Interpreting Dyno Data

[Doug Axelrod]

All,
Is it more appropriate to use the corrected numbers for torque and horsepower when communicating dyno test results, or do people use the raw numbers?
Thanks,
Doug

[Roscoe]

Doug,
I believe the lack of response here is due to the lack of info you provided. Could you be more specific and give us some more information???
Roscoe

[Dan Stryffeler]

Doug, I think most numbers are corrected (by SAE J1349 standards) for temperature, humidity and altitude (distance above/below sea level) because all these factors affect air density. Mine worked out as shown

Per SAE J1349 Standard
Abs. Barometric pressure 29.23"
Humidity 00.0%
Temperature 77F
Air density ratio 93.4

My SAE correction factor was 1.0487 for 29.55", 36.6% and 82.3F

At least, that's what I was told...

Dan

[Doug Axelrod]

Folks,
Sorry, I thought the data were sufficient. The engine was dynoed, I believe on a DTS4000 (I can find out more, if needed). The printout lists:
Speed (rpm)
Power (hp)
CPower (C_HP)
Torque (lbft)
C_TQ (lbft)
BSFC2 (lb/hpr)
and then columns for water and oil temp, and oil pressure

It's my understanding that the corrected terms account for ambient temperature and air pressure to some agreed-to baseline. I assumed that all dyno tests provide this sort of corrected data and want to know which terms folks use for engine to engine comparisons.
How's that?
Doug

[Bob Putnam]

Doug,

In a verbal horsepower contest, it's always fairer to use corrected bhp - unless your engine has been tested at 31" of mercury and 0 deg C.

Unfortunately, most dynos do not exactly correlate with other dyno results, so the best bhp comparison comes from two engines tested on the same dyno on the same day...

[cobish]

amen, brother Bob,
Dough,
standard dyno parameters, what were the numbers ofthe run?

[Richard Hudgins]

Doug,

It is most important that the standard correction factors be applied when doing any dyno testing as part of an engine development program.

Without the correction factors applied, you will have fairly significant variations in your numbers. Therefore, you would not know if a jetting change helped or hurt you. This applies to any change you make, headers, intake stacks, valve clearance, etc.

Also, dyno rooms can change temperature and humidity very rapidly during a run cycle. We always recorded the correction factors at each 500 rpm step and applied them to that part of the cycle. (Of course, these days, these things are recorded automatically and everything calculated by computer in real time. In my days the best we had was a HP calculator with reverse Polish notation. (You engineer folks will understand that one : )

With chassis dyno's, you open a whole can of worms when it comes to repeatabilty and serious engine development. I have seen as much as 22 ft lbs of torque difference on the same car from cold transmission and rear end fluids to warm fluids. (928 Porsche S4)

Those curves were really confusing as the driveline started to heat. The worst of was after the system was good and hot, you started losing engine performance due to heat soak.

Therefore, for serious engine development, an electric engine dyno is the only way to go. (Not a water brake, been there, done that, if incoming water pressure changes, you start getting woogie numbers)

For bragging rights, i would recommend a chassis dyno with a hot drivetrain and a cold engine.

[Roscoe]

One inmportant factor I see missing (tell me if I missed it) is Air to Fuel Ratio. When I dyno'd my Cobra I faxed the results down to Demon Carb. Based on the results, and interpreting the air/fuel ratio, I made some changes including rejetting. My next dyno session gave me 60 more hp!!!

Roscoe

[Doug Axelrod]

Excellent points!
I'll use corrected values in communicating engine performance parameters and will check in to see if air/fuel data were collected (I think they were). The engine has a 750 Demon on it and I know the builder played with jets before the dyno run I attended. That was the 12th run, so a lot of optomization had been done. When the car is delivered, I'll share all the parameters. Until then, it's vaporcar..
Doug

[Roscoe]

Doug,
Before you spend anymore money on dyno testing, call Demon. They have a great tech support line. Tell them what you are doing and fax the info to them. They will call you back with suggestions.

Here's what they told me:

1) Change linkage from progressive to one to one. $ 7.50
2) Put 1" 4 hole spacer between carb and manifold $ 20.00
3) Adjust linkage to make sure you are getting full stroke (Free)
4) Jet down 2 numbers (I bought jet kit) about $ 30.00

Result: 60 hp gain

Roscoe