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Regarding the hp on an engine dyno:
Perfect conditions, no alt. no pwr steering, no acc. at all, except water pump, typically open pipes.
chassis dyno has all the acc., trans, drive line gears wheels etc.
chassis dyno also see's rotating inertia, wheel weight, tire diameter, drive shaft etc.
So I have learned recently chassis dyno do corrective numbers to compensate for all of the above variables...netting higher numbers so customer are not disapointed....
Case in fact, a friend 69 roadrunner, original Hemi car, modified exhaust with headers, reworked two four barrels, mild cam increase, professinaly tune( these guys are good). He made with his 426 HEMI 310hp at the rear wheels at 2000' elevation. No corrective math,it is what it is to the rollers.
So, I read about peope putting 400+ to the tires, are these corrected numbers or are they getting that out of cleveland. the guy's doing non corrective numbers claim typical%24-26 depending on the trans. they dont play those #, just pure HP to the rollers.

Any thoughts?

I am curious what other chassis dyno guys do out there. I had my pondtiac engine dynoed, then chassis dynoed and was surprises, like all I thought I'd have more. but I did video it, kinda've cool, especially when nitrous is hit. I anyone wants to see it for kicks and giggles let me know and I will try to post it

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I'd like to see your video.

FWIW - I had my car dyno tuned. It was done on the kind of chassis dyno that bolts directly to the axles (wheels and tires are removed). My original stock 90,000 mile engine with headers, Performer intake, Holley Avenger 670 carb, and Mallory Unilite electronic ignition registered 250hp on the chassis dyno. But I don't know about any "corrective algorithms."

I think most chassis dyno's report 'to the rollers' numbers, at least the ones around here do. That said to make an 'apples to apples' comparison we all need a standard set of criteria to compare to and elevation is probably one of the biggest factors in that with 3% HP (generally accepted) loss per 1000 ft elevation. So at our local Carson dyno (~4,700 ft) I'd get about 15% less than someone at sea level, which I well believe as at Monterey last year my car was noticeably different. Some guys have to re-jet the carb to go.

We also have a guy here who ran 510HP at the rear wheels on the same dyno (i.e. at 4,700 ft), he can get away with 12:1 comp and pump gas at these elevations.

The drivetrain loss in Pantera is 19-20%.

The drivetrain loss in Pantera is 19-20%.

Hmmm, seems we covered this a year or two back and I remember the drivetrain loss as being 23%.

This was determined by several engine/chassis dynos on actual cars.

That is, a 400 FWHP engine would show 308 RWHP

.77 times 400 = 308

And using the RWHP, convert to FWHP by multiplying by 1.3

1.3 times 308 = 400.4

My observation Larry has been about 19%, I like to round it off to 20%.

Keep in mind, besides the losses in the transaxle, the air temp in the engine bay, the weight of the wheels & tires and the efficiency of the exhaust system will be three major contributors to the differences between the figures for engine dynos & chassis dynos. So I wouldn't personally expect the losses from one car to another to be consistent, I would expect some variations.

Yet all of the figures I've seen published in PI Mag since about 2000 have all been remarkably right around a 19% difference.

cowboy from hell
This is very interesting info for me. My friend who has a 71 cuda, 340 4spd, factory hot solid lifter cam (purple cam) holy 750, edlebrock RPM intake and aluminum heads, headers & free flow exhaust.....power tuned on the dyno by the shop, who are outstanding....266hp at the rear wheels.
Same concept on my friend 69 hemi roadrunner, upgrade cam, factory two fours, 2 1/8 headers etc., 4spd. 310hp at the rear wheels.
Ok, whats wrong with this picture, corrective numbers? this well known race shop say's the do no corrective numbers, they basically say who cares, it is what it is at the rear wheels.
Not to get to funky, but it is hard to imagine some of the rear wheel numbers out there, or is it just because our local, well respected, race tune etc. shop is a realistic number? I dunno....My Trans-am made 392hp at the rear tires on this same dyno, pretty much nothing not done to this engine less supercharging. 440cu., long H-beam rods, steel nitrated crank, studed top to bottom, race ported aluminum heads, roller cam, 950 race demon carb, 2" headers NASCAR 3 1/2" exhaust, BG fuel system, 11-1/2:1 compression, o-ringed block on, and on, and on...TKO600 5spd thru 3:73 12 bolt.
All this, and only 392 rwhp.
Now, that said, it without question spanks the living @@$%$#@ out of my friend Hemi road runner, a 70 hemi cuda, 67 GT500 etc.
Oh, 11.13 1/4 mile on street tires. 3655lbs.
392 rwhp.
So maybe someone can see my confusion, the dyno by the way is always kept up to snuff and certified.
I'm also at 2000ft elevation. All shift points were at 5500rpms.


P.S. 525rwhp when we hit the Nitrous and spun the tires off the dyno.

P.S.S. Be nice guys, keep the pontiac jokes to a minumum. You won't want to know how many clevelands, 428fe's, 460's and any stroked ford combo I spanked so far.
the weight of the wheels & tires

Okay, I'll bite. Why would this make a difference in the figures?

Air temp = a difference, check.
Exhaust = a difference, check.
Transaxle friction loss = a difference, check.

But how does wheel/tire weight affect chassis dyno horsepower? I can see it affecting how fast you get rpms up, but the actual horsepower?

enlightenment please...

Originally posted by #7260:
Pantera's don't have drivetrains, they only have driveshafts. A 20% loss of HP from a mid engine car to the rear wheels seems high. Chuck


everything in front of the driven wheels is the drive train, all cars have a drive train. In the Pantera that includes the motor, the clutch, the ZF, the half shafts and the stub axles.

20% loss includes the difference in the motors performance exhausting into the engine dyno exhaust system (normally a set of headers exhausting into short sections of exhaust tubing, but no mufflers) and the car's real exhaust system; the air heated in the engine bay, which accounts for 1% loss of bhp per 10 degrees F, the frictional losses of the transaxle and the weight of the wheels & tires.

A front engine rear wheel drive car can have loss numbers in the 20% range too, using an aluminum drive shaft, titanium axles, light weight wheel/tire assemblies. The standard front engine RWD car has quite a bit more loss, more on the order of 25%.

cowboy from hell
> So I wouldn't personally expect the losses from one car to another
> to be consistent, I would expect some variations.

A case in point. On the last 408C we dyno'd we had a difference of
50 HP between the Pantera Euro GTS mufflers and 3" inlet/outlet
Magnaflow mufflers. That's a 10% variation on a 500 HP motor.
The conversion numbers are meaningless.

Dan Jones
bhp = Brake Horse Power

a "brake" is just another term for a dynomometer.

To distinguish between an engine dyno and a chassis dyno, some folks use the terms fwhp for flywheel horsepower (engine dyno) and rwhp for rear wheel horsepower (chassis dyno). Normally when I'm talking informally with folks I'll say a car is "on the rollers" when I'm referring to the chassis dyno.

doesn't help at all, does it? have a great weekend!

cowboy from hell
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