i started on an in-depth explanation, which i will post on my website, but i'd like to tackle this part right now...

congratulations! you and red rocket win the prize! you've just explained why HP is the all-important measure.

with torque, you have to also know overall gearing in order to figure out how hard the car is accelerating. power is a more basic measure. it's all you need to know in order to determine a car's performance potential.

if you maximize the time you spend in the peak power range, you're maximizing the car's acceleration. if you maximize time at the torque peak, you're maximizing fuel economy. (i'm talking WOT only, of course.)

so, let's go through some math to show what i mean... in fact, let's take red rocket's exact numbers, since this will be a really dramatic effect to make my point.

we have 400ft-lbs at 3k rpm, or 375ft-lbs at 6k rpm. we'll do this from 2 different perspectives.

first perspective, both cars are going the same speed, one is making 400ft-lbs running at 3k rpm, the other is running at 6k rpm and making only 375ft-lbs. in order for road speed to be equal, the overall gearing on the second car is double (numerically) that on the first car. we can assume any speed and this still works, but we do need to assume some speed in order to do the math. so let's say 45mph. let's also simplify things by assuming that the tire is 24" tall, so the radius is exactly one foot, making the whole gearing factor for the tire equal to 1.

one revolution of the tire is 2*pi*1ft = 6.283ft. so in one mile, the tire turns 840.36 times. times 45 miles is 37816.2 revolutions, then divide that by 60 to get per minute and it's 630.27. that's 630.27 rpm of the tire at 45mph.

so the overall gearing for the first car is 3000/630.27 = 4.76:1.

the overall gearing for the second car is 6000/630.27 = 9.52:1.

see where this is going?

now let's calculate hp for both cases.

400tq at 3k rpm is 400 * 3000 / 5252 = 228.5hp.

375tq at 6k rpm is 375 * 6000 / 5252 = 428.4hp.

almost twice the hp. (remember that!)

let's figure the acceleration:

car 1: 400 * 4.76 = 1904lbs of force at the tire. let's say it's a 3000lb car, so the acceleration rate is 1904/3000 = .635g. (remember, the tire is 12" radius, so the foot drops off and you get straight pounds of force.)

car 2: 375 * 9.52 = 3570lbs of force, so acceleration is 3570/3000 = 1.19g.

let's try something... 1.19 / .635 = 1.87. 428.4 / 228.5 = 1.87.

holy moley! so the torque at the engine in each case was different, and the second car had less torque than the first car. but the second car accelerated 1.87 times harder than the first. and, damn i love it when i'm right, it just happens to have 1.87 times more hp.

the other perspective is to realize that even with the same gearing, the car with double the hp will be accelerating the same at 80mph as the lower hp car will at 40mph. which is an eye-opening experience to feel firsthand.

 

:eek: Damm, I'm enlightned :lol: :yesnod: Anyway, to throw another variable, which I think would'nt really be a fully precise outcome. But anyway, I was reading Maximum Boost by Corky Bell, and in it, it said that you should have large intake runners and short ones with high flow intake and exhaust port for a NA engine. But for a Turbo a long runner intake engine will be better b/c it helps produce massive amounts of TQ at low end. And then when the turbos spool up, they produce enough power even if they had high flow or Low flow intake runners.

So what I'm saying is that if you only accelerate both cars to only 2500 or 3000rpm which one will win? Because after the 2500 or 3000rpm, the turbo takes over and would just toast the NA engine.

 

IMHO, you need both, torque for launching and hp @ high rpm for high speed acceleration. Thats why I fell in love with LT-5. 405 hp AND 390 lb.ft. :flag

 

Very good discussion and thanks for the education. All I know is that when I let out the clutch at 3000 RPM's with 400 LBS of torque and a 1.6 60' there are alot of High HP 6000 RPM+ cars in my rear view mirror trying to catch me in the 1/4 Mile. :thumbs:

 

But the 2nd car ISN"T geared 9.something to one in real world apps.

I know you did that to make a point- and it was a good one- but they're going to have similar gearing "in the real world." So how does that affect the differences in acceleration and power?

If HP is all that matters, why does GM spend so much time and money worrying about runner length vs flow?

:confused:

Good thread though

 

ya, that was the second perspective, where i cheaped out by just making a single statement instead of going through the math. i'll do it tomorrow or sunday, i don't have time to do it right now... it still shows higher hp wins, but i'll prove it tomorrow (sunday at the latest). note, though, that since i haven't gone through it yet, i don't know if i can clearly state hp superiority without using calculus. i can take a bunch of points to show a trend, but i might not be able to come up with a single number that shows more speed over time unless with calculus... dunno, maybe i can, we'll see.

red rocket's last statement is just another way of saying you need a lot of power at all rpm. more hp up top with less power through the middle will only help if you manage to spend the majority of your time up top. but take, for instance, a car making 20ft-lbs less torque at the 3k launch rpm, but averaging 20hp more from 4k to 6k rpm. this theoretical car will catch him in a big hurry, probably by the top of first, part way through second at the most.

it's really the area under the power curve with respect to the time you spend at each point. (hence my calculus statement above, if you integrate the function it immediately pops out that hp wins.) in my example of his car vs the theoretical, he's spending a second or two at the point where there's more power. so my theoretical +20hp car will kill him, since the other 10-11 seconds i'm putting down +20hp. but, if we're in a really tight auto-x where the whole time is between 2k and 4k rpm, but my theoretical engine doesn't make more power than his until over 4k rpm, i'm at a disadvantage. (note L98 vs LT1 in auto-xing, relative to this example.)

i'd like to illustrate the point where both cars have equal gearing by using real examples. can anyone send me dyno sheets for an L98 and an LT1? don't the L98s have about 40ft-lb more peak torque, and roughly that much less power? should be a good illustration, then...

thanks.

 

Ask and ye shall recieve.

Here is a dyno chart of my previous car, a 1990 6 speed coupe with some mods, weight reduction (3100lbs raceweight without me in it), a 3.33 rear end gear, and a 1996 LT4 that with the exception of muffler elims, is stock with the 3.54 rear end gears. I used the same dyno for both runs and all numbers are corrected. My L98 car was a beast in 1st or 2nd gear and fun to drive but nosed over past 5k rpm. The LT4 is just starting to eat. FWIW, I had alot of runs in the L98 car at the strip and it trapped at 80-82mph in the 1/8th and 105 with an occasional 106mph in the 1/4. Fastest et was a 13.5x at 105.x. I've had limited practice and time in the LT4 but the first time at the strip (1/8th mile) it trapped anywhere from 80-82mph.


[Modified by Dr. Evil, 11:28 PM 4/4/2003]

 

At the drag strip you want to accelerate a mass (your car) to the highest speed in the shortest time. Putting mass in motion requires adding kinetic energy (doing work) to the mass and kinetic energy is measured in foot pounds and time is measured in seconds. Horsepower is the time rate (per second) of doing work (foot pounds) and 1 hp is 550 ft lbs per second. To go the highest speed in the least time requires horsepower. So the winner will select the highest hp engine and will select the total gear ratio in each gear to allow the engine to keep its average rpm (for the range of rpm the engine operates through in each gear) centered on the maximum hp rpm. This provides the highest average hp and therefore the car accumulates the highest speed in the shortest time. Engine torque is not a factor! The gearing will multiply the engine torque to apply the maximum average torque over the 1/4 mile run.

 

well, crap, it's on the axis of speed instead of mph. (edit: i mean "rpm," not "mph.") working with that is just working the same problem i already worked. i already proved that, for a given speed, the hp tells you how hard the car accelerates. so if you look at the graphs and take the whole range from 48mph to 118mph, figuring out the area under the power curve tells you which car is going faster.

the question becomes, will the extra top end hp make up for less power at lower rpm by the end of the race? at a dragstrip, once you're in stride after launch, you can maintain the engine rpm in the optimal area, so it's clear the higher hp car will win. if talking a 48-118mph roll-on, in 4th gear only, between these two cars, it's not so clear which one will win.

i can't eyeball that graph and say which one has more area under the curve. if you carried the race to the top of 4th in the LT4 (meaning a shift in the L98), it's pretty obvious the LT4 would win.

there's further complication, however, hence the need for calculus. knowing the finish speed doesn't tell you which one is further ahead. so you have calculate the distance traveled as well. but since the distance traveled is a function of the car's speed (which constantly changes) and the time, it gets ugly keeping track of everything, mathematically. (for me, anyhow.)

(sidenote, i have a very interesting observation. i was trying to think of a clear way to equate the graphs to performance, and something struck me. if my first assertion is correct, then when you look at power and torque vs speed, assuming the cars have identical gearing, then the power curves will intersect at exactly the same speed as the torque curves. since there's a slight difference in this graph, then either the gearing is different, or the inductive pickup for rpm has errors. or both, as it's very common for the pickup to have errors. reading his description, the gearing is slightly different, so it fits. doesn't the LT4 have 3.45 gears, though?)

back to the dragstrip, just look at these 2 cars in a race. drop off the launch and first 60', i'll even concede the L98 gets the nod there. what i need to do is show that the LT4 car is consistently maintaining higher acceleration.

ZF6 gearing is 2.68:1, 1.8:1, 1.29:1, and 1:1, right? also, forget that the L98 has taller gears, just make them both have 3.45s. but, i need rpm and torque vs rpm for both cars in order to show this, so i first need to take the graph and backtrack to rpm numbers.

i first started to do this by calculating engine rpm from knowing the gearing and tire size until, duh, i realized the dynojet already did that for me. it put torque on the graph. so to know rpm, just take 5252 * hp / tq. i thought that would be easy until i realized how difficult it is to read the graph, and that speeds on readable lines wouldn't coincide with nice steps in rpm. so, please allow me to fudge together a table, using the graph as a rough guide.

to get into the ballpark, i took the 50mph for the L98 as being 125hp and 300ft-lbs of torque. doing 5252 * 125 / 300 gives us 2188rpm. those points for the LT4 i took as being 124 and 285, so 2285rpm.

sanity check: 2188 * 3.45 / 3.33 = 2266. pretty close.

this means the rpm per mph for the L98 is 43.76 and for the LT is 45.7. L98 peak hp is roughly at 95mph, so 4157, and 125mph for the LT4, so 5712rpm. so, fudged table, for torque:

RPM L98 LT4

2000 300 275
2500 325 270
3000 340 275
3500 325 285
4000 310 295
4500 275 290
5000 220 280
5500 260
6000 220

let's say the L98 shifts at 4500rpm, and the LT4 at 6000rpm. again, give the L98 an even hand by making both have equal gearing, say the 3.45. also, we need a rough estimate of rpm drop per shift. 1st to second will be 4500 * 1.8 / 2.66 = 3045rpm so about 1500rpm (6000 * 1.8 / 2.66 = 4060, so 2000rpm for the LT4). for 2nd to 3rd it's 1250rpm for the L98 and 1700rpm for the LT4. hell, close enough, let's call it 1500rpm (2000rpm for the LT4) on all shifts.

ignore the launch, let's start them in 1st gear from 2500rpm.

RPM(gear) L98 LT4

2500(1) 325*2.66*3.45=2980lbs 270*2.66*3.45=2475lbs
3000(1) 3120 2520
3500(1) 2980 2615
4000(1) 2840 2705
4500(1) 2520 2660

now the L98 shifts, drops 1.5k rpm, but gains rpm at a lower rate, so a little more fudging:

5000(1) 2570
3750(2) 1970
5500(1) 2385
4000(2) 1925
6000(1) 2020

now the LT4 shifts, let's sync the rpm again:

4250(2) 1850 1815
4500(2) 1710 1800

shift L98:

5000(2) 1740
3750(3) 1410

i think that's enough to show the trend. since the L98 has to keep shifting earlier, it keeps losing torque to the wheels, so the LT4 maintains higher acceleration from a certain point onward. (before the first shift, actually.) which, again, shows why hp is such a wonderful unit. it encompasses all of this manipulation with the myriad of variables. with just the hp, you know the performance.



[Modified by MSR, 9:57 PM 4/5/2003]

 

You're the man, MSR.

Now all I need is a supercharger and a six-speed....

:jester

 

Hmnn, didnt realize that by posting it in the MPH format it would cause you to do all that extra math. Would it help if I posted charts based on rpm? If so then here it is. Its the same 2 charts, but instead of based on mph it's by rpm. I know it's alittle harder to read but it's the best I could do for now.

 

looks like the table i fudged is close enough to the rpm graph, so i'll leave it the way it is. note, though, that the spacing/formatting was lost once posted. for instance, when the 2 cars are in different gears, the force numbers lose the column spacing. ditto on the torque, the 5500 and 6000 entries should be in the LT4 column.

 

All i know is the more the better :jester

 

With the other calculation you posted. One could figure out how hard his/hers car accelerates in G forces depending on Gear ratio, Rear end gear, and at what rpm does this amount of G force acceleration occur depending on how much TQ the engine makes at that rpm. Is that right? :confused:

 

Pretty much. Just one thing, though. Don't forget about wheelspin. Your tires and suspension can only provide a certain level of grip, and if you're trying to figure acceleration on a road course or auto-x where you're turning also, you begin to delve into friction circles, which becomes another discussion altogether.

 

The Honda S2000 makes 240 horsepower (the same as my '86) and pulls mid 14 second quarter mile times. My old Fiero with a 160 horsepower engine would match that. How? The higher horsepower engine in the Honda has an axle ripping 153 ft/pounds at 7500 rpms while the 3.4 v6 had 200+ at 4000 rpms. The Fiero was heavier and had to be shifted more often, but torque made it equal. Horsepower sells cars, torque wins races.

 

You apparently missed an important part of the thread....
The point is that which ever car is making more horsepower on average during the race will win. Period. This means that if the S2000 could launch at 7500 and run the entire race (including hole shot) from 7500 - 9500 (or whatever it peaks at), it would beat your older Fiero. True, in a quarter mile drag from a dead stop, the lower power band of the Fiero may have equalized things because the Fiero could get a hole shot on the S2000. Take a look at the trap speeds though. I'd bet you money that S2000 was going faster. FOR THAT REASON, if you make that a 1/2 mile drag race from a dead stop, or from a roll, the S2000 would eat the Fiero alive. Hell, my L98 Vette runs 1/8 times nearly identical to just about every LT1 I've ever run against at the drag strip, because I ALWAYS get the hole shot because I'm making more RWHP at low rpms. The LT1's always eat me alive on the 1/4 because they have more horsepower in their powerband than I do, and my Vette is LIGHT compared to most street-driven Vettes (3050 wet weight).

As MSR pointed out, the nice thing about using horsepower rather than torque to compare two cars which are otherwise equal is that horsepower allows the other things (such as gear ratios) to wash out (assuming the two cars are travelling at an equal speed), whereas with torque you must multiply all of those factors together, and in the end, all that extra math got you to the same answer as you would've reached by simply using the horsepower comparison. The simple fact of the matter is that the car that is making more average horsepower during a race will win, all else being equal.

 

I did not miss a point, I just made another one :)
For giggles, I put some numbers into desktop dragstrip. 3500 pound car, 700 R4, 2.59 gears, basic stuff. The only things I changed from the runs was the horsepower/torque and shift points to reflect the peak H/P of each engine. First run was a 444H/P @ 8000 and 303 TQ @ 7500, the car ran a 14.99 second quarter mile at 101 mph. Next I threw in a 265 Hp @4000 and 350 tq @2500and set the shift points accordingly. It pulled off a 14.64 @ 94mph. Gearing and tranny would make the high H/P engine come alive, but all things being equal, torque won.

 

The reason "Torque won" is because those gears KILLED first car. That's not a real comparison because a car with that powerband would never run those gears. If you're going to make a comparison, at least make it a reasonable one.

 

<Removed what I wrote it didn't make sense>

What would happen if you increased the weight on all these cars? One would think that you need more torque to get a heavy car rolling.... Then again we all drive cars that way as much as a Neon :yesnod:

Guess it all comes down to how far you race. Kinda like that Viper VS the rocket car. Viper = more torque than the rocket car, but the rocket car had more HP. Viper killed it for like a mile till the HP of the rocket car caught up to the viper and it blew it away.