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#Post#: 500--------------------------------------------------
Relation between CR and Pressure
By: Robin_16v Date: October 17, 2012, 9:23 am
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Hi,
I measured my compression of my 2l 16v yesterday, and it pumped
a average of 10,5 bar on each cilinder.
Now my question is, can you look at the 10,5 bar and say that's
10,5:1 CR or not?
Robin
#Post#: 508--------------------------------------------------
Re: Relation between CR and Pressure
By: JMR Date: October 17, 2012, 4:58 pm
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Nope.
Blimmey thats a bit low Robin...in fact quite a lot low...around
155psi. A quick and easy test is if the c.r & cams are matched
then the cranking compression should be around 200 psi...13.5
bar.
Cat cams 280 something in yours arnt they?
#Post#: 512--------------------------------------------------
Re: Relation between CR and Pressure
By: JMR Date: October 17, 2012, 5:42 pm
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Just had a quick look at yr 2ltr build thread...looks like the
pistons protrude like a KR 1800...so C.R should be around 11:1.
Based on that and a quick calculation for the cam duration you
should have enough c.r.
Try another compression gauge Robin and see what reading you get
from that. Also...when cranking make sure you have yr foot full
down/throttles fully open...crank for around 8 cranks / 6
seconds.
Confidant the cam timing is right Robin?
#Post#: 517--------------------------------------------------
Re: Relation between CR and Pressure
By: Robin_16v Date: October 18, 2012, 7:12 am
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Ok now we found the second problem why its down on power..
Transmision is of, and i brought the engine to my dad,s
workshop. I will measure the volume of the chambers etc to make
sure what the cr is.
What compression does a standard abf? 12bar?
Also i do not trust my head gasket.
283 degree inlet cam and 279 exhaust i thought .
Keep you posted.
#Post#: 519--------------------------------------------------
Re: Relation between CR and Pressure
By: Stooza Date: October 18, 2012, 9:54 am
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Right here we go, ready to be shot down :) I reckon 13-13.5 bar
mate
#Post#: 526--------------------------------------------------
Re: Relation between CR and Pressure
By: JMR Date: October 18, 2012, 2:25 pm
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That's what i based my rough calculation on...found em in your
post robin.
Before you pull the head check the current cam timing...lift at
tdc on both cams.
#Post#: 536--------------------------------------------------
Re: Relation between CR and Pressure
By: Robin_16v Date: October 21, 2012, 7:22 am
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checked the cam timing today, and it was of timing.
inlet cam showed 2.6mm at TDC needed to be 2.95mm
exhaust cam showed 2.2mm at TDC needed to be 2.7mm
Cilinderhead chamber showed 42cc
headgasket 7.6cc
and the head of the piston is 5.7cc above block.
And that gives me a total volume of 43.9cc
give me a cr of 12.3:1 if im right ;D
head gasket was not pressed together at all...
#Post#: 537--------------------------------------------------
Re: Relation between CR and Pressure
By: JMR Date: October 21, 2012, 6:26 pm
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The phasing is a touch advanced over the recommended Cat Cams
setting but only by 5 thou...the Cat setting is 10 thou advanced
and you have the inlet 15 advanced over the exhaust...which is
fine. The inlet tho is around 10 thou retarded from the
potential ideal position...tweaking the main external vernier
would do the job, bringing both cams more advanced...setting the
inlet to the Cat recommended setting of 2.95 (116 thou) would do
the job.
That compression ratio is more than fine with the cam spec.
The basic ingredients look right, so I'd try another compression
tester. If the results are the same then the rings are leaking.
We rolling road tested an 8v engine earlier in the year which
made 104hp at the wheels with only 150psi comp pressure. I built
another bottom end from second hand parts with new piston rings,
and even with a tamer cam than before the power cam up by 20hp.
.ring seal is very important.
Better ring seal (if there is an issue), a tweak of the cam
timing and the 3-angle seats sorted in the head and the power
will leap up. Peak power should be around 7,400-7,500.
Did you pop up a carb jetting spec Robin?...if it's over rich it
may well have suffered ring wash causing excess wear.
It can all be sorted ready for the next season Robin... ;)
#Post#: 538--------------------------------------------------
Re: Relation between CR and Pressure
By: Robin_16v Date: October 22, 2012, 11:24 am
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Piston rings are one race old and tested them, there fine.
I think its a head gasket failure. As you know i skimmed the
block to get the CR for my cams, when we assembeld the engine it
looked fine, i never thought about the headbolts... when we got
the cilinderhead of it was clear we had some oil leakage between
the sides of the head, and looking at the gasket, it was not or
just a little pressed together at all. How it runed i dont know,
but i think we found the problem. After measure the head bolts
it was clear that there to long. so the gasket could never be
prest together right... (not easy building a race engine)
Carb spec from memory is:
160 mains
37 chokes
showed 12:1 afr with 39 degree inlet temperature.
Probably running lean in normal race condition about 25 degrees.
I dont fully understan you're cam story. Do you mean setting the
inlet cam to 2.95mm and advance the exhaust cam? about 5 thou?
#Post#: 546--------------------------------------------------
Re: Relation between CR and Pressure
By: JMR Date: October 26, 2012, 4:32 am
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Ahh...there's the fault then...
Consider using ARP stud & nut kit during the next re-build
Robin.
Wow!...that inlet temp was glowing!...you'll never get the
jetting right with that air temp on the rollers....you'll get a
false reading.
I'd say 160 mains were too rich on 37mm chokes. I'd look at
going to 38mm chokes now. Pop up the full jetting list Robin.
Both cams are retarded from the potential ideal. If you advance
the main vernier so both cams advance, but measure the tdc lift
of the inlet cam to the recommended Cat setting, the current
exhaust cam position will be a good starting point...so dont
change the relationship between the two cams, just adjust the
main vernier to get the correct inlet reading and re-check the
exhaust.
So bigger chokes, maybe some jetting, better cam timing, 3-angle
seats and a head clamped down and that should be a lively
engine.. 8)
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