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Measuring Compression Ratio

Ducvelo

Well Known and Active Forum Website User
VOC Member
Is there a standard way of measuring compression ratios on Shadow (series C)? On my other bike beginning with a V, there is an elaborate ritual of measuring oil, greasing top of piston to seal, and pouring in from measuring cylinders until it just reaches bottom of spark plug hole. This accurate measurement seems to be important (and of course it helps that the cylinder is vertical). But I can't find any talk of this on Vincents. Is accurate setting of compression ratio not important? What's the easiest way of finding out what pistons I have in? And what is general advice on best compression ratio on a shadow - used for fun and trips rather than racing.
Thanks for thoughts, Malcolm
 

vibrac

Well Known and Active Forum Website User
VOC Member
I always use play dough. mould about 80cc in your hand plonk it on the piston, put the head with valves in place hold down with a couple of head nuts (take the spark plug/s out!) turn the engine over watch the snakes come out of the plug holes lift the head (you can check the clearances now if you are worried) peel out the dough in the dome allowing for volume inside where the plugs would go roll it into a nice ball (I knew play school had a purpose) fill up a burette with water to a measured line pop in ball read off cc
of course to get the right ratio its easier to use compression plates than pistons I make up an Excel spread sheet and just read it off
 

ogrilp400

Well Known and Active Forum Website User
VOC Member
Yep, Vincent, Velo, Honda Triumph, Norton etc, etc ........ it is the same for all. Its the spark plug hole that needs to be vertical. The exact figure of the compression ratio is not important but setting the same compression ratio between cylinders is desirable.
 

Big Sid

Guest
On a choice of pistons , comp. ratio for a strong running motor but still sweet , we have built several at 8 to 1 and they are stout runners but still very pleasant running .
For skirt clearance go with a nice 4 thou. , no less if the standard cylinder muff with iron liner is used . Sid .
 

Panama

Forum Website User
VOC Member
Why can't it be done by measuring pressure with a compression gauge? One of the gas laws (I don't recall if it was Boyle's Law or Charle's law) states that P2/P1=V1/V2. Using 15 as P1 (atmospheric pressure), if the gauge registered 120 when kicking it over, that would indicate a compression ratio of 8:1. Someone would have to closely watch the gauge while someone else kicked it over, but it would work.
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
The Dennis Minett notebook has all the volumes for the engines he built. Some examples; 77 cc= 7.48-1, 64.5 cc= 8.73-1, 83cc= 7-1. Just do the backward maths from there, or these three will probably be a good range for a road machine anyway.
 

Big Sid

Guest
My recollection compression readings with a screwin type gauge , not manually held in place , is for a Rapide about 125 psi , a Shadow with 8 to 1 pistons 140 / 145 , and mine with 9 to 1s , 32mm carbs and port work showed 175 and a bit . The larger carbs and porting inhales more so reads higher to my way of thinking .
Done with the carbs held wide open or better yet taken off the heads thus unimpeded inlet ports . And it takes up to six boot overs to reach maximum reading .
This process takes a strong leg on a powerful lad , not for wimps .
How do these results compare to those others have seen ? Sid .
 

riptragle1953

Well Known and Active Forum Website User
Non-VOC Member
The compression ratio that mean's anything is the ratio developed when running and warm. All this preparation ritual is meaningless. The guys above are right..... warm it up, screw in the gauge and see. It's like doing a leak down test with a combustion chamber full of grease otherwise.
One of my in looking at Vincent tech advice after all these years was to see how things have developed. Lord, it's gotten worse. Perhaps it's best to ignore the years of Vincent drivel (as if it's something oh so very unique) and lean how an Otto engine works.
 
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