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      01-23-2008, 09:56 PM   #1
T Bone
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Question How to measure an Engine's ability to Rev Quickly?

In looking at the performance of the E92 M3, there are things that can easily be measured.... HP, Torque, Area-under-the curve.....

But my question is that one of the beautiful design elements of the S65 V8 is how freely it revs and how quickly it revs.

As you know Revs determine the output of an engine. The higher the revs, the more power an engine produces. If want to understand the basics of the how horsepower is calculated, please do a search.....

So my questions are....
  1. Is there a way to measure how quickly an engine revs? I.e. the ability to generate more power quicker
  2. Does a fast revving engine with the same power / torque ratings as a slower revving engine accelerate the same car faster?
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      01-23-2008, 10:25 PM   #2
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Quyci revving vs. acceleration

Quote:
Originally Posted by T Bone View Post
In looking at the performance of the E92 M3, there are things that can easily be measured.... HP, Torque, Area-under-the curve.....

But my question is that one of the beautiful design elements of the S65 V8 is how freely it revs and how quickly it revs.

As you know Revs determine the output of an engine. The higher the revs, the more power an engine produces. If want to understand the basics of the how horsepower is calculated, please do a search.....

So my questions are....
  1. Is there a way to measure how quickly an engine revs? I.e. the ability to generate more power quicker
  2. Does a fast revving engine with the same power / torque ratings as a slower revving engine accelerate the same car faster?
By "fast revving" I assume you are referring to free-revving under no load? Once you apply full load of brisk acceleration, the rate of change for revs is governed more by power to weight and gearing than any unique characteristic of the engine. An engine that revs quickly in immediate response to the throttle is more "responsive" to driver inputs, but I don't think that ultimately makes it significantly faster. So my answer to #2 would be no, accept for the fractional quicker response to throttle inputs.
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      01-23-2008, 11:05 PM   #3
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what greg said, but add torque.

on the other hand there is also throttle response. the M3 should have great throttle responce with the 8 individual throttles.
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      01-23-2008, 11:32 PM   #4
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Quote:
Originally Posted by GregW / Oregon View Post
By "fast revving" I assume you are referring to free-revving under no load? Once you apply full load of brisk acceleration, the rate of change for revs is governed more by power to weight and gearing than any unique characteristic of the engine. An engine that revs quickly in immediate response to the throttle is more "responsive" to driver inputs, but I don't think that ultimately makes it significantly faster. So my answer to #2 would be no, accept for the fractional quicker response to throttle inputs.

This is what is troubling me..... An engine with a heavy crankshaft can have the same power output as an engine with a lighter crankshaft at steady RPM but the engine with a lighter crankshaft will change revs faster over a period of time since there is less inertia to fight....this would mean it will get in the power band quicker, which means faster acceleration.

Anyone else want to chime in? I am truly confused.
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      01-23-2008, 11:38 PM   #5
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Rotational inertia is just another type of load the engine experiences. It is not any different than hauling a trailer, climbing a hill, or pushing through air in the sense that they are all loads the engine needs to oppose. The only difference is that rotational inertia only becomes a load during acceleration. It is a non-issue if the engine is running at a steady rpm. An engine accelerates/revs faster when it experiences less "total" load.
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      01-23-2008, 11:42 PM   #6
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yes - the less weight of the rotational mass of the engine parts will result in faster acceleration and that will also show up as higher HP and TQ.

you are not confused. you are on the right track. but the actual measurement is still HP and TQ.
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      01-23-2008, 11:47 PM   #7
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Quote:
Originally Posted by lucid View Post
Rotational inertia is just another type of load the engine experiences. It is not any different than hauling a trailer, climbing a hill, or opposing drag in the sense that they are all loads the engine needs to balance, and overcome if it is to accelerate. An engine experiencing less "total" load will accelerate/rev faster.

Understood.....using the heavy vs. light crankshaft example below....we are talking about the rate of change in revs. At a steady RPM, the only thing the engine is fighting is internal engine friction (plus the load on the drivetrain). At a steady RPM, it is not dealing with the inertia of say heavier components.

When constantly changing RPMs, the air-fuel charge would be better used to accelerate rather than overcome inertia caused by heavier internal parts.

Not trying to argue, I would like to get a better understanding....
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      01-23-2008, 11:55 PM   #8
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Quote:
Originally Posted by T Bone View Post
Understood.....using the heavy vs. light crankshaft example below....we are talking about the rate of change in revs. At a steady RPM, the only thing the engine is fighting is internal engine friction (plus the load on the drivetrain). At a steady RPM, it is not dealing with the inertia of say heavier components.

When constantly changing RPMs, the air-fuel charge would be better used to accelerate rather than overcome inertia caused by heavier internal parts.

Not trying to argue, I would like to get a better understanding....
Yes, rotational inertia only becomes a load during angular acceleration. Otherwise, it is not experienced as a load. (I was editing my post to make that clearer when you responded). If you discount tire friction, the same thinking applies to the mass of the car; the engine experiences the mass of the car as a load only when you accelerate.
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      01-24-2008, 12:33 PM   #9
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Quote:
Originally Posted by T Bone View Post
In looking at the performance of the E92 M3, there are things that can easily be measured.... HP, Torque, Area-under-the curve.....


  1. Is there a way to measure how quickly an engine revs? I.e. the ability to generate more power quicker
  2. Does a fast revving engine with the same power / torque ratings as a slower revving engine accelerate the same car faster?
Very simple: the horsepower- the horsepower nicely summarizes the efficiency of the engine in a duty cycle.
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      01-24-2008, 02:15 PM   #10
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Quote:
Originally Posted by chonko View Post
Very simple: the horsepower- the horsepower nicely summarizes the efficiency of the engine in a duty cycle.
An engine with less internal inertia will be a quicker engine if all other factors are equal(HP& Torque) as it will have less resitance to change speed.I know this from 1st hand experiance with a E30 race car many years ago.We went from a 30 lb flywheel to a 12 lb one in combination with a 3 puck lightweight clutch setup.The car was a joy to drive with the lightweight setup and the speed of the shifts and braking was improved greatly.In quanitive terms we picked about a second in lap times on a 1.30 lap and over 7kph at the end of a 1 km straight.But on a dyno (not a dynojet) it showed no difference in RWHP or torque.A reduction of rotational weight is almost is a free lunch!
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      01-24-2008, 03:51 PM   #11
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Quote:
Originally Posted by chonko View Post
Very simple: the horsepower- the horsepower nicely summarizes the efficiency of the engine in a duty cycle.
Quote:
Originally Posted by Gearhead999s View Post
An engine with less internal inertia will be a quicker engine if all other factors are equal(HP& Torque) as it will have less resitance to change speed.I know this from 1st hand experiance with a E30 race car many years ago.We went from a 30 lb flywheel to a 12 lb one in combination with a 3 puck lightweight clutch setup.The car was a joy to drive with the lightweight setup and the speed of the shifts and braking was improved greatly.In quanitive terms we picked about a second in lap times on a 1.30 lap and over 7kph at the end of a 1 km straight.But on a dyno (not a dynojet) it showed no difference in RWHP or torque.A reduction of rotational weight is almost is a free lunch!

Chonko, any comments on Gearhead999s response?

Are all 400 hp engines equal?
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      01-24-2008, 04:07 PM   #12
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I could only hope this is what my M3 sounds like This a good example of an engine of low internal inertia.

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      01-24-2008, 04:17 PM   #13
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Interesting discussion....just cant wait to hear this thing in the flesh
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      01-24-2008, 07:26 PM   #14
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Quote:
Originally Posted by Gearhead999s View Post
An engine with less internal inertia will be a quicker engine if all other factors are equal(HP& Torque) as it will have less resitance to change speed.I know this from 1st hand experiance with a E30 race car many years ago.We went from a 30 lb flywheel to a 12 lb one in combination with a 3 puck lightweight clutch setup.The car was a joy to drive with the lightweight setup and the speed of the shifts and braking was improved greatly.In quanitive terms we picked about a second in lap times on a 1.30 lap and over 7kph at the end of a 1 km straight.But on a dyno (not a dynojet) it showed no difference in RWHP or torque.A reduction of rotational weight is almost is a free lunch!

I understand what you are saying and is absolutely correct but these are second degree variables.
Fundamentally, engines do not generate HP, they are actually generate Torque. The torque generated at any rpm is what drives the shaft. Power is how fast that Torque can be applied within a time frame, ie torque applied per unit time. So HP relates to efficiency of the engine, how fast the engine takes in fuel, and how it quickly expels the burnt fuel. If an engine can perform this cycle faster than another with the same amount of fuel intake, then that engine will naturally have more Power.

Like you rightly pointed out, at the engine the mass of the components(piston+cylinder heads etc) in the combustion chamber does affect the amount of energy(torque) that is required to initiate motion.
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      01-24-2008, 07:42 PM   #15
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Quote:
Originally Posted by T Bone View Post
Chonko, any comments on Gearhead999s response?

Are all 400 hp engines equal?
Given two engines that have the same displacement and both use petrol as fuel.
1. 3.0 litre V6 with 200 ft.lbs with 300 HP
2. 3.0 litre V6 with 200 ft.lbs with 250 HP

Engine (1) will be the faster revving engine.

All 400 HP engines are not equal as you will have normalize for the displacement- the engine with the higher HP/litre will be the faster revving engine. I will say the engine with the higher HP/litre is the most optimized engine.
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      01-24-2008, 07:47 PM   #16
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Quote:
Originally Posted by chonko View Post
All 400 HP engines are not equal as you will have normalize for the displacement- the engine with the higher HP/litre will be the faster revving engine. I will say the engine with the higher HP/litre is the most optimized engine.

Very interesting discussion....can you please explain your statement above?
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      01-24-2008, 08:18 PM   #17
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Quote:
Originally Posted by T Bone View Post
Very interesting discussion....can you please explain your statement above?

A good example to my statement is the RS4 engine and the M3 engine.

1. 4.2 litre V8 with 414 HP
2. 40 litre V8 with 414 HP.

Just looking at the HP numbers will tell you nothing about the engines, however when you normalize for the displacement:

1. 414/4.2 = 98.57 HP/litre
2. 414/4.0 = 103.5 HP/litre

After normalizing for the displacement you can see the difference in the engines- this is purely from the engine perspective decoupling gearing and transmission. The second (m3) engine is a more optimized than the first (RS4) engine.
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      01-24-2008, 08:46 PM   #18
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Quote:
Originally Posted by chonko View Post
I understand what you are saying and is absolutely correct but these are second degree variables.
Fundamentally, engines do not generate HP, they are actually generate Torque. The torque generated at any rpm is what drives the shaft. Power is how fast that Torque can be applied within a time frame, ie torque applied per unit time. So HP relates to efficiency of the engine, how fast the engine takes in fuel, and how it quickly expels the burnt fuel. If an engine can perform this cycle faster than another with the same amount of fuel intake, then that engine will naturally have more Power.

Like you rightly pointed out, at the engine the mass of the components(piston+cylinder heads etc) in the combustion chamber does affect the amount of energy(torque) that is required to initiate motion.
Everybody loves to overcomplicate how to make engines produce morepower,but to put it simple terms(1)an engine is a big airpump.Put more air through it with the proper amount of fuel being ignited at the right moment it produces power!More air& fuel= more power.(2)Reducing weight in recripocating parts will equal more power because of less weight being flung around(pistons & rods and to some degree the crank)(3) Reducing weight in a rotating piece(flywheel,clutch) will not have any effect on horsepower,but only the ease that it will change speed at(faster revving).

I have been around hottrodded V8's most of my adult life and I can not wait to get my hands on M3 engine as it is an engine that I have allways tried to get emulate with modding.Geez over 100bhp litre normally aspirated and with warranty!
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