Hey guys! I've been reading a lot on the forum lately about people trying to grasp the relationship between torque and HP. More importantly, I've noticed people trying to figure out what's better -- monster torque at low RPM, or smaller torque but a higher RPM. And I have seen a lot of misunderstanding -- most of the people theorizing have no idea what they are talking about.

That's okay, though. One Crazy Dave had posted a link to VetteNet with the single best explanation I have ever read. You can read it for the first time, or again, at http://www.vettenet.org/torquehp.html.

The author of this article reaches the correct conclusion, which is that the ONLY thing that matters is torque at the drive wheels. So, concerning the ongoing discussions of low-end torque vs. high-end HP, etc., the only thing that matters is torque at the wheels. This is why a Honda S2000 handily outruns a stock V6/Getrag Fiero. By the time 57 MPH rolls around, you are on your second shift, and he is barely past his first. He may not make as much torque at the engine, but he gets to multiply it a lot longer than you do.

Now, I have wanted to run some calcs using a real Fiero dyno test to figure out what the optimal shift points should be. I realized years ago that the optimal shift points had nothing to do with the engine's peak torque or peak power. Instead, I wish to shift so as to maximize the torque at the drivewheels.

So here's what I did:

(1) I did some simple equation writing to convince myself that torque at the wheels is simply the torque at the flywheel, multiplied by the total gear ratio (gear ratio multiplied by differential), multiplied by driveline losses (I used 15%).

(2) I used lou-dias' 3.4L dyno graph. I enlarged it, printed it out, and read off torque at 250 RPM intervals. I entered those values into a spreadsheet.

(3) Next, using 841 revs/mile, I computed the MPH in each gear, by engine RPM.

(4) Finally, I multiplied the torque read from the dyno graph by gear ratio and efficiency (0.85) to determine the torque available at the wheels, for that gear, at each RPM. I now had three datapoints -- RPM, torque available at the wheels, and MPH.

To optimize torque at the wheels, I need to shift gears before the torque at the wheels in the lower gear drops below the torque at the wheels in the next higher gear. So I plotted wheel torque vs. MPH, and sure enough, they cross in every single gear. So I read the MPH points off of the graph, and went back to my spreadsheet.

Here's what I found (values are not exact):

Shift 1-2: 31 MPH, 5500 RPM, drops to 3250
Shift 2-3: 50 MPH, 5250 RPM, drops to 3500
Shift 3-4: 74 MPH, 5150 RPM, drops to 3500
Shift 4-5: 103 MPH, 4900 RPM, drops to 3750

This is with the Getrag. Now, I know that the 2-3 shift is going to bother a lot of people. Remember what this graph says: assuming no time for shifting, shifting at these points maximizes the torque at the wheels, which optimized acceleration. Using the Getrag means that you MUST shift twice to hit 60 anyway, so this IS the optimal shift strategy for maximum accleration, and this particular dyno graph.

I hope this helps some. The quantity you wish to optimize for maximum acceleration is torque at the wheels. Period. So talking about porting the Caddy 4.9L heads, etc. and the impact on acceleration, is really smoke until you have a projected dyno graph, and can see whether or not you get more overall available torque at the wheels. This, of course, would be the area under the graph, and this is another calculation not covered here.

By the way... does anybody have a dyno graph for a bone stock Iron Duke Fiero? I would love to do this so *I* know where to shift my daily driver!

[This message has been edited by Dave Rodabaugh (edited 05-09-2002).]

Dave, I'm glad you said this before I did. I was watching all the different threads on Tq vs. Hp and I was just itching to get in there about gearing ratios and such, but i didn't want to start any more arguments and flame wars. Btw, that vettenet article is indeed well written, and definitely describes the situation well. I would love to see the proper shift points for the duke as well, if anyone has the dyno graph.

For all those that always say Torque wins races, Hp sells car, your wrong. The proper combination of torque, Hp, gearing, tyres, driver, etc. wins races.

EXCELLENT article!!! Many thanks for taking the time to post that.

Did your analysis include elapsed time? Can you give us a comparison if you shifted at 4500 or 6000rpm?

HEY THAT'S GREAT! Can you do it now for a 4 speed like mine. Very curious what the differences are. I've usually had 5spds. Thanks.

Phil

------------------
GTDude
25 years GM experience

horsepower sells cars!!
torque wins races!!

------------------
Rich
Sioux Falls SD

86' 2m6 4.10-too many extras to list
92' BMW 325i-cold air intake, borla exhaust,strut tower brace,
98' Acura CL 3.0-, cold air intake

horsepower sells cars!!
torque wins races!!

I have been wanting to comment on this. All I am wondering is that IF this statement is true, then where are all the diesel race cars? It is the proper combination of both, not one or the other.
Chris
------------------
Check out my site at: www.customdynamix.com
Turbo Q4 GT Project

[This message has been edited by MoneypitGT (edited 05-09-2002).]

Its torque that gets you going, but HP that sustains you,
Plus you should never do anything that increases HP, at the cost of torque with a 4 banger.

The point is that you need a high TQ curve over a wide rpm range.
Neither a high winding 4 (without vvt or a vairiable intake) nor a diesel that only revs to 3 or 4k rpms is going to do alot of good without alot of close ratio gears.

I think the correct assesment of the torque vs. horsepower debate is you can never have too much of either one.......lol......JMHO!!!!

Torque makes you the first person to the next red stop light but horsepower gets those speeding tickets to add up. A V8 sucks in the city sometimes

[This message has been edited by Her86GT (edited 05-10-2002).]

quote Originally posted by Randolph: Did your analysis include elapsed time? Can you give us a comparison if you shifted at 4500 or 6000rpm?

My analysis does not include elapsed time. That is much more difficult to do. I would need to include the weight of the vehicle, and eventually, include the values for wind and rolling resistance also. And, since I would likely use some type of discretized method to compute the elapsed time, I am not sure how accurate it would be, given that I read the torque samples only every 250 RPM.

This is a greate article. It also shows how important it is to pick the right combination of components when doing a build up.

Most parts are sold by how much horsepower they can produce. The correct way to choose parts is based on torque peak. If you have a cam that produces a torque peak at 5000 rpm, but a manifold that peaks at 7000, you don't take advantage of either.

HP is how fast you hit the wall, Torque is how hard you hit the wall! Give me low end tourque and hih end HP every time!!!

------------------
Peace and Happiness
Frank and his fotoflash, an '84 SE
Member NIFE

quote Originally posted by fotofrank: HP is how fast you hit the wall, Torque is how hard you hit the wall!

I heard that one as, "horsepower is how fast you hit the wall, torque is how far you take the wall with you"

Dave

It's more like torque is how much of the wall that you move when you hit it, and horsepower is how fast you moved it.

Boy, all that deteriorated to meaninglessness pretty fast.

Back to reality, horsepower is torque times RPM divided by 5252. Anyone with an elementary-school math education can figure it out.

Dave

Heh, bhp is just (Torque * RPM)/5252, that's all. No magic, no fairy dust. Twist@speed.

That's it! PS: nice analysis of shift points. You want to maximize area under the curve (not nessicarly torque curve either, or rather, gear corrected tq curve...)

Best!
Ben.

quote Originally posted by Pontiaddict: The point is that you need a high TQ curve over a wide rpm range. Neither a high winding 4 (without vvt or a vairiable intake) nor a diesel that only revs to 3 or 4k rpms is going to do alot of good without alot of close ratio gears.

------------------

Ben Cannon
88 Formula, T-top, Metalic Red. (2:13.138 at Sears Point) "Every Man Dies, not every man really Lives"
88 Formula, Northstar, Silver, In-Progreess. -Mel Gibson, "Braveheart"

quote Originally posted by crazyd: Boy, all that deteriorated to meaninglessness pretty fast. Back to reality, horsepower is torque times RPM divided by 5252. Anyone with an elementary-school math education can figure it out. Dave

Thanks for that enlightenment. Think what Dave R. was trying to point out is the fact that peak HP OR Torque doesn't mean a thing, it's torque at the wheels, how much for how long (otherwise known as "area under the curve") combined with gearing, that matters. An engine that develops 500 ft/lbs of torque, but only for a 200 RPM window with steep falloff before and after, won't make for a fast car, unless you have a 15 speed tranny behind it. Conversely, an engine that will rev to 9K+, with a much lower amount of torque, but available thru much of the engine's RPM range, can make for a faster car, if geared properly.

Which is why we don't see many mack trucks winning a bunch of races.

BTW...WRT the meaninglessness that this has deteriorated into, I personally am fond of the way Freddy puts it:

Power is how fast you're going when you hit the building
Torque is how far into the building you go....

Take care, all...
Jeremy B.

PS---Dave-I'm carrying around a printout of your original post, so when the cops ask why I'm doing 104 MPH, I can say, with documented proof, that I NEED to, in order to use my car most effectively, and hit fifth gear...
Also...I'd say your figures are pretty close. If you pay close attention to where the car *feels* like it needs to shift, just isn't pulling as hard as it was, you seem to be spot-on. I've been driving my wife's Formulated coupe with the 4cyl tach in it, and after reading your post started paying attention to the MPH I was shifting at when thrashing on it (IOW, all the time...) and I can say 1-2 and 2-3 shifts are usually within 2 or 3 MPH of what you "calculated". Haven't paid enough attention to the 3-4 shift, and can't honestly say WHERE the 4-5 shift is since I'm still running the coupe speedo...does "straight down" tell you anything? LOL...
Jer