Technical Why horsepower and torque curves cross at 5252RPM

Yes, I agree that shifting at maximum HP rather than torque peak will produce a faster ET.

The end result of where to shift is different with every car and engine combination and gearing definitely will affect how the engine responds..........but the engine itself has to be capable of a certain amount of torque at a certain rpm, and there is no way around that. I'm not saying that a real engine would drop 50% of its power at 5,000 rpms vs 7500 rpms. I merely used your example for explanation purposes.

All gearing does is allow you to use the engine at the point where it is making the most HP, it does NOT increase the HP thats available. The only thing that will increase HP at any given RPM is more Torq. You get the most Torq/HP out of any engine, then you try to find what gearing works best. In real life (as you know I'm sure) you also have to deal with multiple gears in the tranny, and often the gap between gears pulls an engine below its best rpm range. Two of the gears may be near perfect for the application but that other gear may kill your et if its too far from the lower gear. Lots of automatics suffer from that problem.

Anyway, I'm sure I'm not really telling you things that you don't know already, just trying to put my comments in context.

 

Shifting; I think the bible in the book of "Grumpy" said you must shift through the power band (power curve) to achieve the best acceleration. As I understood it, the engine may be at full output at 6800 RPM, so you'd shift at 7000 which is fully through the power. The engine doesn't go limp at 7000, in fact good hipo stuff has a nice chunk under the whole curve. Take it to 7800 or into valve float and it probably goes limp. This of course is specific drag theorem focused on MPH (work) and ET (time). Without playing Klatu from Earth Stood Still with math, does that make sense how I said it?

 

Klaatu barada nikto

 

Everything being said is correct, from both sides of the discussion, just approaching it from different directions.
The way I see it is that an engine is only capable of producing torque and rpm. How much of each you have, and the way they combine in each individual engine, is what gives the horsepower rating. It is a derived figure that indicates how the engine uses it's available torque to move the vehicle mass a certain distance in a given time. This is pretty much what @squirrel is also saying. Well, I hope it is, or I am misunderstanding his approach.

Gearing, in my view, is a separate branch of the overall picture. Here, I agree with @ekimneirbo. It has nothing at all to do with the available power that the engine can create. It's job is second in line, and is to make the best use of the power to do the job you require the car to do. Gearing can not create power. That is the sole job of the engine. The gearing simply enables the best use of that power, so it should not be considered as an actual contributing factor of power generation. There are three major steps. Power generation (engine), gearing (ratios - trans/ring & pinion/tire size), and getting the power to the ground (traction/suspension/tire contact area and compound). While all three things work in unison to get the job done, they all play a separate roll.

And, @Weedburner , your post makes my head hurt. Impressive stuff. I will be looking into the info in your post in the hope of learning more.

And for those guys that say they can't get their head around the math, here it is in it's most dumbed down form.
Engine-wise, if you want it to go hard, make it pull hard, and make it spin fast.
The basic concept really is that simple............

 

Here's an idea I've floated many times, and I'll try to be concise. Sample 1 engine makes 650 HP at 7000 RPM. Sample 2 makes 725 at 7000 RPM. Our car is a speed trails car (Bonneville, El Mirage). We change ONLY the engine. Does it go faster? No gear, trans, weight, aero changes, engine only. I have a theoretical answer, and I'm really almost alone in my "...why and how..." reasoning. Go ahead, I'm sincerely interested your outlooks and ideas. I'll post my answer later today, and up front I'll say I don't 100% know I'm right. Let's play...

 

Speed trials cars are not a very good thing to use as an example of how fast you can go, based on how much power the engine makes in what gear...because they can be traction limited, at full speed. The aerodynamic drag can be greater than the tractive force of the tires on the racing surface, at high speeds. So, I won't even guess at which is faster.

 

The engines I'm used to working with make 600hp at <2000 rpm. Peak torque is >2000 ft lbs at ~1200 rpms. These engines never see north of 3000 rpms. It's all about what job the engine is designed and intended for. Is that a recipe for quickest 1/4 mile ET? Probably not, unless you're trying to move 80,000 #'s

 

The same rules apply, even if the RPM numbers are lower overall. Note that the HP peak is at much higher rpm than the torque peak...when you're trying to get that 40 tons moving, you need to keep the revs up

 

2000 + ft lbs of torque - 650 + Hp - 1700 rpm, https://bangshift.com/bangshiftxl/g...rn-out-big-power-and-huge-torque-on-the-dyno/ Cubic inches rule !

 

As a side note:
Just remember...safety is 3rd, Torque is 1st and HP is 2nd!

 

While some smarter than me people seem to be on this topic, I am going to stretch it a little. I have always wondered about overall ratio. Is there any difference in making it in the trans or the rear end? Using Mopar ratios...
example 1...2.45 auto trans first gear and 4.30 rear gear equal 10.53 overall
example 2...2.77 auto trans first gear and 3.73 rear gear equal 10.33 overall or almost the same.
I know firsthand, the second example will give you better cursing RPM in high gear.
But will one give you better take off at the track, assuming traction isn't an issue and everything else stays the same??
Thanks, Gene.

 

There is no answer per se, I have a theory about what happens and why to the terminal speed of my said examples. I will say the majority of folks say I'm wrong, yet I don't see how. Clue? It's not an electric motor

 

Basically an engine operating WOT in an actual car spends it's time between two acceleration rate extremes that affect it's torque output. Keep in mind that the engine's rotating assy is a flywheel style energy storage device that absorbs and stores energy internally as it accelerates, but then also returns that stored energy when slowed down by external resistance.

...At one extreme, the engine produces its maximum net torque operating WOT steady state at it's torque peak, a condition that it might only see if the car is operating flat out at Bonneville! The engine's rotating assy is not absorbing any energy while operating at a constant speed, so all of the engine's torque potential gets applied to the transmission's input shaft.
...At the opposite extreme, the engine produces net zero torque WOT at it's maximum acceleration rate, a condition it will only see WOT if dis-connected from the rest of the drivetrain. All of the engine's torque production is going into accelerating its own rotating assy, with no torque left over to do any external work.

Here's a link to my page that explains where this graph's torque numbers come from https://grannys.tripod.com/clutchtuning.html



Grant

 

Lots more to consider than just the ratios. The distance between the shafts in different transmissions allows the use of larger (stronger) gears. Also the number of teeth on each gear affect its strength. When you change the basic size of the gears (and maybe even the shafts), you affect the ability of the gear to handle the torque being applied.

As an example, Tremec sells the 5 speed TKO in two different torque capacities. One is 500 ft/lbs while the other is rated for 600 ft/lbs. They are basically the same transmission with only the first gear ratio being different. The lower (higher numerically) geared trans will always have a lower torque capacity. Same thing applies to older wide ratio vs close ratio 4 speed trans.

Moving to the rear end, the gears employed are much bigger and heavier because they have to deal with the torque thats been increased by the transmissions gears. I can't really say that one ratio is stronger than the other, because I don't know how many teeth they have and given their size.......whether the lesser tooth gears may have much more thickness. Lot more room in there for adding some material than between tranny gears.

I would say that it doesn't matter which way you do it, the outcome in first gear should be basically the same. What you have to look at is the second and even the third gear ratios when racing. The gap between ratios is important. When you use a trans with a lower (higher numerically) first gear, there may be a wider spread between ratios when you hit 2nd gear. If its too wide, your engine may drop below its best power band. Same thing going from second to 3rd. Generally, having less of a spread between all of these ratios is beneficial in racing.......so the higher numerical first gear should work best. You have to multiply all 3 gear ratios X the rear ratio and look at the overall ratio you get and how far apart they are. Thats where you will see what I'm talking about.

The 3.73 will always give the best cruising with either overdrive trans. You should however look at what the overdrive ratio is in each trans, and whether the rpm vs speed you cruise is compatable with the cam you are using. Some people cruise at 80 mph these days, and its nice to have your engine running below 2500 rpms when doing so.

Hope this helps you.

 

5252's only practical purpose is to make the resulting number smaller, as typical hot rodder doesn't really care much about how his engine might compare to a "standard" horse.

When you leave 5252 out of the equation, it boils down to torque x rpm- highest average number wins.
400tq x 4500rpm = 1,800,000
375tq x 5500rpm = 2,062,500
The 375tq example features less powerful combustion cycles, but there are more of them in a given time period.

Grant

 

In the most basic terms, torque is the actual force that performs the work, while HP is a useful calculated number based on torque and RPM that can be used to measure the amount of work performed. Big Diesels have been mentioned, which I have a lot of experience with. An early D8 Cat would only be "rated" at 185 HP from over 1200 ci, but that is at 1200 RPM- and it's strongest, due to torque rise, was at 800 RPM- they will get down and chug. Gearing will let the engine work in the proper RPM range to accomplish the desired work using the power range of the engine. The epitome of this in the real world was the early Mack Maxidyne engine/ Maxitorque 5 speed trans combination. They beefed and hopped up the old Thermodyne engine with a turbo etc., "rated" 237 HP at 2100- but with peak torque at 1200, and a torque rise of over 50%. The Maxidyne 5-spd trans was set up with ratios that let it drop from 2100 back to the 1200 in the next gear, and it would roll coal, sound incredible and move out. It was difficult for drivers used to 10 and 13 spd trannys and engines with a lot less lower RPM torque to get used to, as they would want to downshift on hills at 1700-1800, had to learn to trust the engine's torque rise and let it chug. Gearing a drag car is similar, you gear and RPM it until it will have enough torque to pick up the next gear- a torquey engine can run a 2 or 3 spd and less rearend gear, a little screamer will take more gears and more rearend gear to stay in it's range, as we all know. Most of us are old enough to remember the Modified Production cars with leetle engines over 10K RPM and gears 6.0 and lower- it ain't rocket science

 

That would have less to do with holding it for the extra 500 rpm, and more to do with what the rpm drops to in the next gear. You have more revs at the base of the next gear, and as a result you are also a little higher up the torque curve. The slightly higher rpm x the slightly higher torque adds more advantage (or available horsepower) than you are losing between 6500 and 7000 rpm at the top of each gear. So you are a little quicker overall.

If you held it too long at the top of each gear, the opposite would start to happen, and you would lose advantage.

 

If both of the HP ratings are calculated at 7000 rpm it would mean that "sample engine 2" would be generating a higher amount of torque. This would enable it to accelerate a bit quicker, as it has more muscle to move the same mass. The car should also achieve a higher terminal velocity, as it should be able to push to a slightly higher rpm before it hit the limits of resistance (wind/friction).

Of course, all of this is assuming that the torque curve of both engines are a perfect match shape and duration wise, just one higher than the other. Highly unlikely, but we are working hypothetically..........

Your clue referencing an electric motor alludes to your theory having to do with the torque curves. An electric motor generates maximum torque from zero to maximum rpm. You have me intrigued. Waiting with baited breath..............

 

Torque in itself... is useless. (Taking cover from all the rotten tomatoes coming my way.)

Torque, as a static force, does nothing. It says there's a twisting force on something, nothing more, nothing less. So, you are applying a s**tload of torque to the rusted solid screw, nothing moves, the torque itself does nothing.
If you apply more torque you'll eventually get movement, either the screw comes loose, or something breaks. Now you have two things, torque and movement. Torque multiplied by movement (such as rpm) is power.

Same thing with a car. Torque is the force (on the wheels) that makes the car move, but once you have movement you have (horse)power, and it's the power that performs the job of moving the car. You need the torque just like you need the movement, without one or the other there's no power and no work gets done.

The big misunderstanding seems to be that motors with lots of torque are "stronger". Nope. 100hp is 100hp, no matter if it's a motorcycle screaming at 10000rpm or a tractor chugging along at 1000. The tractor has to have 10x as much torque (at that particular rpm) to have the same power, but at exactly that rpm both engines can perform exactly the same amount of work.
The difference and the origin of the misunderstanding comes when we use those same engines on a different rpm.
The tractor will have a nice, flat torque curve over a wide rpm, so it'll have good power over a wide rpm range. Drop down to 750rpm, it still has lots of power, increase to 1500rpm, still plenty of power. The motorcycle on the other hand, it has been optimized to run well on a high rpm, the compromizes that comes with that means it works relatively bad outside that rather narrow rpm range. It may be good at 8000rpm, good at 12000rpm, but power output plummets once you go outside that.

On a light motorcycle, with a close ratio gearbox it may be absolutely fine, maybe even fun to go full throttle down a winding road and change gears all the time to stay in the narrow rpm range where the engine performs well. Put the same engine in a tractor and you may find you need 50 gears to keep the engine in that rpm range while using some heavy farm machinery. You'd go crazy before lunch.

At the same time, the 10x heavier, huge tractor engine would make the motorcycle a horror to drive, even if it has the same peak power and higher average power throughout the rpm range.

An engine that has peak power at a low rpm has to have a high torque relative to the power, we saw that from the start (torque * rpm = power). Engines with the peak power at low rpm will almost always have the power spread over a wider rpm range, it's simply their nature, and the higher rpm engines go, the narrower rpm range they work well over.
So, are engines with high torque stronger? No... and yes. At peak hp rpm they have the same strength and can do the same work, but they usually have more power once you end up at any other rpm and THATS why they feel stronger while going up a steep hill pulling a heavy load at very low rpm. It's still the horsepower at work, they just have more power available at that low rpm.

 

It’s math.

You wouldn’t understand.

 

And here's something to toss into the mix. A single horse has a maximum of around fifteen horsepower, while a human has a maximum of about one. So when you next brag about how much power your car has, remember that it is really only the equivalent of 'people power'. It is widely believed that James Watt deliberately fudged his calculations, and overstated the 'horsepower' virtues of his newly invented steam engine to boost enthusiasm and sales.

 

Ok, I believe the car will go faster. I think it's a power delivery thing. To get to the ppwer the engine cycles through the chosen number of cylinders and creates a total amount of work. It "punches" mechanically and each punch has more force available to achieve the power or work. Were it just an electric motor more power does nothing because of linear output. Being really over simple, a buzz vs a bang, or a bunch of bangs. And yes it will accelerate to the terminal speed faster. Now how much faster would be an experiment I can't afford. Most will tell or have told me I'm full of shit so no new ground to be had.

Harder hits to the same delivery mechanism. Let the games begin...

 

@theHIGHLANDER , I agree.
Quicker acceleration due to more torque being applied to the crank per engine revolution.
Greater terminal velocity because the extra power can push the rpm higher before the car hits the resistance wall.

 

I don’t know. But I installed wheel and driveshaft sensors on one. Some smart guy used an electronic brain to measure those numbers. Rear wheels were losing traction around 220mph.
The solution. Add a bunch of lead weights behind the rear axle.
Went faster weighing more.
Would the extra hp push that weight more or just lose traction again but maybe faster.

this thread hurts my nogin

 

What my engine would look like after testing that theory

 

In theory perhaps, but not in practice. Go ahead and hook up the theoretical 10k rpm motorcycle engine and see what happens; it will immediately fall on it's face. Scroll up the thread to see where @MeanGene427 talked about Torque Rise. With either engine, as you apply the load the rpms will fall, with the 1000 rpm tractor engine the torque will INCREASE as the revs fall, the load will start to move and the rpms will increase back up to max hp again; with the small high rpm engine the rpms will drop, torque will drop along with the revs, and the engine will never recover. And good luck finding the drive train that will sustain the 10,000 rpm input to move the load.

 

I hear that- I have never had mine past 3500.

I don't understand, why would you want to be in such a hurry?

 

My "experiment" makes no adjustments for traction, aero, weight, RPM or ratio. If we get into this could be this and that could be that then the question is moot (it is anyways). Sure, you could adjust parameters and change conditions and in most cases hit the mark. The cyclical nature of the power developed.

 

It does seem like if a feller got up a little earlier in the morning a lot of this could be avoided, don't it?

 

I do land speed racing on paved tracks but it’s vintage ohv bikes..But anyways, on a Superflow dyno known for being accurate, the engine makes the best Rear wheel hp at 7150 rpm and then falls off sharply. Max torque is at 5100 rpm and at 7100 rpm 90% of the torque is still then, then falls away fast. The HP and torque curves are proportional to like a SBC
on the track the fastest speeds are when it’s geared for 7400 rpm