Hey guys, I searched but couldn't find anything on this. Let's get a discussion going about these two things, since it seems kinda confusing to newbies like myself.

I know there's things like displacement, head flow, compression ratio, camshaft profile, gearing, boost vs NA, vehicle weight, etc. that all play a factor. Does torque converter stall speed play a factor also?

I guess my question is: How is HP and torque related? What's more important? Can you use one to your advantage if you lack a bit of the other?

I had all my questions typed yesterday but lost the page. I'll edit it later. I think I'm forgetting some items.

 

HP is a calculation using torque.

Torque matters for the bottom end when you need to get moving. HP keeps you moving faster.

There is no one better than the other. All dependent for situation.

You can give some up to get more of the other.

 

HP=torque*rpm/5252

want more HP? Spin the motor faster! NOTE: this only works if you can keep moving air. If you are "out" of head, cam, exhaust, intake and valve control; you are only going to make noise

you need to carry that torque to a higher RPM to make more power

Stall speed allows you to couple engine power to trans at (or closer to) peak torque. They shouldn't necessarily affect the numbers the dyno reads, if the dyno is being measured after stall speed. Lower RPM stalls probably show more torque because the engine is coupling sooner and less is lost spinning the converter, but I can't prove that. When the converter flashes on a low vs high stall, any "gain" is out the window because the high stall already has the engine that much closer to peak torque

low torque/ high rpm engines were my jam before I entered the LS world. I have had 1000cc engines making 225hp but they needed 13k rpm to do it. I have a 365ci diesel making 235hp but it redlines at 3400

LS's aren't even that "torquey" because I think every single one makes peak torque ~4k

Kind of the walnut shell version of what I know on the subject

 

To really understand it, forget about horsepower. Just think of an engine having either a lot of torque at low rpm, or a lot of torque at high rpm. Or if forced induction, a lot of torque at low rpm and high rpm probably. Just replace the word "torque" with "power". As Arthur indicated with the formula, HP is just a pretend number because you have to make sense of why the engine is pulling harder as the rpm rise even though the torque number is dropping. It is because there a lot of "torques" , lol, happening at higher rpm.

Did that add to your confusion?

 

so a torque converter's stall rating changes the number in the conversion equation, right?

e.g., stock transmission uses a 5252 stall, but changing to something like a 2800 stall (higher or lower depending on what your head/cam/exhaust mods match to) changes the torque-to-horsepower conversion factor. so 300lb @ 4000rpm normally = 228hp... but 300lb @ 5000rpm with heads/cam/2800 stall = 536hp

 

I can assure you there is no factory/OEM stall at 5252 lmao

they are like 12-1800, depending on application 😂

5252 is the "cross over" point where HP and torque cross on the dyno. Even if your engine doesn't get to 5252rpm, that's where the lines would cross

I don't know how or why, but that's the math. I can't explain it any better than I have without spending some time on google learning

EDIT: https://www.enginelabs.com/engine-te...s-at-5252-rpm/

 

5252 is the crossover point due to the equation, we should really be using Newton/meters but America!

 

But AMERICA aha

 

Great responses. I'll take a stab at the question, although I think @arthursc2 nailed it already in a lot less words :-)

Let's start with definitions, as there are some key things to consider. I apologize this may be too basic to some.

Torque: Is a measure of force that can cause an object to rotate about an axis. Units are typically ft-lb.
Think of torque as strength. It's simply how much force can be exerted by an engine. Analogy would be how much (lbs) can you leg press. Since everything is spinning inside an engine, the units are relative to the crankshaft axis. The piston(s) will produce a force that get's multiplied by the crankshaft stroke to produce the unit of lb x ft. A larger displacement engine will almost always generate more torque than a smaller displacement engine, all other things being equal.



Horsepower: Is a unit of power equal to 550 lb-ft per second or 737.5 watts (watts = 1 Joule per second.) The important thing here is that now we have units of TIME, seconds. Analogy going back to the human body would be HP would be how long can a runner run in a race. Here, strength is not the only consideration, there's things like your lungs and heart, etc. Obviously, in a race, it's all about time. HP is a unit of work being done in a given amount of time. In graphical terms, HP is the area under the HP line with time on the X-axis.

https://en.wikipedia.org/wiki/Horsepower


So from a purely mathematical standpoint, comparing 2 engines and disregarding everything else, the one with the highest HP is better one, right? Yes, by definition, considering both are equally efficient. But things get more complicatedness with things like RPM and TQ curve. Why is it that TQ usually stay constant and then falls off as you increase the RPM but HP increases to a peak? As was mentioned earlier, HP can be derived or calculated by HP = TQxRPM/5252. The 5252 in the denominator is simply to get the units correct. It's a conversion factor.

Here's a LM7 HP/TQ curve:

Source:https://www.motortrend.com/how-to/12...-mild-to-wild/

You can see in that equation that even though TQ falls off, you are spinning faster and faster and that's why the HP curve keeps increasing. But the other limitation is airflow going into an engine and head efficiency. Recall the phase that an engine is "just an air pump." Head airflow is NOT a linear relationship to valve lift, duration, etc. which is dependent on cam profile. Look at the LS1 head airflow: You can clearly see the CFM at 0.200" lift is not half of the CFM at 0.400" lift. If you draw a line on those bars, so can see the general curve profile and at some point, the CFM of the head maxes out.

Source: https://www.lsenginediy.com/ultimate...r-heads-guide/

Why is head airflow important? Well, because if will help create more HP. Note airflow units: CFM (cubic foot/minute.) You have units of time. Comparing 2 engines with same displacement and everything else the same, the one with the higher flowing parameters will theoretically make more power. This is where camshaft profile also plays a big role. In fact, aside from better machining and materials today, the bottom end of reciprocating internal combustion engines hasn't really changed in a while. The head design it seems to me is where all of the research has been done, especially with things like SOHC/DOHC, VVT, DOD, direct injection technology, etc.

So what about gearing? That further complicates things because you can use gearing to manipulate torque to the wheels. Archimedes said: "Give a lever long enough and a fulcrum on which to place it, and I shall move the World." It's the same principle. You can have a weaker engine that spins faster outdone a stronger engine spinning at lower RPM, within reason. But the HP is the same. How is this so? I haven't done much thinking about it, but I think again has to do with the units: RPM and time. Two extreme cases would be a chainsaw or motorcycle engine or other smaller displacement engine spinning really high to make big HP, relative to engine displacement. Conversely, a strong diesel engine with huge torque but low redline needs to rely on a gearbox with a lot of gears and leverage to get work done.

What about boosted application? That just simply increases everything because now you have ambient air that is normally in vacuum being pressurized going into the engine (along with more fuel, of course). Nitrous has a similar effect because the gas has more oxygen than regular air. More air/fuel = more power. It seems like the HP/TQ curves mostly get shifted upward depending on boost level.

I haven't even looked at vehicle weight.