Odd Exhaust

[AnnapolisAirhead]

I set out to sync the carbs on my '71 R75/5 and it dawned on me that it has no exhaust cross-overs. I thought all '70-95 boxers had at least one cross-over.

Here is a picture. The picture isn't really all that clear, but the headers are smooth. Not like they were cut off and welded.

Has anyone else seen anything like this or know if this was stock or production numbers?

A friend who has another '71 R75/5 and who rode mine for a year or so told me that my bike feels like it has better mid-range power--so I was wondering if this could be the result of no cross over?
Seems to me that the balancing of the carbs would be even more critical.

[DennisDarrow]

That is strange and yes syncing is even more critical.....Something happened and am sure you will hear all about it from the "know alls".........Surely IS NOT a carry over from the /2's as they have one even though it's in the rear........HMMMM, one wonders.....Dennis

[AnnapolisAirhead]

That's what I was thinking Dennis, the /2 carry-over....but I seem to recall cross-overs there too. One other thing, the Bing 32mm carbs didn't have a vacuum port, but I've seen that before. The vacuum port kit was installed last year to make balancing easier.

The bike is pretty much stock except for the repro seat, lower bars and headlight mirrors (so you can confirm that your knees still exist). Runs like a top, but the mufflers are getting a bit long in the tooth. I like the stainless exhaust systems, but thinking that I may now keep the existing headers.

Here's another pic.

[20774]

My R69S has no front cross-over, just the aft one. I don't recall if the R69S and the early R75/5s exhaust diameters were the same. If they were, I could see where stocks might have been used up with the first few '70 models, but hard to believe they would still be doing that for the '71 models.

[AnnapolisAirhead]

Kurt, this bike doesn't have any cross-overs. Maybe the picture doesn't show it very well, but its a clear shot straight to the silencers. The nubs you may see in the picture are just the motor mounts.

It looks to be stock BMW headers, just wondering if it is stock or not. European maybe?

There's is a URL to check VIN numbers, but I'll have to dig that up.

[dbrick]

I have a very vague memory - CRS Syndrome, anyone? - that I considered buying some aftermarket headers in the mid-70s with no crossover points. Maybe a prior owner installed some; they look pretty shiny for an almost-40-year-old machine.

[20774]

There's is a URL to check VIN numbers, but I'll have to dig that up.

http://www.bmw-z1.com/VIN/VINdecode-e.cgi

or you can just go to RealOEM and enter you VIN...it will then show you the month/year of build.

Also Phil Hawksley site lists the VINs by model:

http://www.bmbikes.co.uk/enginechassis.htm

[kmkahuna]

That's what I was thinking Dennis, the /2 carry-over....but I seem to recall cross-overs there too. One other thing, the Bing 32mm carbs didn't have a vacuum port, but I've seen that before. The vacuum port kit was installed last year to make balancing easier.

The bike is pretty much stock except for the repro seat, lower bars and headlight mirrors (so you can confirm that your knees still exist). Runs like a top, but the mufflers are getting a bit long in the tooth. I like the stainless exhaust systems, but thinking that I may now keep the existing headers.

Here's another pic.

The vacuum ports on the Bing flattops are here... Don't know if that's from a kit or not, though.

[44006]

I have bought stainless headers for /2 conversion from Epco with no crossovers
for use with R75/5 and R100/7 engines - I just ordered them that way for less
cost and ease of assembly, tuning idle and balance by ear - suits me fine as a
tourer not interested in ultimate available performance.

Maybe I am not the only one with that frame of mind.

I have a 1970 R75/5 stock parts bike that came with single crossover forward.

[LOSTBOY]

MCM-now MAC-exhaust used to make headers for BMWs without cross-over pipes. Yours were probably made by them.

[AnnapolisAirhead]

Thanks guys. I've seen complete stainless exhausts for /5 from Epco, he makes a fine set.

[VANZEN]

The cross-over tube is, in fact, designed to improve low to mid RPM torque delivery.
The second cross-over that was added in '81 (?)
was intended to compensate for EPA mandates
that otherwise might have compromised power delivery
by boosting torque delivery over an even larger (and frequently used) RPM range.
Placement and number of cross-overs will affect the "shape" of the torque delivery curve.

In the days of old, high RPM racing machines such as the the RS54 were without a cross-over,
but it will be interesting to note that Udo Gietl experimented
both with the number and placement of cross-overs,
and ultimately got rid of all of them –
Seems that he favored a tapered header
claiming that this technique improved torque delivery throughout a wider range of RPM
than was possible with cross-overs.

Consequently, when one sees his R90S Superbike,
one might assume that it is fitted with conventional single diameter headers as stock ...
but it is not !

Only recently has Gietls's performance technique been applied to production motorcycles.

[AnnapolisAirhead]

Thanks Vanzen. I'm not a speed freak, but I am a curious soul so I enjoyed reading that and all the other input.

So, now the conundrum. Do I put it back to stock and order a complete SS exhaust, or just a new set of silencers. Maybe I should pick up a pair of used stocker headers and test a bit.

Anyone have an idea what a set of these headers are worth (besides that common "whatever the market will bear" kinda response)? Could I get a cup o' coffee for them?