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350 compression

2K views 16 replies 7 participants last post by  earlf 
#1 ·
does anyone have any idea what kind of compression i should be running?? i have a 350 bored .060 over,comp extreem energy cam 246-3(lift 490,duration at .050 230 intake, 236 exhaust,tappet lift 274 intake ,286 exhaust,dish pistons with .067 x 3.160 cc relief with 041 heads 64 cc..this works out to be about 9.4 to 1 on paper..i have about 130 psi ,thought i would have more... thanks earlf
 
#3 ·
My little machineyThingaMajigger said 10.4 with a fel-pro blue head gasket.

Anyways at 130 you need to do a leak down test or at least squirt some oil in the cylinder and retest to see if the rings have seated.(if oil doesn't raise you pressure you may need to check for leaky valves. If all the cylinders are equal leaky valves are probably not the cause.) I hate to say it since Comp is my favorite cam but if both of those(rings/valves) checkout you may need to look at you cam advance or that split duration bleeding off cranking pressure.
 
#4 · (Edited)
poncho62 said:
130 does seem a little low...................perhaps the overlap of your cam is causing it.

i haven't built one of these in 40 years .every thing has changed is there a way to see this???

NXS said:
My little machineyThingaMajigger said 10.4 with a fel-pro blue head gasket.

Anyways at 130 you need to do a leak down test or at least squirt some oil in the cylinder and retest to see if the rings have seated.(if oil doesn't raise you pressure you may need to check for leaky valves. If all the cylinders are equal leaky valves are probably not the cause.) I hate to say it since Comp is my favorite cam but if both of those(rings/valves) checkout you may need to look at you cam advance or that split duration bleeding off cranking pressure.

your right on the 10.4 with flat top pistons i have dish with 8 cc relief ,everything else up to par must be cam overlap which i never understood thanks earlf
 
#6 ·
YEA! 400s always run high cranking compression! I love it!

That 350 should have 150-170 with his set up. I think the cam is retarded:p or the rings haven't seated yet.



<split druation..I steer clear from those when I can.>

Strange, it seems like that cam would have a wide lobe seperation?(112-114) raising static compression. Sir, what is your lobe seperation by chance??
 
#7 ·
NXS said:
YEA! 400s always run high cranking compression! I love it!

That 350 should have 150-170 with his set up. I think the cam is retarded:p or the rings haven't seated yet.



<split druation..I steer clear from those when I can.>

Strange, it seems like that cam would have a wide lobe seperation?(112-114) raising static compression. Sir, what is your lobe seperation by chance??
lobe sep is 110
 
#8 · (Edited)
earlf said:
does anyone have any idea what kind of compression i should be running?? i have a 350 bored .060 over,comp extreem energy cam 246-3(lift 490,duration at .050 230 intake, 236 exhaust,tappet lift 274 intake ,286 exhaust,dish pistons with .067 x 3.160 cc relief with 041 heads 64 cc..this works out to be about 9.4 to 1 on paper..i have about 130 psi ,thought i would have more... thanks earlf
i have run similar cam specs with 330 cubes and low comp,, worked ok when tuned,,.. Tourqe range was good

Is your cam timing ok,???
 
#9 ·
compression

I ran the numbers and with a 10 cc gasket and assuming your heads are right on 64 cc you are correct at 9:4.1 compression. I am not sure if you cc'd the heads but they are almost always bigger then the advertised cc's.

I ran the numbers throught the dynamic and you cylinder PSI should be 160 psi @2000 feet altitude. I have never found any engine to come out right on the number so i would think 150 psi would be more realistic... Now if your heads are around 67/68 cc witch is closer to a 9:2.1 static compression. and a dynamic around 146 Psi ( corrected as above) now your getting close to your 130 PSI. I would think you have the cam installed late.

Did you degree the cam????? If so what was your closing numbers for the intake valve???

Keith
 
#10 ·
Did you run your compression check with the plugs removed, ignition hot wire off and the throttle locked in the wide open position? Depending on what kind of piston your running, this too can have a effect. If there "rebuilder" cast or hyper pistons, they likely have .020 off the tops, the compression distance of a stock 350 piston is 1.560, the rebuilder jobs are 1.540, this lowers the compression all the more.
 
#13 · (Edited)
Re: compression

k-star said:
I ran the numbers and with a 10 cc gasket and assuming your heads are right on 64 cc you are correct at 9:4.1 compression. I am not sure if you cc'd the heads but they are almost always bigger then the advertised cc's.

I ran the numbers throught the dynamic and you cylinder PSI should be 160 psi @2000 feet altitude. I have never found any engine to come out right on the number so i would think 150 psi would be more realistic... Now if your heads are around 67/68 cc witch is closer to a 9:2.1 static compression. and a dynamic around 146 Psi ( corrected as above) now your getting close to your 130 PSI. I would think you have the cam installed late.

Did you degree the cam????? If so what was your closing numbers for the intake valve???

Keith
Keith I used a dial indicator on the rocker arm and watched the no# 1 intake open all the way, it was at 106 degrees on degree wheel
 
#14 ·
Re: Re: compression

earlf said:
Keith I used a dial indicator on the rocker arm and watched the no# 1 intake open all the way, it was at 106 degrees on degree wheel
I am not real familar with this way..... How did you make sure that the zero on the degree wheel was at tru top dead center????

keith
 
#15 · (Edited)
Re: Re: Re: compression

k-star said:
I am not real familar with this way..... How did you make sure that the zero on the degree wheel was at tru top dead center????

keith
actually i didn't use a degree wheel .i marked the harmonic balancer at 106 degrees from 0 mark and turned engine over till no# 1 came up on compression stroke and continued till 0 mark linged up with cam cover 0 mark . crankshaft keyway is at the 1:30 position ,set dial indicator and turned engin over till dial indicator quite moving ,iam now looking at 0 mark on timing cover it is at the 106 degree mark i made on the balancer and no #1 intake is completly open..if i can't do it like this let me know thanks earl
 
#16 ·
degree the cam

I looked through your process and i don't see a way to confirm tru TDC.

I would find tru TDC by using a stop in the spark plug hole. Rotate the engine by hand untill it stops, mark the balancer. rotate in the oppisite direction untill it stops and mark the balancer again. Exactly half way between the 2 marks is tru TDC.

I would then use a degree wheel and check the cam centerline to see if it is in fact 106 degrees.

I am not saying your way will not work but i would need to do your process on an engine to confirm it. but either way you neeed to find tru TDC...

Keith
 
#17 ·
Re: degree the cam

k-star said:
I looked through your process and i don't see a way to confirm tru TDC.

I would find tru TDC by using a stop in the spark plug hole. Rotate the engine by hand untill it stops, mark the balancer. rotate in the oppisite direction untill it stops and mark the balancer again. Exactly half way between the 2 marks is tru TDC.

I would then use a degree wheel and check the cam centerline to see if it is in fact 106 degrees.

I am not saying your way will not work but i would need to do your process on an engine to confirm it. but either way you neeed to find tru TDC...

Keith
Keith thanks for the reply.When i had my heads off i marked tdc on balancer ,but the more i read on cam degreeing and compression i'am going to pull off waterpump & ect and go through the whole procedure just to make sure.will let you know what happened ..Thanks again Earl
 
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