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Old 03-28-2012, 06:14 PM
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305/350 Compression Psi

Hey guys,

I have a 76' 350 4-bolt main with 416 305 heads on it. The 58cc 416's are freshly rebuilt & ported but have the stock 1.84 valves. The engine is not bored & the pistons are flat top's with the 4 valve reliefs. Bottom end was rebuilt in 1995 with maybe a few thousand miles on it since then. Head gasket is .039 thickness. Valve springs have 136psi seat pressure. Carb is a Holley 600cfm on a dual plane intake. Cam specs are 224/224 dur. @ .50, .450 / .461 lift. Now for the question! What should I be seeing as far as cylinder pressure? I'm reading 130-135 on 7 of the cylinders, 150 on the 8th. This seems too low. Any opinions before I bury my head in the sand?

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Old 03-28-2012, 08:09 PM
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Yeah, how did you test it? When doing a compression test you should pull all the plugs, pull the hot wire off the ignition so it doesn't start and block the throttle wide open so it can get air. If you tested it with the throttle closed it don't get much air. Next is the cam you have is fairly healthy and the loss during the overlap period causes the low psi reading but it's overcome in the upper RPM band where you can't test it.
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Old 03-28-2012, 08:56 PM
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The cranking compression is low because the cam you got has a late intake valve closing point. Its 224@.050" 114LSA but is installed 1deg retarded from true straight up, when installed normally with a stock timing set. 115 in C/L 113 ex cl. Its a retarded cam in more than one sense.

This cam was orignionall designed for the 1969 corvette L-46 11:1 compression ratio 350-350hp motor and stock cast iron exhaust manifolds
w 4 speed trans a tiny bias ply tires of the day.

This cam is a dud.

Your "58cc" heads are likely not 58cc. Most of the 416 heads I have done measure at 61-62cc finished. ( unless I mill them down)

The piston deck clearance height @TDC may be as far as .055" in the hole
(typical with rebuilder pistons and non decked block.

Couple things you can do.
Install your heads with a felpro 1094 thin .015" shim gasket to get the cr up.
CC the heads and mill if you want more cr. I'll bet money they are not 58cc.

I have flat milled them down to 51cc ( valve seat edge)

Degree in the cam to verify the true .050" timing events and advance the cam about 8degress. 107 in C/L 121 ex C/L

This will close the intake valve sooner and raise cranking compression and make more low mid range torque.

Recam the motor
This specific camshaft is not very good for a 350 w 305 heads and headers
Find a 224deg cam ground on a tight 106 to 108 LSA and degree it, install on a 104deg in C/L
EG Isky 270H MEGA cam #201271

Big big difference. The 350 w 305 heads combo wants a tight LSA cam.
especially when using the stock 1.84" valves.

106 to 108 LSA. 102 to 106 intake C/L

The cam you got is killing it.

Most people don't go near far enough when porting these heads.
A quick pocket port job only gets you half way there.
They need to full porting and larger 1.94" valves.
If you heads were only lightly worked on there is much more to do.
These heads work really well when done right
I 'd ditch that cam.

uneven and low compression test. Something is leaking (rings, valves, head gasket) If you cannot get the rings to seat you may have to hone an rering it
If the cylinder walls are to far gone you will have to bore and replace the4 pistons.


The only way to get rings to seal is to make full power WOT CONTROLED runs.
They won't seal if the motor has only been babied.
You don;t need to over rev the thing but it needs some good WOT blasts to seat the rings.

34-36deg max WOT timing, good gas and Champion RV8C spark plugs. ( gap at .035")

The carb is too small for max performance . get a 750

Did I mention Ditch that cam?

Last edited by F-BIRD'88; 03-28-2012 at 09:24 PM.
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Old 03-29-2012, 04:26 PM
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I had a feeling the LSA was too much. All of the other motors I've heard with this swap have a more choppy idle. But this is what my engine guy said work. The engine feels a little stronger than it did with the 882 heads on it but not much. The pistons are rebuilder pistons and the block has not been decked. When I told my engine guy what the compression was, he said it was too low. But I put oil in a couple of the cylinders & the reading didnt change, so I think I'm good there. Still not sure why the one cylinder hit 150psi??

The heads have been fully ported. Pocket ported, as well as the runners. I will look into that Isky cam. I like the sound of it as well! I appreciate all the help with this guys! p.s. F-bird, what do you mean when you say C/L?
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Old 03-29-2012, 05:02 PM
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Quote:
Originally Posted by Suprx125
p.s. F-bird, what do you mean when you say C/L?
(lobe) centerline
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Old 03-29-2012, 08:31 PM
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I have 2 compression gages (same make and model) and one reads 20 to 30 psi lower than the other.

what you want to see if is equal pressure over all 8 cylinders.
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Old 03-29-2012, 09:05 PM
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Yea, I used the same gauge for the whole job. All cylnders were the same, except for one which was 150psi.
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Old 03-29-2012, 09:07 PM
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Quote:
Originally Posted by 454C10
I have 2 compression gages (same make and model) and one reads 20 to 30 psi lower than the other.

what you want to see if is equal pressure over all 8 cylinders.
True that!

Forget trying to determine anything from the reading you get from a pressure gauge, I have three calibrated pressure gauges...one mechanical, one digital and the other is a Heise meter.

All of them give different peak pressure readings on the same motor, all that matters is the variance between cylinders...it's a diagnostic tool to determine cylinder sealing equality.
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Old 03-30-2012, 02:07 PM
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C/L is centerline. LSA is cam lobe separation The cam you got will check at 115 in 113 ex C/L's and 114LSA
(cause its retarded by 1 degree). Degree it, the .050" intake closing point will be 46 or 47 ABDC. Thats too late.

Then: Make the intake valve close earlier.

Advancing it by 7 to 8deg will move the .050" intake closing point to 38 39 ABDC. The intake exhaust C/L's will be 107 and 121
Increased cylinder pressure and noticably more torque.

do not guess, degree it then move it.

Then get the isky cam Its much much better. 108 LSA 104 intake c/L 112 ex C/L big torque.

Get the real measured cr up (felpro 1094 gasket, cc and milled heads)
find out why the cylinder cranking PSi is so low and so uneven (rings,valves etc)

150 to 180psi. is where you want to be. I suspect the basic build quality is very low.

what converter and gears? what car?

Last edited by F-BIRD'88; 03-30-2012 at 02:21 PM.
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Old 03-30-2012, 05:29 PM
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Its on a 76' GMC K25 pickup 4x4 with 4.11 gears 4 sp. manual trans. Not a rocket ship! haha. I don't really want to have to lift the heads off, but if I need to install a thinner gasket then so be it. Would I just advance the cam on the timing chain? Is there another cam that would work thats cheaper than the Isky?
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Old 03-30-2012, 05:53 PM
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Quote:
Originally Posted by Suprx125
Its on a 76' GMC K25 pickup 4x4 with 4.11 gears 4 sp. manual trans. Not a rocket ship! haha. I don't really want to have to lift the heads off, but if I need to install a thinner gasket then so be it. Would I just advance the cam on the timing chain? Is there another cam that would work thats cheaper than the Isky?

google search on degree ing in a camshaft.
You need to verify the .050" lifter rise valve event points
you will need a degree wheel, dial indicator, TDC piston stop
either two solid lifters or read off the edge of the hyd lifter body.
you adjust the cam advance using a set of cam gear offset bushings.
you must establish the cam timing before moving it.

There is no short cut and you cannot just wing it.

The Isky 270H mega cam is only $135 at summit.
You get what you pay for. Thats a very good price for the right cam.
use new isky hyd lifters or new speed pro HT817 lifters.

knock .030 off the 305 heads and reinstall with a felpro 1094 gasket.
If the rings are fubared, rehone and install new hastings rings.

Sell that 224deg 114 lsa .450 .460 lift L-46 L-82 cam and lifters to someone you don;t like.

It will be a whole different motor.

If its got tall mudder 4x4 tires it can use a lot more gear.

Last edited by F-BIRD'88; 03-30-2012 at 06:02 PM.
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Old 03-30-2012, 06:16 PM
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You can buy the whole summit 350 SBC engine rebuild kit for $232
Includes good flat top pistons with the correct compression height.
all the gaskets, bearings etc
All you got to do is bore the block, and regrind the crank and resize the rods.
You can 0 deck the block or use the felrpo 1094 gasket.

put it back together with the isky cam and it will R O C K!!!.
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Old 03-31-2012, 12:18 PM
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*update* So I did another compression test this morning & propped the throttle open. Compression shot up to 150-155psi very quick so I think I avoided a rebuild. These are rebuilder pistons that are in it so they do sit lower below the deck. I'm thinkin that combined with a thicker head gasket might still get me lower than desired compression. My new goal will be the cam swap over to a 106-108 LSA. I'll keep you guys updated.
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