This may be a stupid question, but... Lets say that a certain engine is dynoed (on an engine dyno) and found to produce X peak HP and Y peak TQ. From a stand still launch in first gear, does the differential "see" a SUBSTANTIALLY higher lever of HP and TQ? Intuitively, I want to say that the driveline (after the first gear set) will experience a torque close to (first gear ratio) times Y. And similarly, (first gear ratio) times X for the "seen" HP at the differential. This leads into my next question... When the rear-end manufactures say, "This tracloc unit is good for 500HP...", does that take into account the multiplication of the lower gears? How is this stuff related?
OOOh good question. Big difference in TQ multi. from 2:79 to 4:11's. Your still at 500 hp but the torque changes through the gear ratios. I have a feeling this is going to get involved.
Oohh...Can I throw an iron in the fire??? Torque converters are torque "Multiplication" devices right??? So would that have to be figured into the equation as well...
I'll just comment on one part of the question: torque on launch. If the engine has built up RPMs when the clutch is dumped and the throttle is opened the drive line can see more torque than the engine rating because some of the energy in the rotating assembly and flywheel is dumped as the engine RPMs are abruptly slowed. That's why the tires chirp when power shifting even when they don't continue to spin with WOT.
i believe that the torque gets multiplied by the gears but the horse power is actually reduced. horse power is a measurement of torque applied over time. so as gear multiplication increases, the time that it takes to get through the rpm range gets reduced. to see this in action go talk to a chassis dyno shop. if you dyno a car with 3.0 gears then change the gears to 4.10s, the 3.0 gears will put out a higher hp number on the chassis dyno than the 4.10s will. so when a drive-line part is given a hp rating its really not an accurate way to consider the force it will see. an 8" track lock could survive over 1000 hp. if the weight of the car is super light like under 1000lbs. the same track lock could easly be broken with just 200 hp in a 5000lb car. this all may sound good and it may be right but it has just as good of a chance to be very wrong. its just what i have observed and picked up over to many years of doing this stuff.
Good points made... Bryant, it makes sense that weight definitely plays a role. On the chassis dyno, is that what "corrected" numbers mean? That was the reason I was trying to think of an engine on an engine dyno, that way the results are "true" engine numbers, without having to worry about the driveline influence, etc.
i dont know of an exact way to figure true engine power from a chassis dyno. there are correction percentages that people say. ive heard anywhere from 12% to 20% loss from the drive line. it just not a simple correction factor across the board. every car will be different. the true value of a chassis dyno is for tuning.
I'm pretty sure HP is totally irrelevant to acceleration, it's torque that counts there. If your engine makes 500 ft-lbs at peak and you have a 4.00 axle and 3.00 first gear, then your axle(s) will see 6000 ft-lbs at that peak. There used to be a good explanation on Moser's website, but I can't seem to find it now...
This is the question that had me puzzled when I was trying to think about optimum shift points. (Deleted thread from a while ago.) It seems like torque would be the important factor for acceleration but shift point discussions are all about the HP curve. Here's one of many discussions found by Googling "torque HP acceleration" http://www.houseofthud.com/cartech/torqueversushorsepower.htm
Strange that a rear (or trans) would be rated in hp since hp is a function of torque and rpm. How do they know what rpm you're gonna turn?
I think that the rear end maker is just trying to speak in laymans terms by rating a rear carrier in HP. Dang Bros This thread is a dynomite read. If I had not been at Dyno Day Id be confused. This is real Rocket Science.
This is a cool thread. I'll add to the complications of weight vs actual torque with impact. At the instant the maximum torque is applied(ie: launch) you can calculate torque produced by the engine times the trans 1st gear ratio times rear axle ratio but to that you would have to add the load of inertia from the weight of the rotating components just before the line lock or clutch is released. The instantaneous applied force is incredible considering some of the engines and tbe weight of some cars, and say it dead hooks on launch, WOW!
Yeah, this is a neat discussion. I appreciate the responses. I also think that the rear end ratings are "normalized" to help the buyer get an idea of thier capability. But there is no doubt that the applied torque can be 10 times the engine torque curve through first gear.
