Technical Why horsepower and torque curves cross at 5252RPM

Math?

 

gravity ... torque is measured in pounds.

 

What if the engine can’t do than many RPMs?
Math makes my head hurt.

 

ne'er the twain shall meet

 

If the engine in my '48 ever got to 5200 RPMs, it would be raining pistons in Phoenix.

 

Well, I went back to my latest pull and shore enough they crossed between 5200 and 5300 rpm!






Bones

 

The formula

HP=torque * RPM / 5252

makes it so, every time. Because

5252/5252 = 1

.

 

Well... 7024, for horsepower and newton meter.

 

I'm often blinded by science.

 

I'm glad we figure our horsepower and torque in U.S. units and not metric....it makes the final numbers bigger. Kind of like my Grampa used to say:" Marry a girl with small hands, it makes your dick look bigger!"
It probably also explains why Lucas electrical components don't work on hot rods.... they are made for metric voltage.
This stuff is making sense to me now..

 

So everyone is not really making horsepower...........they are making only torque.

Then we calculate the HP based on the rpm. If you want to have more HP you have to increase the torque thats available for that rpm.

 

^^ yeah ! What he said ! ^^

.

 

Torque rules!!!! Measured in foot-pounds.

 

Sort of........
Horsepower is a function of torque and rpm.
HP=torque x RPM / 5252, as @squirrel posted earlier.
Therefore, as you say, more torque will yield a higher horsepower number at the same rpm.

But, why do you think there is an obsession with making engines capable of higher rpm? Big torque, along with the ability of the engine to achieve and to hold together at high revs, is the key to the wow factor.

Torque generates acceleration. Horsepower (torque x rpm) generates speed.

Of course, it will all depend on whether your horse is as good as James Watt's horse was...

 

Actually, horsepower makes acceleration. Think about two engines with a flat torque curve of 400 ft lbs, one that has a 5000 redline, vs one with the same 400 ft lb flat torque curve, but revs to 7500 RPM. The high speed engine will make 50% more HP, even though the torque is the same. If it is properly geared to spin at high rpm while going down the track, the high RPM car will be much quicker.

But....if for some reason you want to only run your engine at 5000 rpm, then build it as big as you can, with as much torque as you can. A 5000 rpm 450 ft lb 454 is going to make about the same power as a 7500 rpm 300 ft lb 302.

 

The 5252 number comes from Watts 33.000 number

 

Old circle track saying...Horsepower is how fast you were going when you hit the wall. Torque is how far you moved the wall!

 

What you said is correct, but lets say its the same engine in both cases. The key is being able to maintain that 400 ft/lbs from 5,000 to 7500 rpms. If someone cannot maintain that 400 ft/lbs thru the upper range, the additional
2500 rpms will not produce that additional 50% more HP. Most naturally aspirated engines will not maintain the same torq level as rpms increase to the upper range.

Virtually any dyno chart will show a marked decrease in torque after it peaks as the rpms increase. Some have very flat torque curves and maintain peak or near peak for a long time.....but eventually they too begin to fall off.
Even though the torque begins falling, the increasing rpms can make more HP for a while.......then both reach a point where they fall and no further gain is made.

So what you said is theroretically correct, but I don't think its mechanically possible so far. Generally any engine I'm aware of will not make the same torque at 7500 rpms that it makes at 4000 rpms.

 

Crap! Now my head hurts!

Dave

 

Yes, I definitely do not play with cars for maths. I have hobbies to avoid maths. I does more maths at work than I want to already... Same with English. Mine English already be honed to perfectness.

 

haha my grand father used to tell me the same thing.

 

Here is a simple straight line comparison showing a hypothetical flat torque curve (curve? it's flat ) They call it a curve for a reason.

Anyway you can see that IF Torq was to remain constant at 400 lbs/ft all through the RPM range that your resulting HP is simply a constant rate of increase. Double the rpm and you double the HP. Thats pretty simple.

Look at the attachment below.
I started with 1,000 RPMs and got 76 HP
When I triple RPMs to 3,000 I get 228 HP (which is 3 times 76= 228)
When I go 5 times the RPMs 5,000 380 HP (which is 5 times 76= 380)
When I go 7 times the RPMs 7,000 533 HP (which is 7 times 76 = 533)
When I go 9 times the RPMs 9,000 685 HP (which is 9 times 76 = 685)


What you can see here is that HP will ALWAYS increase the same amount no matter what type the engine you are using (naturally aspirated) IF TORQUE REMAINS THE SAME. The only thing that can change the HP is if the TORQUE changes because 5252 and RPM are always predetermined.

I'm simply trying to put this in a format thats easy to grasp, so I hope my rambling here helps some to see that the math involved is actually pretty simple. You have two things that are predetermined constants and one thing that is variable. In the real world the torque curve will not be constant at 400 lbs/ft. Given the Formula HP= T x RPM divided by 5252, RPM and 5252 are predetermined. Torque is the only thing that is changable to get more HP.

 

Horsepower is a bit of a unicorn. It only exists as a function of torque and rpm. The torque and rpm are the real factors that we have to work with, and are the only variables we can manipulate. The horsepower we get is merely a figure that we calculate. It is the result of the torque and rpm we have. Changing the torque and/or rpm parameters of the engine will affect both the acceleration and speed achieved, and we call this resulting effect the horsepower. Gearing should not come into this discussion. It is an entirely different subject.

To keep it simple, build the engine with as much torque as you can, and to spin at the highest rpm you can, and the result will be high horsepower.
Like they say, "You want to go fast? How much money do you have?".......

 

When you actually put that 400ftlb engine in a car, the flat/constant 400ftlb "curve" number becomes meaningless as you introduce engine acceleration rates.

Here's a graph showing how different rates of accelerating the rotating assembly can affect the net torque output of a 425ftlb engine...



Note that only 204ftlbs reach the input shaft at the 1st gear engine acceleration rate of 3685rpm/sec. But in 4th gear, torque improves to 403ftlbs accelerating at a much slower 356rpm/sec rate.

Below is some Racepak data collected from my personal 2525lb driver/car, with some rough calculations applied. Sbc power, but exact same car/trans/flywheel/clutch/gearing/tires. Exact same heads/intake/carb/cam/compression. Only difference between these two engines was the rotating assy...

Scat cast crank (49lbs), steel rods, 1863g bobweight, WOT no-load rev rate of 8500rev/sec.
In 3rd gear @ 535rev/sec accel rate, 31.47ftlbs absorbed by the rotating assy, 468.53ftlbs twisting the transmission's input shaft.
In 1st gear @ 1975rev/sec accel rate, 116.18ftllbs absorbed by the rotating assy, 383.82ftlbs twisting the transmission's input shaft.

Scat F43 lightweight crank (42.1lbs), aluminum rods, 1492g bobweight, WOT no-load rev rate of 11,515rev/sec.
In 3rd gear @ 541rev/sec accel rate, 23.49ftlbs absorbed by the rotating assy, 476.51ftlbs twisting the transmission's input shaft.
In 1st gear @ 2217rev/sec accel rate, 96.27ftllbs absorbed by the rotating assy, 403.73ftlbs twisting the transmission's input shaft.

Comparing the two...
...both engines put out the same torque when operating steady state @ 5000rpm.
...WOT 4500-5500 at 3rd gear acceleration rate, the lightweight rotating assy hits the transmission's input shaft with about 7.98ftlbs more torque.
...WOT 4500-5500 at 1st gear acceleration rate, the lightweight rotating assy hits the transmission's input shaft with about 19.91ftlbs more torque.

Grant

 

Has anyone seen my fire extinguisher? All this math just scorched my little friggin' pea sized brain...

 

I've been told in the past that Lucas electrics are made for both imperial and metric voltage. The reason Lucas doesn't work in hot rods is because they use whitworth terminals. Everybody knows that whitworth terminals are too large for American smoke which lets the smoke escape from your wiring circuit.

 

Yeah...but most engines that are set up for high RPM use, don't drop torque output by 50% in the upper RPM range.
Look at the HP curve, after 5252, it keeps climbing after torque falls off. If you gear the car to keep the RPM up there, where HP is increasing as torque decreases, you'll go quicker.

My Chevy II goes quicker if I shift late, like 7000, vs. at the 6500 specified power band of the cam.

Every engine is different....but if you want to go quick, you should have a good understanding of what HP is, torque times RPM.

Power is work over time. The more work the engine can do in the given time, the quicker you'll go.

 

Gearing is a vital part of the discussion. It's what makes it so you can do the most work with the engine, by keeping the RPM near it's peak, as often as possible, on whatever track (or street...wait, did I say that?) you're racing on. Building a high HP engine that you only rev part way is a waste of time/money.

 

You crack me up!