Someone told me today that a car with 3.73 gears will make more power than the same car with 2.73 gears. Is this statement true? If it is I dont get it.

 

If the same car was dyno'd with 2.73's and 3.73's. The car with 3.73's will dyno lower.

A car with shorter gears will not make more power

 

Can you tell me why?

 

the gear ratios have a way of tricking the dyno. The motor doesn't actually make more power. the dyno software calculates HP by using different types of information it obtains during the pull. One of these types is how fast the car accelerates. That's one of the reasons that you use a chassis dyno to tune the engine and drivetrain, not measure HP.

 

Anyone else

 

more power-no........better launch off the line-yes

 

doesnt matter what gear you put in a car it will always make the same hp steaper gears just allow faster accel which is always nice. for a 350 tpi 3.27 is probably best if you go with hsr the steaper the better keeps you in higher rpms. tpi likes lower rpms keep it between 2500 and 4500 and you will pull harder.

 

Cool. thanks for the responses

 

A 3.73 gear will have a higher mechanical advantage. This helps you accelerate (unless you select a gear that does not match your powerband) doesn't mean that it will "dyno" higher. You increase axle torque. One of the reasons why you see numerically higher gears dyno lower is because they have a greater parasitic loss. There are more teeth and thus more contacting surfaces which equates to higher drivetrain losses.

 

Huh? You mean to say a half a dozen more teeth on the ring gear is going to show a measurable difference on the dyno? What ever loss that creates will more than negated by the rear wheel torque created by this mechanical advantage.

 

I will use a stock LO3 to help illustrate this a bit.

Assumptions.

170hp at 4200rpm
80% driveline efficiency
245/50/16 wheel
Gear swap to have a final drive ratio of 2.73 to 3.42 (we will assume the dyno will be in 3rd gear for the 700R4 and or 4th gear for the T5. Both being 1:1 gears).

We know that a gear change increases the mechanical advantage of a car. This can easily be seen with a tractive force (Ft) comparison through the procedure below.

Ft = (e * HPe)*(375/V)

Where

e = engine efficiency (assuming .80)
HPe = peak hp at the engine
V = speed at peak power rpm (with final drive and 1:1 tranny gear)

V = 60*N/R*Nt


Where,

N = peak HP rpm
R = total gear reduction (our changing factor for this example)
Nt = Tire rev/mile
rt = radius of wheel and tire

A 245/50/16 tire has a sidewall height of 4.82” and adding half of the wheel diameter gives you the radius of the rolling wheel.

rt = ((4.82+8)/12)*.98 (for tire squish) = 1.047ft
Nt = 5280/(2*rt*pi)

So for our application Nt = 802.5

So back to V.

V = (60*4200)/(2.73*802.5) = 115mph (with 2.73 gears)
V = (60*4200)/(3.42*802.5) = 92 mph (with 3.42 gears)

Ft = (.80*170)(375/115) = 443 (lbs)
Ft = (.80*170)(375/92) = 554 (lbs)

You can see the increase in driving force at the wheels. A 3.42 gear gives you a 25% increase in that regard.

Now to equate that to HP at the wheels.

HPwheels (HPw) = (Ft * V)/375

So using our calculated Ft for both gear ratios you have.

HPw (2.73) = (443 * 115)/375 = 136 hp
HPw (3.42) = (554 * 92)/375 = 136 hp

136 at the wheels is consistent with stock LO3 dyno runs (see below)

You can see there is no gain at the wheels. However, as stated above, the numerically higher gears will have an increase in coulomb and viscous friction thus decreasing the driveline efficiency compared to a numerically lower gear. So in the real world you cannot use the same driveline efficency factor. The difference is small though which is why when you switch to a different gear set you should see little to no change on the dyno. You will see a difference at he track and in your butt dyno because of the mechanical advantage increase.

Here is a dyno sheet of Dewey316’s stock T5 LO3 with 3.42 gears. The run is in green and validates the 136hp calculation.



Here is a dyno sheet of Xceleratemaro’s stock A4 LO3 with 2.73 gears. The driveline efficiency differences between the two transmissions (T5 is roughly 5% more efficient) is one of a few causes to explain the slight power differences. None of the variance is from the gears themselves. Adding a 5% increase to his driveline efficiency would yeild around 138hp thus showing a slight power drop with a gear swap.

 

[QUOTE]Originally posted by ShiftyCapone
[B] You will see a difference at he track and in your butt dyno because of the mechanical advantage increase.

Its funny some kids around here with TPI cars get lower gears and run the same times as they did before. The tpi powerband is strange like that

 

[QUOTE]Originally posted by 89IrocZ350TPI
[B]Seeing the same ET is common (gear swaps on an otherwise stock 3rd gen) but they should still see an improvement in their trap speed.

 

Yup, as I think back the 2.77 gears were actually pretty decent. Got a 13.7 with those at 96. Then with 3.42's I got a 13.5 at 99. BUT I had better tires when I ran with the newer 3.42's so its tough to say how much the gears really did. Could smoke the tires with both gears either way so its all good.

 

What was your trap speeds with each gear? Trap speed will take traction and most driver error out of the equation and let you really know what you gained with a mod.