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Old 12-22-2007, 08:28 AM
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What is correct compression number for sb

Hello,
Can someone tell me what I should be reading--in psi--on my actron compression tester. I have a 350 sb .040 over, 6 inch rods, KB hyperutectic 11:1 pistons, and new rings. The motor only has about 90 minutes of run time on it, and has never been on the road, so my rings have not seated yet.

I want to do an across the board compression test on all 8 cylinders to try and isolate a possible a bad cylinder.

Just to bring everyone up to speed, the motor is still backfiring out of the drivers side header, not much just a little. I have rechecked/re-adjusted my clearances between the roller rocker tip and valve tip, .024 exhaust, .022 intake. This is what the manufacturer recommends on the cam spec sheet. I also checked inside the distributor cap where the rotor makes contact with the terminals on the distributor. Two out of the eight had just a little burnt mark. The distributor is a new HEI unit from summit.

Any and all responses are appreciated

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Old 12-22-2007, 08:38 AM
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Cylinder pressure will depend on the compression ratio and cam timing.

For example, a 11:1 cr engine with a 244/244 degree at 0.050' cam could have cranking pressures around 175psi. However, if a smaller cam is used, say 218/218, then pressures would be well over 200 psi.

Popping out the exhaust could be a rich mixture. Try running about 25 degrees of timing at idle and turn the idle speed down to 800 rpm then adjust a/f mixture screws for best idle (smoothest). Running more timing at idle allows the engine to run with less throttle opening which keeps the carb in the idle circuit. Also check for a broken valve spring.
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Old 12-22-2007, 11:34 AM
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Actual compression ration is about 9.98:1. I am using 1.94 76cc chamber heads--for the time being--in addition to a shim gasket and the 11:1 KB pistons to get my compression down to about 9.98:1. The cam was installed "dot to dot", not advanced or retarded. No, I did not degree the cam this time. The specs on the this cam are as follows;

Solid.

Advertised duration: Intake; 274 degrees, Exhaust; 282 degrees

Duration @ .050: Intake; 242 degrees, Exhaust; 248 degrees

Gross lift using 1.5 roller rockers from Harland Sharp:
Intake: .482 Exhaust: .504

I know the heads have to go. This has been my first motor that I rebuilt myself. I learned some things along the was which I will change on my next motor.

My total initial and mechanical timing is set as follows; 35 degrees at about 2900 rpm.

I did not see a broken valve spring when I set my lash clearance. Also, I am using a 750 holley with vac sec with a one size larger than stock jet. I mentioned in a previous post that all 8 of my plugs are black, not sooty black, just black. Having a rich condition is starting to look like it could be part of my problem.

With the additional information, should I still be looking for about 175psi for a good reading on my compression tester? At what reading should I say, I have a problem with this cylinder?

Last edited by newdad1; 12-22-2007 at 11:55 AM.
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Old 12-22-2007, 02:14 PM
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You're barking up the wrong tree. The black plugs are fouled. Replace them.
replace the power valve in the carb. As long as you have compression on all cylinders, you're fine. You have an ignition or carb problem. Check the spark colour. Should be blue and strong. yellow is weak. (low voltage, bad coil etc)
Timing should be 34-36deg total and around 20-24deg at idle.
Use a 4.5" power valve. 750cfm holley jetting 70-72 pri 80-83 sec.
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Old 12-22-2007, 09:43 PM
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Quote:
Originally Posted by F-BIRD'88
You're barking up the wrong tree. The black plugs are fouled. Replace them.
replace the power valve in the carb. As long as you have compression on all cylinders, you're fine.
Probably right about the fouled plugs but I'd like to note tot he OP that even compression on all cylinders is good regardless of the number.if they are all 150 psi no worries, but if one is 170 and another is 135 a problem.
I had a friend with a 500 ci BBC that had 150 psi cranking and it ran a best of 9.11 @ 151 mph with a 250 shot.
http://www.youtube.com/watch?v=s0flVsWucIs
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Old 12-22-2007, 11:19 PM
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Check your compression, it should be even, between 165 and 190, depending on your valve overlap.

I had a 1974 Ford F350 460 today that was running very rough. It turned out to be running too lean, being caused by the carb. idle speed screw set too high. The mixture screws were turned in almost all of the way in. It's air/fuel ratio was 2.568 (way too lean), and after adjustments, it was 1.002 with 1.000 being a perfect 1/1 air fuel ratio.

Just food for thought!
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Old 12-23-2007, 09:50 AM
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Quote:
Originally Posted by carsavvycook
Check your compression, it should be even, between 165 and 190, depending on your valve overlap.:
The amount of overlap doesn't effect cranking pressure. The engine isn't even making compression during that part of the 4 stroke engine cycle. What effects cylinder pressure is the intake closing point on the compression stroke. Longer duration cams typically have later closing intake valve which lowers pressure because less air is trapped. The intake valve on longer duration cams is open while the piston is moving up wards so the engine can't build and pressure while the valve is open.

In fact, if you test 2 cams with the same long duration but one cam has more overlap (more narrow lobe separation angle) then the cam with more overlap will make more cylinder pressure due to an earlier intake closing point. It is all about the intake closing point.
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Old 12-23-2007, 12:23 PM
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If its burnt exhaust valve..'s or its badly seated a compression check will show one or more cylinders with a big drop in comparison to the rest.

Had the same on a Datsun 1200 and it turned out that a exhaust valve had a groove burnt into it.

Just another possibility
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Old 12-23-2007, 01:35 PM
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I am going to check the cylinder pressure now, and change the plugs.
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Old 12-23-2007, 09:10 PM
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Ok, this is what I got when I did a compression check on each cylinder. I took two readings on each cylinder, then averaged the two:

Cylinder: PSI
1 186
2 210
3 201
4 187
5 197
6 206
7 195
8 183

The plugs were all black which doesn't come as any surprise. I think the primary jet is a little bigger than what is called for, I think its 81 or 82. This is what the guy who rebuilt the carb put in. Anyway, I will put a #71 in.

I also noticed that the spark plug wire boot that goes on the number one spark plug has some of the insulation burned off from rubbing up against the header. That will also get fixed. I will replace the powervalve also.
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Old 12-23-2007, 09:15 PM
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those numbers are very border line. You need to get it running leaner and cleaner and recheck. 10% is about the maximum allowable difference between cylinders.
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Old 12-24-2007, 09:19 AM
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That cam seems to have a short advertised duration (274/282) which will make a high cranking pressure on a 11:1cr engine. However, the duration at 0.050" is fairly large (242/248) which indicates a very intense cam (fast ramps).

My 11:1 sbc has a compcam 292H (292/292 advertised and 244/244 at 0.050") and cranking pressure is around 175 psi. The extra late closing point of the 292 degree intake valve bleeds off pressure (18 more than your cam).

If you have detonation problems then get a longer advertised duration cam.

also, any difference in the valve lash adjustment will make the numbers vary by quite a bit.
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