Pipe Dreams or Dream Pipes?

Bewarethemoon

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I know this has been discussed from time to time before but I thought I'd go ahead and create a specific thread anyway.

So I'm approaching the stage in my build where I'm looking into exhaust design. I'm not looking for styling cues as I'm set on TT style pipes and I'm sure it will come as no surprise that I'm planning on having them constructed from my favourite material Titanium.

What I'd like to talk about is the performance aspect of their design so EPO's/Exhaust stubs, stepped tubing and final length.

I know the topic of stepped headers has come up before and I'm very interested in this. If I'm going all out, and I think it's apparent that I am, then why not?

I'm looking forward to hearing anybody's thoughts.

Daniel.
 

Bewarethemoon

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Gary Hoos, who is on tops on performance sells TT pipes that are 1-3/4" by 39" in length. This also depends on engine mods.
Hi @MrBultaco

I’ve seen the pipes that @hooser sells and I love them, unfortunately I don’t think they’ll quite work with the sump filter conversion kit I’m using from Smedspeed.

I’ve seen talk before about stepped pipes and wondered if it was exploring that route.

Daniel.
 

arcticXS

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I

My engine is pretty far from stock. I’ve read between 1-5/8” and 1- 7/8”.

Daniel.
In Craig Weeks' tuning manual, some dyno graphs show very little hp gain from 1 3/4" pipes vs 1 1/2". Same for 36 or 38 mm VM carbs vs 34 mm. So unless you are building a full blown race bike, I'd go for 1 1/2" thin wall tubing (1 mm or 1.25 mm wall thickness) Either all the way, or stepped up to 40 or 41 mm tubing somewhere near the midpoint of the header
 

Bewarethemoon

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In Craig Weeks' tuning manual, some dyno graphs show very little hp gain from 1 3/4" pipes vs 1 1/2". Same for 36 or 38 mm VM carbs vs 34 mm. So unless you are building a full blown race bike, I'd go for 1 1/2" thin wall tubing (1 mm or 1.25 mm wall thickness) Either all the way, or stepped up to 40 or 41 mm tubing somewhere near the midpoint of the header

Thanks @arcticXS! That’s exactly the kind of information I was hoping for.

I’ve read that with Titanium you can use relatively thin tubing and get a great, metallic note to the exhaust, that coupled with the rephased firing order should should incredible!
 

Signal

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The XS 650 Club of Australia website has a drawing for a stepped exhaust pipe designed I believe by MMM of 650 Central. It is in the race bike section.
I have copied this design with an added muffler after the megaphone. It works well.
Here is a link.https://old.xs650.org.au/Technical Info/exhaust_port.htm
 
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Bewarethemoon

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This
The XS 650 Club of Australia website has a drawing for a stepped exhaust pipe designed I believe by MMM of 650 Central. It is in the race bike section.
I have copied this design with an added muffler after the megaphone. It works well.
Here is a link.https://old.xs650.org.au/Technical Info/exhaust_port.htm

This is exactly what I was after @Signal

That would be great in a TT style!

You have one of Craig’s heads too don’t you? How is working out?
 

Signal

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I have 2, one was being used on a 650 Central long rod 750cc conversion with 36mm VM carbs and a Megacycle 250-30 cam. Exhaust as above with muffler after diffuser. Best pull on the dyno was 77 hp at the rear wheel.
That number is highly suspect as no before/ after or calibration data but is around what can be expected with those mods.

However the motor is a gem, had a few problems with the shift shaft but once sorted it went really well. Very flat torque curve we always got great starts we even got on the podium at the Cemetery Circuit one year.
The motor was taken out to sell the chassis with a stock 650.

The other head is for a new 880cc motor, that will go in a new 13 inch wheel chassis eventually. The long rod motor will be a spare to take away to 2 day meetings. That is the plan if it happens or not remains to be seen. Racing here has been ruined by covid restrictions. So a great deal of motivation on my part has been lost.
The last time the 700 2 stroke was raced was Feburary 2021.
 

halfmile

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I built these TT pipes with 42mm OD tubing with a 1.57 ID. Both pipes exit to the right side.
TT Pipes 006.JPGTT Pipes 003.JPGTT Pipes 009.JPGcalendar stuff 015-001.JPG
 

Bewarethemoon

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Signal

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Did you make the anti reversion inserts too?
Yes I bought anti reversion inserts from 650 Central to run in my road bike. Before I installed them I made copies and ran them in the long rod motor. However in hindsight I suspect that they are not needed and may even be a minus with the Lillie replica head. The only way to really check will be on a dyno with and without inserts and we did not do this. That being said the motor runs really well with them.

The other day I had some time to try my flow bench and was flowing a stock head to get some baseline figures. Just playing about really as it is the first time I had used it. I had made further copies of the 650 Central AR inserts and fitted one to the exhaust. The flow numbers dropped I suspected this would happed as the insert reduced the diameter of the outlet. However the insert as I understand its purpose is about reducing reversion and improving quality of flow.

The previous owner of my road bike had fitted VM36 carbs. Fitting the inserts made it run noticeably better quite a remarkable change.
 

toglhot

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Horsepower gains from things like exhaust, carbs, porting, stacks and so on are so small so as not to be noticed, however, with pipes you obviously get a sense of more power and speed due to the noise output. Put all these things together and you may notice a difference, albeit small.
 

Bewarethemoon

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Horsepower gains from things like exhaust, carbs, porting, stacks and so on are so small so as not to be noticed, however, with pipes you obviously get a sense of more power and speed due to the noise output. Put all these things together and you may notice a difference, albeit small.

I’m not sure that statement’s particularly accurate @toglhot, in essence I see your point but I think you’ve made a rather broad sweeping statement.

If this was the case then surely decades of effort by tuners and novices would of been a complete waste of time no?

It’s up to you whether you believe the figures quoted on here but if you can take an engine with 45bhp to 65bhp, I wouldn’t call that a small difference, 45% increase in fact.
 
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toglhot

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That would depend on how that 20 horsepower increase was obtained. Manufacturers generally exaggerate their products positives, it's in their interest to exaggerate. That said, there is absolutely no way an exhaust system will increase hp from 45 to 65. You'd be lucky to get that from a cam and bigger bore.
 

arcticXS

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That would depend on how that 20 horsepower increase was obtained. Manufacturers generally exaggerate their products positives, it's in their interest to exaggerate. That said, there is absolutely no way an exhaust system will increase hp from 45 to 65. You'd be lucky to get that from a cam and bigger bore.
I guess that for most, getting 65 real rwhp from an XS 650 involves at least 750 cc, higher CR, hotter cam, AND aftermarket carbs, filters and exhausts.
 

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Horsepower/torque gains with an exhaust are 'almost' ALWAYS associated with the need to add fuel in the mixture to take advantage of the better breathing. Ergo the gains from the exhaust cannot be credited to the exhaust alone. Arguably its the added fuel thats providing the power gains, with the exhaust just being a necessary part of that addition. Dyno Testing by most exhaust suppliers will cite a 10% to 15% gain, which with proper tuning is probably pretty close to accurate. But I'd agree, no way will an exhaust take a 45 hp motor to 65hp! Further, the aim for a particular exhaust note, or 'sound', is often counter-productive to performance levels. Straight pipes are a good example of this, as those pipes are really only appropriate on motors where intended performance gains are targeted at sustained wide-open-throttle, as in the drag strip! But, guys love the way they sound, so you see 'em everywhere on the street! For my money, and for best 'street' performance, a properly built 2-into-1 header in a 2-cylinder motor is the best way to make both usable power and torque. Everything else is all about just what you like, not 'what's best'! Needless to say, thats a very good reason to do anything!!!
 

arcticXS

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Looking at some more recent engine designs, the Yamaha 700 twins, and the BMW F800 models, it strikes me that those bikes have fairly small header dimensions, maybe 35 mm OD. And still make 65-85 horsepower. Since those are modern 4-valve designs, they may not be directly comparable, but anyway....If anything, they have less cam durations, so the scavenge period is shorter.
 
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