.

Francisco Soto