MikeSammy

I am planning an engine swap soon. I have a chevy 350 engine block # 3970010 with factory 76cc heads ( # 3973493 ). I wanted to know whether I should use the 76cc heads or use a pair of 305 heads (#14014416 ). I believe that the 305 heads will bring up compression. I plan to use the car for constant highway usage. I was going to use the 305 heads and use the intake valves from the 76cc heads and then port them since people complain that the 305 heads dont give enough air flow.
oh yeah, and the engine is a carbed engine. The engine is also completely stock except for a cheap aftermarket comp cam and a cheap weiand intake.

engine build A: 400 76cc heads stock

engine build b: 305 heads with the 400 intake valves and full port.

bluers91

The 305 heads should be 58cc and will raise compression quite a bit. When ported they can make good power with the right camshaft.

mlynch001

As long as you do not want any top end power, the 416's will be fine. You will have compression. However, with those small chambers you will find it very hard to stuff the big valves in without causing severe valve shrouding issues. The intake ports of the 416's are very small and as long as you do not expect much above about 4500 RPM you will be fine. There is a guy on this site who is doing a bang up job of home porting 416 heads, there is some food for thought on that thread. Check out "Doom 86" and the thread "ported 416 Heads" in this section, good info and good pictures as well. Again, they would work fine on a 305 ported, but on the larger displacements, they become a serious restriction. Also, 416's are prone to cracking in the exhaust seats.

MikeSammy

Oh, well thats ok about top end power since my rpm limit is only 5000 with the yellow at 4500 so I won't be going much above that. Also, I am not well educated in the engine so what do you mean by valve shrouding issues. Do you mean that the intake valves from the 400 heads wont fit? And if I port the heads to their maximum will I still loose the top end power?

MikeSammy

Does porting heads decrease compression by raising the cc.

mlynch001

Valve shrouding is basically where the OD of the valve is so close to the side of the chamber or cylinder that the wall of the chamber or cylinder wall actually blocks the flow of air from the valve. The 400 will likely have 1.9375 intake valves and either 1.5 or 1.6 exhaust valves. Those valves can be placed in the chambers since the guide centers on all stock SBC heads are the same. 305 engines came with 1.750 or 1.8 intakes and 1.5 exhausts. Careful porting Might arrive at a port that will flow enough air to support the 350, but it will be hit or miss. Porting without a flow bench AND without any experience is a complete shot in the dark. Proper porting is much more than just HOGGING out the ports! "Doom 86" has a great thread that covers what you want to do, and discussion of the pitfalls of such a project. This is going to be a time consuming project that will demand a lot of patience to get it right. I am NOT saying that it cannot be done, just that it will take a lot of careful work, thought and planning to get it right. Much more difficult than starting with a better set of heads and going forward from there. It may be more work than you want to take on, depending on your experience level. I would start by doing some reading of some basic "How to Hotrod SB Chevy" or other stuff related to High Performance Engine building. Once you understand the concepts of airflow, valve shrouding and other things, you will be able to make better decisions as to which way you want to go.

MikeSammy

I think Ill just take it to my machinist then to get ported

mlynch001

No, porting is inside the intake and exhaust ports and behind the valve, so it does not increase the chamber volume or change the compression ratio.

These changes to the head reduce chamber volume and increase compression:

Milling the head surface.
Installing valves with larger and thicker heads or that protrude deeper into the chamber.
Installing seats that raise the stock sized valves higher in the chamber.
Using a thinner than stock head gasket on one with a smaller than stock bore diameter.

These changes increase chamber volume and decrease compression:

Cutting chambers to clear and un-shroud larger valves.
Smoothing off the interior or polishing the combustion chamber
Cutting or grinding the valve seats and lowering the valves deeper into the chamber.
Installing valves that are thinner or recessed on the combustion chamber side.
Using a thicker than stock head gasket or a gasket with a larger than stock bore opening.

Hope this helps.