Competition H.P. verses Gearing?

[1dslram]

My question is if a CTD was to dyno at 500 H. P with a 3. 56 rear gear, and changed the rear gear to 3. 73 with that being the only change what would the truck dyno at? This is to answer a difference of opinion between a friend and myself.

Thanks

Bruce

 

[Huff N Puff]

You'd end up with 500hp

You'd end up with the same 500hp,cause the engine would rev faster youd make less torque but more rpm therefore same horsepower. Merv

 

[XJSuperman]

In a rock crawling situation having the T-case in LOW range will increase the torque to the axles BUT the engine HP/torque output is unchanged as should be your dyno numbers



Basically changing gears will change the RPM range at which the engine is in while driving.

A higher gear ratio (numerically lower) will allow lower engine RPM's for the given tire size... . meaning you can achive a higher top speed.

a lower gear ratio (numerically higher) will allow the engine RPM's to come up quicker and thus allowing for a lower top speed overall... of course this is all based on the engine and its rev capabilities as well.



For instance: using the formula



(tire dia * engine RPM) / (336 * differential ratio* transmission ratio * transfer case ratio) = MPH



336 is a constant and I'll use a 1:1 3rd gear auto trans ratio and the T-case 2WD high ratio of 1:1



So take your 3. 73 geared truck, stock tires, with the auto trans.....



You would have (31. 8 * 2000 RPM) / (336 * 3. 73 * 1 * 1) = 50. 7 MPH



with a 4. 1 geared truck you would have



(31. 8 * 2000 RPM) / (336 * 4. 1 * 1 * 1) = 46. 2 MPH.



So at 2000 RPM's the truck with 3. 73 gears will be going 4. 5 MPH faster.



Bascially it puts the engine's useable torque (to a degree) at a different MPH range. The 4. 1's will get you to 50 MPH quicker BUT the 3. 73's will give you a slightly higher top speed in the end provided the engines are an equal platform. BUT these 2 trucks should produce the same dyno numbers just at slightly different RPM/MPH.

 

[KLockliear]

Actually, you would show slightly less power since a little more HP is used to accelerate the drivetrain faster with the shorter gear.

 

[Forrest Nearing]

the deeper (higher numericly) gears will impose more parasitic drag than the lower... now, whether or not they'll impose enough to be seen on a 500hp diesel or wether or not the altering of the load on the engine will alter the curve is beyond me at this point, but it's been proven time and time again that the higher the gear multiplication, the higher your drivetrain losses.



Forrest

 

[XcumminsX]

ok now how about this... . I have 4. 10 gears but offset them higher gear with a tire with a 34. 5" diameter... making the final ratio about 3. 70 (something).

 

[Forrest Nearing]

well, you're still going to have the drivetrain losses of the deeper gears, and then you're going to have more inertia at the wheels to overcome.



but, I really think that all this is splitting hairs... doesn't matter how much power you're making, all that matters is who gets to the finish line first! (yes, I understand that a dyno is an invaluable tuning tool, I'm just saying that gaining or losing a few horsepower on a 500+hp setup is no big deal)



Forrest

 

[KLockliear]

XCX, that's even worse. When I used to play with my TA, I went from a 17x9" rim with 275/40/17 tires to a 17x11 rim with 315/35/17's and lost 4rwhp with just that change.

 

[XcumminsX]

4 rwhp? i went from 265/70/17 to 315/70/17. Either you had a typo or your really lost 4 rwhp. I dont care about 4 rwhp.

 

[Huff N Puff]

Hey Keith

This is Merv in Springfield just wanted to say Hi and thankyou to one of the people that started me on this addiction. By the way your loss of 4hp could have been a change in the weight of rotating mass. Merv

 

[Forrest Nearing]

it could also be from different atmospheric conditions, different resolution on the dyno software, etc. etc. etc.



Forrest

 

[KLockliear]

LOL -> Merv.



You are exactly correct Merv. Since I didn't change the gearing, the 4rwhp loss experienced was strictly due to increase in intertial mass. More tire/rim weight at the same distance from the centerline. The two tires were only . 02" diameter different, so no gearing due to height either.



Those dyno runs were done about 30 mins apart too. Here's the page:



Tire Testing



XCX, you will likely see a bigger loss than my 4hp due to how much more tire you will have, and how far it is from the centerline of the axle. You are stepping up 2" in width, but that 2" and it's inertial weight is now 17. 15" from the centerline (34. 3" dia tire), where the old tires were only 15. 8" from the centerline (31. 6" dia tire). Just moving the weight farther out increases the inertia, PLUS you added size/weight.



Changing gearing as well will only compound your loss. It still may be 10hp or less, but 10hp is 10hp. It may be more when you add it all up.