Following on from a recent thread here about a chop with drag pipes, I'm interested in the science behind it.
I've seen lots of customs running on drag pipes, the scavenging effect of the collector and tailpipe length on a four is well known, also a 4-1 sound is music to my ears -pure heavy metal.
Whether this means certain death to the exhaust valves if running on drags is uncertain. Interesting thread here:
www.jalopyjournal.com/forum/threads/shor...iction.265982/page-2
A couple of things I'd add:
1)The ww2 Merlin aero engines would overheat quickly, within minutes, when not airbourne, this is what I've read elsewhere,
2) From the last post in the thread:
"After running hard and getting things extra hot (especially if you are lean because you didn't re-tune it), when you come down to an idle, the reversion pulse (yes, even in a 3/4" long port there is a reversion pulse) carries ambient temp air to the extra hot valve, and causes a slight warpage. Now you have a slight exhaust valve leak that will eventually (it may take a few thousand miles or so) erode the valve (a 'burnt' valve).
The cold ambient air "carried" to the hot exhaust valve sounds wrong, it is a negative pressure
wave, it is about wave motion. not movement down the exhaust port/pipe. Rather like if I chuck a kettle of boiling water into one end of a cold bath, the wave from the splash travels to the other end and back again, but the water remains pretty much where it is .
