Exhaust crossover pipe

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Alan L

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Hi all,
I believe the purpose of the cross-over pipe on the exhausts of later Commandos is to reduce noise. Does it serve any other purpose? Does it affect performanc?
I replaced the standard pipes on my Bonneville T140V with non cross-over pipes some time ago. It was certainly louder but also seemed to pull better although it can be difficult to tell.
I wonder if the large red hot cross-over pipe directly in front of the cylinder head affects cooling.
al
 
They were a biatch to install and they leaked at the crossover clamps (black soot) and scratched the chrome on assembly. However, my 850 MKII lost a few MPG of fuel economy and a bit of midrange torque when I switched.
 
PROPERLY DESIGNED crossover pipes can reduce noise AND increase power. X type crossovers do that better than H type so the X is the common configuration nowadays. Whether the Norton crossover is an "optimum" design for the engine is a different question... ;) I suspect it is too close to the exhaust valves and, not being an X, it may not do anything with power though it does reduce noise.

FWIW, the Dunstall tuning guide from the early 70's states that a crossover will increase power on a Norton 750 per the dyno. The guide recommended one be installed for performance work. The style of crossover in the tuning guide is the same design that Norton later incorporated into the 850.
 
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I recently went to single pipes and found improvement in sound and pull. And SOOO much easier to work around.
Would not go back to crossover.
Jaydee
 
I think the reason why a well designed crossover both reduces noise and increases performance is that both silencers are then handling flow from each cylinder.

A poorly designed crossover may cause standing wave issues.
 
By GPS speedo, my 850 saw an extra 3-4 kmh at top of dyno hill with the crossover plugged. It is a 4000-4500 rpm pull.

Glen
 
One other "feature" is the tendency to crack the header pipe at the crossover branch. Ive swapped to straight through pipes and love the look, simplicity and sound.
 
worntorn said:
By GPS speedo, my 850 saw an extra 3-4 kmh at top of dyno hill with the crossover plugged. It is a 4000-4500 rpm pull.
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I would expect the jetting would have to be different between straight/crossover systems for optimum performance. Did you happen to do a before/after plug chop test?

In '12 I removed the straight pipes on my 73 850 and installed a NOS set of crossover pipes. I could easily hear the difference in a reduced exhaust dB level. As far as power - no idea, didn't dyno or otherwise test. But they haven't cracked in 8 years of being there and the cobblestone roads in our area are really rough on such things.
 
At the risk of repeating myself (myself!), I wonder if the cracking is due to some perhaps neglecting to slacken then tighten the crossover fittings EVERY time the exhaust roses are checked?
 
I've spent quite awhile experimenting with a full range of jets in order to get the optimum for both setups.
The balance pipes just couldn't catch the separated pipes, no matter which jets I used. 260s are best on this bike for the balance pipes with open peashooters.

Glen
 
xbacksideslider said:
It must be less restrictive?
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Yes but ‘less restrictive’ is not an infinite benefit. An engine with any given cam needs a given back pressure. Possibly the Commando cam just wants that bit more back pressure and / or pressure wave that’s provided by straight pipes?
 
Yes, Eddie. My point is just that the crossover necessarily reduces back pressure by roughly doubling the volume of the muffler for any one exhaust pulse
 
But again, per the Dunstall guide back in the day, the X-over pipe made more power than straight pipes and was a recommend mod for increasing power. Maybe that included other mods as well, I can't recall. I have an original of that tuning guide but it's in Mexico and I'm not. ;)

Since properly designed X-over pipes make more power on other 4 cycle engines, it's reasonable to assume it will on these engines.
 
MexicoMike said:
But again, per the Dunstall guide back in the day, the X-over pipe made more power than straight pipes and was a recommend mod for increasing power. Maybe that included other mods as well, I can't recall. I have an original of that tuning guide but it's in Mexico and I'm not. ;)

Since properly designed X-over pipes make more power on other 4 cycle engines, it's reasonable to assume it will on these engines.
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Well, that may or may not be the case. Allegedly the system was designed by Dr Gordon Blair, so it should work well cos he certainly knew his stuff. But some folk report nil gains having tried ‘em. Either way, it’s a long way from the stock cross tube which is the topic here.
 
I read those claims and believed them. Then I read the actual quote from Doug Hele where he explains that the crossover helps to restore some of the power lost to overly restrictive, quiet silencers ( Black caps).
That's why Norton fitted them, it was a response to decibel level requirements, not a hop up item.
Unless you call getting a bit of lost horsepower back a hop up.
Once the restrictive silencers are removed and replaced with free flowing items, the cross pipe isn't needed and actually sucks away some midrange, from my testing, at least.
The crossover does quieten the bike at low revs, even with open pipes.
I didn't get anything great from the Dunstall full system either, despite Dunstall advertising claims of 11 second quarter mile et'd just by fitting his system to a stock bike.
With many runs trying various MJ sizes, the best result from the Dunstall system was the same as the stock MK3 balanced pipe and open peashooters. The very best result so far is with the stock pipes, open peashooters and crossover blocked.
I intend to try RGM 1&1/2" next. If Norton folklore is correct, this should slow the bike down.
The GPS will tell us the truth!

Glen
 
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Fast Eddie said:
Well, that may or may not be the case. Allegedly the system was designed by Dr Gordon Blair, so it should work well cos he certainly knew his stuff. But some folk report nil gains having tried ‘em. Either way, it’s a long way from the stock cross tube which is the topic here.
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If you look at the pic of the X-oover in the tuning guide, it is identical to position/style of the Norton OEM crossover that came out a year or two later. It seems clear that Norton either copied it for production use or came to the same design on their own.
 
MexicoMike said:
If you look at the pic of the X-oover in the tuning guide, it is identical to position/style of the Norton OEM crossover that came out a year or two later. It seems clear that Norton either copied it for production use or came to the same design on their own.
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Sorry, I misunderstood, I thought you were talking about the Dunstal 2:1:2 that joins under the engine, that was the Blair pipe.

Crossover pipes near the head, like on Triumphs too, I believe are more about noise reduction than performance gain.
 
Hi,
For what it’s worth my T140V did seem to pick up hp and just feel sharper with straight pipes.
It certainly seems to make sense that if the bike is fitted with restrictive mufflers for the purpose of noise reduction that a crossover would help share the gas flow between the two of them and reduce performance loss.
An an amateur it hard to see that the gaping hole in that the cross-over creates in the side of the pipe would t create some rather odd turbulence of the gas flow (for good or bad). As I also mentioned, it’s hard to imagine it doesn’t affect cooling.

how do you blank off the cross-over pipe?
regards
Al
 
I made up some temporary aluminium plugs, just for the test.
So this was not exactly like switching to a pair of smooth separate header pipes.
That could be a bit better or worse. It's possible that turbulence at the cross pipe junctions helped somehow.

Glen
 
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