Engine/Transmission (1998.5 - 2002) Big Tires Stock Gears

[Sick Boy]

i am running 35" tires but am still running the factory gearing 3. 55. is this affecting my horse power num. on the dyno, since i didnt regear the truck? truck does 438hp at best an a 14. 1 at the strip. what i want to know will the truck show more power an be faster if i regear the truck? thanks

 

[RustyJC]

Larger tires and/or wheels = more inertia. This means that (1. ) RWHP will be less on an inertial dyno since you're fighting the higher inertia of the larger tires/wheels, leaving less HP to overcome the inertia of the dyno rollers, but (2. ) RWHP readings on a load dyno should not be affected significantly.



For the reasons in 1. above, ETs as well as braking are normally hurt by larger tires/wheels that have significantly more inertia than stock. It takes more power to accelerate or decelerate the higher inertia tires/wheels. Regearing will help overcome this, but won't reduce the increased inertia. Gears can increase rear wheel torque, but not HP.



Rusty

 

[Sick Boy]

rusty, the type of dyno was a dynojet they told me it was one that is built in the floor, not portable. so is my hp ture? thanks

 

[RustyJC]

A Dynojet is an inertial dyno, so larger tires/wheels would cause it to read low. This is because the Dynojet measures RWHP (and calculates torque) based on how fast your truck can accelerate the dyno rollers. With the higher inertia of the larger tires/wheels, you won't be able to accelerate them as fast, and the Dynojet will interpret this as lower RWHP and torque (since the torque isn't directly measured but is calculated from the RWHP).



Rusty

 

[SKneeland]

find someone with a mustang dyno

 

[Sick Boy]

so the mustang dyno will give me a true reading. why do they use the dynojets if they are not right?just tryin to learn the diff. between them. thanks just found a shop w/ a mustang 1100 dyno

 

[RustyJC]

The Mustang (or any load dyno) can measure torque (and calculate power) under steady state conditions where the drivetrain is neither accelerating or decelerating significantly. Therefore, tire/wheel inertia isn't a factor like it is with an inertial dyno like the Dynojet where the rate of acceleration of the rollers (which is affected by tire/wheel inertia) is what determines the measured power/calculated torque.



Rusty

 

[Rman]

the two dyno's just get their numbers differently. neither one is truly wrong, they just get different answers the formula has two variables, they each find the one and calculate the other, based off a known RPM and a constant. Rusty can chime in on the origins of the constant. It's been too long since my last physics class.

 

[RustyJC]

The basic equation used by both is:



HP = (Torque x RPM) / 5252



A load dyno measures torque at the rollers, roller RPM and plugs them into the equation above to calculate BHP. If it knows engine RPM, it can then take the BHP reading and calculate engine torque as below.



An inertial dyno measures HP as a function of the rate of acceleration of the dyno rollers of a known rotational inertia. It then calculates torque (if, in the case of the Dynojet, it knows engine RPM) by using the formula:



Torque = (5252 x BHP) / RPM



The 5252 is 33,000 lb-ft/minute (the definition of 1 HP) divided by 2 x pi radians in order to convert from a linear to a rotational measurement frame of reference.



Rusty

 

[Fishin Guide]

The basic equation used by both is:



HP = (Torque x RPM) / 5252



A load dyno measures torque at the rollers, roller RPM and plugs them into the equation above to calculate BHP. If it knows engine RPM, it can then take the BHP reading and calculate engine torque as below.



An inertial dyno measures HP as a function of the rate of acceleration of the dyno rollers of a known rotational inertia. It then calculates torque (if, in the case of the Dynojet, it knows engine RPM) by using the formula:



Torque = (5252 x BHP) / RPM



The 5252 is 33,000 lb-ft/minute (the definition of 1 HP) divided by 2 x pi radians in order to convert from a linear to a rotational measurement frame of reference.



Rusty

I have dynoed on the same dyno with different tire heights and the same gears.



There was a 2HP gain from dynoing with 315's vs the stock 265's.



Dave

 

[Sick Boy]

thanks everyone for the help! greatly app. !

 

[tURBOdIEZEl]

hey dino, here is a chart you might be interested in checking out... ... hope it helps ya out.

http://www.oly4x4.com/gearratios.htm

 

[RustyJC]

I have dynoed on the same dyno with different tire heights and the same gears.



There was a 2HP gain from dynoing with 315's vs the stock 265's.



Dave

Inertia or load dyno?



Rusty

 

[JR2]

DF2- I don't know anything about dynos, but I know that with Mach 4. 5's, an edge comp and a bd Rad you ought to be doing better than 14. 1 in the 1/4 mile. I have DD3's and a drag comp and I am at 13. 7 +/-. Your big tires are killing you off the line I bet.



I have always wanted to regear the truck with 4. 10's and then run bigger tires to get about a 3. 73.

 

[Rman]

The basic equation used by both is:



HP = (Torque x RPM) / 5252



A load dyno measures torque at the rollers, roller RPM and plugs them into the equation above to calculate BHP. If it knows engine RPM, it can then take the BHP reading and calculate engine torque as below.



An inertial dyno measures HP as a function of the rate of acceleration of the dyno rollers of a known rotational inertia. It then calculates torque (if, in the case of the Dynojet, it knows engine RPM) by using the formula:



Torque = (5252 x BHP) / RPM



The 5252 is 33,000 lb-ft/minute (the definition of 1 HP) divided by 2 x pi radians in order to convert from a linear to a rotational measurement frame of reference.



Rusty

SEE! Oo. I told you he'd chime in with some stuff out of his genius bag.