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Old 08-16-2021, 08:21 AM   #1
philcobill
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Question High Compression Head Replacement

We are considering getting a high compression (actually more of a medium compression) head to replace the aluminum head on our Model A's engine. Specifically I have my eye on the 5.5. to 1 ratio head offered by some of the on-line model a stores. I assume that the aluminum head already on the engine is high compression. (we measured the compressions on each cylinder and they were around 75 psi each.) Should I be able to just simply replace the head or are there other considerations that would need to be done with the other internal engine parts, given that it already had a high compression head? If we do this we will get the higher quality studs etc. What about a gasket? What is recommended? I think that Snyders had one they recommend.

If we went up to a 6.0 to 1 ratio head is that a game changer?


Bill
Walworth, New York

Last edited by philcobill; 08-17-2021 at 11:48 AM.
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Old 08-16-2021, 08:45 AM   #2
alexiskai
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Default Re: High Compression Head Replacement

I personally would keep the head you have, both because at 75psi it's already above the stock compression (so the change you see from the Snyder's head will be reduced) and because it seems to be a vintage aftermarket head, which gives the car some extra personality. Unless your goal is to shift it back to looking absolutely stock, or your existing head is cracked or unusable, in which case the 6:1 head is a fine option.
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Old 08-16-2021, 10:34 AM   #3
J Franklin
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Default Re: High Compression Head Replacement

I agree, keep what you have if it is working.
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Old 08-16-2021, 10:36 AM   #4
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Default Re: High Compression Head Replacement

If using an aluminum head be sure to coat the studs and the insides of the stud holes w/anti-sieze, to facilitate later removal of the head.
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Old 08-16-2021, 10:42 AM   #5
alexiskai
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Default Re: High Compression Head Replacement

Also make sure you have a fresh waste metal cathode.
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Old 08-16-2021, 10:50 AM   #6
Jim Brierley
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Default Re: High Compression Head Replacement

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Compression doesn't hurt anything in your engine, detonation will kill it. Use less spark advance, and don't let it knock.
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Old 08-16-2021, 11:13 AM   #7
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Default Re: High Compression Head Replacement

Suggest you leave it like it is why change?
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Old 08-16-2021, 11:40 AM   #8
Ernie Vitucci
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Default Re: High Compression Head Replacement

Good Morning all...I have read in at least one book that 75 pounds is stock compression...not sure this is correct...what is the collective wisdom on this subject? Ernie in Arizona
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Old 08-16-2021, 11:49 AM   #9
oldredford
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Default Re: High Compression Head Replacement

The new heads have a better combustion and intake flow. The difference between stock and 6.1 is amazing. Also if you plan to install a Mitchel overdrive you need the higher compression head.
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Old 08-16-2021, 11:51 AM   #10
nkaminar
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Default Re: High Compression Head Replacement

I believe that the 5.5:1 or 6:1 heads from Snyders will provide about 110 psi compression. I could be wrong. One advantage is that they are cast iron and not aluminum so you will not have to worry about galvanic corrosion. I used the graphite head gasket recommended for the high compression head with the copper spray gasket cement. This worked the best for me of any of the various head gaskets.

The high compression by itself will not cause any problems in the bottom end if the bearings are in good shape. Cruising at part throttle does not add significantly more force on the rods and crank. Of course, an engine built for cruising will last longer.

There is more to head design than the compression ratio. The combustion chamber design is important as well as the clearance for the gas flow through the valves. In my opinion, the Snyders heads are a good design. I don't think there is too much difference between the performance of the 5.5:1 and 6:1 heads.
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Old 08-16-2021, 12:44 PM   #11
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Default Re: High Compression Head Replacement

75 would be high for stock compression, people typically report 55-65. Somebody on this board reported 45 a couple days ago, although I'd be a little concerned about a number like that. Similarly 110 is more than I would expect from a 5.5 head. Maybe 100?

The OP is ultimately the one to weigh the tradeoffs. You'd get slightly better performance from a 6:1 head, but is it worth the cost of the part, plus shipping, plus you might need to have it milled flat, plus you gotta paint it, and you're off the road while you're waiting for it, etc., etc.
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Old 08-16-2021, 04:39 PM   #12
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Default Re: High Compression Head Replacement

I had my 6:1 Snyder head checked for flatness at an auto machine shop and he said it was very flat. Don’t use antifreeze with the Best graphite gasket for a few hundred miles, after that it’s ok.
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