ok FNG's... oops I mean new guys. Sorry. Here is the deal with "back pressure" A engine needs 0 back pressure to run and run well. What pipes and mufflers are for is to reduce noise and rapid cooling of the valves. Have you ever cooled off a frying pan in the sink when it was hot and it warped? Well I have and that is what valves do if cool air can get to the valves after the engine is shut off. Thats why you see drag racers put balls or cups over the pipes after the engine is shut down. YOU NEED EXHAUST PIPE ON THAT ENGINE!! a couple feet may not be long enough to prevent rapid cooling. YOUR ASKING FOR IT!
Love Westy.
Ahhh, yes... the mythical 'short exhaust pipes burn valves' story...
If there were any truth to this story, then every piston-powered warbird from WW1 & 2 would have toasted/bent valves, as would straight-piped Harleys & V8 car motors used in dragboats that utilize unmuffled header pipes, etc.
In the instances where I have seen bent/burnt valves on engines with straight pipes, the damage occurred due to leaner fuel/air mixtures caused by the removal of the exhaust restriction; e.g., no more muffler - thus, the engine was out of balance on its air/fuel mixture & it was never tuned back to the proper A/F ratio... it wasn't caused by the mythical 'shock cooling' myth.
Consider this: how does cool air suddenly rush back up into the short exhaust pipe & hit the valves? You've got a lot of heat-mass sitting there in the cylinder head at shutdown that will take a lot of time to cool down. Most exhaust valve heat transfer occurs through the valve face to seat; less of the heat transfer occurs through the stem to guide.
And straight pipes sound like crap - run some mufflers.
'Tanner'