I just checked the price of compression testers over here in the UK. Cheapest was about £14 up to £30 ish. Google compression testers and you will probably find a local supplier, otherwise I expect you could get one sent from the UK. Have you checked that with the plugs out the engine is easy to turn over? Just wondering if a valve is hitting a piston or something is hitting inside either the tappet inspection caps or the valve spring caps.
Compression ratio
- Thread starter Bill Cannon
- Start date
Yes the engine turns over freely with valve lifter actuated or with plugs out. I checked all the clearances, valve to piston, tappets to covers, piston to squish etc when I built the engine. I've got a compression tester at work but without some means of turning the engine over at a reasonable pace it will be no use!
Bill
Bill
Just a thought, if you made sure the piston was at TDC and the valves were closed and the bike lent over so the plug hole was uppermost and filled the combustion chamber with engine oil measuring it as you put it in, that would give you the compression volume. then sucked out as much as possible any remainder would drain by and not contaminate anything.
Chris.
Chris.
Bill,
I know a few owners that use electric starters on Eglis that have high performance engines. I am assuming that they have high compression pistons in place because one rarely builds a 1300cc magnesium cased engine and installs 8:1 pistons. Does the starter manual say that there is a limit of 9:1 on compression?
David
I know a few owners that use electric starters on Eglis that have high performance engines. I am assuming that they have high compression pistons in place because one rarely builds a 1300cc magnesium cased engine and installs 8:1 pistons. Does the starter manual say that there is a limit of 9:1 on compression?
David
Hello Bill, I know you have had Vin's a long time, Would it be worth putting a pair of known Normal carb's on, Just to get it started, Maybe it's just the drag of new low clearance pistons, what is the clearance ?. I have built engines in the past, Not Vin's, That were very tight, But once started they were fine. I used to kick start 12 to 1 pistons with a bent up kicker with not a lot of leverage, But I always use the method of kicking down and letting the Valve lifter go at the last sec', Just trying to think of a way to get you going, All The Best, Bill.
David, Francois Grosset assures me 9:1 should be no problem, although it have belt drive primary with non standard ratio which reduces the mechanical advantage. My concern is that the compression ratio may be more than 9:1 because of 500 heads on 600 barrels.
Bill, skirt clearance is 0.003". I have a pair of rollers, with which I have already had the engine running so any initial tightness should already have been reduced.
Thanks guys.
Bill
Bill, skirt clearance is 0.003". I have a pair of rollers, with which I have already had the engine running so any initial tightness should already have been reduced.
Thanks guys.
Bill
Why not use the rollers? You only need to turn the engine over about 3 - 5 times with the throttle wide open in order to get the reading, which, if you have a compression tester like mine, retains the highest reading. Alternatively roll the bike down a hill or get a tow and let the clutch out once moving in second gear. Remember the throttle must be wide open. You can make life easier by taking out the plug for the cylinder not being tested.
Well I have just measured the cylinder head volume! I used engine oil in a syringe and filled to about half way up the plug thread with the bike on side stand. I may have overfilled slightly so my nominal reading is 62cc.
I have done calculations based on a possible error of +\- 2cc.
Swept volume+head volume divided by head volume = compression ratio.
(600+60) /60= 11:1
(600+64)/64= 10.4:1
To one significant figure.
If it were a 1000 not 1200
(500+62)/62= 9:1 which is what the pistons should be, so I think this proves my conclusion that 500 heads have a different volume to 600 heads. So I have been supplied unsuitable heads!
I don't fancy a road bike with compression ratio in excess of 10.4:1.
I also don't really fancy sending the heads back to Australia.
Next question, how do I reduce compression ratio, bearing in mind Terry Prince barrels seal to the crankcase with a fine o ring rather than the conventional gasket?
Bill
I have done calculations based on a possible error of +\- 2cc.
Swept volume+head volume divided by head volume = compression ratio.
(600+60) /60= 11:1
(600+64)/64= 10.4:1
To one significant figure.
If it were a 1000 not 1200
(500+62)/62= 9:1 which is what the pistons should be, so I think this proves my conclusion that 500 heads have a different volume to 600 heads. So I have been supplied unsuitable heads!
I don't fancy a road bike with compression ratio in excess of 10.4:1.
I also don't really fancy sending the heads back to Australia.
Next question, how do I reduce compression ratio, bearing in mind Terry Prince barrels seal to the crankcase with a fine o ring rather than the conventional gasket?
Bill
For my money you have three choices, easist, compression plate, you can get one made at your local laser cutters, machine the pistons, remove metal from the combustion chambers with a die grinder(very messy). The aermacchi I'm building right now has the same problem, it's on 12.7-1 so a compression plate is being made now. Wynns black gasket maker will seal the plate to the cases, guaranteed.
Bill,
It is becoming clearer, but I am having difficulty focusing on the issue. The bike should kick start for you it seems to me, because you are using a decompression lever. If you are not using a decompression lever you need to use rollers of some sort (or bumping.) I know some electric starter owner who use the decompression lever with the electric starter, although I do not know whether this is an "approved" method. So, assuming there is nothing wrong with your starter, the only solution is to use a decompression lever to initiate starting. I say this because, and this is a guess, the compression ratio may be high, but I don't think it is wildly high.
You are talking about a mechanical compression ratio of 9:1. I don't know when your intake closes, but the corrected compression ratio is probably 8:1. If your combination of parts were to jack up the corrected ratio to 9:1, it would still run well on 92 octane pump gas. If the compression ratio were higher you would hear 'pinking.'
I think that attempting to measure the net volume in place is impraticable. The mechanical compression ratio is measured assuming the valves are closed. This is not what occurs. The corrected compression ratio, which starts the calculation when the intake valve closes can be done by formula, but it is always lower than the mechanical compression ratio. This leads me to think that there is something else going on.
David
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