HP Numbers

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.

I agree with you on the HP numbers. The biger question is, how much HP does it take (or torque) to obtain the goal? (10 sec qtr mile, 0-60 in 4 secs, etc).

My 79 lincoln with the boat anchor, i mean factory 400, Ford says in its lit that it does 160hp. Now, 1979 should be NET numbers, not RW measurements. The only one i ever heard about bein dynos was in a truck magazine. C6 trans and 9" rear, got a whole 60 hp to the wheels!!!!! After reading recently (here and in a magazine) about trans and rear end HP 'useage' the 60 is probably about right!

SOOO, if 60 hp can move 4800 pounds of beast (not quickly mind you) then if'n i do the basics (intake, cam, elec fan, headers/dual exh, mebbe a carb) then i add conservatively 60 HP (160 top 220, right?) then i should get that extra 60 DIRECTLY to the wheels (no added drains in the drivetrain) - so if i DOUBLE rear wheel HP, how much improvement should i see in actual use?

So there should be a correlation between HP, weight and 1/4 mile time? So if we know any 2, we can calculate the third, right?

Good post Mike. I've always enjoyed seeing people post their Levi's Dyno impressions.:rolleyes:

It's especially fun to think about those who believe they'll get an extra 20 hp from headers, 10 more from the K&N filter, 15 more from the electronic ignition, 5 more from changing engine paint color, and so on, and so on.................... Add them all up..............who needs a supercharger?:p

I had a mildly built 440 a few years ago. Never dyno'd it, but one of my employees ran it through his Mr. Gasket Computer Dyno. Came back with something around 420 hp!!! While the car ran pretty well, I seriously doubt it was anywhere near that big of a number. But hey.........................the computer said so, right??:LOL:

Some real good points, Mike. All the horsepower in the world doesn't matter if ya can't hook it up. Engine dyno numbers are a good way to check yourself and your ideas, but chassis dyno's are probably more relevant to "real world" applications. I take the poor man's way out and check mine with 0-60 times or et slips. The hp numbers take a distant second to the way the whole car reacts when the horsepower is applied. Most horsepower numbers bantered about are nothing more than a WAG anyway, just one more thing to chuckle about with the guys at coffee time!!!

I don't know Bob you might have actually have under-rated your 440. I had one of those on again off again old customers of mine come by the shop yesterday (you know the type..... after he can't figure it out or find some one to fix it for free he'll finally bite the bullet and pay shop rate :CRY: all the way).

According to him his (STOCK) 440 Magnum (which it really is) is making 500 HP of course he "believes in balancing and blueprinting all his engines". He's even thinking about putting on "them aluminum heads" to get 700 HP out of it :rolleyes:

It's so nice to tell people I'm semi retired and send them somewhere else. :p