Secondary Pipe length matters but there are trade offs
kevinsrv7.com said:
James,
Looks good, and sounds like your happy with the 4/1. Could you trim a few inches off the aft end of that pipe or is that a no no? Thanks
Good question, go to the cafe foundation ORG web site and look up the reports they did. Excellent stuff.
http://cafefoundation.org/pdf/EPG PART IV.pdf
http://cafefoundation.org/pdf/epg.pdf
http://cafefoundation.org/v2/pdf2/EPG2.pdf
(It is pretty technical but if you commit the effort you will learn something; I know I did. One thing you learn is there are variables and goals. Are you trying to make max power or best efficiency at cruise and at what power and altitude.)
Short about 19"-29" at 2.25" for the secondary pipe. The primaries, pipes coming off the heads, are about 32"-36" by 1.75" dia. There are computer programs that will calculate this stuff. With bore, stroke, rpm, valve size, cam lift, duration and overlap and power you want peak efficiency you can calculate the theoretical best pipes. The above is about right. We have one wrinkle, we fly at altitude. The lower power as we climb with lower pressure (at the exhaust exit) changes the calculations.
There is no perfect 4-int0-1 pipe. We have to contend with practical aspects (like 29" tail pipes) and fitting it under a cowl. With that said most have a pretty good ball park set up. My secondary is 19" long but I may cut it down and do some testing of my own. I think the longer pipe has some positive effect on not beating the belly up and cockpit sound. On the other hand the few HP gain is probably offset with a little more drag. The no free lunch rule applies.
Many systems cut the secondary (the big pipe past the collector) down to say 10", for visual appeal. It will still work but you lose some "scavenging" and change the best efficiency or effective power band. In exchange you get less pipe hanging out in the breeze.
As Allan said you can go with megaphones and reverse cones (necked down exit. Drag racers do this and it not only has affect on sound but performance. Cafe Foundation tested megaphones and found some or little gain, but it was at the cost of more noise. Sounds like that is what you want, no pun.
They also tested a Coanda a cone inside the megaphone. When combined with a nozzle you thrust from acceleration of the (exhaust) gas. It has no negative effect but did lower noise.
Cafe Foundation also looked into the effect of thrust from the exhaust it self. If you had the ability to change the "nozzle" shape in flight you could get more thrust out of the exhaust. Not suggesting this is practical or worth while but just interesting. Kent Paser in his book, "speed with economy", touches on thrust from the exhaust with variable nozzles. The issue is at altitude the secondary pipe exit is too large (velocity is too low do help). Ever see a jet fighters engine nozzle change shape during large change in thrust or after burner, Coanda effect.
I had a megaphone on my Honda SL125 when I was a young kid. It was loud and it growled. I personally would not go with anything that makes significantly more nose because I fly in populated areas. To each his own, just be a nice neigbore. If you are loud and proud climb up fast and away from folks on the ground that want to shut down airports.
