Pelican Parts Forums - View Single Post

Walt Fricke · 09-08-2011, 12:36 PM

Camlob

For sure, reset the cam timing to see what it does. Not that big a deal.

And your brother's 997 is not a fair comparison. A full on air cooled 911 race motor can make about 110 hp per liter. A water cooled, 4 valve 997 can make from 165 to 175 hp/liter. Its volumetric efficiency is just so much better. Of course, your's isn't a full on race motor with that cam, and you may well not want that anyway. Shifting up around 8,000 greatly reduces engine life between rebuilds.

You are wrong about your "rev limit" thinking. Assuming that the mechanical limits of the engine aren't exceeded (and with race valve springs and race rods and bolts I'd expect that it will hold together fine at 7,500), the real question is how does the shape of the torque curve determine optimal shift points for your gearing. Which depends on your gearing too.

Without doing the math (which isn't all that tricky), what you want to do is have the torque multiplied by the gearing in the lower gear match the torque multiplied by the gearing in the new, higher gear after you shift.

For any kind of torque curve likely from a 911, this is always going to be above the HP peak. Unless you have a race gear box with the gears evenly spaced, the optimum shift point can vary from gear to gear. First gear's optimum, because it is usually way low compared to second, is apt to be way up toward the mechanical limit.

I had a race motor which I shifted at 8,000 rpm. Then I finally got the car on a dyno, and did the math for shift points. Oh - peak HP was about 7,100, and the calculated shift points were about 7,600. I'd been wasting time running it up to 8. It felt like it wanted to keep going (unlike CIS motors,which you can feel just run out of steam if you disable the rev limiters), but that was misleading - there were the facts.

Anyway, that's probably what your tuner is trying to tell you about where to shift. You see "rules of thumb" like "shift at 500 rpm over HP peak" and so on, but with dyno information a guy really should just do the math. Then you will know.