Trying to determine compression ratio on a 305. Figuring 58 cc heads, stock bore 12cc dish piston gives me 9.0 to 1 and using a 4 cc flat top gives 9.9 to 1. I am using total figure of .045 for deck height/gasket. 1.560 comp height pistons What was the factory deck height and gasket commonly used, and does anyone have a part number for a 305 steel shim gasket ? I thought the factory specs were about 8.5 dish and about 9.5, flat, and usually factory specs are a little high, why am I off from typical spec ? I would like to run the flat tops if possible, with enough cam would that run on 87 octane ? Did the factory flat top enigines require premium gas ? Thanks in advance.

If you use the flattops with a ROL hg31600 and deck the block for .02 in the hole with your 58cc heads you will have 9.8:1 CR. If you want less compression use ROL hg31010ht and that will give you 9.4:1 CR. Heres a link for the head gaskets.
HTTP://WWW.STORE.PARTSDINOSAUR.COM/PAGE70.HTML

Oh BTW the factory 305 with 9.5:1 doesnt need 93 octane although it runs better on it because the computer will give you more timing without detonation.

So am I correct to assume that the 305 did NOT use a thin steel shim gasket ??

The last three 305's that I have dissasembled all had steel head gaskets. Two of which were 84' blocks with 2pc RMS, and an 86' block 1pc RMS.

Back to the question of my first post, assuming factory shim gaskets, how do you come even close to the factory advertised compression, with either the flat tops or dished ? I just don't get it. If you use typical specs on most any 350, 327, or even any other engine, chevy or non chevy, the actual compression ratio that the factory lists is usually on the HIGH side of what it really is. Every time I try to figure a 305 I alway show about .5 ratio higher than what the nominal compression should be, I know my math is good, but at the risk of repeating myself, piston -4cc, bore 3.73 , stroke 3.48, deck .025, gasket .020 = 9.9 compression - advertised as a 9.5 motor, what gives ? If you run the math on any tyoical 350 advertised as 8.5 compression you usually end up with real compression being lower than advertised, not higher - I'm missing something here I think, either the pistons sit lower, have more relief cc, gasket ist thicker, head is bigger than stated etc. Whats up with this anyway ??????

Any additional input ??? - Thanks

I think the reason your numbers are off is because you need a different compression calculator. As far as what's wrong with yours, why it's giving you those bogus numbers, I can't see it, so I have no clue what its malfunction is. But it obviously has one.

I come up with 8.67:1 for a motor with 3.736" bore, 3.48" stroke, 58cc chamber, .039" gasket, .025" deck clearance, & 12cc dish.

Then I get 9.28:1 for the same combo except with -6cc pistons (flat tops with valve reliefs), and 9.51:1 with -4cc pistons.

Both go up about .1 with the typical .030" overbore.

Both of those are assuming that the gasket bore diameter is equal to the cylinder bore diameter, which is of course not true. So the actual CR will be slightly lower.

I have not found the factory's CR numbers to be particularly off. Not GM's, or anybody else's, really. I'm a pretty arrogant SOB, but I'm not quite ready to accuse the various factories of not knowing what they're building.

I would not (in fact I do not) run steel shim gaskets, EVER. The regular FelPro blue gasket that comes in the gasket set will work fine. But if you just really like changing failed head gaskets alot, there's no reason you couldn't run a 350 steel-shim gasket.

I've never had a steel shim gasket fail.

I've never yet pulled apart a motor built with them, factory or otherwise, where they hadn't already failed; whether the owner knew it or not.

My original 1983 chevy van 305 had about 200,000 on it when I pulled the heads to replace a rusted through steel shim gasket. I have never seen another fail.

Some 305s came with 52-53cc heads on them too.

Thanks sofakindom for checking my math, my calculator using your figures gives me in order 8.68 - 9.28 and 9.49 which is within rounding errors in the program. The difference is that in my original figures, I used .045 total deck which assumes a .020 gasket which I thought was stock. When you figure a .039 gasket ,the figures come out pretty close to factory numbers. I have never personally had the heads off a 305 yet, but every original 283, 307, 327 and 350 I have ever had apart and I talking dozens of engines, have had a factory steel shim gasket that measured about .020, the exception is every 400 I've had apart has had a compostion gasket. It seems kind of weird that the factory apparently figured the comp ratio assuming a .039 gasket but still used shim gaskets in many of the engines ???? Going to the shim figure adds about .4 to any of the above figures. As mentioned above by 89formula, the factory nominal rated 9.5 engines did not require premium, but if some were really 9.9 compression, even with the small bore that would be pushing it.

My next build is going to be a 305 come hell or high water just cause I want to do it. I try to look a what the factory did to get some idea of how to procede. I will be using a cam that closes the intake valve later around 33 - 37 ABDC similar I beleive to the L69 cam. I want to keep total quench/deck between .040 to .050
but don't want to put in 12 cc dish pistons and find out I could run kerosene and not ping, but don't want to put in flat tops and have to run premium all the time or retard the timing a bunch, that s the tough part here.

Try it with these specs:

Head gasket: 4cc (the actual volume of a factory .020 steel shim, not it's calculated volume using bore size- which will always be too small).

Piston dish: 6cc

You should come up with about 9.6:1. The actual gasket volume might be slightly larger than 4cc and/or the piston's valve reliefs may be slightly more than 6cc. I mean if you only add ONE cc anywhere in the calculations from this point you'll be dead-on at 9.50:1

That, my friend, is close enough for government work.

The old L82 cam pulls like a demon in my 312. 300 RWHP through open headers and TBI speaks for itself.