compression ratio

1931 flamingo

1931 flamingo

Well-known member
I have enjoyed the recent dialogue about hi-compression heads, Mr Brumfield's input/knowledge, etc. Question? If we take a stock model A head and "shave" it .125-.150 (or more if possible), would we not be increasing the compression ratio, or not enough to justify expense. Of course, I would imagine with a new hi-comp head the design of the combustion chamber has a lot to do with it also. I don't want to open a can of worms, just curious.
Paul in CT
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It would, indeedy, increase the compression ratio.
The police and aftermarket heads' manufacturers would tell you that they have also developed the perfect head chamber to make the gas mixture swirl around and create the smoothest burn and power. Probably something in the argurement, too!
Your "shaved" head could also interfere with the pistons as they reach tdc. You'd know. It would make a funny noise!!
I once had a nice Ford script head that had been shave down until the relief above the pistons was gone completely. I did not try to use it, but maybe I missed a bet.
 
'I once had a nice Ford script head that had been shave down until the relief above the pistons was gone completely. I did not try to use it, but maybe I missed a bet."

I have one like that. I ran it on my roadster for several years and it ran very well. It blew a head gasket (between 3 & 4 ) and I did not have it checked for flatness so I did not re-install it. Have no idea what the compression ratio is. Any idea what it might be?
 
1931 flamingo said:
I have enjoyed the recent dialogue about hi-compression heads, Mr Brumfield's input/knowledge, etc. Question? If we take a stock model A head and "shave" it .125-.150 (or more if possible), would we not be increasing the compression ratio, or not enough to justify expense. Of course, I would imagine with a new hi-comp head the design of the combustion chamber has a lot to do with it also. I don't want to open a can of worms, just curious.
Paul in CT
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You cannot machine down a stock head enough to make a noticeable difference. The combustion chamber size is very different in a high compression head, and it has only about half the volume of the stock head. If you machined a stock head that far, you would be well into the water jacket. Pony up for a new head, it is worth the money.
 
2manycars said:
You cannot machine down a stock head enough to make a noticeable difference. The combustion chamber size is very different in a high compression head, and it has only about half the volume of the stock head. If you machined a stock head that far, you would be well into the water jacket. Pony up for a new head, it is worth the money.
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When some people discuss their concern regarding the relief over the pistons they do not take into consideration the thickness of the compressed head gasket. The ones I use have a compressed thickness of .060 according to the manufacturer, Best Gaskets. The manufacturer's name is posted as a reference to my statement and not to advertise or endorse a product.
 
Hi just posted in the other thread about bearing loading so I won't belabor that point!
.120-.150 WOW! But I guess you can take this much off a ModA head. Back in the day, I used to cut Greymarine 4s (Continental 163) and .010-015 with quite a noticable change, so much that I had to have the prop reblocked from a 12/12 to a 12/13 to slow the engine back down to 3200 RPM. The little 16" Correct Craft would fly with a 13" pitch to the prop! (Or it felt like it was flying anyway)
I have always wondered about the Deck height VS the piston protrusion beyond. My conclusion was that the pistons are flush with the deck height (Except on a flathead V8 where the pistons have a conical protusion above the deck height). Here you would have interference. But where the tops of the pistons are flat. no matter what you took off the the head shouldn't matter. Plus we can add the approximetly .060 for the head gasket. However! let us not forget the valves! They go up and down too. Shave the head too much and it isn't the pistons that will go bangedy bang bang, It's the valves! And with almost the same amount of destruction! Find a way of cheaply and accuratley filling some of the enormous head cavity before you chop .150 of your head. ( .150= .006 less than 5/32 .156) Better still, buy a HC head and leave yours stock.
Terry
 
Here is some info on milling stock model A heads. I think I gleaned this info a few years back off the Ford barn. I didn't list who posted this info I think it was Larry, but it may have been Ned. Here goes. Stock ratio 4.22 mill .025 and you get 4.40 , mill.050 you get 4.59 , mill.075 you get 4.81 and if you mill .100 you get a compression ratio of 5.04

The minimum thickness of the head after millingis 240 Measure the thickness of the casting from the underside in the edges of the water jacket holes near the distributor near stud holes 4 and 5 . The original thickness of the unmilled head was 394 .

On a stock engine the pistons rise about .030 to .032 above deck height. I think that the metal clad gaskets are about .060 when torqued down. The un milled head has a counter bore or squish area above the piston. I am not sure what the depth of the original counter bore was. I have had heads milled so that the counterbore depth was .030 with good results. That is if the block hasn't been decked and the pistons don't rise above deck height more than .032 . With the metal clad gasket this will give around .028 to .030 clearance between the piston and the head counterbore. this won't necessarily work with the composite B gasket because they are not as thick when torqued.

If you have pistons that don't rise above deck height. you may gain a little increase. If bored way out you will get more.

I don't think it is worth the trouble. It would be best to buy a higher compression head.
 
The New Ford Barn as a SEARCH at the top. It works real nice just type in a key word.
I think this is the 3rd time I posted this pic
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If you mill the hell out of a stock head (or any head for that matter) you will make it more prone to warpage and cracking.

Moreover, a high compression head made to maximize performance, such as a Brumfield Head, is more than just an increase in compression. There's greater air flow, an increase in turbulence, etc., etc. that combine with the compression increase to make them really go. You can't get that from simply milling a stock head. Also, depending on how much is milled, the dist shaft may have to be shortened and the studs may protrude way high above the nuts and look odd in my opinion. Of course the studs could be shortened as well.


Larry B.
 
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Estimates I've seen are too high. The average depth of the original counterbore was .093" on the couple I've checked. That means the most gain from milling is that first .093" because beyond that point you are removing much less volume for a given depth. Removing that first .093" should reduce the chambers to about 218 cc and provide a ratio of about 4.6:1. As noted that would leave a static piston clearance of about .018"-.028" on a block that hasn't been decked depending on the gasket used.

It's not worth it in my opinion.
 
Hi Marco. I see you're still doing a great job of correcting all the misinformation inherent with these message boards. It's a never ending battle.

Yes, depending on the gasket and the original deck height, you're correct. The Fel-Pro 7013 would probably allow enough clearance and would probably work in many cases. A quench distance of .035" is the minimum practical to run without engine damage on an engine with tight bearings and pistons, steel rods and modest RPM.


Larry B.
 
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