was curious how much gain i could expect changing rear gears to 3.55 or 3.91 i have 3.23 now 69 roadrunner 4-speed 28in tire 440 510 hp/ 540 tq flywheel 310 hp/ 415 tq rear wheel 4400, redline thought would have done better but dyno crew says my rear gear is killing it,

I don't think there should be any change.

It cant change H.P. but you will see a performance gain. I would recommend the 3:55

It will change the horsepower and torque level your system operates in at rpm levels.

3:91...

a gearset won't change the Hp/Tq that the chassis dyno actually sees? I always heard that it would, as gears "multiply torque"

if it doesn't matter what the gear ratios are, then why do they always do the dyno pull in the 1:1 gear?

I would change the dyno crew first

So...if you made a dyno pull in second gear, wouldn't be the same as pulling it in high gear with 4.68 gears? I always wondered if the final gear made a difference on a wheel dyno. Do they have to input final gear ratio and tire diameter before they start?

I realize that most of Kansas doesn't have a lot of elevation, but if the flywheel horsepower is a corrected number and the wheel horsepower is observed, that could explain some of the huge loss.

3.91s will theoretically give more parasitic than would 3.23s, all else being equal. Maybe 5%. Probably less.

What is type/brand of chassis dyno? Mustang dynos give notoriously conservative, but arguably more accurate numbers than a Dynojet.

Are those engine numbers verified on an engine dyno? If so, those chassis numbers are low regardless of dyno type or gear ratio. I would expect to see 400+. You've got some serious parasitic loss in the drivetrain. Bad bearings, brake drag, driveline misalignment, etc. Is AFR being monitored during the pulls and is it good?

...and of course, the correct answer is zero.


Dave

how do you change an engines potential by adjusting the rearend ratio ?? You'll change speed/rpm with different ratio in all the gears but the horsepower potential remains the same.

Different coverters(for auto guys/gals) will only enhance or decrease the "potential" EXCHANGE from engine to tranny to rearend.

It's like taking a full 16 oz glass of water and putting it in a 32 ounce glass...You still have 16 oz.

Also, the type of air filter, headers, and mufflers on the car compared to the engine dyno could cause some loss...especially if the air cleaner lid was so close to the carb it was choking it.

So they didn't take it past 4400? that would explain the okay torque differential but bad HP differential.
BTW that dyno operator probably failed math class.

Sheldon

and maybe it's a combo of a happy engine dyno and a stingy chassis dyno. if the engine dyno was reading 20-30 high, and the chassis is 20-30 low, that would also account for the variance.

If you really ARE losing that much Hp through the chassis, then start looking for problems...as mentioned, brakes dragging, driveline mis-aligned, bad bearings, maybe a bad torque converter, etc.

also, make sure nothing in the motor has gone bad from engine dyno to chassis dyno. if you are starting to wipe some lobes and have less lift now, or the the intake/exhaust is too restrictive, etc.

I know that how my engine was tested on the dyno, it'll NEVER see that kind of set-up in the vehicle unless I turned it into an all out race car. on the engine dyno there was no air cleaner whatsoever, no turbulance of intake air because there was no hood, no moving air over the carb, etc. just sitting static in a room. on the engine dyno it had the same headers as in the car, but they were open, with a vacuum system over the collector to take the fumes outside, vs a full exhaust in the car. it also had an electric water pump, with no other accessories, and the fuel pump, ignition, etc. was all driven by a stand-alone power source, so the flywheel hp was a pure, free number with NO loss anywhere.

add in a full exhaust, air cleaner, underhood turbulance, alternator, water pump, PS drag, and it wouldn't surprise me if suddenly my flywheel hp was down by 30, and I haven't even begun to account for parasitic loss through the transmission, U-joints on drive shaft, rear end bearings, inertia of the parts, etc.