why does everyone want to spin the coyote up to 8000 plus?

[tjm73]

Remember that MT article comparing an e90 M3 and the S197 coyote mustang back in 2011? The cars weigh about the same, make about the same hp, but the Mustang makes about 100 plus more lb/ft of torque. 0-60 times are identical. Comparison: 2011 Ford Mustang GT vs. 2011 BMW M3 Coupe (motortrend.com) If torque were king the Mustang should smoke the m3 in acceleration but it doesn't.

You can't use two different cars to argue this point. There are too many variables. Gearing and traction being two.

 

[tjm73]

The gearing does not compensate for the torque output.

The gearing allows for utilization of the HP production.

That's exactly what gearing does. If you don't have much torque you add gearing to multiply the torque at the axles. It's why 4.10's accelerate easier and quicker than 3.08's in a Mustang, all other variables being equal. But you have to shift more often.

 

[Norm Peterson]

The gearing does not compensate for the torque output.

Of course it does, but you have to look beyond the engine alone. Torque by itself at the crankshaft doesn't tell you enough, but that does not matter because purely on a torque basis it's torque to the drive wheels that matters.

In the Mustang vs M3 case, the M3's gearing allows it to use the torque curve around that engine's peak power rpm with about the same effectiveness as the Mustang's gearing allows it to work with the Coyote's peak power rpm. In both cases, the drive tires end up seeing similar amounts of torque.


Norm

 

[stevbd]

You can't use two different cars to argue this point. There are too many variables. Gearing and traction being two.

For sure you are right, there are a lot of variables with the M3. But in a general sense it makes the point that a car with a LOT less torque but similar HP is posting similar acceleration times.

How about the Boss vs. GT comparison? Those two cars are pretty much identical in terms of drivetrain, trans, gear ratios, traction, etc. -- except for the Boss having less torque, more HP, and faster acceleration.

 

[stevbd]

Of course it does, but you have to look beyond the engine alone. Torque by itself at the crankshaft doesn't tell you enough, but that does not matter because purely on a torque basis it's torque to the drive wheels that matters.

In the Mustang vs M3 case, the M3's gearing allows it to use the torque curve around that engine's peak power rpm with about the same effectiveness as the Mustang's gearing allows it to work with the Coyote's peak power rpm. In both cases, the drive tires end up seeing similar amounts of torque.


Norm

I think it's more helpful to not think about transmissions and final gearing, etc., it just puts too many variables in play. Imagine the s65 in the M3 and the coyote in the Mustang are just hooked up 1:1 to the wheels. I am pretty sure, assuming the weight of the cars is the same and you could keep each motor in its optimum rpm range for maximum HP, they should accelerate about the same even though the s65 makes 100 lb/ft less torque. I think that's because they make similar power - i.e., similar horsepower. (420 for M3, 412 for mustang)

Now as a practical matter, having to buzz around at 8200 rpm to make the target HP could get pretty damn tiresome. I prefer the coyote. But mathematically speaking, I think they should accelerate very similar.

 

[eighty6gt]

torque to the drive wheels that matters.




Norm

matters for what?

acceleration?

completely wrong

4.10's work because you enter the high hp areas more quickly, and are able to spend more time with the rpm's in the area where the most horsepower is produced. Say for instance you go through the 1/4 at the top of 4th instead of the top of 3rd - you got to the top of 1st more quickly and spent less time in an area with low horsepower production, and made an entire extra pass through the power range. That's a lot more power producing cycles, more fuel burned.

 

[Midlife Crises]

When I look at your graph I see banging 4 gears and operating between the torque peek and horse power peek. Or there abouts!

 

[Norm Peterson]

I think it's more helpful to not think about transmissions and final gearing, etc., it just puts too many variables in play.

No, all those things do matter and it's an over-simplification to think otherwise. I get that if you don't have an engineering mindset that you're going to look for "easy" rather than "rigorous", but that doesn't make "easy" as good of an approach.

All those HP to weight and 1/4 mile estimators? What they're working with in the background is some assumptions about torque in the vicinity of peak power rpm and gearing. Don't forget that power varies more with rpm than torque does until you get past peak power rpm.


Norm

 

[Norm Peterson]

View attachment 75774 View attachment 75775

matters for what?

acceleration?

completely wrong

4.10's work because you enter the high hp areas more quickly, and are able to spend more time with the rpm's in the area where the most horsepower is produced. Say for instance you go through the 1/4 at the top of 4th instead of the top of 3rd - you got to the top of 1st more quickly and spent less time in an area with low horsepower production, and made an entire extra pass through the power range. That's a lot more power producing cycles, more fuel burned.

Sorry, but while those plots may be useful for correlating engine rpm with road speed they aren't giving you any acceleration answers.

I will give you that they provide a reasonable illustration of what you want to achieve, but you're missing out on a whole lot if you stop there.

For one thing, acceleration (what you feel and what throws you back in the seat) roughly follows your engine's torque curve, not its HP curve. It's not an exact relation, since drag increases with speed. A HP curve by itself cannot even hint at that.

I've been tinkering with mathematical simulations of acceleration off and on for over 50 years at this point, so I do have a pretty good handle on this matter and a torque basis is by far the simpler approach.

Peak HP (along with an idea of how much drag force your car creates) is all you need to estimate your car's top speed. Though in order to build your car so it can actually get there you still need to work with gearing and drive tire size.


Norm

 

[eighty6gt]

no matter what you do, you are using torque to get to calculate a power unit to use to calculate acceleration. It's there in whatever math/spreadsheets/etc.. are involved.

How can you not realize this after 50 years?

 

[Norm Peterson]

Like I said above, I've been at this for a long time.

Here's maybe half of the input data for one of two cases I can compare against each other. I'm curve-fitting to peak HP and Torque data and their rpms but I could use exact dyno data without too much extra work.


Here I'm only using it to compare the gearing of one transmission and axle gear ratio against another transmission gearset with a slightly different axle ratio, but it can compare two entirely different cars. SS Camaro against Fox Mustang? No problem. Even estimate the time differences from the exit of one corner to the braking zone for the next (there's a little scrap of that table showing at the bottom).


I can even plot the differences between the two cases as if they were running wheel to wheel from a roll


You aren't going to be able to do that from knowing only the HP and the car's weight.


Norm

 

[eighty6gt]

why not

as soon as you put gearing and time into a calculation with torque, bmep, whatever

you are now calculating using power. If you have power, you can get back out of it anything you'd like.

have any calculations with steam turbines in there?

 

[Norm Peterson]

no matter what you do, you are using torque to get to calculate a power unit to use to calculate acceleration. It's there in whatever math/spreadsheets/etc.. are involved.

How can you not realize this after 50 years?

I'm not using power for calculating acceleration. I suppose it would be possible, but it would be a good bit more involved. Why is that so hard to understand?


Working backward from acceleration to the information you need about engine output, it goes like this:

Acceleration is [drive wheel traction force] divided by [car mass]. Let's ignore things like drag and traction limits at least for now. Car mass you know and remains constant.

[Drive wheel traction force] is [drive wheel torque] divided by [drive wheel rolling radius]. Drive wheel rolling radius is also known (and for all practical purposes constant).

[Drive wheel torque] is [engine torque] times [transmission gearing] times [transmission gear efficiency] times [axle ratio] times [axle gear efficiency]. Transmission and axle parameters you either know or can estimate with reasonable accuracy.

Nowhere do I need to use HP except that my curve-fit approximation of the torque curve needs it. If I had a dyno'ed engine torque curve to begin with, I wouldn't even need to know what peak HP was.



Norm

 

[Mike Mckay]

OH MY HEAD HURTS just go WOT and hope it don't blow up...

 

[LarryJM]

Is there anyway to get the rev limiter to 7000 RPM vis 6850 on a stock Mustang GT without a tune? Since I removed the sound tube and have Lloyd thick carpets, it's EZ to hit the rev limiter in passing on a two lane. The Rev Limiter must cut the fuel off so it gives you no warning.

 

[stevbd]

No, all those things do matter and it's an over-simplification to think otherwise. I get that if you don't have an engineering mindset that you're going to look for "easy" rather than "rigorous", but that doesn't make "easy" as good of an approach.

All those HP to weight and 1/4 mile estimators? What they're working with in the background is some assumptions about torque in the vicinity of peak power rpm and gearing. Don't forget that power varies more with rpm than torque does until you get past peak power rpm.


Norm

Norm, you are over-thinking this. Putting all the variables of downstream gearing into play isn't "rigorous," it's "unknowable." There are infinite variations and possibilities from car to car. With the right gearing you could theoretically move a train with a motorcycle engine.

The question was, what is more important torque or hp? The answer is both.

You need to simplify and just focus on the engine, because everything after the crank can be modified one way or another via gearing. At the crank, torque is just a measure of twisting force. HP is a measure of how quickly that twisting force can be delivered, and thus how quickly an object can be accelerated. HP just adds the element of time in the form of rpms.

So yes, a lot of torque per power stroke is helpful for making a lot of HP but you also need rpms to deliver, or repeat, that torque and twisting force quickly and generate a good HP number to accelerate the car.

To use a ridiculous example, if you have a massive engine that delivers 5,000 lb feet of torque per power stroke but only fires once every 15 minutes, you could move a house but you won't accelerate that house very quickly. You would have a ton of torque but not much HP.

Assuming vehicles have the same gearing, weight, and traction, I can't think of an example where a model that has more torque and less HP accelerates faster. I think the opposite is true.

 

[eighty6gt]

I'm not using power for calculating acceleration. I suppose it would be possible, but it would be a good bit more involved. Why is that so hard to understand?


Working backward from acceleration to the information you need about engine output, it goes like this:

Acceleration is [drive wheel traction force] divided by [car mass]. Let's ignore things like drag and traction limits at least for now. Car mass you know and remains constant.

[Drive wheel traction force] is [drive wheel torque] divided by [drive wheel rolling radius]. Drive wheel rolling radius is also known (and for all practical purposes constant).

[Drive wheel torque] is [engine torque] times [transmission gearing] times [transmission gear efficiency] times [axle ratio] times [axle gear efficiency]. Transmission and axle parameters you either know or can estimate with reasonable accuracy.

Nowhere do I need to use HP except that my curve-fit approximation of the torque curve needs it. If I had a dyno'ed engine torque curve to begin with, I wouldn't even need to know what peak HP was.



Norm

there is power all over in there, right in your formulas you have listed. if you dyno the torque curve, you know the power.

thank you stev, for the house bomb engine example, I will use that in the future.

 

[Norm Peterson]

there is power all over in there, right in your formulas you have listed. if you dyno the torque curve, you know the power.

Sure, I could extract power from the torque curve - I actually did that in for the HP plot - but there is no point in using it for anything beyond illustration and a sanity check on the curve fit. Working from what you need at the contact patches to make acceleration happen tells you all you need to know, and HP isn't one of those things.


XenForo strikes again


Norm

 

[eighty6gt]

Is there anyway to get the rev limiter to 7000 RPM vis 6850 on a stock Mustang GT without a tune? Since I removed the sound tube and have Lloyd thick carpets, it's EZ to hit the rev limiter in passing on a two lane. The Rev Limiter must cut the fuel off so it gives you no warning.

just smack the limiter. That's what it's for. Of course you'd need a tune to remove it...

Alternatively could paper over the tachometer!

 

[Norm Peterson]

Norm, you are over-thinking this. Putting all the variables of downstream gearing into play isn't "rigorous," it's "unknowable." There are infinite variations and possibilities from car to car. With the right gearing you could theoretically move a train with a motorcycle engine.

Perhaps you should try to stretch your understanding of this. I don't expect you to be able to do the math in your head, or realistically on paper. Just at least be able to picture how it works.

None of those other variables are "unknowable" . . . unless a person is unwilling to either track them down or develop a decent basis for estimating them.

My spreadsheet is based on physical quantities involved in vehicle acceleration and the mathematical relationships among them. It doesn't care what the car is, or what engine is under its hood, or what any of the other nearly 80 independent variables I'm including have as values.

Your HP-only approach and speeds vs rpms can't even begin to give you the information that's in the charts a few posts back.

The question was, what is more important torque or hp? The answer is both.

I would never consider torque alone when dealing with acceleration, and acceleration for me is a much bigger picture than just drag racing.

You need to simplify and just focus on the engine, because everything after the crank can be modified one way or another via gearing.

Just focusing on the engine is the way people new to the car hobby think, because they don't know any different, and they haven't had exposure to the math/engineering involved that would lead them think there might even be more to it.

To use a ridiculous example, if you have a massive engine that delivers 5,000 lb feet of torque per power stroke but only fires once every 15 minutes, you could move a house but you won't accelerate that house very quickly. You would have a ton of torque but not much HP.

What's your point here? Working with something like a sixteeth of a HP doesn't tell me anything I couldn't do with less effort straight from the basic data of 5000 lb*ft @ 1/15 of an rpm.

Assuming vehicles have the same gearing, weight, and traction, I can't think of an example where a model that has more torque and less HP accelerates faster. I think the opposite is true.

As long as you keep the engine running optimally around peak power rpm, sure. But if you're operating down at peak torque rpm it's going to be the other way around. Not all acceleration happens at a drag strip, not all acceleration that matters happens at the drag strip, and not all acceleration involves rpms in the vicinity of peak power rpm. Drag racing tends to get in the way of understanding this, and there's plenty of history of drag racing enthusiasts downplaying rigorous thinking in favor of an "everybody knows this" kind of thinking.

I was once young enough to lack the knowledge to think any different myself. I still have the Hot Rod special publication that changed my mind on this forever.




Norm