Estimating RWHP from FWPH?

Ron Earp

Admin
I bet the difference it makes can't be measured on a dyno.....and, that was proven a couple of years ago when a magazine ran about 4-6 types, including the high and mighty Redline Synthetic, on a chassis dyno with no difference.
 
Ron

A credible MOPAR magazine did a back to back dyno
test on a healthy Road Runner and picked up (6) RWHP.
They switched both the tranny and rear end fluid to Redline.
But that Hemi trans/Dana rear is a lot larger/heavier
than one of our transaxles. So I would think in a GT40
the gain would be even smaller.

MikeD
 

Ron Earp

Admin
Saw the same thing on the Mustang but no gains. I imagine if there is a difference it is below the statistical variability of a dyno.
 
DynoJet's measure power by timing how quickly you can accelerate the drum. The drive train losses are the ammount of power it takes to accelerate your drive train, including a big set of wheels. There is your percentage loss, without looking for where all of the heat is going. The faster you accelerate the wheels and tires, the more power is stored in their kinetic energy.

If you want to see a gain on a chassis dyno, try using smaller, lighter wheels and tires.
 
Very true. A flywheel dyno measures torque directly and is slightly influenced by inertia. A chassis dyno can be heavily influenced by inertia. A way to think of it is a chassis dyno shows what HP you can put to the ground. If you choose relatively heavy wheels and tires your car will show less HP and accelerate and brake slower, etc.
 

Howard Jones

Supporter
So I guess what we are saying is since HP is a mesurement of changing the speed of a given objects mass over a given span of time then the heavy tire/wheel combo can really eat up HP while the engine in increasing in rpm during a dyno run.

But why would the tire/wheel weight have a influence on peak power after the engine has reached a given max RPM.

I guess I have always looked at dyno sheets that say somthing like 400Hp@6000RPM and believed that the motor makes 400hp when the engine is running at 6000RPM. BUT really thr sheet is saying that it took 400HP to get the engine to 6000RPM in a given time @ a sontant load.

Is this right?
 
Most rear wheel dynos do not hold the engine at a particular RPM and measure torque. They allow a sweep and look at resonse time to spin up a drum. Some models have a disipation source mounted to the drums and can hold the engine at a particular RPM and look at torque. This is rare. The engine dyno do measure torque directly and also do a sweep. However the mass that could give false readings of "engine" HP during a sweep are small (e.g. flywheel inertia, etc.).
 
Here are a couple of numbers to chew on. Recently I dynoed my Cobra with exact same setup and components on the same dyno with two different motors. the only drive accessories were water pump and alt. the first motor was a 428 fe which made 468 at the crank and 354 at the rear wheels. roughly a 24% loss. after losing the bottom end of the motor i had a 427 built. it made 649 at the crank and 511 at the rear wheels. now down to a 21% loss.

I have a ford 9" with 4 link. on the same dyno most cobras with independant rears were losing between 25-28%.

What does this mean, absolutely nothing but it is fun to talk about.

I am about to build a 351w for my 40, same builder, same dyno. give me some time and i will report back those findings. I am targeting 500 hp at the crank and about 375 at the rear wheels, assuming a 25% loss. we will see. looking for a balance between power and keeping the g50/50 alive.
 
Here is what my own personal experience with a variety of different types of racing machinery has shown: In the typical 60-early 70’s era muscle car (300+ flywheel horsepower, top-loader/T-10 style manual box, 9”/12-bolt real axle) the typical measured loss…assuming that all components are in good working order…is typically about 62-65 horsepower at 100 mph tire speed. Now, looking at current Nextel Cup and BGN cars which use the same basic driveline technology but with some very significant differences that I’ll outline in a moment, we usually see driveline losses of only around 40 horsepower…and at 180 mph tire speed. What makes the purpose-built drivelines so much more efficient? The biggest single factor is component stiffness. These gearboxes have much stronger cases and shafts, as well as drastically increased bearing surface area to maintain proper shaft and gear alignment under high load...along with stronger gear sets to maintain the most efficient gear-tooth contact pattern. Certainly, better lubrication and thermal management help out also, but the difference between the very best “qualifying only” gearbox lubricants we’ve ever tested and commonly available O.E.M. stuff is only about 4 horsepower (not minimizing the significance of that…most Nextel Cup crew chiefs would shoot their own mother for a 4 hp advantage the week before Daytona and smile while doing it).

While no; there is no logic or data to suggest a direct, linear percentage of flywheel power lost through the driveline, Howard’s example did get me thinking about what might happen actually happen if we DID couple a “known” 6000+ hp T/F engine to a street-type 4 spd box and rear axle and attempt to measure the losses. It wouldn’t surprise me at all if – in the fraction of a second that those components did actually transfer torque before failing catastrophically – we measured power losses approaching 200-300 hp due to deflection-induced internal friction.
 
You can measure drivetrain losses (and inertial losses) in rundown mode on a Dynojet where the drag of the drivetrain decelerates the roller from its peak speed (160 mph on a 248 Dynojet). The operator declutches immediately at the redline and allows the vehicle to coast so as to produce a negative horsepower figure. I ran a Dynojet equipped facility for some years, mainly testing Porsches; A 993 would show peak losses of 32 to 34 bhp (4th gear/7000+ rpm) which added to the wheel horsepower usually matched the exact flywheel figure quoted by Porsche themselves. By way of comparison my race GT350 with a RUG toplaoder and 9" rear had a peak loss of 43 bhp and that was the most lossy drivetrain I ever tested. The most powerfull Porsche I tested was an unrestricted GT2 with 840 bhp; transmission losses 36 bhp.


Colin Artus
 

Ian Clark

Supporter
We've just had a motor come off the dyno at 530hp.That was with a Holly 750 and 4>1 headers. What's it going to do with Webers and 180degree sequential exhaust? Each dyno is different, works differently and calibrates differently. Throw in temperature, humidity and barometric pressure and you've got a pile of variables.

Common sense says the hp loss can't be a constant proportion given that you could test the same car twice on the same dyno with the same driveline, change only the motors. Replacing a 300hp motor with a 500hp motor (flywheel) won't increase the hp loss by 50hp on the same driveline. Of course I could be full of @#$ too.

Seems to me it doesn't matter what the numbers are, it's how fast you can get down the road that counts. If you've got bags of power and can't keep it on the ground you're going nowhere fast and the power is wasted.

So this is where drag strips are so benifical. No B.S. it goes or it doesn't. How much power gets to the ground is easy to calculate or you can pony up for a G-Tech Pro.

Problem is the real numbers seldom come with big numbers and bragging rights. At the end of the day, there's just not a more exciting car to drive than a GT40 and that's what really matters:)
 
John Ross, the dyno you're referencing wouldn't happen to be the one at KCM in Dallas, would it? I ask because I just got the dyno results from KCM for the SBC in my Lister yesterday. I take them with a LARGE grain of salt because I had the engine dynoed almost three years ago, on a completely different dyno by someone else, but the results I got yesterday showed exactly a 25% loss to the rear wheels (542 on the engine dyno, 407 on the chassis dyno)...
 
Just finished tuning my car on a chasis dyno. We maxed out at 332hp at about 7000 RPM. It tested on the engine dyno at 460 ie about a 28% loss. Interestingly we used a carb set up(demon) for the engine dyno because my builder didn't have an engine dyno harness to run the Motec computer and fuel injectors. Perhaps some of the loss is due to the fuel injection instead of the drive train. Never the less the car is a hoot to drive. I had 318 chassis tested hp in my Cobra but would frequently spin the rear tires. With the GT40 it all hooks up and goes like a bat out of hell! /ubbthreads/images/graemlins/cool.gif
 

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An Audi 5n016. But it's not a fair comparison since we used a different carb/fuel injection set up.
 
The following is my question to the tech department at Superflow Dyno, and is followed by their reply.
Harold, Is the percentage of horsepower loss due to drivetrain friction
the same on a 500 hp engine as it is on a 300hp engine in the same car,
same drivetrain, same chassis dyno, etc.? By the same I mean, if it's 18%
on the 500 HP will it be 18% on the 300 HP. The 500HP & 300HP being engine
dyno numbers. Also, HP readings seem to be closer, chassis dyno to chassis
dyno than they are engine dyno to engine dyno. Does this have to do with
operator input etc.? Thanks, Al

Al,

This question is one that makes me have a good time in answering. It is of common interest to many and unfortunately, it is not addressed enough for folks to get a better understanding of the variables.

The loss across the drivetrain is not a fixed percent. It is a loss that is the same at the same speeds. I have attached some drivel that I have written on the subject. I hope that it is of some use to you.

Although operator influences are of some effect, most variations come from calibration differences and room conditions, exhaust back pressure or exhaust gas recirculation.
Normally speaking, good procedures will produce good results.

Numbers mean something, but only if you know where the numbers come from.

I had answered another question concerning the driveline losses in the following fashion and I think that it certainly applies in answering yours.

"I will attempt to answer them categorically and although I will try to be brief, I am generally not successful to that intention.

The issue of drivetrain losses happens to be a very complex problem and most people that are interested in these types of problems want rapid or simple answers, not complex ones. That is one reason why the fixed % loss (relative to power input) has been so popular over the years albeit an incorrect process of evaluation.

In the first example that I used, based on real circumstance measurements, the loss in the drivetrain was indeed 200+Hp. The major portion of the loss was a very "sloppy" torque converter that had a high percentage of "slip". Racing type torque converters are a mystery in themselves and I do not know how to address that issue other than with what we measured. The issue becomes even more confusing when testing with the converter "locked out" and the coupling becomes just a fluid coupling (according to what has been reported to me from our Customers). What I neglected to state (in a more clear fashion) was that the loss via the drivetrain should be somewhat fixed relative to speed (ie: varies with speed) and temperature reference points those losses are represented by a curve that is not linear with speed.

Almost everything in the drivetrain is a basic power resistor, absorber, or extractor. F=uN is certainly a common way to evaluate simple sliding friction, but as an example, if applied to the tires of a racecar, the coefficient changes with temperature and loading through drivetrain (the tire patch is torque input sensitive) and racing tires can generate a coefficient that is in excess of unity and maximum adhesion is typically at 12-15%(on some as high as 18%) of tire slip (according to race tire manufacturers). The losses for the tires themselves vary for a number of reasons and are important issues when considering total driveline losses. Most racing gearboxes (manual) utilize straight cut gears and as such do not generate the thrust forces on the case that the helical or hypoid type gears generate, but the losses (windage and viscous drag) are a function of friction, speed, and temperature. Additionally, dragging brakes or changes in pinion angle are among the many variables and "power extractors" in the system. The engineers and technicians at the OTC here in Colorado Springs work with exactly the same challenges for the bicycles used in competition (EX: Lance Armstrong's bike used tires that were approx 5yrs old).

The 12 to 20Hp losses that I mentioned were for 4speed gearboxes, the cars were using 9" Ford drive axles, and the power varied on the three race cars that we tested in the excercise from a high of about 500Hp to a low of about 350Hp as I recall. The point that I was attempting to make in my comments was that the gearbox is an absorber and perhaps more of an absorber than choosing the correct gear ratio for the drive axle so the racecar could be raced in direct gear instead of 3d gear.

The power losses for the trucks that I cited were passing on comments from manufacturers, not testing that we had done. However, because the axle that we use in our AutoDyn product is a large industrial axle that is used on some large over-the-road trucks, we have good data on the losses on that type axle. We even found out why it is better to use a synthetic lubricant in the axle assembly. The lube volume is about 5 gallons and with a ratio of 3.9:1, the measured power to roll the system at 200mph is typically about 36+Hp. Standard lube causes substantially larger losses. Not all manufacturers measure the losses in their systems, but we measure both the losses and the inertia (including aero losses) of our chassis dynamometer systems we do not use estimates or calculate what the inertia of the system "should" be. We also provide a method of measuring tire slip by using a non-contact speed sensor to relate to the tire patch to roll interface.

Inputs shared from some professional race teams seems to indicate that Nextel Cup cars and Busch GN cars (an example of a good one) has a loss of about 50 - 60+Hp range at 200mph. That figure is drivetrain loss only and is not including any aero or complete rolling losses although it does include the drive wheels and tire patches. The same car tested at the same speeds with a lesser powered engine has virtually the same power loss in the drivetrain. The power losses plotted vs speed is not linear at all. The friction power that should be considered when evaluating power systems for a reciprocating engine is also not linear with rpm.

Most of the advanced level testing that goes on at some levels of motorsports these days has a very tight evaluation window of acceptance (typically a maximum of +/-1%) in all that they do. Although the same teams have CFD programs, they still rely on testing to refine their engine airflow designs and spend time testing in full-size wind tunnels as well. The basic analysis for driveline losses is applied via testing the engine(s) so that they have a BHp reference (FWHP) curve and when the installed power package is tested, they have the power at the tire patch reference (RWHP) curve. The delta between the two is a driveline loss curve for that particular vehicle. Various corrections are typically applied so that evaluations are done with some standard reference, but in general, comparisons are done in the same fashion."

Regards,
HB2
 
Hey Snakeeyes, How are you doing. I drool over your lister every time i visit KCM. To answer your question, yes it would be the same dyno Rob just put your car on.
 
Thanks for the kind words, John. I just picked it up last week, and it's nice to have it back in the garage. The fact that the tank is full of racing gas doesn't hurt--I started it a couple of times over the weekend just to enjoy the aroma of "race track perfume."

Rob at KCM told me that a 25% loss is fairly typical on their dyno for an IRS-equipped car. After seeing the dyno sheets I wonder if my overall loss was actually a bit smaller than that, because for some reason KCM only ran my car to 6000 rpm. When my engine was initially dynoed by the builder it made max power at 6500, and picked up about 20 hp between 6000 and 6500. Either way it would be a pretty insignificant difference.

What was astounding to me was how down on power my engine was, in a relative sense at least, when I brought the car in to KCM. The baseline pull only resulted in 326 hp! The air/fuel ratio was way out of whack and the engine was accordingly way, way too rich. By the time Rob was done he had "found" over 70 hp, which is nice. I've only had time to drive the car once since I brought it home, but with a curb weight of just under a ton (well, probably just over a ton now that the roll bar is installed), it's a screamer. Of course, the power just drives home the point that the car is woefully undertired. On to the next stage of the project...

Of interest to me, I had Rob dyno my car with open headers, since I've heard horror stories about the losses associated with many sidepipe setups. After re-jetting to compensate for the open pipes my engine only picked up about 13 hp, so I am satisfied that the mufflers in my sidepipes are acceptably efficient (phew--one less thing to spend money on).
 

Doc Watson

Lifetime Supporter
As far as I can see percentage loss is a bad indicator and should not be used. If we take 3 engines which all produce the following hp at say 6500rpm.... and assume that the loss is directly related to the speed of the transmition, then...

HP loss (HP) % loss
500 50 10.00%
400 50 12.50%
300 50 16.67%

At constant speed we would expect the loss to be the same (in this case 50hp, again a figure I've just invented to illustrate a point) we will also see that the percentage change varies from 10-16% due to different HP engines.

Of course there are loads of assumptions in there, but I'm trying to think of reasons why the losses in a drivetrain would increase with increased HP (and torque) and not just speed. Yes the drivetrain will accelerate up to the max rpm faster with a more powerful engine, possibly causing a difference compared to acceleration from a less powerful engine due to the speed at which oil can be ejected from between bearing surfaces and the rate at which the oil and case can dissapate heat. At a constant speed the losses would be the same for any hp engine. The difference would be how quickly those losses are reached. I bet if you got a 300 hp car and accelerated up the gears the gearbox would be cooler than it would in a 500 hp car, all other factors being the same. However if you slowly accelerated both cars the gearboxes would probably be the same temp wouldnt they?

So dont use percentage indicators for losses in drivetrains, note that Roy from RTB quoted a loss for a given speed no mention of HP or how quickly he got to 3800 rpm, im guessing he measured this when the transmission was in a steady state (i.e. had been running at constant speed for a while).

Taken to an extream... suppose you could accelerate the car to its max rpm in a very short time (which of course would require a higher HP engine) the speed at which the losses are created would produce localised 'hot spots' within the transmission, the oil and metal not having enough time to conduct the heat away from these hot areas, and local hot areas are not good for transmissions.

So I suppose I've talked myself around to the idea that at constant engine speed, drivetrain losses are constant for any HP engine, however when accelerating the faster the acceleration the greater these losses would get while accelerating. When the car reaches a constant speed the losses would then drop down and would again be independant of engine hp.


Im just thinking aloud here so please feel free to pick holes in my argument.

Doc Watson
 
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