Author Topic: Compression Ratio  (Read 299 times)

Online Slymo

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Compression Ratio
« on: 17.11. 2018 10:13 »
My 58 SR took its first run since a total overhaul today and it appears to be a bit over advanced my having set the timing at 13/32" btdc which I have read since is too advanced. I also understand that it is dependant on what octane of fuel and what compression ratio the machine has. My motor came to me with new pistons in a rebored barrel but I've never actually been certain what compression they were. I've assumed that they are 9:1 based  not much but I would love to know so am posting a pic of them with head off looking for wise counsel
NZ

Online muskrat

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Re: Compression Ratio
« Reply #1 on: 17.11. 2018 10:53 »
G'day Slymo.
Look like 9:1 https://tinyurl.com/y6vjyhgy
Yes too advanced, 3/8" max but would probably be happier with 5/16" on 95 octane. Too advanced will cause pinging (detonation, knocking) and destroy the pistons very quickly DAMHIK.
Cheers
'51 A7 plunger, '57 A7SS now A10CR, '76 XT500, '77 AG175 '83 CB1100F, '81 CB900F project.
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Online RDfella

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Re: Compression Ratio
« Reply #2 on: 17.11. 2018 10:56 »
That profile looks like 9:1. Any part nos? Difficult these days I know, as aftermarket suppliers rarely mark anything.
BTW - comforted to see my bench is not the only one with stuff all over it. All my good intentions to have a clean, tidy bench soon evaportate. Only time it was really clean was in the days (long ago) when I used to overhaul BW auto gearboxes.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online Slymo

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Re: Compression Ratio
« Reply #3 on: 17.11. 2018 23:06 »
Cheers all. Yes I work in a rather squalid environment which I rarely seem to be able to get on top of. Fortunately now the bike is out of the shed I can do a tidy up which will last until the next thing I do in the workshop.
NZ

Online Slymo

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Re: Compression Ratio
« Reply #4 on: 19.11. 2018 04:05 »
Set the timing at 5/16" and it does seem to be a sweet spot. Idles like a stationary engine when retarded and seems very willing and responsive on advance. Just need a seat now and I can start road testing proper.
NZ