BC

Uh huh. Try doing when a fumbled shift means 98lbft instead of 160.

SharkSkin

Sorry Brendan, ya lost me there...

BC

Well, the story goes Dave that you said that a fumbled shift in a BMW would mean 250 instead of its peak, but a 928 would still be a 300. Well, i was remembering my track time with the S2000, where I fumbled a shift while watching an ***-wipe almost run into me in a Z-06 at Streets of Willow because he was burning his brakes. I ended up in a VERY wrong gear, I think 3rd or 4th, at just under3k rpm. It was a long hill to climb out of. I could have down shifted, but I didn't want to disturb the snorting bull behind me anymore.

So thats my torque peak I think in the S2000, 160. I probably had less then 98 at that low RPM to pull the higher gear, and all of a sudden I realized I WAS driving a very unhappy car at that rpm.

SharkSkin

Ah, I get it now. Those aren't the numbers I mentioned, but as soon as you said S2000 I got it. Fair to say the attached pic sums up how you were feeling at that moment?

mark kibort

Ive been racing for 10 years, and have video taped every single race. Ive never missed a shift where that happend. ive missed a shift that took longer than it should, I found 2nd gear where 4th should have been for a split second, but those two or three times a broader hp curve wouldnt have helped or made any difference. If you are talking about a brain fart, where you leave it in 3rd gear when you should be in 2nd, then actually, if you look at the chart, both cars would have the exact same torque at the wheels, even though one would have a lot less engine torque than the other. BUT, the HP used would be the same and wouldnt favor either car. thats what the "same shaped HP curve" means.

lets say the 928 was at 1/2 the max rpms, say 3000rpm, whats the HP? now look at the BMW at 1/2 the max rpms, say 3900rpm, and what's the HP there?
BOTH cars are near 170hp at half their max rpms.

both the same! this means, torque to the driven wheels will be the same. this mean the accerative forces are the same.

It goes back to acceleration =power/(mass x velocity)

so to answer your question, the torque curves are the same shape and value, so there is nowhere the 928 has any "grunt" advange. to answer your question, i would be perfectly fine with any engine with a similar hp curve as the 928. engine torque doesnt matter. (unless you start talking about rpms too)

MK

mark kibort

This is the same type of misconception about engine torque vs HP that got a few of my BMW racer buddies in trouble too. once they figured out that its all about hp and peak torque or torque curves dont mean anything, they started to run faster. if you miss shift and get in the wrong gear, any car would be at a disavantage. one with a broader hp curve would be at less of advantage, but the traffic will be blowing by no matter what the "shape" of your curve.

remember max acceleration at any vehicle speed is at the greatest hp available, not at max torque. There are times ive down shifted to 1st gear at turn 11 just to get a little jump out of the hole. its a 5k to 6300rpm range, but its 20% more torque to the wheels than if i just kept it in 2nd gear. minus the shift time, its a net gain. however, there is some risk. mis that down shift to 1st and you loose time! so, when i do it, i make sure im dead on with the blip and the brakes and gas to make it work.

Mk

SharkSkin

Mark, I was talking more about the "Oops, I'm in too tall of a gear, but if I shift right now this guy's going to pass me" scenario. You have a few car lengths to grab the line you want or you're S.O.L. You're right, you should never get in this situation, but you asked a hypothetical question so I stuck our hypothetical M3 and S4 drivers into that position to answer your question. Let's say some debris appeared on the track and they had to slow unexpectedly.

Initially, I just eyeballed the charts looking for the points about .7(one gear change) further down the charts. I gave it a SWAG and tossed out 3500RPM. You make a good point about the RPM range in use, so I'll recalculate taking that into account. I'm referring to the charts you posted in post #11 above. Now I'm checking myself, checking my assumptions, let me know if I've missed anything:

Gear spacing: .7

Ideal RPM ranges from your charts:
M3: 5500-7800 RPM (I see where you rounded up from 5460)
S4: 4200-6000 RPM

And your original question that I was referencing:

OK, with that out of the way, our hypothetical cars are one full gear too low, meaning they should be at 5500RPM(M3) and 4200RPM(S4) in the next lower gear -- but they're not. So, using the .7 factor, I come up with our guys being at the following RPMs, listed below with the HP figures off your charts in post #11. The torque to the driven wheels isn't that interesting to me for this scenario. I'm interested in horsepower, because that's the measure of the engine's ability to accelerate.

M3: 3850 RPM 160 HP
S4: 2940 RPM 170 HP

So, leaving aside possible dyno variation because I took your question to mean that only these particular charts should be compared. I come up with a slight edge to the S4 here. 6-1/4% isn't much, but it's still an edge.

mark kibort

Nice job!

you broke the code. however, the only difference is that i had to work with a 300rwhp 928 vs a 285rwhp BMW. if they both were the same, say, like the Holbert car before mods (ie 285rwhp with only a cat missing), the BMW actually has a slight edge! Hows that for the V8 compared to I6 comparisons!! kind of blows all tall tails of V8s having some god given edge, right out of the water! I know, i live it every race weekend!!!!

as a matter of habit, talking about the "unexpected occurance of debris", any time i press the brakes hard, its followed by a nee jerk reaction of a downshift. Ive had lots of these types of things happen during racing, never has it been even a concern. But if you made the mistake with either the BMW or 928, you see that there is no difference in accelerative forces, even with the 928 being 50ft-lbs richer in engine torque (not at the rear wheels at any speed torque i.e. equal HP at any same speed )

Now the long wait 'til spring!

mK




MK

SharkSkin

Good stuff, but to be fair it seems that l-6 engines have a broader powerband than similar-displacement l-4s. You're comparing two basic engine geometries that have many good qualities in common, though the exhaust note of a v8 is more musical to my ears.

GlenL

Thanks, Mark. I'm feeling much better now.

I mean, I had a cold and have been drowning in work. Got a moment now.

Through many, many (many) posts on this what I don't think you get is what your oft-repeated "identity" really says. Add a pair of additional equations and see what happens:

Power = engine RPM * engine torque (There are other forms but this one works here)
velocity = rear axle RPM * wheel radius (times 2 times PI times unit conversion factor, but let's simplify for a moment)

So we quickly get

a = (engine torque * engine RPM) / (rear axle RPM * wheel radius * mass)

Now we know that the ratio of engine RPM to rear axle RPM is simply the tranny gear times the final drive ratio or the drive train gear ratio so:

a = (engine torque * drive train ratio ) / (wheel radius * mass)

I've left out all the constant conversion factors and the wheel radius is also a constant. Let's lump that together into Z, an all-purpose constant containing all the constants. So....

a = ( torque * drive train ratio) / mass * Z

Looking at it another way, torque divided by length is force. (l*F=t or F=t/l) So pushing gear ratio into constant "Z" and pulling the wheel radius out...

a = F / m * Z

So ya see that the "Mark equation" really just says that F= ma, although you restate it a = F/m. And that's what I find so perplexing. Why complicate one of the most important Newtonian fundamentals? That's something I've asked all along. Once you have the car in gear, and that's essential for applying power, the whole thing boils down to F=ma. Voila! Simplicity! Grace! Perfection!

mark kibort

Hi Glen,

Yes, but i think you miss one important point.
It is A=F/M, but at a the rear wheels, and this is what is so confusing to most when comparing or thinking about engine torque. engine torque is the force generated at the engine, not at the rear wheels.

Power, incorporates all the factors needed to determine relative acceleration rates of two different engines as far as torque, but the same as far as HP.

as far as identities, combinations of identities are still identities.

gear boxes basically are used to maximize HP utilization over varied speed ranges. The reason i posted the two curves, one from a BMW and the other from a V8 928, was to show that engine torque is irrelavant when comparing like vehicles, with out knowing speeds. however, knowing HP you can determine torque at the rear wheels at ANY speed, so it becomes a very simple way to compare and look at acceleration potential of any two lke vehicles.

Again, i ask the question, with the BMW having 100ft-lbs less engine torque, is there any place, at any speed where the "mighty " v8 will have any advantage, even having 50% more torque at the engine?? the answer through all of this is no!
why????? because acceleation =power/(mass x velocity)

Its that simple. Hp is proportional to power at any given speed, and will determine torque at the rear wheels so that you then can look at the Torque or Force .(or the Force that is used in "A=F/M")
The A=F/M, can only be used if you know the F!! knowing engine torque does not give this information, with out knowing the speed AND rpms (BOTH!) But, with Hp, i dont need to know anything else to compare acceleration abilities at any same speed.

simple, by design. HP incorporates torque. it is more useful as it has 2x as much information to draw from.

Glad you are feeling better

mk

SharkSkin

Actually if you want to nit-pick, we don't have enough info to answer that question, because you've brought "speed" into the fray. Even my carefully derived answer above has to be thrown in the circular file since it does not(and cannot) take speed into account.

What is needed to fully & accurately answer that question is at least final drive ratio & tire size. Better would be to have those plus gear charts for both cars so we don't have to make the ".7 factor" assumption. I've already shown a slight edge for the S4 at a certain point on the power curve but you have not provided enough info to relate this data to speed.

mark kibort

Actually, all the information is there. If you are at the same speed, and the same HP, regardless of ratios, tires, etc, if you are at the same hp , you are going to accelerate at the same rate. This, again, is because acceleration is proportional to power at any given speed.

You only found a spot that had slightly more hp, becuase the curves for the two engines are not the same, the 928 had more than this difference in its rwhp numbers ( 300 vs 285) but the point was to show dramtically, that even thoguh the BMW has much less engine torque, at any speed, at any same HP level, they will accelerate the same. actually, gearing is incorporated in the answer.

If you start changing ratio spacing and shifting the up and down the speed range, then you wont have the same HP at any same speed, there will be trade offs. (another discussion, gearing and its effects. gearing is a maximizer of available HP over any desired speed range)

since our discussion was how engine hp relates to acceleration, it stands to reason, we have to keep all the comparitive variables the same . (i.e. same car, weight, gear spacing, %max rpm/speed, etc. )

this is a great example because the BMW has the same weight, same gear spacinig, close to the same %max rpm/speed, etc Yet, the BMW has much less engine torque. its HP curve is the same shape too, just as broad as the 928S4. So, this way, all missifts, all starts, all shifts all produce the same acceleration rates at any given speed through the entire speed range.

So, we have enough to answer your question. you give me speed and hp at that speed, ill provide you torque at the driven wheels, then, its up to you to give more information to find engine torque by using the rpms, wheel sizes, gear ratio, etc.

I have given you the gear charts so no assumption is nessasary.
BMW vs 928 max speeds in each gear based on redlines. if you want the ratios because you dont trust me, they are:
928 /mph in each gear
4.04 55
2.68 80
1.92 112
1.45 149
1 :1 with a 2.2 rear end
BMW
4.2 36 (almost like a GTS with a 3.09)
2.49 56
1.66 85
1.21 116
1:1 with a 4:10 ratio

so, you can see the speeds vs % max rpm is near the same. (except 1st which is not used in road racing, in a drag race it would have an advantage here, from 0-36mph)

mk

edit:
PS. in your example, 160 vs 170hp based on the S4 having more hp to begin with, but more importantly to the example, the repective engine torques are 300ft-lbs porsche vs 225ft-lbs in the BMW!! yet both have near the same acceleration in a "bogging" gear because the HP is near the same at that same speed.

SharkSkin

Whoops, OK, I somehow completely overlooked post #14. Those are pretty close, so I concede the point. If his max speed in 2nd was say 45, 3rd 65, etc then surely there would be a number of speeds where one car or the other might have a significant advantage, but averaged out over a couple of gears on a long straight it would be a wash.

mark kibort

same hp curves and looking at the gear spacing and shift points, there is really no place where the Bmw or the 928 have any "significant" advantage.

the 80 to 85mph shift point favors the 928 by more than 5%, but then again, from our gear ratio discussion, the 80 to 85mph 5mph spread, would favor the BMW by 20% so in the end, there is no signifcicant advantage.

The real point, is if both cars had the identical shift speeds and spacing, there would be no differnces due to the engine torque differences. as long as the Hp is the same shape, acceleration will be the same with the same spaced gear box. try and find one point where you would have different forces at the rear tires. you wont, because HP will determine torque at the rear wheels at any speed.
MK