DaveFJ
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| posted on 25/9/10 at 11:07 AM |
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low compression woes
after ages of messing with other bits i have finally got around to checking my compression ratio.... 
not good i think...
now i wasnt sure if i was supposed to do it with all 4 plugs out or one at a time so i did both.... here are my results...
all 4 plugs out
1 = 7.7
2 = 7.9
3 = 8.1
4 = 8.1
one plug at a time
1 = 8.6
2 = 8.5
3 = 8.6
4 = 8.9
It's a 2.0 litre pinto that had the bottom end opened up to 2.1 by scholar engines and oversie pistons fitted. rebuilt by them so I am pretty
sure that is ok (about 2000 miles ago)
the head was 'cleaned up' by me following the many guides online so may explain that slight variation across cylinders...
head is from a 2.0I with a Kent fast injection road cam. and i had a full 1mm skimmed from the head.....
so. opinions... guessing i need to find a way to up the ratio quite a bit but with 1mm already off the head i am a bit worried about taking more off
in case i get piston-valve contact...
I should probably add that i have no blue smoke and am not loosing oil...
[Edited on 25-9-10 by DaveFJ]
Dave
"In Support of Help the Heroes" - Always
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brianthemagical
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| posted on 25/9/10 at 11:14 AM |
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Do you mena you've used a compression tester, and they're pressures?
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DaveFJ
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| posted on 25/9/10 at 11:23 AM |
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yes.....
Dave
"In Support of Help the Heroes" - Always
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adithorp
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| posted on 25/9/10 at 11:26 AM |
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Did you have the throttle open when you did it?
adrian
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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DaveFJ
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| posted on 25/9/10 at 11:27 AM |
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b*gger...........
will have another go and post results....
doh!
Dave
"In Support of Help the Heroes" - Always
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brianthemagical
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| posted on 25/9/10 at 11:30 AM |
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I don't see what the problem is?
Cams have an effect, as do other aspects of the engine.
They all seem fairly close so as long as it runs ok i wouldn't worry.
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40inches
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| posted on 25/9/10 at 11:32 AM |
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Have you checked the pressure gauge against a known pressure? mine is about 2bar low!
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ash_hammond
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| posted on 25/9/10 at 12:10 PM |
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Agreed!
quote:
Originally posted by brianthemagical
I don't see what the problem is?
Cams have an effect, as do other aspects of the engine.
They all seem fairly close so as long as it runs ok i wouldn't worry.
.: www.mac1motorsports.co.uk | www.m1moc.com :.
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MikeR
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| posted on 25/9/10 at 01:03 PM |
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If your answers are within 10% then you're ok (although i'd personally be a little worried at 10%). Yours are under 5% so there should be
no problem.
If you want a giggle, do it again (all plugs out to make it easier for the starter to turn the engine, fuel line disconnected and throttle fully open)
but put a tea spoon of oil down the bore being tested. It will help show how worn the bores are. After that you need to do a leak down test to
identify what is wrong with the head.
(although as i and others have said, i think the engine compression is fine).
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DaveFJ
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| posted on 25/9/10 at 03:23 PM |
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I was just worried that the ratio seemed pretty low....
been trying to sort my performance out... just seems a bit.. slow...
Dave
"In Support of Help the Heroes" - Always
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MikeR
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| posted on 25/9/10 at 03:29 PM |
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tbh - i've no idea what the numbers are, i'm used to PSI, just looked it up and you're around 110 psi which is (i think) low.
Try the same test with a tea spoon of oil down the bores, see if they're much higher.
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DaveFJ
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| posted on 25/9/10 at 04:01 PM |
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using the calc in this link
http://www.pumaracing.co.uk/comp.htm
my average CR is about 7.3 .... seems very low to me....
but as i say - i forgot to open the throttle! so will try again and post results
Dave
"In Support of Help the Heroes" - Always
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rusty nuts
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| posted on 25/9/10 at 06:11 PM |
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quote:
Originally posted by DaveFJ
using the calc in this link
http://www.pumaracing.co.uk/comp.htm
my average CR is about 7.3 .... seems very low to me....
but as i say - i forgot to open the throttle! so will try again and post results
How do you work that out? the only way to work out the compression ratio is to measure the swept volume /combustion chamber ratio IIRC
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mark chandler
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| posted on 25/9/10 at 06:28 PM |
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So is it a hotter cam, this will lower the figures.
When i did mine years ago for my capri I had 1/4" skimmed off the head + oversize valves on an otherwise standard engine and got 135BHP.
Also how have you set the cam, you need 5 - 7 degree advance to maximise the middle range (where you drive).
Regards Mark
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MikeR
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| posted on 25/9/10 at 07:33 PM |
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quote:
Originally posted by DaveFJ
but as i say - i forgot to open the throttle! so will try again and post results
Missed that bit - that will lower the figures considerably.
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MikeRJ
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| posted on 25/9/10 at 11:06 PM |
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You can't get a reasonable indication of compression ratio from a compression test, far too many variables to take into account.
A long duration cam will lower the static CR, but raise the dynamic one within the power band.
The faster you spin the engine the more the compression tends toward diabatic, i.e. you raise the temperature of the compressed gas which increases
pressure.
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DaveFJ
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| posted on 26/9/10 at 10:47 AM |
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i hear what you are saying but...
everything i read and others results say i should be getting a cranking pressure closer to 200 psi not the 120-140 i am seeing.....
given the long duration cam could it be that my cam timing is slightly out?
[Edited on 26-9-10 by DaveFJ]
Dave
"In Support of Help the Heroes" - Always
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MikeR
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| posted on 26/9/10 at 11:24 AM |
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Having the throttle closed will REALLY affect your compression test. Repeat it with the plugs out, fuel disconnected and throttle open. I'm sure
you'll get a higher figure.
(guess who's also done it with the throttle closed once and panicked about the low figures)
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DaveFJ
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| posted on 26/9/10 at 04:05 PM |
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OK....
finally found time to get the engine warmed up again and re-do the tests...
results are (throttle open this time!)
1 = 145 PSI
2 = 144 PSI
3 = 144 PSI
4 = 145 PSI
note in PSI this time as this is what people on here seem to want 
I also tried a squirt of oil - on cylinder 1 only, that brought the pressure up to 154 PSI. I assume this is an acceptable increase and i dont need to
worry too much about wear?
So.. using the calcs from the link above to the PUMA race engines site I am getting a calculated CR of 8.2:1 which does still seem low to me... engine
is running OK, I am just looking for a bit better performance
FYI the stated levels (Haynes) for my engine in an un-modified state are:
CR 9.5:1
pressure at crank speed 10-11 bar = 145 PSI - 159 PSI....
fom everything I have read my ideal for performance would be between 185 PSI - 200 PSI.
Question is, do I need to worry about this and if so how to go about it.... head gasket is bog standard so i suppose I could find a quality thin one,
but how much increase can i expect from that?
Also - with my cam (FR34 i think) and 1mm already skimmed from the head, how much further dare i go?
alternatively do i just take the otehr advice I have been given, based on the compression i am seeing, and fit a turbo/supercharger?
Dave
"In Support of Help the Heroes" - Always
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Ivan
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| posted on 26/9/10 at 04:46 PM |
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I would say that you have nothing to worry about - any attempt to convert cylinder pressure to compression ratio is pretty meaningless and should not
inform any decision you make.
The evenness of the pressures on all four cylinders looks very good. Remember that these readings are taken at low cranking speeds that cause lots of
leakage past the rings and are very affected by cam timing.
Also the gauge is almost guaranteed to be inaccurate and its seal to the plug hole to be inconsistent at best assuming it's a press fit so
don't put too much reliance on the readings.
I doubt very much that your motor would have been built with an 8.2:1 compression ratio unless turboing was the specific intention so approach
turboing with caution unless you have opened up the motor and cc'd eveytrhing and refreshed the bores and rings.
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