Gear swap vs. RWHP

[rcdice]

Would a rear gear swap, say from stock 3.07 to 3.55 have ANY effect on RWHP results on a dyno? My understanding is no.

I have an appointment next week to have my bone stock car dynoed (DynoJet) to establish a baseline. I want to re-dyno it each step of the way with a few mods to see exactly what each one does. First tubes and filters, dyno again. Then high flow cats and cat back, dyno again. When I get the exhaust done, it makes sense to do the gear at the same time as the car will be on a lift with the exhaust out of the way. My question is, will I get a pure reading on what the exhaust mod did to RWHP or will the new gear skew it? Any opinions? Thanks,

 

[ViperRay]

The short answer is no. In theory though, there may be differences in frictional losses with the different gear ratios but I don't know that it would be measurable. Perhaps someone here has dyno tested this.

 

[opnwide]

I usually hear of slight LOSSES in rwhp when going to 3.55s, but not sure why. I've got a set I'm fixing to install, and I'll definately dyno afterwards so any future mods have a good baseline.

 

[Cris]

I would guess that a difference would be more apparent with a Dynojet or other inertial dyno. That is because the lower gears allow the engine to speed up faster during the test, making the driveline and engine inertia a larger player.

I have measured mine in 3rd and 4th gears and there is a significant difference. I know some is due to trans efficiencies in the different gears, but the difference in driveline inertial losses is significant.

 

[FE 065]

I've always been puzzled why a gear change would show less hp.


If your torque multiplication is increased with the gear swap, shouldn't you be levering the dyno's big wheel around easier/faster-just like added hp does?

 

[Martin D]

You will lose 2-3 percent with the change. Which is really not much...until you start talking BIG numbers. But also, you can lose power with how the car is strapped to the Dyno.

Regards,

 

[Cris]

With a lower gear ratio the dyno is spinning slower throughout the test than it is with higher gears. Because it is spinning slower the rate of change in drum speed must be higher to absorb the same level of applied power. If the vehicle had no rotational inertia (engine, trans, differential, axles, wheels) and the differential friction losses were constant, then the power measurements would be the same. But, because there is driveline inertia, the fact that the driveline must speed up quicker means that more power is being absorbed (really stored) by the driveline during the test. If you wanted to spin the dyno at incredible speeds you could do the test in 5th and reduce the influence of the driveline inertia.

 

[rcdice]

Wow. This is getting is getting heavy for my non-engineer brain. Fascinating though. Perhaps I'll test it after the exhaust mod with the stock gear then again after the gear swap. That would be interesting. As stated, I kind of wanted to do the gear at the same time as the exhaust but I would like to get an accurate reading. If RWHP could read 2-3 percent less with the gear, then I'll do them seperately.

From what I've read, 4th gear is the most common to dyno with. Any reason I shouldn't use 4th?

 

[FE 065]

I'm going to add a Borla catback, then go back under the car and add a 3.45 gear, measuring changes at the strip each time.


Then the Belangers

 

[joe117]

4th is a straight thru drive for the transmission at a 1:1 ratio. The shafts are locked and there are no gears between the input and the output. No gear loss.

 

[AJ]

you will lose about 3-7 rwhp due to higher friction with the shorter gears.

alex

 

[Jerry Dobson]

Transmission has nothing to do with the rear end gear change. The rear end is a "mini gear box"....like 3.07:1

Yes there is about a 3% loss.


"The Chassis Dynamometer measures power that reaches the tires, not Engine Power. Therefore, any changes you make to your drivetrain may result in a change in your chassis Dynamometer results. A steeper gear ratio will provide the following,

1. Higher peak force to the ground
2. Higher average force to the ground
3. Lower top speed (MPH)
4. Higher peak acceleration rate in all gears
5. Lower peak horsepower

The reason you lose your peak hp is based on higher acceleration rates, increased gear mesh/losses, higher tire losses. These will affect your peak horsepower number in a negative fashion. Losing approx. 5% is expected when changing to a significantly steeper rear gear. Engine power was not reduced, power to the ground was. In exchange, you have more force available which is what allows you to accelerate faster off the line. If you enjoy the feeling of Gs forces as you accelerate, you made a wise choice. Do not be discouraged by the peak power numbers—they are just a reference"

 

[IEATVETS]

My understanding is that yes you do lose 2-3% rwhp but gain it back in torque. In other words horsepower goes down but torque goes up.

 

[joe117]

HP is calculated from rpm and torque.
In a perfect world a gear ratio that traded rpm for torque would have no effect on horsepower.

You can't trade horsepower for torque with gears,
you can trade rpm for torque.

If there is a change in dyno horsepower when you change gears, it must come from one of two things.
A change in gear friction.
Or,
A flaw in the dyno's design.

 

[Ulysses]

Loss of traction on the drum could also be a factor.

 

[GR8_ASP]

Joe the flaw in the dyno design is that it is not measuring horsepower or torque directly. A normal dyno measures the beam, or torque and the rotational speed. Those combined calculate horsepower exactly. However, the most common types of chassis dynos are inertial dynos. They do not measure the torque but instead measure the speed of the drum, and based on the rate of change in the speed calculate the horsepower. To do this they must be in a dynamic situation (i.e. going swiftly through a speed range). Because of this the vehicle driveline inertia (engine, flywheel or torque converter), trans, prop shaft, differential, axles, wheels and tires) are part of the equation. That is the inertia of those parts "steal" from the instantaneous horsepower available. Thus the higher the rate of change in the drum speed, the higher the "loss" attributed to the vehicle's inertial components. Thus lower gears will always result in a lower horsepower reading with an inertial dyno. It is simple physics.

The same is of course true when you drive. In first gear, as the engine speeds up more rapidly, the driveline is storing energy faster in the driveline inertia than it does in a higher gear. That is why a lower flywheel weight (i.e. inertia) feels like more horsepower.

When you shift into the next gear the stored energy is effectively released and results in a momentary increase in acceleration. All thing held equal that is why you may chirp the tires when shifting. It is the release of the stored energy that provides the torque to spin the tires. The faster you shift the greater the stored energy is available. If you shift slowly the energy is just lost in allowing the engine to slow down slower than it would without the inertia. That is why a light flywheel results in a car with more engine braking.

So to say the difference in measured power (using an inertial dyno) is more friction is not correct. The different gears may affect the differential friction some, but not to a significant degree. The vast majority of the difference is due to the driveline inertia and the dynos measurement method.

My $0.023 (damn inflation)

 

[joe117]

I agree.
The point is, what look's like a change in horsepower, due to gear ratio, is really just an artifact of the test equipment.

Now, did anyone consider that perhaps the same artifact is causing some of the difference between the data from an engine dyno and a chassis dyno?

Every time someone tells me there is 15% loss in the drivetrain, I wonder where all the heat is going. I know there is some loss in the drivetrain but some of the 15% must be due to the fact that there is a gear ratio involved.

 

[GR8_ASP]

There are various formula for estimating the driveline losses. As a generalism they are in the 12-15% range for most manual transmission cars. Also, in general, the higher the horsepower the lower the percentage as the viscous and non-load related friction are a function of speed and not load.

I would guess that for most Vipers the loss would be about 13%.

 

[joe117]

How much of that 13% is real and how much is because of the inertia factor mentioned above?

If 3.55 to 3.08 causes a change in the dyno reading, then a 1:1 connection on an engine dyno must give different readings, due to inertia, than the chassis dyno.

So some of the 13% conversion factor must be an artifact and not real loss.

 

[Jack B]

Has anyone with 3.55's or 3.73's tried a fifth gear pull. It is not direct drive, therefore, has some associated losses, but would have less inertia losses. It would be interesting.

Inertia losses may not be quite accurate, perhaps a better description would be the extra hp required (due to the delta rpm) to rotate the, engine, transmission,drive shaft and the pinion gear