Newbie Question: Can Cams "Shape" The Torque Curve?

[Wrecked944]

I have been wondering...if you were building your motor strictly for street use (or maybe street and infrequent autox) and you wanted to maximize low end torque for acceleration...would it be possible to achieve this effect using only a custom cam grind? Can one lower the peak torque point to 3000-ish rpms? I know this may be sacreligious but I am curious about how cams influence the torque curve.

[Bill Verburg]

to a point yes, that was the way that the old T, E, and S models were differentiated, there were also differences in cr and port flows. But the primary determinant of the character of the torque curve was the cam.

Displacement of course would be another way to modify a torque curve, that was the path used on the '73RS wich had the same cams and heads as the S of that year but had considerably more torque due to the extra 300cc of displacement.

[911pcars]

A simplified answer for more midrange torque would be higher lift and less duration. Higher compression ratio and cubic inches do wonders too.

Sherwood

[Eagledriver]

Yes the cams have a big effect on the torque curve. For best acceleration you need the torque at higher RPMs. Acceleration is a function of HP. Big torque will only get you off the line fast. For example an F1 car with a 3 litre engine probably has less than 250 ftlb of torque. It gets that torque at 12000-18000 RPM. This results in 800 HP and amazing acceleration.

Think of it this way would you get better acceleration at 60mph in second gear or third gear (assuming your engine can rev that high). If you had that F1 motor you could be in second gear at 150 mph!

-Andy

[911pcars]

"For best acceleration you need the torque at higher RPMs."

However, you first have to get the engine to high rpms if that's where the torque is, and unless your gearbox has the proper gearing (low gear ratios), it's going to take awhile to get the revs, thus the need for torque at a lower rpm. "Low" is a relative term. For our engines, low speed would be around 2000 - 2500 rpm. An engine with the proper valve timing (among other variables) can help produce more low-end torque for your particular needs.

An F1 engine has a very narrow (and peaky) torque range, thus the need for a 6 or 7-speed gearbox to keep the revs in a certain rpm range for the best acceleration and throttle response. For a street engine, the ideal is a wide and flat torque curve that begins at a relatively low engine speed along with enough breathing to continue creating power at high rpm. "High" is also a relative term. 7,000 - 8,000 would be considered high for our engines. 7K rpm might be off-idle speed for the current 8 cyl. F1 engines that are already into the max. 20+K range. That's high.

Modern engines with variable valve timing and valve lift attempt to optimize horsepower and torque at both ends of the rpm range. Our old-design, fixed valve timing engines must be content with a compromise between bottom and top ends. Like I said, displacement and compression ratio do alot to minimize the compromise.

Sherwood

[cnavarro]

To further extend Sherwood's simplified answer, properly set up heads are also worth their weight in gold and greatly influence the performance of any given cam. In an ideal world, you would set up the heads and have a custom billet grind made up for the cams. We do this all the time with the Porsche 914. If I'm not mistaken, don't you do this Camgrinder? And yes, compression and displacement help tons- why do you think 2.6+ litre type 4s are so popular, at least after we made Nickies for them. :-)

[911pcars]

"...To further extend Sherwood's simplified answer,...."

Simplified answers from a simplfied mind. Glad to assist.

Sherwood

[camgrinder]

I can make a set of cams to move the powerband down. The trade off is losing high rpm hp. If you want 10 ft lbs of torque at 3000 rpms, it make cost you 15 hp at 6500.
The octane requirement of the engine will go up, due to the increased cylinder pressure.
On an auto-x car this might work well for short, tight corner tracks.