BHP Explanation...

[Zektor]

Hi All,
I've read, that in the past BHP on the Corvette used to measured in "gross" format. I presume this means BHP at the engine crank (like on a dyno). But in later years they changed the format to "net" BHP. Which I think is supposed to mean BHP at the wheels???

I'm a little confused by how much BHP my 1982 Corvette has. Factory spec is supposed to be 200 BHP right? Now is this 200 BHP at the wheels ...or 200 BHP at the crank?

If this is 200 BHP at the wheels, then approximately how much would it be making at the crank? ...or how much BHP is lost due to transmission losses?

Cheers
Dazza

 

[Stallion]

I'm not definite, and I'm surely not an expert on this but I think base horsepower is right from the crank. And I think net horsepower is from the wheels, you know because hp is lost throughout everything like the transmission, I think.

Like I said, I'm not a master at this sort of thing, but this is what I think is meant. Sorry if I'm wrong.

TR

 

[smurfvet]

Your bhp is at the wheels. The horsepower rating change occured in the 70's when the gross hp numbers from the engine alone weren't thought to be "fair". So they went to the rear wheels which incorportated the losses in the entire drive train.

 

[BigBrakeFuelie]

I believe in the 60`s and early 70`s they rated engines on horsepower just by their self without exhaust or smog equipment.Today they rate it more accuratly.Your 82 should be 200 net horsepower.They changed the ratings in 72.

 

[Bob Chadwick]

This was done in an effort to get insurance rates down. The crank HP had gotten so high that the carriers were jacking up the rates. I have read a good rule of thumb for camparison is that the wheel HP is 85% of crank HP. So at 200 wheel HP, you would have 235 crank HP (200/85x100).

I have read different things on the C5's with some publications stating that the 405 HP on the Z06 is at the crank and others stating at the wheels. Anybody know for sure?

Bob

 

[Nut]

Dazza,

Bobchad's short cut calculation for crank HP is what I have been told by my Dyno shop too. 85%.

........... Nut

 

[Tom73]

Dazza,
Gross HP is taken at the flywheel on an engine dyno. It is done with a open air intake, headers and no mufflers, no emission equipment, and no accessories. Engines were rated at Gross HP through 1971. In '72 they went to a Net HP rating. This is also taken at the flywheel on an engine dyno. But the Net rating is with the engine as it would be equiped in the car. It has the stock air cleaner assembly, stock exhaust manifolds with full exhaust, all standard accessories like the alternator, and all emission equipment in place.

Gross is the max potenal of the enigne while Net is what the engine puts out under real world conditions.

The factory never rated any engines as RearWheel HP. That is a speed shop thing done with a chassie dyno.

So your 82's 200 hp is a net rating taken at the flywheel.

tom...

 

[Bob Chadwick]

Tom,

Now that the misunderstanding on gross v. net has been cleared up does the 85% rule still work for comparison purposes?

Bob

 

[smurfvet]

Well, I stand corrected. Tom73 is correct. Too bad I heard what I offered as an explanation from a GM service manager at the time.

I found a copy of a 1972 article at corvettearchive.com under 1972 model year, titled "Corvette overview in Corvette News, Oct. 1971". That has the explanation, which is as Tom73 said.

The actual ratios work out to between 73% to 77%, based on the engine. The "new" 200 hp engine was the "old" 270 hp. engine, for instance. 70 hp. (74%) seems to be a BIG whack to take for the net testing, but the other two engines in the article have similar decreases.

Check the article out.

 

[Tom73]

.... does the 85% rule still work for comparison purposes?

Bob,
I really don't know. There are way too many varables to try to go with one percentage. Also the new car are much more efficent with their electronic controls then the older engines. On the modern engines you are more then likely looking at only a 10% difference in Gross and Net.

For the older engines, check out the 71's, they were rated both Gross and Net. That will give you a basic for percentages but the percentages are not constant across those either.

tom...

 

[Ken]

I've always thought the horsepower loss was more like 15-18%, but who's quibbling over a few percentage points? :L

_ken

 

[lytemup]

I have read different things on the C5's with some publications stating that the 405 HP on the Z06 is at the crank and others stating at the wheels. Anybody know for sure?

Bob

it's net hp, or 'at the wheels'. that's for sure. it's the standard way (SAE) everyone measures it. ford got burned a few years back when they overratted the cobra- i believe they may have even offered buybacks.

 

[LT1Vettepilot]

A general rule of thumb to get a ball park figure for the HP at the wheels is to take your net HP and subtract 15% for a manual transmission and 20% for an automatic.

 

[sscam69]

Bob

As far as the Z06 is concerned its rated at 405hp at the flywheel. They typically put about 360-370 to the wheels.

You also have to take into account if its a manual or automatic. The torque converter fudges numbers , thats why manuals are prefered for dynoing.

Nowadays I don't really know how they are rated. Some are rated differently.

Ford REALLY screwed up when they overrated thier cobras! They have had nothing but problems with those things the last couple of years. Lots of quality problems.

As far as that percentage is concerned, from what I have seen the manual T-56 trannys are eating up the same constant ~50hp/~50ftlbs no matter what the power/torque. Just an observation.

Frank

 

[Tom73]

it's net hp, or 'at the wheels'. that's for sure. it's the standard way (SAE) everyone measures it. ford got burned a few years back when they overratted the cobra- i believe they may have even offered buybacks.

Nope, they are rated at the Flywheel (see my post above). The car companies have NEVER rated cars as rearwheel HP, that is a speed shop thing. So the Z06's 405HP is at the flywheel with all accessories as installed in the car.

tom...

 

[lytemup]

Tom-
you're right, i was confusing net and rear wheel as the same...i should have said it's definetly NET and just NET.
:duh

 

[GerryLP]

point for clarification

People, :xmas

I recognize BHP as brake horsepower, so termed because early on the use of a friction brake was utilized; however, the important thing is that by definition, brake horsepower is the USEFUL or ACTUAL horsepower produced from an engine. In other words, parasitic horsepower (the power needed to drive engine components) is taken into account. This was the net horsepower.

Nevertheless, I agree that this is normally done at the flywheel. In the early 90's I tested aircraft Jet Fuel Starters (JFS), and I kid you not, our test cell was a cab about half the size of a cargo trailer that we typically see on 18 wheelers. The cab was divided by a wall, and on one side was the JFS stand and on the other the control panel. The turbine on the JFS was connected to output to a flywheel that resembled more a large disc brake, and we applied the load to the JFS with a glorified disk brake caliper.

I also agree that car manufacturers would tend to define their engines output horsepower at the flywheel. Too many options that could change the rating at the rear wheels (tranny, power assist components, and the like).

Now lets all have nice drink....:L

:bar

 

[Ken]

Hot Rod magazine has an article this month on chassis dynos. I posted a message about that a little while ago: Chassis Dynomometers Explained...

_ken

 

[jmp]

When you say "with accessories", what exactly are you talking about?

For instance, power steering, AIR, AC? These are all belt drive, right, so wouldn't they use up power?

If this is the case, then wouldn't your HP be altered with which 'accessories' your car is equipped?

(If my assumptions are correct, my next question will be: which accessories where included in the factory test cars? My question after that will be: what kind of hp differences are we talking about when comparing e.g. fully equipped cars with fully unequipped cars?)

 

[Tom73]

jmp,
I believe that the accessories on the engine for the Net HP ratings were the ones that came standard on the car. This would include the alternator, the water pump, and the A.I.R. pump plus, if standard, the p/s pump and the a/c compressor. Net HP is the engine as installed in the car.

tom...