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compression and psi


 
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need help please
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PostPosted: Sat Sep 29, 2012 4:29 am    Post subject: compression and psi Reply to specific post Reply with quote

I have read the archives, and I know that many "variables" will effect cranking pressure, but maybe someone can give me their best guess.

I have a super m with dome pistons and a 450 lp head. It cranks at 200 psi. Any guess as to what the compression ratio would be? The engine is a 281 with stock crank and stock rods if that matters.
 
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Geo51B
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PostPosted: Sat Sep 29, 2012 6:02 am    Post subject: Re: compression and psi Reply to specific post Reply with quote

On the back of my compression tester is a chart .8.0 is 140psi,9.0 is 160psi,10.0 is 180psi,11.o is 200psi. So it should be 11 to 1 compression.
 
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Mopower
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PostPosted: Sat Sep 29, 2012 8:36 am    Post subject: Re: compression and psi Reply to specific post Reply with quote

Stock cam?
 
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need help please
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PostPosted: Sat Sep 29, 2012 10:45 am    Post subject: Re: compression and psi Reply to specific post Reply with quote

no. 198 Dur. 102 ICL.
 
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Mopower
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PostPosted: Sat Sep 29, 2012 11:11 am    Post subject: Re: compression and psi Reply to specific post Reply with quote

This is just an inexact estimate... if it were a stock cam moline, it'd be in the neighborhood of 9.5:1. +/- 0.3
 
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Dblair
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PostPosted: Sat Sep 29, 2012 1:00 pm    Post subject: Re: compression and psi Reply to specific post Reply with quote

the Nebraska test number 620 which is the 450 lp test says the cr is 8.35 to 1 . if you go to Weber's tractor works you can find the compression listed at 197 with fire crater pistons and with flat top 223 . you will have to go to a 30 cc dome to get to 10 to 1 , 42 cc to get 11 to 1 and 51 cc to get to 12 to 1 , by my calculations anyhow.
 


Last edited by Dblair on Sat Sep 29, 2012 1:07 pm; edited 1 time in total
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The Wizard
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PostPosted: Sat Sep 29, 2012 1:57 pm    Post subject: Re: compression and psi Reply to specific post Reply with quote

The Formula for converting compression ratio to psi is
compression ratio x 14.7 + 14.7+ 10= cranking pressure
 
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BushogPapa
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PostPosted: Sun Sep 30, 2012 4:51 pm    Post subject: Re: compression and psi Reply to specific post Reply with quote


An engine with longer Int and Exh Valve Events will show a lower cranking pressure than one with shorter valve events..
At "Designed" of "Built For" RPM, the cranking pressure may well be much Higher than starting RPM pressure..due to more efficient filling of the cylinders because of improved valve over-lap/Lift and intake charge velocity..
Detonation may be your first sign..
Ron..
 
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buickanddeere
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PostPosted: Mon Oct 01, 2012 4:34 am    Post subject: Re: compression and psi Reply to specific post Reply with quote

The Wizard wrote:
(quoted from post at 17:57:36 09/29/12) The Formula for converting compression ratio to psi is
compression ratio x 14.7 + 14.7+ 10= cranking pressure


Sorry but as previously stated. Your formula doesn't work unless it allows for cam timing.
 
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High Octane
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PostPosted: Mon Oct 01, 2012 9:29 am    Post subject: Re: compression and psi Reply to specific post Reply with quote

cam grinds can deflate cyl pressure at cranking speed, and increase as rpm increases. warm up your engine, then back your valve lash off to .025 hot then recheck your compression. if it were me, at them specs, that cam is a door stop till you get over 470ci or over 280psi, or, 2400rpm or more?
 
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PostPosted: Mon Oct 01, 2012 9:41 am    Post subject: Re: compression and psi Reply to specific post Reply with quote

think high velocity. high cyl pressure, slow rod ratio. cause thats what u got now with a deflating cam at low rpm. i had a cam in a farmall, i could gain 20psi of cyl pressure by adjusting lash at .025, vs stock lash. stock lash, the grind deflated cyl pressure. a stock cam would have gained me 20psi more at cranking speed. i needed to deflate cyl pressure in mine to a resonable level, so i ran the lash at stock. but mine had a rod ratio of 1.39 vs your 1.90. but my psi to mechanical conversion scale was still accurate in both. lol.
 
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Mopower
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PostPosted: Mon Oct 01, 2012 11:16 am    Post subject: Re: compression and psi Reply to specific post Reply with quote

That's from a service bulletin chart from the factory comparing different heads etc.
 
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buickanddeere
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PostPosted: Mon Oct 01, 2012 1:10 pm    Post subject: Re: compression and psi Reply to specific post Reply with quote

Rod angle and length doesn't mean anything to torque and HP developed.
Same goes for two otherwise identical displacement engines. One long stroke small bore and the other short stroke and large bore. They will make the same power, at the same rpms with the same cam and heads.
The long rod advantage is less rotating friction and rounder/better sealing cylinder walls due to less side thrust.
 
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