Cross overs

[Guest Brian Robson]

Can someone explain as to why an exhaust system on a LeMans actually requires a crossover at all?

Does it smooth out the pulses for EPA tests?

Does it improve power response at a certain rev range?

Could the Guzzi run as well and sound even better without a crossover?

I'm sorry if this has been answered before in a long past thread, or if it causes those of you with vast technical knowledge to collectively raise your eyebrows, but it came up at our regular Friday round table and resulted in quizzical pensiveness.

[dlaing]

I have posted this question regarding guzzi's in general and got no clear answer.

I theorize that:

The crossover is supposed to reduce back pressure without increasing noise.

On a Boxer twin the pulses are evenly balanced between cylinders and great benefits can be had with a crossover or two into one exhaust.

On a V twin the pulse are not evenly balanced or timed so a crossover or two into one exhaust will result in the cylinders not being as well balanced under all revs and loads.

So a V twin may in fact do better with no crossover, if you are not concerned about noise.

But the uneveness of the cylinder balance may be negligible. I really don't know.

 

Also the crossover serves as a mounting point for the mufflers, so if you do away with the crossover, you should make a bracket to support the exhaust where the crossover was.

 

I also posted that I suspected that the reason that my vaccuum gauge indicated that the cylinders balanced unevenly throughout the revs, was because of the crossover and possibly the airbox.

 

The same argument may apply for individual air filters rather than one air filter for two throttle bodies..

 

Look at the Quota, One throttle body and a two into one exhaust possibly causing a lack of top end power but producing more low end power, unless it is all due to a cam or something else.

 

If you remove the crossover, the crossover location is an excellent spot for a battery or two....

[al_roethlisberger]

Exhaust "cross-overs", collectors, h-pipes, and bridges... all various names for similar exhaust parts, are part of a well-designed exhaust system.

 

Cross-overs are designed to tune an exhaust such that the exhaust pulses can scavenge exhaust gasses from the other header runners. Different header lengths, cross-over/collector configurations, and tube diameters can be chosen to move the torque-curve and peak HP curves where the designer targets.

 

Here are some useful specific links:

 

Exhaust Fundamentals Treatise on the NSX Forum

 

Exhaust Theory Overview

 

As with any performance part and modification, different MG V11 cross-overs deliver different torque/HP peaks in the RPM range because there is always a trade-off with the different designs.

 

There are several other threads in the Forum that discuss the characteristics between the popular cross-overs such as the Mistral, FBF, and Stucchi.

 

Hope that helps...

 

al

[docc]

quizzical pensiveness

I love that! A Guzzi trait if ever I've heard one.

 

From reading Kevin Cameron,Sportbike Performance Handbook, I gather that a well engineered back pressure ( the crossover will provide that ) sends shock waves back up the header and when reflected in the combustion chamber during valve overlap, contributes to evacuating the chamberr generating additional vacuum for increased cylinder filling. ( whew, now I'm dizzy!)

 

Of course this only effective at those particular rpm ranges which are affected dramatically by exhaust changes. Moving the back pressure impulse a few just a centimeter or so can shift the 'power band' . Hence the big differences from different crossovers.

 

I would expect ( wildly guess) that a straight through 2 into 2 exhaust would favor the top end howl but rob the mid range.

 

Hopefully I've made my contribution to our collective quizzical pensiveness.

[Guest Jeff Kelland]

Theoretically speaking, a crossover allows each cylinder the benefit of having 2 mufflers, i.e. reduced backpressure and noise. As docc points out there is a suction created behind the slug of spent gas as it travels down the pipe. if this negative pressure coincides with the valve overlap event, then that suction can be used to pull fresh intake air in. The trick is to get it to happen when we want it to. I'll bet it's not happening between 4 and 5 k rpm on my Le Mans! There is also another wave created when that slug of gas exits the pipe, a positive pressure wave traveling back up the pipe. I bet that wave hits my cylinders right about 4 to 5 k rpm! This wave inhibits intake flow into the cylinder if it gets there at valve overlap. (add this into the equation of intake tuning previously discussed) Yamaha and Honda have used variable exhaust tuning to play with these waves. So does the new Buell. Any "fixed" exhaust system can only be good at some rpms. A straight 2 into 2 would I believe enhance upper rpm ranges at the expense of the midrange. The stock system on our bikes is probably the best all around compromise between lower, midrange, and upper power ranges also considering noise and economics (imagine the costs involved in this kind of research and development).Now if anybody out there has a few million or so to develop a variable exhaust for our geese I'll get in line to buy one. Check out this months issues of Cycleworld and Roadracing for pictures of the new Buell's exhaust system.Pretty neat stuff I think

[Guest Brian Robson]

After reading the links kindly provided by Al, the first and seemingly obvious conclusion is that the best pipe for power is a straight through pipe, without any obstruction or backflow.

After looking through several magazines and books, there are many motorcycles in the past that ran two seperate pipes, and while these bikes were of course unrestrained environmentally, they were forced by the demands of the day to run with lots of torque and mid range.

I do suspect that much of this crossover/manifold technology has enviro rationales, and that with an efficient silencer we may get the same if not more power and the same emissions.

Watch and listen to the start of "Lawrence of Arabia" and hear two pipes on a V-twin without a crossover.

Some more quizzical pensiveness with a smattering of verbal inebriosity

[gthyni]

This is my interpretation,

 

with an unrestricted flow we should not need a cross-over/balance pipe.

but straight pipes isn't a practical solution unless you are Valentino Rossi

 

The more the mufflers restricts the flow the counterpressures is reduced by using

"the other" muffler to flow part to the exhaust pressure.

I have a hard time understanding how pressure waves could help much

given the uneven cycle of an V2 motor. I rather think the differences between

different crossover is due their capacity to reduce counterpressure.

 

I also think that the result you get from a specific crossover will vary with

the flow characteristics of the mufflers, cam profiles and so on.

Would be interesting to see how this is solved an Ducati 998SP or similar

race bike with V2 and 2into2 system.