milling heads for comp ratio increase

nm9stheham said
I'll apologize if I have missed this, but I don't have a clear picture of what heads you actually have. Where are you getting the info that the heads are/were 60 cc's? If you are reading that off of a lot of sites, they spec out the NHRA minimums, not actual factory cc's.

The only stock 273 heads with nearly that small a chamber are those with closed chambers. But what I see here are the milling numbers you need for removing so many cc's from OPEN chamber heads.

And what pistons? Are they flat or do they have a mild dome on them? If we don't look into all of this, I fear you are waay off in your assessment of CR.

I see that you are going away from the milling idea. So much of this may be not useful to you anymore. But know that the 3% HP change versus 1 point in CR is quite misleading:
- That 3% number comes from a Wallace online calculator that makes oversimplying assumptions and looks at only one aspect of HP changes from CR, and so is not accurate to start with
- The number is ONLY for peak HP. It says nothing about HP and torque at lower RPM's. If you are road racing, rallying, or street cruising, then those lower RPM numbers are what are important, not the peak. BigBlock Mopar referred to this in post #38. BTW, how are you using this vehicle?

Thanks for your post - Answer to the last question first - I might have already explained that previously - it is going into a 1970 ute (what you call a pickup) which is a daily driver and is also on the light side, 2750 pounds. And I don't race or overly heavy on the throttle constantly like some people. But I do have a bit of fun every so often.

CR, yes I do have the open chamber heads, as I will explain now. Well I got this info from several mopar sites that list the different casting numbers of the heads. I have seen a lot of differing information. Eg that Allpar site is very sloppy with info, I have seen around 4 errors or differing numbers in just the engine area.

Quote from Paul Pitcher Porting LA Cylinder Heads. "Production heads used on 64-65 LA engines have closed chambers 57-65cc in volume... with average closer to 64cc"

Table 1. Chrysler Small Block Cylinder Head Volumes (cc)

Casting Number Year Chamber Intake Port Exhaust Port
2465315/2658920 65-66 64.5 60 127
2843675 67-70 68 65 138

3418915 71-72 71 69 149
4027596 80-84 71 65 149
4323345 85-91 74 62 150
4323302 85-91 62 54 118

The table above has different numbers to the info I saw, which was that the early 64-65 273 had closed chamber heads and they are saying they had 57cc Comb chambers for a 9.2:1 CR. 1966 and 67 each had different casting numbers - not sure what type of head (maybe something to do with intake bolt hole angles), 1968-69 they went to the hydraulic cam and open chamber, also used on the 318, casting 2843675. Which is what I have and were 60cc (? That's what it said - must be wrong) and 8.8:1 . Because I was originally going to use a Felpro 318 head gasket I took off a couple of tenths of CR for that. My engine is just the standard 180hp 2BBL. So the pistons are the flat top (with valve reliefs). Yes I should work out the CR accurately for my engine and cc every Comb chamber. What is actual and what is written on sites can vary. I have to work out how to do that with what I have here at home, as I don't want to buy the kit. It shouldn't be too hard to do with some thought.