Bhp?

[Badsnake427]

Is there a way to figure out what the bhp would be if the actual hp from a dyno is known? What is the loss in hp, due to the clutch, tranny, etc?

[mr0077]

RWHP (rear wheel horsepower) is usually somewhere between 80% and 85% of flywheel horsepower...I think they're both BHP (brake horsepower)

[Burgs]

From what I've seen around here, it's pretty high at around 25% or more. I think a good chunk of that comes from restrictive side pipes and sometimes small air cleaners. As ken said above, 80-85% is where I'v seen cars other than cobras.

[Badsnake427]

Thanks guys. Is the hp rating from a dyno, the hp at the flywheel? Thanks again.

[Burgs]

That would depend if it's an engine dyno (flywheel HP), or a chassis dyno, (rear wheel HP).

Do you have a specific example??

[Badsnake427]

I'm thinking of an engine dyno, where there is no actual car involved.

[Burgs]

Ok, so let's say you have an engine that dyno'd at 500 HP on the engine dyno, which is considered FWHP, (flywheel HP). The parasitic loss from the transmission, rear end, ect., is around 15% for a manual trans, 20% for an automatic. Throw restrictive components on the car such as side pipes, air cleaners, ect, you will loose even more. The end result can be measured on a chassis dyno as rear wheel HP, (RWHP). I have no idea how BHP, (brake HP), relates to FWHP or RWHP.

[DAVID GAGNARD]

I believe "brake hp" and flywheel hp are different names for the same thing..........Seems to me in the "old days" the manufacteres reffered to the output of the engine as brake hp, which was nothing more than flywheel hp..........

David

[mr0077]

David, thanks, I stand reluctantly corrected.

I'm not sure I trust Wickipedia for much, but they define it as flywheel hp, absent losses, since the band brake ("brake" HP) was applied to the crankshaft to measure the torque. I thought the term applied to all HP readings since they all use a form of "brake" to measure torque and arithmetically determine HP from there...

I bet there's a dyno genius out there that can set me straight :-).

[CobraBoy]

Originally Posted by mr0077

I thought the term applied to all HP readings since they all use a form of "brake" to measure torque and arithmetically determine HP from there...

Not so! Rolling road dynamometers use an inertial mass that is accelerated by the engine spinning the driven wheels to determine horsepower from the rate at which the engine can actually accelerate the mass. So your brake dynamometer is using friction applied to the engine, whereas the rolling road uses an additional mass coupled to the engine, which are different principles.

Otherwise, brake horsepower measured by a brake dynamometer is the same a flywheel horsepower. Differences creep in when people begin to talk about SAE (Society of Automotive Engineers) or DIN (Deutsches Institut für Normung) horsepower, which are measured according to different test protocols, engine ancillary loads, etc.

When I was at college, I ran a series of experiments on reformulated gasolines on an engine running on a test bed, connected to a brake dynamometer. Some use a simple rope tied around the flywheel, whilst others use friction clutches or even hydraulic systems (like a torque converter but permanently stalled) to measure torque. Then, basically, torque times speed equaly horsepower.

Hope this helps!

Paul

[mr0077]

Cool, thanks for the explanation.

[double ugly]

CobraBoy did you get a PHD or something?

[olddog]

One of the biggest variables in measuring Hp is the fact that they correct the measured Hp to what the engine would have produced at sea level, at a given temperature, and a given barametric pressure. This is done to standardize the result so that no matter where the test is done everyone would get the same result. The problem is the formulas to correct for this have been changed numerous times, and they know that the formulas are still not exact. Then you have errors in the measurements. And most importantly how honest is the man doing the correction. One dyno expert told me that if he wanted to he could move the numbers a bunch, and still claim it was done correctly.

[Suomien]

Aren't there all kinds of "rules" regarding what accessories are present during testing to determine HP? Whether there's a belt-driven alternator, water pump, nowadays even the A/C compressor might be included.

Back in the mid-60's, it was all engine, no accessories, and "special" carbs even. Back then, they probably tested with race headers even (remember the picture of the Cammer with the glowing headers?). Now, measurement is more standardized, so that the numbers actually mean something. Theoretically.

[Traveller]

Here's a simple equation I use:

Horsepower = never enough

[Badsnake427]

Originally Posted by Traveller

Here's a simple equation I use:

Horsepower = never enough

I like your thinking!

[496fe]

Another factor is the manufacturer of the dyno. Dynos can be as much as 5% different from each other. Some dynos even have conversion tables to show what this particular manufacturer's HP would be using other dynos.

If you compare a "high rating" flywheel dyno to a "low rating" chassis dyno, the HP difference becomes greater between FWHP and RWHP. There are no standards for the regular guy. The manufacturers have their own standards.

[496fe]

Here's my numbers as example:

FWHP = 608
RWHP = 474 so I divide 474 by 608 = .779 (call it .78).

With this reasoning, I have a 22% loss from transmission, driveshaft, rear end, wheel inertia, water pump, alternator, power steering pump, air cleaner, ambient air temp. (chassis dynos are usually in air conditioned rooms), mufflers, pipes, etc.

I can now back into my post-NOS FWHP by dividing the new RWHP number by .78 to arrive at FWHP from RWHP:

575/.78 = 737 FWHP (I usually roudf up to 740 .

So what this all means is that you need to have a FWHP AND RWHP to know for sure.

Brian

[eschaider]

Don't forget the Flux Capacitor (FC). A partially charged FC can produce indeterminate changes in the sub-space field surrounding the oil filter and negatively impacting the dyno session.

Among, of course, a myriad of other things that result in having to re-torque your license plate bolts.

[Gene Gorman]

Originally Posted by Traveller

Here's a simple equation I use:

Horsepower = never enough

I used to think that untill I built a 700hp supercharged C6 Corvette and now with my 484ci. Alum. Shelby block Superformance. You can over power a car. But it sure is fun.