The great torque/HP debate

[Motor Daddy]

I'm having a little problem understanding why people are constantly saying, "peak power is not important because the engine spends very little time there". If I hear that statement one more time I'm gonna puke. I have discussed this issue at length with some very knowledgeable people, and some that have serious math skills, none of which can dispute this example. Maybe some of the Pro's here would like to tell me why I am off my rocker? The discussion started as me saying that the power is in the heads. I am a traditional Pontiac fan, and I have a Pontiac 400 in a Firebird. Of course, Pontiac makes a 455 with basically the same bore (455-4.151, 400-4.12), but the 455 has a 4.21" stroke compared to a 3.75" stroke on the 400. I've noticed a trend in Desktop dyno that shows the 400 as having as much or slightly more peak power than the 455 when only switching the displacement.

The common rebuttal to that is, "peak power means nothing, the 455 has a greater torque curve so it will be faster". So I started thinking about what torque really is (totally different thread), and I concluded that people don't seem to realize what torque really is. So I made this example to try and show people WHY peak power is so important to acceleration in a vehicle:


Can anyone tell me exactly which portion of this example you disagree with? Are you also of the mentality that peak HP means nothing? Can you explain to me why the only thing engine B has going for it is a little bit of additional "peak power", and yet, it has a greater torque at the wheels at EVERY MPH? Do you recognize engine A as totally incapable of beating engine B when both are optimally geared, even though engine A has a greater torque curve?


Here is a prime example between two different engines with completely different torque curves.

Engine A in second gear ( 1.98 ) with a 3.23 rear gear and 26.5" tire.

2000 464 lb-ft 176 hp 24 mph 2967 RWTQ
2500 482 lb-ft 229 hp 30 mph 3082 RWTQ
3000 496 lb-ft 283 hp 37 mph 3172 RWTQ
3500 511 lb-ft 340 hp 43 mph 3268 RWTQ
4000 506 lb-ft 385 hp 49 mph 3236 RWTQ
4500 481 lb-ft 412 hp 55 mph 3076 RWTQ
5000 427 lb-ft 406 hp 61 mph 2730 RWTQ
5500 363 lb-ft 380 hp 67 mph 2321 RWTQ


Engine B in second gear ( 1.98 ) with a 3.73 rear gear and 26.5" tire.

2309 410 lb-ft 180 hp 24 mph 3028 RWTQ
2887 430 lb-ft 236 hp 30 mph 3175 RWTQ
3464 460 lb-ft 303 hp 37 mph 3397 RWTQ
4041 465 lb-ft 357 hp 43 mph 3434 RWTQ
4619 465 lb-ft 408 hp 49 mph 3434 RWTQ
5196 445 lb-ft 440 hp 55 mph 3286 RWTQ
5773 390 lb-ft 428 hp 61 mph 2880 RWTQ
6351 330 lb-ft 399 hp 67 mph 2437 RWTQ


Two identical cars (except for engine and gears, same weight) are moving along at a steady 24 mph in second gear side by side. They both punch it at the same exact time. Which one will pull ahead immediately, and get to 67 mph the soonest, and cover the greatest distance in the same time period? The one with the greatest engine TORQUE, or the one with the greatest engine HP??

Here is the dyno sim of the 455 and 400. The 455 has exactly the same parts, I just switched the CID. Notice the lower torque curve on the 400, and that the 400 has just a bit more "peak power", and yet, the 455 (engine A in the example) is TOTALLY INCAPABLE of beating the 400 (engine B in the example).

Comments?

[hotrod]

Assuming best gearing for both, the engine with the highest area under the power curve in the rpm range it uses to go from x to y.

Torque and power are tied together by a mathmatical relationship you cannot have one without the other (if rpm is non-zero) the engine that has a higher torque at x rpm, must also have higher power at that rpm it is a mathmatical certainty.

People use torque because it is easy to think in terms of torque and a bit less natural to think in terms of power.

Due to the fixed relationship between power rpm and torque you can specify any 2 and the other is also specified. Low rpm torque translates to low rpm power.

Power is the ability to do work and to accelerate a car takes work.

In your chart you can see if the 400 and 455 engine are both geared to spend their time above 4500 rpm the 400 has a clear advantage. If they are geared to stay below 4500 the 455 has a slight advantage. If the gear split runs the engines from 3000 - 5500 then it gets pretty close as total area under the power curve for that rpm range is going to be nearly the same.

Larry

[blykins]

I think in terms of peak power, it's more important in race cars than street cars. Race cars go from idle to wide open and stay at wide open. Street cars spend a great deal of time at partial throttle and at low rpm's.

Since I deal mainly with heavy/well mannered street vehicles, I find it counterproductive to build engines that make gobs of power at peak rpm and have no hindend whatsoever at low/mid range.

[hsutton]

Larry says,
"If they are geared to stay below 4500 the 455 has a slight advantage."

Let me repeat, so we are perfectly clear on this:

The 455 is INCAPABLE of beating the 400, regardless of the MPH or the gear the 455 selects. TOTALLY and utterly INCAPABLE!

In Chevrolets the 454 will beat a 396 unless the gearing is radically different because the torque is much higher on the 454. I had a 396 with 4.88 gears and a ZL-1 cam which would always trap at a higher speed than an aquaintences 454 in the same basic car yet i could not pull with him through mid range and was appx. 2 tenths slower in the E.T. department over the full quarter. He had 4.56 gears and on one run he went 11.46 @ 114 MPH while my car ran 11.64 @ 117. Three hundred feet out, even though i left on him he was a car of daylight ahead of me and wound up beating me by about 3/4 of a car length. I had a Camaro after this car with a 454 and the same basic insides as the 396 and only 170 lbs. less car weight, it went 11.15 @ 126.76 MPH, (same compression, cam, carb, intake and way less gear, 4.10) but way more power. The bigger motor should have way more torque at any given RPM if enough cam is put in to use the extra inches.

[hsutton]

Motor Daddy, I think your example is unrealistic in nature because the spread in torque is going to be way more than 20-30 lb. ft. at any engine speed. More like 60-100 lbs. at the same speed on the longer stroke engine if it's not severely undercammed or exhaust restricted.

[Motor Daddy]

Did anyone happen to catch the Musclecar shootout? That sure was a nice 69 GTO RA-IV Pontiac 400 engine, wasn't it? Talk about an awesome car!

[F1Fever]

which ever motor has the greatest AVERAGE HP over X amount of time will win.

the only exceptions are weight and drive train efficiency.

the can of worms about a smaller motor with the same heads...well that's another can of worms...

[Motor Daddy]

Motor Daddy, I think your example is unrealistic in nature because the spread in torque is going to be way more than 20-30 lb. ft. at any engine speed. More like 60-100 lbs. at the same speed on the longer stroke engine if it's not severely undercammed or exhaust restricted.

The point is not how the power is made, it's that these are dyno RESULTS. It doesn't matter how the engine is made. I am not talking so much about the engines, but the actual torque and HP curves.

[Motor Daddy]

Does it matter if I am a PRO. After you made a statement I replied. You then changed the rules from your first statement. Once you get all of the rules for the comparisons then maybe we can talk about it.

I didn't change any rules at all. I haven't even stated any rules. I couldn't understand what you had written in the first sentence, and I thought I sensed a little hostility in your tone.

[bigjoe1]

I would rather have the MOST horsepower every time, I do not care about the best average or anything like that. Then you must figure out a way to make your car work to take advantage of the most HP. ((gearing, torque converter, whatever )As far as I can tell, the most HP wins every time.
JOE SHERMAN RACING ENGINES

[nitro2]

At any vehicle speed, the combination that makes the most power (RWHP) at that vehicle speed, will make the most RWF, RWTQ and the most acceleration at that vehicle speed.

Acceleration = (RWHP x 550)/(vehicle weight x 1.466666 x vehicle speed)

Acceleration in g's
Vehicle weight in lbs.
Vehicle speed in MPH
1.466666 is the conversion factor to MPH to FPS


RWF = Weight x Acceleration = (RWHP x 550)/(1.466666 x vehicle speed)

RWF is the rear wheel force in lbs. applied to racetrack


Note air drag, tire drag, component acceleration is not included in this formula.

Clint Gray
TFX Engine Technology Inc.
(Combustion Pressure Analyzers)
www.tfxengine.com

[Motor Daddy]

Nice, I like those words. Keep 'em coming!

[axegrinder]

Its very simple to me, but I'm not sure what your point is in your original post. It's been proven too many times to count: MORE RPM, MORE HP, LOWER GEARS. Just look at Pro Stock: over 10,000rpm,NASCAR: 10,000rpm, and F1: 20,000rpm before the rules held them to 19,000. Is there really anything to talk about? The only thing I don't understand is how these motors overcome all of the friction at these rpm to make more power, but they do, so its still the correct way to win races.

[Motor Daddy]

Its very simple to me, but I'm not sure what your point is in your original post. It's been proven too many times to count: MORE RPM, MORE HP, LOWER GEARS. Just look at Pro Stock: over 10,000rpm,NASCAR: 10,000rpm, and F1: 20,000rpm before the rules held them to 19,000. Is there really anything to talk about? The only thing I don't understand is how these motors overcome all of the friction at these rpm to make more power, but they do, so its still the correct way to win races.

Thumbs up, Guy. That's music to my ears!!

[rskrause]

If, for some strange reason, you shifted at 5,000rpm the 455 would accelerate quicker. But in general, the 400 in this example is going to be quicker as long as the gearing is anything close to optimum and the engine is operated in the correct rpm range,.

Richard

Do you mean that once the 400 was ahead of the 455, the guy in the 400 would let off the throttle at 5,000 RPM, or that the throttle cable broke, or that he mistakenly set his rev limiter to 5,000?

I think I was actually agreeing with you that the 400 is generally going to be faster/quicker. It still seems to me that this is the case unless you postulate some odd conditions such as an artificial rev limit, or very odd gear ratios. I'd take the 400 for racing, the 455 for towing. In a street car, there would be more to consider.

Richard