Hope I posted this in the right section. Could someone explain to me how changing the gears in your rear end affects rear wheel horsepower?
(For example, going from 3.23 to 3.73, or 4.10 to 4.30 - would you see more or less at the wheels and would it bee sooner? How about tq?)
On a related topic, what's the difference in dynoing in 3rd or 4th gear?
I imagine 3rd would boost sooner but 4th boost more or something? How about N/A applications? Thanks.

 

It doesn't affect horsepower or tourqe at all. Gears make your rear tires turn faster or slower. It also effects your top end speed and milage.

The reseaon you dyno in 4th gear is becouse its a 1-1 ratio.

 

+1.

From my understanding a higher ratio equals quicker acceleration, and a lower ratio equals higher top speed

 

The rear gear will multiply the torque coming into it by whatever the ratio is. As for HP to the wheel, that would involve some math that I don't feel like doing right now.

rx7twinturboboy has the acceleration and top speed stuff right though.

 

Very informative.

 

HP is always unaffected by the gear ratio. Torque, however, is.

Remember that power is force times speed (distance/time). Or, more specific for our needs, power is torque times RPM. Changing the gear ratio will alter the output torque , but it will affect output speed exactly opposite, so power stays the same.

Simple example: Given: 1lb-ft @ 5252 RPM. Run this through a 2:1 reduction gear: torque doubles to 2lb-ft but speed halves to 2626 RPM. 1lb-ft at 5252 and 2lb-ft at 2626 both equal one horsepower.

 

Exactly like peejay said. Just image that you are back in school and doing multiplication. If you multiply by something larger then 1 you will get a bigger number (120*1.2=144) and vise versa for a number smaller then 1 (120*0.5=60). What that means is that when all is said and done, because the twisting force is coming through the gears, when it is reaching the ground the number has changed according to the gear ratio. This way it helps make more force to get you moving in lower gears and takes less force to keep you moving in higher gears.

 

Thank you.

 

a shorter gear ratio will equal into a faster car. but your mpg will suck due to a high constang rpm

 

No.

A shorter gear (high numerical number) will equal faster acceleration, but a LOWER theoretical top speed.


A taller gear (numerically lower number) will result in slower acceleration, but a higher theoretical top speed.

(I am being picky here. "faster" generally refers to top speed, "quicker" refers to acceleration. Im operating under the assumption that you are using these same principles. after re-reading your post I realise you may not be)

I say theoretical because, for instance, our car will do something like 198 mph at redline in 5th gear with a 4.10:1 rearend, but will do over 200 with a 3.90 rearend. In either case few of our cars create enough power to overcome drag at those speeds so a 4.10 rear is preferable beucase it doesnt effect real top speed.

Dynoing in 4th gear is close to a 1:1 ratio so the transmissions torque multiplying is nearly zero after the rear end ratio is accounted for. 3rd gear would show an increased torque on the dyno because of torque multiplication. You would likely boost at a lower rpm at 4th gear due to the increased load v rpm. In 3rd you would race past the 4th gear boost threshold before the turbo could spool, theoretically. This is why sometimes with turbos its advantageous to run a taller rear end as it creates a greater load at the engine, resulting in the turbo spooling lower in the RPM band. In reality however the difference between 3rd and 4th gear boost threshold is probably insignificant.

On a side note: Mustang dynos can be run in any gear. Some companies do theoretical 1/4 miles be varying the load in relation to speed to simulate a 1/4 mile run and can give you pretty accurate times.


BC

 

exactly! simply said: its a torque multiplier.
-J-

 

Thanks.

 

Jees. The gear has absolutely NOTHING to do with power! A motor makes so much power always regardless to what the **** the motor is hooked up to.

If gearing had anything to do with it you could not accomplish a flywheel hp rating. Flywheel power is exactly what the motor is capable of making. There is no math = to power of gearing multiply gibberish.

Power is Power regardless.

Gearing only affects the amount of rotations per RPM. Wheel hp is simply measurement of power found at the wheels. If the gearing is to steep for the motor to handle you can lose power to the wheels from sufficient lack of power being provided to turn the setup. You can not make the motor magically make more power. TQ is measured by the amount of instantaneous force applied at any giving moment. HP is measured by the amount of work done giving over a period of time.

 

That answers my original question of how gearing may affect wheel hp. No one thinks (hopefully) that changing the gears will increase engine hp.

 

It does not actually make more power but it moves the power around.
On the dyno it does effect the power you will see. It's like tuning a car in 3rd or 4th gear. It also affects spool up time on the dyno and will result in more or less low end torque/power depending on gearing. Automatic cars are effected greatly by rear end gear changes.
For example let's say a certain car makes the most power in 3rd gear. Change rear end ratio and now the same car will make the most power in 4th instead of 3rd. When the right ratio is choosen you can actually even the power between gears.

 

I hope your just refering to turbo cars crispeed. Any other variations would just be from dyno error or dyno operator error. Turbo cars can have issues on inertia dynos because they don't have enough time to build full boost in lower gears. The dyno spool up happens too fast.

 

No one is suggesting that gearing CREATES power. Power is defined as horsepower, and is the same no matter what gears you choose to use. However gearing WILL effect torque applied to the ground.

Let say that an engine produces 10 ft/lb of torque with every rotation. If the output shaft rotates 3 times per every rotation of the wheels then the applied torque to the ground is 30 ft/lb. If the gearing is changed so that the output shaft rotates 4 times per single wheel rotation then the applied torque to the ground is 40 ft/lbs. However in both cases the horsepower hasnt changed, and the engine hasnt created anymore torque.

What? You can get a flywheel hp number by a) pulling the engine and hooking it to an engine dyno, or b) taking an assumption of drivetrain losses and ESTIMATING what the power output is at the engine based on wheel hp. Power doesnt change with gearing, but you will have to adjust for torque multiplication. This is why a reputable dyno shop wont estimate flywheel hp. its too inaccurate and is only good for ricers to brag about. If you want to know exactly what you engine is doing you have to pull it.

Your lack of proof reading has made this arguement almost impossible to understand.


As has been stated already. Also power and torque are different.

Technically you have it backwards. Gearing effects the amount of RPMs per wheel rotation.

Your other sentance is so gramatically screwed up as to be hard to understand, but Ill try. It sounds like you are saying that if your gearing is too TALL (not steep) that you can lose power to the wheels. No. You just got through saying that power is power. Now you are contradicting yourself.

If your rear end is geared too tall it makes it more difficult for the engine to rotate against the mass of the car. This is why it is harder to pull away from a light in second gear than it is in first gear. In first gear your torque is being multiplied to help move the mass of your car, but the engine revs up so fast that you have to switch gears in order to go any faster. In second gear the torque multiplication is lower, but since the car already has momentum it is easier for the engine to rotate against the mass of the car.

However you never LOSE power to the wheels. You can reduce applied torque to the ground, but you dont lose it.

What do you think headers or more boost does, convinces God that he should help push you along? You can make the engine make more power, but you are right, its not magical.

Also, a motor is driven by electricity, an engine is driven by mechanical force. We dont have motors driving our cars. we have engines. The biggest motor on your car is your alternator.

Seriously, before you reply to this with your inevitable gnashing of teeth and **** pouring out your mouth do me, and everyone else, a favor and proof read your post. this isnt the first time I have seen a post from you that makes no sense because you dont take the time to proof read it. You lose half your credlbility in your self-aggrandizing anger in every reply you make, and lose the rest of your credibility in your inablility to put together a properly constructed sentance. It took me less than 2 minutes to proof read my post, correct my errors, and make the entire thing clearer for others to read. Take the time and do that for us.


BC

 

all that and sentence is spelled wrong... "sentance" im just messin with ya... lol

 

Noted I was at work with like 4 people trying to get me to do things. I missed a single word in an entire post. Ill work on it.


BC