It's easier to derive an equation for acceleration if you work with torque, given that you'll know the other things like gearing, tires, weight, drivetrain efficiency, etc., anyway.

HP can give you a faster "quick and dirty" estimate of approximate acceleration at any given speed. But it is somewhat more cumbersome to work with once you start considering rotational inertias (wheels, tires, clutch/TC, etc.) that essentially siphon torque from the engine output as they accelerate rotationally.


Norm

 

Well I just talked to justin at JPC today and he was telling me that he ran 4.10s until just recently when he started hitting the rev limiter in 4th at 156 mph. He uses 28" tires and he is really pushing that I go with 4.10s and that my car has plenty of room with those gears.

 

Plenty of room? Not with a 6800 rpm rev limit and 26"(?) tires unless they grow a bit at speed. 126-ish at 6800 with 4.10's and your tires if they don't grow.

If his tires really do measure 28" tall, is he really running his engine up to almost 8000 rpm? Or do they grow noticeably at 150-ish mph?

Not picking on you, but with the numbers we have it doesn't work.


Norm

 

He does run his car to around 7800 rpms, but he does that with the same enigne that I have but with after market heads and a bit of work to the block too. Justin and I have talked about the durability of my engine b4 and he thinks that I'll be good up to 7000 rpms which as the charts stated b4 is 132 mph with 4.10s.

 

Understand that that chart is 3% to 3.5% optomistic because it is based on the 20168 theoretical factor rather than the 20900 factor that accounts for real-world tire tread compression effects. That's my big gripe with all of the online speed calculators that I've ever seen - they are theoretical rather than real, and there is a difference. Your case in particular points out the need to use a closer approximation, as a 3 to 5 mph "error" at 120-ish really could make a difference where being off by 1 to 3 at 65 is just noise.

Working from your 6000, 3.73's, and 122.3 mph data instead of any of the calculators (even mine) I'd plan on having no more than 129 mph capability at 7000 rpm with 4.10's and your current tires, which correlates to 128 & change trap speed if you hit a 7000 rpm limit at or just barely past the finish line.


Norm

 

Totally agree that rwhp and rwtorque are related by equations involving engine rpm, gearing, rotational mass inertia, drivetrain mechanical losses, etc.
However, what I was referencing in a purely general sense is that inertial weight/rotational mass and torque are the primary forces that respectively prevent an object from accelerating and cause it to accelerate from a standstill and at lower speeds, while air resistance and hp are the primary forces that respectively prevent an object from accelerating and cause it to accelerate at higher speeds(60 mph +/- and faster).

 

Let's go through an analysis of the units.

1 HP is 550 ft-lb/sec, which can be rearranged to lb-ft/sec. That's a force times a velocity. When the total drag (force, lbs) times whatever speed (velocity in ft/sec) equals 550 times the peak HP of your engine, you are at the maximum possible speed for that car and engine. The details of whatever gearing, tire size, etc., it takes to put that car at that speed with that engine and its peak HP rpm are not involved. Powertrain efficiency, however, is involved.

HP does not care about engine rpm for determining theoretical top speed. Gear ratios are dimensionless and drop out. At top speed, rotational acceleration is also zero, so all of the rotational inertia things disappear as well. IOW, HP is directly tied to top speed, but indirectly related to acceleration (since you then need to know all the other stuff).


Torque is ft-lbs (or lb-ft, doesn't really matter). Since it is a rotational item and vehicle straightline acceleration is linear, you need to divide by a distance (ft) in order to get a force. The appropriate distance in this case is drive wheel radius, which leaves you with a force at the drive tires' contact patches. Opposing that is the total drag force. The difference represents the force available to accelerate the car mass.

Here, the gearing, efficiencies, torques lost to rotational acceleration consistent with the car's forward acceleration do matter, as they affect the force applied against the pavement.

You can work from torque to determine top speed, but this is the less direct method for doing so. Essentially you have to close in on the speed at which acceleration equals zero.


You could work from HP to determine acceleration, though you'd need to know the HP involved to rotate all of the rotational inertia things consistent with the forward acceleration. It can be done, but it's cumbersome.


Long post short, HP is more directly related to top speed, torque is more directly related to acceleration. So it's not a matter of either torque or HP being 'wrong' with respect to what you're looking to find out (speed or accel), just which one is better suited to doing the required math.

I hope this makes sense. I can't look at it from a newbie perspective any more. Been at it too long.


Norm

 

ive got stock 17's with the 3.55 gears and have done 158 mph. that was using my gps for the speed

 

Norm, what type of engineering work do you do? That's a nice quick and dirty write up. Unfortunately it is so much easier to explain when you can draw pictures and show some dimensional analysis.

 

By education, I'm a civil engineer (structural).

By the last 30 years occupation, I'm somewhere between civil/structural and mechanical, working as a pipe stress analyst. Technical writing (aka preparing calculations and other reports) is a part of that job, as is reviewing calculations and reports prepared by other engineers.


Norm