[ace.cafe]

Ace:

Sounds too exotic for me.  I would be afraid of the piston hitting the exhaust valve at the top of the exhaust stroke, even with cutouts in the top of the piston.  I also don't understand how, if both valves are open on the intake, part of the incoming fuel air mix isn't simply pulled out the exhaust and down the tube by the partial vacuum created by the outgoing exhaust, since there is nothing to stop that.  But like I say, too exotic for me.

I'm sure the instructors at our local technical college will have some enlightening comments for me about this thread.

Thanx & good luck.

Paul

The movements of the valves are controlled by the cams and valve train to be only open a certain amount at TDC, and the piston is made to have proper clearance for them.

The issue involves the needs for the valves to be open a suitable amount when the piston is going down. Since the valve cannot be opened instantaneously, it is opened in a controlled manner which begins prior to TDC.  This helps it to get open as far as needed in time for the maximum inlet charge to come in. And it also does as BW explained, which is to use the inertia of the exiting column of exhaust gas, with its vaccum-like effects, to help scavenge the remaining exhaust gasses out of the combustion chamber, and help to "jump start" the intake flow into the cylinder by pulling some in via this method.

Regarding the loss of some intake mixture out the exhaust valve during this process, yes that does occur. It is controlled by the design of the cams which takes into account these "crossflow losses", while also gaining as much benefit from the other aspects of overlap design as possible. It is a trade-off of losing a little mixture out the exhaust, to gain a better overall result, while trying to lose as little mixture out the exhaust as you can and still get the benefits. These effects actually work differently at different exhaust speeds, so you get different levels of efficiency of this design at different rpms. The art of this kind of design involves getting what you want at the rpms you need it to work best at, while not losing any more than you have to at the other rpms.

This is a very deep subject, and as BW says, there are many entire books written about it. It's one of the most effective tuning tools for performance.

[ERC]

Maybe he'll learn something from the technical instructors?    ERC

[D the D]

Not if he thinks rejetting the carb will somehow cause his valves to strike his piston.

[barenekd]

Without wading though all the responses, No, the bike will be fine without the muffler, albeit, loud. The valves won't hit the psiton unless you rev it up to about 7 grand. Even then I'm not sure they get there with as little lift as they have. As for richening the engine, turning a screw isn't going to do it for your. If it's off that much, it would need to be rejetted. I used to run my Matchless with the stock muffler, then shift to an open megaphone, then a baffled megaphone and none seemed to do much to the mixture. Sure never burnt anything up! I don't think you will have much of an issue here, unless the cops object. I rode another Matchless about 80 miles on the freeway with no exhaust pipe whatsoever, when my exhaust fell off one day. Caused no damage to the bike. Some people's hearing may have been affected
Bare

[Samir72]

Thank you all for the lively debate!

Samir

[REVirginia]

+1. If nothing else, the debate scares us newbs into rejetting, learning the bike and not violating any local noise ordinances. These are a great help.