Loss of rwhp on a dynojet with 4.10s compared to 3.27s (debate with friend)

[StangDriver5650]

He and several other of my friends are saying that it doesn't make a difference because it is all corrected and some say that you will dyno higher because it changed the 1:1 ratio of 4th gear and makes it run through the gear faster so it makes the dyno think you're putting down more HP.

I said that I know for a fact that 4.10s (or 3.73s) will dyno a few rwhp lower than a 3.27 car. I am not sure why this is but I just know that this is what I've been told many times by other Stang guys on other boards. I'm correct aren't I? Could someone explain why you dyno lower with gears? I think I'm going to get and told to use the search feature but I'm going to post this anyway.

 

[raykrv6a]

I would have thought the opposite. I'm running 3.27's and he said it was hitting 140 mph on the speedo during the pull. It was damn loud with the tire howling. I would have thought 4:10's would be more efficient at getting hp to the tires as they don't speed the axles and tires (and dyno) so damn fast.

 

[Craig Mack]

The dynojet does correct for the gearing. The difference may be 1-3rwhp IMO.

 

[JonJon]

The dynojet does correct for the gearing. The difference may be 1-3rwhp IMO.

which could be dyno variance from run to run anyways

 

[StangDriver5650]

Found an answer on corral.net

" http://www.shane.roberts.net/dynojet.htm

These questions have been long standing debates among car enthusiast, so I recently emailed DynoJet and asked these exact questions. Below are the responses I received from the DynoJet representative.

QUESTION:
A car having 3:27 rear end gears is placed on a DynoJet chassis dyno and has a dyno performed. Then this same car is taken off the DynoJet chassis dyno and has the rear end gears changed to 4:10. The car is then placed back on the DynoJet chassis dyno and another dyno is performed. Will the second dyno show a loss of horse power caused by the 3:27 to 4:10 gear change?

ANSWER:
Yes

EXPLANATION:

"The 4:10 gear will show less horsepower than the 3:27. The reason is due

to rate of acceleration changes. The rate of acceleration is quicker with

the 4:10 because of torque multiplication being increased to the rear

wheel. The horsepower will show less because the targeted RPM is met

before the horsepower has a chance to overcome the rotational mass (dyno,

drive line, etc.) or moment of inertia in speed. Because the speed is

decreased and the RPM is met faster, the horsepower never has a chance to

catch up with itself, so to speak. The overall ratio of 1:1 will always

produce the most horsepower on the chassis dyno. Having said this, a

similar problem can occur with horsepower loss when the rear gear is too

high. The horsepower is being absorbed in just trying to keep the

rotational mass spinning. Please keep in mind that your engine's

horsepower never changes but what gets to the dyno or drive surface

does. If you have any further questions please don't hesitate to

ask. Thank you."

 

[00s281sc368]

That is correct, there was a very instructive long post about this a while ago.
With shorter gears, all the wheels and drivetrain will have to spin faster (at the same 4th gear on the dyno) with 4.10 gears, thus increasing inertia and getting less HP at the wheels. Although the engine crank HP remains the same, there is more loss on the drivetrain.

 

[MAC[H]Z]

I have done dyno runs with my stock tires and 4.30 gears and then changed to my taller race tires with my 4.30 gears and I gain up to 5 RWHP difference just with the gear change from the tall tires.

 

[white99gt]

Gear changes effect the acceration rate.But the difference is so small it could be considered a run to run variance on a dyno pull..

I ran 3.27's the went to 3:73's ..NO difference in power exactly the same..

But what does 1-3 rwhp matter anyway if the car is faster..