Since all car mags and dealers refer to engine HP, and not to RWH it would be interesting to know what some of these mods add to the engine HP.

It appears as though there is a 20-25% HP loss between the flywheel and rear wheels.

Assuming this, would it be safe to assume that a RWH gain of 5HP would add 20HP to the 287HP for the Z, giving us 307HP? I know this makes no difference in "getting there" but I am constantly being asked what HP my Z has, and never refer to the RWH. Putting it pass the magic 300 number would help bragging rights.

If there is a better method of figguring this let me know.

 

5 rwhp doesn't equal 20 fwhp. Let's say that a stock car dynos at 230rwhp. That's a 19.86% drivetrain loss. If you add 5 rwhp, that becomes 6.24 fwhp.

Edit: On your calculation of 5 rwhp to 20 fwhp, you made it a 75% drivetrain loss, not 25%. With a 25% drivetrain loss, it would be 6.67 fwhp.

 

OK thanks. So I guess the rule of thumb being to add 20% to whatever RWH gain you have to get the FWHP, ie 12rwhp = 14.40FWHP. Roughly.

 

We figure about 17 % (+/- a couple) with the 6MT driveline loss. TO calculate take RWHP/(1-%loss) In my case I baselined 238 . . .so 238/(1-.17) = 286.7 Now I am at 264 so 264/.83 = 318 CHP.

RWHP - Rear Wheel
CHP - Crank or engine

 

The percentage is an estimate. The only way you will know true HP at the motor is by using an engine dyno. The only way you can truly calculate drivetrain loss is by using an engine dyno, installing the motor in the car, and using a chassis (wheel HP) dyno and doing the calculation for the difference. The original percentage comes from the manufacturers claimed HP rating vs. what the given car dynos to the wheels stock. Notice that all the guys doing dynos after they mod the cars get their wheel HP numbers and reverse calculate to the motor to get their motor HP. It is just an estimate for the motor HP number unless they are using an engine dyno (which they didn't). Drivetrain loss has to be taken at each individual circumastance to be accurate. It seems logical that drivetrain loss would become smaller as power increased. The fact is, you can't tell unless you use both types of dynos (engine and chassis) and caluculate your percentage. Any motor HP claims based on WHP are estimates based on the stock loss percentage. Ok?

Also keep in mind that it takes 20 percent of 100 to make 80 but it takes 25 percent of 80 to make it back to 100.

VandyZ, I would think you are closer to 313 crank HP than 318 but that is just a 5HP diff. Unless you change the drivetrain, it isn't correct to assume the amount of loss goes up with the amount of power. The only time this would really happen is with an automatic trans. Remember, you are putting HP to the wheels, not percentages.

 

I've read somewhere that drivetrain loss is more of a set number particular to your car instead of a percentage. Not sure how true that theory is either. Can you ellaborate on the drivetrain loss on the autos a bit more?

 

Why do you always have to do this?

What formula are you using. Is it something superior? Come on. I gave the formula I used. Its basic algebra. You don’t have to try and correct every thing. I do under stand what you are saying but if you don’t offer your algorithm you look like you are coming up with the numbers off the top of your head. I don’t think either one of us has enough experience with this particular car to say how the % loss changes with mods.

Geez

Oh and by the way the 17% I got was not just from my car baselines . . . I could tell you but I would get yelled at. It just so happened to match my car near perfectly.

 

When they ask you how much HP you have just smile and say,
"Enough." Always works for me, keep em guessing.

 

I'm usually going to fast for them to ask.

 

Vandy, only one sentence was directed to you. In fact, 95% of my post was copy and pasted from my post in the nitrous forum regarding this very issue from much before this thread ever popped up. I am just saying that all we can do is estimate. Don't be so defensive. Besides, at low HP levels like 3XX the difference is marginal (5 HP) and doesn't matter. 17% is pretty much the generally accepted loss % for the Z. That is not my case. My case is that loss should not be measured as a % in the first place. Drivetrain loss is parasitic and it takes what it takes whatever the HP level and the only way to accuratley see motor HP is to use an engine dyno. I don't care if you are making 318 or 313 or 413 HP. That is not my point. It's not all about you.

Automatic transmissions have torque converters. Torque converters slip (except in high gear - they lock up). The converter will slip harder with more power applied to it (slippage is measured in percentages and is adjustable with rebuilds). A converter will never be locked on a dyno situation with an auto since the trans will downshift out of high gear into a lower gear that allows the converter to slip. The more power you make, the more it will slip unless you rebuild it to your spec (whatever it may be). Race cars are not allowed to have lockup converters (NHRA/IHRA).

With a stick, you're in the gear you are in. The clutch does not slip (unless it sucks/old) and will transmit the power the motor is making.

Vandy, this entire post is not dedicated to you. I think I will dedicate it to my mom. She is great and without her I could never have learned to type this post. Thank you.