WOT, peak torque and rpm
 

I am wondering how far past the peak torque number should I be propping my engine? I have found a lot of talk on the subject but failed to see this come up other than you don't want to run at peak torque. I am currently running B-1's 26" and showing 5100 rpm on one and 5300 rpm on the other. Peak torque is at 4400 rpm (both on the dyno) and is about 520 ft lbs. At my stated rpm's the torque is still 508 ft lbs on the one running 5100 and the other is showing 501 ft lbs. Is a drop from peak torque of only 14 lbs +/- decent? Is there a number you guys shoot for? Just for reference, mine are still running 476 and 478 ft lbs at 5700 rpm. My AFR gauge quit working but last I saw both are running in the upper 11's. Thanks for your comments.

If your engine won't live at 520 ft*lbs, it will not live at 508 ft*lbs either.

Impressively flat torque curve.

Torque is one thing to consider.
Rotations (and certain positions) over Time is another. :wink
Gasoline engines and gears don’t enjoy being at full load at low(er) rpms.

The reason you don't want to run your engine at WOT at the point of maximum torque is simply the stress that the torque puts on the components.

Whether that maximum is at 2000 rpm or 4000 rpm doesn't really matter from a stress perspective. It is not like rotating the driveline faster reduces the stress from the torque.

Most 2-stroke and 4-stroke gasoline engines have peak power above peak torque, so this is not a major issue with correct gearing (and wheel size or propeller size depending on whether the engine is in a car or a boat).

Thanks for your comments Markus and SB. Markus, your comment is why I asked the question. It seems to me that 2 torque numbers are very close together and why should it matter. From what I have read and heard, you shouldn't run at peak torque. There's not a definitive number or rpm above that point I should run. SB, I believe I am reading that running at 4400 rpms even though it's my peak torque number is fine because the engines are spinning fast enough. If my peak torque was at 3500 and that was also my WOT rpm that would be a problem because of the slow rpm's? If this is true, is there a definitive rpm point where engines should and should not run at WOT? Lastly, If you were in my shoes, would you run the B-1 26's at the stated 5100 and 5300 rpms or the B-1 28's @ 4500ish rpm? For my setup, the 28's were faster. Thanks again for the comments.

Sure it does. :thumbs
(within reason of course.)

Peak cylinder pressure is momentary and falls quickly. It falls faster as the piston moves, but the peak should now be meaningfully impacted by piston speed. And the more rotations, the greater the number of peaks.

Run the 26’s at the higher rpm, depending on fuel and boat load weight you could be lugging the bigger props. Or lab the 28’s 😀

I think the 26's will be the best bet too although the 28's were also easy to get on plane with, mid range cruise speed was better along with top speed being just a little better. I don't think I'm going to spend the money on "labbing". I'm going to install the 28's and see where the rpm's are now and go from there. Thanks for your comment.

Here is the combustion engine design theory:

What limits the ability to rev any cross plane V8 is the exhaust, with the exhaust pulses stumbling on each other, as you can hear already at idle. The next limiting factor is the heavy rotating assembly with crank counter weights.

A push rod V8 is crippled by the valve train with lots of heavy components moving back and forth even before these things come into play.

Considering all that, you should rev your engine as high as your valve train can go.


I'm not a push rod V8 guy, but 5100-5300 rpm shouldn't be excessive. So go for the 26.