Whipitnow

I have been trying to understand something for a while now, I am to lazy to setup an experiment to see if my hunch is correct.

I have always been impressed with what a rig does rated at 400 HP, to be able to drive 80,000 pounds to 80 MPH and be so unaerodynamic is impressive.

Could a 400 HP Chevy 350 with 350 FT pounds of torque and the proper amount of gears do that or no ? Something tells me if you were to do this the 350 ci 400 HP Chevy with only 350 FT pounds of torque would only be able to drive the 80,000 pound load to say 50 or so MPH in such a unaerodynamic package.

I understand all the work time formulas and so forth.

Thanks....

4XLR8N

The Chevy 350 with only 350 ft/lbs of torque probably could not move that 80,000 lb rig at 80 mph.

It could definitely move it, and with the proper gearing could move it just as quickly (acceleration) as the big torque diesel engine...up to a point. Don't know exactly when that point is, but it is definitely well south of 80 mph. Eventually, the gearing necessary to keep accelerating (and keep the Chevy 350 in its power band rpm-wise) will require more torque than the Chevy has. At that point, the diesel pulls away.

For that matter, the same limitations are true of the diesel...but it's practical limits (in terms of mph) are much higher due to its higher torque output.

Whipitnow

So what you are saying is HP is not HP, a engine that has more torque to back the HP up has a higher peak output in real world use even though the math of the work time says they are the same.

Your torque is your acceleration and HP your MPH I was told and can see. The diesels are slow at accelerating because the stroke is so long they rev real slow and have real slow rev gain ?

This is a interesting ? and the only way I can see solving it is for me to rig up a test using electric motors. I think it is a carefully gaurded secret because in here lies the master tune to understand how to tune engines for race applications to the TEE!

Whipitnow

You say gearing, so the rig motor has lets say 4 times the torque of a Chevy 350 and the rig uses 10 gears so you just give the Chevy 40 speeds and on paper it says it is the same ? but I do not think it is at all and there are loses in there and the math lies ?

4XLR8N

Gears are nothing more than glorified levers...and there purpose is to either multiply torque (underdrive) or multiply output speed (overdrive).

A diesel with 800 ft/lbs of torque running through a set of gears that provides 10:1 multiplication would effectively apply 8000 ft/lbs of torque to the wheels.

The Chevy 350 with 400 ft/lbs of torque running through a set of gears that provides 10:1 multiplication would effectively apply 4000 ft/lbs of torque to the wheels.

BUT...

Change the Chevy's gearing to 20:1 and now you have 8000 ft/lbs of torque being applied. Of course, this gear ratio mandates that the Chevy is now turning twice the rpm as the diesel.

Whipitnow

Thanks and I understand all that, if the peak HP output is the same the torque at the wheel will be the same if all the gearing is lined up on both vehicles the same.

On paper it says if you were to push the Chevy powered 80,000 pound rig up to lets say 80 MPH and had all the gearing right it should be able to maintain that 80 MPH just like the diesel because the peak output of the engine is the same on paper as far as work time goes.

The diesel engine makes like 4 times or more torque than the Chevy as in 1400 to 1600 FT pounds. We know the diesel has 4 times the ability to drive a load to peak output because of this but could the Chevy motor maintain the speed on flat ground under that load ?

I think if you had the Chevy all geared right and going 80 MPH under a 40 ton load in a rig it would immediately start losing MPH becuase the HP formula is wrong as far as real world app, in other words the math lies.

I am just guessing at this and do not know but from my real world experience something tells me you really cannot gear very well around a lack of torque for a application even if the math says you can.

cv67

That sbc might make 400 lbs tq at 5k+ the diesel is doing it way way sooner.
Diesels redlines are very low.

nickalltogether

400hp is 400hp and it can do the same amount of work regardless of how its made. I'd estimate the difference here is that the small block peaks at 400hp and the rig engine probably has a wider rpm range where it's at or near 400hp. The math/physics behind it are accurate- if you could lock both engines at their 400hp rpm they would have the same capabilities.

Whipitnow

So you are saying the math does not lie, so you are saying with enough gears and like a air shifter so the Chevy engine could stay in the sweet spot of the power it could do it ? The Semi does make way more average HP over it's narrow RPM operating range.

The math does say it can but I have a hard time believeing it. I was talking to a man who operated the sled at a tractor pull and he told me the higher HP classes using V8's got their asses handed to them by the lower rated HP diesel class.

He was claiming the rig motor class on diesel could make a full pull with more weight over the higher rated HP V8 style car motor....

He claimed the V8 gets out of the box faster but then it gets choked down as the sled moves up and the diesels don't get out of the box as fast but they don't get choked down and maintain the power much better under load. Is that because the average HP is higher because how much low end torque they have ?

cv67

I may eat my words but there is no 350 on the planet thats going to move a big rig well lol

Whipitnow

According to the math it can with enough gears, it would need 40 or 50 gears but I have hard time believeing it.

If it can I know it will lose the MPH faster when you hit a hill.

I would need to rig a test with eletrical motors to test this out but I believe there are loses in there. I asked this ? at many sites and got alot of bogus information so I think I may have stumbled onto a gaurded secret of some sorts as to be able to correctly read a dyno and understand what it really means in real world app....

NSFW

Your question translates to:

"How do you measure what it takes for a giant brick to push air out of the way at 80mph? Is that a question of torque, or is that a question of power?"

It's a question of power.

So, yes, if a 400hp diesel can do it, then a 400hp Chevy 350 can also do it.

Whipitnow

I agree with that but something tells me it really is not like that. I could very well be wrong about this as well just looking for the right answer. I had some people saying HP is HP and others who said no it is not and the engine that makes the power at lower RPM and does it with torque rather than RPM is a more powerful HP.

What they were essentially saying was take a Pro Mod motor that makes 2500 HP at 9000 RPM and take a ship motor that makes 2500 HP at 250 RPM, the ship motor would make around 50 thousand FT pounds of torque and they were saying the torque is your back support to your HP.

They were saying the Pro Mod is a Kraco 100 Watt 5 pound car amp that can only drive an 8 OHM load or 1 woofer per channel where as the ship motor is a 100 Watt Mark Levinson that has a 75 pound power supply and can drive a 1 OHM load or many woofers per channel to the peak output. The peak output is the same but how much load you can drive to that output is detemined by how much torque you have to back your HP up..... They said you cannot gear around a major lack of torque. Rehere Morrison told me this....

crf450x

Torque is a measure of force. Horsepower is irrelevant and boils down to just a made up value. Torque can be measured and hp is calculated using the torque value and rpms. (Hp=tq*rpm/5252)

So if you have a 350 that makes 1500 lbft, sure it will do the same work as a 1500 lbft diesel. It would be terribly inefficient but in theory would be capable of moving the same load. Another advantage a large diesel has, though, is a very flat torque curve. Say the 350 might make peak torque at 3000 rpm, the diesel will make it from 1100-1500 rpms which is damn near the whole rpm range.

(All numbers were just made up off the top of my head)

Whipitnow

I understand all that, and yes a diesel makes way more average HP across it's narrow RPM operating range. On paper it says you just add gears to the 350 to keep it in it's best RPM range....

crf450x

Assuming the 350 has the same torque output, I agree.

NSFW

That is their confusion speaking, not their understanding. Power is torque multiplied by RPM.

If an engine makes the same power at higher RPM with less torque, it's still making the same power.

Sometimes there are practical considerations that are more important than theory, for example that higher-revving engine is going to wear out faster. If you just need to do a job that requires X horsepower for an hour or five, then it doesn't matter. If you need to do that job for 20 years, then it matters.

The tradeoff is, if you have half the torque, you'll need double the RPM to match the power. That's easy if you're talking about doubling 2000 or 3000 RPM, but it's hard if the baseline was taken at 4000 or 5000 RPM and you want to match that power with half the torque.


That's like saying you can't really measure the volume of a box. The only things you can really measure are the length, width, and height. Volume is just a made-up value that we calculate from other numbers.

But, for some scenarios, volume is the most useful way to compare boxes.

And for many scenarios, power is the most useful way to compare engines. Going back to the aerodynamic question in the first post, for example. If you need to push a big brick through the air at 80 mph... How much torque does that take? Well, that question makes about as much sense as asking how many inches it takes to fill a gallon.

It's not a perfect analogy, but it'll do.

nickalltogether

You have this backwards, but it is a common misconception. Put your car on a dyno without an rpm pickup and you will only see a horsepower reading. If it's a "made up value" then how can horsepower be measured, but torque can't? You can shift the car to a lower gear, which would increase torque at the wheels, but the horsepower reading will stay the same. Horsepower, as the name implies, is a measure of power; torque as you said is just a measure of force. The physics definition of power is the rate at which work is done. If you care to read more on the topic, this link provides background to the horsepower measurement: https://physics.info/power/.

Along with the brewery example of horsepower provided in the link above, another explanation that I find useful can be seen here: https://auto.howstuffworks.com/horsepower.htm. One horse can (theoretically) lift 33,000 ft-lb of coal per minute. So it can lift 3300lb 10ft, or 330lb 100ft, or 33lb 1000ft, etc per minute. The comparison here would be to gears in a car, or even to yourself on a bicycle.

Tying it all together: You could theoretically take one horse and move 3.3lb 10,000ft, but the horse would have to be running 113mph, which is impossible. The same holds true for the big diesel vs the Chevy 350. Both are at 400hp and should be able to do the same amount of work in the same amount of time, but the 350 would need to be geared such that it could stay around that 400hp rpm at any speed from 0-80mph. Frictional losses and wind resistance would be your biggest obstacles. You could theoretically have the 350 move 1/4 the load at 4 times the speed of the big rig, but friction and wind resistance would eliminate that in the real world as well.

NSFW

I was in complete agreement until this point. But if you've got a 350 that makes 400 horsepower, gearing it to make that power at 80 mph won't be hard at all.

Wind resistance is the dominant factor in pushing a truck through the air at freeway speeds, and it'll be the same regardless of which engine is under the hood.

nickalltogether

Yeah, I think you're right there... I didn't fully think the reasoning through on the rpm thing. It would definitely be wind resistance and probably some kind of drivetrain losses due to the complexity of a transmission that could keep you in the powerband from the whole 0-80 spectrum that would be the biggest obstacle. I'll edit my original post to avoid future confusion.