|
|
| HP is not important? |
Subject: HP is not important?
by crispdollaz on 2009/12/10 7:23:57
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.
|
|
