example: 400lbft at 4000 rpm vs. 200lbft at 8000 rpm
looking for some real science answers here
example: 400lbft at 4000 rpm vs. 200lbft at 8000 rpm
looking for some real science answers here
Do you mean to ask whether it's the same amount of horsepower? Yes it is.
Is it the same 'thing?' That depends on how you define that thing.
I suspect "and I gear it for the same wheel speed in both cases" would also be part of the question.
In reply to Keith Tanner :
well I just mean purely engine power wise not considering transmissions or gearing or what have you
In reply to white_fly :
by "thing" I mean speed, setting aside gearing and tires as variables, would this example have similar times? (0-60, 1/4 mile, etc)
In reply to manladypig :
Not if you're setting gear aside....twice as more torque is twice as much torque.
HP is basically a measure of what you can do with the torque and the hp is the same.....but to use it you need the correct gearing. With the correct setup the 2 engines should give you about the same 0-60, 1/4mile etc...but ONLY with the gearing matched to the engine to allow you to use the torque from each engine.
Yes, half as much torque at twice the speed will accelerate you at the exact same rate.
Fun tip: If you are calculating acceleration you don't need to know torque at all. Power (or Torque and delivery rate) is all you need.
manladypig said:example: 400lbft at 4000 rpm vs. 200lbft at 8000 rpm
looking for some real science answers here
Technically yes. What you lack in "torque" you make up by increased leverage with the lower gear needed to run the same speed.
In the real world, the higher RPM engine will be easier to "pedal" in low traction conditions. Think of it like the difference between pushing hard on a short ratchet versus gently on a long breaker bar - you have much more control when you are controlling less force and more leverage.
manladypig said:In reply to white_fly :
by "thing" I mean speed, setting aside gearing and tires as variables, would this example have similar times? (0-60, 1/4 mile, etc)
If the gearing is identical, one of the examples is geared wrong.
And in fact they WON'T be geared identical, because the high RPM engine will hang onto lower gears longer, and/or the low RPM engine will need to upshift sooner. Even if the gearing were identical, that would go out the window the first time the low RPM engine upshifted. Now it's losing mechanical advantage, while the high RPM engine is still in 1st gear.
This is why I get a kick out of people who like slow engines "FOR TEH TOOOOORQUE". Sure, great, your TDI makes 200ft-lb at 1200rpm or whatever. It still makes exactly as much horsepower as an ABA. So you're running in 4th up that hill instead of 3rd, but you're not accelerating any harder.
I would much rather have a 500hp/ 250lbft motor over an 250hp /500lbft motor.
All day long.
Power can be geared to make any torque, torque can't be geared to make any power. Conservation of power is pretty important.
alfadriver said:Power can be geared to make any torque, torque can't be geared to make any power. Conservation of power is pretty important.
I like it.
The late, lamented VW Quantum that I had did not have a high revving engine. It did not make much torque, either, and as a consequence was geared relatively short. Running WOT out to the point on the tach where the engine would stop pulling, you were in FOURTH gear before you got to 60mph. Somehow, the short gearing did not imbue the vehicle with massive acceleration 
Contrast this with my late, lamented bridge ported 13B, which, even with 4.78 gears in the rearend, would cross the traps in the quarter mile at 104-105mph while in THIRD, with some RPM left to go. More torque, more RPM, and a lot more mechanical advantage.
In reply to alfadriver :
Sure, but hat wasn't the example, its 500hp/500ftlb or 500hp/250ftlb which is a much harder choice.
Depends on the situation. If it's in a vehicle where the weight difference if the engine is barely noticed, then the 500/500 engine is a better choice. But if that's a 427 side oiler vs. a Cosworth DFV in an F1 car, I'd take the Cosworth- as the weight would matter.
Still the game is always a power game and how you use it.
Weight could be the same too......turbo or spin the ever loving E36 M3 out of it.
So as you say/imply there re many other variables to be looked at then just the hp number.
mke said:Weight could be the same too......turbo or spin the ever loving E36 M3 out of it.
So as you say/imply there re many other variables to be looked at then just the hp number.
Turbos introduce the problem of throttle response, or lack of linearity thereof. I wouldn't want a turbo without AWD to cover my tail, personally.
It all depends on what you are doing with it. If you are dyno racing, or running on perfectly groomed surfaces with good traction under all conditions, it probably does not matter, although one account I read noted that turbos needed to have almost 50% more power to equal the speed of a naturally aspirated car, depending on course design, strictly due to power delivery characteristics.
Turbos add weight, as well, in the form of the turbo and the heavier-duty exhaust manifold, and wastegate, and intercooler.
Knurled. said:Turbos introduce the problem of throttle response, or lack of linearity thereof. I wouldn't want a turbo without AWD to cover my tail, personally.
It all depends on what you are doing with it. If you are dyno racing, or running on perfectly groomed surfaces with good traction under all conditions, it probably does not matter, although one account I read noted that turbos needed to have almost 50% more power to equal the speed of a naturally aspirated car, depending on course design, strictly due to power delivery characteristics.
Throttle response is something I care a lot about on a street car but is almost irrelevant for say roadracing where you know exactly what you're planning well ahead of time and can control what rpm you will be at everywhere.
Its not an easy question. On paper the engines are equal speed wise....more details are needed to understand what fits the application best.
Wouldn’t a low torque high rpm drivetrain use lighter components than a high torque low rpm. I’ve broken more stuff with torque than rpm.
akylekoz said:Wouldn’t a low torque high rpm drivetrain use lighter components than a high torque low rpm. I’ve broken more stuff with torque than rpm.
Sorta. Behind the trans the torque will still be the same.
Inside the trans to get proper gearing you'll have to use larger gears. The weight savings from less force is likely offset by this.
In reply to ProDarwin :
The real issue is finding synchronizers that like to shift at high RPM. They usually don't.
Also, if you have a wide gear spread in the trans, the mainshaft's gears really need to be on needle bearings and not bushings. The main gears are always tied to the engine speed in any constant-mesh trans. So when the engine is at 9000rpm, a 3.6 1st gear's main gear is always turning at 2500rpm, and a .7:1 5th gear is always turning at ~12,850. So at 9000rpm in 1st gear, the 5th gear's driven gear is going to have an over 10000rpm relative speed between it and the output shaft. A bushing is not going to tolerate that for very long at all.
This is much of why my transmission lift is so short, between the synchros rapidly wearing and filling the fluid with debris, and the RPM problem causing the transmissions to end up with horrible vibrations/knocking in 5th. (That and I never bother to rebuild as long as I can get junkyard transmissions of questionable quality for under $200)
Well, one way around the synchro problem is to put a primary reduction in the trans to bring the gearset down to normal speeds (I know the S2000 has one, not sure if it is before the standard gearset though). However, this is A) an extra gear required and B) multiplies torque to the rest of the gears.
In reply to ProDarwin :
Or D: Close ratios in the trans, and a separate overdrive unit. The Laycock de Normanville units are kind of lossy with their fluid pump, but there aren't really any better options out there.
The current MX-5 has what is probably the best compromise: No overdrive in the trans at all, just tall gears in the rear, 1:1 top gear, and steepish 1st. Yes, 1st gear is 5:1, but the tallest gear-reduction gear is in the 1.2 range, and this ends up being a better spread than 3.6 to .7, so it should in theory handle higher RPM internally just fine. As a bonus, you're not spinning the driveshaft at high RPM with all of the harmonics issues that come with that.
The 'torque' rating of engines must've been devised by the marketing department of the car manufacturer who's cars made less peak power than the others.
I've said it before and I'll say it again, hp is what matters.
What most people call 'torque' is just hp at 'low' rpm.
mke said:Weight could be the same too......turbo or spin the ever loving E36 M3 out of it.
So as you say/imply there re many other variables to be looked at then just the hp number.
If you are making the exact same weight engine make the exact same power but at twice the engine speed, but at half the peak torque, something is wrong.
If "all things being equal" weight would be proportional to displacement- so the materials to make engine A would be the same as engine B. And since torque is proportional to displacement (it's just an air pump), the engine that makes half the peak torque would be really close to half the displacement. Therefore, close to half the weight. (yes, the valvetrain would be more robust, but relative to the block and head, they don't add much weight to make them stronger.)
Turbos make the engine not equal- as peak cyl pressures would mean more materials. They also don't mean half the torque, as they improve the efficiency of the engine as an air pump. That's kind of their point- you can make a 2.5l engine have the same power and torque as a 5.0l engine.
So to really have an engine that makes the same power, but half the torque, you have to spin the E36 M3 out of it for this to be a realistic example.
Robbie said:I've said it before and I'll say it again, hp is what matters.
What most people call 'torque' is just hp at 'low' rpm.
Yep.
In reply to alfadriver :
No, you could easily run 15 psi boost to double the torque and you can tune the peaks to any rpm you please...mostly. A 2.5l turbo deisel might well be designed for 3000 rpm all day everyday vs an NA 2.5l type S engine designed for what 8k. Its just a thought experiment, not real because nobody would be likely to put a skidsteer engine in their car...but hp is hp.
...but its just a thought experiment.
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