STL94LT1

I have heard that lower gears (4.10's) will produce a lower rwhp on the dyno. If so, how much lower and why?

black bart

That is total :bs

LT4Sam

I've never heard that and based on what I know of basic physics, it's impossible. HP is HP, gear changes won't effect what the engine output is.

Sam

No Go

I have heard of it as well-on the inertia dynos (Dynojet, etc.)

I believe running in different gears also will give different numbers (1, 2, 3, 4th etc.)

:confused:

black bart

When running on a dyno you always use direct drive which is 4th on a 6 speed drive on auto. If you use a lower gear that would show much higher torque numbers since the computer on the dyno jet is programed for direct drive. Different rear ratios would change the torque reading if it were not for the fact that the computer on the dyno is connected to your engine and it reads engine rpm and wheel speed and computes the gear ratio and then calculates the torque and power. That is why gear ratios won't make any difference it is automatically computed by the dynojet.
Talking about dyno"s I have a good story about one. I went to funfest at Mid-America and they were running vettes on a dyno from Breathless performance so I got in line but when it came my time to run the guy running the dyno looked at my car and said you are not going to put that on my dyno you will break it. :D Turned out that the dyno was only good for 400 rwhp I told him I could make it read tilt. :smash:

1985z51

bring your car to me......the record on my dyno is 670rwhp. or i can bring it to you! there are situations where having a lower gear ratio will show up as less output.....the engine has to be loaded to make hp,the engine will see less load with 4.10 gears than 2.59 gears...of course the car will accelerate harder with the 4.10's. on some inertia dynos there can be strange dyno numbers...two identical cars on the dyno,one with a heavy flywheel and heavy rotating mass...one without. on this particular dyno the heavy drivelined car shows more output..obviously theres a problem here...the light car uses less energy to rotate the mass. hope this helps :seeya

LT4BUD

If we are plotting HP vs RPM, it is the other way around, RWHP will increase with a 4.10 gear.

I don't know by how much, but if we quess drive train losses around 40-50 hp (15% of 300), I would guess the difference would be small but measureable.

RWHP is essentially net engine horsepower less drive train losses.

RWHP for a given vehicle will be a function of what transmission gear you are using for the test and the rear end gear ratio since these gear ratios effect the speed at which driveshaft(s) and wheels are spinning for any given motor rpm.

So if you changed a 3.45 rear end to a 4.10 rear and kept everything else the same, the RWHP should increase. This is because the rear wheels are now spinning slower for any given engine RPM, so less power is being used to accelerate the tires/wheels/half-shafts.

On the other hand if we were to "plot" RWHP against MPH, changing to a 4.10 gear would decrease RWHP. This is because for any given MPH the driveshaft, motor, etc are spinning faster so more HP is being used for these functions and less is available at the rear wheels for acceleration.

Bottom line is you will get better acceleration with 4.10's because......

Nathan Plemons

I believe people have hit on this here but nobody has really nailed it. Has anybody ever looked at a set of 4:10 gears vs a 3:45 or something smaller?

Start counting teeth and looking at the contact area. You'll see that the 4:10's have much more teeth and thus appear to have more surface area contact at all times which also means more friction. Friction reduces horsepower period. The things is that the frictional losses are not enough to negate the benefit of the torque multiplication.

Your car accelerates faster because effective torque to the ground is increased. The dyno numbers however don't increase because it ignores the rear end ratio. It calculates backwards to give you the esitmate of FLYWHEEL numbers based on what goes to the rear wheel. The total torque to the ground goes up, but the gears are actually less efficient and thus there is a higher drivetrain loss and your dyno numbers will show lower.

It's a proven fact, but it's also miniscule. You're talking about 5-10 hp on the absolute maximum.

Monks

I don't know why, but when I dynoed my car at Norris
Motorsports in Florida, Mike told me my numbers would
be lower with my 4:10 gears.

Russ

MrNuke

My Automatic vette's 4th doesn't engage till 116 when WOT, so the dyno sill tell me Hp and Torque from 116+? For real? Considering most TPI cars have WAY more power under 116MPH this kind of makes no sense to me at least... I wanna go to a dyno soon, but if I have to accelerate in 4th only, it will be low I think..

scorp508

He wrote that a little funny. He means...

4th on a 6-speed
DRIVE (3rd gear) in an Auto
4th in a 4+3 also.

p.s.

Whoever said Dyno's are calculated for direct drive what the heck did you mean? Even with a tranny in direct drive you still have the rear gears to deal with throwing it way out of direct again. :confused:

LT4BUD

I'll restate what I think we all agree on which is RWHP = motor HP less drivetrain losses.

Drive train losses do include frictional losses in the transmission, differential, rear axle. But the hp required to accelerate the "weight" of the driveshaft(s), wheels, tires, etc must also be viewed as a loss. Even if you had a frictionless drivetrain you would still use hp from the engine to accelerate the weight of the drivetrain. So drive train losses in this theoretically frictionless system would be strictly a function of the weight & relative speed of these components.

So, if we reference RWHP to motor RPM, and do nothing but go to 4.10 rear end, the rear wheels will spin slower relative to motor speed. so less hp is being used to move the rear wheels which reduces drivetrain loss & increases RWHP.

If we reference RWHP to MPH, then RWHP would go down since the motor & drive shaft are moving faster relative to the rear wheels.

While tests are normally performed with the tranny in 1:1 ratio, you would find similiar comparisons between testing between say 1st gear & 4th gear. If we ran a test in 1st gear we should see more RWHP vs RPM since the drive train is moving slower relative to motor RPM.

My conclusion is that the energy consumed in accelerating the mass is more significant than a change in frictional losses when only changing gear ratios. By the way my 4.10's have 41 teeth & 10 teeth.


[Modified by LT4BUD, 11:56 AM 5/4/2003]


[Modified by LT4BUD, 8:24 AM 5/5/2003]

MrNuke

Ahh thank you.. good my 3rd gear pulls liek a beast.... :D

STL94LT1

Thanks guys, now I am really confused. :D

Nathan has talked about changing to 4.10's. When he does, maybe then we can see some dyno results on the 3.45 to 4.10 gear change.

Nathan Plemons

Your theory sounds good but it's flawed. The only thing a dyno does is measure how quickly you accelerate that drum from a given speed to another given speed. This is power, no matter how it gets to the ground.

If the dyno does not have an RPM input it can still measure horsepower, it just can't display torque because it has no way to calculate it. It all boils down to drivetrain loss, pure and simple.

Numerically higher gears are not as efficient at lower ones. For every benefit there is a tradeoff. I just remembered I have this dyno graph, it should nail the point home.



Look at these 4 dyno runs. They were all made within 10 minutes of each other. You will notice that 3 runs are very close to each other and one is not. The reason is that the 3rd run was made in 3rd gear! Since 3rd gear is numerically higher than 4th this is exactly the same thing that would happen if you changed rear end ratio's.

Notice the dyno actually reads a LOWER HP and Torque rating for the run made in 3rd gear. This is because the car really does put less power to the ground for this gear, it has more loss through the drivetrain.

I know it's hard to fathom that the car puts down less power to the ground. Actually the car puts down MORE power due to torque muliplication, but when you divide out that multiplication you actually get a lower power at the flywheel. The drivetrain just gets more efficient as you approach closer to a 1:1 drive ratio, hence the more horsepower in 4th.

jz94

NP, are you talking about 3rd gear in a 4L60E? I just had my '94 LT1 dynoed and got numbers like your 3rd gear run. I don't know what gear my car was in, but I did see the speedo indicate 129mph during the run. The dyno guy never asked what the rear axle ratio was. I figured 277RWHP was about right for a 300HP rated engine with minor (Corsa cat-back and SLP claw intake) mods. :U

Nathan Plemons

No that is a 93 Z-28 6-spd so it's a T-56. That's the difference between 3rd gear which is 1.35:1 vs 4th which is 1.00:1 Runs 1,2 & 4 were made in 4th gear, run 3 was made in 3rd gear.

Hence third is the numerically higher gear. The exact same effect as adding a higher gear out back. :cheers:

BTW, the dyno operator will never ask what gear you have. They dyno calculates the effective gear ratio assuming it has an RPM signal. It does not break this down by rear end and transmission ratio, it has no way of knowing. It only has the total gear ratio which is the combined effect of your rear end gear and tranny gear.


[Modified by Nathan Plemons, 2:11 PM 5/4/2003]

MSR

i see there's something else i need to put on my website, right after the tq vs hp thing... :rolleyes:

the difference in gearing changes the rate of acceleration. since power is work over time, and work is the change in energy, you're losing power to the wheels with shorter gearing, since you're changing the energy level of the rotating mass at a faster rate.

example: the rotating mass at 2000rpm has a certain amount of energy, the same mass at 6000rpm has a much bigger amount of energy. the faster you go from 2000rpm to 6000rpm, the more power it takes. the more power that goes to changing the energy state of the rotating mass, the less that goes to accelerating the dyno drum.

this happens on an engine dyno, too. if you do a steady-state pull, it measures more than if you do 300rpm/sec, which in turn does more than 900rpm/sec, and so on.

the differences in drivetrain losses (from one gearset to the next) due to friction are so low order that they're probably below a dyno's noise level.

jz94

Oops! I didn't mean to use that puke thingy, guess I should have looked at it longer. DUH!

LT4BUD

It is sure hard to dispute back to back dyno runs!! Less RWHP in 3rd gear vs 4th gear is contrary to what I have been saying. It should be higher in 3rd since the driveshaft, wheels, etc are spinning slower thus using less of the motor hp.

So when we shifted to 3rd gear either the friction in the tranny really increased, that is more than the hp going into the rest of the drive train decreased or something else came into play.

I don't really take any issue with the fact friction will change with different ratios, I just don't think in the rear end that friction is the most significant change when you change ratios.

What I suspect happens in the tranny is that when it is 1:1 you have the fewest gears involved, so when you go to 3rd gear you have more gears involved which means a significant increase in friction but also you have more shafts & gears turning which means a significant increase in the weight of shafts/gears turning.

Don't know this to be true but it is a plausable explanation in my mind. Maybe someone who knows the inner workings of the T56 can comment.