Redvette_22

I have heard many time of HP loss on the dyno when changing gear ratios. Can anybody verify what HP loss you would see on a car when changing from say 3.15's to 3.42's, then 3.42's to 3.73, and so on up to the new 4.10 that is available for the C5 ? :confused: You always see mention that if a gear ratio was higher it would produce better HP on the dyno. Has anybody done just a gear change on their car and gotten before and after dyno results ?

MitchAlsup

The engine does not loose HP when you change the rear gears (or other gear ratio changes). What happens is that more of the rotational mass* of the drive line has to be accelerated with higher gear ratios. This extra inertia is measured as a loss in HP and TQ by the dyno even though the car is accelerating faster. So what is really happening is that the conversion factor (85%) fo flywheel HP to rear wheel HP is being subtly modified by the gear ratio change.

* First moment of inertia

rhoeven

The way I look at it is as follows.
hp=torque x rpm/5252
Now assume that you change the diff gear with 10%. This means that the rear wheel rpm( dyno drum speed) goes down with 10% and the rw torque should go up with 10% so the result should stay the same theoretically.
The side effect of higher torque however, will also create higher friction which will increase the drivetrain loss hence a few ponies get lost.

Same would happen if you'd increase the engine's torque with 10 % and leave everything else unchanged. The resulting rwhp is not going to increase with 10% but maybe only 8 or 9%.
Contrary to general belief, drivetrain losses are not constant. They additionally are influenced by the speed of the rotationg parts.


Anybody disagree or has other explanation?

Roger

my00c5

From watching my tuner <cartek> do before and after pulls on cars with various gears .. I have observed the following :


3.42 to 3.73 swap is worth 8-10 rwhp.
3.42 to 3.90 swap is worth about 14 rwhp.
3.42 to 4.10 swap is worth about 18 rwhp.

Could be more of a loss with automatics due to different converters.

Scott01C5

That is a little extreme in my opinion. I've had a lot of experience with gear changes and dyno's from my Mustang days, and this is just MY opinion

3.15 to 3.42's - lose about 3-4HP
3.25 to 3.73's - lose about 4-5 HP
3.15 to 3.90's- lose about 5-6 HP
3.15 to 4.10's - lose about 6-7 HP

my00c5

We are talking vettes here not mustangs :) Totally different trans and rear configuration.

Redline Motorsports

Bottom line is that your NOT loosing HP! It's just be measured without being corrected. I don't know why DynoJet doesn't provide there software with the capablity to compensate for gear changes. The same way it compensates for corrrected HP numbers vs. actual. It would at least provide a means of equality between comparisons. It seems like everyone needs to OVER clarify the fact that the gears are robbing them of power. The cars are going quicker..... :lol:

jbsblownc5

I did some dyno before and after gear change from 3:42 to 3:73 and found it to be around 10 rwhp.
JB

no cure

I disagree with most said here, especially rhoeven's entire thesis. that has nothing to do with how it works. nor MitchAlsup saying 'So what is really happening is that the conversion factor (85%) fo flywheel HP to rear wheel HP is being subtly modified by the gear ratio change'. what?! un huh. similarly, 9D9LS1 mentions 'correction factor' but there's no correction for final gear ratio. none. changing the final gear to a "higher" ratio (it's actually lower but everyone knows what we're talking about) resluts in a slight loss of sae corr rwhp due to increased frictional loss caused by the tighter teeth spacing on the pinion gear.

if you take C5 x and it's making (for ie) 500 flywheel hp, and you do 20 pulls over an afternoon (on the same dyno), you will have a range of sae corr rwhp numbs, because there's a 'normal' variance in dynoing the same car, even right back to back on the same dyno. just how car engines and chassis dynos work. so worrying bout 3 or 5 or even 7 sae corr rwhp is meaningless cause it's within the 'standard range of measurement (error)', but anyway, to get back to my hypothetical example, now take this same car, no other changes than quickly swapping the 3.42 rear gear for a 3.73 ring & pinion, and now strap her back on that same dyno and do 20 more pulls, one will see a generalized loss of about 1% sae corr rwhp, still having a whole bunch of slightly varrying numbers. this loss is caused by an increase in the parasitic loss, ie, now more rwhp is going up as heat instead of power, because of the teeth spacing/angle which very slightly changes the coeffecient of friction. that's all. the car is still making the exact same 500 flywheel hp, and there's nothing going on with any 'correction factors'.

btw - imho, the numbers given by Scott01C5 are closer to the truth than any others, that's about what's going on. of course, the more power one starts with, the slightly greater one will loose changing gears.

David

Scott01C5

But we are still talking about gear swaps and dyno's. The totally different trans and rear configuration makes it different on both before and after the dyno, shouldn't make any difference on a before and after dyno session.

Scott01C5

This is exactly what I was going to say. I've put my Vette on the dyno on two different days without doing anything to it and have seen two different results. The second time my numbers were down almost 5 HP. A friend of mine did the same thing with his, he was down almost 7 HP

mackeyred96

any numbers yet????????????????????