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Old 06-10-2003, 04:02 PM   #1 (permalink)
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Interpolated flywheel horsepower?

Can someone please explain the following excerpt to me. This is from an ad on a website for an aftermarket mod, let's all just leave it at that in terms of what/whose product it is. But am I misunderstanding the concept of RWHP and how it relates to flywheel HP. I thought that on 2 cars with basically the same drivetrain setup, such as on the regular C5 and the Z06, if both cars are in a 1:1 gear that the drivetrain loss should be basically equal between the 2. That somewhat constant drivetrain loss % could be used to interpolate the flywheel HP once the RWHP is measured via chassis dyno.

"Our LS1 made more flywheel horsepower than our stock 2002 Z06 today. How? ..............system* with our new......... , made 324.7 (304.6 stock) Rear Wheel HP and 333.3 ft. lbs. torque(311 ft. lbs stock). Interpolated* flywheel HP and torque= 416.3 HP and 427 ft. lbs. at the flywheel! Our stock 2002 Z06 dynoed at 350.8 RWHP which translates to 412.7 HP at the flywheel. Click on Dyno above for more information. 20.1 RWHP and 22.3 RWT over stock! (Dyno available upon request)"

How does 324.7 RWHP on the LS1 translate to 416.3 flywheel HP, but the Z06's 350.8 RWHP only translates to 412.7 flywheel HP? Am I confused or missing something obvious?
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Old 06-10-2003, 04:46 PM   #2 (permalink)
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Okay, upon further review it appears that the LS1 was an automatic so it does have a higher loss %. Still, are you all reading this the same way that I am? Is this company claiming that with just the addition of this single mod your stock LS1 motor will be stronger than a stock LS6? I'm having a hard time swallowing this even though the comparison stated is an auto vs. a manual.
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Old 06-10-2003, 06:24 PM   #3 (permalink)
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Well - without giving away the product or the company - all I can say is Google is an amazing tool... :D

The first hit that "Our LS1 made more flywheel horsepower than our stock 2002 Z06 today" returns happens to be a mod for LS1 motors... :P :cool:

LOL

Enjoy...

-Paul *stirring the pot* K

p.s. Even though it could be construed as "Ricey" - I like the look of this C5...

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Old 06-10-2003, 06:44 PM   #4 (permalink)
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% of lost HP from flywheel to rear wheels is a flawed formula IMO.
If a car dyno's 350 HP at 6500 RPM at rear wheels and makes 400 HP at 6500 RPM at flywheel there is a loss of 50 HP. That is the amount of HP to turn all the parts at 6500 RPM. If you add a different intake and same car now makes 370 HP at 6500 RPM at rear wheels it has gained 20 HP at rear wheels. Since we know that at 6500 RPM it takes 50 HP to turn all the parts flywheel has only increased same 20 HP. % is now different. Exceptions to this are mods that require HP to operate. Ie. supercharger uses HP to operate but again % does not work. Only way to know flywheel HP is to dyno at flywheel.
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Old 06-10-2003, 06:54 PM   #5 (permalink)
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Quote:
% of lost HP from flywheel to rear wheels is a flawed formula IMO.
That is the way I look at it also. If you go by percentage, then the more HP you add to the motor the more loss through the drive train. When in fact, it should only take X amount of HP to turn all the parts no matter what the HP is at the flywheel.

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Old 06-10-2003, 07:21 PM   #6 (permalink)
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The math doesn't add up. Something sure looks bogus

Dynamometers come in two basic flavors, "Chassis" and "Engine".

An Engine dyno usually measures the HP of the bare motor, no accessories or driveline. You have to take the engine out of the car to put it on an Engine dyno. Engine dynamometers measure the raw HP the motor produces at the Flywheel.

A Chassis dyno measures the HP the car puts down at the rear wheels. This HP is the engine HP minus the losses for the driveline and engine accessories (alternator, water pump, PS pump, etc.).

The losses associated with the driveline and accessories are considered to be "constant", they don't vary with engine RPM. This isn't quite true, but it makes the math easier if you treat it this way, so everybody does.

If you know how much power is required to turn the accessories and driveline, you know what the losses are, and you can compute the raw power that the motor is making by measuring the HP at the rear wheels and "compensating" for those losses.

While the accessories on all Corvettes are the same, the drivelines are not. The Automatic transmission wastes more power than the Manual because (real technical jargon here) it's got more "stuff" that has to be turned. That extra stuff creates more drag, and requires more power to overcome.

In addition, the power consumed by the accessories on an LS6 is the same as the power consumed by those accessories on an LS1. Since the LS1 makes less HP, the power lost to the running gear is a larger percentage of the overall power produced by the motor. Example: if it takes 40HP to turn all the running gear, that's about 10% of the LS6's 405HP, but it's 11% of the LS1's 350.

No one seems to know what the actual accessory and driveline losses are for a Corvette. I've seen numbers ranging from as low as 12% to as high as 15%. They're different for the LS1 and LS6, but here's the key... when you increase the Engine HP with aftermarket Mods, the running gear power loss stays the same (it's 40HP whether the engine makes 200HP or 500HP). This means that as the engine HP increases, the percentage loss given up to the driveline and accessories decreases.

In the Ad you quote, the modified LS1 made 324HP, which they "interpolated" to 416HP at the Flywheel. Simple math says they're assuming a 22% loss! (1-324/416=0.22). That's kind of generous if you ask me. Consider... If their "Stock" 304RWHP rating is accurate, and we believe GM's numbers and the LS1 makes 350HP at the Flywheel, the actual loss in this "stock" LS1 Corvette is only 13%. (1-304/350=0.13%). Accordingly, their actual "interpolated" Flywheel rating should have been 324/(1-0.13), or a much lower (but still respectable) 372HP.

Assuming you copied their Ad correctly, by their own admission their Mods are good for 20HP. If the stock LS1 is 350HP Gross, that's a 370HP Gross when you add 20HP of Mods.

Their estimated 416RWHP is clearly in error.

Finally, the Z06, which made 350HP at the rear wheels, is "interpolated" to 412HP at the Flywheel. This says the assumed losses are about 15% for the Z06. The Z's losses actually *should* be a smaller percentage. But curiously, if you assume the stock LS1 made 304HP, and that's actually a Gross of 350HP, the LS1 has about a 13% loss. If the Z06 makes 350HP, and that's actually a Gross of 405HP, the LS6 has about a 14% loss.

Since the LS1 had an Automatic transmission, and it has lower base HP than the LS6, the LS1 has to have *greater* losses than the LS6, not less.

So, even their "stock" Dyno numbers don't add up.

That makes their assumed 22% loss figure, and 416HP Gross estimate *MIGHTY* fishy.
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Old 06-11-2003, 06:41 AM   #7 (permalink)
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Thanks for the feedback, I was hoping that I was not the only one that thought the claim and data to back it up seemed a little odd/off.
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Old 06-11-2003, 08:02 AM   #8 (permalink)
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FWIW, while getting my BG delivery, a GM engineering told me to figure a 13% drivetrain loss for a stock Z06. As always, vendor marketing "numbers" are to be viewed with caution.
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Old 07-04-2003, 04:14 PM   #9 (permalink)
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Quote:
Originally posted by Dave
% of lost HP from flywheel to rear wheels is a flawed formula IMO.
If a car dyno's 350 HP at 6500 RPM at rear wheels and makes 400 HP at 6500 RPM at flywheel there is a loss of 50 HP. That is the amount of HP to turn all the parts at 6500 RPM. If you add a different intake and same car now makes 370 HP at 6500 RPM at rear wheels it has gained 20 HP at rear wheels. Since we know that at 6500 RPM it takes 50 HP to turn all the parts flywheel has only increased same 20 HP. % is now different. Exceptions to this are mods that require HP to operate. Ie. supercharger uses HP to operate but again % does not work. Only way to know flywheel HP is to dyno at flywheel.
Well, your thinking is flawed ... this is why. From what you say in your example above, it should take 50 HP to turn all the parts behind the flywheel at 6500 rpms. So does this mean it still holds true that it takes 50 HP to turn all this stuff when you're at 6500 rpms with a cracked throttle (i.e., you have the rpms/speed, but NOT the HP going through the driveline)? The frictional HP losses inside a tranny or differential is a function of the HP (ie, load) going through them. There is a lot less HP loss through the driveline if you were coasting at say 100 mph then there would be if you were pulling a trailer at WOT at peak HP at 100 mph. In which case do you think the tranny and differential would get the hottest? If I understand what you're saying, it shouldn't matter since they are both running at the same speed (100 mph but with different loads) ... but that's not the case. :D The one with max HP going through it is making more friction and sucking more HP from the rear wheels and also turning all that extra HP loss and friction into heat.
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Old 07-04-2003, 04:28 PM   #10 (permalink)
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Here's some info from Lingenfelter on this subject. It's talking about ZR-1's, but the concept is the same.



Lingenfelter Performance Engineering, Inc.

--------------------------------------------------------------------------------
March / April 1996
Race season is well underway and the shop is as busy as ever. The engine dynamometer has been especially busy with the drag race engines - including our own - as well as the street engines and R&D projects. Our Dynojet chassis dynamometer continues to get more and more use as well. We have now chassis dynamometer tested many stock and modified ZR-1 Corvettes along with a variety of other cars and trucks.

As part of this testing, we have been able to measure many different variables that effect rear wheel horsepower. One such variable that we have been able to quantify is the horsepower losses due to elevated vehicle operating temperatures. The LT5 engine seems especially sensitive to this. This highlights the need to cool the intake manifold and get the water temperatures down between drag runs. We have also been able to measure and quantify the losses between an engine dynamometer test run and a chassis dynamometer run of the same engine. Even when an engine is run on the engine dynamometer with all of the accessories and the catalytic converters (the way we usually run most engines), you will usually see losses of between 15% and 20% between the engine dynamometer and the chassis dynamometer. These losses are due to the additional backpressure of a complete exhaust system and the frictional losses in the transmission, differential and tires. We have even been able to measure the differences in friction due to changes in tire pressure - in one case a customer had left his rear tires roughly 15 psi low after leaving the dragstrip and then came to the shop. We chassis dynamometer tested his vehicle with the low tire pressure and then brought the tires up to proper street pressures and gained close to 15 horsepower at the rear wheels.

The Dynojet chassis dynamometer is an inertia type chassis dynamometer. This means that it calculates horsepower and torque based on how quickly a given inertia - in this case, a set of rolls of given mass and dimensions - is accelerated. The length of time it takes to accelerate from one rpm level to the next is the sweep time and the rate that you accelerate from one rpm to the next is the sweep speed or sweep rate. Because it takes more power to accelerate the mass faster, you will see lower horsepower figures when a car is tested in a lower gear (1st gear for example) then when it is tested in a higher gear (3rd gear for example). This is because at higher speeds and higher gears, it take longer to accelerate from one rpm level to another (for example, 2000 rpm to 6000 rpm). This remains true until the gains in horsepower from increased sweep time are offset by the increased frictional losses of the transmission, differential and tires. As speeds increase, the frictional losses in the transmission, differential and tires increase. The higher the horsepower of the car, the faster it will accelerate the rolls and the higher the speed (and therefore the higher the gear) will need to be to get the sweep time long enough to give an accurate reading. Because the Dynojet chassis dynamometer is an inertia type chassis dynamometer it does not allow you to perform fixed rpm or step type horsepower tests - you can not hold the vehicle at a given rpm or speed and check the horsepower level. Despite this limitation, the inertia type dynamometers give you a very accurate measurement of what the vehicle sees in real world situations. An inertia type dynamometer will show the effects of reduced driveline inertia (such as lightweight flywheels, driveshafts and wheels) while a steady state test does not show these improvements.

Because the Dynojet chassis dynamometer uses one large diameter roll per wheel, overheating the tires and the tendency of cars to try and jump off the rolls is not a concern. This design also means that the cars do not need to be loaded down against the rolls - further reducing tire heat build up and increased frictional losses through the tires. This means that the risk of tires exploding from too much heat and load is virtually nonexistent.

In our continued effort to offer more horsepower for the LT5 engine - we will soon be offering larger, lightweight stainless steel valves for the LT5 engine packages. The larger valves will offer improved airflow and the reduced mass will provide higher rpm capability to the LT5 engine. Keep an eye on this Shoptalk section to get more information on the improvements provided by the larger valves as well as from the titanium rods mentioned in the last issue. Many other improvements for the Corvette and the LT5 engine are also in the works.

For those ZR-1 owners that would like to upgrade to the newer 1994 through 1995 ZR-1 wheels or want an extra set of wheels, we have available a limited number of the GM 5-spoke ZR-1 wheels. A set of the 17 x 9.5" front and 17 x 11" rear wheels sells for $1099 while a set of four of the 17 x 9.5" wheels (for 1988 through 1996 Corvettes other than the ZR-1) sells for $1079. We also offer polishing and powder coating for $995 a set.

In our continued brake package improvements, we will soon be offering a brake package designed to bridge the gap between the stock ZR-1 brakes and our complete 13.5" Alcon brake package. The new packages will keep your stock 13" rotors but use the 4 piston Alcon calipers. These calipers offer increased stiffness over the stock PBR two piston floating calipers and should provide better heat dissipation and pad wear. You will be able to later upgrade to the 13.5" two piece rotors without having to change calipers. These Alcon caliper/13" Corvette rotor packages are also available for the Camaro, Firebird and Impala SS as well. The Alcon calipers are now also available powder coated. We now also offer the complete line of Performance Friction "-4" and "Z" compound street pads along with the Performance Friction racing compound pads.

With the weather finally starting to look better and spring already here (and summer not far away), it is time to start enjoying our cars again.
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