Performance Tweaks

The cannons pretty much done now. Normally I’d leave it at that, but this time I’m going to stick it out a little longer and do some finish work.

One thing I’ve noticed is that the piston doesn’t retract all the way during firing. If there’s half an inch left behind the piston after firing that means that half an inch had to be vented unnecessarily, which slows down the piston movement.

The first thing I tried to fix this was to replace the QEV with something with a higher flow rate. I borrowed a sprinkler valve from one of my PVC cannons for this. I modify these valves a lot for speed, so I know for a fact that this valve can move a lot of air.

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The results: no change in the firing impulse or piston travel. I guess the QEV is working pretty well after all.

Here’s what’s inside a quick exhaust valve btw. It’s surprisingly simple.

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So.. all that’s left to do really is lengthen the piston. This will make the valve open less widely, but it will also make it open faster.

First, I fired the cannon with nothing in it, then disassembled it to check the piston position.

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The piston had retracted only .42″ out of 1.28! The piston is way too short. I decided to cut the travel down to .65″, which meant lengthening the piston shaft by .63″.

I had some extra half inch rod lying around in anticipation of this. I spent an hour and a half or so on the lathe and made a new piston shaft .63″ longer than the first. This is the 3rd one I’ve made…

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I’ll put a comparison video here soon. Basically, with shorter travel the discharge became much shorter and sharper. The deep, wall-shaking voooom sound was reduced as well unfortunately. The difference with a grapefruit loaded was HUGE. I don’t have a chrono for it yet, but subjectively, cannon went from “pretty powerful” to “scary powerful”. It pretty much vaporizes grapefruits now, leaving only a little mound of rind stuck to the wall at the impact point. The kick is similar to that of a shotgun with a 12ga slug in it. I’ll do more experiments with this once I have a chrono, but I’m very satisfied with the current performance.

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