tuned length

hi sjef,i'm also interested in improved exhaust set-up but nothing happening on your thread?

Hmmm. This calcultor at nightrider might be OK if you have 7 0r 800 cc in you cylinder, like a Harley.
Has anyone ever tried this trick told to me by an engine builder who was, before he fell victim to metal fatigue at 140 mph, very famous and well respected in South Oz speed boat racing?
Apparently, you draw a line down your header/exhaust pipe with chalk. Take your boat for a hot lap or two.
When you come back, your chalk line will be burnt to a certain point on your header. This is where you cut it off.
Or maybe the line is done with soap. Memory fading. I'll check with my boat racing mates and get back to you.
Mick
When you come back

Hi,

In his book, "Performance Tuning in Theory and Practice" A. Graham Bell, gives the the following formula for determining the tune length for a exhaust header pipe;

Length = (850 x ED / rpm) - 3

ED = 180 degrees plus the number of degrees the exhaust valve opens before BDC
rpm = revs at which max toruqe occours

In a table of calculations in the book, for an exhaust valve opening 75 Deg before BDC (the std SR cam is 76 Deg BBDC) the following lengths are listed;

Max torque rpm length

6000 33.1"
6500 30.4"
7000 28.0"
7500 25.9"

In the book he does not state it but these lengths are normally taken as being from the centre of the exhaust valve, not from the actual exhaust port / flange, so you have to deduct the length of the actual exhaust port from the pipe length.

The diameter of the pipe is calculated using a formula that I can't seen to type into the computer, but for a 33.1" long pipe, you use a pipe with a ID of 1.68" or the closest available pipe. Generally, altering the ID of the pipe will shift the max torque down about 500 rpm for a 0.125" reduction in diamater and up the same amount for a similar ID change.

The author also states that on at "standard" road tuned engine, an error of 10% in pipe length either way will only result in a bout a 3% power reduction from that with the optimum tune length. He also goes into the advantages and power gains that are possible with a tuned megaphone, which probably isn't applicable in you case as you want to remain road legal, and a pipe that I have made based on his work is even too loud for racing now days.

CJ

The pipe calculations are based on designing a pipe to give max torque at the rpm that you choose and use in the calculations. So you choose the rpm you want max torque, for a street engine, or max power for a race engine and calculate the length of the pipe for that rpm. The rpm that that max torque and/or HP ocours is based on the cam design and other things like the carbie etc as everything is so inter-related. The author makes no mention of the effect of bends on the power, but you wouls have to think that an "up" pipe with tight bends would mack a bit of a dofference compared to a "down" pipe with fewer and less tight bends.

When I have made pipes, and I think a lot of tuners do the same, I have made the header pipe a bit longer the tuned length and then cut it back as a bit of trial and error. Sometimes you actually have to make the pipe a bit longer than the tuned length so you can get the end to a position that you can actually fit a megaphone / muffler.

So in reality, it all gets a bit too technical unless you are chasing the absolute last little bit of HP out of an engine.

The small Supertrapp mufflers are actually very close in dimensions to the megaphones that suit the 500 Yamaha engine. From memory the last megaphone that I made for mine was about 180mm long, with a 7 degree taper, which as I said is close to the size of a Supertrapp. Since it was so loud, I ended up fitting a small Supertrapp, with about 18 plates in it to keep the noise nazi's of my back, although now even the Supertrapp would be too loud for racing with the new noise rules.

CJ

PS.
What do you bend your pipes with? The guys up here tell me that an ordinary hydraulic pipe bender like you could buy from say Catford Engineering in Jamestown, will just crimp thin wall exhaust pipe.
I wondered about putting a spring over the pipe, like plumbers do with copper pipe.

Hi guys, the place where i get my tubing from supplies in mandrel bends with 5 degree increments from zero degrees to a full donut of variying radius. Build it on the bike piece by piece and you will get it exactly how you want it. On the reverse cone on megas, keep the angle as close to 22 degrees as you can get, as apparantly this is the optimum for back pressure issues. There is an old guy down the road from me who is a tank maker, and he has all the gear for rolling cones in sheet metal, takes all the guesswork out of making a megaphone. Also i believe that exhaust tube suppliers (in aus anyway) always work in outside diameter. Cheers Franky b.