Do C heads give significant performance improvements or other benefits over Gs, assuming same valve sizes, no porting and mild engine builds or is the difference negligible for a street engine?

The only two advantages I've seen in favour of the G's are hardened valve seats (which the C's should get during a rebuild anyhow) and the use of valve rotators to extend valve life.

 

In stock form, all BBO heads except J castings probably flow within 10% or so of each other. You won't feel the difference if you swap heads, assuming both have the same size valves.

 

There are some differences between the heads. For example, the C head in stock form has a volume of 177ccs, whereas the stock G head has 169cc runners. The difference in volume seems to be in the roof near the guide. The G head has a noticeably shorter roof in that area. However, as Joe stated, can't see it making much of a difference on the butt dyno.

I wouldn't worry about the induction hardened seat aspect. If you install larger valves, or even do a valve job on a set of G heads, you will probably be past the induction hardened surface. They are not actual seat inserts like machine shops install today. Besides, I have never seen a sunken exhaust valve on a stock set of heads, and I have seen a lot of Oldsmobile heads. And the rotators are just extra weight on the valvetrain.

 

I disagree with your analysis of the C/G intake port above. Measure the port from the deck and see. I do agree that it is nothing to get in a twist over, less than a few CFM/horsepower.
The induction hardened seats are nice to have, but I have seen many castings of ALL vintages with sunken seats. However I live where ambient temp hovers around 110 for months on end. The induction hardening should be .050 deep from the factory and if a machinst digs this deep in one valve job, he better have a reason. Lots of people say it will be gone but have they ever ground seats? Grinding should only take off up to .025, MAX. If the engine was towing a trailer, or had marginal cooling it would be worse off. It's not going to be an issue in Canada at all where OP resides.

What you/ your machinist should really check are the valve spring seat depths.

 

I did measure from the deck surface to port. I got .160" difference from C to G......hence the reason for commenting on the thread on the difference between ports. What did you come up with for port volume when you measured the ports? And if you had any difference, where did you find it to be? And moreover, did you even measure the port volume??!??!?!??

The reason you probably see seats sunken in on all vintage heads is because back in the day, every corner auto repair facility had a valve grinder/seat grinder.......it was common practice to restore heads after 70k-100k miles or so (maybe you saw heads with 3 valve jobs already). And when you say sunken, we are not talking way up in the head where an insert is mandatory, right? On top of that, when we talk performance heads today with our Oldsmobiles, it usually consists of oversized valves......and a 1.71 exhaust valve probably goes past the induction hardening from the factory. And yes.....I grind seats.

So tell me what me and my machinist should check on valve spring seat depths. Or how about what I should check.....

 

There are subtle differences in small block and big block heads. Dave the Freak has been porting a lot of heads, and we have been analyzing all of them.......lots of interesting things we came up with, especially the short turn on small block stuff.

But hey, what do we know?

 

Lighten UP

My 70 deg cutters would always hit the long side bowl on a B/C casting, never hit long side of bowl on later castings. Your #'s look correct to castings. C casting should have the lower port (.160 like you said), G has higher taller bowl.
Valve seat depth comment is for the OP on G heads.
When I say sunken, I mean .200" down into the port, YES. The deeper the owners foot was into the throttle on a continual basis did this. Heat, load, throttle. Not speculating, a fact in AZ. Even seen BBC truck heads with ind hardened seats sunken. Chevy eventually went to machined inserts in factory heads.

"The reason you probably see seats sunken in on all vintage heads is because back in the day, every corner auto repair facility had a valve grinder/seat grinder.......it was common practice to restore heads after 70k-100k miles or so (maybe you saw heads with 3 valve jobs already)."
Really? I can tell factory cut guides, seats, deck. Don't speculate, I was not being snotty.