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Old 12-18-2011, 05:52 PM   #1
Voltaire OP
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R90 Airhead 2 into 1....

Hi everyone, I'm building an R90 track bike and I would like to fit a 2 into 1. The guy at the custom exhaust shop seemed to think it didn't matter much if it was equal length of not....
I'm from the world of air cooled VW motors and equal length are the standard....
Spec of the motor is 336 cam, 36 mm Dellortos, 81 clutch, 1 mm off the heads.
The bike is not for road use...its made up of parts I bought locally and off Ebay, the core motor is a 1973 R90.
Cheers in advance, John
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Old 12-18-2011, 09:31 PM   #2
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Uneven header lengths will cause the two cylinders to make power in different ranges-in other words, it will probably broaden the power band, but peak torque and horsepower should be lower. Honda did this deliberately on the second year CBX if I recall correcly, and BUB Exhaust did something similar on their K100 exhaust, although I think BUB did this to make their job easier.

The more "built" your bike is, the more important exhaust and intake tuning becomes. A Triumph tuner from the '50s used to bring several sets of headers to each track, along with a hacksaw; he cut the pipes until the bike made the best power for each track.

Gregory Halenda's uber-cool bike had the most beautiful 2:1 exhaust I've ever seen and he used to frequent this forum. Maybe we can get him to weigh in.
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Old 12-18-2011, 10:51 PM   #3
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For a definitive answer, you might want to have a chat with Chris Hodgson at San Jose BMW.

Of all of the guys who raced or race Airheads, his is the first name that comes up for me when the question is Airhead performance and IINM, all of his race bikes were fitted with twin pipes and silencers. He also manufactured the old Dual Supertrapp systems and those allow you to tune the two sides of the engine individually and relatively easily.

Lufty made the 2 into1 systems and those were known to reduce power out put, while sounding louder. If you intend to go in that direction,get someone who knows what they're doing to design your exhaust system for you.
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Old 12-19-2011, 06:49 AM   #4
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These guys seem to have some interesting theories and products; Black Hole Exhaust

The rest of the site makes for interesting reading.

I'd like to hear from anyone that has dealt with them.
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Old 12-19-2011, 06:54 AM   #5
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I'm not an expert and I don't know the answer but the Mac Performance guys apparently think that equal length matters. This is the 2 into 1 that came with my R75/5 (the last section is modified now with a supertrapp disc end):

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Old 12-19-2011, 11:01 AM   #6
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thanks for the feedback guys, I took a copy of the Black Hole one to the exhaust man yesterday.....got a grunt of approval.....
I might just put the std headers back on and try out my Nortons peashooters in the short term just to get running.
Triumph Thruxton ones fit perfect and look good but they weigh 4 kgs each and sound awful.....as in quiet.

Exhaust man kept going on about 'boxers'....as though it was somehow different to conventional engines....surely its much the same a Brit twin ( apart from the vibration and leaks) with both pistons rising and falling together with alternative firing.....
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Old 12-19-2011, 11:35 AM   #7
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Quote:
Originally Posted by Voltaire View Post
I might just put the std headers back on and try out my Nortons peashooters in the short term just to get running.
Triumph Thruxton ones fit perfect and look good but they weigh 4 kgs each and sound awful.....as in quiet.
These guys make some pretty light ones. I'm guessing they're pretty loud too:



http://coneeng.com/motorcycle_components.html
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Old 12-19-2011, 04:11 PM   #8
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Originally Posted by mymindsok View Post
For a definitive answer, you might want to have a chat with Chris Hodgson at San Jose BMW.

Of all of the guys who raced or race Airheads, his is the first name that comes up for me when the question is Airhead performance and IINM, all of his race bikes were fitted with twin pipes and silencers. He also manufactured the old Dual Supertrapp systems and those allow you to tune the two sides of the engine individually and relatively easily.

Lufty made the 2 into1 systems and those were known to reduce power out put, while sounding louder. If you intend to go in that direction,get someone who knows what they're doing to design your exhaust system for you.
The problem is that Chris has so many people calling him up and asking stuff like that. I think he gets a bit tired of it sometimes and I don't blame him. I suggest reading articles and looking at photos of his setups.

Speaking of photos, take a look. Lots of his race bikes had two into ones on them. I have welded up quite a few of them myself and then helped him check them out on the dyno. According to Chris, we had Wrecking ball running better than ever before up until then and it had had two into ones on it of varying designs for quite some time at that point. I never knew Chris early on but later on he came to favor two into ones. We have discussed the matter at length many times while working on stuff. Two into ones sacrifice a bit on top for a much more tractable power everywhere else. It was always easier to get the things pulling real hard and strong right off the low end with two into ones he said. Sacrificing top end I joked back. My dad always said that the thing to remember about top end power is that first you have to get there! Everyone of them that I helped him with had two into ones including his oilhead at the time.

Dr. Curve, on the other hand, reported favoring two into two's but he was really even more concerned with top end than Chris was if that is possible. At any rate, each setup has its own pros and cons which all depend on everything else. My advice, again, is to look at photos of airhead race bikes here in the states and overseas. Tons of them are winning their classes with two into ones! Same with two into twos although I think the two into ones have done quite a bit better in the last 30 years.
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Old 12-19-2011, 05:54 PM   #9
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Quote:
Originally Posted by oldroadie View Post
I'm not an expert and I don't know the answer but the Mac Performance guys apparently think that equal length matters. This is the 2 into 1 that came with my R75/5 (the last section is modified now with a supertrapp disc end):

I've never seen a Mac like that before. Are you sure it is one?
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Old 12-20-2011, 06:47 AM   #10
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Originally Posted by mattcfish View Post
I've never seen a Mac like that before. Are you sure it is one?
I know very little, the whole BMW experience is new to me. I was informed it was a Mac but what do I know? Now it's a 2 into 1 with a Supertrapp disc tuning set up:




Might even get it running again this afternoon once I torque the right head a set the valves.
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Old 12-20-2011, 08:10 AM   #11
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I have a two into one with equal length headers, a different design than the one above: I am told that is/was an ABC? The muffler has a removable baffle inside the megaphone... (Well, it did until I had it "customized" by cutting it off and welding a peashooter to the unit!..... just for the looks!!)

Original:



Now:


I have a hard time keeping the pipes from rusting, in spite of painting with high-temp etc... am thinking of sending it out for ceramic coating... anybody done that?
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Old 12-20-2011, 09:01 AM   #12
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I haven't done the ceramic coating but I saw a /5 that had the stock system done that way. It was fantastic but it may have been a special system? This looked almost like chrome. I'll try to find the info maybe somebody knows what I'm talking about.

Jet coating or something like that?

Charlie
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Old 12-20-2011, 09:29 AM   #13
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Yes, Jet-Hot is the company that I am aware of... have not yet contacted them, it may be atrociously expensive, but it sure looks like it works!

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Old 12-20-2011, 09:47 AM   #14
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Yes Jet Hot. Depends on what you think is expensive, doesn't it? I don't remember now but I thought it was reasonable. I thought it was Chrome. Until next to some real Chrome. Then it's just not quit as shinny.

No idea how well it ages. I only saw this once that I know of, when it was brand new.

If you have a system that is basically sound then coating is cheaper than new. It's a good option.

Charlie
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Old 12-20-2011, 12:46 PM   #15
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I've seen a lot of Airheads but I've never seen a Supertrapp that looked like that!

Who built it? The clearance looks very tight. Does it clear the axle bolt and brake lever?
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