rob75383

I've estimated that I've got about 400+ hp on tap.
At Bloomington Gold, I went to the chassis dyno and only ran with 291rwhp and 332rwtq.
I guess that I've still got tuning issues, but I thought I'd get more power than that.
How come a 355 with the same cam, intake, carb, better heads, makes 455hp and almost 500lb on 87 octane, and the dyno operator says I'm running about 340hp at the flywheel?

What is the estimated amount of parasitic loss that an average automatic has?? 10%, 20%, 25%, 30% ??

A 427 C2 had about 260rwhp, which made me feel a little better, but not much.

carter200

I remember hearing 20% but not positive :seeya

CFI-EFI

Power absorbsion of the drive train remains a hotly debated topic. Some swear that it is exactly X%. The power loss varies form car to car, and the components that make up the drive train. Anything between 15 and 20 % is a reasonable approximation. You could pull your engine and have it dynoed, then you would know for YOUR car. Or would you? All dynos are different, and you would have one hell of a time hooking that engine to the chassis dyno, or vice versa. Good luck.

L79vette

The chassis dyno operator told me 16-20% for a manual. Didn't ask about automatic, but would expect at least 20%. He has an engine dyno also, and this figure was from his experience running the same engine on the engine dyno and then the chassis dyno. Also, recall that TH400's have a reputation for taking a lot of power to run, as well as being bulletproof.

I think you do have considerable tuning to do, though. My car made 281RWHP, 307RWTQ with a mild 327. This is with 2.5" cast iron exhaust manifolds, pocket ported 462 heads, 235degrees at 0.050 cam, LT-1 intake manifold, 650 Holley. But, I also have a manual trans.

One suggestion: Those repro mufflers may be costing you a lot. I put Dynomax Super Turbos on, which are rated at 410CFM each. David Vizard suggests using mufflers with total flow rating equal to 2.2CFM per open header horsepower. Dynomax is one of the few companies that will give you flow ratings for all their mufflers. Super Turbos are pretty quiet at idle and cruise, too.

[Modified by L79vette, 6:14 AM 6/19/2002]


[Modified by L79vette, 6:16 AM 6/19/2002]

rob75383

I forgot to mention that I had ran my headers uncapped for the dyno test.

Smitty's75

This issue really drives me nuts,,wouldn't you think it would take the same amount of HP and TQ to turn the same driveline no matter what engine it's behind???? An auto behind 400 HP would take 80 HP (20%) to run, but the same auto behind 600 HP takes 120 HP and 700 HP takes 140 HP, where does it all even out?????? With this way of thinking, if Merlin gets up to 1500 HP, he would lose 270 HP (18%). The same transmission and driveline would consume 72 HP behind a 400 HP car???!!!! There has to be a ceiling where it all stops!!!

Lets hear some theories on this!!

427Hotrod

Well it's real messy, no matter who you talk to. First.. try to get a clear answer on "which way" they want to go.

Example: If you have a 400 hp engine and lose 20% through drivetrain, that makes 320 RWHP.

If you chassis dyno 320 RWHP, and then add 20% to it, you get 384 engine HP. It adds up quick.

Remember, that most engine dyno tests are pure gross, yet they are corrected to std SAE #'s in most cases.

I know many of the chassis Dynojet guys use current factory rated numbers to "back into" the numbers. Most new LS1's chassis dyno at 295-305 or so stock. Since the factory called them 345 or so HP they have decided that about an increase of 15-19% is about right for manual trans cars.
Note that the chassis dyno number is more like 12-14% or so less than the advertised rating if you back into it that way.

I've dyno'd mine on a freshly calibrated Superflow dyno with 2 1/8" headers, a belt driven stock water pump. It made 732 HP at the crank.

I've since chassis dyno'd it on two different Dynojets and got similar results on both. There were some differences though, so it's not a pure apples to apples comparison. In the car I used 2" headers, a Team G aluminum water pump, a fixed 7 blade mechanical fan, the alternator was in operation as well as a mechanical fuel pump( I know big deal... but I'm trying to be as accurate as possible!) It made 615.7 RWHP.

That gets you:

615.7 + 15% = 708.5
615.7 + 16% = 714.2
615.7 + 17% = 720.3
615.7 + 18% = 726.5
615.7 + 19% = 732.6

If you go the other way...

732 - 12% = 644.1
732 - 13% = 636.8
732 - 14% = 629.5
732 - 15% = 622.2
732 - 16% = 614.8

Either way you look at it, I think the numbers are realistic, considering all the variables. Smaller headers, alternator and fixed blade fan could easily contribute to the differences in max HP as well as any tuning variables and two entirely different types of dyno's. I also use a Doug Nash 5 speed which I am sure is not the most efficient piece as compared to a T-56 and I really doubt the Vette IRS is the most effecient setup for power tansmission through U-joints, halfshafts etc as compared to a 8.8 Mustang rear etc.

Mine seems to be in the 16% total loss as compared to engine dyno and all the variables, or in other words about 19% above RWHP IF I put it in dyno mode with less accessories and bigger headers.

All seems plausible.

I discussed the same issue with many folks about the seemingly exponential increase in loss as measured HP increased. You have to remember that as HP goes up friction etc also increases in relation to the load. From everything I can gather the %'s are pretty accurate through the Hp ranges.

Hope all this helps some.


Jim


[Modified by 427Hotrod, 10:14 PM 6/19/2002]

L79vette

If you were uncapped you really have some tuning to do.

Now, just uncapping can hose your tuning to some degree. If you set the air-fuel ratio correctly for the capped up exhaust, then when you uncap you will lean out, which costs power.

Mine only pulled 255HP when I went to a "dyno day" where you got a couple of pulls but no time for tuning. That wasn't what I expected, so I had some tuning done on a dyno. Turned out there was a major problem with fuel control. The fuel pump was overwhelming the needle and flooding the float bowls. Don't know how I would have found that in road testing. On the chassis dyno we were doing a run with the hood up and air cleaner off after making an adjustment. When fuel shot out the vent tubes, we knew what was wrong.

You should get a leakdown test to make sure you aren't losing cylinder pressure through the valves or rings. Then get the timing and fuel curves set right. You should see a lot more power.

71roadster

Jim
I think you are making a small error in your calculations when you try to add %20 to the rwhp numbers.

320hp +%20(320) is not the same as 400fwhp - %20(400).

if you want to assume a %20 drivetrain loss you need to figure that 320rwhp is %80 of the fwhp. That is 320 = .8 * X so to figure out your fwhp (X) you need to divide 320 by .8 to get (X) your fwhp of 400.

Try it that way and the numbers should agree.


As far as the thread topic is concerned there is a great article in the July issue of Circle Track and Racing Technology about drivetrain losses and where you can pick up power. Its a good article and the mag has articles like this every month. Its my new favorite magazine. Check it out.

Chris

427Hotrod

I agree with you. I guess I should have been more clear. When I deal with chassis dyno operators, they are generally less than clear on any correction factors. You get several different versions. Some say literally to add 20% to the raw RWHP number, others back into it by multiplying as you said. Others take a engine dyno number and try to make the chassis dyno number fit somewhere.

That was kind of my point. It's pretty tough to do accurately. I was just trying to illustrate what they say and how they related to some real dyno numbers. If I do it the other way I end up with 769 crank HP which I wish I had seen at a 20% loss. Hey , maybe my small headers really did help! :-) It still works out to around a 16% this way for my car.

We've done several cars with fresh engine dyno numbers and then straight to the chassis dyno. One made 740 hp and then made 610 RWHP. But it was an auto with a sloppy converter. About a 18-20% (depending on the way you look at it) loss. It later made 641 RWHP after head work. At that time it engine dyno'd at 794. Very close. The car also ran 9.60's in a 3500 lb car on drag radials, no power adders.

Once again, hard to be exact, but good trends to look at.

I guess the best and only number that really counts is a trip to the old asphalt dyno and get a MPH number on a time slip. There's not much doubt then!

Great tip on the cirlce track mag. I also find they have more "real tech" than the others. In fact, the various 5.0 Mustang books are putting out some pretty hardcore stuff too.

Thanks,

Jim


[Modified by 427Hotrod, 4:25 PM 6/24/2002]

1985z51

having a chassis dyno is a funny thing...no one is ever gonna be happy unless they get the number they think they should have. the only thing i tell people when they use my dyno is that they should use this as a comparator. make a change and run it again to see if youre goin the right way. :)

L98IROCZ89

General rule of thumb is 20%.

Some cars eat up more power than others. Some of the newer cars will eat up less HP than some older ones because they are much more efficient. Relativley speaking, a C5 puts more power to the ground than a C3... relativley.

Also, an automatic tranny will result in less RWHP than a manual because the torque converter eats up more power.

Chris O'

Comparing desktop dyno and an actal dyno run, mines about 30% (TH350). :eek:

BBA

Ok...you guys are completely ignoring physics here.

You have to add each parasitic loss item individually.

Say for example, you have a TH350 trans. I think the numbers for a TH350 trans with stock line pressure takes about 16 HP to turn in third gear at 5000 RPM, go up on line pressure ( IE: shift kit installed ) and it might take 20 or even 30 HP at the same speed. Now look at the alternator, with no electrical load and depending on the drive gear ratios, it might take 10 HP at an engine speed of 5000. Now look at the power steering pump...oh, lets say 15 HP at 5000 engine RPM. Water pump? Hmmm....20 HP at 5000 RPM?

Ok, assuming these numbers are right, you can say you have 16+10+15+20=61 HP at 5000 RPM. So, if you have an engine that makes 400 HP at 5000 RPM, you will have 339 HP at the wheels. You might see a little less for drive shaft/u-joint/rear gear friction.

If you noticed, those losses are not relevant to the particular engine horsepower. The same drivetrain above with a 500 HP at 5000 RPM engine would make 439 RWHP.

What the losses are relevant to are engine RPM...IE: The faster you turn a water pump, the more power it takes to turn it. So the same system at 6000 RPM might take 80 HP to turn.

Basically, the whole percentage thing is totally non-transferrable from ANY car to ANY other, but all cars with similar drive accessories will have similar parasitic losses, regardless of the horsepower of the particular cars engine.

BBA

BTW: I only expect a few people to understand and agree with this when they first read it...but the more you read it, the better the understanding you will have.

C4racing

I agree and disagree. :crazy:

Yes, parasitic losses of rpm related components do go up as rpm increases. But if "average" stock C4s or C5s are put on a chassis dyno, or if the engines are removed and run on an engine dyno, then it is an apples to apples comparison. The only caveat is that there can be slight component to component variations (ie, one alternator might use a bit more horsepower than another at the same rpm).


Once you start changing engine components that result in different torque and horsepower curves, THEN you get into the rpm effects. If max power occurs in a stock engine at 5500 rpm, but with modifications the same engine's horsepower max now occurs at 6200 rpm, then the additional parasitic losses DO come into play.

So when you say:
....I disagree. Take 2 identical cars with identical drive accessories. Modify one of them so it makes more power (usually higher torque at a higher rpm). Since the modified car makes more horsepower at a higher rpm then that car will have a greater rpm related parasitic loss. (Read your own argument.)

I think you're missing the point of like components on like chassis. Again, excluding minor variations in individual components, the difference in parasitic losses of 2 stock Vettes when compared to one another on a chassis dyno would be statistically insignificant.

But who on this forum really has a "stock" car? :lol:

Once people start modifying their engines and start changing the rpm at which max horsepower and torque are made, then the car to car variations start.

Btw, do you thing that OEMs when quoting engine (flywheel) horsepower actually strip the engine of all rotating accessories? (alternator, air pump, smog pump, fan, power steering pump, etc) I don't think so. But of course I don't work for an OEM so I suppose anything is possible......
:seeya

L79vette

It also really does seem to be a percentage loss. I cannot explain why, but if you build your engine to make 25 more HP at 5000RPM on the engine dyno and you had 20% loss with your previous setup, you really will only pick up about 20 on the chassis dyno. I think it may have to do with more friction due to the higher torque at the same RPM.

We all wish that those extra HP fell directly back to the wheels, but they don't.

blue_stingray

i can understand parasitic HP loss from such accessories as a water pump or power steering pump-- after all they are pumps, and the faster they are turned, the more they pump. at higher rpms, more HP is needed to overcome higher fluid pressures. one way to overcome that is to either switch to an alternate powered method-- IE: electric water pump, or switch pulley ratios to allow the pump to turn at a slower rpm at higher engine rpms.

what i can't get is parasitic HP loss from an alternator. the shaft rides on roller bearings, regardless at what rpm you spin it, it will require the same amount of HP, since there is no load. when you consider the current it produces, how does that introduce load on the shaft? i'm getting this theory simply because you can easily spin an alternator with your hand. hook up a small electric motor to it, like a window fan motor, and it still spins easily. where's this parasitic loss coming from? and this is just a rough guesstimate-- if the engine is spinning 5000 rpm, the alt is spinning around half that-- where's the load that justifies 10 hp loss from the alt??

L79vette

The alt spins easily out of the car because it is not driving any electrical load. When you hook up a load to it, it becomes hard to turn as the mechanical energy you are putting in is converted to electrical energy to drive the load.

That said, 10HP is way too much. 1-2HP is more like it. 1HP=746Watts. Even with an inefficient energy conversion, alternators are a small drag. Doesn't take much electrical power to run a car.

BBA

I think the efficiency of hte alternator is around 50-70%, so if an alternator was running at 100 AMPS, 12V, it's producing 1200 Watts, and if it is 50% efficient, it is using 2400 Watts of power. Thats equal to about 3.2 HP...so maybe my example is a high estimate of parasitic loss from the alternator.

True...but think of this as well: When for example, some one uses N2O on a stock engine, it keeps the same power curve, just higher levels of power are made at the same RPM's. Now the 300 HP engine at 5200 RPM is making 400 HP at 5200 RPM, the parasitic loss is the same, so the full added HP is deliveredd to the wheels...not some lower percentage, unless what L79vette said is more the case.

Hhmmmmmm...