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2 stroke header and expansion chamber wrap

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Posted by: KXaggerator---------------------

I have a bunch of extra 2 stroke pipes lying around and I was thinking of experimenting with wrapping the header and expansion chamber areas of my one of my old exhaust pipes with E-Glass tape or something similar to reduce resonate noise. Is there any disadvantage to doing this other then extra weight or possible fit issues? Does anybody know of a suitable resin or epoxy? Is there not a slight increase in engine efficiency with increased header heat retention? Yes, I know it will be ugly.


Posted by: Sean Hilbert---------------------

If you're going to wrap to reduce noise, I say go for it -- the less noise we all make the better. However, if you expect a performance gain, I wouldn't count on it. You are correct in that insulating the pipe will cause it to retain more heat, and that will in turn change the pipe's tuning characteristics (the speed of sound is dependent on gas temperature). So, unless the pipe was developed with a heat retaining wrap in place (highly doubtful), then you will actually see a performance drop by insulating the header section.


Posted by: Mark Parsons---------------------

I'm not sure about wrapping 2-strokes, but I wrapped my 4-stroke head pipe with thermo-tec exhaust wrap and got a perceived increase in power. Stroker Speed Performance recommended it and I thought it seemed to help some.


Posted by: Rich Rohrich---------------------

The rules Sean pointed out apply to both two and four stroke engines. If you can raise the internal gas temps with a thermal barrier on a four-stroke you are basically just tuning the pipe to a slightly higher rpm.


Posted by: Boit---------------------

Would there be a perceptible change in power characterisitcs by wrapping the header pipe on a bike such as a YZ426? Would it be noticeable by the rider?


Posted by: Rich Rohrich---------------------

In past experiments on various four-stroke engines I haven't seen anything substantial. So while I haven't specifically tested this on the 426, I see no reason why it would respond any different. Jet Hot makes a very nice thermal barrier coating. It's applied well and has shown itself to be quite tough. There are a lot of reasons to like it, but power for your dollar isn't one of them in this case


Posted by: Boit---------------------

That's pretty much what I suspected. It would seem to me that it would take a substantially larger displacement engine for this to make a viable difference. If I was a fast pro looking for every ounce of power advantage and someone else was paying for the R&D, this might be a "must have".


Posted by: Rich Rohrich---------------------

Quote:
Originally posted by Boit
If I was a fast pro looking for every ounce of power advantage and someone else was paying for the R&D, this might be a "must have".


It wouldn't matter then either. If that were the case you would just build the pipe to peak at the correct rpm and not concern yourself with vague tuning effects that may or may not be present. In the end thermal barriers on pipes (coating or wraps) may have some potential use for keeping heat out of places it doesn't belong (your leg, the shock body, etc.), but trying to keep heat in for a performance gain is a dead end.


Posted by: David Trustrum---------------------

On RR GP bikes it was fashionable to wrap the pipes & it was beneficial as the speeds were enough to cool the pipe changing the characteristics in higher gears ie: peak power occurs lower. Apparently snowmobiles were quite affected for obvious reasons. Then came along titanium which was lighter & retained heat much better anyway so didn’t require wrap, apparently. So much for big buck race bikes though.

For road race bikes I have used fibreglass insulating wrap covered by aluminium tape, all available where your plumber shops. On a very peaky small 2 stroke it seems I can run a touch higher gearing. The effect didn’t seem marked on the dyno but the cooling effect wouldn’t have been the same, esp over a bit of time.

There has been some counter belief that the portion of gas spilled out into the chamber & stuffed back in would be heated if the pipe is wrapped, though I would have thought that the chamber was hot enough for the difference to be negligible.

Also the downside is it looks crappy soon & promotes rust underneath the wrapping.

On a dirtbike there is no way I’d bother as the speeds are no where near the same.


Posted by: bwalker---------------------

Pipe wrap can be very usefull in a snowmobile due to the cold airs cooling effect and the fact that many stock pipes have stinger diameters that are too large. The oversized stingers are there to ensure a motor doesnt frag when some crazy lake racer from Mn holds it wide open for ten miles straight on a subzero night. Needless to say this large stinger does nothing for acceleration. When I hill climb my sled I warm the motor up to operating temps and then when I am at the line I put the sled on a track stand, peg the throttle wide open and apply the brake in a effort to heat the pipe up. I have found that the performance gains from a hot pipe are very noticable. In fact most of the mfg's use some sort of heat shield on there pipes now.


Posted by: MACE---------------------

I'm interested in this from a sound perspective. I would think that there would be a performace advantage only because the pipe temperature would be consistent. For instance, you wouldn't get a sudden chilling effect from a creek crossing.

Ultimately, I'd envission that the metal of the expansion chamber would be replaced by composite totally. The inlet pipe would probably be retained.

In aerospace, we use some high temp resins that would be appropriate, but I understand that they are very expensive and harder to work with. I think it would be worthwhile to start the wrapping well back from the inlet where it is cooler - probably at the tail end of the divergent cone and use a 250 degree cure resin. Check Aircraft Spruce for supplies.

I hope you actually give this a shot. If you have access to a sound meter, It would be interesting to find out how much difference it makes.


Posted by: Jaybird---------------------

I just looked at two Scott Summers racers that were not used and for sale at a local shop. He had the xr650R(water cooled) wrapped, and the xr650L(air cooled) not wrapped.



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