Horsepower questions

[blowngasket]

First let me say that I'm not a professional. I'm more of a hobbiest, but that wasn't on the list when I signed up, so you'll have to excuse my lack of knowledge.

1) My first question is how horespower (and torque) are measured, exactly. I don't know exactly how a dyno works, but to calculate power I would think that you would measure how fast the engine can accelerate a known mass and in what amount of time it takes to do so. If this is this correct, what's the formula here? The next thing is torque. It's measured in ft/lbs, so I'm thinking it is a measurement of force? If that were true and I have an engine that produces 300ft/lbs of torque, is that to say that it produces a force of 300 lbs one foot away from the center of rotation? I don't really know, but that doesn't seem like very much to me. Also, I have a friend who thinks he read somewhere that all horsepower and torque curves cross at 5,250 rpm because of the forumla's used to compute the two. My logic arugment for this was what about engines that don't rev to 5250, such as diesels. Is there any truth to this, or is it just an invalid conclusion based similarities in observed data?

2) The next question is where it is measured. There seems to be something about gross and net horsepower measurements. I understand that before 1971? they measured it terms of gross and after that it was net. Gross being without accessories and net being with all the accesories (such as alternator, water pump, fan etc). So, are these measurements taken at the flywheel, output shaft of the transmission, at the wheels? If the measurements are taken at the same place, what is an average loss throughout the drivetrain system? It seems to me that both of these measurements would be greatly affected by gearing. So, if it's measured anywhere other than the flywheel how is that considered in any formula?

Lastly, it seems (through my experimentation) that an engine produces more torque if the exhaust is restricted or has more back pressure. However, I'm at a complete loss to figure this out. I would think that the easier the piston can move up during the exhaust stroke, the less loss there is and therefore would produce more torque and horsepower. What's up with that?

I know this is a lot to answer so I'm greatful to anyone willing to take some time to explain what might be the "basics" to you.

Thanks in advance.

 

[MaxRaceSoftware]

Hp = ( Torque * RPM ) / 5252

or HP = Torque * RPM * .000190404

Torque = ( HP * 5252 ) / RPM

RPM = ( HP * 5252 ) / Torque

Study the relationships ..try different values and you
will gain learning experience

Your friend was correct...HP and Torque Numbers are equal and cross each other at 5252 RPM due to equation
.....its also the total AVERAGE of HP or Torque over
an RPM histogram of the actual acceleration run that counts

Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[MaxRaceSoftware]

Lastly, it seems (through my experimentation) that an engine produces more torque if the exhaust is restricted or has more back pressure. However, I'm at a complete loss to figure this out. I would think that the easier the piston can move up during the exhaust stroke, the less loss there is and therefore would produce more torque and horsepower. What's up with that?
============================================
How are you measuring "BackPressure" ???
and "Exhaust Restriction" ??? Could you clarify a little more what these terms mean to you and how you are measuring them in your experiments ??
....Then I might be able to answer your questions

Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[blowngasket]

Well, I haven't actually measured the back pressure but can induce it's a higher or lower based on certain situations. For example, when there is an exhaust leak or a car is run straight off the header there seems to be a noticable decrease in low end torque. Conversely, if the exhaust is restricted, like through a bent pipe, it seems that there is more torque. Perhaps these are incorrect observations, but I don't think it was imagined.

Also, the equations you posted were interesting, but don't really answer the heart of the question. In the equations, HP or torque were both defined in terms of each other. So how does a dynomometer measure either one? I can see however that if RPM equals 5252, then HP and torque would equal each other.

 

[blowngasket]

Thanks for the response by the way.

 

[MaxRaceSoftware]

Depending on where the "leak" occurs, it can change
exhaust tuning and exhaust gas temperature (EGTs)
and influence/change wave timing during overlap period
...you might be nameing this "BackPressure" , but it might not be an increase in exhaust system pressure your are experiencing but a positive-pressure wave occuring during the overlap period..reducing torque/hp at that rpm range

The Bent_Pipe .... if you bend a pipe a little one way, the
diameter increases a little the other plane, the net effect is the pipe still has the same area, different shape.
If its badly crushed or bent, it will create a flow-restriction.

What you might be "seeing/noticing" is that your primary and/or collector lengths or even your total exhaust system lengths and diameters are wrong for the RPM-Range you are interested in using ???

1=The "lower" the operating RPM-Range , the more the primary and collector pipe diameters/lengths become critical

2=The "lower" the Compression Ratio the engine has , the more Torque and HP LOSS can occur from improper primary and collector diameters/lengths..because of the increased "clearance-volume" above the piston and its relationship to total engine swept volume

3=The "lower" the Engine RPM-Range and the Greater the "overlap" period,...the greater the time exists for
exhaust and intake reversion, flow-reversals,..due to improper pipe diameters/lengths

Dyno====> measures Torque by applying a "load" from a water-brake , controlled by a servo-valve, which in turn, controls the water discharge rate out of the water brake .
The operator can rotate the water-brake discharge servo-
valve manually or by the dyno's computer during a dyno test

An strain-gauge sends voltage to dyno computer that then
is converted into Torque. The front half of the water brake houses an RPM counter/pickup...so along with Torque readin and RPM reading ..you can now calculate HP from the equation
..that will be Raw HP from raw, uncorrected Torque...
then thru HP weather correction equation you get
corrected HP, Torque, etc readings

I don't know how this message board REPLY's box
formats sentences..but I can try to post a dyno test's output and let you see what it looks like


Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[blowngasket]

Could you elaborate on what exactly "wave timing during overlap period" is? How do "positive pressure waves" affect HP/torque in terms of net forces on the piston? It seems that if it were to affect the HP/torque, it would have to be creating a force on the pistons. If you can easily post a dyno test's output I would be interested in seeing it. Again, thanks for taking the time to respond.

 

[MaxRaceSoftware]

I tried posting a dyno test , but being 9 columns in width
it didn't format correctly...would have to change font size
Will post portion later on "Exhaust wave action during overlap" effects from 2 different dyno tests so you can see
effects.

During the overlap period, both the intake and exhaust
systems are "connected" ....you want a negative-pressure
wave's peak psi dead-centered on the overlap-period, during
the most important engine RPM-Range you've determined your
type of racing requires.

SuperFlow dyno includes an "Air-Turbine" that looks like
a tall velocity-stack , but houses a white plastic fan
on jeweled-bearings thats attached to an rpm-counter,
it has honeycomb air-straightening layers above and below
the fan...there are different diameter air turbines to fit different size carbs...during a dyno test, you put the air turbine on your carb...then you can measure the Volumetric Efficiency of the engine, the CFM amount the engine is using
at different RPMs , and along with fuel-flow data, you can determine Air/Fuel Ratios for jetting, and also can determine Brake Specific Air Consumption ( BSAC ) which is
Air in Lbs./Hour divided by raw uncorrected HP
Brake Specific Fuel Consumption ( BSFC ) which is
Fuel in Lbs./Hour consumed divided by raw HP

During a dyno test, if an engine has the wrong length
header collector...that is causing a Positive-Pressure
acoustical wave to occur at valve overlap period.....
the effects of this wave can easily be seen in the
Volumetric Efficiency % data column ...you will see
Ve% percent "too-low" or the "Ve % RATE" too-low at
low rpm of dyno test then Ve% RATE will climb quickly
or you will see Ve% data climb the fall or stay the same
for a couple hundred RPMs...thats a definite indication
of exhaust pipe length being incorrect ( also diameter
might be incorrect). Basically something like this ;
RPM Ve%
3000 85.0
3100 86.0
3200 87.0
3300 88.0
3400 89.0 at 3400 and below=effects of wrong pipe length
3500 94.0 <--notice large Ve jump from 3400 to 3500 RPM
3600 96.0 this tells me there is a positive pressure
3700 97.0 wave occuring below 3500 RPMs
3800 98.0
..... ....
5000 93.0

I could go in the dyno room and add about 6 to 10 inches
of collector length and pickup the Ve % , Torque , HP ,BSFC,
BSAC, from 3000 to 3500 RPMs and even higher in RPM range
Depending on how low RPM dyno testing was done , the Compression Ratio, the overlap-period,....will determine how much more Torque and HP you can gain with proper lengths...I've seen as much as 60+HP with some low-compression , large overlap period roller cam, SuperStock
engines , especially below 4500 RPMs.





Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[MaxRaceSoftware]

Think about this ...
a typical engine under load during a dyno test
will show 1000 to 1500 deg F exhaust gas temperature
exiting exhaust port about 1 to 3 inches away from
the head's exhaust port flange inside the header tube
....during the overlap period, when both the intake
and exhaust systems are connected and communicate with each other.... Why doesn't the hi-temp exhaust gas ignite
the incoming intake air/fuel mixture ???
The exhaust gases are hot enough.

Even under hi-vacuum part throttle cruising conditions with
hi exhaust system pressure ...there is still enough
correct exhaust gas momentum out into the exhaust port
to prevent incoming air/fuel mixture from igniting
during overlap (along with acceptable iginition timing) !!!

What you &quot;see&quot; during exhaust gas reversion occuring from
the wrong pipe length is a positive pressure acoustical wave
causing some of the low-temp exhaust gases to
be pushed back up the header pipe ,,sometimes all the way
to the intake manifold's plenum chamber , killing HP and torque at the rpms that it is occuring

At the least, the postive pressure wave will disrupt
intake charge momentum and tuning...it will reach the
air turbine fan and reduce Ve % readings....
the pulsing been so severe on a few engines I've tested
that the positive pressure wave has actually pushed
the honeycomb top layer out of the air turbine

If you take apart a cylinder head that was already
dyno tested or run down a race course...look at the
exhaust bowl area and then at the intake bowl area
..the 2 should be day-and-night different appearing !!!
The intake bowl and entire port should be like the day it was ported without any traces of exhaust gas residue Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[GregLocock]

Nice work Larry.

Back to the original poster's other questions.

How a dyno works.

The old fashioned waterbrake is probably the easiest to understand. A large pump is attached to the flywheel of the engine. It is supported on the centreline by good bearings, and restrained from rotating by a lever arm with a force gauge in it. The back pressure on the pump is adjustable. to give a varying resistance. Therefore at a particular operating condition the torque is directly measured = force times lever arm, and the engine speed is known. SO

power = torque * speed* a fudge factor (by definition)

The fudge factor is dependent on the units involved.

Yoou are right 300 ft lbs is not very much, that's why cars need gearboxes.

Average loss in the drivetrain: Figure on 98% efficiency MAX for any one set of gears and in practice it is rather less. The overall drivetrain efficiency for a manual car is 92-95%. First and second will tend to be less efficient. Auto trans could be worth up to another 10% off that.
Cheers

Greg Locock

 

[blowngasket]

Thanks again for the posts guys. Anyone know about the difference between net and gross horsepower and where it's measured? Also, what figure, gross or net, is represented by manufacturers when they list the specs?

 

[fjbalaux]

1 hp is by definition the work needed to lift 33000 lbs at 1 foot/minute. When you measure the torque in feet lbs the work done for each revolution is 2*pi*1 (load*distance).
33000 /(2*pi) = 5252,11

Some of you guys seem to have a lot of experience with dynos. What do you think of inertia dynos?
Francois

 

[MaxRaceSoftware]

Some of you guys seem to have a lot of experience with dynos. What do you think of inertia dynos?
Francois
=======================================

It depends on the engine size CID and RPM, Torque/HP
the engine is making ?? as to dangers of inertia wheel diameter and weight you need

What would the dangers be involved with an inertia-type dyno
-VS- a water-brake dyno when it came to testing
a NHRA ProStocker at 1260 to 1280 HP output at 600 RPM/Sec acceleration rate at RPMs to 9300 ???
How practical would that be ??

Some of those racers are testing 600 to 750 CID engines at
1500+ HP at 8000 rpms , sometimes more HP with Nitrous
..how would you dyno those on inertia dyno ? Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

 

[MaxRaceSoftware]

Some of you guys seem to have a lot of experience with dynos. What do you think of inertia dynos?
Francois
=======================================

For small engines the inertia dyno is the practical way to go !!

But with a water-brake dyno such as a SuperFlow SF-901 computer dyno,
I can Steady-State test the engine
I can chose different RPM/Sec acceleration rates
I can see the effects of lightweight components like your inertia-dyno can

But an inertia dyno won't let me simulate a real-world engine as closely as water-brake dyno

Example, if i dyno test at a non-acceleration rate
(Steady-State)...Air/Fuel Ratio might showup as 12.5:1 for best power .......but the same engine at 600 RPM/SEC acceleration test ...would show 14:1 Air/Fuel ratio because
of the greater fuel's inertia

With computers hooked up to water-brakes ..you can have a bunch more variety of testing situations than an inertia-type dyno . Now days, some teams can simulate the entire race on the dyno . Larry Meaux (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers