Thread: New to the game and got a question.

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What is the differents in fly wheel power and rear end power?

Flywheel horse power is the horse power rating with NO LOAD. Rear wheel horse power is the horse power under load.

Flywheel horse power is usually more than rear wheel horse power.

It is generally accepted that the drivetrain (includes everything from the backside of the crankshaft to the tire) absorbs about 25% of the power that is measured at the flywheel. So, if a motor makes 400 flywheel horsepower, the power at the rear wheels might measure 300 hp. (400 x .75)

I wrote this a few years back on a day that was too cold to be out in the shop


Beings as how I've made a couple of comments about about HP numbers here in the last couple of days, I thought I should explain my position on them and the reasons for it.

Do you know what HP your talking about??

Gross HP (engine HP as determined by an engine Dyno without any accessories and usually under ideal conditions and with a "laboratory" intake and exhaust).


Net HP ( more real world with all the accessories connected and and standard intake and exhaust systems).

Rear wheel HP (What a Chassis Dyno will see at the tires after the drive train eats up it's fair share of HP)



Brake HP (HP= RPM X Torque divided by 5252) is a mathimatical formula that needs to be based on real values. Without those real values (and the only way I know to establish those are on a Dyno) you can skew the numbers to say almost anything you want to.

Even factory HP ratings from the 60s (and after seeing some of the numbers on the new preformance cars I think some of those too) were/are often either optimistic or grossly under rated. In most cases the numbers were based on a blueprinted engine with optimised fuel and exhaust systems. These tollerances are of course not carried over to the assembly line. In the case of the under rated engines (the 426 Hemi and some of the COPO Chevy big Blocks come to mind) the HP number was pulled at an RPM much lower than the actual peak HP was generated at.

This is also pretty much the same story with engines and engine kits you buy that advertise a certain HP. In many cases they had ONE optimised engine that really did generate those HP numbers, at that specific altitude, at that particular temp, on that particular day with a specific intake and exhaust.... AND in many cases these are "CORRECTED" numbers to take certain variables into account (and usually raise the HP numbers that are then advertised).

IMO the computer programs are little more than an educated guess. There are certain variables in even 2 "identical" engines (friction loss, cylinder imbalance, cylinder leakage etc come to mind) that the program would have to take into account. Many people also have a tendency to be a little optimistic when plugging in the variables. Additionally, without taking into account things like intake/exhaust runner volume, legnth and texture (i.e. the actual efficiency or tunning of the intake and exhaust systems) the numbers become a best guess.

Finally, an engine on a dyno is little more than a fancy coffee table. It's what the CAR (truck or what ever) actually does that matters. All the HP in the world does not mean anything if the rest of the combination is not "dialed in" to use it.

The whole thread is here

http://www.clubhotrod.com/forums/showthread.php?t=2484

Thanks guys for the info. I have seen people ask this numereous times but never understood it but it makes sense now thanks agian.