Max power/torque of gearboxes and diffs
Discussion
Does anyone know the maximum power and torque capacity of the Ford Sierra 7" and 7.5" diffs. Also in a more general sense I'd like to compile, for everyone's benefit, a list of capacities for all gearboxes and diffs generally available to the car modder. I appreciate that uprated gear sets make it difficult but to start with let's assume the standard gears, crown wheels and pinions.
What weight of vehicle, what tyres are you using, and what are you using it for ?
You cant answer a question like that withouth specifying what you intend to use it for.
As for a list of boxes, and diffs available ?? While that would be nice, the list could be huge, as there are so many of each.
Is there any particular car you have in mind for the list ? A gearbox to fit one car, might never fit another anyway. Fwd, Rwd, 4wd ?? Road use, drag use, track use ? How much power etc etc
You cant answer a question like that withouth specifying what you intend to use it for.
As for a list of boxes, and diffs available ?? While that would be nice, the list could be huge, as there are so many of each.
Is there any particular car you have in mind for the list ? A gearbox to fit one car, might never fit another anyway. Fwd, Rwd, 4wd ?? Road use, drag use, track use ? How much power etc etc
When you quote capacity for a unit there are two factors. Neither is power. One is input torque, one is output torque.
Input torque is basically peak torque of the engine times 1st gear ratio. So in the case of a Cossy making 400 lbft torque with a 3.6:1 first gear the torque the diff sees is 400x3.6 lbft which is 1440 lbft.
Output torque is input torque times the axle gear ratio. If the axle was 4:1 then a 1440 lb ft input torque becomes 5760 lbft.
If the tyres can take only 4000 lbft torque then the axle will never see more than 4000 lbft torque.
Some very powerful cars can use fairly weedy axles as they have a tall axle ratio and therefore don't need the strength in the side gears, but in many cases the car manufacturer has also reduced power in 1st gear. You won't notice it, but it will stop the gears from fatiguing off the blank.
Back to the Sierra axles and what you will find is that if you only use full throttle in 2nd and above, they can survive use in an engine with 500 lbft torque. Where the problem comes in is if you use warm soft compound slicks and are starting a race or hillclimb and the axle sees full multiplied torque. If the unit is undersized, it will break, or start it's fatigue cycle.
This is why GPA Sierras were homologated with a 9" axle for the RS500.
The Escort Cosworth went to a 7" axle because the torque it saw was less due to 40% ish being sent to the front.
But if you are compiling a database I would start by compiling input and and output torque and that will give a good guide as to what works and what does not.
Input torque is basically peak torque of the engine times 1st gear ratio. So in the case of a Cossy making 400 lbft torque with a 3.6:1 first gear the torque the diff sees is 400x3.6 lbft which is 1440 lbft.
Output torque is input torque times the axle gear ratio. If the axle was 4:1 then a 1440 lb ft input torque becomes 5760 lbft.
If the tyres can take only 4000 lbft torque then the axle will never see more than 4000 lbft torque.
Some very powerful cars can use fairly weedy axles as they have a tall axle ratio and therefore don't need the strength in the side gears, but in many cases the car manufacturer has also reduced power in 1st gear. You won't notice it, but it will stop the gears from fatiguing off the blank.
Back to the Sierra axles and what you will find is that if you only use full throttle in 2nd and above, they can survive use in an engine with 500 lbft torque. Where the problem comes in is if you use warm soft compound slicks and are starting a race or hillclimb and the axle sees full multiplied torque. If the unit is undersized, it will break, or start it's fatigue cycle.
This is why GPA Sierras were homologated with a 9" axle for the RS500.
The Escort Cosworth went to a 7" axle because the torque it saw was less due to 40% ish being sent to the front.
But if you are compiling a database I would start by compiling input and and output torque and that will give a good guide as to what works and what does not.
stevieturbo said:
I would also say that the driveshafts or CV joints will break before the actual crownwheel and pinion break, which is what the measurement actually refers to.
In some cases you are right.
But what I have seen on a fair number of powerful and abused cars is the teeth fatiguing off the pinion headset, and in other cases the pinion gears crack through and fail.
Every case is different. If the point is to compile a database on what a final drive unit can take, do that, then do a separate one for driveshafts and their components.
A mate snapped his diff pin in his Cossie on a standard car. He had only owned it a few weeks.
The driver plays a part. But the fault or weakness there wasnt in the CWP, its one small part of the diff. He fitted a quaife into the 7.5" CWP, and it lasted for months of abuse, eventually selling the car.
I'd actually think a Quaife 7.5" Cossie diff would be strong enough for most cars if thats the sort of route you were going.
The driver plays a part. But the fault or weakness there wasnt in the CWP, its one small part of the diff. He fitted a quaife into the 7.5" CWP, and it lasted for months of abuse, eventually selling the car.
I'd actually think a Quaife 7.5" Cossie diff would be strong enough for most cars if thats the sort of route you were going.
If a diff pin snapped I'd say it was a manufacturing defect - over hardening possibly making it brittle.
I have seen many differential pinion gears (ie they mate with the side gears and are not part of the cwp set) fail. Typically the traction control is switched off in snow, the driver wheelspins the car and lubricant can't get onto the diff pin surface fast enough. You get a lot of heating and the surface starts to break down. The part cracks from the inside out.
Having tried pretty much every 'advanced' diff out there as part of my job my feelings are about the Quaife are that it is much better than an open diff but that there are many other diffs that work better when one wheel completely loses traction. The Quaife can't cope with that. My favourite stand alone unit is the BTR Hydratrak, as used by TVR and Morgan. Superb.
If you really want to have the ultimate then active diffs are the way to go, but the control system is complex and costly, and takes a lot of time to tune.
I have seen many differential pinion gears (ie they mate with the side gears and are not part of the cwp set) fail. Typically the traction control is switched off in snow, the driver wheelspins the car and lubricant can't get onto the diff pin surface fast enough. You get a lot of heating and the surface starts to break down. The part cracks from the inside out.
Having tried pretty much every 'advanced' diff out there as part of my job my feelings are about the Quaife are that it is much better than an open diff but that there are many other diffs that work better when one wheel completely loses traction. The Quaife can't cope with that. My favourite stand alone unit is the BTR Hydratrak, as used by TVR and Morgan. Superb.
If you really want to have the ultimate then active diffs are the way to go, but the control system is complex and costly, and takes a lot of time to tune.
Diff pin breakage is quite a common thing witht them. I know a few guys locally that have done it. They love doing burnouts and diffing.
Im not sure about the quaife itself, but Im sure you can get a torsen diff that has some form of preload built in, so it can cope with a wheel in the air, and still transfer drive to the other wheel.
As a wheel in the air isnt a common thing, I think the Torsen design, in theory anyway is excellent, and I run one on my car ( not Quaife )
Im not sure about the quaife itself, but Im sure you can get a torsen diff that has some form of preload built in, so it can cope with a wheel in the air, and still transfer drive to the other wheel.
As a wheel in the air isnt a common thing, I think the Torsen design, in theory anyway is excellent, and I run one on my car ( not Quaife )
Thanks again, really facinating stuff for the inexperienced. I'd started the thread off by mentioning the Sierra diffs because I couldn't find how an individual gets hold of a BTR unless stumbling over one on eBay or similar and then finding someone to check it over and get the bits??
I'd heard about the issues with the Quaiffe ATB and lifting wheels, but had assumed that would never happen on the road?
Most suppliers/overhaulers claim that both the 7" and 7.5" are "unbreakable" with a Quaiffe unit in them and that anyway apart from the c/w+p they are very simlar inside?!
You get led toward a 7" on the grounds the choice of ratios is better and then to a 7.5" if theyv'e got one.
I'd heard about the issues with the Quaiffe ATB and lifting wheels, but had assumed that would never happen on the road?
Most suppliers/overhaulers claim that both the 7" and 7.5" are "unbreakable" with a Quaiffe unit in them and that anyway apart from the c/w+p they are very simlar inside?!
You get led toward a 7" on the grounds the choice of ratios is better and then to a 7.5" if theyv'e got one.
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