What's the secret to torque/hp ratio?

[OddJob]

I was searching around and stumbled across this article from 2010:

http://www.nsxprime.com/forum/showthread.php/136712-ScienceofSpeed-3-5L-NSX-608-whp-603-wtq

What's the secret to getting a higher torque-to-hp ratio? Most setups seem to get quite a bit lower torque-to-hp.

 

[E-Nough Logic]

The secret is the engines inability to flow air efficiently above 5252 rpm.

If an engine can consume air efficiently above 5252rpm then horsepower should exceed torque, this is a good thing don't shy away from it! Honda engines typically do this very well which is why it is rare to see this. Your example shows a stroked 3.5l engine where torque drops rapidly after 5700rpm. Stroking the motor will demand more air at every rpm. Once airflow demand increases if the heads or other components can't keep up with the flow, torque will drop. Additionally when turbocharged if the turbocharger can't keep up with the flow at that rpm and boost level then again torque will drop.

Hondas can get a bad rep for not having torque but I disagree. The s2000 for example many claim doesn't have a lot of torque. IMO it is the opposite, it has a fair amount of torque when compared to other 2.0l engines, it's the other engines that have No horsepower! When given a displacement it is very difficult to make a lot more torque than any other engine because there is only so much air you can get into the engine each stroke. Horsepower however is only limited by your ability to flow air overall, so as long as the engine can keep filling the cylinders at more rpm then you will continue to make power and sustain torque.

 

[OddJob]

Thanks!

 

[Mark911]

In addition to VE (including pumping losses), don't forget about frictional losses which increase exponentially with RPM.

 

[tof]

This isn't nearly the mystery you make it out to be. TORQUE X RPM / 5250 = HP

So horsepower is dependent on two things, torque and RPM. The reason many honda engines produce "bigger" HP numbers than torque numbers is because they maintain peak or near peak torque at higher RPM. High RPM X Good torque = more HP.

So your question is absolutely valid but the way you asked it could confuse future readers of this thread. The ratio of torque to HP is fixed. The ratio of PEAK HP to PEAK torque is what you are interested in.

Generally longer stroke engines tend to produce more torque lower in the RPM range. These engines "pull" harder down low and in the mid-range. But a long stroke limits the safe limit on RPM because it means more mass moving at faster peak speeds. A shorter stroke engine can rev higher, all other things being equal, but it tends to produce less torque and peak torque tends to be higher in the RPM range. This can be a good thing if you want a high-winding engine like the B16 of the 99-2000 Civic Si or the crazy F20C S2000 AP1 engine that could spin safely to over 9K RPM.

Since HP is a function of torque AND RPM, as long as you can wind an engine out to a higher RPM, you can produce more HP with the same amount of torque.

There's obviously a lot of other factors in determining the torque curve and maximum torque of an engine like breathing, as Logic pointed out. That's why the NSX has a variable volume intake plenum and why VTEC was invented.

 

[ENT]

The trick to all of this is to look at torque at the wheels, not at the crank. You might have less TQ at the crank but if you're making it at a higher RPM then you can gear it down and make more TQ at the wheels. That's really what HP is - a relative measure of the potential amount of TQ at the wheels after factoring in gearing.

Cheers,
Ian

 

[Rope]

Torque is measured at the crank because the amount of torque at the wheels changes with each gear change as they are a torque multiplier. Tof nailed it on answering the OP's question

 

[OddJob]

Ah, thanks, I'd forgotten about the equation. Everywhere I punched in the dyno graph numbers from the aforementioned article, it's spot on. I guess I was more interested in how I could make 600 ft-lb of torque at 4000 rpm.