Routy wrote on 04/17/11 at 06:52:29:I'm sure not a tech on it, but I doubt the lack of back pressure in itself will do any valve damage. Its would be the cold air dumped on the valves at shutdown that can cause damage. I can remember rodders capping stacks quickly after shutting down.
But, God knows I've run a lot of strate pipes in my day, and I never seen any valve damage.
Thats the difference between cars and bikes. Cold air isn't the issue unless you have a very short pipes (like open headers on a car), then you would have to worry about bending valves.
You may have run straight pipes for years, but you have evidential ran you carb rich too. I have run straight pipes for many years also, and have become very good at valve jobs because of it. if you read the above explanations, you'll see that there are many factors that come into play.
Torque cones that go in at the pipe at the head have not been proven to do anything. A 4" baffle is the best place to start. The end (outlet) of the baffle should be no closer than 2" from the outlet of the pipe. If you pipe is long enough, you may be able to get away with 'lollipops'.
And here, this is from a burnt exhaust valve (open pipes and improper tuning) from a lean condition. Valve face split bounced around long enough to do this....
And for torque cones....
Anti-reversion cones are a device you put into your head pipes, near the cylinder head, to discourage reversion (i.e. a positive pressure wave in the pipe that interferes with cylinder fill if it arrives during overlap). Some are bell shaped like pictured above and others are a pure cone shape.
At various times I've tested these things (both styles) on exhaust systems that both showed and didn't show evidence of reversion. The results have always basically been about the same ...
They always seem to either cause a small loss over a wide range, or on occassion I'll see a small loss at only higher rpm's.
My guess is they'd be more effective on a system where the pipe was oversized for the motor size and target rpm. I'll continue to test these as I get time and if I get a result on a particular setup where they show a real advantage. In the meantime, it's not high on my list of worthwhile performance mods.