I agree with your computation procedure but 15% loss is not only accounted to the parasitic drivetrain but also the weight (which I mentioned).

The back pressure thing:
Exhaust valves open and close, exhaust gas will flow, then stop, and then flow again as the exhaust valve opens. The more cylinders you have, the closer together these pulses run. In order for a "pulse" to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The "body" of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient. The pressure differential is what keeps a pulse moving.
As you know by now, that exhaust gas is actually a series of pulses, we can use this knowledge to propagate the forward-motion to the tailpipe. How?
I agree tuning the the headers like equal length. But that's not the only engineering tricks we are so fond of come in to play here. Since majority of the mufflers today are not too efficient, to overcome the loss of efficiency, you have to introduce to the system called back pressure.
Racing car like Ferrari has the best sounding engine. Their research in the exhaust area allow some back pressure to attain a sonic sound that is needed to produce power.
Maybe I am wrong.

 

First of all are you talking weight of the car or weight of the drivetrain and rotating assembly..(wheels tires rotors) as weight of the car itself would have no affect on a dyno.. weight of the parts would be contribution to parasitic drivetrain loss and startup inertia.. the biggest affect would be in first gear as far as lightening the flywheel light weight wheels etc etc.. once up to speed drag would be the limiting affect onthe road.. but

as far as exhaust noises and backpressure if what your stating was that simple they would throw in another cat to induce more back pressure which wouldnt result in a better sound or improved performance.. sonic sound or whatever you want to call it doesnt produce power its all about decreasing back pressure to increase flow.. as far as the noise thats all in the can /muffler

 

Vehicle mass does not affect drivetrain loss, however it does affect rotational mass. That is, anything that weighs anything, and is attached directly after the flywheel. this includes the flywheel, clutch, driveshafts, rear rotating diff assembly, and the weight of your wheels. This is all compounded into the assumed -15%.

As far as exhaust pulses, you are right. But I think you're wrong in thinking that the more cylinders you have, the quicker these pulses come behind each other. Remember, there is always one cylinder firing at all times -- it does come quicker when you start putting a load on it. That's why turbo cars build boost under load, but does not when standing still.

I will also argue, that most modern mufflers are pretty efficent today -- but that's another topic for another rainy day. The fact of the matter is, exhaust tuning is done from a sense of 'least restrictive path possible' IE: no backpressure. Ferrari motors produce the best sounds b/c of their engine layout and careful consideration to the diameter of the exhaust from header all the way back. They never, ever try to "add backpressure" because that breaches the first rule: never add backpressure. Backpressure is counter-productive -- why would you want to block exhaust flow if you're trying to make it go faster?

The only time you would want to have a smaller diameter pipe, is at lower RPM speeds, when not enough velocity is present to scavange the exhaust from the exhaust pipes...that would be the only time you would want a smaller diameter pipe -- some manufactuers do this by adding a valve.

 

"Vehicle mass does not affect drivetrain loss"

Then it will be very interesting to see if I will lost 15% hp if I dyno my project Lotus super 7 that weigh in (990) just under 1000 lbs.

Do you think I will average between 197 to 205hp ?

 

Easily - vehicle mass affects acceleration times, as that is hp to weight ratio. But has no bearing on your hp @ the wheels. 990lbs/200whp = fun!

On the dyno, you will output the same as us, maybe slightly more, since you have a SuperSeven, they have smaller, lighter wheels then us S2k owners.

And you can't count out that there are always factory "freaks" and factory "duds" as well as climate, elevation, engine condition all have a bearing. But, anywhere from 195-210, I'd expect to be correct.

 

First of, thanks pilot!

Read the very bottom
http://www.indy2000.co.uk/march2006therollers.htm

Just loss of 4.5% as the 15% you mentioned.

Do you think the dyno is right?
Do you think wt. distribution is the factor?

 

Ahhh, I see where the confusion comes from...

UK dynos are different from US dynos. I don't know how they do it, but their dynos "estimate" flywheel hp somehow (atleast that is how I've come to understand it -- please don't quote me!)...If you notice they've measured it with a "BHP" or brake horsepower. It's usually interchanged between flywheel hp. Plus, that motor sounds like it isn't stock, and I read "tuning" involved, so that means there's either a piggyback or a standalone ECU.

I'm pretty sure they'll find that last 4.5%. Interesting read.

 

Pilot, you probably right, that engine has some steriod parts.

But anyway (just for information).
BHP - Brake horse power - it means measured horse power. Where as HP is theoretical calculated horse power. Both units are equivalent to 746 watts.
Its hard to differentiate who is accurate.


I need to research a lot, so I can be successful in installation. I don't know much about S2000 engine that's why I need some feedback from a S2000 guru like you,
thanks my friend!

 

No problem. I'm by far an S2k guru though, lol.

Also, I just re-read that article -- he's getting a lot of wheelspin. Looks like he can't get all his power down Usually, when a dyno slips like that, the dyno isn't big enough to handle all the available power.

FYI: They call it brake hp, is because engine dynos are spun up using a big drum brake to measure tq, and thru math figure out BHP. For entirely too much reading, go here: http://en.wikipedia.org/wiki/Horsepower