Real Fiero Tech

Pyrthian wrote:

Kohburn wrote:weight should be irrelivant when not moving and measing hp at the wheels. would only need weight to take that measurement and then calculate estimate 1/4 mile time

yeah, that what I was thinking as I hit Submit.....
the HP at the wheels is the HP at the wheels.

Sort of. Dyno measures how fast the wheels are accelerating, it's the only thing it has to go by. That would change alot if there was only 1500 lbs of weight compared to 3000 lbs of weight put on by the rollers. Mustang Dyno is alot more accurate, since the DynoJet only uses 2000 something punds to simulate the weight of the vehicle.

S8n wrote:

Pyrthian wrote:

Kohburn wrote:weight should be irrelivant when not moving and measing hp at the wheels. would only need weight to take that measurement and then calculate estimate 1/4 mile time

yeah, that what I was thinking as I hit Submit.....
the HP at the wheels is the HP at the wheels.

Sort of. Dyno measures how fast the wheels are accelerating, it's the only thing it has to go by. That would change alot if there was only 1500 lbs of weight compared to 3000 lbs of weight put on by the rollers. Mustang Dyno is alot more accurate, since the DynoJet only uses 2000 something punds to simulate the weight of the vehicle.

what? the equipment should be calibrated to the rollers not the vehicles weight since the vehicle itself is not moving..

ie the equipment is measurign the acceleration of a drum - based on how fast the drum accelerates and the known inertia it should be able to tell you exactly what the power ouput at the tires surface is. has nothing to do with the weight of the car.

There's no simulation about it... The drums have a known mass and known moment of inertia. The engine's power can be calculated directly from their acceleration.

Now, if the dyno's trying to match the car's acceleration on the road so as not to misstate the impact of flywheel effects on the output at the wheels, THEN it might be appropriate to use vehicle weight, but that's the only reasonable way to use vehicle weight on a dyno.

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Originally posted by donk_316
Well which one is right?

Mustang too low?
Dynojet too high?
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This is a question that is endlessly debated across the internet...

We're obviously biased toward the Mustang, as we own one, but when we didn't own a dyno we weren't biased towards one or another and researched all different types and brands of dynos we could, with price never being a factor, and we decided on the Mustang.

The main difference between the dynojet and mustang is that the mustang calculates horsepower as accel/time with load applied, whereas the dynojet calculates it as accel/time against a mass of known inertia.

The reason we went with the Mustang is that the Mustang results are more 'real-world' as they are against a dynamic load that is as close as your vehicle will see on the street, whereas the dynojet results are against a fixed-mass inertia which, unless your car happens to weigh exactly as much as the mass and is as thin as a piece of paper (IE: no wind resistance) is not going to be the same as your car would see on the street. But don't just take my word on that, if you do your own searching I believe you'll find that this is the general concensus.

In any event, if you wanted to do a 'fixed-inertia' test (Just like a Dynojet) the Mustang can do that as well. I just don't know why you'd want to, unless you just want bragging rights numbers

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Originally posted by McFly
If subracings mustang dyno was inertia like the dynojet(which is actually made) the nthe numbers would be almost identical.
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This is correct. Dynojet vs Mustang static inertia numbers should be the same.


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Originally posted by McFly
A Dynojet will be consistant everywhere where as a mustang is all dependant on the opperator.
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It's not so much the operator as much as correct vehicle weight. To obtain the most accurate dyno test possible, it would be best to have the car weighed with the driver in it and that number used for the Mustang load test.


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Originally posted by donk_316
but why does "wind resisitance" and "weight of vehicle" matter when all you want to know is how much "power" your car has or "how fast does it spin the drums"
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The amount of drag produced by wind resistance is rather important. The amount of drag produced will slow your car down and create a force your car must over come, therefore creating a greater load and requiring more power to overcome that force.

The tune created on a dyno that don't take these factors into account may not be accurate on the street, thats why you often people having to make "street adjustments".


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Originally posted by donk_316
AHHHH ok... i understand that..

But what if your not tuning? The car is tuned and you want to see what it lays down?
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Sort of depends, I suppose, on what 'lays down' means to you. If it means, correlates to a 1/4 mile time, or similar, then I would suggest you want a dynamic load. Anything other than that is just 'theory' as far as I see it... sort of like, jacking your wheels into the air and seeing how fast they spin up, I suppose . It might look cool, but what does in mean in terms of how the car is going to be used?

Also, as an aside, you can do simulated 1/4 mile runs on the Mustang as well, complete with launching as hard as you want, and it will even give you a simulated timeslip . Some day, I'm going to do some back to back testing (next year, looks like) and see how close the 1/4 mile simulation is to reality.


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This is a conversation posted on 780 with me and the Mustang Dyno Operator at Subracing

dynamic load makes sense for tuning

but adding in calculated rolling and wind resistance does not change the actual power output of the car, that part is just stupid. the power output is the same but taking that power output and the estimated resistance would give a more accurate 1/4 mile estimate.

suggesting that more wind resistance reduces your power output is rediculous


i do like the idea of dynamic load for tuning purposes though

I agree on the irrelevance of wind resistance. Garbage in, garbage out.

Donk, would changing the weight the operator inputs change the power the dyno reads? Do you think the output of your engine actually changes with the weight of your car?

donk_316 wrote: It's not so much the operator as much as correct vehicle weight. To obtain the most accurate dyno test possible, it would be best to have the car weighed with the driver in it and that number used for the Mustang load test.

So why doesn't the dyno have a scale built into it? That seems like the logical thing to do since the car sits ON the dyno. Without accurate weight, the results are apparently garbage.

The amount of drag produced by wind resistance is rather important. The amount of drag produced will slow your car down and create a force your car must over come, therefore creating a greater load and requiring more power to overcome that force.

This is *junk*. The car doesn't make enough more power to worry about with a small acceleration vs a smaller acceleration. The only impact "load" (IE, acceleration) will have on the power output the dyno reads is through the flywheel effect of rotating components.

So how do you accurately measure wind resistance without accurate coefficient of drag data for the car? Do they take basic measuremens to even estimate frontal area? This claim is just another case of garbage in, garbage out. The idea that bad information is better than no information is fundamentally flawed.

Feel free to quote me.

Thats what i dont understand... if im not "tuning" who cares what my car weighs? If it weighs in at 5 tons it would still be xxx hp.

I think once i go to this shop and see what kind of increase i get with full boost and new coils, Ill go to a Dynojet and get "socially acceptable" numbers that arent dependant on the dyno operator. As IF my car is only putting down 220hp.

Well get an explanation of how the variables affect the output. If the horsepower figure is significantly dependent on the input weight, then we know it's crap. I don't see how the dyno can be *that* fucked up and the MFG still be in business, though.

donk_316 wrote:Thats what i dont understand... if im not "tuning" who cares what my car weighs? If it weighs in at 5 tons it would still be xxx hp.

If you're not tuning and just want a number, then it's fine. But, especially with a turbo car, the weight of the car determins the load on the engine. That affects how quickly the turbos spool, which affects your power output. Makes a big difference when trying to properly tune a vehicle.

No, the ACCELERATION changes boost/RPM relationship. The acceleration changes as a function of gear and speed. However, trying to take a half-assed stab at it without having the correct data from which to work is worse than not dealing with it at all, IMNSHO.

Programming a different weight into a Mustang dyno will effect the results of a dynorun.

People like to use a Load bearing dyno like a Mustang for tuning because it helps to simulate a load on the motor like out on the street. Simple laws of physics state that it takes energy to accelerate an object through air at an ever increasing spead. The mustang helps to simulate air and ground resistance as well as the weight of the car accelerating through space on the drivetrain.

Turbos spool faster on a Mustang dyno then a dynojet because a laod is placed on the motor instead of just a drum spinning. Turbo guys like to use Mustang dynos as it simulates the real world better I suppose with a load being on it. Helps fine tuning everything.

I dont' know how much a dynojet drum weighs, but if its less then your car, then its not a very good representation of the load your engine will see while in gear driving down the road. My car accelerated in third gear on the dyno then it did out on the street. I am guessing the drum doesnt' weigh what my car weighs.

The Dark Side of Will wrote:No, the ACCELERATION changes boost/RPM relationship. The acceleration changes as a function of gear and speed. However, trying to take a half-assed stab at it without having the correct data from which to work is worse than not dealing with it at all, IMNSHO.

The engine doesn't have to change rpm for the load and boost levels to change......vehicle doesn't even have to accel or decel for load and boost to change. As the load on the engine changes, boost will be affected.

If your car weighs a lot more the turbo will spool at a noticeably lower RPM in the same gear because your motor will be under full load in the low RPMs longer (since it takes longer to accelerate).

Not to throw things off base here but my local shop uses Dynapacks. Do I throw all this weight BS out the window when I take my car in?

ah yeah - it would make a difference for the real worl drag racing senario whp output of a turbo engine in that specific gear, so do they do a run in each gear then? the turbo will spool differently in each gear and hold boost differently based on how fast they shift and the turbo setup.

S8n wrote:

The Dark Side of Will wrote:No, the ACCELERATION changes boost/RPM relationship. The acceleration changes as a function of gear and speed. However, trying to take a half-assed stab at it without having the correct data from which to work is worse than not dealing with it at all, IMNSHO.

The engine doesn't have to change rpm for the load and boost levels to change......vehicle doesn't even have to accel or decel for load and boost to change. As the load on the engine changes, boost will be affected.

At WOT, which is the condition under which dyno runs are conducted, LOAD == BULLSHIT. Load doesn't vary. The engine's always putting out it's max power at that RPM. Acceleration is what varies.

For instance, if you program the dyno to hold the engine (naturally aspirated for simplicity of example) at 4000 RPM, and then allow the engine to accelerate from 4000 RPM, is the engine putting out MORE power when it's accelerating? Is it putting out less? NO. The engine is putting out the same power, but when the engine is accelerating, some of that power is going into spinning up the engine & rotating components of the dyno.

The easiest way to tell would be to log the mass air flow readings and go from there. The only way an engine makes power is under load. Sit in your car in neutral and go WOT. Might not make it to WOT before you hit redline. Is the engine making peak power and torque? No, not even close. Its an extreme example, I know, but less load will accelerate the engine faster, and the rear wheels ( not in neutral though) faster. And that is the only thing the dyno measures is how fast the rear wheels are accelerating.

okay so how does all this help me? Im supposed to go back for another run this afternoon. but the only things that will have changed will be new coils (IF they get here in time) and 4-5psi more boost. I should make sure the same calibrations are used as last run to get a fair comparison.

Then when i want an all out HP war, ill let him know...lol! or go down the street to the dynojet