Torque curves

[Steve D.]

I recently saw an add for a Subaru with a 2 liter engine with advertised horsepower of 227. I have also seen various other ads for relatively small displacement engines with 225-320 HP. What do the torque curves look like on these engines as compared to a 327-350 small block with similar HP ratings ?

Steve

[Duke W.]

Re: Torque curves

First, modern engines (since the early seventies) are rated using the SAE net method, which is based on the "as installed" engine configuration in the car including all accessories, emission controls, and the actual exhaust system.

Vintage Corvettes prior to '72 were rated using the SAE gross method which is based on testing the engine on a laboratory dynamomter with no accessories or exhaust system. Fuel flow and spark are set for optimum torque across the rev range and observed data are corrected to standard sea level condtions which means higher air density and bigger numbers than standard SAE condtions.

As a rule of thumb approximation, net horsepower is in the range of 80 percent of gross.

Torque and power are uniquely rated via the relationship:

T = (HP x 5252)/ RPM

and the expression can also be algebraicly rearranged to compute power when torque is known.

For a naturally aspirated engine, peak torque is basically a function of displacement and compression ratio and the shape of the torque curve is a function of valve timing. Conservative vavle timing means high low end torque, but modest power because the torque curve reaches a peak and falls off at modest revs. Aggressive valve timing shifts the torque curve up the rev scale, which reduces low end torque/power, but improves top end power.

The engines you referenced are turbochanged, so torque is primarily a function of displacement and boost pressure. The problem with turbocharged engines is that they do not develop full boost at low revs, and in transient situations there is "lag" - a second or so is required to reach full steady state boost, so these engines can feel slow to respond and soggy down low.

Big naturally aspirated V-8s like Corvette engines produce lots of torque because of their large displacment, so they have have a lot of power at low revs - what I call "lazy horsepower", and they also have virtually instant throttle response, which means the torque/power is there right now when you step on the throttle - no waiting for boost to build.

Duke

[Duke W.]

P.S.

Engine flexibility is important to good driving perception and the most flexible engines have both high and broad torque curves. The best special high performance engines in vintage Corvettes have 80 percent torque bandwidths from about 2000 to 6000. By comparison, the current LS6 in the Z06 has a NINETY percent torque bandwidth from about 1500 to beyond the 6500 RPM rev limiter.

That's why you can't beat a big displacment pushrod engine like Corvette V-8s. They pack the best torque and broadest torque bandwidth in the a small lightweight package and all the valves, cams, and turbos in the world can't beat a big naturally aspirated V-8 when it comes to high and broad torque curves.

Duke