Forrest Gump 9

iTrader: (19)

Dynos are tuning devices!! Get over it.

CandCPerformanc

Thats not so true, our Dyno granted is only a 2 wheel dyno has the option for gear roll on pulls and also full 1/4 mile pulls including launch run off of a porta tree light and all and you can log in online with another Dynocom Dyno and race another car in real time as well. Yes we have tried the race function with our race car and it was 1 tenth off from what our average run is it said we would run a 9.17 and we usually run 9.06-9.09 average dut the dyno does not account for head wind and or wind drag.

9sec9

iTrader: (2)

I agree with Forrest Gump 9's last post. With that said, I would like to make one post for those trying to 'convert' from one dyno to another when trying to make comparisons.
1) It should be understood that it would be good for only YOUR car under the exact same conditions. Not good for YOUR car vs someone else's on different days and conditions.
2) When converting FROM Mustang Dyno TO Dynojet or FROM Dynojet TO Mustang Dyno, the percentages are different depending on WHICH dyno your convertin FROM. Simple math. For example using actual figures of the OKIX car:
Mustang Dyno = 425whp
MPH 127.80 @ 3180 lbs indicates 518.1 mph, based on calculator.
If the 518.1 indicates CRANK Horsepower then, reduce it by 18% which would yield 424.84 wheel horsepower. It dyno'd 425 whp on the Mustang. EXTREMELY accurate if the calculator and times are used. Remember, these are independent to his actual results.
Now, Let's assume that the 425 whp is to be converted TO Dynojet numbers. (why, I don't know, doesn't matter, but let's assume it just for sheets-n-giggles).
If the 425whp is multiplied by the assumed 18% conversion factor, you would only arrive at 501.5 Dynojet number. WHY? Because you're trying to arrive at 518.1 and the percentage MUST be increased when converting from the Mustang TO the Dynojet, if you expect to arrive at the same CRANK (if that's what it is) horsepower. It would need to be increased by nearly 22 % (425 + 22% = 518.5)
The opposite is true when converting FROM Dynojet TO Mustang. If a Dynojet reads 518.1 whp, then to arrive at the Mustangs 425 whp, you would need to REDUCE the percentage to arrive at the Mustang number ( 518.1 - 18% = 424.8).
With all of that said, I'm certain that most calculators consider the MPH vs horsepower to be using a final drive of 1:1, not the Evo VIII 1.03 or the Evo IX 1.09. This alone will make a difference when on the same dyno and conditions. An Evo IX registering 400 whp on ANY dyno, would register MORE whp if the 4th gear were changed out to the VIII 1.03. Not much, but still a difference. Again, simple math. My 2cents to the conversation.

Mad_SB

iTrader: (8)

That has always been my observation. In my area, dynojets seem to be between 15 and 20% higher than our dynodynamics ( we don't use any additional correction where many dynodynamics shops use 1.1 or 10% higer correction).

On our dyno a stock evo will dyno about 23% lower than the factory rating so we figure add 23% to get your flywheel. FWD and RWD are in the 10-15% range lower than advertised on our dyno.

I would assume that dynojets assume a 10% drive train / friction loss for all vehicles.

Not to mention the operator can always change correction factors to make their tune look good or someone else's look bad.

Mad_SB

iTrader: (8)

At least one exists, ours

Seriously though I'm not sure about the certification but we do not use any additional correction factors and are happy to do shootout mode graphs. They are very accurate for flywheel hp, within about 10 hp in most cases.

4TUN8

iTrader: (18)

My post was about pulls for dyno sheets, IE you make a 3rd gear pull and print out the sheet. Anything else that you read into my statement is just that.

CandCPerformanc

4TUN8 I was not trying to argue with you on that at all I was just stating that there are dynos that will average out all gears if you want and they will give you a semi real world number for quater mile time. Our dyno states our car only makes 886 whp but the car weighs 3465 per class rules and we run a 72mm turbo but the 1/4 mile times are pretty darn close if you run it in 1/4 mile simulation. I know Dynodynamics is setting up software for the very same thing I just know how many people you would find that would trust strapping their car down and doing a full blown 1/4 including launch.

lemmonhead

Chassis dynos are great tuning aids but they only give a approximation of power output as some of the important variables are not accurately controlled. Certain magazines seem to think that results obtained from chassis dynos are the gospel. They are not. 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. Tire alignment has been shown to affect results up to 3% as well. Note that Engine hp DID NOT change here yet the dyno recorded an increase in hp at the wheels. A change in wheels/tires also does not affect true, wheel hp either, only the rate of change on the rollers and the vehicle acceleration on the road. One can only conclude that inaccurate moments of inertia and correction factors are being used.

On intercooled, turbo cars, there is usually insufficient airflow to ensure accurate results due to charge temperature variation which can be substantial. Even coolant temperatures may not stay down during the run which can affect power outputs considerably. The rate of acceleration is also important on turbo cars to be sure that the boost is not lagging the engine rpm. With RPM climbing too quickly, the boost has not reached a peak value so the hp figure is again inaccurate. Turbo cars should therefore be tested in top gear.

Without proper temperature stability and accurate moments of inertia on the rotating components, there CANNOT be accurate results as the scientific method is no longer being applied.

When all things are kept the same between runs and you get a tangible gain, it is a gain at least. How much, is open for discussion. It is important to note that as the oil temperatures in the engine, transmission and differential increase, friction usually decreases. This manifests itself as an increase in power at the rollers on each subsequent test. This factor should be accounted for when doing back to back runs. It may look like you are gaining some power on each run by making other changes when in fact this is due to reduced oil viscosity. When using a chassis dyno, always use the same gear and tires and wheels and start the runs from the same speed or RPM. Re-baseline periodically to see what temperature increases have done to power output.

Chassis dynos are quick and easy to hook up but have many of the above failings. They do not possess the accuracy of a properly calibrated engine dyno which has a more carefully controlled environment and condition set. Obviously, most non-professionals don't want to be yanking engines to use an engine dyno so chassis dynos do have their place.
Comparing the Numbers

Many novices are quick to compare hp numbers between chassis and engine dynos and come up with all sorts of wild conclusions about drivetrain losses. These comparisons are essentially meaningless. Inertial dynos are based on the sound scientific priciple of accelerating a certain mass with a known moment(distance) over a given time. The rate of acceleration of that mass and moment is a result of the force applied (torque). If the RPM is known, HP can be calculated. 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. Even with coastdown drag measurements, these cannot be accurately calculated as different factors are affected in different ways. Some are proportional, some are inverse squared functions etc. Inertial engine dynos offer a very accurate figure if properly calibrated as only the flywheel's moment of inertia needs to be calculated and added to that of the billet. Water brake or eddy current dynos generally measure force (torque) directly through a ram or strain gauge so moments of inertia are not important on these in fully loaded tests.

Concluding that there was a 25% drivetrain loss by comparing HP achieved on an inertial chassis dyno and that obtained on an engine dyno is fundamentally flawed in that the chassis dyno numbers are highly suspect in the first place.

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.