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Does tire size and gear ratio affect the dyno??

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Old 04-21-2003, 10:50 PM   #1
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Does tire size and gear ratio affect the dyno??

My buddy has an LS1 Trans Am, that dynoed at 380 HP at the rear wheels. (Heavy engine mods, T56 tranny, 3.42 gear). He just got the Holley LS1 intake manifold and a ported throttle body put on. He also got a 4.10 rear installed and I believe a little bit taller of a tire. It dynoed again at nearly the same as before, actually a tad bit less. I would of thought for sure the intake would have helped tremendously, considering he has a different cam and heads than stock. So my question is, can the tire size and rear end ratio have an effect on the dyno results? If so, how? Thanks in advance.
355, AFR 195cc heads, Comp 230/236 @.050 cam, Edelbrock Victor E EFI manifold, FAST MPFI XFI 2.0, 39 lb/hr injectors, MSD 6AL ignition
Pro-Built 700R4 trans, Strange 3.73 posi rear, Spohn sub frame connectors, lower control arms w/brackets, adjustable torque arm
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Old 04-21-2003, 10:55 PM   #2
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Location: 51N 114W, 3500'
Posts: 16,208
Car: 87 IROC L98
Engine: 588 Alcohol BBC
Transmission: Powerglide
Axle/Gears: Ford 9"/31 spline spool/4.86

Dyno results are always corrected to sea level. To make a proper dyno run, specs need to be imputed into the dyno. They want to know tire size and gearing so that the dyno can compensate for each setup.

Changing from 3.42 to 4.10 gears will increase the amount of torque multiplication. Dyno results are base on torque then hp is calculated from the results. Putting in the lower gear then increasing the tire size will still give the same amount of torque but the mph should be higher because of the taller tire.

Not all dynos are the same. Getting a dyno run at one shop then getting another the next day at another dyno can give different results.
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All engine, no power adders! Bests: 9.029@150.45 (at altitude)
Theoretical sea level performance 8.623@157.05
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Old 04-22-2003, 12:14 AM   #3
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Join Date: Feb 2002
Location: Central Coast Calif.
Posts: 2,635
Car: 91' Camaro
Engine: 383 ci
Transmission: T-56
Axle/Gears: 9 bolt / 3:70

I am about to dyno my Yukon... 4:56 gears and 38.5" tires... We will see..

When I had my Camaro on the dyno, they never asked tire diameter or gear ratios.. ??

1991 Camaro RS/Z28 weekend playtoy...
1969 Camaro SS ...454/ muncie 4 speed 12 bolt w/ 4:11's
1997 Yukon, Supercharged , 12" Cognito Motorsports lift , Chameleon pearl ghost flames, too much to list..
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Old 04-22-2003, 08:18 AM   #4
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Car: 1987 Black IROC-Z (original owner)
Engine: 350 Crate with TPI
Transmission: 700R4 with 2400 SLP Stall

NO, tire size and gear ratio have no effect on Dyno results. You are measuring Horsepower & Torque. There is a SAE correction factor.
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