View Single Post
lucittm lucittm is offline
Manassas, VA
lucittm's Avatar
Join Date: Apr 2007
Posts: 1,211
I have a similar car without the X33 option. It is interesting to me that you want better off-boost performance and response. I believe the purpose of the X33 option was to provide better top-end performance and a sacrifice of some off-boost power.

I think the X33 cams (and heads) cause your manifold vacuum to be lower off-boost (The reason for variable cam technology). The cylinders will not fill as fast under the lower vacuum and the performance suffers slightly. On boost, which is really at higher RPMs, the power is greater because you are "on the cam" and forcing the air through the manifold.

According to Adrian Streather's book, Porsche Enthusiasts Companion, the X33 option also changed the valve timing. You could advance (increase) the valve timing to improve your bottom-end performance. I would be curious what the factory recommended for valve timing with the X33 option. Mine is .60-.65mm.

Could you also explain your item: Bypass waste gate?

I am afraid if you increase your C/R above 8.0 and run at 1 bar, your street car (from the picture) may not be too reliable. For track purposes it would be fine.

As fas as CIS and C/R, I don't think there is a relationship there. CIS just controls the AFR and regulates how well the engine can flow air to the intake side of the head.

The graph that 125shifter provided is useful if you assume a 50% efficiency for the intercooler. At 1 bar, your C/R should be below 7.0 with premium fuel. With racing fuel, you might be able to push a C/R of 7.5.

If you change out your cams, heads, etc. (X33 option) let me know, I may want to buy them.

Good Luck,
1991 964 Polar Silver Metallic Turbo Coupe
Old 11-25-2008, 08:35 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)