Crossover type exh HP vs non-crossover

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mike996

mike996

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Just wondering if anyone has ever seen an actual dyno test comparing an850 motor HP with the oem crossover/peashooter exhaust vs an 850 with non-crossover (750 type) pipes. Dunstall stated in his tuning guide that at 5500 RPM the crossover type exhaust was worth 5.8 HP on the dyno over a megaphone equipped engine and recommended a crossover exhaust as the best thing for a Norton street bike. The factory then essentially did that with the 850 - maybe they figured it out on their own or they used his research. But I don't know how the HP figures he quotes compares to a a normal 750/peashooter exhaust. He goes on to say that at 7000 RPM, the meg-equipped engine has 2-3HP over the crossover. But obviously for any normal riding, the crossover is dramatically better. Has anyone seen/done a dyno test directly comparing a 750/peashooter exhaust with an 850 crossover/peashooter exhaust on an 850?
 
Someone on another thread here was just commenting thats its rare to see any meaningful dyno tests of any bikes
- so we will be lucky to see such specific tests on Nortons AND with the required exhausts.
Someone will have done them, its just they never get into print - or into the public domain ?

It has also been commented that the crossover exhaust was to split the exhaust gases to quieten down the exhaust, for noise testing.
So whether that will have any hp benefits at all will be interesting to see..
 
dyno test on same dyno and bike with just one thing changed is very meaning, for the bike, just not directly comparable w/o same bike tested in similar conditions. Why not try both a crossover and megaphone or 2-1 mega
 
Neat valve!

If it had proven to improve hp at 5000 RPM with valve open, and better HP at higher revs with valve closed, you could automate it and have yourself the Norton equivalent of the old YPVS(Yamaha Power Valve System). :)
 
That's pretty neat!

Do you still have it?

Since reading The Dunstall Norton Tuning manual some time back I have been curious about the crossover exhaust. He clearly states that the dyno showed the increase in power so I don't doubt it existed as described - the crossover vs a megaphone-equipped bike. But maybe a standard 750 type exhaust is also better than a "megaphone" exhaust at 5500 RPM and therefore crossover is not much better than the standard exhaust for street use...though Dunstall says the crossover is the best performer for a street bike. The quote from Dunstall is: "By far the best system for fast road work is separate exhausts joined by a balance pipe up close to the cylinders." He then goes on to talk about HP differences.

Of course, his test was undoubtedly on an engine dyno, not a chassis dyno so that 5.8 HP would have been considerably less at the rear wheel. But I would think that if you had that valve, ran the bike up to say 5k in 3rd gear, went to WOT and as the bike accelerated from 5-6k RPM you flipped the valve open/closed you might be able to detect some change in the rate of acceleration...or maybe not. ;)
 
Crossover dramatically better for everyday riding? Don't think so, binned mine for singles, felt better, refitted them in later years when I wore a hole through a single, didn't like them. Then they cracked, so i binned them for good, fitted new singles. Had the same set of TOGA peashooters the whole time. Certainly didn't lose any performance. It has been said the balanced pipes were fitted to any commandos (Mk11a, MK111) fitted with the annular discharge (beancan) mufflers, as part of noise abatement controls. Don't know if it is true, but certainly doesn't produce any more horsepower, less, if anything. IMO, Balance pipes actually make it harder to do the idle balance thing with the palms over the idling exhaust outlet--I could never set mine up really well, and the exhaust gases from one side were always cooler than the other. Dunno why, but that disappeared with twin singles. YMMV.
850Mk111, standard twin 32mm Mk1 Amals, pre-Mk111 aircleaner,twin pipes, peashooters, Tri-Spark, CNW replacement single coil, XS 650 breather.
 
ludwig said:
Ages ago I made a crossover tube with a valve , operated with a choke lever , so that I could open/close the tube while riding .
To find out if it was worth the trouble to keep the balanced pipes that kept cracking .
Std 850 with open peashooters .
Apart from a change in exhaust note , I felt no difference .
The balance pipe went ..



First saw this mod over 35 years ago on a 500 Gold Star, the owner used it around town and opened it up on the open road, strange then, that since the A.C.U. imposed noise limits on the racing bikes so that all 350/500 Manxs, 7Rs e.t.c. have had to use a quieter end can on their racers and gained an extra 2-3 bhp, as verified by Dyno testing :!:

First saw this mod over 35 years ago on a 500 Gold Star, the owner used it around town and opened it up on the open road, strange then, that since the A.C.U. imposed noise limits on the racing bikes so that all 350/500 Manxs, 7Rs e.t.c. have had to use a quieter end can on their racers and gained an extra 2-3 bhp, as verified by Dyno testing
Does this mean that all the information on open Megas is a load of hot air :?: :shock:
Click to expand...
 
Strictly from an engineering standpoint a exhaust crossover is about flow volume.

The crossover allows the individual cylinders to flow out of both pipes. In general (non race type) mufflers are restrictive. Allowing the cylinders to breathe better by flowing out both mufflers is a good thing from a performance standpoint. Better flow is always a good thing. In the old days (and still today :D ) Air flow in and out of the engine was what we strived for.

Crossover pipes also allow the engine to run quieter because the exhaust note is being split through both mufflers. This was one of the reasons they were fitted originally to meet noise requirements.

If the crossovers for the Norton were better quality (stainless steel maybe), I think there would be less question about value.
 
I know that the cracking of the oem's was a problem. The picture in the Dunstall book of his crossover system shows the crossover pipe itself to be flex-pipe, not solid pipe like the Norton oem crossover pipe. I't appears from the pic that by using the flex-pipe, Dunstall considered vibration as an issue though Norton did not appear to use that approach when they incorporated a crossover into the 850 system. A flex pipe would probably have eliminated the cracking and maybe the impression of the system would, as Dennis noted, be better.
 
There is a large difference in power if You are using restrictive mufflers like the black cap mufflers or the late peashooters with the small opening.

If you are using mufflers with low restriction there is very little difference. Jim
 
I've done exhaust cross overs on all my V8's to find it helped take out the hi tones for deeper softer note and helped the low down grunt w/o hurting the top end. I'd think a 2 into one would work similar. The ancient wisdom for best tap in point was to run crayon down header lenght then run to desired RPM of most interest and see where it melts first and cut there. Usually too much stuff in the way to do this on most street craft but close as can does help some.
Tri Y headers come to mind.
https://www.google.com/#bav=on.2,or.r_q ... +y+headers
 
The factory crossover is not positioned to help with resonance tuning of the exhaust. It only provides help with volume of flow when you have restrictive mufflers. Jim
 
Bernhard
'
Does this mean that all the information on open Megas is a load of hot air ?

If you fit megaphones to the pipes on a twin, they emphasize whatever power characteristics your cam, and pipe and inlet length dictate. In short if your bike has a nasty top end motor without much midrange torque, it can become an even nastier piece of shit to ride. If you have a harsh cam spot, a megaphone can cause the rear end to step out, especially if you let the motor drop off the cam and have to slip the clutch to get it going again. Before we had 'gumball' tyres, the older compounds used in triangulars made bikes fitted with megaphones more hazardous to ride. When I built my Seeley 850, I designed a two into one pipe from my previous experience. It is not set up to pass down one side of the pipe because getting the header pipes equal is difficult, and getting it tight up under the bike means that getting to the drain plugs is a problem. With the cam advanced it is too loud for street use, even with a decent open muffler, however it works very well - the bike accelerates extremely rapidly, however the power transitions are gentle. The tail pipe diameter has a large effect on performance. If you like I will measure my pipe and post the details - for what they are worth.

I don't believe the crossover system ever did much on Triumphs, I think Jim is correct - if your mufflers are stifling the motor, the crossover could help. Makes a quieter bike.

This photo shows an accident about to happen. The short stroke 500cc Triumph motor had radical cams which gave a power band from 6,000 RPM to 10,500 RPM. The bike is fitted with 4 inch megaphones, and severe 7R AJS brakes and a close ratio gearbox. . The photo was taken at Calder Raceway near Melbourne, - in front of the bike, the main straight runs for about one kilometer with a fairly open hairpin bend at the end of it . When you reach that bend you are travelling at high speed, and even if you get back to first gear for the corner, with the close box at 6000 RPM you are travelling too fast especially with those old hard tyres we used to use. So you slow down, and the motor drops off the cam. If you slip the clutch to get it back, the bike is instantly sideways, and it all happens at about 60 MPH. I was a fairly decent rider when I started racing, the megaphones turned me into an instant dud. I didn't get respectable lap times until I developed a two into one exhaust to suit that bike. I think I crashed at the first five meetings at which I raced.
Crossover type exh HP vs non-crossover
 
The crossover type exhaust in the Dunstall tuning manual works very well but you need to follow the plans exactly to get the benefit. You would also need to use a cam with similar timing specs as the exhaust is very sensitive to cam timing. Jim
 
Are the specs for the mufflers used with Dunstall's crossover system known ? The correct back pressure could be a major factor, especially if the exhaust is opening early.
 
acotrel said:
Are the specs for the mufflers used with Dunstall's crossover system known ? The correct back pressure could be a major factor, especially if the exhaust is opening early.
Click to expand...

I think he had his Decibels in mind. There was a picture with his system on one of his pamphlets that showed Decibels.
At least that is what I used when I built his system some years back. It was very strong in the 4500 to 5500 range. Jim
 
comnoz said:
acotrel said:
Are the specs for the mufflers used with Dunstall's crossover system known ? The correct back pressure could be a major factor, especially if the exhaust is opening early.
Click to expand...

I think he had his Decibels in mind. There was a picture with his system on one of his pamphlets that showed Decibels.
At least that is what I used when I built his system some years back. It was very strong in the 4500 to 5500 range. Jim
Click to expand...

Do you know what cam the Dunstall system was designed for?
On the other hand I guess a system could be designed from the Blair 4st book for any cam and be reasonably successful
 
Cheesy said:
comnoz said:
acotrel said:
Are the specs for the mufflers used with Dunstall's crossover system known ? The correct back pressure could be a major factor, especially if the exhaust is opening early.
Click to expand...

I think he had his Decibels in mind. There was a picture with his system on one of his pamphlets that showed Decibels.
At least that is what I used when I built his system some years back. It was very strong in the 4500 to 5500 range. Jim
Click to expand...

Do you know what cam the Dunstall system was designed for?
On the other hand I guess a system could be designed from the Blair 4st book for any cam and be reasonably successful
Click to expand...

I never did get that answer but since it was designated as a street system I figured it would be his stage one cam.
I used it with a Web-Cam 12A with about 5 degrees of advance. Jim
 
The factory crossover is not positioned to help with resonance tuning of the exhaust. It only provides help with volume of flow when you have restrictive mufflers. Jim
Click to expand...

Yep I realize Norton was restricted in where to cross and the main reason crossovers are said to help low end is allows two pipe & outlets to pass the gas so less restriction. Crossover are not based on harmonics like 2>1's and various muffler types lengths and tappers and outlet size. With more volume and outlets to pass the blasts crossovers then to soften and low the tone so double good idea if not very dramatic.

I'm a bit confused on exhausts of open headers vs muffler or mega with some restriction of out let. My 1st Combat blew it rusted mufflers off on 7th day on trip with the Harley rough necks and it woke up the power especially top end, so much so it started to wheelie on me and get loose on first two shits. Otherwise besides rather loud it didn't seem to bother starts or putt putt response bogging. Same thing with the P!! dragster that I kept at 600 rpm base line and would plow a furror with a wake deep in Fla sand ruts no throttle in 4th at a slow jogging speed.
The mystery part...
After I accidentally detuned Ms Peel form her prime, 1st by switching 34 Miki for dual 32 Amals, then swtiched small port std head for CHO-Combat It dropped to normal Cdo power but when the opened up Desible mount cracked off th 2>1 1,5" header it became obnoxious to me to have to slip clutch to idle arosss pasture or among rally crowd, had to ease on throttle to get to high way speed and it never cleared its throat into a powerband, just hard throttle to loudly gather speed. Had a couple roll start drag races at 2004 INOA rally and got smokes off line sounding like a short shifting HD, ugh. The biggest drop in power was loosing the long megaphone. Later tried a peashooter but it only helped the noise not much else. Some work with or w/o muffler others just can't seem to live w/o one or two.

Dunstall Notes details his various systems and tests that cover the range discussed above. I and others have posted in somewhere prior.
 
If you fit a race cam with a very restrictive muffler, the cam spot may be suppressed, however there usually at least a minor increase in power across most of the rev range. I would expect a better result with the crossover, however I doubt the resonance effects would be anywhere as good as with an open pipe without baffles.
 
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