Ok I have a reall good question here. Your transmission and rear axle both multiply the torque from your engie to tires. So I have to ask why are people so obsessed with horsepower alone? I realize HP does play a role in lateral acceleration, but as most street engines are tuned, they hit their peak torque well before their peak HP, and race engine are the exact opposite. If you have 400 HP at 5500 RPM and your engine only generates 200 lbs.-ft. of torque at that same RPM, wouldn't that be as useless as a car that has 400 HP at 5500 RPM, and 450 lbs.-ft.? Both cars would shift into second, and in theory the street engine would be dropping back to it's peak torque and the race enigine would drop out of it's useful powerband, right? Wouldn't they be close at the finish line, if both engine were installed into the exact same car with exactly the same gear ratio and tires sizes? Wouldn't the sreet car take the racecar off the line, but the race car would catch up in the top end of each gear right? Kinda like this:


Car 1     =:-------/------/----:Finish  12.75 @ 99 MPH
Car 2     =:-------/------/----:Finish  12.56 @ 104MPH

/ would be a shift point(i.e. 1st to 2nd)

Notice I put the shift points at the same spot for each car. I'm estimating what my theory is. I'm trying to understand the theory of different tuning types for an engine, and I appeciate any help.  

HP and torque are directly related to one another.  You can't have 400 HP at a set RPM and different torque numbers.

While gears will multiply torque...they also change the RPM you are operating at.

Torque in and of itself is worthless.  If you set the parking brake on your car while it sits on a hill...the brake is applying torque to the wheel.  Torque at a RPM is what we care about.

Torque is a rotational force.  HP is the rate at which that force is exerted (the rate is accounted for by RPM)...  

Ok. So how can a street tuned chevy 350 have 400 HP @ 5500 RPM, and 400 lbs.-ft. of torque @ 3800 RPM. A race chevy 350 would be tuned so that peak torque would occur at like 5500 RPM and peak HP is at say 7000 RPM. Meaning that the street 350 still has torque at 5500 RPM, or else that flywheel wouldn't twist, but it doesn't have 400 lbs-ft at that RPM. So if gears multiply torque a street 350 would see torque at the rear wheels like so: 1000 RPM 275 lbs-ft, 2500 RPM 360 lbs-ft, and mear 4000 it would see the full 400, at 5500 you would be back down to say 350. When you shift your engine RPM would drop back to about 3600 rpm with a THM-350 trans, thus allowing that full 400 lbs-ft of force won the wheels, therefore pushing the car harder after the shift and droping off slightly as the gear winds out. The racing 350 would come off the line slower as there is say only 210 lbs-ft at 1000 rpm, hence the need for a stall converter, as the gear winds out it would pull harder, cuz the engine is closer to it's peak torque. However when the engine shifts back down to 3600 rpm it would drop 1200 RPM lower than it's useful powerband and therefore not push hard until engine speed neared 4800-5000 RPM. My Quad 4 in my achieva behaved in that manner. The DOHC was rated for 150 HP @ 5200 RPM, and like 140 lbs-ft @ 4800 RPM. It didn't accelerate hard until about 3500 RPM. So inevitably the racing 350 would come out slow, but catch up in the last 1/8 of a mile, where the street 350 would pull as hard as the engine RPM is well past it's peak torque.  

First off...use paragraphs.  What you just typed is incredibly difficult to read.

Second, you are talking about a power CURVE...

HP = TORQUE X RPM ÷ 5252

Horsepower and torque are directly related...you cannot change one without changing the other.  Gear ratios change the speed by the same amount they change the torque...so you cannot gain or lose HP by just changing the gear ratio.

Lets say you have an engine that produces 300 ft-lbs at 4000 RPM.  You put in a 3.0 rear end ratio...

You've now got the axles spinning at 1333.33 RPM and 900 ft-lbs of torque.  

Now lets say you step up to a 4.0 rear end ratio...

You've now got the axles spinning at 1000 RPM and 1200 ft-lbs of torque.

BOTH produce the exact same HP at the exact same engine speed.  Difference is the 4.0 rear gears at 4000 RPM have the car moving slower.



Everyone already knows that a flat torque curve is preferred for a street car...for a race car you tend to not care about really low gears or a high stall torque converter to help launch the car...

Point is this: "If you have 400 HP at 5500 RPM and your engine only generates 200 lbs.-ft. of torque at that same RPM, wouldn't that be as useless as a car that has 400 HP at 5500 RPM, and 450 lbs.-ft." is impossible.

400 HP at 5500 RPM means you have 381 ft-lbs of torque.

An engine with 250 ft-lbs of torque at 5500 RPM has 261 HP
An engine with 450 ft-lbs of torque at 5500 RPM has 471 HP  

Ok I'll try to use paragrahs next time. That is what I wanted to know though. That is much clearer than your first post.    

HP= Torque X RPM/5252. Okay so no matter what, an engine will ALWAYS have less torque than HP, if the engine revs over 5252 RPM. In other words, it is not possible for an engine to make 500 lbs-ft at 6000 rpm and only 450 HP, right? Do I got that correct?


Second I think I am wrong here but if I use that formula, a race engine that generates 875 HP @ 9000 RPM, would only have 512 lbs-ft??? 875= 512 x 9000/5252??? Or does the formula change when you go from natural aspiration to a blower or turbocharged application, and from gasoline to race fuel or alcohol?


Lastly these formulas are based off laws of physics, right?  

Ok and for the axles, that can really clear things up. Tires would spin faster with less torque, which is great for flat straight line performance, right? Where as you would want the 4.0 rear end for towing a trailer? Even though the tires spin slower, they help produce more torque to overcome the load, right?  

You are correct that an engine will never have more torque than HP above 5252 RPM.

The formula is entirely based on physics.  HP is simply the rate at which work is performed.  In this case, it is the power generated from a rotational force.  The formula is the same regardless of induction.  

The lower rear axle ratios are "better" for launching, towing, etc.  I use quotes as there is such a thing as too low...even when you need a great launch or plan to tow a bunch.  But as a general rule, that holds true.

Sorry I was not able to post the formula in my first post...didn't have time to look for it.  

Ok that is what I'm trying to get to, now on a final note. If you took a Honda V-tech 1.6L and fully tuned it for racing and it had 450 HP at like 7000 RPM, and another guy takes the exact same car and puts a 350 Chevy with 450 HP at 5500 RPM, let's say both drivers use a 79 Firebird, with a THM-350, and 3.73 rear ends. Obviously a firebird of that year would need work to put a FWD 4 banger in it. Would the 350 win in a straight up race, since it should have more torque throughout it's RPM range over the 1.6L V-Tech?

I mean 450 HP at 5500 RPM would equate to 429 lbs.-ft.
The other engine has 450 HP at 7000 RPM, and would only have 338 lfb-ft. It would take longer for a 4 banger to generate the rate(HP) that the work is being done right?  

Don't focus on the torque for this.  The important number is HP.  The reason we concentrate on torque is to give us an idea of what the HP curve will look like.  Because the Honda is so small, it requires those high RPM's to compensate for the lack of torque.  The key for your race scenario is that the Honda has a VERY steep HP curve.  This will result in a huge drop-off in power during every gear change.  The 350 won't have such issues as its HP curve will be fairly flat...meaning its drop-off after each gear change will be much less than that of the Honda.  

Ok. So in essence, the 350 would lose less power on it's shifts, and accelerate more consistently, or the driver wouldn't notice a sharp change in performance. Where as the driver in the Honda would be slightly bogged down after changing gears, until that smaller engine got it's revs back up into the higher ranger where the power us actually generated, right?
i.e The 350 would probably shift at like 6000 rpm, back to 3620 rpm, with a flat powercurve, not much would be noticed as far as performance, but the Honda would be shifting from probably 7500 rpms, back down to 4520 rpm, and dropping really low from 450 HP, to about say like 320?? Is there a formula for caculating HP at a specific RPM as well, or are there too many variables for a consistent calculation? Varibales, being air charge temp, barometric pressure, humidity, air/fuel ratio, timing, etc.?  

Well, you'd pretty much have to use dyno results to really get into it (a dyno measures torque and calculates HP for you from that and the RPM readings).

But yeah...you could basically say the Honda, over the course of a gear, would average a lower HP (even if the peak was higher than the larger motor).  

You have to consider the moment of inertia of the rotating mass of and attached to the engine as well. The weight of the system helps keep the revs up during the time of a shift. You know about inertia, as in Newton's first law saying that anything with mass is resistant to a change in motion. Moment of inertia is just the analogous term used when speaking of objects rotating...

You mentioned a 350 shifting at 6000 rpm and dropping to approximately 3500, in that case it would be a bad situation for performance as a 350 making peak horsepower at 6000 rpm would most likely be making peak torque higher than at 3500 rpm, and ideally in a drag car the engine should fall to about around its peak torque during a shift as around this point it will more easily be able to "pick itself up" from the reduction in rotational speed.  

GibTG:
You have to consider the moment of inertia of the rotating mass of and attached to the engine as well. The weight of the system helps keep the revs up during the time of a shift. You know about inertia, as in Newton's first law saying that anything with mass is resistant to a change in motion. Moment of inertia is just the analogous term used when speaking of objects rotating...

You mentioned a 350 shifting at 6000 rpm and dropping to approximately 3500, in that case it would be a bad situation for performance as a 350 making peak horsepower at 6000 rpm would most likely be making peak torque higher than at 3500 rpm, and ideally in a drag car the engine should fall to about around its peak torque during a shift as around this point it will more easily be able to "pick itself up" from the reduction in rotational speed.

Most likely the RPM would only drop to 4500...probably higher than that even.  But the principle remains the same...an engine with a flat power curve will handle it better than a "peaky" one will.  

Very true...

"Peaky" engines benefit from a lot of gears, to keep those revs consistently high.