How much hp will it take?

[bullfrogjohnson]

4th gear gains a ton more wheel speed than 3rd. When the truck is in 3rd the engine spins over around 1.4 times to every one time the tires turn over. When the truck is in 4th it is one to one. Now imagine going down the track at 4000 rpms. That .4 adds up quick.

 

[Leadfoot]

Why is 4th better than 3rd though? It's just a gear change. It's like the ford guys claiming 4.88's pull awesome in OD....isn't that the same as 3.73's in 4th?

Final gear ratio is final gear ratio nomatter how you want to slice and dice it with tranny gears, t-case range, ring and pinion, tire size, etc. BUT I have been told that pulling in 1:1 in the tranny (direct drive) gets more power to the ground as there is less power wasted (parasitic loss) inside the transmission (planetaries).

Again this is what I have been told, a tranny expert should be able to shed more light on this. If it is true that there is less "loss", is it a decent amount or is it negligable?

The only other thing that pops into mind is the effect of "where" the gear reduction occurs.
Example:

Two guys run 28:1 final gear reduction. One guy does it using 4.56 ring and pinion and the other guy does it using 3.73 ring and pinion (obviously they are using different tranny and t-case gearing). The guy running 4.56's is less likely to twist his driveshaft. I don't know if that plays into running direct drive vs a lower gear in the tranny or not......

 

[whitetrash21]

Example:

Two guys run 28:1 final gear reduction. One guy does it using 4.56 ring and pinion and the other guy does it using 3.73 ring and pinion (obviously they are using different tranny and t-case gearing). The guy running 4.56's is less likely to twist his driveshaft. I don't know if that plays into running direct drive vs a lower gear in the tranny or not......

....but more likely to strip the teeth off the ring gear. It's torque being displaced to different parts of the drivetrain. With 3.73's, the ring gear will have more materiel for each tooth, less likely to rip them, thus the driveshaft twists, u join pops, etc.

The drive line parts listed here are just examples. It could be any part of ones setup that goes, all based on which parts are gonna be stronger with the amount of power placed against them.

You're right about final gear ratio, doesn't matter how you get there, 1:1 is still 1:1. Like bullfrog said, its about wheel speed. The faster the tires are turning, the quicker you can build up groundspeed and the more momentum the trucks gonna have before the sled drags it down.

 

[White Duramax]

The whole reason of running 4th is 3rd gear is just not fast enough of a gear. Have to jump up to 4th but its a big jump. Changing the gears would make it a little closer of a gap.

 

[duramaximizer]

Final gear ratio is final gear ratio nomatter how you want to slice and dice it with tranny gears, t-case range, ring and pinion, tire size, etc. BUT I have been told that pulling in 1:1 in the tranny (direct drive) gets more power to the ground as there is less power wasted (parasitic loss) inside the transmission (planetaries).

Again this is what I have been told, a tranny expert should be able to shed more light on this. If it is true that there is less "loss", is it a decent amount or is it negligable?

The only other thing that pops into mind is the effect of "where" the gear reduction occurs.
Example:

Two guys run 28:1 final gear reduction. One guy does it using 4.56 ring and pinion and the other guy does it using 3.73 ring and pinion (obviously they are using different tranny and t-case gearing). The guy running 4.56's is less likely to twist his driveshaft. I don't know if that plays into running direct drive vs a lower gear in the tranny or not......

That's what I thought. It doesn't matter. Tire size can make up the biggest differences. Once you get to 35's though, you have to do something. I just think people are going the wrong way. JMO

 

[smith6.6]

There are ahand full of us running 4th 35's and 3:73's all the trucks are in high 700 to 800 hp trucks, no one unlocks there converter and we spin out. If your not atleast 750 and your tracks bite hard like ours you won't hold.

 

[JoshH]

Here's a spreadsheet I made a while ago that is pretty handy for this type of discussion. If you look at the bottom, you will see three tabs. The first tab will show you your RPM at a given wheel speed for a single setup (tire size, final drive ratio) in all gears for your truck. The second tab allows you to compare RPM and wheel speed for two totally different setups. The third one isn't really relevant to the conversation.

 

[Micheal Tomac]

I never had a problem spinning out in 4th low with 3.73 gears with 33x12.5x17 Trxus STS or 315x70x17 bfg AT's . That was running with a stock cam and the mods listed below. The only time I'd downshift from 4th to 3rd at the end was in an 8500# class on a tight clay track.

 

[POUR'N COAL]

Here is 4th on a loose track 3.73 gears
I would go 4:56 for pulling but on a 5spd it would be to low for the 1/4 mile

[YOUTUBE]h-QBqHwJogI[/YOUTUBE]

Here is 4th with no unlock
[YOUTUBE]QmUv0J2NS3E&feature=channel[/YOUTUBE]

 

[ecc_33]

Ray, I still drive mine also tow some trailers. 4.56 I can still cruise at 75mph on the freeway at around 2500 rpms. Its not to bad

 

[dmaxalliTech]

Josh, nice spread sheet

Ray, 700 hp minimum to run 4th.

 

[duramaximizer]

FWIW 600hp is a poor place to be with 3.73 gears.:cruch:

 

[White Duramax]

I never tried 4th this year and was probably a little shy of 700hp. I am switching chargers and few other things and should be closer to 750hp or so. I will probably get all the engine stuff done and do some test hooking to the sled, helps when theres 3 sleds about 10 miles down the road. If I can't get 4th to hold then I will probably look into changing gears.