Calling all Dyno Experts - Negative Power?

[Ron]

<FONT face="Comic Sans MS">Perhaps I have the term wrong, but our local Dynojet operator graphed our "negative power" (coasting back down from 6k rpm in fourth with clutch in).

He said that this represents the actual driveline loss and should be added to the rear wheel HP in order to calc flywheel Horsepower. We dyno'd 7 Vipers today and all came in at between 24 and 30 average negative horsepower.

In my case, my max run was 435.29 HP SAE corrected plus say 30 negative equals 465 HP. Using the typical 15% driveline loss, you come up with 512 HP.... Big difference.

So which is correct, 6.5% or 15%, 465 or 512?

Thanks....</FONT f>

 

[Russ M]

The real number is the one you get from taking the motor out and performing the test on an engine dyno. The decel test is not very accurate so dont bother putting too much faith into it.

Usualy independent rear drive car loose anywhere from 12-18%.

 

[SUN RA KAT]

435 is your rear wheel horsepower.

30 horsepower is driveline loss.

465 is your flywheel horsepower.

450 is the advertised factory flywheel horsepower.

You owe the factory 15 horsepower.

 

[treynor]

&gt; He said that this represents the actual driveline loss and should be added to the rear wheel HP in order to calc flywheel Horsepower

Err... horsepucky. Driveline loss under full acceleration is far greater than it is during coast-down. Remember that frictional power losses = velocity * normal force * coefficient of friction. The normal force in the driveline under full acceleration is obviously much greater than it is during coast-down, and thus so is the frictional loss.

 

[RoyV101621]

I know of only one person that has done a engine dyno run and a chassis dyno run on the same engine before beginning modifications. Sean Roe did this with his '95. I hope he will chime in on this one.
I feel that because he was the first to make the effort to create a true correction factor, it should be called the "Roe Correction Factor"
<u>What I find more astonishing:</u> Sean is a Viper tuner and his correction factor is much lower than the theoretical 12 to 18 percent. Most tuners want a large correction factor because it sells FREE horse power. Most owners want a large correction factor because it makes you go faster, at least when you're bench racing.
Your dyno operator is a technician whom is also selling a service. The larger "feel good factor" would have been advantageous, but what is the point of a dyno if you chose to ignore the results.
Brent, the tech, said that the deceleration number is not wholly accurate, but is a **** bit closer than a theocratical industrial average which includes Geo Metros and Ford Pickups. Trucks.

Hey, What's a VCA for? I propose that the VCA fund an independent engine to chassis dyno comparison on a GEN2. Anyone second the motion?

 

[MES]

From what I recall the real loss was 11.5% so your 435 = 491.5 still very strong. That also means the cars are NOT that overrated like everyone thinks. 450HP=398 to the wheels, stock vipers are typically 400-410 to the wheels.

 

[ronviper]

Ron take the car to the track your mph will tell you what is really going on . That is the true power the car is putting to the rear wheels, give or take 1 to 2mph.

 

[RoyV101621]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by MES:
From what I recall the real loss was 11.5% so your 435 = 491.5 still very strong. That also means the cars are NOT that overrated like everyone thinks. 450HP=398 to the wheels, stock vipers are typically 400-410 to the wheels.

<HR></BLOCKQUOTE>

No disrespect: Indirectly your making Ron's point. Ron is looking for a verifiable source for drive line loss. If it is 11.5% that's great. Were did this number come from?

Assume Ron's car is a stock '96 with K&N and tubes
435 rwhp = factory 450hp + 15hp - 30hp drive line loss
or 435 rwhp = underrated 477hp + 15hp - 42hp drive line loss
Truth is, this is a debated technicality. The car puts 435hp to the pavement and it's one of the fastest cars on the road. So how much horse power does the engine produce?

I repropose that the VCA fund an independent engine to chassis dyno comparison on a GEN2. Think about it. The cost is minimal. Someday, someone unfortunate will have to send there stock GEN2 engine to Arrow to be gone though. Arrow typically verifies there work on a dyno. After the engine is reinstalled, in stock configuration, the owner only need allow publication of the results. The cost is only the chassis dyno and the truth be told.

 

[Sean Roe]

Here are the dyno sheets from our '97 GTS a couple years ago.
Engine built by to SCCA specs (blueprint, etc). Engine run on the dyno with the car's exhaust & PCM. Engine put in car and run with stock trans, clutch / flywheel, rear gear and tires. Race fuel used in both tests. Same air filter setup used in both tests. The engine dyno test was comparing a modification to the exhaust manifolds, so disregard the lower #'s as that exhaust did not go in the car.

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Sean

 

[onerareviper]

Sean,

So a good formula would be :

1.) 539.7 / .90 = 600 HP

2.) 513 / .90 = 570 Torque

* I always used .85, but maybe the Viper drivetrain doesn't drain that much power.

 

[Ron]

<FONT face="Comic Sans MS">Thanks everyone for your comments.

Looks like 11.3% is what Sean's factor worked out to be and if it's good enough for him it's good enough for me.

I especially like Roy's idea, hopefully someone will pull that one off soon as another data point.</FONT f>

 

[Sean Roe]

If everyone wants to use percentages, that's fine. But, it seems to me the drivetrain uses about 60 HP and TQ, irregardless of the #'s the Viper engine is making. The math doesn't look right adding the same percentage when an car is making 400 RWHP, then gets modified up to 550 RWHP.

400/.89 = 449
550/.89 = 618

So, the 400 HP example had a drivetrain loss of 49 HP and the higher 550 HP example had a drivetrain loss of 68 HP, with the same drivetrain? Looks good for the HP tuners, but, is it correct? I add 60 myself, but generally just tell people the rear wheel numbers and let them do what they wish.

Sean

 

[treynor]

Sean,
You'll have larger frictional losses with the same drivetrain if you ask that drivetrain to pass larger forces. Consider the alternative -- that the drivetrain soaks up 60 HP all the time, even when you're cruising along on the freeway with the throttle barely off the stop?? Clearly not.

There is a legitimate question whether the friction in the drivetrain obeys the theoretical linear behavior: force of friction = coefficient of friction * normal force between surfaces.

 

[onerareviper]

Treynor,

I see your point. But wouldn't it have more to do with RPM, than actual power? How would your drivetrain know if the engine had 400HP or 800HP? It's just a matter of the fictional loss due to spinning parts, right? So Sean's adding 60 HP would seem to be accurate, unless you raised the max RPM's.

P.S. - I have no idea what I'm talking about, but I had to comment anyway.

 

[GTS Dean]

Don't forget about gear tooth loading and pressure angle. More HP = more gear/bearing loading = more friction = more heat = greater power loss. This even includes the splines on the drive and half shafts.

 

[Sean Roe]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by treynor:
Sean,
You'll have larger frictional losses with the same drivetrain if you ask that drivetrain to pass larger forces. Consider the alternative -- that the drivetrain soaks up 60 HP all the time[/B], even when you're cruising along on the freeway with the throttle barely off the stop?? Clearly not.

There is a legitimate question whether the friction in the drivetrain obeys the theoretical linear behavior: force of friction = coefficient of friction * normal force between surfaces.
[/B]<HR></BLOCKQUOTE>


Our engine lost about 60 HP / TQ after going in the chassis and doing another full throttle pull (comparing full throttle to full throttle) and I agree with your example of drivetrain loss being lower cruising down the highway at low RPM.

If we pulled two plugs out of the engine making 400 RWHP and did another full throttle pull where it now made 300 RWHP, would the drivetrain loss be reduced to 37 HP?
300/.89=337.
Fact is, I'm not sure. So, I compare SAE corrected RWHP to SAE corrected RWHP and don't try to get into any arguments about what the true engine HP is. For my own piece of mind, I add a flat 60 to the peak #'s. I think the only way we'd be able to tell any better would be to test a higher and lower HP engine than our 571/600 and compare the results.

Sean

<FONT COLOR="#ff0000" SIZE="1" FACE="Verdana, Arial">This message has been edited by Sean Roe on 06-19-2002 at 06:54 AM</font>

 

[Steve-Indy]

Here we are, again.....where is Team Viper/PVO when we need them?

 

[GTS Dean]

Why does it make any difference what the flywheel HP is to most people? An engine without a drivetrain attached to it won't get you down the road.

If you're bigtime experimenting with engine mods, then an engine dyno is the way to go. It's much simpler to swap parts with the thing sitting on a stand at waist level than to go in and out of the chassis a hundred times to get data. Also, the quality of the data from an engine dyno is much better.

 

[Janni]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by GTS Dean:
Why does it make any difference what the flywheel HP is to most people? An engine without a drivetrain attached to it won't get you down the road.<HR></BLOCKQUOTE>

Dean, Dean, Dean... of course people want to know the flywheel horsepower because the number is BIGGER!

 

[Steve-Indy]

Good points, Dean. The "rub" comes when guys buy HP upgrades based on PRINTED tuner claims, only to find out LATER that there is quite a fudge factor that can be/has been manipulated into the final quoted HP claim. I have encountered 14-20% claims on the drive line losses...all of which could be used to jack up the perceived flywheel HP claim, thus SEEMINGLY delivering the quoted "goods" to the customer. Then, when said customer happens into a club dyno day at a disinterested shop, there is a BIG SURPRISE in terms of the measured rearwheel HP and the theoretical flywheel HP as calculated by THAT shop that day...even with the "correction factors". Now the customer feels deceived and disappointed. If we could stick with rear wheel HP and torque quotes, things MIGHT be simpler and folks happier with their mods.

In MY opinion, the bottom line here is performance on the pavement, NOT bragging rights on the numbers. (Most of this has been pointed out by Roy in the above posts)

 

[GTS Dean]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Steve-Indy:

In MY opinion, the bottom line here is performance on the pavement, NOT bragging rights on the numbers. (Most of this has been pointed out by Roy in the above posts)

<HR></BLOCKQUOTE>

Bingo, Steve! That's why I like to dyno with the t-stat wide open and the motor good and hot. Whatever the reading is is what you have under your foot most all the time. It's how you use it that matters. The only time I've ever done mine was at TNT on a hot fall day. 111F dyno cell temp and right off the street, I turned 413/449 corrected with tubes and filters only.

 

[Ron]

<FONT face="Comic Sans MS">Flywheel HP is of interest because that's how the car companies quote and we are forever wondering how we compare. If car companies quoted RWHP, flywheel HP would become irrelevant just as gross became irrelevant when Net HP figures began being published back in '71.

While I'd agree that the pavement is where the action is, verifying the affect of mods are more easily done on a dyno than by seat of the pants. Minimizing data corrupting variables is the hard part.</FONT f>

 

[Steve-Indy]

" Minimizing data corrupting variables is the hard part."


....such as who checks the dyno itself and how often ??? Don't get me started on "calibrated for life" black box devices !!

 

[davem]

Guys,
Nice to see Sean's dyno work. On our dyno, we usually "guestimate" 15% loss just to make the customers feel better, 25% if it's an automatic. Most engine dynos are brake dyno's that step and hold the motor at fixed rpm, and interpolate between the points (you can see that in Sean's flywheel graph). The dynojet 248 chassis dynos are inertial (unless you have the load cell) which I think is a little closer to what the car actually sees in real use (neglecting wind loading), meaning you accel the engine against a heavy drum of known inertia and measure the rate of change of the drum's RPM to get hp, and use engine RPM from that to get torque. So it still is not a great way to estimate driveline losses, but it's the best I've seen yet.

Really need to get Sean's Viper on a Superflow dyno that can re-create the brake dyno's RPM stepping. That would put the debate to bed fully, loss per RPM at the same static load!!

The only time the brake hp (crank) number really matters is when your trying to size injectors, but whenever we do that we usually go a little big anyway, just to be sure. It still is a fun debate though!

later,
Dave.

 

[Keller]

Hey guys, I just came across this interesting discussion and thought I would chime in.
I do agree that with increased loads (HP) frictional losses will increase but lets ignore this for a second and think about the other way we lose power in the driveline, accelerating masses.
Now if we accelerate a given mass at a given rate it will take X amount of time to perform this function and if we decrease the time required to do this we must increase the energy input.
Think about this, if we call the driveline and wheels and all other accelerated parts Y and we accelerate Y from velocity A to velocity B in a given time it requires X amount of HP. Now decrease the time required to get from A to B. Huh.. we increase the rate of acceleration which requires us to increase the amount of HP to perform this. This is an oversimplified explanation but you should get the point. The faster you accelerate any mass the more energy it requires to perform that acceleration. So the more HP you have driving the driveline, the more HP it requires to get from velocity A to B because your rate of acceleration increases!

 

[Keller]

PS
this is also why when you are cruising down the highway at a constant speed that driveline losses are much less than on the dyno. You are not accelerating any mass, just maintaning the enery required to keep these parts moving.

 

[Keller]

 

[jamie furman]

I think the only time you can use a constant formula say 11% or whatever is on the same motor using the same induction and exhaust on both dyno's.What usually happens though is the engine dyno is set up in an air conditioned room in optimum conditions with big headers and no accessories or drive train running cold water through the block and the numbers are huge.I do alot of old car stuff hemi's and FE motors and such and there installed horse power from engine dyno to rwhp is usually 30% loss on carburated cars in my experience.I think if you dynoed a viper motor like these race shops dyno their race engines they would all be making 550 gross hp with open headers and no accesories.
On a stock viper maybe 11.3 is a good number. I think the only thing that really counts is what is on the rear wheels the other numbers are just for the engine builder to get paid.

 

[Mike Brunton]

Ding Ding Ding Ding Ding!

We have a winner! Keller, you win the prize!

I've been saying this forever but nobody believes me. Your explanation is right on. Basically it takes a given amount of power to overcome inertia. You wanna do it quicker? Gotta dump more power in. The TOTAL POWER CONSUMED doesn't change in the driveline much (i.e if you accelerate from 0-60 in 4 seconds or you do it in 20 seconds the energy used up is similar), but given that, when time goes down, energy usage MUST go up!

If I walk a mile or run a mile - I exert the same energy either way... but it's a lot harder to run it because the time is lower so the average energy output must be alot higher, even though it's for a shorter period of time.

 

[Keller]

Sweeeeet! I knew my brain was good for something