Quote:
Originally Posted by Bob KellyIII
I agree that the triumph is probably running around 110 psi to 120 psi
and your gauge says 60 psi..... ( about half uh? ) but I seriously doubt the hoses are flexing that much to give you the low reading ( they would be blowing up like a balloon !) it's gott'a be the gauge itself ! just test it with the air compressor try to get a good seal and
then fill the compression gauge with it and you'll see just how far off the gauge is when you compare it to the air compressor...
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Gauge will read spot-on when tested with air-compressor. But will be off when used to test compression... huh.... what?@!#€!?!
Ok, let's step back and see how compression in engine works, in simple terms; not gonna worry about gauge-pressure vs absolute-pressure or anything. Let's say each piston displaces 125cc of volume as it moves and down. Then at very top of stroke, it
squeezes all of that 125cc into tiny 10cc pocket at top in head. That's combusion-chamber volume, eh?
So there's some sort of factor there, some sort of ratio? Let's just divide original swept-volume by final volume: 125cc/10cc = 12.5:1 ... wow! That's compression ratio!!! Manual says we should see something like 150psi pressure inside that combustion chamber when piston sucks in full breath and compresses it (WOT). Ok, great... ideal scenario....
However, what happens when you stick hose where spark-plug used to be? It
adds volume to combustion-chamber. Let's say volume between tip of hose to bleed-valve at gauge is about... 10cc. Now your combustion-chamber is 10cc original +10cc of hose = 20cc !! Now those pistons are shoving 125cc of swept-volume down to 20cc instead of 10cc. So compression-ratio is now 125cc/20cc = 6.25:1 instead of original 12.5:1 !!!
Wow! So pressure in head+hose must be lower now than before, right? How much lower? Well, let's take ratios of ratios: 6.25:1/12.5:1 = 1/2. So 150psi*1/2 = 75 psi!!! So gauge is working! It's just not able to account for extra volume of hose!