Effect of changing differential

[AM1990]

If I am going to change my truck differential and find some suitable one for it, What could be the possible effects on vehicle performance of that differential..??

For now I haven't plan for it. Just want to know the basic Idea of what could be affected.

 

[Mech_Engineer]

Do you mean you're going to change the differential's gear ratio?

 

[rcgldr]

A higher gear ratio increases acceleration (if not traction limited), increases rpm for a given speed, and lowers the speed at which upshifts need to occur.

A higher gear ratio may require a larger differential, since the driving gear will generally have fewer teeth engaging the driven gear if the gear ratio is increased and the differential size remains the same. This is why car guys may switch from an 8 inch differential to a 9 inch differential if going to a 4.10 gear ratio differential.

 

[cjl]

A higher gear ratio increases acceleration (if not traction limited), increases rpm for a given speed, and lowers the speed at which upshifts need to occur.

A higher gear ratio may require a larger differential, since the driving gear will generally have fewer teeth engaging the driven gear if the gear ratio is increased and the differential size remains the same. This is why car guys may switch from an 8 inch differential to a 9 inch differential if going to a 4.10 gear ratio differential.

It only increases acceleration in first gear before you get into the powerband. After that, assuming you don't have a ridiculously narrow powerband and assuming you have a sufficiently closely spaced gearbox to stay in the powerband, you'll accelerate pretty similarly. It will also improve controllability and torque when starting, which is good for towing or off-road use. Highway gas mileage will suffer though, since the engine will be spinning faster to maintain the same cruising speed.

 

[AM1990]

Do you mean you're going to change the differential's gear ratio?

yes desire differential have greater gear ratio

 

[AM1990]

increases rpm for a given speed,

RPM of engine..??

and lowers the speed at which upshifts need to occur.

upshifts mean gear shifing..??

 

[rcgldr]

It only increases acceleration in first gear before you get into the powerband.

Acceleration is increased in every gear (if not traction limited), but the top speed in each gear is reduced. The time it takes to shift make reduce the advantage of a higher gear ratio.

increases rpm for a given speed, and lowers the speed at which upshifts need to occur.

Increases engine rpm? gear shifting?

Yes if the gear ratio is increased by 10%, then the engine rpm at a specific speed in a specific gear is increased by 10%, and the top speed in each gear is reduced by 10%, meaning you have to shift gears at a lower speed.

 

[Mech_Engineer]

yes desire differential have greater gear ratio

But WHY do you want to?

 

[AM1990]

But WHY do you want to?

actually somebody proposed me to change differential. currently gear ratio is 4.3 and proposed one have 5.857. But I am not sure what are pros and cons of changing it.

 

[AM1990]

But WHY do you want to?

reason behind proposal is less gradeability. To enhance it new differential with higher gear ratio is proposed

 

[AM1990]

Acceleration is increased in every gear (if not traction limited), but the top speed in each gear is reduced. The time it takes to shift make reduce the advantage of a higher gear ratio.
Yes if the gear ratio is increased by 10%, then the engine rpm at a specific speed in a specific gear is increased by 10%, and the top speed in each gear is reduced by 10%, meaning you have to shift gears at a lower speed.

Thanks for your reply.
Is there any mathematical calculation for it..??
or It is just a rough estimation.

 

[Mech_Engineer]

Is this an off-road vehicle like a jeep or Toyota Land cruiser ?

 

[cjl]

Acceleration is increased in every gear (if not traction limited), but the top speed in each gear is reduced. The time it takes to shift make reduce the advantage of a higher gear ratio.

Acceleration in every gear is increased at a given RPM, yes. Acceleration at every speed is unchanged though, since you'll be in a higher gear at the same speed (unless your gearbox doesn't have good ratios for your powerband).

 

[rcgldr]

actually somebody proposed me to change differential. currently gear ratio is 4.3 and proposed one have 5.857. But I am not sure what are pros and cons of changing it.

That could result in the engine running very fast if you ever drive that truck on the highway.

Acceleration in every gear is increased at a given RPM, yes. Acceleration at every speed is unchanged though, since you'll be in a higher gear at the same speed (unless your gearbox doesn't have good ratios for your powerband).

With a 10% to 20% increase in differential gear ratio, the speed range for the normal and higher gear ration differential lower gears overlap, until you reach a gear where the higher gear with the higher gear ratio is the same as a lower gear with the lower gear ratio. Within the speed ranges where the lower gears overlap, when the overall gear ratio is higher, the torque multiplier is higher, and the acceleration is greater.

 

[cjl]

That could result in the engine running very fast if you ever drive that truck on the highway.

With a 10% to 20% increase in differential gear ratio, the speed range for the normal and higher gear ration differential lower gears overlap, until you reach a gear where the higher gear with the higher gear ratio is the same as a lower gear with the lower gear ratio. Within the speed ranges where the lower gears overlap, when the overall gear ratio is higher, the torque multiplier is higher, and the acceleration is greater.

Only if the torque is the same at higher RPM as it is at lower RPM. If you run most engines to redline, you'll be past the torque peak nearly 100% of the time, and therefore at higher RPM, the torque will be lower. Whether or not the acceleration will be greater before or after the diff change will then depend on how sharply the torque drops off.

You can bypass all these concerns by simply looking at the horsepower the engine is delivering at any speed. In all cases, the acceleration will be greater where the horsepower is greater. If at a given speed, the car was running at the horsepower peak before the gear change, and after the gear change at the same speed, it's running above the power peak (but below the redline), it will accelerate slower at that speed after the change than before it. If before the gear change, the car was running below its power peak, and after the change at the same speed, it is now running at the peak horsepower RPM, it will now accelerate faster than before.

 

[Mech_Engineer]

reason behind proposal is less gradeability. To enhance it new differential with higher gear ratio is proposed

Changing the differential gear ratio won't necessarily increase the grade you can go up at speed since you may be limited by your engine's maximum power output; it may help with the absolute maximum grade that can be ascended very slowly.

We need to know more about the vehicle before we can really give useful feedback. What kind of vehicle is this, and what sort of grades are you hoping to ascend? Is it 2WD or 4WD, and what sort of transmission and/or transfer case does it have?

 

[AM1990]

Is this an off-road vehicle like a jeep or Toyota Land cruiser ?

I am working with a truck modifier. Just started to work here

 

[AM1990]

Changing the differential gear ratio won't necessarily increase the grade you can go up at speed since you may be limited by your engine's maximum power output; it may help with the absolute maximum grade that can be ascended very slowly.

We need to know more about the vehicle before we can really give useful feedback. What kind of vehicle is this, and what sort of grades are you hoping to ascend? Is it 2WD or 4WD, and what sort of transmission and/or transfer case does it have?

It is a 4x2 truck with Transmission model MF06S. having 4.3 rear axle ratio.
suggested rear axle ratiion is 5.857.
Do you need anymore..??

 

[Mech_Engineer]

I think a 5.85 gear ratio will be way too high for your truck, it will severely change your engine rpm on the highway and likely a detrimental effect on your mileage. I suggest looking into engine improvements like a supercharger if you need more power for going up hills.

 

[AM1990]

I think a 5.85 gear ratio will be way too high for your truck, it will severely change your engine rpm on the highway and likely a detrimental effect on your mileage. I suggest looking into engine improvements like a supercharger if you need more power for going up hills.

the proposal mentioned that already this engine is using with that differential in a standard vehicle of the manufacturer (not locally modified, means company is offering that differential with same engine with high price)

So I am now confused that if the engine is same and we are going to make differential to be same then what could be affected with our vehicle.
dose drive train (propeller shaft) have any effect on it..??

For your suggestion, we have to work for engine side. will try to check it whether it is possible for us or not.

 

[SteamKing]

It is a 4x2 truck with Transmission model MF06S. having 4.3 rear axle ratio.
suggested rear axle ratiion is 5.857.
Do you need anymore..??

Suggested by whom?

A rear axle ratio of 5.857 is fine if you are going to be pulling stumps out of the ground with your truck. You'll get quicker acceleration off the line, but your top speed is going to take a hit. Also your gas mileage.

For example, 15" passenger car tires turn about 720 RPM at 60 mph on the highway. With a rear axle ratio of 5.857, the engine must turn 5.857 × 720 ≈ 4200 RPM to make the car go 60 mph. That's about twice the RPMs that an engine turns in most cruising situations.

If you are dead set on making this change, your transmission should have one, or maybe two, overdrives so that you can use the truck on the highway without blowing up the motor.

PostScript: The axle ratio for most street racing cars is about 4.11, and some street cars set up exclusively for drag racing will use 4.56 gears.

 

[AM1990]

Suggested by whom?

A rear axle ratio of 5.857 is fine if you are going to be pulling stumps out of the ground with your truck. You'll get quicker acceleration off the line, but your top speed is going to take a hit. Also your gas mileage.

For example, 15" passenger car tires turn about 720 RPM at 60 mph on the highway. With a rear axle ratio of 5.857, the engine must turn 5.857 × 720 ≈ 4200 RPM to make the car go 60 mph. That's about twice the RPMs that an engine turns in most cruising situations.

If you are dead set on making this change, your transmission should have one, or maybe two, overdrives so that you can use the truck on the highway without blowing up the motor.

PostScript: The axle ratio for most street racing cars is about 4.11, and some street cars set up exclusively for drag racing will use 4.56 gears.

so I should consider Mileage and Top speed as both will be affected.
And If the engine is going to be run on High RPM than it will decrease its durability and causes high engine room temperature to be occurred.

 

[SteamKing]

so I should consider Mileage and Top speed as both will be affected.

Like I said, if you want to pull stumps, this is your truck. If you want to drive it a lot on the highway, you're going to notice some differences in how it runs.

And If the engine is going to be run on High RPM than it will decrease its durability and causes high engine room temperature to be occurred.

Well, you should make sure the radiator and the water pump are in good working order. Installing a heavy-duty radiator might be a good idea.

You might also want to put a deeper sump pan on the engine to increase oil capacity, or install an engine oil cooler. Engines running at high RPM all the time are hard on oil and the cooling system.

 

[AM1990]

Like I said, if you want to pull stumps, this is your truck. If you want to drive it a lot on the highway, you're going to notice some differences in how it runs.

Well, you should make sure the radiator and the water pump are in good working order. Installing a heavy-duty radiator might be a good idea.

You might also want to put a deeper sump pan on the engine to increase oil capacity, or install an engine oil cooler. Engines running at high RPM all the time are hard on oil and the cooling system.

I will try to pull stumps if modification would done.. :-)
Thanks for your kind information..
Have a blessed life.. :-)

 

[rcgldr]

The axle ratio for most street racing cars is about 4.11, and some street cars set up exclusively for drag racing will use 4.56 gears.

In the case of the 4.56 gear, a lot of these cars are using drag slicks which have a larger diameter than stree tires, so the higher ratio is used to compensate for the larger diameter tires. For the faster street racing cars that get around 10 or 11 seconds or less, they're making a lot of power and need a lower ratio differential due to the higher top speed on such a run.