I'm sending the head of my 1961 225 slant six in for a valve job, hardened seats and a general going through. Should I also have them shave the head for a little more compression? If so, how much and still run on 87 octane? I'm not sure if it would make any difference but I'm also currently running the Offenhauser dual 1bbl carb set up with 2.25" exhaust

Can't see any harm in getting it to 9.5:1 comp.
I'm sending my head in as well for the same stuff but I am going to get the head shaved to 10:1 comp. approx 0.100" off the head is going to do this, still have to take more measurements.

Out of interest how much is that going to cost you?

well I just got off the phone with the machine shop and he said about $150 barring surprise damage to guides, valves etc.. that doesn't include any major shaving, just enough to ensure that it's flat. If you're taking off .100 and it gives you 10:1 compression I would probably have .030 taken off just to be safe. Sound logical? If anybody here thinks this is a bad idea please stop me now.

Taking .080" off my cylinder head got me up to 8.5:1. You must measure the piston to deck clearance and the combustion chamber size to know what the current compression ratio is and what it would be with a given reduction in cylinder head thickness. I had to have .080" taken off the block and .100" off the head to reach 10:1.

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Joshua

Yep, Totally agree.

I've been told between 0.080" and 0.200" to get to 10:1 compression.
Think it's definitely worth it though unless you want to get high comp pistons etc.

Since I'm not supercharging or turbocharging I'm not concerning myself with the exact compression ratio. I've always heard that there is a substantial amount of milling that can be done on these engines before they become boat anchors. This car doesn't get driven often enough or hard enough for me to want to explore those limits. I just figured while he's flattening the head he might as well improve it a little.

Just be sure that if you are removing an original steel shim head gasket and replacing it with a newer/thicker head gasket the amount that gets shaved off needs to be added to that difference or you may end up with a wash or less compression...If you are trying to improve a little be safe and do some measuring. Good luck, dual one barrels are neat!

artracer.com,

There are several compression calculators on line to help with determining a ratio. Measure the volume of each of the head’s combustion chambers in cc’s.

Volume can be determined by covering the cavity with a small square of plexi glass stuck down by a skim of grease with a 1/8th inch hole drilled in it. (get a chunk of scarp from a hardware or glass shop) Inject colored water into the plexi covered chamber (with valves still in place) via a 60 cc veterinarian syringe. Tractor Supply sells them two for less than five bucks.

You will come up with about 54 cc’s, if I remember correctly, for a stock unmolested head, less if it has been previously shaved. Someone will correct me if I’m wrong.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

54 cc is the "blueprint" number that the factory used to calculate the 8.4 to 1 advertised compression ratio.
Trouble is, most factory heads have larger chambers, when you actually measure them, hence, a lower compression engine.

The difference between a shim and a composition head gasket is roughly .025 so in my mind, that is a minimum cut to keep your current CR.

As noted, a little more CR will help so ask the shop how much they are willing to take for their "standard" charge... I can usually get them to take .030 to .050, after that, they want to charge me extra for more "passes".

So all that aside... here is the quick answer, cut .060
This is my best "guess" based on many early engines I have measured.
I use 58 cc for the factory head (not 54 cc) and -.160 and the piston deck height. (Not -.140 as advertised)
With those numbers, we have a 7.9 static CR and a 7.3 dynamic CR based on a stock cam.
Cut .060 off this combination, you end-up with 8.67 CR and 8.025 dynamic CR, that about right for 87 oct. pump gas.
(check your engine to these numbers)
DD

the gasket kit that I have appears to include the thicker composit head gasket. Do I spray head gasket glue on these types as well? Should I instead search for the "shim" style?

It is best to have both the block and head surfaces clean & flat (freshly machined) when using a steel shim head gasket.
A thicker composit gasket is more "forgiving" on used surfaces.
DD

If you want/need a steel shim gasket I have some available. OEM new old stock Mopar.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
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64 Valiant 4dr 225