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| .090" off a standard 12G295 head = ?? https://www.ausmini.com/forums/viewtopic.php?f=5&t=51451 |
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| Author: | sgc [ Sun Mar 22, 2009 1:32 pm ] |
| Post subject: | .090" off a standard 12G295 head = ?? |
Quick question: Has anyone taken 90 thou off a standard '295 head, and if so what was your chamber volume? It's pretty common to take .060" off, does anyone know what chamber volume that returns (again on an otherwise unmodified head)? |
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| Author: | drmini in aust [ Sun Mar 22, 2009 2:44 pm ] |
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I have taken .100" off these heads in the past, but the more you rip off `em the thinner the head deck gets, and the more potential for gasket problems. As for the chamber volume, the only safe way is to put the valves in, pour in the volume of liquid you want and measure down to it. This is because these heads are all getting old now- valves are often sunken into the seats, or it's had bigger flat headed ex valves fitted, and/or the chamber shape modified in the past. |
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| Author: | sgc [ Sun Mar 22, 2009 3:19 pm ] |
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I'm just trying to get a handle on the head which is on my car. I've measured the height of the head at about 2.660", which means it's been decked by about 90 thou.. that distance in a perfect cylinder equates to about 7.5cc, but I'm just wondering if anyone has measured what that much off a '295 actually does to the chamber volume. An exercise in theory mainly, I'm not going to yank the head off just to cc it 
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| Author: | drmini in aust [ Sun Mar 22, 2009 6:53 pm ] |
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Best to do this trick with engine cold...... To see what your C/R is now, accurately: Point car a bit uphill. Put at TDC firing (ie. valves closed), on #1 cylinder. Remove spark plug. Get a horse syringe or similar, fill it with ATF or light oil. Fill the chamber until 1/2 way up the plug hole. Note the reading, this is total combustion chamber volume (CV). C/R = [cylinder displacement (eg 998/4) + CV]/CV. To get the ATF out, just put some rags on front of motor and crank it over with the coil lead off.
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| Author: | sgc [ Sun Mar 22, 2009 6:59 pm ] |
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Yep, think I'll give that a go, if I can find a hill and a horse who'll lend me a syringe
I think I'm up around 10:1 somewhere, based on the pressure returned by a compression tester -- btw 150 and 155 psi depending on the cylinder. |
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| Author: | gafmo [ Sun Mar 22, 2009 7:23 pm ] |
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Nice and even with the tester 
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| Author: | Morris 1100 [ Sun Mar 22, 2009 8:31 pm ] |
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A compression tester is only a rough guide. The bigger the cam the worse the result. 
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| Author: | sgc [ Sun Mar 22, 2009 8:33 pm ] |
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Morris 1100 wrote: A compression tester is only a rough guide.
Hence I'm digging deeper
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| Author: | Morris 1100 [ Sun Mar 22, 2009 8:45 pm ] |
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If you have enough ATF and rags you can follow the Dr's method to cc the head and then you can turn it to BDC and cc the cylinder and minus the head to get the displacement. |
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| Author: | carter [ Wed Mar 25, 2009 8:46 am ] |
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i had about 70 thou off my 295 head and goes fine.runs a little hot dont know if that has anything to do with it but runs well otherwise |
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| Author: | sgc [ Wed Mar 25, 2009 8:51 am ] |
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This engine is bulletproof, it's had one rebuild in 15 years and is still going strong
Must get it on the dyno one day, curious to know how many bananas it'll produce. |
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| Author: | simon k [ Wed Mar 25, 2009 10:18 am ] |
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sgc wrote: I think I'm up around 10:1 somewhere, based on the pressure returned by a compression tester -- btw 150 and 155 psi depending on the cylinder.
my tester shows 200, I wonder what CR that makes mine... 2 rebuilds in 12 months though 
last year I got a couple of big syringes from the vet I work for occasionally - just haven't gotten around to doing this test - the head's been off twice in the last month, you'd think I could have CC'd it while it was off, but no
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| Author: | drmini in aust [ Wed Mar 25, 2009 11:44 am ] |
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There's no way a compression test can tell you what the compression ratio is. The bigger the cam and the more overlap, the less effective compression stroke you have at low speeds. eg a 731 cam (pretty tame, really) has the inlet valve closing 64° after BDC. Bigger cams it's more like 80°, ie near 1/2 the compression stroke is wasted from a static compressing (psi on gauge) point of view. This is the main reason you can run 12:1 C/R with race cams. |
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| Author: | sgc [ Wed Mar 25, 2009 8:18 pm ] |
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drmini in aust wrote: The bigger the cam and the more overlap, the less effective compression stroke you have at low speeds.
Excellent point, I hadn't considered that
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| Author: | sgc [ Wed Mar 25, 2009 8:20 pm ] |
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simon k wrote: the head's been off twice in the last month, you'd think I could have CC'd it while it was off, but no
Heh.. you'd think Not that you had bucketloads of time to play with it, though..
I'm going to have mine off in the next couple of weeks I think.. I want to get to the bottom of my oil consumption before tackling the Muster run. |
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