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Old 06-19-2003, 05:26 PM   #1
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Default Dyno Documented Loss of RWHP From Gear Swap

I've been curious about this for a while and some of you may be as well.

A buddy of mine just stepped up from stock 3.42 gears to 4.56's and documented the rwhp change on the dyno. Dyno runs were made at the same shop one week apart (not totally scientific but best we could do.

Dyno Before (Borla, Lid, stock rear, y pipe)
315rwhp

Dyno After (only changes were 4.56 gears, LS6 intake)
306rwhp

Isn't the LS6 intake normally good for about 10-12rwhp? If so looks like stepping up in gear cost around ~20rwhp! He says the car feels a ton peppier though!

Man, I wonder what I'm losing with my 4.33 gear and 9 inch!
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Old 06-19-2003, 05:36 PM   #2
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

just curious, what was the torke change?
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Old 06-19-2003, 08:31 PM   #3
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

I installed an LS6 intake on a bolt on car and saw no improvement on the track. Never dynoed it. I think it's a different story w/ cam and heads/cam cars.
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Old 06-19-2003, 09:16 PM   #4
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

I THKNK THE LOSS MAY BE DUE TO THE GEAR RATIOS OR SOMTHING OF THE SORT BECASE IN AN M-6 THEY DYNO IN 4TH WHICH MAKES A 1-1 TARIO
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Old 06-19-2003, 10:10 PM   #5
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

John, I lost 27rwhp going from the stock rear and 3:42's to the Moser 9 inch with 3.89's. Lost another 5 or 6 when I stepped up to 4:30's.
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Old 06-19-2003, 10:45 PM   #6
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

I lost 15 hp from my 3.42s going to a hybird 8.8 with 4.10s out of a 6m but gained it back after LS1 edit.
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Old 06-19-2003, 11:42 PM   #7
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

You will loose hp with an increase in gear (or even a change in gearing).

http://home.columbus.rr.com/trackbir...no%20small.jpg

This is a dyno run on my 2002 Z28 (M6 car, nearly stock and only a few months old). This dyno run was done in 3rd, 4th and 5th gear. If you notice, I made peak HP in 4th gear, direct drive, no over or under driving the transmission. The reason you run better times (or potentially should) with "more gear" is because the car gets into the power band faster. If you hop on a 10 speed (21 speed?) in top gear, it takes forever to get into your personal powerband. If you use more gear, you can crank up to a reasonable speed faster (though less effeciently). I hope that makes sense (that is part of thy that dyno pull exsists...it was on a dynojet and is rear wheel HP). Just my experience....

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Old 06-20-2003, 03:01 AM   #8
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

Quote:
You will loose hp with an increase in gear (or even a change in gearing).

http://home.columbus.rr.com/trackbir...no%20small.jpg

This is a dyno run on my 2002 Z28 (M6 car, nearly stock and only a few months old). This dyno run was done in 3rd, 4th and 5th gear. If you notice, I made peak HP in 4th gear, direct drive, no over or under driving the transmission. The reason you run better times (or potentially should) with "more gear" is because the car gets into the power band faster. If you hop on a 10 speed (21 speed?) in top gear, it takes forever to get into your personal powerband. If you use more gear, you can crank up to a reasonable speed faster (though less effeciently). I hope that makes sense (that is part of thy that dyno pull exsists...it was on a dynojet and is rear wheel HP). Just my experience....

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Old 06-20-2003, 10:22 AM   #9
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

Quote:
John, I lost 27rwhp going from the stock rear and 3:42's to the Moser 9 inch with 3.89's. Lost another 5 or 6 when I stepped up to 4:30's.
WOW.

I dynoed 475rwhp/505rwtq with old 422ci iron stroker thru a 9" with 4.10's and on slicks... Must have been close to 500rwhp thru a stock rearend!
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Old 06-20-2003, 10:32 AM   #10
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

PSJ, yeah it sucked big time when I found that out. I figure I've got around 460-465 with the stock rear. Surprised the hell out of me.
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Old 06-20-2003, 10:36 AM   #11
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

One thing I just thought of though. When I did the dyno comparison between stock and 9 inch, the 9 only had a few miles on it. Maybe it's not so bad after it gets broken in some. Still, it has to be around the 20 mark.
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Old 06-20-2003, 11:46 AM   #12
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

A dynojet measures acceleration of a known weight and figures horsepower from that. A 9" has heavier parts inside (and the offset pinion causes about 2-3% more loss than a 12 bolt) and it takes more energy to accelerate those parts. Just as a light driveshaft, light flywheel or skinny tires on the front will reduce your ET (less hp is wasted accelerating the flywheel or driveshaft and is then used to move the car). If you ran the same vehicle on a dyno that uses eddie current or a water brake, you will not see nearly as much loss through the 9" as you do on a dynojet. If you put the car on jackstands and let out the clutch in 4th gear, hold it at 1000 rpm and floor it....time how long it takes to get to redline, then do that in neutral, it will spin up faster in neutral since it is not accelerating the weight of the drivetrain, which takes extra horsepower). Also, the reason steeper gears "absorb" more power is reduced effecincy and extra heat. Also, you are accelerating the driveline faster in the same amount of time and the internal losses become more prevalant (think of the time from 1000 rpm to redline in neutral, it is not instant and therefore uses horsepower to accelerate the crank, rods and such). Anyway...just some rambling thoughts that may help explain what has been proven (and why it is that way).

Good luck!

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Old 06-20-2003, 12:00 PM   #13
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

haha...I dynoed 444 RWHP thru my 9" with 4.71 gears
Maybe I should redyno now that I have 4.30s
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Old 06-20-2003, 12:00 PM   #14
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

Some good points here for sure, does an out-of-balance tire lower your rwhp?

I saw some 'considerable' bouncing when they let off my car after one of the pulls, maybe it was just slightly out of balance but looked bad since it was going 100+mph?
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Old 06-20-2003, 02:35 PM   #15
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

That just may be the dyno rollers "pushing" the tires....kind of a "reverse wheelhop". If you have an out of balance tire, I'd think you would feel it in the interstate (or at high speed). I don't suspect you would see much of a hp difference....(I'm guessing less than 3 hp). Just my thoughts....

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Old 06-20-2003, 05:44 PM   #16
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

i cant see how you loose hp with diff gearing
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Old 06-20-2003, 08:53 PM   #17
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

Quote:
haha...I dynoed 444 RWHP thru my 9" with 4.71 gears
Maybe I should redyno now that I have 4.30s
Fireball, how noisy are your gears?
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Old 06-20-2003, 11:15 PM   #18
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

Quote:
i cant see how you loose hp with diff gearing
With more gear, you are actually accelerating the wheels and all the rotating parts of the engine, transmission and rearend (driveshaft, clutch etc) more quickly. Now, since a dynojet measures acceleration of a weight and the faster you can accelerate that weight, the more horsepower it will record. Now, since we are now asking the driveshaft and axles and tires and flywheel to accelerate faster as well you are "wasting" more horse power accelerating car parts and not the car.

This is why I said that HP would measure different on a "brake style" dyno. That type of dyno "holds" the motor at an rpm by increasing load until the RPM begins to drop (the load is more than the available horsepower), you remove the "acceleration" factor (having to accelerate the weight of the drivetrain). At that point, you will still likely see a slight reduction in HP due to the frictional losses of the steeper gearset.

If you want to test this theory, find a set of lighter wheels and tires and use them on the dyno. If they are 10 lbs lighter each, you may find 15 HP (you are accelerating less drivetrain weight and that leaves more HP to spin the dyno rollers). If you can keep them from spinning on the dyno, you will see a change. Then, if you know anyone with a super heavy set of aftermarket wheels (same overall diameter), dyno again with them, you'll likely lose 5-20 hp from the original baseline with your wheels and tires. This will demonstrate the relationship between dynojet HP and rotating weight. Once you see how rotating weight affects dyno measurement, you can start to consider how much HP it takes to accelerate all of the drivetrain parts on your car, and the faster you try to accelerate them, the more horsepower you will need to do it. And....the steeper gears accelerate the wheels (brake discs, etc) faster than the stock gears and that is where the "missing" horsepower is getting "absorbed", and why it is not being used to spin the dyno rollers.

Is that explanation any better? It is a hard thing to explain, but once it starts to make sense...you'll wonder why it was so hard to figure out the first time.....

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Old 06-22-2003, 07:09 PM   #19
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Default Re: Dyno Documented Loss of RWHP From Gear Swap

Quote:
Quote:
haha...I dynoed 444 RWHP thru my 9" with 4.71 gears
Maybe I should redyno now that I have 4.30s
Fireball, how noisy are your gears?
4.71s were noisy

4.30s are pretty quiet considering the solid torque arm (spohn spherical end)
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Old 06-22-2003, 07:09 PM
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