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Old 07-23-2009, 07:37 PM   #1
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Cam Timing VS. Cranking Compression... Cam Pro's Please Chime In.

Quick question.... I have a 347 with a blower. I replaced a N/A cam (TFS stage 3 .575/.593 236/248 @110 l/c) with a Comp blower cam (.603/.613 248/260 @114 l/c) I installed the new cam straight up (although comp said go 4 deg. advanced... duh!) now with the intake closing too late i'm only getting 130 psi cranking comp. (cold, forged pistons, throttle WOT, with all plugs out) I feel this is a bit low, don't you all think? now for my question... how much more cranking compression should I get with this cam degreed correctly? 4 degrees advanced (give or take) from where i'm at now? i realize the sooner the intake closes, the less compression is lost, but how much? thanks for any input in advance guys.
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Old 07-23-2009, 07:48 PM   #2
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Comp Cams said install it 4* advance or it was ground with 4* advance in it? 130 PSI doesn't seem that low considering its a cold motor and a big cam. What's its adv. duration? Put a little oil in the cylinder and do a compression test again.
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Old 07-23-2009, 08:33 PM   #3
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A little more info?
Motor setup, cam specs, etc?
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Old 07-23-2009, 08:56 PM   #4
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Quote:
Originally Posted by rstoff View Post
Comp Cams said install it 4* advance or it was ground with 4* advance in it? 130 PSI doesn't seem that low considering its a cold motor and a big cam. What's its adv. duration? Put a little oil in the cylinder and do a compression test again.
Agreed..

need more info. forged pistons when cold can allow for some blow by. did you do a compression check before the swap? is the compression consistant in all cylinders? Given the duration of the cam you could lose some cranking compression if not installed per specs.
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Old 07-23-2009, 10:41 PM   #5
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lots of details here can make a difference.


lets just say that you can advance or retard the cam opening / closing pointsd of refrence in relation to the cam card and eEVENTUALLY you will in fact see a definate change in cylinder / cranking compression...and then at that point, you will also have no more piston to valve clearance... then you will bend some valves..

depending on the originating cranking compression..
and the rod length in relation to the stroke...
the resultant compression as seen on a gage installed in thru the spark plug hole
WILL VARY because of the relatively SLOW rpm that the starter will spin it..

something else then happens... what occurs is that ar an actual relative runniong rpm...lets say 2000 rpm's...the valve train still moves as it should but the DYNAMICs of HOW the air flows in..and into the cylinder changes....

so, in essence ...what you see on a compression gage "at starter speed"...
does not allways equate to a similar change at RUNNING rpm's.....

a lot of this is related to the number of degrees of overlap that the exhaust to intake lobe has.......
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tires are the great equalizer
a degree wheel, EIGHT INCH dial caliper & adjustable pushrods should be just as normal as a 1/2" wrench in your toolbox
there aint no shortcuts to doing it RIGHT....
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Old 07-24-2009, 02:29 AM   #6
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is this too much for you to understand???


there are cams that have differnt shapes of the low lift area ramps that can be put on a larger or narrower lobe seperation angle...and not bleed off cylinder pressure....
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there aint no shortcuts to doing it RIGHT....
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Old 07-26-2009, 09:33 AM   #7
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thanks for the responses guys. (BTW thats some good info on engine dynamics Kato.) I just found the cam card to provide more info. The specs are as follows:

INTAKE EXHAUST
Valve Adjustment .016" .016"
Gross Valve Lift .616" .628"
Duration @ .015 lift 285 297
Duration @ .050 lift 248 260

Valve Timing Open Close
@ .050 lift. INT. 10 BTDC 58 ABDC

EXH. 64 BBDC 16 ATDC

All specs are for cam installed @ 114 Intake Center Line

This is on a 347 (4.03" bore 3.40" stroke), Forged 8.5:1 pistons, 58 CC heads .050" head gasket. (2.08"/1.60" valves) checked cold, all plugs out, dry cylinder walls, fresh battery. This same engine with a Trick Flow stage 3 cam .575/.593 236/248 with 110 L/C made about 150 PSI cranking comp. I did have to flycut the pistons prior to running this new cam, is it possible that reduced the static compression significantly? (all 8 holes measure the same by the way) I also have to run a tremendous amount of timing now, and the motor just feels flat. I'm just trying to figure out (BALLPARK figure) how much more cranking compression i stand to gain by advancing 4 deg. or do I need to raise static compression with a new set of pistons. Thanks guys for any help.
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