troppo

what would be the easiest (I realize there might not be an "easy" way) way to lower the compression on a 993 engine? Any thoughts on the matter would be appreciated, and yes this is prior to sc install.

thanks-

SwayBar

The 'easiest' way to lower the dynamic compression ratio (..versus the static compression ratio) is to install a camshaft which holds the intake valve open longer past bottom-dead-center than another. Actual compression of the charge cannot commence until the intake valve closes. Until that happens, the piston is rising higher and higher in the bore, decreasing available volume to compress once the intake finally closes.

In the big scheme of things, the factory-rated static/mechanical compression ratio really means nothing compared to the engine's dynamic compression ratio, which is a function of both the static compression ratio, and the intake valve's timing, specifically, when the it closes on the compression stroke.

I would think popping in a new camshaft would be cheaper than new pistons.

Also, one does not want to decrease the static compression ratio by installing a thicker head gasket(s) which will upset the squish area.

993_Pilot

Disclaimer: I am probably not the best one to answer this question... but I think you can lower the compression slightly by using thicker head gaskets. (This works for Small block chevys and such but the porsche motor is a whole different animal and I have never had one apart) You could also have the heads modified, but that won't be cheap if done right. The best and most expensive way would be to have the engine purpose-built for the SC with new pistons.

Edit: I told you I wasn't the best one to answer this... someone gave a better more knowledgeable answer as I was typing!

troppo

I just read an article in excellence where ruf is supercharging 997's and 996's and they actually use a thicker head gasket and say it is bulletproof.
I guess could you please give more detail as to why using a thicker head gasket would not be appropriate? Squish area??

thanks again

SwayBar

There are many articles on the topic, and here's a quickie I found:

http://www.nhra.com/dragster/1999/is..._Speaking.html

Major point from above link:

"High compression also tends to make the cylinder more active by increasing the turbulence in the combustion chamber. This turbulence can forestall detonation by creating a finely atomized, homogeneous fuel/air mixture that burns quickly. It is a serious mistake to cut the compression ratio by installing a thicker head gasket or running more piston-to-head clearance because this destroys the "squish" between the head and piston that is essential to producing turbulence."

As for RUF, they surely know what they're doing. Nonetheless, the best-case-scenario for lowering compression ratio does not include thicker head-gaskets for the above mentioned reasons. However, depending upon the application, etc, there must be some wiggle-room as demonstrated by RUF.

troppo

let me ask this then, where can I get a camshaft that would accomplish this. Any ideas on costs? Also anyone know where a thicker head gasket could be found?

thanks-

CP

Greg,

May we know why you wish to lower the compression ratio in an NA 993? Enhance engine longevity?

Thanks.

CP

troppo

I plan on supercharging this winter and would feel more comfortable if I lower the compression

CP

Greg,

Then it is a great question. I don't know the technical answer. However, when RUF converted my 993 C2 to the BTR (added K-27 turbo charger) they lowered the compression to 8.4 to 1. I am sure that is to avoid engine exploding. If you found out how, please let us know.

CP

Red rooster

Gregg,
Maybe the first question is how much do you want the compression lowered ?
That will depend on the boost level and whether you will have a linear supercharger like an Eaton or a centrifugal square law like a Vortex .
If you are going for a linear, then lowering the dynamic compression by cam closing point may be a problem.
Generally late closing is accompanied by more overlap which you do not want
because of the resulting boost loss !
Most production motors will take some piston crown machining with no strength issues arising.That is a way of loosing compression but not squish .

Maybe a talk with a really good motor machine shop could be the lowest cost option.
Good luck with this project. Hope to hear more !!

Geoff

troppo

I plan on a tpc unit, this is a screw-type. Not familiar with a vortex? I plan on keeping the boost quite moderate either 4.5 or 6. However, I would start with the 4.5 for sure.

Geoffrey

The easiest way is to machine the piston down since there is enough material to do it. 993 engines DO NOT have a head gasket so installing a thicker one is not an option. Also, I would never use a thicker head gasket to lower compression because it affects the quench and squish of the piston and will often result in lower engine performance and one that is more prone to detonation due to the loss of the quench/squish area.

PS speaking from experience, 9:1 might be a good compression ratio to aim for your supercharger experiment.

SwayBar

Would you care to expound on your statement since it doesn't make any sense?

This is not even remotely true. Overlap occurs when the intake valve has just opened, and the exhaust is closing. Overlap is completely independent of when the intake closes.

This is a good idea. Find an old piston and have someone cut it open to check the thickness of it's crown.

SwayBar

According to PET, there is a head gasket, part number: 964.104.115.20

Martin S.

buy lower compression pistons...voila, install 993TT pistons! I believe they are lower compression than the NA motor.