Exhaust system design

[68C]

Does anyone on this forum have a good understanding of vee twin exhaust design and so help me get my mind round how our systems work?

 

Parallel twin four strokes with 360' cranks output one exhaust pulse per crank revolution. When running these exhaust pulses are evenly spaced prompting the use of two-into-one exhaust sytems whereby the first pulse is followed by a drop in pressure which helps the second pulse leave its cylinder and so on.

 

Vee twin four strokes of course don't have evenly spaced pulses so to make the pulses extract the next exhaust charge the exhaust pipes would have to be of different length before joining at the 2-1 so that the pulses appear evenly spaced.

 

Most of the exhaust systems on our V11's have equal length pipes before the 2-1,or crossover/balance pipe in our case.

 

I imagine this tuned exhaust system will only be most efficient at one rpm and possibly throttle position but it must have advantages, look at the snakes nest of exhaust pipes on a racing car. The Buells appear to have some attempt at altering the pipe lengths from the original Sportser.

 

So why don't we have 'tuned' manifolds or exhaust systems on our Guzzis?

[gstallons]

IDK..............

[helicopterjim R.I.P.]

 

So why don't we have 'tuned' manifolds or exhaust systems on our Guzzis?

 

I don't know either but the factory Ducati race exhausts are equal length to where they join whereas the MGS-01 has unequal length before they join.

 

When I raced a Ducati 900SS many years ago I built exhausts that were equal length, some with balance tubes, some without, some 2 into 1 exhausts with equal length and some unequal before they joined and I was slow with all of them.

 

As I said - I don't know!

[GuzziMoto]

With a 90 degree v twin there will pretty much always be a difference between the exhausts effects on scavaging from one cylinder to the other. Even if you use unequal length primary tube the distance between one cylinder exhaust valve and the other is the same, regardless of which direction. It takes the scavaging pulse the about same amount of time to get from cylinder one to cylinder two as it does from cylinder two to cylinder on no matter what the pipe lengths are. The primary scavaging effect is from the merge between the two pipes. There is a secondary effect from the exhaust exiting the end of the pipe, but from my experience working with Herb Gebler (RIP) on our Ducati race bike exhausts that was not as important. You can use changes in diameter and other tricks to help even it out but I don't think it will ever be even on a V twin. Some Ducati's used what we called a Spaghetti system with twin cross-overs on a two into two system. I don't have much experience with those.

What I do know is nobody on another 750 Ducati ever passed me in a straight line with more power. Our bikes ran two into one systems built by Gebler. I know there is more to power then just the exhaust, and we had all that as well. I'm just saying I heard a lot of people with two into two systems talking about how their bikes had more power then ours did with its inferior two into one system (two into one systems were not common back then) but no one ever backed up the talk by motoring by.

By the way, I wanted a twin exhaust system with separate pipes but testing showed us the two into one had nearly as much top end but a wider spread of power. If you looked at the area under the curve you had more total power with the two into one.

[68C]

Hi GuzziMoto,

 

I think I understand your main point that we can arrange things to scavenge well on one cylinder but not on both, unless we swop the pipes around every revolution!

 

You mention the point where the two pipes meet is more important than pipe length, do you mean that acts as a venturi, the passing gasses suck on the other pipe rather than the void left be the passing slug of gas causing a depression?

 

I tried to draw a sine-wave graph of the position of the piston and valve opening over 720' of crank revolution but bogged down completely as I could'nt work out where to start the other cylinder, 90' before, 90' after, 630' ... ???

 

I suppose I like the idea of a lighter system, however my mate reckons the twin exhaust looks cool, my wife thinks weight could be lost from another part of the bike/rider set up.

[GuzziMoto]

As it was explained to me, by the afforementioned legendary exhaust guy, the primary pulses are the strongest part of the equation. In a two into one set up as the exhaust pulse from one cylinder gets down to the collector and starts up the mid pipe there is a negative pressure wave that goes up the other primary pipe. There is also a negative pressure wave created when the exhaust pulse exits the end of the exhaust system but that wave may or may not make it up to the exhaust valve before the next positive pressure wave comes down. Most street systems are too long for that and your more likely to see that wave making to the collector and meet up with a positive wave there that helps get that pulse out and generate a stronger negative wave to go up the other primary tube to help the pulse begin. Our race systems have virtually no mid pipe. They are just a two into one system with either a mid pipe of less then a foot or no mid pipe at all (just a collector with a muffler like device attached).

The key is to get negative pressure waves to be at the collector and at the exhaust valve at the right moments to help the positive pressure pulses get out quicker. The negative pressure pulses that go back up the other primary pipe and make it to the exhaust valve as it is opening are the most important pulses. The secondary pulse that make it to the collector are not as strong, they are split between both primary pipes if they make it to the collector before the next pulse.

But on a motor as crude as a Guzzi I doubt the details of the exhaust layout will make much difference. We were running serious cams, head porting, and carbs, on our Ducati's. The Guzzi lump is much less highly strung. On a motor like that it will be much less sensitive to exhaust tuning.

 

On a side note, my wife prefers a twin pipe set up on her V11 while my Griso has one of Todd's right side exit kits currently. On my Daytona it has the StainTune system but I want to build something like what my Griso has because I prefer the sound of a two into one system. To my ears a two into one system has a meaner, raspier, snarl. The two into one system are deeper and more booming with the individual pulses more recognizable.

[rocker59]

A few of observations from over the years:

 

Dr. John ran a 2:1 on his Guzzi race bikes.

 

The Sport 1100 I saw running in the AHRMA races at Park City years ago was running a 2:1.

 

Most of the 883 cup Sportsters in the 1990s ran 2:1 systems. Supertrapp, in particular.

 

My 1994 model XLH883 ran great with the SuperTrapp 2:1.

 

I'd put a Dr. John 2:1 on one of my Guzzis, if I could get ahold of one. Saw a vendor in Japan a few years ago with them, but no habla Englais and no ship to USA.

[Guzzirider]

I know someone who has made his own Dr John replica exhaust for his LM1000- I can put you in touch if you like.

[68C]

Altough I mentioned this as an aside in a centre stand thread I thought I would repeat it here.

 

Has anyone tried fitting/modifying a Grisso or Breva 2:1 exhaust system to fit a V11. I imagine the header side of things would not be too difficult assuming clearance around the oil cooler - might have to be moved but hopefuly not into the ugly side mount. I thought to rotate the conection pipe to the silencer to exit on the right, more usual for left hand drive roads and to ease fitting of a centre stand so the lever 'wot yew stand on' fits close. I suppose going out to the left clears the drive box and enables a closer silencer fit. Could feel a bit odd having the centre stand on the 'wrong' side but could live with it. Obviously all this this requires some welding and won't be a simple swap.

 

Many bikes have the end can bolted up to a three bolt flange, might be possible to fit that and have a wealth of silencers to choose from.