numbers from chassis dyno vs. engine dyno

[Mr. P.]

So would an automatic putting down 625 at the rear wheels be about 750 at the flywheel then?

Depends on which automatic & converter; some loose 20% and some loose as much as 23% (4L80E) due to design. Which transmission are you using exactly?

Mr. P.

[SWR]

For hub-type dynos like DynaPack and Roto-Test on manual boxes a typical rear wheel drive car has about 8-10% loss. Big,complex 4WD with a ton of angled differentials etc was 12.5%.

Tire friction in "normal" benches most probably gets close to doubling the figures I mentioned above.

[Duner]

So would an automatic putting down 625 at the rear wheels be about 750 at the flywheel then?

Depends on which automatic & converter; some loose 20% and some loose as much as 23% (4L80E) due to design. Which transmission are you using exactly?

Mr. P.

It's in a '99 Dakota
The transmission is a 46RE
2600 Hughes convertor
3.92 rear gears in a 9.25"

[RL]

Did a 351C manual 601fwhp got 384rwhp, thats when we noticed the mufflers were restrictive and changed to Borla. We never retested with the Borlas but ran 10.8

Remember the full exhaust is added in the car and the engine is confined(more heat soak), tyres, the losses will vary.

[GOSFAST]

GOFAST, are those numbers with manual or automatic transmissions?

Our own tested units were all T-400's, not too many "stick" cars around here anymore!

The "Vette Doctors" are involved with mostly late Vettes and see more sticks than we do!

Thanks, Gary in N.Y.

[John W]

My 427sbc made 758hp on a SF902. In the same configuration with the exception of adding the alternator and x-pipe/mufflers, it made 583hp on a Mustang dyno thru a T350.

Later, I changed the intake and installed a Coan 350XLT tranny. I ran it on a Dynojet during a car club "dyno day" and it made 648hp. The difference on the track between the 2 intakes was nill.

[MadBill]

It's been reported here more than once that Mustang dynos give more conservative numbers than D-J and others..

[Ed Wright]

It's been reported here more than once that Mustang dynos give more conservative numbers than D-J and others..

Bill, that seems to depend on who is running the Mustang, or maybe who's it is. I have a Dynojet, and know of two shops I tune for that see bigger numbers when they get their customer's cars back on their Mustangs than they made on my Dynojet.

Btw, I hade a "10.5 Shootout" customer's R&M bbc engine show about 22% less on mine in the car one Friday than it had made on their Super Flow engine dyno that Wed. 3200 unlocked tc, T400 and 9" Ford rear. 1375 flywheel in Arlington vs 1080 at the tires in Tulsa, if I remember the number correctly. I know the 22% is correct. That was just on motor, mine won't read high enough for the nitrous. Just shows a straight line at 1200. Newer DJs are supposed to handle up to 2000 rwhp? Mine is ten years old. Some newer Dynojets appear to give out bigger numbers than mine. One in Memphis showed an LT1 Camaro from there to make 418 rwhp, mine showed about 370. I re tuned it, made 385 (and it would even idle <G>) and he saw 430 when he got back to Memphis. If I was going to a dyno race, that is the one I would want to take.

[1989TransAm]

Ed, due you dyno tune the older speed density TPI type cars? That would be with the 7730 computer. Might be worth a vacation trip to Tulsa from Southern California.

[runawy9]

Why does it take a percentage of the power?
If a 400 hp motor uses 20% on the chassis dyno that is only 80 hp.
So why does it take a 600 hp motor 120 hp to due the same job?

[ClassKing]

Westech -Ca.
From their SF 901 to their SF chassis dyno - 118 hp loss.
T-400 - ten inch Continental conv. - spool - street tires 28 ".

[SWR]

Why does it take a percentage of the power?
If a 400 hp motor uses 20% on the chassis dyno that is only 80 hp.
So why does it take a 600 hp motor 120 hp to due the same job?

I don't get that either. Why would a ball bearing increase it's friction with 50% just because a force 50% stronger is transmitted through it?

And the heat...say 1200 hp ....882.353 Kw...then a 20% loss...176.47Kw. Quite some heat... Why aren't we seeing more coked oil in gearboxes and differentials? But that's because people factor in the wheel resistance too,which is about half the loss so the 'box and diffs don't get heated that much. And the wheel resistance is the main reason I don't want to use a wheel-driven dyno for any R&D,ever again.

[MadBill]

This question has the makings of another "Torque Vs. Horsepower" debate, guys!

Nonetheless, here's a couple of thought starters:
o Friction relates to the normal (right angles to the surfaces) force and the coefficient of friction between the surfaces. That's why it's harder to drag a diesel block across the shop floor than an aluminum LS Chev one. Clearly, when you're transmitting more torque through a gear pair, etc., there is a greater contact force, thus more friction. As the RPM rises, this translates to more power loss.

o This is also why with your car (to simplify, lets say with a manual tranny) on a chassis dyno, you could take the plugs out and rotate the engine, driveline and inertia roller with a breaker bar, even though the same components would absorb hundreds of horsepower at high loads and RPM.

o A really precise analysis of all the drivetrain losses would likely reveal a range of characteristic variables Vs. speed and load ranging from fixed, to direct percentages with either and/or both RPM and load, to ones increasing at some power over one (e.g., squared) Consider a 4,000 RPM stall torque converter with lock up. At 1,000 RPM, it absorbs 100% of the engine's power. Between there and lock up, it's a variable, dependent on RPM and engine torque. After lock up, it becomes a much smaller factor..

o Obviously, just as on an engine dyno, everything we can do to standardize the test conditions [tire pressures, actual (test-specific?) tires, hold-down strap tension and angles, all lubricant temps, atmospheric correction factors, etc., etc.] will help with consistency, but with many more factors in play than for an engine dyno, the opportunities for variations will remain higher.

o Bottom line, an approximate percentage loss is the best when can do for a general case, and unless dyno racing is the sole object, it serves the purpose of showing proportional changes as the tuning process proceeds.

[Duner]

Thank you, that actually helped ME at least better understand why the "losses" go up and the power increases.

My main reason for looking for an "approximate guess" has more to do with how much power my fuel system needs to support. That requirement will be based upon engine performance, regardless of rear wheel numbers of forced induction type. I know that switching from supercharged to turbocharged - "freed up" about 125 hp that was previously "spent" on driving the blower. Or at least that's my guesstimate based on the 6 mph gain thru the traps on basically the same boost number. My fuel system has always been my limiting factor. It would only support a limited amount of "flywheel" horsepower, even if it wasn't all going to the rear tires. Not pulling the blower let that power get used at the rear wheels for an improvement in ET and MPH.

[bigjoe1]

I heard from a nascar guy that the loss on a chassis dyno was not a percentage, but just a net figure like 70 HP on an 849 HP engine-- this is with all the trick running gear ect-- I dont go along with the percentage loss thing either

JOE SHERMAN RACING