numbers from chassis dyno vs. engine dyno

MadBill · Post by **MadBill** » Sat Dec 13, 2008 6:21 pm

We need someone to test their motor on an engine dyno, not only at WOT but at 3 or 4 specific part-throttle settings as well, then repeat on a chassis dyno...

FASTFATBOY · Post by **FASTFATBOY** » Sat Dec 13, 2008 7:51 pm

Has to be a percentage, and that percentage really cant be pinned down. You can get closer with a manual trans though.

Cant be a set number, an FBody with a manual trans, stock 10 bolt, 16 inch street radials, 3.42 gear, aluminum driveshaft will put down more power than the same engine combo with a 9 inch geared with 4.11's, detroit locker, steel driveshaft and slicks. The latter combo sucks up more power to the wheels.

Also what alot of you dont consider, most race setups dont even have an alternator on the front of the engine. Then guys like me have EVERYTHING on the front when on a chassis dyno, that sucks up power vs an engine dyno.

I was told my percentage was closer to 25% because of the 9 inch rear, 4.11 gears, steel driveshaft, detroit locker, 325/50 drag radials, 4L60E trans and ALL the accesories on the front of the engine.

Mr Ed, you saying my car would push 500 rwhp on a newer dyno? If so I might have to try that.

SWR · Post by **SWR** » Sat Dec 13, 2008 7:53 pm

MadBill wrote:We need someone to test their motor on an engine dyno, not only at WOT but at 3 or 4 specific part-throttle settings as well, then repeat on a chassis dyno...

Yup. I've seen many results that says not a net value like 90 hp,but not a direct percentage either. More like if you triple your existing wheel hp,you get a percentage that's about 50% of what you started with. Say 250whp,and you assume a 20% loss in the drivetrain = 312.5hp. 62.5hp loss.

Now triple the whp to 750. A 20% loss then would be 187.5 hp. What I've seen many times is that you end up with about 830-835hp at the flywheel. In relation to whp output that is about 750 / 0.9 = 833.33 hp. Which is a 10% loss at 83.33 hp,give or take a little. Half the percentage,but an increase in friction nonetheless.

That's about 28288 Watts more of heat energy to remove through the walls of your gearbox / rear end. Easily enough to heat most normal-sized houses...

FASTFATBOY · Post by **FASTFATBOY** » Sat Dec 13, 2008 7:59 pm

Duner wrote: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.

I think your 125 hp gained may be generous. Most cars pick up around 10-11 mph per 150 horse of NOS. At least in th 9-12 second range.

ALWAYS better to have more fuel system than you need, gives you a safety factor if you ever have a pump go weak.

Is the car EFI? If so monitor injector duty cycle, O2 voltage. How much power is it making(guesstimate)?

Duner · Post by **Duner** » Sat Dec 13, 2008 8:28 pm

FASTFATBOY wrote:
I think your 125 hp gained may be generous. Most cars pick up around 10-11 mph per 150 horse of NOS. At least in th 9-12 second range.

ALWAYS better to have more fuel system than you need, gives you a safety factor if you ever have a pump go weak.

Is the car EFI? If so monitor injector duty cycle, O2 voltage. How much power is it making(guesstimate)?

That 125 hp number very well could be too generous.

It's a '99 Dakota. The truck weighs 4050# in race trim.
With the blower the trap speed was 120 - and dyno'd at 582rwhp
With the turbo the trap speed is 126 - no dyno time yet
The online calculators all give different numbers, but most call it a rearwheel difference of 100 hp.

It was originally an exercise to see how quick I could make the truck go using all the original parts. The fuel system is all wrong and will be entirely revamped in the near future. It's presently maxed out for sure.

new engine builder · Post by **new engine builder** » Sat Dec 13, 2008 8:33 pm

FASTFATBOY wrote:Has to be a percentage, and that percentage really cant be pinned down. You can get closer with a manual trans though.

Cant be a set number, an FBody with a manual trans, stock 10 bolt, 16 inch street radials, 3.42 gear, aluminum driveshaft will put down more power than the same engine combo with a 9 inch geared with 4.11's, detroit locker, steel driveshaft and slicks. The latter combo sucks up more power to the wheels.

Also what alot of you dont consider, most race setups dont even have an alternator on the front of the engine. Then guys like me have EVERYTHING on the front when on a chassis dyno, that sucks up power vs an engine dyno.

I was told my percentage was closer to 25% because of the 9 inch rear, 4.11 gears, steel driveshaft, detroit locker, 325/50 drag radials, 4L60E trans and ALL the accesories on the front of the engine.

Mr Ed, you saying my car would push 500 rwhp on a newer dyno? If so I might have to try that.

MR. Ed???
Now that's funny.

,but it's not right.

FASTFATBOY · Post by **FASTFATBOY** » Sat Dec 13, 2008 8:44 pm

new engine builder wrote:MR. Ed???
Now that's funny. ,but it's not right.

Hey Hey I show respect to my elders I was raised in the deep south. And Mr Ed is one nice guy. He deserves it.

And whats not right? Elaborate please.

randy331 · Post by **randy331** » Sun Dec 14, 2008 12:14 am

I don't see how the HP loss on a chassis dyno, vs engine dyno, could be a fixed %, or a fixed amount of HP.

When you increase acceleration rate, on an engine dyno, there is a HP loss, just because of the increase in the power needed to accelerate the engine at the higher rate.
On a chassis dyno your accelerating not only the engine, but all drive train mass, so any acceleration rate difference will show a larger gain/loss.
Now throw in the inertia wheel type dyno, vs a fixed acceleration type dyno like a super flow, and how could you ever come up with a fixed loss.

The HP loss on a chassis dyno isn't just friction etc, but accelerating all the extra mass of the drive train. AS that acceleration rate goes up, so will the difference between the HP on a engine dyno, and a chassis dyno.

Even with a relitively effecient drive train, the loss could seem high if the acceleration rate on the chassis dyno, is higher than that used on the engine dyno.

Randy

Ed Wright · Post by **Ed Wright** » Mon Dec 15, 2008 3:32 pm

1989TransAm wrote: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.

Yes sir, I do.

Ed Wright · Post by **Ed Wright** » Mon Dec 15, 2008 3:40 pm

bigjoe1 wrote: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

Joe, I have heard the same thing. Makes no sense to me either, why an 800 hp engine looses more through the same drive train than a 400 hp engine. But, I'm just a dumb Okie. I try to tell kids to stop worring about that anyway. I tell them it's just a tool for measuring changes in that car that day. It's a safe place to drive it WOT while logging data and testing changes. The weather stations are not dead nuts, nor is the SAE correction formuli. My bone stock '98 Trans Am has about a bazillion pulls on it (with zero changes), and from a 15 deg day in the winter to over 100 in the summer, the corrected numbers vary over 30 rwhp. I tell them "This aint a lab, it's a tin building in Oklahoma".

Ed Wright · Post by **Ed Wright** » Mon Dec 15, 2008 3:44 pm

MR. Ed???
Now that's funny. ,but it's not right.

I keep telling myself David is too young to remember that TV show.
That is what I keep telling myself.....

But, probably not.

MadBill · Post by **MadBill** » Mon Dec 15, 2008 4:45 pm

Realizing that it's not necessarily obvious that more transmitted torque = more driveline friction, how about an example: If you drive down the freeway at a steady 90 MPH (pretend you're in Montana and that Smokey still looks the other way at such speeds), your axle seals won't cook and will never start spewing lube and the gears won't turn blue.
Now try to do a few dozen laps in a high-powered circle track or road race car at a 90 MPH average, without a big oil cooler. I've measured almost 300° diff lube temperature with a decent cooler!
Doesn't that extra heat equal umpteen more watts of lost power?

1989TransAm · Post by **1989TransAm** » Mon Dec 15, 2008 6:36 pm

"Yes sir, I do."

Thanks Ed. I'm in the process of building up a highly modified TPI motor using the new Dart SHP block at 370CI and with the new AFR 195 Competition heads. I expect to be well past the 400rwhp mark. A trip to Tulsa would be a nice break in drive.

Probably be in the middle of next year as these projects go. Thanks, Allen

plovett · Post by **plovett** » Mon Dec 15, 2008 7:51 pm

I think the answer is that friction increases with hp, but not in a linear relationship. So 1.5 times the hp does not equal 1.5 times the friction. I think the equation is more complex than that.

I don't have the answer, but in this case I think it is in between the two extremes:

1) hp loss is constant and not related to the hp produced.
2) hp loss is a percentage of the horsepower produced.

Neither is correct IMO,

paulie

Sprinter99 · Post by **Sprinter99** » Tue Dec 16, 2008 1:14 am

I dont think this is something that can ever be figured with a percentage accuratly,there are too many variables, trans, rear end type, tire size, gear, tire pressure, I can only imagine what a converter can do to the numbers the chassis dyno spits out. Almost no 2 vehicles are exactly the same so none of them can really be compared. The only reason i really chimed in is because none of the formulas anyone has mentioned will even come close on a sprint car, mine made 539 on a Mustang chassis dyno and i would be real surprised if it even made 600 on an engine dyno.