Can someone give me the basic version of the HP vrs.Torque explanation. In otherwords, lets say there are two engines - one puts out 145HP and 75 ft. lbs of torque, at lest say 10K RPM. The other engine only puts out about 92HP (at 7500 rpm) but similar (72 ft. lbs.) of torque but again, at only 5500 RPM. Obviosuly we're not talking Vee (or even First) motors here. But I'm trying figure out if torque or HP is more important in a road racing application and I assume someone where would be able to "splain it to me Lucy"
Thanks
HP vrs Torque for Dummies
-
Larry Bradley
- Posts: 248
- Joined: June 25th, 2006, 12:26 pm
Re: HP vrs Torque for Dummies
The horsepower number is calculated by the mathematical equation: torque X RPM divided by 5252.
So all HP and torque curves will cross at 5252 RPM since at that point 5252/5252 will be 1
"one puts out 145HP and 75 ft. lbs of torque, at lest say 10K RPM"
RPM makes big horsepower!
The HP curve will always be less than the torque curve until you hit 5252 and greater than the torque curve above 5252.
"The other engine only puts out about 92HP (at 7500 rpm) but similar (72 ft. lbs.) of torque but again, at only 5500 RPM."
That one wouldn't, couldn't happen.
75 foot pounds at 5500 would yield just a bit over 75 HP.
5500 / 5252 = 1.047220106626047 X 75 = 78.54150799695351
As a rule, and I know there are some who disagree with this and maybe there are some exceptions, the maximum torque occurs at the point when volumetric efficiency peaks. The point when the cylinder is getting in all the air / fuel it can. from that point on, the demand from the cylinder for more air / fuel is not getting met.
And is decreasing. Say for instance the induction system can only deliver 450 cfm.
And because of cylinder displacement and rpm the motor sucks in 450 cfm at 4500 rpm.
100% volumetric efficiency. From that point the cylinders are still yelling for more than 450 cfm but it is not possible to flow more. Volumetric efficiency starts to fall. 95% 90% 85% ect.
But you still make more HP due to rpm.
Torque stops building, but because of rpm horsepower is increasing.
So wind the whistling piss out of it for more power.
This is just a quick thumbnail explanation.
Do a google search, and there are lots of books written that can explain just how it works and what will effect torque / HP.
Looking at a few real torque/HP curves will help you see what is happening.
Larry
So all HP and torque curves will cross at 5252 RPM since at that point 5252/5252 will be 1
"one puts out 145HP and 75 ft. lbs of torque, at lest say 10K RPM"
RPM makes big horsepower!
The HP curve will always be less than the torque curve until you hit 5252 and greater than the torque curve above 5252.
"The other engine only puts out about 92HP (at 7500 rpm) but similar (72 ft. lbs.) of torque but again, at only 5500 RPM."
That one wouldn't, couldn't happen.
75 foot pounds at 5500 would yield just a bit over 75 HP.
5500 / 5252 = 1.047220106626047 X 75 = 78.54150799695351
As a rule, and I know there are some who disagree with this and maybe there are some exceptions, the maximum torque occurs at the point when volumetric efficiency peaks. The point when the cylinder is getting in all the air / fuel it can. from that point on, the demand from the cylinder for more air / fuel is not getting met.
And is decreasing. Say for instance the induction system can only deliver 450 cfm.
And because of cylinder displacement and rpm the motor sucks in 450 cfm at 4500 rpm.
100% volumetric efficiency. From that point the cylinders are still yelling for more than 450 cfm but it is not possible to flow more. Volumetric efficiency starts to fall. 95% 90% 85% ect.
But you still make more HP due to rpm.
Torque stops building, but because of rpm horsepower is increasing.
So wind the whistling piss out of it for more power.
This is just a quick thumbnail explanation.
Do a google search, and there are lots of books written that can explain just how it works and what will effect torque / HP.
Looking at a few real torque/HP curves will help you see what is happening.
Larry
Now a promise made is a debt unpaid, and the trail has its own stern code.
Re: HP vrs Torque for Dummies
There are a lot of variables, and the "horsepower" vs. "torque" argument is as old and contentious and "Ford vs. Chevy" or "tastes great vs. less filling," but my position has always been that given basically equal car variables, "horsepower wins races." A lot depends on what other mods you can make, especially to transmission gearing. If you can take advantage of a high-rpm, high-HP engine by regearing with a lower numerical final drive, you can make up for a little less low rpm torque and still accelerate off a corner, but also carry top end speed. In a class like FV, where you are limited to relatively few ratio choices, I would want as much torque as possible and as much horsepower as possible with as little high rpm dropoff as possible.
-
rstackjd
Re: HP vrs Torque for Dummies
Ok thanks guys - I think I get it, or at least have a place to start. Again, this has nothing to do with FV or FST. Honestly, it's a wild ass idea for an FB car, but the motor I have in mind is weak in HP relative to the "favored" motors.
I am confused about about Larry's comment on the Torque ("wouldn't/couldn't happend") because I believe those were the numbers psted by the manufacturer. But perhaps I was wrong - will have to go back and look.
Thanks again.
I am confused about about Larry's comment on the Torque ("wouldn't/couldn't happend") because I believe those were the numbers psted by the manufacturer. But perhaps I was wrong - will have to go back and look.
Thanks again.
Re: HP vrs Torque for Dummies
I'm not an engineer, but the 7500 rpm engine doesn't seem impossible to me.
Assuming the engine really is producing 92 hp at 7500 rpm, the torque should be 64.4 lb-ft at 7500 rpm. At 5500 rpm, the point of peak torque (75 lb-ft), the engine should be producing, 75.4 hp. The 7500 rpm engine will result in the world's slowest FB.
Assuming the engine really is producing 92 hp at 7500 rpm, the torque should be 64.4 lb-ft at 7500 rpm. At 5500 rpm, the point of peak torque (75 lb-ft), the engine should be producing, 75.4 hp. The 7500 rpm engine will result in the world's slowest FB.
69 Beach Solo Vee, #65 FM
85 Lynx B Solo Vee
71 Zink C4 Solo Vee
85 Lynx B Solo Vee
71 Zink C4 Solo Vee
-
Larry Bradley
- Posts: 248
- Joined: June 25th, 2006, 12:26 pm
Re: HP vrs Torque for Dummies
Opps sorry, I didn't pick up all you wrote about the second motor.
"The other engine only puts out about 92HP (at 7500 rpm) but similar (72 ft. lbs.) of torque but again, at only 5500 RPM.
I didn't pick up you were back at 5500 rpm.
Larry
"The other engine only puts out about 92HP (at 7500 rpm) but similar (72 ft. lbs.) of torque but again, at only 5500 RPM.
I didn't pick up you were back at 5500 rpm.
Larry
Now a promise made is a debt unpaid, and the trail has its own stern code.
-
rstackjd
Re: HP vrs Torque for Dummies
Great - now I have to wipe the soda that out my nose off the keyboard!Lynn wrote:The 7500 rpm engine will result in the world's slowest FB.
That was good.
Re: HP vrs Torque for Dummies
rstackjd,
For racing applications, in general it's hard not to side with the engine with the highest peak HP. But, as you pointed out, the application (e.g., the specific race track) can make all the difference. Let me add that without also specifying the transmission gear ratios, that the question can't be answered.
Let me give you my take on HP vs. torque. When racing, the only thing you care about is power (HP). At any time, you want to choose whatever gear puts your engine at an RPM where it's putting out the most power. That's all there is to it. Whenever you choose the gear that provides the higher HP, you will also have the higher torque at the rear wheels.
That may not seem right, but it is. Dyno curve torque is quoted at the crank and the RPM is the engine's RPM not the rear wheel's RPM, and needs to be corrected for each gear ratio you use. Without correcting with the gear ratios, you don't know what torque you will have at the rear wheel. That's what matters.
That being said you can look at this by 1) always considering only HP or 2) always considering only torque at the rear wheel. Either way, you'll get the same answer about which engine is better. When HP is higher given a gear ratio and RPM, then so is torque.
Like I mentioned before, the problem with using torque is that it has to be multiplied by the gear ratios, so every time you upshift the torque instantly decreases by that multiplier, which makes it difficult to see what's happening without graphing the torque curve separately for each gear and overlaying them. But, power has no such multiplier and doesn't care what gear you're in, so you just have to go the dyno curve and read HP directly. That's it - nice and easy.
It's all about power. The only reason we even talk about torque is because we have a rotating assembly. If we didn't, we'd have no torque to talk about.
Okay, with that in mind, to answer your question, let's assume that 1) the 145 HP engine is very peaky and the power falls off fast - well below 92 HP - if you drop RPMs a little, and 2) that the 92 HP engine has a broad spread in power and the power is very flat and doesn't change much with RPM. I think this is what you were trying to convey with your engine choices. If you have a broad enough choice of gear ratios to keep that 145 HP engine close enough to 10K RPM on your roadrace tracks of choice, then that's what you want because you will be providing more power than the 92 HP engine could at any RPM. If your gearbox is wide ratio and perhaps less number of gears you often have a choice of two gears - one gear that puts you at an RPM way before the before the HP peak and one way after, then the 92 HP engine could conceivably be better because on average you'd be putting out higher HP. This is assuming that often the 145 HP engine would be operating far away from it's HP peak and putting out way less than 92 HP due to a lack of gear ratio choices. As was stated by others this tends to be the situation with the Vees where we have usually only two gears to choose from.
I don't know the transmission options in FB, but I would find it hard to believe that your gear choices would be so limited that you shouldn't go with that 145 HP engine, given the two example engines that your stated. I'd put the 145 HP engine in my Vee in a heartbeat because it would give me vastly more HP someplace down the straight and would allow a pass, even though it might be a dog in other places.
I hope that helps. If you do have some dyno curves of engines to consider, you can send them to me and I can help make the choice. Just PM me. Sorry for the lengthy posting!
For racing applications, in general it's hard not to side with the engine with the highest peak HP. But, as you pointed out, the application (e.g., the specific race track) can make all the difference. Let me add that without also specifying the transmission gear ratios, that the question can't be answered.
Let me give you my take on HP vs. torque. When racing, the only thing you care about is power (HP). At any time, you want to choose whatever gear puts your engine at an RPM where it's putting out the most power. That's all there is to it. Whenever you choose the gear that provides the higher HP, you will also have the higher torque at the rear wheels.
That may not seem right, but it is. Dyno curve torque is quoted at the crank and the RPM is the engine's RPM not the rear wheel's RPM, and needs to be corrected for each gear ratio you use. Without correcting with the gear ratios, you don't know what torque you will have at the rear wheel. That's what matters.
That being said you can look at this by 1) always considering only HP or 2) always considering only torque at the rear wheel. Either way, you'll get the same answer about which engine is better. When HP is higher given a gear ratio and RPM, then so is torque.
Like I mentioned before, the problem with using torque is that it has to be multiplied by the gear ratios, so every time you upshift the torque instantly decreases by that multiplier, which makes it difficult to see what's happening without graphing the torque curve separately for each gear and overlaying them. But, power has no such multiplier and doesn't care what gear you're in, so you just have to go the dyno curve and read HP directly. That's it - nice and easy.
It's all about power. The only reason we even talk about torque is because we have a rotating assembly. If we didn't, we'd have no torque to talk about.
Okay, with that in mind, to answer your question, let's assume that 1) the 145 HP engine is very peaky and the power falls off fast - well below 92 HP - if you drop RPMs a little, and 2) that the 92 HP engine has a broad spread in power and the power is very flat and doesn't change much with RPM. I think this is what you were trying to convey with your engine choices. If you have a broad enough choice of gear ratios to keep that 145 HP engine close enough to 10K RPM on your roadrace tracks of choice, then that's what you want because you will be providing more power than the 92 HP engine could at any RPM. If your gearbox is wide ratio and perhaps less number of gears you often have a choice of two gears - one gear that puts you at an RPM way before the before the HP peak and one way after, then the 92 HP engine could conceivably be better because on average you'd be putting out higher HP. This is assuming that often the 145 HP engine would be operating far away from it's HP peak and putting out way less than 92 HP due to a lack of gear ratio choices. As was stated by others this tends to be the situation with the Vees where we have usually only two gears to choose from.
I don't know the transmission options in FB, but I would find it hard to believe that your gear choices would be so limited that you shouldn't go with that 145 HP engine, given the two example engines that your stated. I'd put the 145 HP engine in my Vee in a heartbeat because it would give me vastly more HP someplace down the straight and would allow a pass, even though it might be a dog in other places.
I hope that helps. If you do have some dyno curves of engines to consider, you can send them to me and I can help make the choice. Just PM me. Sorry for the lengthy posting!
Re: HP vrs Torque for Dummies
Most motorcycle engines are very peaky and have a narrow width of rpm in which the hp is high. Hence the 6 speed trannies. Same thing with diesel trucks, narrow rpm width and 15 speed trannies.
The above post is for reference only and your results may vary. This post is not intended to reflect the views or opinions of SCCA and should not be considered an analysis or opinion of the rules written in the GCR.