I've been going over this in my head and think I made a decision but I figured I'd throw it out here so everyone could tear me to shreds.

Consider the stock exhaust: a healthy looking 2" OD pipe. Pop it off and it's somewhere around 1.125-1.187 ID.

From what I've gathered the Raask pipe is a 1.5" ID. Lancer mentioned a post where someone is running a 1.625" ID pipe.

I have some prebent stainless tubing. Both with 1.375" and 1.5" ID. I plan on upgrading to Lancer's VM36 kit by the end of the month.

When I do port work and gasket matching for my cars the goal is to eliminate any bottlenecks and/or restrictions. I figured a 36mm inlet (1.416" ID) would benefit from the 1.5" ID exhaust. There's no pulsewidth to match or blend, not sure backpressure becomes an issue or I'd consider the 1.375" ID but I don't know what effect backpressure would have on a single cylinder engine.

Next would be the muffler. I think I'd have a better shot at finding a 1.5" ID muffler rather than a 1.375" ID muffler.

Questions, comments, suggestions? Just thinking outloud.


--Steve

Check out step headers. Start with the smaller tubing at the head & 8-12" down go to the larger tubing. This will help you keep some low end torque & the step helps with exhaust reversion pulses. If I was going to build my own head pipe I would build a step type.

I run a Patriot step header on my Ford 2.3 L mini-stock motor. It gives the car more torque off the corners than my Esslinger big tube header.
This motor runs in a range of about 7000 rpm in the corners to 8500 at the end of the straight on a 1/4 mile asphalt oval.

My setup at the time I replaced the stock header (1.3" ID) with my homemade 1.75"OD/1.65"ID header was with a 36mm VM w/ UFO, outer exhaust port opened to about 38mm, Stage 1 camshaft and good flowing muffler (test unit for new HD design).
Compression ratio was still stock.
Cylinder did have silicone carbide treatment.
Header was basically stock length.

Power and torque were substantially increased from idle to wide open.
I found no negative symptom in any throttle range at all.
Once jetting was sorted out it was a very strong runner.

Scott_H wrote on 05/10/12 at 08:39:40:



Making a new one using the same 1.75" OD/1.65" ID pipe

OK!
http://www.physicsforums.com/showthread.php?t=594484
This cat tried to design his own exhaust for a 153cc 7200RPM single cylinder bike.

He references this formula for the length of the primary pipe:

LengthOfPipe = (129540 x E.T) / (R.P.M. x 6)
Where ET is exhaust valve timing BBDC and RPM is peak RPM-500.

I have no idea what constant that 129540 number represents.
ET is exhaust timing before BDC plus 180* to come back around the guiii-tar.
64* referenced in the first post plus 180* brings us to 224 which matches Lancer's exhaust valve duration spec.

As for peak RPM I've read a couple posts that say the bike peaks at 5400 but once a few upgrades have been equipped people say they have no problems at 6500.

For my calculations I used 6500 RPM, minus 500, bringing that value to 6000.

I got 877mm/34" as the length of the primary pipe. Note that this includes the length of the exhaust runner.

Quote:


Add displacement of 652cc to that guys formula to solve for diameter and I get 44mm/1.76".

Now here are the problems I have with this:
This formula is to solve for a free flowing exhaust. This doesn't factor in restrictions like the filter, intake, head design, runners, etc.

He does mention this is for a free flowing exhaust. Meant to be a straight, unwaivering pipe. He states that bends reduce flow 10-15%. I believe that bends restrict flow but I find 10-15% hard to believe. I know that in my experience with racing that a 90 degree bend roughly equates to an 8% loss in horsepower.

His formula states that's for the OD of the pipe. As we're concerned with volume and flow I think he meant to say ID.
I could have 1.75" OD pipe that comes in .050, .080, .120 wall thickness. For instance, you could have a 2" OD pipe with a 1.187"ID (stock exhaust).

All that said and done I'll be using 1.625 OD .080 wall stainless tubing which should be ~1.485 ID.
I have 1.625 OD .050 wall which should be ~1.525 ID but I want the extra wall there because that's my style. Ladies don't mind a little extra thickness.
I think I'm going to end up in the 34-39" length area.


--Steve

If you want some really good solid info/advice about what size & length header will best suite your engine then go to this site and read/study what 50 years of experience of making headers produces.

http://headersbyed.com/index.htm