I've been reading a bit on tuning an exhaust. It's crazy all of the different exhaust and sizes and tube lengths. They all seem to work but supposedly some work better than others. I've had three different cams in the past year on two different engines. That is key in determining tubing size/length. Does anyone know what the optimal dimensions are for a stock 912 ULS?
While cleaning up the garage I found a paper with a very thorough report on the cam in my Zipper Big Bore. I'm excited to tak to Hal about it. (He didn't just slap those kits together--there's so much science to it all!)
I'm guessing the dimensions are a pinch different for it. You have to know things like BBDC and ATDC and the target RPM. Since airplanes don't climb out at redline, should the climbout RPM be used to calculate?
Also Courierguy comes to mind with his Swiss muffler. One can lengthen the primary pipes and see better midrange power (and reduction at the top end). Maybe that's why he likes to throttle back and his fuel burn is awesome? I guess that Swiss muffler isn't a primary pipe but maybe it helps? Speak on it TS. Does it change/hurt anything noticeable other than sound?
We used redline as the calculation for the 140 HP turbo experiment. It wasn't stock but the magic length of primaries was 32" or 16".
Anybody know anything? Hook me up!

Courierguy will respond as soon as he returns from the mountain top.

I was told Rotax mufflers/headers are the best option and you can't improve on it ... but after doing the 114hp high comp zipper kit on my 912uls I decided to modify the exhaust a little and see what happens anyway. My rans has the long muffler that sits behind the engine, I don't think this design is as good as the later rotax muffler that sits underneath... but anyway, I cut it open, pulled out the tiny pipe with a million holes in it, replaced it with my own 2" stainless pipe that I drilled a boat load of holes in, and then opened up each end and ran a 2" dump pipe out of each side for not just more flow, but more even flow hopefully. Picked up 100+ ft a min climb if not better. It's a little louder, though not loud, and sounds much "beefier" than a stock 912. This design still sucks IMO... it creates a lot of turbulence... more than is necessary IMO. I think there are plenty of gains that could be had, but the hassle involved to build the headers, collectors, and find light mufflers and stuff it all behind my engine is not something I feel like tackling right now - I made some changes, saw some gains and I'm good with that right now.

I know there is more gains to be had by running equal length headers etc. I don't know what the optimal size of the headers would be, I would guess 1/8 to 1/4" than the stock size for the big bore kit. But that's a guess from past experiences with other engines etc.

I've read some well known engine builder articles about other engines where the guy said having different length headers wasn't all bad because then different cylinders are more efficient at different rpm ranges and you cover a wide area that way! fatter, wider powerband (not airplane related stuff here) That's kind of crazy and not really the way I like to do things but...ok..

I don't know anything that you already know, just thought I'd throw in my 2 cents about my own experiences thus far... at some point I might experiment further. I'm about to install a Rotax Airbox and see what kind of gains that provides on a zipper 912. Then maybe back to exhaust.... we'll see... interested to see where this goes.. what cam are you running now? what is your setup on your 912??

The one I'm selling is Rotax stock. The Black and Tan one in my videos (as of last May) is the new zipper big bore cam. I think Hal and I might be the only guys with it but I'm not up on all that. He said the original zipper was around 120 HP and this might be 124 HP. I believe that is ballpark based on a 914 prop I can easily redline.
I too dislike my stock muffler and it's off center tail pipe and stainless Swiss-cheese guts.
Your theory could be right about a wide range of rpm help with variable lengths. I think the reason for the off center tailpipe is for a heater muff. That's what snowpants are for.

Recently I cut my modified muffler in half, and capped each end. Since both sides have dumps I put in, I decided to seperate them and cut down on turbulence. I think I have seen climb numbers and rpms to support saying this helped.. but at most, it wasn't much. I did get louder again, it sounds really good.. but a bit loud.

But why I really wanted to post again, and get your thoughts wyoming, is to talk about pairing the proper cylinders headers together to increase scavenging. I've done turbo headers before into twin scroll turbos and it's best to pair the correct cylinders, and obviously it's good to pair them in a n/a application as well. After looking at the firing order of the rotax, unless I'm crazy, it would be beneficial to pair the front cylinders together, and the rear cylinders together, instead of the way rotax has paired the banks together....

what are you thoughts on this.. it would be harder to do obviously, but in theory there would be slightly more power / economy to be had doing this!

The problem is, I have a lot of failed experience and am not formally trained to know anything. So I not only shouldn't but can't give suggestions. I'm a trial and error kind of experimenter, so I would say get flex tube and try it both ways. All on the ground of course.
Then once you find the best pull at a certain RPM build it out of the good stuff.
I really wonder if the longer Swiss muffler helped at mid range RPM. I wasn't saying that to be snarky and stuff.
We both need an engineer it seems!
I was entering my cam's data into this but am not even sure I was doing that correctly.
http://www.mezporting.com/exhaust_length.html

It seems like putting money into the right prop is the best way to milk HP. There are too many halfassed options and too many people giving suggestions with small amounts of experience to know what is right though.