Again, the point of the article! Thank You!

And for those paying attention and now understanding the point of the thread, there is a DEFINITE pattern numerically when trying to understand and calculate Gross HP to Net HP to RWHP. It is not a precise science/math but generally speaking, as stated earlier in the thread, the pattern is VERY clear, although not totally accurate.

For older cars with less efficient drivetrains ( not newer cars, AWD, FWD etc) , the numbers in the article, and others who have posted their Gross HP to RWHP, the percentage drop is roughly anywhere from 30-35%-there are various variables, BUT you can get a ballpark number. The Gross HP to Net HP drop is roughly 15%. The Net HP to RWHP is anywhere from 15-20%, depending on exhaust, headers, transmission.

So time next time you talk to some guy who says he has a 500 HP motor, you can ask the right relevant questions like "is that Gross HP, a Crate motor, rebuilt by an engine builder, Net, RWHP etc"? Almost always on older cars with non stock engines, it will be gross. You now know that a 500 HP crate motor, most of the time, would be 325-350 RWHP, which helps explain why his 500 Gross HP engine would be no where near my 10 Z06 which has 450-460 RWHP but rated at 505 NET.

 

Better yet, ask for a time slip, a lap time or his autocross class and times...because otherwise, it's all just exhaust anyway.

 

Thanks for sharing.

 

Many years ago...the first version of my 540 made 732 HP on the Superflow 901 engine dyno with open headers and a belt driven water pump. No air "hat", spacers or anything else.

Dropped it in the car and it initially made 561 RWHP on a Dynojet with closed 3" exhaust with Flowmasters, same water pump and pulleys...but now driving a fixed 7 blade flex fan. This was through a Doug Nash 5 speed (runs made in 4th gear- underdrive to keep tire speed down) and 3.36 IRS setup. I had done some back road "butt-o-meter and plug reading tuning". It was interesting to see how it acted on the chassis dyno with various tunes. I was able to pickup a couple of peak HP leaning it out...but the overall curve was much better with my butt-o-meter tune.

Then I dropped the headers loose and the power immediately hit 615 RWHP with no other changes! It wasn't too wild about the Flowmasters it seems.

It's also interesting to note that the % difference between the two dyno's varied a good bit throughout the range.

The previous 427 made 423 RWHP and ran low-low 11's@123 mph capped up through 2.5" pipes. It was fun after some of the dyno sessions to "play" with some of the big HP C-5's with blowers. Those guys were always able to lay down a big number on the dyno..but from a 40 MPH roll...the "lil 427" would walk away from them. At that speed it's a pure "HP" race.

I've found supercharged and turbo stuff can put up incredible numbers on the chassis dyno that don't always equate to real world performance. It takes work to get all that power to actually accelerate properly.

There are so many variables to ET's and track times. On the dragstrip...trap speed is a good indicator of what is actually happening. My car has 3.07 rear gears, is heavy and isn't the strongest in the first 60' or so....but after that it gets moving fairly well.

JIM

 

Keep in mind that the Gross to Net percentage drop will vary based on accessories as well.

Water pump, Alternator.
Water pump, Alternator, Power Steering.
Water pump, Alternator, Power Steering, A/C.
Water pump, Alternator, Power Steering, A/C, A.I.R. Pump.