Building exhaust - ID and reversion/pressure wave questions

Tommyjoking · New Member Joined: Apr 19, 2009 Posts: 7

Oh boy

I got my 81 XS Special in pieces all over my garage. I will be turning up a custom exhaust in stainless. I love the look of "mile" pipes as that says flattrack to me but I have seen a couple of interesting examples with 1 pipe on each side being perfectly symmetrical. At any rate my ??? revolve around tuning issues.

I am planning on using 1.402 ID tube. I have already welded up some short megaphone cans which will use just enough packing to take the edge off (exit diameter is larger then OD of headers). The stock carbs will be ditched in favor of 34-36mm flatsides or roundslides with K&N velocity stack type filters. Cams will never be more then a Hot street grind, I do not intend to shorten valve guides or grind reliefs in pistons.

My questions revolve around the ID of the exhaust: Due to limitations on rpm (valvetrain, bottom end, rods etc.) I doubt this engine will move enough air to need more then my ID of 1.402 . However I do not want to have the exhaust "open" and the midrange fall flat on its face, after all thats the appeal of this old school twin isn't it? The real question is do I need to add restriction at the head or downstream to keep my torque #s as high as possible Wink

. If to add past the head (downstream) how far for the best possible pressure wave timing, generally speaking of course (cams/timing/ignition)? Experimenting at the head is not that problematic, but tuning reversion timing elsewhere could be a nightmare.......any experience is GREATLY appreciated.

Thanks for any responses
Tommy

Tommyjoking · New Member Joined: Apr 19, 2009 Posts: 7

Oh cmon

Surely some of you have raced these motors and have a guideline/tip or two....I am not trying to build a blueprinted race spec motor for sale!!! I am building a daily driver I want to get as much as I can out of and have it run the next day. These engines have been around for about 50 years......the homework has already happened Shocked

help a fella out please.

650skull · Support Staff Joined: Jul 19, 2007 Posts: 1186

Mikes sells a set of reversion cones to keep the back pressure up.............here is another way to do it if you are not worried about cutting up the old exhaust.......... gently cut off outer pipe where it is welded to the gasket inserts, cut off the old inserts and place into the new pipe .........how long ? Don't know, this is info i have found and other people have done it

Tommyjoking · New Member Joined: Apr 19, 2009 Posts: 7

THANKS

the stock exhaust is trashed...no problem cutting and using as you describe. It would not be too much trouble for me to turn out some different length/dia inserts and see what they do.

Since I really can't turn out 20 different bends and weld in inserts in multiple locations..$$$$..experimenting right at the exhaust port is my only realistic option. I will try wide open and start working my way back down from there I reakon.

Thx again
Tommy

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

deleted

nj1639

Hey 5twins,
Are they 2 into 1 Mac's? If so, any carb changes from stock?

650skull · Support Staff Joined: Jul 19, 2007 Posts: 1186

5twins..........Couldn't remember where i had got that info from..........will make note of the extra insert for when i need to use this ...........Thanks

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