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Join Date: Jul 2003
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Also worth bearing in mind the effect cam profile/overlap has on dynamic compression; I won't pretend to know how to start to do the math, but John Dougherty said:

Originally Posted by camgrinder View Post
I can calculate a dynamic compression ratio based on the rod length, stroke length, cylinder bore diameter, static compression ratio and intake valve closing point. For example,
Stock 930 3.3 turbo with 7-1 compression and stock cam = 6.12-1 dynamic compression ratio.
Swapping to an SC profile cam lowers the dynamic compression to 5.55-1. Going further and swapping to a 964 profile reduces the dynamic compression to 5.19-1.
Off boost the power will be down with the 964 profile. But on boost you will see a jump in HP. Now if you bump the static compression up to 8-1, the dynamic compression will also jump, up to 5.89-1. This new dynamic C/R is slightly lower than the original turbo engine.
in this thread 8.0/8.5 CR for 930 ???

So the more radical the cam, the more boost you can run. I figured >99% of my street driving is off-boost - so kept SC cams and 8:1 for my 3.4 Motec conversion. I like the way it drives.

CIS 3.3 930 with 8:1 and SC cams drives like a sports car with a big old, torquey, willing engine - until it spools. How I imagine an SC with a big bore kit would feel. You can drive this around all day off-boost and it doesn't feel like a hardship.

Strap on a better turbo etc as well, and it's a different ball game. Given enough load on the dyno, my K27/HF made 6 psi @ 2400 RPM. In contrast to the 3DLZ (which hit like a hammer to the chest and wouldn't be completely spooled until 3300, 3500 in lower gears - 5 speed), boost onset is linear and progressive - hard to tell exactly when it starts, boost just comes in as you rev. More throttle/load, it just comes in faster/harder.
'77 S with '78 930 power and a few other things.
Old 01-09-2020, 05:52 AM
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