38mm to 40mm exhaust port

[awaffa22003]

Can it be done? I have a R90/6 motor and the bike that it was in got in a wreck, slightly bending the headers. So I cut out the crossover and got the holes welded up but now i'm noticing that I have a exhaust leak at my right port. Further examination found that my right header is a bit more bent than I thought. Now I have a set of 40mm headers in great condition from a '77 RS that's just sitting in my basement...could I bore out the exhaust ports to 40mm on the R90 to fit them? I wouldn't have to worry about a muffler fit because I'm running shorty Emgo's anyway. Would I gain anything from this?

[vanzen]

Can it be done?

Sure – for the cost of set-up and machining at a reputable machine-shop
or the time – if you have the machine tools and the skills.

Would I gain anything from this?

Yes, an empty space in your wallet where the cash used to be.
Oh ! You meant performance ? Zip, No, Nada ...

OEM 40mm headers are quite rare !
Sell them on flea-bay or elswhere and then use the cash to get 38's
and a quality pair of mufflers.
Sorry, but the above option represents the path of least resistance,
the maximum return on time invested,
and the minimum necessary amount of cash expended.

Are the header pipes causing the leak ? Or was the port damaged ?
Fixing either would still be more cost effective than the option you have proposed.

[awaffa22003]

Ok, in that case, explain to me the reason for a 40mm exhaust on the RS. Please.

[vanzen]

Ok, in that case, explain to me the reason for a 40mm exhaust on the RS. Please.

1st of all, in order to put this into a realistic perspective,
not all RS models were equipped with the 40mm ports & pipes ... only some Euro models.
2nd, the R90/6 engine does not have the same "big valve" heads and 40mm CD or 38PHM carbs as the RS,
i.e. the 90/6 has a "different" engine.

The functional purpose of the 40mm diameter head pipe (vs the 38mm) on some RSs
would be to affect power delivery such that a bit more HP and top end speed is gained
at the expense of torque delivery (street tractability) below 5k rpm.
Even on the big valve RS engine – the tangible results were marginal if appreciated.
A /6 could likely suffer at both ends of the performance spectrum ...
losing torque at the bottom –
by virtue of the increased pipe diameter & subsequent decrease in exhaust gas velocity,
incapable of delivering any benefits at the top –
by virtue of valves & carbs that are not designed to support the increased "flow" demand.

[awaffa22003]

Okay now we're getting somewhere. No, my bike doesn't have Dells...but what I do have is a pair of Mikuni 36's that I want to play with, as far as my heads...my friend owns a bike shop where I can get some work done. I was gonna have him port them this winter and bigger valves wouldn't be a problem. Not that I'm trying to jump into this but I have a street/touring bike now, and what has been my street bike before is now my project bike.

[awaffa22003]

everyone's probably seen it but for those that haven't, this is my 1983 R65 frame with a '75 R90/6 motor in it. I still run the 32/9 final drive.

[Chuey]

Another avenue that may work would be to cut a good stub out of the 38mm pipes and cut a similar length out of the 40mm pipes, say two inches. Then weld the 38mm pieces into the 40mm pieces. That would make use of your 40mm pipes without having to modify the head.

I have no idea of the hot rodding science involved but I think I've seen some high performance engines with pipes that expand soon after leaving the port. To be fair, the one I'm thinking of was a modern inline/transverse four cylinder and that is quite a bit different from an airhead.

For me, rather than butt weld the two pieces of pipe, I think I'd leave a little extra (1/4" or so) and braze the pipes together. I think that would make a longer lasting joint. If I did that, I would internally taper the smaller section so it was a smoother transition, though it gets bigger instead of smaller in the direction of the flow, so it may make little difference.

Chuey

[vanzen]

Another avenue that may work would be to cut a good stub out of the 38mm pipes and cut a similar length out of the 40mm pipes, say two inches. Then weld the 38mm pieces into the 40mm pieces. That would make use of your 40mm pipes without having to modify the head.

A cheaper and better approach than machining the heads, IMNSHO.

I have no idea of the hot rodding science involved but I think I've seen some high performance engines with pipes that expand soon after leaving the port. To be fair, the one I'm thinking of was a modern inline/transverse four cylinder and that is quite a bit different from an airhead.

Udo Gietl used this technique in the late 70s - early 80s on the airhead
to enhance torque delivery in lieu of crossover pipes –
Extensive research and experimentation produced a specific design that worked.
A common technique today.

[mattcfish]

Another avenue that may work would be to cut a good stub out of the 38mm pipes and cut a similar length out of the 40mm pipes, say two inches. Then weld the 38mm pieces into the 40mm pieces. That would make use of your 40mm pipes without having to modify the head.

I have no idea of the hot rodding science involved but I think I've seen some high performance engines with pipes that expand soon after leaving the port. To be fair, the one I'm thinking of was a modern inline/transverse four cylinder and that is quite a bit different from an airhead.

For me, rather than butt weld the two pieces of pipe, I think I'd leave a little extra (1/4" or so) and braze the pipes together. I think that would make a longer lasting joint. If I did that, I would internally taper the smaller section so it was a smoother transition, though it gets bigger instead of smaller in the direction of the flow, so it may make little difference.Chuey

How about 45mm pipes? Is this just a sales gimmick?
http://www.rbracing-rsr.com/blackhole/b ... austs.html

[Major Softie]

Those are the guys who devote a whole page ranting at (presumably) Matt Capri. I don't know if their exhaust is any good, but it certainly looks nice. The whole position they take that this is a "race exhaust," while being perfect for touring, definitely has the "sounds too good to be true" kind of ring to it, but, who knows.

They certainly think very highly of themselves...