350 heads on a 305?

Bill, You can install small valve 350 heads only on a stock bore 305. This is because the bore difference as you mention. The large valve heads will not work, for example 2.02 1.50 intake and exhaust valve diameters because they may hit the cylinder wall. I never tried to install a set of 350 heads on a 305, however here are some suggestions.

Check the intake valve and make sure when it is open through the full range of valve lift that it does not hit the cylinder wall. The exhaust valve should not be a problem, but check this also. Chevrolet had this problem with the 396 big block, when it was first introduced for the 1965 model year and GM cut a relief in the deck of the block so the intake valve would not hit it. This is poor for performance because the flow of air/fuel gets disturbed and only flows 270 of the full 360 degrees of the total circumference of the intake valve head. Also small cubic inch displacement engines do not work well with large flow/port cylinder heads, unless of course they are designed as a combination to work well together, as in a high rpm road race Nascar or high rpm drag racing application for which you would use aftermarket cylinder heads, unless you had access to a flow bench or dyno, used to monitor your improvements on stock casting cylinder heads.

The problem with small displacement and large valve cylinder heads, if both are stock and unported or unmodified, is that the combination does not work very well unless the camshaft and compression ratio, as well as the induction (intake) and exhaust are all matched together as a system to work in a vary narrow rpm range which gives maximum power. Anything below this rpm range and the engine does not run very well.

Double check both valves through their full range of motion and compare the exhaust ports on the 350 and 305 heads on the end ports. The newer 305 and 350 end ports had the bolt hole location revised for later models (where you bolt the headers or exhaust manifolds to the cylinder head) and they do not interchange. Therefore if you already have a set of exhaust manifolds or headers you plan to use, make sure the heads have the matching holes, otherwise you will either need to make a set of plates for the heads, so the headers/manifolds will bolt on, or you will have to modify the headers/manifolds.

Bill, If you want to pull off the original heads and still use the stock 2 barrel, mill the stock heads, which is normal if your local machine shop runs a straight edge across the edge of the deck of the cylinder head and determines they are more than .005-.006 warped. Have them milled .020-.025 which will shrink the size of the combustion chamber, therefore increasing the compression a little bit. You will also have to do the same to the intake so it matches the cylinder head mating surfaces. Then clean up the casting flash in the intake ports and port match the intake gasket to the intake and exhaust ports. Since the heads are off, have a good 3 angle valve job done and have the valve springs checked or replaced.

Install a little better performance camshaft and lifters by calling Crane cams for suggestions on the power you want at 1 386 258–6174. Ask him for a good combination which will produce approximately 250–290 horsepower with your 2 barrel.

Now, you can make a modified 2 barrel flow big numbers, like 650–750 cfm. Take a look at past circle track articles, which did a step by step modification process, with pictures. This was about 2 years ago for stock modified series circle track cars where drivability is a big factor. They milled off the choke horn, where the air cleaner stud screws into, and it looked just like a fuel injection throttle body. Next they removed the small 5/16" fuel line and fitting and replaced it with a 3/8" line and used a modified needle and seat to accept more gas flow into the carb. Surf Circle Track's web site for carburetor shops. There are plenty that can dial you right in.

You can do wonders and keep all you stock parts, except for the camshaft and lifters, without touching the bottom end of the engine to produce close to 300 horsepower and drive the car everywhere.

These steps will wake up your 140 hp 305 and make it come to life.

Good Luck!

Good Luck!

The small block will be lighter and fit right in place with no modifications and dollar for dollar will give you maximum for your money, unless of course you are building a full blown race car, where the big block would make more power, cheaper once you get into the $10,000 and above price range of building a race engine. First figure out your budget. If it is under $10,000 go with the small block. You may have to dent in the firewall or move the engine forward of the stock, factory installed small block location, if you choose to run a big block with the larger HEI distributor cap, as the big block is longer and wider than the small block.

Either engine will give you outstanding performance if you choose the correct combination, it just depends what you prefer. For engine parts or complete engines go to Summit Racing, open 24/7 at 1 800 230–3030 or Jeg's also open 24/7 at 1 800 235–3535. Both are in Ohio and both offer fast, friendly service.