Stupid and unrealistic "Which is faster?" question

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.

I think you've put enough restrictions on the question at hand that the answer is they're exactly the same. Same hp at the speed in question.

I would think Car B would get there faster due to gearing.

I'll always remember Andy Nelson saying: "My cars aren't fast, they're quick".

I know what it is, just can't define it......

tpwalsh wrote: I think you've put enough restrictions on the question at hand that the answer is they're exactly the same. Same hp at the speed in question.

So the gearing difference doesn't mean anything there?

914Driver wrote: I would think Car B would get there faster due to gearing. I'll always remember Andy Nelson saying: "My cars aren't fast, they're quick". I know what it is, just can't define it......

I think i may have worded this all pretty terribly.

I'd like to know which car will be accelerating faster AT their respective 600hp points. Car A with a 3.9 rear end at 8500rpms, or Car B with a 3.23 rear end at 6500rpms.

You can't race a car against a carb.

They're probably about the same. I'd go with the taller gears if it were mine if the car can pull 70mph at light throttle.

acceleration = force / mass.

force = torque / tire diameter

if your peak torques are the same and the driving tires are the same then your peak acceleration would be the same.

slow wrote: acceleration = force / mass. force = torque / tire diameter if your peak torques are the same and the driving tires are the same then your peak acceleration would be the same.

So i'm to understand that the gearing difference doesn't matter at all somehow?

In reply to Swank Force One:

I'm guessing B.......because the world doesn't need to hear an f2/fe hybrid queafing at anything more than 6500.

Do they both have the same cd number? Same tires?

Also... The gearing is already in the torque number unless you meant at the crank. In that case - identical driveline losses?

In reply to Swank Force One: sorry it does. i was using the torque at the driving wheel instead of engine torque. Maybe some real engineer can point out how to convert engine torque to wheel torque (the gearing will show up in the calculation).

Giant Purple Snorklewacker wrote: Do they both have the same cd number? Same tires? Also... The gearing is already in the torque number unless you meant at the crank. In that case - identical driveline losses?

Cars are identical save for details listed. Same transmission, same car, same weight, same drag coefficient, same color, same driver, etc etc etc.

Power numbers measured at crank. Identical driveline losses.

In reply to Giant Purple Snorklewacker:

I think its the exact same car, just with different rev limits on the otherwise same engine, same trans, and with different rear gear ratios to match.

yamaha wrote: In reply to Giant Purple Snorklewacker: I think its the exact same car, just with different rev limits on the otherwise same engine, same trans, and with different rear gear ratios to match.

Bingo.

Same car. Two different ring gears, same peak HP, just at different RPMs.

Not answerable without the dyno graphs.

Yeah we'd need dyno graphs to give a proper answer...but if I had to guess, assuming these are real-world cars, I'd think B would be faster because high-revving screamers don't tend to be big on torque.

wvumtnbkr wrote: Not answerable without the dyno graphs.

Are 1rpm range dyno graphs a thing?

I'm only asking which one accelerates faster at exactly their peak HP numbers.

Car A @ 8500rpm makes 600hp and therefor 371ftlbs, 3.9 rear end.
Car B @ 6500rpm makes 600hp and therefor 485ftlbs, 3.23 rear end.

Swank Force One wrote:

slow wrote: acceleration = force / mass. force = torque / tire diameter if your peak torques are the same and the driving tires are the same then your peak acceleration would be the same.

So i'm to understand that the gearing difference doesn't matter at all somehow?

Yes, the gearing matters. But you've phrased the question to make the gearing part of the equalizing arrangement. If they were geared the same, each car would be accelerating harder at its power peak, but those would be at two different speeds...

Gimme a sec (if I haven't already taken too long typing this) and we can work out how the gearing-equalized torque at the wheels is the same...

Car B is accelerating harder at 6500 than Car A is at 8500RPM.

A: 600 5252/8500 = 371 torques @ flywheel * 3.9:1 rear end ratio = 1447 lbft at the wheels

B: 6005252/6500 = 485 torques @ flywheel * 3.23:1 rear end ratio = 1566 lbft at the wheels

So, Car B is accelerating 8% quicker.

bradyzq wrote: Car B is accelerating harder at 6500 than Car A is at 8500RPM. A: 600 *5252/8500 = 371 torques @ flywheel * 3.9:1 rear end ratio = 1447 lb*ft at the wheels B: 600*5252/6500 = 485 torques @ flywheel * 3.23:1 rear end ratio = 1566 lb*ft at the wheels So, Car B is accelerating 8% quicker.

Hrm. That's interesting.

So i guess my followup question is: despite making the same peak horsepower, these cars are not making the same power at peak?

Insufficient data, faster how? Traction? Gearing? The 3.23 car might go to 60, or down the 1/4 faster, having one less shift. It might even have a higher top speed if the 3.9 car runs out of gears before aero.

In reply to Swank Force One:

No they are not a thing.

accelerates faster implies more than one point in time or at least a derivative of velocity. We are trying to take a snapshot of which one accelerates faster. You can't do it like that.

Basically if I told you that both of these cars are being drag raced and you are holding a radar gun and you can pick one spot to tell their speed and distance traveled, can you tell me who wins?

Nope.

Example:

car a is going 100mph at the 1000 ft mark.

Car b is going 98 mph at the 1000 ft mark.

Which one wins?

Which one is accelerating faster?

Who knows...

Kenny_McCormic wrote: Insufficient data, faster how? Traction? Gearing? The 3.23 car might go to 60, or down the 1/4 faster, having one less shift. It might even have a higher top speed if the 3.9 car runs out of gears before aero.

None of those were the question, though.

I just want to know which accelerates faster at the point of peak HP. Nothing else matters.

Swank Force One wrote:

wvumtnbkr wrote: Not answerable without the dyno graphs.

Are 1rpm range dyno graphs a thing? I'm only asking which one accelerates faster at exactly their peak HP numbers. Car A @ 8500rpm makes 600hp and therefor 371ftlbs, 3.9 rear end. Car B @ 6500rpm makes 600hp and therefor 485ftlbs, 3.23 rear end.

It has to get there somehow...say I have two identical cars, and on one I put an electronic throttle plate before the "real" throttle that is mostly closed except for in a particular rev range. The cars make the same power in that range, but they won't be accelerating equally hard in that range if you put them against each other.