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Walt Fricke Walt Fricke is offline
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Join Date: May 2004
Location: Boulder, Colorado
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Porsche Crest


Putting a slightly "hotter" cam like the 964 in a 3.0 CIS motor should give you some more horsepower. Reading posts, it seems the concensus is that this works and doesn't have nasty drawbacks like terrible idle.

But why do you want stiffer valve springs? You only need stiffer springs if you are using a really radical cam which slams the valves closed or has other tricky characteristics. The 964 is not such a cam. Or if you are going to rev the engine way up. Opinions on what way up is varies. Some get worried above 7,000. I think I got along just fine in a race motor with stock springs at 8,000 rpm.

Now reving the motor up another thousand RPM is a classic method of gaining horsepower. That's why Formula 1 engines can make so much horsepower from such tiny engines. What do they get - something like 800 hp from a 2.0L engine at 19,000 rpm? Something like that - I don't follow those things exactly because I am never going to run an F1 motor. So if you rev your motor quite high for what it was originally designed to do, you certainly need stronger springs to control those valves (I'm simplifying this a bit here).

But if you take a stock 3.0 and rev it up beyond maybe 5,500 rpm you start losing horsepower (on mine the optimum shift points are about 6,200 depending on which shift is involved). Why? Because it is starting to have real trouble breathing. It breaths best down around or a bit below 4,000 rpm, where the torque peak is (the torque peak is always at the point of peak volumetric efficiency because that is where the most air gets pushed - or sucked, if you like - into the engine). Above that the beneficial effects on horsepower of more RPM keep the horsepower climbing even as the breathing and thus the torque falls off, but that doesn't last forever, and at the HP peak the falling breathing takes over and HP goes down above that point (for best acceleration you shift above the HP peak, but that's another topic).

So if you put a 964 cam in your 3.0 engine, we can assume you will raise your torque curve because you will get better breathing. It should move up and to the right on a standard graph. Or perhaps just move to the right. But will it raise the curve such that optimum shift points are now 7,200 rather than 6,200 or so? I'm dubious (did Porsche use stiffer springs on the 964s?), but that is the kind of fact you really want to know before spending money on stiffer springs.

Stiffer springs cost a little bit of power (it takes extra power to compress them compared with stock) and increase valve train wear some. More to the point, they cost money. There might be something else you could do with that money to cut time off your autocross runs, or give you a bit more acceleration for spirited highway driving.

Frankly, if you drive your SC to its limits often on public roads you ought to be put in the looney bin because those limits are so high already they will exceed your ability to figure out when some guy turning left on a country road isn't going to expect that dot in the distance to hit him before he completes his turn, or will think he can see far enough around a curve that he can make his turn or pull out onto the road. Suspension, for instance, will probably help you more in autocross than power. Practice will do more than either suspension or power until you get pretty good at it. But we all do like more power. Power is good (at least as long as you can afford the fuel).

Once you know the crude version of the theory (pretty much all I know - kind of a qualitative understanding), you can do a smarter job of putting the pieces together by taking actual data from others and seeing what their results were. Maybe somebody who has put a 964 cam in a 3.0 engine (and kept the CIS and didn't port the heads or bump the compression and so on) would give you his dyno sheet. That way you could calculate what the shift point RPM is (another subject). If it is above 7,000, maybe the slightly stiffer springs would be a benefit worth paying for.

Walt Fricke
Old 09-22-2005, 01:44 PM
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