chassis (rear wheels) vs bench (fly wheel) HP

[Jerome P.]

In general what is the measured drop (%) in HP of an engine when dyno'd on a bench dyno vs a chassis dyno? Assuming the only difference is on the chassis dyno run the engine had a standard factory exhaust system.

Also what is % loss of HP for various accessories, i.e. PS, PB, A/C, etc?

Another way to put it is: Advertised HP vs actual rear wheel HP

[Joe R.]

Re: chassis (rear wheels) vs bench (fly wheel) HP

In general what is the measured drop (%) in HP of an engine when dyno'd on a bench dyno vs a chassis dyno? Assuming the only difference is on the chassis dyno run the engine had a standard factory exhaust system.

Also what is % loss of HP for various accessories, i.e. PS, PB, A/C, etc?

Another way to put it is: Advertised HP vs actual rear wheel HP

Jerome,

I assume by bench dyno you mean engine dyno. I believe the difference is roughly 20 %. Hopefully Duke will chime in with the real scoop. Here's one thread that has better info than I can provide:

https://www.forums.ncrs.org/showthread.php?t=71413&referrerid=2667&uid=6530

Joe

[Terry D.]

Re: chassis (rear wheels) vs bench (fly wheel) HP

Jerome
If memory serves me there was a post about this a while back. I think the difference is somewhere between 17 to 20 percent, dependind on accessories. Hopefully someone with more info will chime in.. Have you searched the archives?
Terry

[Mike F.]

Re: chassis (rear wheels) vs bench (fly wheel) HP

Lots of variables to take into account, it will be different from car to car.

A couple of examples,

My 91 Coupe, L98, auto, 3:08, was rated at 245/250HP (depending what you read) net (in car, flywheel horsepower). It made 232hp on a dynojet at the rear wheels. Thats only a 6% or 7% loss.
My opinion is that as the L98 was improved over the years it made more than 250HP and Chevy underated it because the new LT1 was coming out and a 50HP bump made for better advertising. (My best L98 ET 13:80@100mph)

My 70 Couple, L46, manual, 3:70 was rated at 350HP gross (engine dyno, no accessories). The cam and pistons have been replaced (discussed here) https://www.forums.ncrs.org/showthre...87967&uid=6179 . It made 213HP on a dynojet.

If you use a 20%(auto) or 15%(manual) loss for drivetrain and accessories my L98 made 278HP net and my detuned L46 makes 245HP net.

Mike

[Michael W.]

Re: chassis (rear wheels) vs bench (fly wheel) HP

Jerry-

there's four commonly used HP figures-

Gross HP- this is measured on an engine dyno at the flywheel and is without normal accessories and typical in-car intake and exhaust systems. Many crate engine retailers and speed shops/engine builders still use this measurement. GM used it on cars up to the early '70s.

Net HP or SAE net- still on an engine dyno at the flywheel but with all accessories and intake/exhaust systems in place. This measurement standard became the industry norm in place of gross Hp mentioned above and is still in place on today's cars. Comparing gross with net results on the same engine can reveal a ~25% difference.

Rear wheel HP- done on a chassis dyno and reflects the above net HP minus drivetrain losses.

Advertised HP- whatever the marketing folks think they can get away with. or or are told to say. Examples of engine being under-rated or over-rated are easy to find especially from the 60s.

A practice used by many to deduct a fixed percentage for a given drive train type or accessory can lead to errors. If an OEM clutch fan when locked up absorbs (ex.) 8 HP at engine red line, that's a 13.2% penalty for a '75 L48. If an L88 truly made 500HP, then using thje same fixed percentage would translate to a 67.5HP used to drive the fan.

Aren't stats wonderful?

[Duke W.]

Re: chassis (rear wheels) vs bench (fly wheel) HP

As a general rule, the driveline/tire efficiency of a front engine rear drive setup is 85 percent for a direct drive manual transmission in top gear and 80 percent for a non-lockup torque converter automatic.

So 100 at the flywheel would be 85 at the rear wheels with the manual and 80 for an auto.

The difference between SAE gross and SAE net on a lab dyno has two primary components. One is the air density correction. SAE gross uses "Standard Temperature, Pressure" (STP) correction otherwise known as standard sea level conditions, which are 29.92", 59 deg. F, dry air.

SAE net air density is 29.38", 77 deg. F. dry air. The difference between these two densities is about 4.5 percent, which results in all SAE net data being about 4.5 percent lower than SAE gross due to air density correction, alone.

Chassis dyno data can be displayed uncorrected, with one of the above two common correction conventions or others, such as DIN and JIS. I often see dyno results compared on the internet when it's clear that they use different correction conventions. It is best to use SAE net correction since dividing these numbers by the driveline efficiency gives a pretty good estimate of SAE net at the flywheel, which can be compared to modern cars.

Beyond air density correction SAE net includes engine accessory and exhaust system losses, which is the most important element. A high flow exhaust system will minimize the pumping loss, which can be signficant.

The best OE exhaust system is the 2.5" system used on C2s. The worst is the single catalyst system from the seventies, which is essentially a single exhaust system. Dave McClellan in his book stated that this restrictive system added "half an atmosphere'" (about 7.5 psi) over the previous full dual 2.5" inch system, which I estimate at about 3 psi at the 300 HP rear wheel power level. This is HUGE and is why one can get little gain from internal engine mods unless the exhaust system is improved, first. Additional power means greater exhaust flow and the internal power increases are eaten up by pumping loss.

It is often stated that the difference between net and gross is 20 percent, including the lower SAE net air density, but it varies greatly among cars depending on their accessory configuration and exhaust system design.

Data I have that includes the same massaged SHP small block that was tested on both a lab dyno (SAE gross) with manifolds and on a chassis dyno indicate that SAE net is 89 percent of SAE gross, and nearly half this difference the air density correction.

This lab tested engine had no accessories other than the coolant pump and alternator with no electrical load. I don't have any data on individual accessories, however, when doing a chassis dyno test it is critical to keep the radiator exit air temperature below fan clutch tightening level. A fully tightened fan clutch can consume 15 lb-ft peak torque and 10 peak HP. High flow external fans and even spritzing down the radiator and fan clutch thermostat with a garden sprayer is important.

See the Fall 2010 Corvette Restorer for more discussion. John McRae's "Special 300 HP" engine made about 280 lb-ft of torque and 220 HP on the chassis dyno test with SAE air density correction. Using the above factors, SAE net was 329/258 and gross was 369/289, so it actually made better than the rated 360 lb-ft, but fell a little short of advertised peak power.

If you put a typical Flint build 327/300 on a lab dyno and test to SAE gross it would make around 250 SAE gross HP. They typically make low to mid 190s at the rear wheels with SAE air density correction, which is about 225 SAE net at the flywheel.

The OE ratings in that era were grossly inflated, and you have to be careful when comparinig different test data to be sure you aren't comparing apples and oranges. In order to make valid conversions, one needs data for the specific configuartion and the more data and the better they correlate, the better the conversion factor accuracy.

Duke