General question about horsepower numbers

[Dallas56]

When an owner posts that my car has X horsepower, is the custom and practice to provide the flywheel number or the rear wheel number? "Tin-man needs some advice" is an example of a recent thread containing lots of posts with horsepower numbers. I can't tell if the numbers are flywheel or rear wheel. In my case, the difference is 30%.

[BBfangs]

I have always thought if not specified rwhp then it is at the flywheel but I could be wrong.

[patrickt]

There are at least three distinct numbers for everyone's build. 1) The engine as dyno'd by the engine builder (usually open headers, maybe no water pump, certainly no accessories), other adjustments that tend to "up" the number; 2) The true flywheel horsepower number that the car has with the engine installed and everything hooked up but as if there was no transmission in the car and a dyno was attached to the flywheel; 3) The rear wheel horsepower as measured with a chassis dyno. 1>2>3, often times by pretty large percentages. #1 might be in the 500s, while #3 might be in the 300s.

[Silverback51]

The dyno also makes a difference. The Mustang dyno is generally about 15% less than the others.

All a dyno is really good for is being able to tell if the changes you make are good or bad.

[WardL]

How's it going Dallas?

I would assume you are going to get the manufacture's "rated" or dyno HP (the highest possible number) unless it is spelled out as something different like Patrickt explains.

[Bob In Ct]

HP is always given at its greatest value which is usually in the 5,000 to 7,000 RPM area. How about specifying the HP at 2,500, 3,500 and 4,500 RPM where most of us drive these cars. If that were to happen more of us would be using dual plane manifolds which tend to work better at reduced (street) RPMs.

[Dallas56]

Thank you for the helpful replies gentleman. I learned something from Patrick. I have engine builder flywheel number (580) and rear wheel number (404, a 30.3% reduction), but did not know that you could take a flywheel number with the engine installed and everything hooked up. It sounds like there is no rule, custom, or practice about what number you should post if you don't say what the number is. If I post horsepower numbers, I guess I'll say what the numbers are, flywheel, rear wheel, or both. Again, thank you for the replies and information. Larry Carlson

[Tommy]

So far as I can tell, it is tradition to post the biggest number you can justify. BTW, the only connection between peak horsepower numbers and performance is that it is one of many indicators of the car's potential top speed. The size and shape of the torque curve determines the car's acceleration potential and is a much better indicator of street performance.

[olddog]

Car manufacturers listed Peak HP at the flywheel with no accessory belts connected and open headers up until 1972 (I think). From 1972 on, the government set a standard that they had to list the Peak HP at the flywheel, with all accessories and mufflers attached as the engine will be operated when installed in the vehicle. It is mistakenly believed that emissions equipment cause a big reduction in power, but it was a new standard in honest advertising.

In addition to this, there are formulas to correct the measure Hp to what the engine would have produced had the test been done at sea level at a certain air temp and humidity. This is because air density is a big factor in the power an engine makes. So to compare different test done at different places on different days (summer or winter) the numbers are corrected to what it would have been at a standard atmospheric condition. It sounds so logical, but notice I said formulas - plural. The formulas are not exact and they keep trying to improve and get it right. There has to be more than a half dozen sanctioned formulas that were used over the years and more than one in use today.

Yep, you can make the numbers come out any where you want and still claim you did it by the book. But which book? I'm not saying there are no honest people trying hard to get an accurate number. I'm just saying there are people who have no honor.

[tin-man]

Hi Dallas, good point, I was quoting Roush, I will find out what their reference point is. tin-man

[mdross1]

Most always crankshaft horsepower numbers are stated,since most motors are dynoed.

[Excaliber]

The flywheel horse, in my opinion, is much more accurate than a chassis dyno. A chassis dyno is like weighing yourself fully clothed, or nekkid, you can never be sure which way it might have been done.

Gear selection is an important factor on the chassis dyno, many other variables as well. The variables are reduced with an engine only dyno, but they still exist.

As stated previously, unless I see RWHP, I assume the number is flywheel

[ha_pe_1968]

Hi!
In most cases the discussion is about the engine HP and not the RWHP. Depends on the car and transmission the difference is about 15 to 35%. And of course the engine HP is more impressive to discuss.
In my case the Roush 427R with dual plane intake and a 870 Holley produces 541,4 HP.
At the Mustang dynamometer the RWHP is 451.7 HP...thats a loss about 16%.
Think thats a good result for a Superformance with stock headers and exhaust and no engine modifications.

Greetings from Austria
Peter

[cobrashoch]

Mostly been said right in the posts above. But my votes for chassis numbers when I am trying to figure things out regarding how a car actually runs. For example when I switched to a glide from a built T.H. 400 I gained about 35 horses across the board. Gained about 8 horses across the board by changing my rear end housing over from a big bearing to small bearing setup. That latter setup also had a much more stout housing BTW. There is something to be said about rolling resistance numbers too. I never think about peak numbers, only compaired numbers. Personally, I think peak numbers are worthless and flywheel numbers are only good on a inital fireup and cursory tune of a new engine. Of couse there is the added cost of continued visits to your local chassis dyno shop to consider if using that method ,,,,,,,,,,,

[Excaliber]

Ron Shockley, very good point, the chassis dyno is king when it comes to setting up your car and finding out what works and what doesn't. Just don't use TWO different, or more, dyno's in the process!

Interesting the Power Glide is so much more efficient than the TH400.

[cobrashoch]

Ernie - Thats the secret reason pro drag racers use glides. That and you only shift once, you loose time slightly each time you shift, esp with a stick. i.e. - consistancy = more rounds you run in ET drag racing. The input shaft on my Coan glide could be spinned by my fingers only. Took a wrench to turn the 400. Don't take much to turn a glide, maybe only 8 or 10 ponys. If your engine makes between 300 and 400 horsepower that drag/resistance number is significant. BTW, most 5 or 6 sp. stick numbers match the glide numbers with respect to what it takes to turn them. Glides on the street are boring, sticksare much more fun.

[Excaliber]

That higher first gear ratio (of the Glide), for me, would also be desirable. I was running a close ratio top loader with a really tall first gear, then switched to a TKO600. While I like the TKO on the street and enjoy the lower first gear around town, at the drag strip, I see no advantage. Tire slip was always a problem and even more so now with the TKO, even 2nd is to low, I can't hook up until third.

[BAD ASP]

This is a mustang dyno reading for my 408w, fuel injected, big stuff 3 ems, 93mm air to air intercooled turbo on 93 octane gas and 6 lbs of boost. I am running with the stock superformance rear end and a tk56 trannie. The numbers are obviously rwhp. The dyno is a great tool for setting up engine parameters and a decent base line. I haven't set up for running more boost on higher octane, I figure whats the point the car is already past my skill level for now.

[PDUB]

Originally Posted by Excaliber

That higher first gear ratio (of the Glide), for me, would also be desirable. I was running a close ratio top loader with a really tall first gear, then switched to a TKO600. While I like the TKO on the street and enjoy the lower first gear around town, at the drag strip, I see no advantage. Tire slip was always a problem and even more so now with the TKO, even 2nd is to low, I can't hook up until third.

Remind us what your rear ratio is again Ernie?

[Excaliber]

The typical 3.31 Jaguar IRS. With the close ratio top loader that was 60-65 mph in first gear. Not the best ratio(s) if you had to start on a hill or a steep ramp into a parking garage. But it made it easier to tame the wheel spin.