Author Topic: H/P Torque Comparrison question  (Read 10676 times)

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Offline Clay Pitkin

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H/P Torque Comparrison question
« on: June 29, 2012, 08:39:09 PM »
Ok question here:

The other day a discussion came up on the subject HP/Torque.

My snowmobile engine in my car produces 185 HP/ and 65 PDS of torque, as we talked the subject of V-8 engines came up. I was told that the HP/torque from a snowmobile engine is totally different that that measured from a V-8 is this true?

I was under the impression that HP is HP, and Torque is torque, regardless of the engine, other than one engine may produce more than the other?

Is this true?

TIA
Clay
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Offline rgn

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Offline panic

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Re: H/P Torque Comparrison question
« Reply #2 on: June 30, 2012, 10:39:20 AM »
What I can't follow is the references (in the link and elsewhere) that identify the force measured by the dyno as "the instant torque at that RPM" or similar language.
If this is true, it's one of those rare engines in which all cylinders fire simultaneously.
Otherwise, it's only measuring the torque of 1 cylinder as the gross, and subtracting the compression and pumping losses of the other 1/3/5/7 for the net.


Offline hotrod

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Re: H/P Torque Comparrison question
« Reply #3 on: June 30, 2012, 11:55:52 AM »
Quote
What I can't follow is the references (in the link and elsewhere) that identify the force measured by the dyno as "the instant torque at that RPM" or similar language.
If this is true, it's one of those rare engines in which all cylinders fire simultaneously.
Otherwise, it's only measuring the torque of 1 cylinder as the gross, and subtracting the compression and pumping losses of the other 1/3/5/7 for the net.


The engine has a flywheel and rotating inertia of its crankshaft and about 1/2 the mass of the connecting rods. That flywheel mass stores the peak torque generated during the power stroke, and then gives up that energy during the exhaust stroke and intake stroke, which allows the engine to "coast" through the rest of the cycle until the next piston fires. You are putting too fine a point on their words, they are talking about the average torque (over a very small period of time) as their "instant". At 6000 rpm we are talking about 100 revs/second so even a one lunger would only coast for 1/50 of a second between power strokes.

There are very high end dynos that folks like the F1 teams use that can actually see and record the power pulses during the power strokes and the decelerations of the crank in between power strokes, but I doubt anyone with a budget less than those sort of teams can even afford time on one, let alone own one.

Larry

Offline Captthundarr

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Re: H/P Torque Comparrison question
« Reply #4 on: June 30, 2012, 12:38:47 PM »
Well put Larry, that helps me as well. thanks

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Offline tortoise

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Re: H/P Torque Comparrison question
« Reply #5 on: June 30, 2012, 12:49:14 PM »
. . .the HP/torque from a snowmobile engine is totally different that that measured from a V-8 is this true?
For a statement to be true or false, it must have a clear meaning.


Offline panic

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Re: H/P Torque Comparrison question
« Reply #6 on: June 30, 2012, 04:37:50 PM »
Yes, I understand how an engine works, thank you.

"Instant" was their choice of phrase.
Average is the exact opposite of instant, as in never instant.
The actual time element is obviously 720° regardless of how many seconds occur.
Again: if it has no time provision (actually instant), it will only indicate 1 cylinder. If there is a time provision, it's indicating horsepower.

Thanks in advance, but repeating the explanation doesn't advance the discussion.

Offline Jon

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Re: H/P Torque Comparrison question
« Reply #7 on: June 30, 2012, 06:19:05 PM »
Hp cannot be measured it must be calculated.

A brake dyno ( be it water, hydraulic, eddy current or physical brake) measures torque and rpm then uses that to calculate hp.

Inertia dynos measure rpm increase of a known weight over time and calculate the hp required to achieve that acceleration, they are only useful for making pretty graphs and selling stuff IMHO.


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jon
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McRat

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Re: H/P Torque Comparrison question
« Reply #8 on: June 30, 2012, 07:19:12 PM »
I've raced engines that make peak power at 12,000 rpm and ones that make peak at 2,800 rpm.

Torque is a pointless number in and of itself.  2100 ftlb of torque is normal for a trash truck with a 300hp engine.

I applied 275ftlb to my crank bolt, why can't I use that power?

Torque won't propel a 1lb car 1 mph by itself.  Don't believe it?  Weld the axle bearings and git er dun.  Or remove the driveshaft, whichever applies.

Not only is a torque number useless, a peak HP number is useless as well, unless you have a CVT (no transmission ratios).

What YOU want is the max area under the HP curve in the rpm ranges you can use.  High average HP in the racing zone.

In the case of our trucks, we go faster by reducing the torque.  And oddly enough, the same thing goes for high RPM engines.  The gearbox in a F1 car would disintegrate if put in a trash truck.  High torque numbers mean everything must withstand huge torsional loads.  From the engine mounts to the tires, and everything in between, if you make small torque, you can make a light powertrain.

Your snowmobile engine with the correct gearing is a better race engine than a normal V8.  Why?  It can make more HP per lb of drivetrain weight.  The V8 must waste a lot of it's power carrying the extra weight of a drivetrain that must hold far more than 65ftlb.

As far as dynos go, and factory HP ratings, here's where things get screwy.  A Dynojet without a retard option will give different numbers than a dyno with a retard mechanism.  Why?  Load and rpm gain will screew up the Dynojet numbers.  I love dynojets for their simplicity and repeatability, but the numbers don't jive with HP/weight vs 1/4mi trap speeds.  It reads too high for a cammed NA V8, but too low for a turbo diesel.  With factory HP readings, it's a Forest Gump thing, "you never know WHAT you'll get", ask Mazda and Ford, both had to buy back a lot cars for not meeting the brochure claims.  GM tends to be very modest, but most mfr's are really mfr's when it comes to brochure numbers.

In any case, I heard people are putting bike/rec engines into antique cars and LSR racing them.  I suppose there is some kind of point to that, but it eludes me.  If torque was important, they would leave the car engine in it.

« Last Edit: June 30, 2012, 07:24:48 PM by McRat »


McRat

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Re: H/P Torque Comparrison question
« Reply #10 on: June 30, 2012, 07:37:32 PM »
...
There are very high end dynos that folks like the F1 teams use that can actually see and record the power pulses during the power strokes and the decelerations of the crank in between power strokes, but I doubt anyone with a budget less than those sort of teams can even afford time on one, let alone own one.


The GM engines we have all do that from the factory.  AFAIK, this is true for all GM engines.  Some engines measure actual chamber pressure during the cycle to do the same thing (VW TDI).

However, you cannot get the info from the ECM, only "balance" numbers for each cyl, and an error code concerning which cylinder is putting out too much or too little power.

Could you do it cheaply?  Yup.  Price depends on how many RPM you need, but you need is a something that will log data and spreadsheet software.  

They sell the cyl pressure systems off-the-shelf, the crank acceleration system I don't know.  I do know you can make your own, I've done it.


Offline Jon

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Re: H/P Torque Comparrison question
« Reply #11 on: June 30, 2012, 07:52:35 PM »
In anything other than a static system torque cannot exist without hp.

Using your tension wrench example, if the bolt is turning you are developing hp, only if you are applying pressure without movement is there torque with 0hp.
The car will move if the force (torque) you are applying when multiplied by the druvetrain ratio exerts a force at the tyre contact point greater than the rolling resistance.

To say hp is more important than torque is like saying thunder is more important tan lightening.
Without torque hp cant exist.

Just depends on how you want to do your maths, when the driving force at the tyre is no longer more than the resistance to vehicle motion (in our case mainly aero drag) the car can't accelerate any further.

Cheers
jon




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Offline manta22

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Re: H/P Torque Comparrison question
« Reply #12 on: June 30, 2012, 08:22:46 PM »
Clay:

"I was told that the HP/torque from a snowmobile engine is totally different that that measured from a V-8..."

What they should have said (and maybe they meant to say) is that the HP/torque curve characteristics of a snowmobile engine are totally different than a V-8.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

McRat

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Re: H/P Torque Comparrison question
« Reply #13 on: June 30, 2012, 08:27:42 PM »
In anything other than a static system torque cannot exist without hp.

Using your tension wrench example, if the bolt is turning you are developing hp, only if you are applying pressure without movement is there torque with 0hp.
The car will move if the force (torque) you are applying when multiplied by the druvetrain ratio exerts a force at the tyre contact point greater than the rolling resistance.

To say hp is more important than torque is like saying thunder is more important tan lightening.
Without torque hp cant exist.

Just depends on how you want to do your maths, when the driving force at the tyre is no longer more than the resistance to vehicle motion (in our case mainly aero drag) the car can't accelerate any further.

Cheers
jon






Here's something odd:

I'm never at peak torque at WOT.  Can't be.  It's physically impossible unless I've forced the transmission using a TECH2 on a dyno.

Why?

Peak torque (605ftlb) for me happens at about 1600 rpm stock.  At no point can you be at 1600 RPM at full pedal.  It will downshift.  There is only 184hp at 1600, so if you ask it for power, it will downshift.  Probably like your family car with an automatic.  You can't do it.

Yet the brochures for all the diesel pickups scream, MORE TORQUE!!!!  Really  :roll:  Exactly what good is 605ftlb at 1600 if it only occurs on a dyno?

Our torque curves fall when dyno testing.  From the time I start to go, to the time I let off, I'm never under 2500 rpm.  A stock truck will stay 2000 and up at full tilt.

So while on paper, torque is required for HP, for me it has no meaning, raising the peak torque number will not make me go faster.  AFAIK, all it does is break things.  Keep torque low.  Keep HP high.

Now in my gas cars, peak torque IS in the racing RPM area.  But I tune for max HP area under the curve.  Peak is nice for bragging rights, but having a wide powerband is better.

Example, I dyno'd our LS1 Camaro while stock.  Found out that peak HP occurred at 6250 rpm, which is where the factory rev limiter is.  By just removing the rev limiter, and shifting at 6600, the car went a bunch quicker.  I didn't increase the torque, or the peak HP.  All I increased was the area under the curve.

Offline manta22

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Re: H/P Torque Comparrison question
« Reply #14 on: June 30, 2012, 08:38:21 PM »
"Yet the brochures for all the diesel pickups scream, MORE TORQUE!!!!  Really    Exactly what good is 605ftlb at 1600 if it only occurs on a dyno?"

It makes a huge difference when you are towing your trailer uphill.

Regards, Neil  Tucson, AZ

Regards, Neil  Tucson, AZ