1981 Mint 400 Flareside Race Truck

[Gary Lewis]

.... Listed as having an 8:1 compression 400 with an estimated 350 HP and 400 ft/lbs TQ, ....

.... I'm having a hard time believing the 350 HP rating - unless they really wound that thing up. Dad's cam was spec'd to give power at low R's, so maybe they used the large flow capacity of the 335 Series heads and really spun that thing.

350 hp and 400 ft-lb means the horsepower peak is significantly below 5252 rpm. The formula for calculating horsepower is torque (in ft-lbs) times rpm divided by 5252. So I'm thinking the estimation is generous.

Very true, Bob. I was aware of the equation, but was thinking backwards.

So, the torque and HP will be equal if the torque peaks at 5252 RPM. And if the torque value is less than the HP it means the torque peak is at less than 5252 RPM. But if the torque value is higher than the HP value then it means the torque peaks higher than 5252 RPM. Did I say that correctly?

To put this into perspective, I tout Dad's engine as having 400 HP and 500 ft-lbs, but without telling what the RPM is for either. However, since you are always quoting the torque peak, then you have enough info to solve the equation: 400 HP comes from exerting 500 ft-lbs of torque at 4200 RPM. And 350 HP comes from exerting 400 ft-lbs of torque at 4600 RPM.

So, Dad's engine has more 25% more torque but only 14% more HP than the Mint engine. That's because the torque peaks at 9% lower RPM than the Mint engine.

 

[1986F150Six]

So, the torque and HP will be equal if the torque peaks at 5252 RPM. And if the torque value is less than the HP it means the torque peak is at less than 5252 RPM. But if the torque value is higher than the HP value then it means the torque peaks higher than 5252 RPM. Did I say that correctly?

To put this into perspective, I tout Dad's engine as having 400 HP and 500 ft-lbs, but without telling what the RPM is for either. However, since you are always quoting the torque peak, then you have enough info to solve the equation: 400 HP comes from exerting 500 ft-lbs of torque at 4200 RPM. And 350 HP comes from exerting 400 ft-lbs of torque at 4600 RPM.

Hmmm? Trying to understand this... 4.9L with ~120 HP and ~260 ft. lb. torque @ ~1600 RPMs?

 

[Gary Lewis]

So, the torque and HP will be equal if the torque peaks at 5252 RPM. And if the torque value is less than the HP it means the torque peak is at less than 5252 RPM. But if the torque value is higher than the HP value then it means the torque peaks higher than 5252 RPM. Did I say that correctly?

To put this into perspective, I tout Dad's engine as having 400 HP and 500 ft-lbs, but without telling what the RPM is for either. However, since you are always quoting the torque peak, then you have enough info to solve the equation: 400 HP comes from exerting 500 ft-lbs of torque at 4200 RPM. And 350 HP comes from exerting 400 ft-lbs of torque at 4600 RPM.

Hmmm? Trying to understand this... 4.9L with ~120 HP and ~260 ft. lb. torque @ ~1600 RPMs?

260*2425/5252=120

In other words, the 260 ft-lbs has to be at 2425 RPM to get 120 HP.

[1986F150Six]

260*2425/5252=120

In other words, the 260 ft-lbs has to be at 2425 RPM to get 120 HP.

Thanks, Gary.

[Nothing Special]

.... So, the torque and HP will be equal if the torque peaks at 5252 RPM.

True, the horsepower at the torque peak will be equal to the torque if the torque peak is at 5252 rpm. But unless the engine has a REALLY peaky torque curve the torque will be not be falling off as fast as the speed is increasing. So the peak horsepower will be higher than the peak torque, and at a higher speed

And if the torque value is less than the HP it means the torque peak is at less than 5252 RPM. But if the torque value is higher than the HP value then it means the torque peaks higher than 5252 RPM. Did I say that correctly?

You are almost (but not quite) backward. First of all, if the torque peak is above 5252 rpm, then the horsepower at the torque peak will be higher than the torque. For example, an engine making 400 lb-ft of torque at 6000 rpm is making 457 hp at 6000 rpm.

But in reality you can't simplify it that much. Because an engine with torque less than hp can still have the torque peak below 5252 rpm if it has a very flat torque curve. For example, an engine with a torque peak of 200 lb-ft at 4000 rpm might only drop off to 190 lb-ft at 6000 rpm, which would be 217 hp.

You can use the 5252 to give you some tendencies, like if torque is higher than horsepower that you don't have a screamer. But the variability in the shape of the torque curve keeps you from being able to pinpoint it.

And that gets to an error in your later hp and torque calculations...

Hmmm? Trying to understand this... 4.9L with ~120 HP and ~260 ft. lb. torque @ ~1600 RPMs?

260*2425/5252=120

In other words, the 260 ft-lbs has to be at 2425 RPM to get 120 HP.

That would be true if the torque and power peaks were at the same engine speed. But that would only be true if the engine had an exceptionally peaky torque curve. Generally the torque falls off fairly slowly as you speed up past the torque peak, so the power is actually going up as the torque goes down.

For instance, this engine might make 260 lb-ft at 1600 rpm, which would be 260*1600/5252=79 hp. But it might only be down to 210 lb-ft by 3000 rpm, which would be 210*3000/5252=120 hp.

That illustrates a useful take-away. Peaky engines can be a kick to drive, but broad torque and power curves are more pleasant. One way you can tell by engine ratings whether the engine is peaky or broad is to look at the rpms at the torque and power peaks. If they are close together, then the engine is probably pretty peaky. But if the power peak is at an engine speed significantly higher than the torque peak, then it's probably pretty broad.

Oh, and by the way, some might notice that I say "lb-ft" while others say "ft-lbs." Torque comes from multiplying the force (lbs) applied to a lever arm (ft). And multiplication doesn't care which order you go in, so ft multiplied by lbs would be mathematically the same thing. So neither lb-ft or ft-lb is incorrect. However, energy is also described in units of force times distance. So to avoid confusion, in the English system of units engineers and scientists use "lb-ft" for torque and "ft-lb" for energy. But as I said, mathematically they are identical, so it really doesn't matter.

[1986F150Six]

.... So, the torque and HP will be equal if the torque peaks at 5252 RPM.

True, the horsepower at the torque peak will be equal to the torque if the torque peak is at 5252 rpm. But unless the engine has a REALLY peaky torque curve the torque will be not be falling off as fast as the speed is increasing. So the peak horsepower will be higher than the peak torque, and at a higher speed

And if the torque value is less than the HP it means the torque peak is at less than 5252 RPM. But if the torque value is higher than the HP value then it means the torque peaks higher than 5252 RPM. Did I say that correctly?

You are almost (but not quite) backward. First of all, if the torque peak is above 5252 rpm, then the horsepower at the torque peak will be higher than the torque. For example, an engine making 400 lb-ft of torque at 6000 rpm is making 457 hp at 6000 rpm.

But in reality you can't simplify it that much. Because an engine with torque less than hp can still have the torque peak below 5252 rpm if it has a very flat torque curve. For example, an engine with a torque peak of 200 lb-ft at 4000 rpm might only drop off to 190 lb-ft at 6000 rpm, which would be 217 hp.

You can use the 5252 to give you some tendencies, like if torque is higher than horsepower that you don't have a screamer. But the variability in the shape of the torque curve keeps you from being able to pinpoint it.

And that gets to an error in your later hp and torque calculations...

Hmmm? Trying to understand this... 4.9L with ~120 HP and ~260 ft. lb. torque @ ~1600 RPMs?

260*2425/5252=120

In other words, the 260 ft-lbs has to be at 2425 RPM to get 120 HP.

That would be true if the torque and power peaks were at the same engine speed. But that would only be true if the engine had an exceptionally peaky torque curve. Generally the torque falls off fairly slowly as you speed up past the torque peak, so the power is actually going up as the torque goes down.

For instance, this engine might make 260 lb-ft at 1600 rpm, which would be 260*1600/5252=79 hp. But it might only be down to 210 lb-ft by 3000 rpm, which would be 210*3000/5252=120 hp.

That illustrates a useful take-away. Peaky engines can be a kick to drive, but broad torque and power curves are more pleasant. One way you can tell by engine ratings whether the engine is peaky or broad is to look at the rpms at the torque and power peaks. If they are close together, then the engine is probably pretty peaky. But if the power peak is at an engine speed significantly higher than the torque peak, then it's probably pretty broad.

Oh, and by the way, some might notice that I say "lb-ft" while others say "ft-lbs." Torque comes from multiplying the force (lbs) applied to a lever arm (ft). And multiplication doesn't care which order you go in, so ft multiplied by lbs would be mathematically the same thing. So neither lb-ft or ft-lb is incorrect. However, energy is also described in units of force times distance. So to avoid confusion, in the English system of units engineers and scientists use "lb-ft" for torque and "ft-lb" for energy. But as I said, mathematically they are identical, so it really doesn't matter.

So, I drive a truck with a rather flat torque curve engine that operates well below the claimed 120 HP, since I rarely drive beyond 2500 RPMs.

It never accelerates quickly, is pleasant to drive [slowly!] and when placed under a strain [load], the engine can be "heard" to say, "is that all you have?"

[Gary Lewis]

.... So, the torque and HP will be equal if the torque peaks at 5252 RPM.

True, the horsepower at the torque peak will be equal to the torque if the torque peak is at 5252 rpm. But unless the engine has a REALLY peaky torque curve the torque will be not be falling off as fast as the speed is increasing. So the peak horsepower will be higher than the peak torque, and at a higher speed

And if the torque value is less than the HP it means the torque peak is at less than 5252 RPM. But if the torque value is higher than the HP value then it means the torque peaks higher than 5252 RPM. Did I say that correctly?

You are almost (but not quite) backward. First of all, if the torque peak is above 5252 rpm, then the horsepower at the torque peak will be higher than the torque. For example, an engine making 400 lb-ft of torque at 6000 rpm is making 457 hp at 6000 rpm.

But in reality you can't simplify it that much. Because an engine with torque less than hp can still have the torque peak below 5252 rpm if it has a very flat torque curve. For example, an engine with a torque peak of 200 lb-ft at 4000 rpm might only drop off to 190 lb-ft at 6000 rpm, which would be 217 hp.

You can use the 5252 to give you some tendencies, like if torque is higher than horsepower that you don't have a screamer. But the variability in the shape of the torque curve keeps you from being able to pinpoint it.

And that gets to an error in your later hp and torque calculations...

Hmmm? Trying to understand this... 4.9L with ~120 HP and ~260 ft. lb. torque @ ~1600 RPMs?

260*2425/5252=120

In other words, the 260 ft-lbs has to be at 2425 RPM to get 120 HP.

That would be true if the torque and power peaks were at the same engine speed. But that would only be true if the engine had an exceptionally peaky torque curve. Generally the torque falls off fairly slowly as you speed up past the torque peak, so the power is actually going up as the torque goes down.

For instance, this engine might make 260 lb-ft at 1600 rpm, which would be 260*1600/5252=79 hp. But it might only be down to 210 lb-ft by 3000 rpm, which would be 210*3000/5252=120 hp.

That illustrates a useful take-away. Peaky engines can be a kick to drive, but broad torque and power curves are more pleasant. One way you can tell by engine ratings whether the engine is peaky or broad is to look at the rpms at the torque and power peaks. If they are close together, then the engine is probably pretty peaky. But if the power peak is at an engine speed significantly higher than the torque peak, then it's probably pretty broad.

Oh, and by the way, some might notice that I say "lb-ft" while others say "ft-lbs." Torque comes from multiplying the force (lbs) applied to a lever arm (ft). And multiplication doesn't care which order you go in, so ft multiplied by lbs would be mathematically the same thing. So neither lb-ft or ft-lb is incorrect. However, energy is also described in units of force times distance. So to avoid confusion, in the English system of units engineers and scientists use "lb-ft" for torque and "ft-lb" for energy. But as I said, mathematically they are identical, so it really doesn't matter.

Bob - I clearly didn't fully comprehend that of which I typed.

But, I do think I understand what you said. And, well said it was. So, I'll leave it at that and cease and desist.

David - Yes, your engine is happy down low, has a flat torque curve, and yet rarely reaches 2425 but probably never does that at full throttle so never produces its max power. It just putt putts along, quite happy to be driven by an almost grandfather.

[Nothing Special]

So, I drive a truck with a rather flat torque curve engine that operates well below the claimed 120 HP, since I rarely drive beyond 2500 RPMs.

It never accelerates quickly, is pleasant to drive [slowly!] and when placed under a strain [load], the engine can be "heard" to say, "is that all you have?"

Keep in mind that the speeds I picked for the torque and power peaks were picked sort of at random so I could illustrate the math. The real point is that you need to know those speeds to really understand the characteristics of your engine. (Actually you need the power curve, showing power at all engine speeds to REALLY understand the characteristics. But if all you have is the torque and power numbers, knowing the engine speeds they correlate with is helpful.)

 

[blackdog]

More Bullnose Flareside desert race truck stuff. Swiss Cheese frame failure perhaps...

wow...i am infatuated with frank vessels beast. thank you for sharing!

[davissmith]

The names on the door are Curt Wilender? And Bill Lind under it.

The guy that posted the pics said Curt Willender I think, with two "L".

curt milinder was the owners name i bought this truck years ago from him it now lives in las vegas