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HP is not important?
Just popping in
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Hello all,

Firstly, I've come across the Datsun B110 because I'm interested in planning a "hot-rod" project for myself, and this car is about the lightest base frame I can find for that purpose.

I was delighted to find this great site and immediately started looking around at technical stuff. One particular area I am concerned about in my initial concepts for this project is the rear-end of the Datsun. However I found the Tech Wiki, and in that there is a section on rear axles with lots of useful information. In that section there is literature stating that horsepower is not a relevant concern when selecting a differential for a car, but instead crank torque is all that matters.

This does not make sense to me, and I hope to verify my understanding of this with the forum's help. Hopefully I am sufficiently clear.

Here is an example to aid my discussion:

Take two drastically different engines:
1) A X horsepower motor which makes 100 ft lb @ 14,000 rpm.
2) A Y horsepower motor which makes 400 ft lb @ 3,500 rpm.

Both of these engines are coupled to a transmission which is essentially a torque multiplier.

The resulting torque fed to the differential is a multiple of the rpm at the motor. Torque is increased, and rpm is reduced to make it reasonable for driving the wheels of a motor vehicle which typically operate at a lower angular velocities.

If rpm is the variable that affects the multiplication, and horsepower is a product of rpm, torque, and known constants, how is horsepower an irrelevant term when determining the ultimate turning force at a differential?

To further illustrate, realize that the turning force at a differential in a high power drivetrain is in the upper 6,000-12,000 ft lbs. Clearly there is a multiplication of the engine torque.

Some seasoned people may know of this formula:
Torque at
rear axles = Torque x First gear
ratio x Rear gear
ratio x 0.90

Notice that torque is an essential variable in determining the torque at the axle, however this figure is meaningless without factoring the first gear ratio. The first gear ratio effectively cuts horsepower out of the equation, but this ratio is determined by many factors, one of which is the engine rpm, and engine rpm dictates power.

Return to the original example of two engines with X & Y horsepower. Set X=Y=400. Would it be advised to hook a differential sized for a 100 (crank) ft lbs engine to the first, and a differential sized for 400 (crank) ft lbs engine to the second?

It seems that horsepower should be used as a rule of thumb, and formula based on maximum torque and first gear ratio (based on a launch scenario) should be used for a more accurate verdict.

Posted on: 2009/12/10 7:23
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Re: HP is not important?
Just can't stay away
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sounds like you know about most of that stuff anyway. I think that the person who made that wiki was thinking along the lines that a 100 lb.ft engine will exert the same strain on the differential in the same gear regardless if it is producing that torque at 2000rpm or 8000rpm.

its the torque that the driveshaft is transferring that is what matters, how fast it is spinning is irrelevant. Perhaps they should have narrowed it down to that rather than engine torque

Posted on: 2009/12/10 7:39
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Re: HP is not important?
No life (a.k.a. DattoMaster)
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its still the torque the matters, it would be comparing the 2 different engines with all else being the same. ie same gear ratios so the torque would the multiplied by the same ratio

and its the torque at the diff thats important anyway. so if you had an engine with no torque but a huge reduction ratio then it would possibly need a similar strength diff to an engine with more torque and less of a reduction ratio

horsepower doesnt matter as the diff shouldnt really care what speed its doing when the torque is applied

Posted on: 2009/12/10 7:45
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Re: HP is not important?
No life (a.k.a. DattoMaster)
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What breaks stuff is force, the unit of force in twisting is torque. Horsepower indicates how far up the engines rpm range that torque is made - gives you a good indications of a cars top speed, but not a accurate indication of how much drivetrain breaking force is available.

"The resulting torque fed to the differential is a multiple of the rpm at the motor." - I disagree, the torque value is not multiplied by RPM - it can be constant across the entire RPM range of a vehicle in some cars. Torque is independant of RPM

Have a think about how big diffs are in trucks, these things have very low horsepower motors with phenomenal torque figures...

Smaller scale any diesel engine really, poor HP, good torque.

In F=ma the F is torque the m is mass - so to determine how fast a car accelerates you use torque, then derive the abstract concept of horsepower.

...I think i've forgotten what my original point is... but in summary force breaks stuff and torque is a force.

Posted on: 2009/12/10 8:01
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Re: HP is not important?
No life (a.k.a. DattoMaster)
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hp = rpm x torque anyway, thats why it matters most, its the force behind the punch that does the damage regardless of the speed, if that makes sense.

Posted on: 2009/12/10 8:09
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Re: HP is not important?
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Hi guys

No matter how much torque or horsepower or whatever you put through a diff wouldn't tyre grip be a determining factor.
Once you have got the tyres spinning you can't exert any more force on the drive train ?
Sounds a bit simplistic but so am I
Cheers
bob

Posted on: 2009/12/10 8:20
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Re: HP is not important?
No life (a.k.a. DattoMaster)
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Your right bob, but I think for arguments sake grip is asumed unlimited.

Posted on: 2009/12/10 8:22
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Re: HP is not important?
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Tire grip is not the limiting factor. Even with skinny 12" tires you can get the rear suspension hopping and break the differential. And hopping (tramping) can happen accidentally so the automaker takes this into account when sizing the diff. Tire grip does have an effect on it, but not usually considered with road cars. With extremes like drag racing slicks it becomes an important factor.

You are all correct, differentials are rated for torque, not horsepower.

The "estimated" ratings given assume a gearbox first gear typical for passenger cars fitted. So for the H145 with a 1.3 liter or less engine, a first gear around 3.7:1. For the H165 cars a first gear around 3.5:1. These gears won't vary too much if it a high-RPM engine, for example if the engine RPM is double, the 1st gear won't be double in practical usage. And Yes, these factors all matter because they are all inter-related, but for practical purposes, a simple torque figure tells the story best.

Posted on: 2009/12/10 8:48
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Re: HP is not important?
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I think what crispdollaz is saying is : given the two engines specified, were put into cars with identical ratioed diffs, and if both have a gearboxes with ratios which meant that at max rpm both cars had the same velocity, the forces the diffs would see would be identicall, because the high revving motor would need to be have a large reduction in speed. this large reduction increases the torque at the tailshaft.

the only time more torque is true...is if both engines have gearboxes with equal gear ratios.

Posted on: 2009/12/10 11:19
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Re: HP is not important?
No life (a.k.a. DattoMaster)
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So he's saying both those motors output the same torque at the output shaft? Well then they would need a diff that could handle that same torque out the end of the gearbox.

That demonstrates that torque is still what breaks diffs, hp is still irrelevant.

They only reason we all talk about torque (lol) is the cars driven in Datsun land will all be about 4:1 out the output shaft in 1st which will send the most torque to the diff.

An engine that spun at 140,000rpm and produced 10ftlb torque that was stepped down the same way would have the same effect - but using out of this world comparison has only managed to distract crispdollaz from the actual issue.

Posted on: 2009/12/10 12:21
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