EFI HP compared to Mustang dyno number

[dbaxter_ss]

Ok just wondering after going to the dyno today and getting extremly different numbers.

Mustang Dyno showed 336.5 rwhp at 6000 and 344.5 @ 3800.
Well this is just a bit lower then what most people are seeing with the magnacharger with fuelrail fixes and tune. ( just side info )

EFI is showing that same pull for HP ( Using vehicle weight at 3775 for the GTO ) HP = 469?

Now even more info on this is that I am showing with EFI constantly the HP over 400, which is what it should be.

So my question is, could the dyno really be that far off?

I was assuming that the efi showed the real world number closer to what a mustang dyno showed.


although very interesting
336.5 /.9 = 373.8 mustang to dynojet rough conversion
373.8 /.8 = 467 ( what is what the fwhp for an auto should be ) Is this what EFI is displaying, just wondering since its almost identical to what was reported.

[Blacky]

Was there any wheelspin when logging EFILive horsepower?
Did you do the logged run on flat ground?

EFILive calculates the horsepower using F=ma (Force = mass * acceleration).
As long as the mass (i.e. total weight of your vehicle) is correctly entered into EFILive and as long as EFILive can calculate the acceleration (based on time and speed - with no wheelspin) then the hp reading *should* be rear wheel horsepower.

Anyone who knows more about mustang dynos (I don't), please feel free to try and explain what is happening...

Paul

[Ira]

Well, since the car is accelerating the dyno and not the car it's likely that the dyno accelerated faster than expected. i.e., the dyno was using less mass than the car. It's likely that the dyno numbers are reasonable and the EFILive numbers are based on faulty input.

Ira

[Delco]

Ok just wondering after going to the dyno today and getting extremly different numbers.

Mustang Dyno showed 336.5 rwhp at 6000 and 344.5 @ 3800.
Well this is just a bit lower then what most people are seeing with the magnacharger with fuelrail fixes and tune. ( just side info )

EFI is showing that same pull for HP ( Using vehicle weight at 3775 for the GTO ) HP = 469?

Now even more info on this is that I am showing with EFI constantly the HP over 400, which is what it should be.

So my question is, could the dyno really be that far off?

I was assuming that the efi showed the real world number closer to what a mustang dyno showed.


although very interesting
336.5 /.9 = 373.8 mustang to dynojet rough conversion
373.8 /.8 = 467 ( what is what the fwhp for an auto should be ) Is this what EFI is displaying, just wondering since its almost identical to what was reported.

A chassis dyno can only report rear wheel HP , EFIlive is calculating a engine HP number , 336 hp is a little low as we see that on good M6 mafless cars and about 310hp on a A4. what did your car run without the blower ?

You cant use the efilive calcualtion when on the dyno of course as the ramp rate determines the calculated power , but it will be a good number to use when on the road.
At the end of the day the number is only a indication of what improvement or loss you are getting from changes , it could be 336 fidgets for all it matters as long as you have 360 fidgets next time you go back to the same dyno with mods.

I assume you are only running about 5 psi which makes sense of the hp you are seeing.

My std A4 with a 10lb whipple was making 420 hp on a load dyno like the mustang.

I would suggest getting a bigger maf or going to the 2 bar when its out , also need to replace the front input of the magnuson to take a 90mm throttle body - it really livens them up :D

[dbaxter_ss]

The dyno was showing boost at 9 pounds every pull once it hit 6000rpms

I guess the other questions I have are :

1. the dyno wanted to do the pulls in 2nd, not 3rd. Said he didnt want to run the dyno up over 120mph?
2. how much will that have affected hp reading. I was told this would mess things up since you are supposed to use a gear as close to 1:1 as possible.

[EFIGUY]

Part of the problem is the way the Mustang Dyno "calculates" horsepower

In any acceleration run on a dyno, a certain amount of the engine's power is "used up" in the negative inertia stored in the rollers as you try to make them accelerate. There is also a fair amount of unknown inertia in the drivetrain of the car that cause the same problem.

So when you accelerate the car on the dyno, it measures {physically} the torque applied to the load cell which is connected to the rollers. The software then has to estimate the amount of inertia that was lost and try to add that figure back in to the HP and Torque numbers before displaying them to you. This is very {VERY} often miscalculated accidently by the dyno operator putting in incorrect values in the software sections that calculate this, or not putting in anything at all.

What gear you test in should not affect the power output recorded at the tires when these values are calculated correctly....in other words, if your dyno guesses correctly the amount of drivetrain inertia, then the right amount will always be added back in so that the power reads correctly.....but you will experience a different amount of inertia in each gear so the calculation needs to be performed for each gear to be 100% correct.

The simple acid test to find out if your inertia has been calculated properly is to do a steady state test and record the power/torque figure at a given point...say 75 MPH, this will be zero inertia added or subtracted because there is no acceleration. The F=MA will not work here because A= 0, but the force is now a real funtion of leverage against the load cell so it is effectively like stepping on a bathroom scale.

Then once you have that value do a run on the dyno and see how it plots the power after the inertia correction happens behind the scenes. If at 75 MPH you have the same number as before, then the inertia was calculated properly. If they don't match.....well you can figure out the rest.

This is exactly why we at EFI University primarily tune our engines using steady state {just like the OEM's do} and also we look at "Tractive Effort" which is the actual locomotive thrust that makes the car move forward....we don't exactly care about torque at the tires because that factor is really a measurement of {torque at the engine - drivetrain loss} when the gear ratios are calculated out.

In other words, theoretically the dyno takes out the difference in torque multiplication by looking at the wheel speed vs Engine speed to determine how to get the engine torque, then adds the calculated inertial losses back in, so no matter what gear you are testing in, you "should" get the same number...

Example:
Engine Torque at flywheel = 400 Ft/lbs @ 3000 RPM
Gear ratio in trans = 1:1
Rear Diff ratio = 3.42

The Torque at the tires would be {{400 * 1}*3.42} = 1368 Ft Lbs, BUT the RPM at the tires is {{3000/1}/3.42} = 877.2 RPM

So in case 1: {400*3000}/5252 = 228.48 HP
and in Case 2 : {1368*877.2}/5252 = 228.48 HP

So we can see that while the gear ratio we test in will produce more torque at the tires {more acceleration in 1st gear than in 5th} it also reduces the RPM at the tires by the same amount, so the net power is the same.

The point is that while chassis dynos talk about "torque at the tires" they really mean "Engine Torque - any parasitic losses in the drivetrain" and this value also has the calculated inertia values added to it and this is where the inconsistencies come from.

Clear as mud, right? :wink:

[dbaxter_ss]

Thank you for the very clear answer.

I appreciate you taking the time to explain this in detail.