This is from SDS efi's website. This backs up what I have been saying for a long time about trusting dyno claims. Now you can still try to plug in numbers and fomulas all you want to try to determine a horsepower level for this car but you still aren't accounting for everything. This article really gives support to the fact that different companies can in fact give different power claims and none of them can be "wrong" as they could only be reporting what their tests showed to be true. Obviously the only truly accurate dyno is the engine dyno but most don't use these. It's still a great article that everyone should read.

http://www.sdsefi.com/techdyno.htm

 

The point about transmission losses "melting the differential case" is a classic!

S

 

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In one recent independent test, hp figures varied by 11% simply by doing the runs in different gears and in another test, results varied by almost 4 % by doing the runs with a different wheel/tire combination.
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shocking... its called changing the inertial loads and its almost like the author has no concept of this



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Most rear drive cars have a 1 to 1, 4th gear which means that the power path goes directly through the mainshaft of the transmission. The only losses here are bearing drag which is less than 0.5% and the viscous drag of the gears running through the oil which is about 1% with hot oil. Indeed, published data indicates a transmission efficiency of 98 to 98.5% for conventional transmissions in 4th gear.

Losses within the driveshaft account for about 0.5% if they are properly aligned, balanced and with fresh U-joints.

Differential losses in the commonly used Hypoid type gearset is in the order of 6 to 10%.
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Those are great numbers for friction losses, but this would confirm the author has no clue about what an inertial loss is



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Other things to watch are correction factors applied for altitude, barometric pressure and temperature. These factors are NOT the same for atmo and turbo engines. Using atmo factors inflates the true, corrected HP figures on a turbo engine. In fact, look at the correction factor applied on your dyno sheets and see if they make sense. Many shady dyno operators simply enter a phantom correction factor to make the customer happy. This is a case where the dyno sheet DOES lie. Chassis dynos are essentially for tuning purposes, they are not well suited to giving an accurate hp figure.

Be aware that SAE correction factors do not apply to turbocharged engines! If your dyno sheet lists SAE corrected HP, ignore it as it is incorrect. You are better off getting an idea of where you stand by looking at observed hp with a turbo engine.
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Bullshit. All power figures in the future will have to be certified by SAE J2723 testing procedures and corrected with SAE J1439 JUN 90 factors- no matter what the engine is. Many manufacturers are voluntarily now using "SAE certified" power figures already which requires an SAE witness and SAE correction factors - GM, Honda, Toyota, and partially Ford are in the category already. If the dyno operator is making up the factors then thats one thing, but the factor is a real equation proven over and over again.

If the dyno is not corrected it is completely worthless as you have no idea if the tester pulled something like dynoing at the hottest part of the day and coldest part of the night


cf = 1.18*((990/PD)*((Tc+273)/298)^.5) - .18


where: cf = the dyno correction factor
Pd = the pressure of the dry air, mb
Tc = ambient temperature, deg C

 

except it doesn't address hub mounted dynos such as Dynapack, Rototest, etc. which are essentially engine dynos that bolt to the drive wheel hubs rather than the crank hub

Sorry, I wasn't impressed ...

 

r0tor: correction factors have never been accurate on turbo cars. You are not supposed to use them. Sadly enough most dyno operators are idiots in thir regard and this stupidity rubs off to their customers. The people the do it are the ones who don't know what they are doing. I've talked about why this is so in the past but quite simply the turbo will help somewhat compensate for altitude by spinning faster. This means power won't fall off as much with altitude and temperature as it would with a nonturbo. Now how can anyone intelligently argue that there is only 1 standard correcton factor that applies to both engines when they each behave differently? A correction wouldn't even be the same turbo to turbo as each turbo has different effciencies. This is the reason why you will sometimes see people claim they are getting more than double the horsepower with less than double the air in boost. It's operator error. That's impossible yet you'll see dyno charts claiming this.

This is actually one of the more knowledgeable articles on dynos I've seen as the author obviously knows how to use one properly. Of he's been racing and using dynos properly for decades so he should know. The fact of the matter is that half the people who actually use them everyday don't know what they are doing. They think you strap a car on a dyno, set a correction factor and then believe what you get. Nope. Experience means nothing if you are doing it wrong the entire time.

Dynapack dynos are farily new ways of dynoing a car. What I mean by that is that they are only now getting more widely used. You see them more now than you used to. It has traditionally only been engine or chassis dynos. The article wasn't written yesterday.

I understand that there would be some hesistation by a few to believe this article. When someone tells you your way of thinking has been wrong the whole time, it's usually very easy to get defensive about it. The fact of the matter is that this is the right way to think about things and that even most dyno owners are wrong. Don't necessarily trust dyno numbers when it comes to advertising. It is very possible to see variation from test to test. Believe what you want but if you don't believe this, you aren't the corect one!

 

well, hate to break it to you...

a) all advertised horsepower figures in the US are "SAE Net" which is slang for the SAE J1439 code - which includes the correction factors!!

b) if you look at the sensitivity of the correction factor, temperature will swing it much more then pressure will unless you have some dyno ontop on Mt Kilamajaro as ambient pressure will only swing about 4% at max due to elevation (resulting in a max correction factor swing of 2%) but temperature can swing 50% easily (resulting in upwards of over 10% swings in correction factors).... and turbo cars ARE effected by temperature

-shrug-

 

FlyinMiata lives and dies by "Correction Factor"

Personally, I never care what my Corrected HP numbers are because I don't drive in conditions they are corrected for.

 

Regardless of correction factors and other variables and drivetrain losses, can the dyno still now be reflective of the power added? GReddy claimed their turbo added 60whp. Most that have this kit have verified just about that increase. We may not know the *real* gross horsepower number, but we do have an idea of what power is now there that wasn't before.

 

you never drive at 70 degrees and atmospheric pressure ?

I hope you then at least pick cold days to get your dyno's done...

 

Well, while I agree that lots of factors that affect dyno numbers, this guy is obviously not an engineer and uses a lot of unsupported "facts" that people might be inclined to believe just because its on the internet (it has to be true, right?)

1) the words "rate of acceleration" appear a couple of times and ought to send-up the red flags right away.

2) saying that a tranny would melt from 37kW but not from 16.5kW is stupid. Why wouldn't it just take twice as long to reach melting temps? BTW 16.5kW is enough power to raise the temp of 500# of steel 3degC/sec. It should be apparent to everyone that there are more losses here than just heat through the tranny and diff. For example, frictional and rolling losses in the tires are probably large, but he leaves these losses out, apparently for added hyperbole.

3) "On an inertial chassis dyno, it is virtually impossible to calculate the the moment of inertia of every tire, wheel, gear, joint , axle and shaft in the power train between the crankshaft and roller, therefore its results cannot offer an accurate HP figure"

This whole sentance pisses me off: a) the same effects would be there for any chassis dyno, not just inertial. b) you don't need to know these moments of inertia. You're measuring power put down by the tires. The effects are taken care-of by Mother Nature. c) of course its not an accurat estimate of crank power. This doesn't mean its not "an accurate HP figure" - obviously its HP put down by the tires

Just another self-appointed expert spouting $hit on the internet - as if we needed more of those... I mean, I see his points, but he really doesn't make his arguments with many well-supported facts. Is he a forum member?

 

Though I like the SDS guys and their site does have a lot of good stuff on it, they are just farm boys for the most part. Their stuff is built by hand from Radio Shack grade stuff. To some degree, I like that because I can take it apart and understand/modify it. However, it does point to the fact that they are not one of the "big boys" and there are probably good reasons for that.

I just can't belive that to this day there is still such a fundamental misunderstanding of the concept of work, the concept of force and the difference/conversion between the two.
Is something as simple a concept as time so hard to grasp?

 

When I dyno, I want the dyno to record what my car does on that day, under those conditions. What I do NOT want is to know what my car 'could' do, under 'ideal' elevations/pressure, and temp.

(shrug).

 

You wouldn't believe!

 

Bah! Hawking is an amateur.

 

Wow, I finally agree with you on something else besides choice of sidearm (1911 worked)

 

so... question

you dyno your car during the day when its 90 degrees out as a baseline and get 187rwhp. You take the car off the dyno and get an exhaust installed and dyno your car at the same place that night when its 75 degrees out and get 192rwhp.... did the exhaust give you any more power????

 

I know better than to laydown baselines in 90 degree weather. I'd never know if I got any gains because correction values simply don't work, despite the idea behind them.

When I dyno my car, I only care about what it's putting down right now. When/if I get a mod I REALLY want to make power, I'll dyno in similar conditions or do a baseline, install the part, then re-dyno the same day.

(shrug).

You're reachin. Correction Values = b.s. pipe dream stuff ppl in high altitudes use to justify their numbers, imo.

 

and the correct answer is you gained 2 hp... 199.5 vs 201.5

correction factor = the only way you can ever fairly compare 2 dyno runs... the Society of Automotice Engineers do not publish "ideas", they published hard and proven facts - many of which make a car as safe as it is