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.

 

Use the ported 416's no question about it.

A mild stage 2 type porting with bigger valves will make good bottom end but should have breath to 5-5.5k with a mild cam. If you plan to do a mild job focus effort on the valve guide and blending the bowls. Before you do anything strip them down and take them to the machine shop to be cut for the bigger valves and magged and cleaned.

I can dig up some mild ported 416 pics if you are interested. They aren't perfect but will give you an idea what they should look like with little modification. I've since gone over-board with them so the newer pics won't be any use for you.

 

I doubt you'll notice a difference with either head. Heads are a lot more than just compression ratio. The compression ratio builds torque but the smaller combustion chamber also shrouds the valves. Since the 305 heads will need bigger valves to help feed the bigger engine, shrouding becomes even more restrictive. Good flow numbers into a large combustion chamber will give better performance than restrictive flow into a small combustion chamber.

 

You think there will be no noticeable difference between stock smogger 400 heads and ported 416 casting? I only ask because you didn't mention the porting that the OP was asking about.

When they cut for larger valves they also run another cutter to deshroud the chamber and hog the bowl for the larger valve. I'm attaching a picture of a set of ported 416's with the cuts made for the larger valves to show what I'm talking about. Talk to your machinist and make sure this is done because Alky is spot on, you can have all the flow in the world and if it isn't able to exit the runner properly it doesn't matter.

 

Thank you Doom86, I would be extremely grateful if you could send the pics of the ported 416's to mikesammy@rocketmail.com. Also, would the 1.94 valves from the 400 heads be big enough for the 416 heads?

 

https://www.thirdgen.org/forums/tech...416-heads.html

Scroll down to post 32, and 33, that is what they should look like for a daily driver with 1.94 intakes. The ports still need light cleanup but you get the idea of shapes. The runner is 170cc in that pic. The runner should be cut tall so the long turn from the roof to the bowl can be as long and gentle as possible.

I made the mistake of opening the bowl up before the cut for bigger valves and misjudged how wide I should go in the bowl, but at the stage I pointed you to in the thread is where you want. The finished product was probably more suited for 2.02 valves given I opened up the push-rod wall all the way to the bowl and finished around 180cc, there was plenty left in this space because I barely even touched it, nothing even past the push-rods.

 

I went from a set of 624 heads (which are basically the same as your 400ci heads) to a set of 601 heads (305 heads) with only 1.94/1.60 valves added, and it made a HUGE difference on my 350. Top end was about the same, as the ports are more or less the same, so they both run out around 5500rpm but just from the jump in compression and better chamber design alone I got a VERY big improvement in torque right from idle up to about 4500 or so, and the throttle response was much better. I had to go from regular fuel to premium because of the compression boost, but fuel mileage actually improved enough to almost balance it out too lol.

Go with the 305 heads.