camburger

Okay hear out my horsepower math for a few moments.

When the C5 was introduced it had an ADVERTISED HP rating of 345hp. We know that was not a RWHP rating. It was some unrealistic dyno number more for the benefit of advertising than anything.

The rear wheel HP rating as we are typically seeing on the stock 345 hp motor comes in at let’s say 310 RWHP on average.

So once a base motor is modified in a typical basic way…LT’s,CAI, 160 STAT, and Tune we are typically seeing a 25-30 RWHP increase, again this is on average.

The question (mostly a curiosity REALLY) I have is what does this 25-30 RWHP jump change the presumed advertised HP rating into now?

Maybe 385-390HP?

sfc rick

No, that's dreaming.

robert miller

Maybe a true 345 RWHP number.

Cosmik de Bris

Using your figures and a constant percentage loss about 370 - 375.

HP = (310 + 25) / (310 / 345)

sfc rick

Generally....it's really about 300 at the wheels in most cases....15% driveline loss with a manual, 18% with an Auto.

Some will show a little more, others with a little less, elevation, temp, all effect the actual numbers.

Also at about 3000 rpm you might be at 200rwhp if that much. LOL!

Rex99Coupe

310+25=335*1.15=385hp
.15 or 15% is loss for manual trans due to friction from transmission and rear end gears etc. For auto trans it would be .25 or 25 %

camburger

I think a couple of you are indicating what I expected the corrected "advertised hp" to be.

Cosmik de Bris @ 370-375

Rex99Coupe @ 385

Not talking RWHP now guys...really just that fantasy number you see in the new Vette brochure and corrected for what we know on average from real world results.

Now taking that extracted data one might logically compare a lightly modified base cars performance with that of the early Z06 from 2001 which I think was right at 376hp. Maybe a reasonably fair comparison. All just numbers obviously...

bobeast

I would suggest that this rule of thumb is great for basically stock cars. However, I don't think parasitic losses are linear. For example adding 30 HP to your engine doesn't suddenly increase the friction in your drive train. Those losses are more a function of RPM.

So if the drivetrain loss from 345 HP = roughly 50 HP, I would suggest that adding 100 HP would not significantly increase that loss, and that almost all of the gain would make it to the rear wheels.

So to put it another way, as you add power to your engine doesn't it seem that parasitic losses expressed as a percentage of total power would decrease?

95wht6spd

300rwhp average, about 50 fwhp (net) drive lline loss on average = about 350 stock. It is not a fixed % loss on all cars, all mods, etc. if you make 400whp, it is about 450 net on a manual C5.

Paul 75 L82

I want pie.

JaxEagle

Why are we concerned about an advertising HP number? I get RWHP and FWHP but Advertising HP? Just curious why?

DanSavage

I've read that back in the early 70s, GM changed their horsepower ratings from gross to net.

I've always taken that to mean that gross was measured at the crankshaft and net was measured at the wheels.

If this is not the case, then what does 'net' horsepower mean?

Lee DeRaud

Wasn't just GM, it was an SAE standards switch (possibly government mandated). The old SAE 'gross' HP was measured at the crank with no accessories; 'net' is also at the crank, but with the engine more-or-less as-installed.
Details here: http://en.wikipedia.org/wiki/Horsepower#SAE_gross_power

allred

I believe "net" horsepower means the horsepower measured at the crankshaft with all ancillary equipment attached (water pump, alternator, power steering pump, air conditioning, stock mufflers system, etc. ) whereas "gross" horsepower was horsepower measured at the crankshaft on a bare dyno'ed motor with optimum jetting and correction factor for atmospheric pressure.

Gross horsepower is "optimum output under ideal conditions"...............net horsepower is "real world output under operating conditions".

The introduction of unleaded gasoline in the early 70s, and other environmental laws, lead to lower compression ratios and leaner fuel mixtures.....and lower power outputs. So the "timely change" from Gross to SAE net "hid" some of this power decline to a certain extent.




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camburger

Well largely speaking "We" are not, it was as I stated "mostly a curiosity really".

JaxEagle

Gotcha. What I meant was, if there was a reason "we" were concerned about advertising HP, then I too was curious because I didn't know the significance of it and thought that perhaps I should know. I think I understand the point of it now.

oldschoolvette

me too

are you asking how to correlate the advertised gross HP to a new higher value following a mod?

How about this: (general and not scientific but would work) Take the net increase in RWHP and add 110% back to the advertised number.

That would get you in the ball park.

I used the extra 10% to account for minor drive line loss

I found this on the web (or I pulled it out of my AZZ after a few beers..in either case) it must be true.

Now where in the world is that pie?

LoneStarLizzard

The math issues (how to extrapolate rwhp into flywheel hp) discussed above are all relevant. Because when I improve my engine's power, my only true benchmark of success is "how much better is it than stock?" Virtually none of us has ever yanked his engine and stand-mounted it in order to measure "as modified" flywheel hp - its too expensive and time-consuming a process. But since I rarely wind out my engine above 5000rpms, the relevant measure of power is torque, not horsepower.

Torque and horsepower are, of course, interrelated. However, virtually all engines provide their maximum torque much lower in their rpm band, and maximum horsepower much higher in their rpm band.

"Bragging rights" are won by those whose engines demonstrate the highest horsepower numbers, whereas the lowest 0-to-60 times are often claimed by those who have the best torque.

So...blown or naturally aspirated, the measure of an engine's power most important to each of us has more to do with how we drive and talk about our machines.

All of that said, the bragging rights awards are generally garnered by those of us who have the biggest rwhp numbers.

Go figure.

The Lizzard

Cybermind

Manufacturers today love to advertise horsepower numbers for their cars never even bringing up the torque or even more important, power-to-weight ratio.
Horsepower just gives you bragging rights but it doesn't win races. Colin Chapman knew this very well and this philosophy carries on with the Lotus cars of today.
This philosophy was also followed by the C5 designers. Lucky for us.

Vetteman Jack

I'd have to disagree with a 25% driveline loss for an automatic. I believe it is more in the line of 18%. Using your percentage, my '03 A4 would only be putting out 262 rwhp which from what I have seen posted about other A4s, would be very low.