Rebuilt M50 - what should compression be ?

I've always seen this as a quick check to guide you. 

Atmospheric pressure is 14.7psi; if you have a compression pressure of 80psi then divide that by atmospheric and it will give you the ratio. [5.44:1]   This is quite loose but gives you a ballpark figure which I would say is not far off for a new built engine of your type.  I think 7.3:1 is stated for M50's depending on year.

Swept volume + combustion chamber volume divided by combustion chamber volume gives it mathematically, but  measuring that in your stage involves burette and fluids....

Cheers

Jon   

The sudden squeeze is 'adiabatic compression'. The rapid temperature rise is too quick to cool off, so it gets to roughly 30% or so higher compression than the volume ratio. So 8:1 ratio gives more than 8x15 (or 120psi). 

80 sounds low but if should improve when run in.  Also remember it must be on fully wide throttle ...it needs all the air it can get. In your case 7x14.7 = 100 (near enough) so you might get 120 or even more.

Attachment is from Garratt Maintenance Book.

Hi - I had a similar issue on my newly re-bored with new piston and rings on my ES2, with 60 psi.

I took the barrel and piston back to the re-bore company and they checked the gap - spot on at 0.003 inch.

The guy said that it will improve as the rings bed in.

I've only done about 30 miles since its been back together - it runs and pulls fine.

I'll check again after a few hundred miles.

Reagrds - Paul.

As well as WOT as David  says, the speed that you can kick it over at is important too. Several fast kicks, as close together as possible is the way to go. Pushing it down the drive is even better. The car mmechanic 'rule' I was taught was to crank for 10 seconds or until reading stabilised, whatever comes first. Also, be aware that an engine that is at room/garage temp will always give a lower comp reading than one at running temp.

If you want to check that the ring sealing is/will be ok for compression, squirt some oil into the cylinder - enough to go all around the piston.  Turn it over a couple of times to spread it around then repeat the comp test. Your readings should improve a lot doing this (as well as stop your exhaust rusting inside!)

Regards, George. 

Hi Mike, et al.,

   In most internal combustion engines the compression ratio is a theoretical maximum pressure calculated on the engine dimensions. In reality this is controlled by the valve timing and engine speed. The inlet valve on your engine closes when the piston is a third of the way up the compression stroke and when turning the engine over at low speed it is only compressing two thirds of the cylinder volume. Bearing this in mind 80 psi sounds quite good, but if you want to see a quick improvement on your gauge, set your inlet pushrod adjuster as slack as possible so the valve doesn’t lift fully and this will bring the valve closing back near bottom dead centre to give you a full cylinder of compression. When adjusted correctly your engine will not get near full compression until it reaches 3,500+ rpm when the gas flow is working at it’s optimum efficiency. This is known as “getting on the cam” or the “power band”. BTW a short stroke Manx has a measured ratio of 11 to 1 but the inlet valve closes half way up the stroke, so 5.5 to 1 on a gauge !

...that's a great explanation of the process and the variables at play.  Thank you Richard.

Jon

Hi Mike, 

Dont  worry too much about the compression tester at this stage, it will get better as the rings/ bore bed in, as you suggest in your opening sentence. I can only dream about 80 psi, but it doesn't seem to. affect the performance of my M50, I had a thread about this a couple of years ago, and eventually just accepted my poor compression results, The kickstarter will only support my weight for a second or two, but the bike (when I thrash it, as I mostly do!) goes very well........just enjoy .

Regards John O