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Location:
Rocklea qld
Registered:
February 2003
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how low can compression be before engine wont run..
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Sun, 14 November 2004 07:39
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ive done a compression test on my new engine since i cant get it going...for some reason number 1 is 30, 2 is about 90, 3 is about 60 and 4 is about 45 psi. its a 18rg with 18rc pistons so i expected it to be a little bit lower but is that to low and would cause it not to run??? does anyone know where is should be at with the rc pistons...
the valves were lapped in before the head was assembled and its got new rings.....what might cause it
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: how low can compression be before engine wont run..
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Sun, 14 November 2004 07:43
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intake cam timing out by a tooth?
if the intake valve closes late, you will get low compression numbers...
did you measure the combustion chamber volume beofre it was assembled? what is the expected CR. what is the expected dynamic CR from cam timing?
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Location:
Rocklea qld
Registered:
February 2003
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Re: how low can compression be before engine wont run..
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Sun, 14 November 2004 07:56
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i was told it would go down to around 8.0-1 compression...would low figures like this cause it to not run at all....im sure the timing was right though.....i put the bottum on TDC first mark on the crank as you turn it. then cams with notches up..
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: how low can compression be before engine wont run..
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Sun, 14 November 2004 08:52
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8:1 should still give you decent cranking pressure..
even with a big cam i'd be surprised if it went below 140psi for a fresh motor..
basically there is something wrong with either the piston rings not sealing (unlikely) or there is something wrong with the valves, ie timing or not closing etc etc...
measure twice cut once 
basically, start from scratch. manually measure cam timing.. notches is not good enough.. manually check that valves are closing properly etc (connect up air to the spark plug hole with both valves closed)
oh.. did you do the compression test at WOT???
Cya, Stewart
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Location:
Gold Coast
Registered:
October 2002
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Location:
Rocklea qld
Registered:
February 2003
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Re: how low can compression be before engine wont run..
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Sun, 14 November 2004 09:44
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i managed to get it to run for a few seconds after some timing moventments and removing the electric pump for a mechanical one....
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Location:
1st street on the right
Registered:
November 2002
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Re: how low can compression be before engine wont run..
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Sun, 14 November 2004 10:28
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I had a 2m running at 35psi max comp in one cyl. All the rest were below that. Acceleration was questionable
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Location:
Adelaide
Registered:
September 2003
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Re: how low can compression be before engine wont run..
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Sun, 14 November 2004 11:41
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I think this is the same problem i am having with my 18RGEU
My compression when the engine hasn't been run (cold) is even across all the cylinders but it is considered quite low (especially according to the green book).
Minimum compression should be around 140 from memory, mine is 120.
On a warm day the car starts with a small amount of effort, but on a cold day it is the biggest pig of an engine to get started ever.
facing the front of the engine, i rotated the engine by hand clockwise until the left most mark on the crank pulley lined up with the timing mark on the block. and i think also from memory the distributor rotor is facing about 2 0'clock
If i remember the pin in the intake camshaft is in the right most hole and the exhaust camshaft has it one back (i think or they could be on the other side... ie. the left most hole for the intake and one back for the exhaust...crap i can't remember).
This is the 88270 head with the smaller of the two domes on the pistons (about 5-8mm above the block face), but as i have mentioned to you before it also has a 1.5 - 2mm copper head gasket which i think is the reason my compression is low (although judging by an earlier comment in this thread i might want to see if i can advance my intake cam a little bit to see if this helps the compression...is it the same for the exhaust as well?)
Compression shouldn't be more than 10 percent between cylinders.
put a bit of oil in each cylinder and give it another test to see if it is the rings (see if the compression value changes ).
but being such low values i would say yes, you do have an issue and this could very much be the reason why it doesn't start.
I have a head gasket for an 18rc and 18rg and they don't seem to be entirely the same (one or 2 holes don't seem to be on one but are on the other), i don't know if that would make a difference?
ummmm i can't think of what else you could look at, you were sure the valves were sealing when the head was off? have you checked the valve clearances...they may be staying open
and not shutting entirely.
exhaust valve clearances should be .29 to .39 mm
intake valve clearances should be .24 to .34 mm
don't give up dude i am waiting with baited breath on how your engine is going.
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Location:
Devonport Tasmania
Registered:
June 2004
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Re: how low can compression be before engine wont run..
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Mon, 15 November 2004 07:43
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compression in these engines is determined wholly by pistons as far as i know, if its an 18rg, or 18rgeu the combustion chamber volume should be the same.
that said, std 18rg pistons have raised top pistons, buy about 4mm i think.
so if u have installed 18rc flat top pistons and just had valve reliefs cut if needed, then your compression is gonna be hell low.
that coupled with the overlaop on the 18rg cams, and youll get pretty low compression i think..
correct me if im wrong ?
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Location:
Rocklea qld
Registered:
February 2003
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Re: how low can compression be before engine wont run..
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Mon, 15 November 2004 08:49
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i got told it would sit around 8.0-1 for compression but that was a 88250 head....i think the main problem is the cam and valve overlap...atleast ive had it running i guess
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Location:
tallahassee FL usOFa
Registered:
May 2002
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Re: how low can compression be before engine wont run..
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Mon, 15 November 2004 16:21
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your cam timing is off, and even then the numbers should be close & consistant, so you've also got an assembly problem.
who put the rings on the pistons, and who put them in the block?
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Location:
Rocklea qld
Registered:
February 2003
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Re: how low can compression be before engine wont run..
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Tue, 16 November 2004 08:18
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i did the test wrng when i did it...im gonna redo it..ive had someone check the timing and they said its good and im thinking its a fuel thing..it runs for about 5 seconds then dies and u cant rev it at all.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: how low can compression be before engine wont run..
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Tue, 16 November 2004 09:21
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| oldcorollas wrote on Sun, 14 November 2004 19:52 |
basically, start from scratch. manually measure cam timing.. notches is not good enough.. manually check that valves are closing properly etc (connect up air to the spark plug hole with both valves closed)
oh.. did you do the compression test at WOT???
Cya, Stewart
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