I have a 350 with a stock compression ratio of 9.0:1. It has dished pistons and 76cc heads (crappy #333882). I'm going to check the bearings; if they look alright then the motor is not being rebuilt. I'm just going to install new heads.

Using 081, 416, or 601 heads seems like a viable option -- many guys on here swear by 305 heads on a 350, due to the smaller combustion chamber size. However, I'm a little concerned about compression ratio. I calculated as much as 10.5:1 compression with these 58cc heads, which I think is too much for my application. This motor is going to be running TBI, and I'd like to put 87 or 89 octane in it. I'm not looking for mad power, just V8 power.

Is my CR calculation correct? Am I forced to go with 350 heads with chamber size in the 60s?

 

Lay the chamber walls of the 305 heads back a bit to unshroud the valves and better match the bore of your 350. That will increase your flow and drop your compression down to a livable range.

 

You'll proabaly be better off taking the heads to shop that works on heads so they can cc the chambers before/during and after they start massaging the chambers to gain some cc, this way everything will be more uniform.
I am having a hard time computing that much compression with your dished pistons though. I run untouched 305 heads with bigger valves and ended up 64 when all was said and done, I've got 4 valve relief flat tops and mines only 9:3.1 with a .039 compressed head gasket. Is your block zero decked? and pistons right up on top?

 

IHI, at TDC the pistons are flush with the head gasket surface. They're two valve relief pistons, and I'm not sure of their volume. When you say "untouched" 305 heads, do you mean exactly that -- new valves, and then bolt them up?

 

okay, to me there's a big difference between a dished piston and a 2 vavle reilef so your probably correct on the comp ratio. The heads I bought for my current "crutch-make it through the season" motor are a crappy cast 170 cc runner 64cc chamber 305 head with 2.02/1.60 valves. When I say untouched it means as soon as they came outta the box, they were bolted on. There has been absoluetly zero port/blending work done on these junkers at all-cant even remember the cast number right now, but do remember they were about as crudy as ya get when I looked them up on Mortec's website. I could've easily had the shop blend around the valves for some unshrouding and gained some flow, plus blended in the runners, but this motor is a joke anyways so I did'nt want any amount of $$$ tied up in it. 305sbc is absoleutly correct on how little material would have to be removed from the chamber for better flow around the valves and lowering the comp. to pump gas standards. If your only wanting 87 octane, you'd better get them down to 9.5:1 to be safe, if your willing to use 91-93 you can easily go 10:1 without problems, just make sure your cooling system is up to snuff to keep the preignition at bay when running at max timing.

 

Yea 10.5 is awful high for 89 octane. 93 octane is probably good. It's only 20 cent more per gallon where I live so for 10 gallons, that only two dollars more than 87 octane. I would run 93 octane and about 10 compression to be safe. Just my .02.

 

I really appreciate all of the help, guys. I think I decided that the motor is going to be fully rebuilt. I haven't checked the bearings yet, but I believe the rings need replaced (there's no more crosshatch on the cylinder walls). Since I'm in there, I might as well do the whole thing.

So, by unshrouding the valves, I can bump up the chamber volume AND match the 350's bore, all at the same time? Let's say that I choose to do that myself -- isn't there a way to check the volume of the combustion chamber by using water?

83 Z28 HO, how I'd love to run 93 octane in this engine! However, my Camaro's the "speed car" that I own, so I can justify spending a little more money on it's gas. This motor is going in my daily driver, a four-cylinder Jeep. I still need some economy, or I'll go broke .

 

You can buy a cc kit from summit or jegs to do it yourself, as far as blending in the cc, just take you 350 head gasket and mark where they seat around the chamber, this will allow you to safely take material out of the cc chamber and a little material goes a long ways when doing this.

 

The hole in most aftermarket headgaskets is 4.16" but the cylinder is only a 4.00" or 4.030" diameter, so don't go by just the gasket as stated above or you'll overshoot the cylinder.
If you find a stock steel shim gasket it might be a 4.00" match.

 

here in colorado altitude f's us out of about a compression point and they sell gas in octane ratings of 85, 87, and 92 and our 85 cost as much as a lower altitude state's 87 octane, not to mention a 13.0 sec car become a 13.9 sec car here, so anyway if you start pinging really bad, just move here

edit: sorry, i had to vent

 

New to this board and I have to confess I've got a 2nd Gen..1981 . Started browsing as you guys have alot more experience of 305's and specifically fitting 305 heads on a 350.
To get to the point. Last week I fitted 601 heads to a 350 block that still had the original -12 cc dish pistons from the 70's. The standard CR was a measly 8:1, which did little for power. I figured all things being equal dropping on the 'ported' and 'unshrouded' 601's would take the CR up to about 10:1. I measured the heads and the volume was around 56cc's. These heads had been skimmed and polishing the chambers and unshrouding the 'stock' valves gave me the chamber size required.
What's it like to drive.....brilliant. It's turned a docile motor into a real torque monster. The engine 'sound' alone is worth doing the change. Other parts are a Crane Energzer H272, Edelbrock performer, Holley 600 and Hedman headers. It's now a great sounding car and performs as good as it looks.
Incidentally Gas quality here in the UK is pretty good but even with 92 unleaded it hasn't caused any pre-ignition problems.

 

Can I quote ya?
This is just the application where a set of budget moded
305 heads is "just right" for wakeing up a lowly low compression smog 350.

 

No problem quoting me. Best thing of all was the low cost of the 601's to start with. Even with freight costs and import taxes to the UK, the overall cost was half that of buying a new set of 'performance' heads.

Cheers.