3.27 to 4.10 rwtq gain... what am I missing?

[jmichalicek]

While my motor is being rebuilt and I'm spending a ton of money anyway, I decided now is a good time to go ahead and swap the gear in my rear end. I decided to do the research and math on how much torque I could expect to be gaining at the wheels and the numbers don't make sense to me.

Based on what I've found, I should be gaining approximately 25% torque at the wheels going from the 3.27 to 4.10 gear. So stock, peak torque in my '99 is around 270-280 at the wheels. Let's go on the low end with 270. 25% of that is 67.5. 270 + 67 = 337. The car doesn't put that out at the motor, obviously it's not going to put that out at the rear wheels.

So, what stupid mistake have I made here or what have I misunderstood?

 

[Chris _Scott]

Rear end gears do NOT add rwtq.

 

[jmichalicek]

Can you explain what I have misunderstood then? "You're wrong" doesn't do me a whole lot of good. I know I'm wrong, that's exactly what I said, I've misunderstood something.

I assume something to do with the torque multiplication with figuring in both the rear end gear and the transmission gearing. That said, that still doesn't make any sense if gears don't affect torque at the rear wheels.

 

[Chris _Scott]

Can you explain what I have misunderstood then? "You're wrong" doesn't do me a whole lot of good. I know I'm wrong, that's exactly what I said, I've misunderstood something.

I assume something to do with the torque multiplication with figuring in both the rear end gear and the transmission gearing. That said, that still doesn't make any sense if gears don't affect torque at the rear wheels.

Gearing does not affect the power output of the engine. I don't know the specifics or exactly how...but I'm pretty sure that a dyno is designed to remove the factor of 'gearing' from inflating the numbers.

gears do not add power to the wheels, they just simply let you go through the gears faster, and most importantly with these motors..puts you in the powerband quicker.

 

[jmichalicek]

I think you pointed me i the right direction there. So now, I think I've figured out what I had wrong. The 25% gain is in the torque that is multiplied by the gear. The actual numbers that we are talking about coming from the dyno are corrected for the torque multiplication.

 

[OzzDOA]

Dyno's don't need to correct for gearing because gears do not affect the actual output of the motor. A dyno works by measuring acceleration vs time and the computer software then makes the horsepower and torque calculations based on the known mass of the roller and recorded engine RPMs. Since all different gearing does is allow the motor to run through its RPM range faster or slower without affecting the actual output at a given RPM nothing will change.

 

[STAMPEDE3]

Dyno's don't need to correct for gearing because gears do not affect the actual output of the motor. A dyno works by measuring acceleration vs time and the computer software then makes the horsepower and torque calculations based on the known mass of the roller and recorded engine RPMs. Since all different gearing does is allow the motor to run through its RPM range faster or slower without affecting the actual output at a given RPM nothing will change.

I may be wrong but I've never been told that they factor out gearing.
Think about this.
That is the key.

If gearing does allow the car to accelerate faster (Which it will) then the Acceleration vs time is faster which will result in slightly higher TQ numbers.

It does not do anything to motor output as said but does change the wheel output due to them spinning up faster.



I can't explain in words what I want to explain. lol

However the 25% is way off, there are many other factors.

Without doing any math it may be closer to 25%- the 18% drivetrain loss.

 

[STAMPEDE3]

Maybe we need a dyno ginny pig.

Dyno with stock gears, do gear swap and re-dyno.
Same shop, same day without any other changes.

 

[jmichalicek]

The 25% is vs the torque using a 3.27, which would also already have the drivetrain loss. I'm certain my confusion was in that the 25% gain is in the multiplied torque (which is what the gears do, multiply the torque the engine makes). The dyno is not measuing the multiplied torque, though.

I believe Ozz is correct and is backed up by Bristol Dyno - What is a dynamometer - How does the Dynojet chassis dyno work. Basically, it's measuring rpms directly from the engine. Same end result as my gear correction thought down to a 1:1 ratio above, different method of getting there.

This sounds correct and the numbers all make sense now. I just needed pointed in the right direction of correcting for the gearing or, really, measuring in a way that ignores the gearing.

 

[STAMPEDE3]

Maybe but I don't think so.

If this is the case the it completly backs the "Track times mean the most" (Which I believe anyway) lol

A 300RWHP car with 3.27s is the same as a 300RWHP car with 4.30s on the dyno?
Then it means absolute crap.

I want to know what the car puts to the wheels WITH mods and gears are clearly a mod.

Think of this,

3.27s to 4.30s is a big difference right?

Now .82 to 1.0 is not that big of a difference but If you make a pull in 5th instead of 4th I'll bet the numbers are lowers. (Just as your article said.)

Gears DO affect how fast the RPMs will rise over time so how could then not effect the numbers?


lol
Forget it, I'm confusing the crap out of myself

 

[OzzDOA]

Think about it. Dyno's don't give you numbers based on minutes or hours or seconds it gives hp and torque curves based on RPMs. Yes, gears will help the car accelerate quicker in "real time" however a dyno doesn't care about that. It is calculating hp and torque vs RPMs. If a car takes 5 seconds to get to 5000 rpm with 3.27's it is the same amount of net work if the car takes 3.99 seconds to get to 5000 rpm with 4.10's.

 

[STAMPEDE3]

I know how they work. What I don't know is how the computer system works and what info they put into it to figure.

For what you are saying to be correct then they must enter the rear end ratio into the computer as part of the calculation.

I never remember them asking what gears I had.

 

[M.P. Cobra Fan]

3.27s to 4.30s is a big difference right?

Now .82 to 1.0 is not that big of a difference but If you make a pull in 5th instead of 4th I'll bet the numbers are lowers. (Just as your article said.)

That is what is confusing me. Depending in the transmission gear selected, the numbers will be different. So why would rear end gears not change the numbers? Thinking about it will make my brain hurt.

 

[1badblownstang]

On a dynojet gear will not make much if any difference in either direction until higher hp levels. On a MD they will drop your hp and tq. They increase your acceleration through gear multiplication, but not actual torque output. You can see actual loses even on dynojet due to less meshing of gear teeth on the steeper gears. KB did some research on this and lost close to 50 hp just going from 3.31's to 3.73s, but this was a700+hp car. I can e3xplain in more detail if needed.

 

[M.P. Cobra Fan]

More detail would be wonderful. I like reading and learning everything I can.

 

[98 N/A 4V]

gears (tranny or rear) are just a torque multiplier. The higher (numberically) the gear the more it "hides" the true output of the motor. Thats why when you dyno a car you put it in whatever tranny gear is 1:1 (or closest) to get the more true reading of engine output and the closer the rear gear is to 1:1 the more accurate it will be.

-Mark

 

[32vApe]

Assuming a dynojet, it measures how fast the car can accelerate the known mass of the rollers. 4th gear pulls with 3.27's will spin the rear wheels on the rollers to aprox. 150mph. at 6500rpm. With 4.10's the car will spin the rollers to the engines 6500 rpm in a shorter period of time but the wheels on the rollers will only going be going 120mph. Another way to look at it is if the shorter gear made more power, you could just do 1st gear dyno pulls and make huge numbers. But the fact is at the top of first the wheels are only going 30-40 mph. The dyno is not measuring how fast the engine revs up it measures how fast the wheels accelerate.

 

[ac427cobra]

I lost 'dyno' RWHP going from a 3:55 to a 4:10 rear gear.

I also spanked cars coming off of turns after the gear change that 'used' to spank me.

A dyno is not the be all end all performance measurement device. It's a tool.

FWIW

:thumbsup::coolman::beer:

 

[M.P. Cobra Fan]

To me a dyno is just a tuning tool and a bench racing my junk is bigger than yours. I am always intersted in how things change and how the dyno reacts to them though. Do I care that I just found out that my 4.10s could have lost me hp and tq on the dyno? no. Because like you said, I can spank cars that would have spanked me with my 3.27's. Thanks for the info gentlemen.

 

[AluminatorSnake]

To the statement of losing some horsepower after switching to 4.10s... Its been done somewhere just cant remember where but they did the gear swap and pre and post dyno results and the explaination for the loss. Basically it said that with 4.10s the rear end is spinning faster than the rest of the rotating drivetrain so it reaches redline slightly faster resulting in the very minor loss in horsepower.