Stupid and unrealistic "Which is faster?" question

Swank Force One wrote: Now... i DID say it was stupid and unrealistic... making it smart and realistic requires data that is not available. I guess in a simpler form: Which car puts down more power to the tire at the point of peak HP? I suppose that's the question i'm asking.

They both put down exactly 600 hp at those respective rpms.

Knurled wrote:

Swank Force One wrote: Car "A" 2100lbs 600hp @ 8500rpms 3.9 rear end Carb "B" 2100lbs 600hp @ 6500rpms 3.23 rear end Both cars have identical transmissions. Consider this a simplified HP vs. TQ discussion, with a twist. I am ONLY interested in which car offers faster acceleration at the "peak" HP numbers given in the above examples. I say Car A is faster at 8500rpms than Car B is at 6500rpms. Sure, at 6500rpms, Car B is making 485ftlbs while Car A is only making 371ftlbs, but over 5252, isn't it the HP that matters? Discuss and learn me.

It's not "over 5252", it is all the time. Look at it this way. 371 times 3.9 is 1446 ft-lb at the axle. 485 times 3.23 is 1566 ft-lb at the tire. At the proscribed engine speed, engine B will accelerate harder. I'm too lazy to do the math, but I'd also bet that it's going slower as well. All HP is, is a convenient way to figure out torque vs. speed. A high winding engine gets more torque by having more gear multiplication.

My problem with this is that HP isn't used to figure torque, rather HP is a synthesized number based on torque/rpm.

And what i may be going at this incorrectly is where i see that there's two ways to make the same horsepower in this case. They're both making 600hp, just at different times, in different ways, but the end result is the same, is it not?

Irregardless, turbo cars are always faster.

In reply to Swank Force One:

It is the same result as far as HP is concerned.

However, what you really want to do is have the FULL dyno charts. Take the entire area under the curve. This will tell you which one is makes more power - total power throughout the rev range.

Without the dyno charts, I couldn't tell you if one of the above scenarios has a peaky engine or not.

In reply to Gasoline:

I hope that the first word their is a joke.

wvumtnbkr wrote:

Swank Force One wrote: Now... i DID say it was stupid and unrealistic... making it smart and realistic requires data that is not available. I guess in a simpler form: Which car puts down more power to the tire at the point of peak HP? I suppose that's the question i'm asking.

They both put down exactly 600 hp at those respective rpms.

The 600hp number is a crank number. You can't expect to put a car on a dyno and come up with the same peak power numbers without changing anything on the dyno for each gear 1-5, right?

I think i'm just really struggling to explain what i'm trying to find out, and it's just not working. That's what i get for trying to make the question as clear as possible with as few offshoot opportunities for debate as possible.

wvumtnbkr wrote: In reply to Swank Force One: It is the same result as far as HP is concerned. However, what you really want to do is have the FULL dyno charts. Take the entire area under the curve. This will tell you which one is makes more power - total power throughout the rev range. Without the dyno charts, I couldn't tell you if one of the above scenarios has a peaky engine or not.

There aren't full dyno charts because the motors don't exist, and i'm not asking about total range.

Like i said, i clearly haven't effectively formed a question, here.

If i was forced to fabricate a range for each of these motors (And i really don't want to, because now there's more variables that will make it confusing for my feeble brain), Car A would read something like this:

75000rpm = 525hp
8000rpm = 565hp
8500rpm = 600hp
9000rpm = 565hp
9500rpm = 525hp

Car B:
5500rpm = 525hp
6000rpm = 565hp
6500rpm = 600hp
7000rpm = 565hp
7500rpm = 525hp

Figure identical HP numbers, just 2000rpm later in Car A.

Miata? The Miata is faster.

In reply to Swank Force One:

Actually, you should be able to. The only difference will be the driveline losses.

What will happen in real life is this:

1st gear, the flywheel (dyno flywheel) will "spin-up" VERY fast to a low peak rpm. It will give you a hp rating (say 600 hp).

2nd gear will spin the flywheel up a little slower to a higher peak rpm (which mathematically gives teh same 600 hp).

This will continue through all teh gears until 5th where the flywheel will spin up the slowest but will have the highest peak rpm (of the dyno flywheel) (same 600 hp).

Due to the characteristics of dynos, I would NOT expect each gear to give the same identical hp (but it should)

The hp is torque with time. (rpm is time)

Does this help?

wvumtnbkr wrote: In reply to Swank Force One: Actually, you should be able to. The only difference will be the driveline losses. What will happen in real life is this: 1st gear, the flywheel (dyno flywheel) will "spin-up" VERY fast to a low peak rpm. It will give you a hp rating (say 600 hp). 2nd gear will spin the flywheel up a little slower to a higher peak rpm. This will continue through all teh gears until 5th where the flywheel will spin up the slowest but will have the highest peak rpm (of the dyno flywheel). The hp is torque with time. (rpm is time) Does this help?

But now we're back to "Then gearing doesn't matter?" when i know that it does. I'm struggling for the term/thing i'm trying to measure. Clearly not power, since that doesn't directly translate into acceleration force on its own.

And i guess not "acceleration force" because apparently that's a problem without a range to measure across.

Maybe it'd be easier to ask "What final drive would Car A need to accelerate faster at 8500rpm than Car B does at 6500rpm?"

But then i said accelerate and nobody likes that either.

Unanswerable question i suppose.

PRE-EDIT: I think I may have guessed wrong about SFO's intent with the original question, based on the latest post, but... maybe there's something useful?

SFO, can you clarify whether I've wandered out into left field? I really thought you were asking whether there was any difference in power between 600hp at 6500rpm and 600hp at 8500rpm everything else held equal (Which I admit isn't captured by your initial question, because your rearend gears mean that these two cars are traveling different speeds at their power peaks)

wvumtnbkr wrote: In reply to ransom: What is your F? There is an F in torque. It is stuck with a linear measurement as well (which cant be ignored).

Which time? In SFO's original question, or my modified version which attempts to make both "cars" make their 600hp peak at the same wheel speed?

In either case, no, I don't think we have enough information to calculate an actual acceleration, but we do have enough to compare the instantaneous force of the two theoretical cars. We do have actual hypothetical axle torque numbers, and if we give them both the same arbitrary wheel diameter, we have force (F) at the contact patch to compare. To do only a comparison, we don't even need a real hp or torque or specific rpm value. We could do this comparison with both cars making X hp, one of them at Y rpm, the other at 1.2Y. As long as the second car had a rearend ratio of 1.2 the first one's, we can say they are applying the same instantaneous force at the same speed at their peak power. Sure, that tells us nothing about what that acceleration is, but it tells us that it's equivalent at that instant. It's not very useful, except that I believe it gets at the crux of the original question...

I could be completely wrong, but my impression is that this whole question was never about deriving actual acceleration or which car would win a race, but comparing what it means for two cars to make the same peak power at different rpm. Everything about actual application to cars is a red herring; this is about the equivalence of peak hp at two different engine rpm.

I agree on the dyno result; 1st gear or 5th gear would give the same results (bar drivetrain losses) on a theoretically-perfect dyno which didn't suffer tire slip and was equally good at reading force and speed across all ranges of operation. Another way of looking at this is to get on a 10-speed and go ride at 10mph. You'll find you can do 10mph in almost any gear on a 10-speed; if you continue doing 10mph, you're putting about the same power into the rear tire the whole time, but in low gear your feet will be spinning madly, but you won't be pressing hard on the pedals. In high gear, the pedals will be moving very slowly, but you'll have to press on them substantially harder. The force at the pedals will vary in inverse proportion to the speed at the pedals. The force and speed at the wheel remain constant.

I think you've stumbled upon what i'm asking.

I'd like to know what it means, and from there, figure out how to determine how one way or the other is advantageous to me. I KNOW how it affects driveability and traction and such.

What i don't know is all other things being equal: "Which is faster?"

The gearing thing i threw into the scenario is confusing because i'm really not asking about ground speed, which makes it confusing because that's normally a big factor in this sort of thing. But what i was trying to figure out is if there's a point where the higher rpm peak could actually be advantageous due to gearing.

Yes, this is for a miata, and yes, this has to do with the other recent thread of mine about SOHC motors.

It's a hypothetical scenario based on a real world situation that doesn't have real world numbers yet. The worst kind.

If we remove gearing, then yes, i know that Car B is making more torque at 6500rpms than Car A is making at 8500rpms, despite both cars making 600hp. Does this ACTUALLY mean that Car B is more powerful at that point, though? I don't think so. But this is probably a big part of why i can't communicate what i'm trying to find out.

My gut feeling if i want to answer my badly worded question in incorrect terms (LOL) is that Car A would be the faster car, all other things being equal besides RPM @ peak and rear end ratio. Why? Because it's still making 600hp, AND it has a gearing (for acceleration) advantage. And yes, that's an incomplete answer to an incomplete question using incorrect terms.

Is this what you want to know:

What rearend gear should I put behind this 600 hp engine with a HP peak at 6500 rpm?

And then you would ask:

What rearend should I put behind this 600 hp engine with a HP peak at 8500 rpm?

This is then going to be used to figure out if you want to go to the trouble to build an 8500 rpm engine or a 6500 rpm engine.

I still say that you can't answer it without knowing the general area under the curve of the two engines.

I would also need to know what you want to do with it in order to tell you which rearend is better. Standing mile? Quarter mile? Autocross? Road race?

The answer to which rearend would be better depends on the highest speed you want to achieve and how many shifts you want to use to get there.

Rob R.

Okay, pretty much everything I've said has been centered around the mathematical equivalence of power for power. None of it has been geared (no pun intended) toward application.

I don't think it would have occurred to me to look at it instantaneously with an eye toward application. I'd be looking for "area under the curve". (EDIT: I get to agree with wvumtnbiker now! )

In your hypothetical dyno charts above, I'm going to stick my neck out and probably be wrong, but suggest that the lower-revving engine will be faster around a track, because its taller gearing will mean that it can accelerate over a larger range of speed without shifting.

A car geared at 3:1 with a 3000rpm usable powerband will be able to use one gear across a road speed equivalent to 1000rpm at the wheels.

If that same car's power peak is enough higher in revs to be geared at 4:1 but has the same width powerband, the usable road speed in one gear will only be 750 rpm at the wheels.

But it seems unlikely that any two approaches will be so tidily comparable, and I'm probably missing a lot of other concerns.

wvumtnbkr wrote: Is this what you want to know: What rearend gear should I put behind this 600 hp engine with a HP peak at 6500 rpm? And then you would ask: What rearend should I put behind this 600 hp engine with a HP peak at 8500 rpm? This is then going to be used to figure out if you want to go to the trouble to build an 8500 rpm engine or a 6500 rpm engine. I still say that you can't answer it without knowing the general area under the curve of the two engines. I would also need to know what you want to do with it in order to tell you which rearend is better. Standing mile? Quarter mile? Autocross? Road race? The answer to which rearend would be better depends on the highest speed you want to achieve and how many shifts you want to use to get there. Rob R.

No, that's not really what i want to know.... i know how to figure out which rear end will work best for my application, but what that doesn't tell is which setup will be faster.

More stupid numbers time:

Say i geared both vehicles to achieve 200mph on the button.

Which one gets to 200mph first? It's something that's hard to answer without dyno charts that don't exist.

And hence my question. My gut feeling tells me that the higher RPM car would have an advantage because it'd have shorter gearing to get to 200mph, but i can't seem to formulate the question i need to confirm that gut feeling.

The reason to go to Higher RPM is becuase the engine "acts" like it has a larger displacement. Which should give more HP.

Therefore, I would say that the 6500 rpm max hp engine would be better. It is making the same hp without needing to go to those high rpms to do it.

I would bet the area under the curve for the 6500 car would be better as well.

High rpms is a bandaid for some other restriction IMHO.

Rob R.

wvumtnbkr wrote: The reason to go to Higher RPM is becuase the engine "acts" like it has a larger displacement. Which should give more HP. Therefore, I would say that the 6500 rpm max hp engine would be better. It is making the same hp without needing to go to those high rpms to do it. I would bet the area under the curve for the 6500 car would be better as well. High rpms is a bandaid for some other restriction IMHO. Rob R.

Powerband would be pretty similar between the two, just shifted up in the instance of "Car A." Both SOHC turbo motors huffing ALL the boost.

I know that Car B would be probably "less stressed" in theory due to rpm and all that, but that doesn't really address the question...

Neither car would be lacking for powerband, or be falling out of it on upshifts.

Both cars making tons of power, Car A has shorter gearing because of higher RPMs.

Swank Force One wrote: If we remove gearing, then yes, i know that Car B is making more torque at 6500rpms than Car A is making at 8500rpms, despite both cars making 600hp. Does this ACTUALLY mean that Car B is more powerful at that point, though? I don't think so. But this is probably a big part of why i can't communicate what i'm trying to find out.

You are correct. The power is the same. Car B is making more torque, but to operate at the same speed, it needs gearing which gives less torque multiplication. The gearing which makes the speed equivalent at the wheels also makes the force equivalent, in this hypothetical case.

Swank Force One wrote: My gut feeling if i want to answer my badly worded question in incorrect terms (LOL) is that Car A would be the faster car, all other things being equal besides RPM @ peak and rear end ratio. Why? Because it's still making 600hp, AND it has a gearing (for acceleration) advantage. And yes, that's an incomplete answer to an incomplete question using incorrect terms.

Ah, but for every 10% lower the higher-revving car is geared, it has to accelerate itself and everything moving crank speed an additional 10% to achieve the same acceleration of the car... For every 1000rpm the lower-revving car gains, the higher-revver must gain 1100. The effective road speed covered by any one gear is smaller on the car with the deeper gears. A powerband of a given width in RPM becomes effectively narrower in terms of speed as you gear the car down.

Moreover, I feel like you're still clinging to the last vestiges of trying to compare apples to apples by gearing everything for power/speed equivalence, but then claiming your orange has an advantage.

What is this gearing based acceleration advantage you're talking about, if we've established that for a given horsepower and road speed, the axle torque is the same?

Do you want to have your cake and eat it, too?

I can't brain around this.

It almost sounds like it'd be advantageous to put together a motor that only revs to 5000rpms but makes mountains of torque, then put like... a 2.73 rear end in it.

Wouldn't be slower, and would be less stress on the motor?

I don't get it. I feel dumb.

I would probably agree with your last post.

However, without the area under the curve, I can't tell you for sure.

Is the 8500 rpm motor making 300 hp at 5000 rpm?

Is the 6500 rpm motor making 450 hp from 2000 to 5000 rpm and going up from there?

I know you said that it would be the same curve, just 2000 rpms higher. I just can not fathom why you would want to choose an engine that you NEED to spin 2000 rpm faster to achieve the same power.

A more interesting question is which would be faster in a straight line... A 575 hp 6500 rpm engine or a 600 hp 8500 rpm engine?

I would bet the 575 hp is faster overall.

wvumtnbkr wrote: I would probably agree with your last post. However, without the area under the curve, I can't tell you for sure. Is the 8500 rpm motor making 300 hp at 5000 rpm? Is the 6500 rpm motor making 450 hp from 2000 to 5000 rpm and going up from there? I know you said that it would be the same curve, just 2000 rpms higher. I just can not fathom why you would want to choose an engine that you NEED to spin 2000 rpm faster to achieve the same power. A more interesting question is which would be faster in a straight line... A 575 hp 6500 rpm engine or a 600 hp 8500 rpm engine? I would bet the 575 hp is faster overall.

I'd choose the engine i'd need to spin faster to achieve the same power (AH HAH!!!! It IS the same power, which means shorter gearing should yield faster acceleration, right? RIGHT?) because i'd be able to put shorter gearing on it which should yield faster acceleration.

I can see how the higher revving motor would suck bad if geared the same, but it's not.

nope.

Check ransoms last post. Specifically his second paragraph.

Rob R.

How much does that matter when you're not using the entire powerband anyways?

Real world example: My MX6 makes big power starting at about 2500rpms all the way up to 6500rpms. Gearing tells me that when doing pure acceleration runs, i'm not really using anything below 5000rpms anyways.

Equal hp (entire curve equal vs road speed) and geared to an equal top speed, assuming same number and of gears, they would accelerate identically.

Though I would generally think two motors like that would be impossible to get power delivery that close at all. I think that's what a few people point out, generally the one with that power at a higher rpm will have a narrower, steeper torque/hp curve due what you need to do to make that kind of power.

Paul_VR6 wrote: Equal hp (entire curve equal vs road speed) and geared to an equal top speed, assuming same number and of gears, they would accelerate identically. Though I would generally think two motors like that would be impossible to get power delivery that close at all. I think that's what a few people point out, generally the one with that power at a higher rpm will have a narrower, steeper torque/hp curve due what you need to do to make that kind of power.

Ok so there's no inherent advantage whatsoever in revving a motor? There were some comments in the SOHC thread i started that made me think otherwise.

Why are the only diesel race cars out there the ones that are forced to be diesel by their class rules, then?

too many other variables....