In keeping with Ford original heads, I'm aware that the early low head produced a higher compression ratio compared to the later high head. My question is, who among us has shaved a low head, and to what limit (depth), to get higher compression comparable with the "Z" head ??? Is this possible ??
Bob, I have had one decked. It depends largely on how much corrosion is inside the water jacket.You don't have a lot of metal there. I have no idea how much it helped on mine,but it runs.
Boy, I'm eagerly awaiting the responses to this one. I don't think that you'd get the performance even if you could mill a stock low head enough to match the compression. The knock against stock low heads, is that they don't have the coolant capacity of the later and Z heads. I hate to think how difficult it would be to keep an engine cool with an extreme milled low head. Plus, the period and modern aftermarket performance heads have much better combustion chamber designs than the Ford heads. It's interesting to note that Ford used a Ricardo combustion chamber design on the Model A, very similar to the Wuakesha Ricardo head for Model Ts.
Eric ,I did it out of necessity,so I had a good gasket surface. I have two aluminum heads,one "Z" and another that is 6.5 to 1. So far I haven't run either.
How much did you mill the low head? By the looks of mine, it could get by with an 1/8th" removed, but I'm not sure it would make a 1/4". I routinely take an 1/8" off the high heads without problems. Taking 1/4" off a high head is hit or miss. Low heads are too expensive for much experimentation.
I removed 3/32 from my 1914 low head. I did so because the previous owner screwed up and left a 0.010 step in the heads surface after he had to take two cuts. It squirted gas from this lip the first time I tried to fire up the rebuilt engine. I don't know how much the previous cut was.
I do know that I can't windmill my engine as some do. I must pull through 1 at a time.The engine runs great and will maintain 45 mph as clocked by another passenger car. It has been in parades and hasn't overheated, even summer in Oklahoma.
I've read where some have milled through the water jacket and simply let the head gasket seal off the opening!!
I'd estimate I've taken off a total of 1/8 or more on my low head without any consequences.
I even going to try a low head on my A cranked new engine for the '14. I need to add a starter.
Oh, yes, It went up Pike's Peak with out any overheating!!
I milled a low head .150. I won't mill that much again. At .100 a dark spot appeared between no.2 and 3 cylinders. I hoped it would go away. It didn't. At .150 it was a hole about 3/4" in dia. I welded the hole up and used the head. It appears the casting is thin in that area. Plus we don't know how much of the head has been eaten away in the water jacket in the past 90 plus years. Or how many times it was milled previously.
According to my calculations, each 0.025" cut increases CR 0.1 point. An uncut, later low head with 0.100" removed will have the CR rise from 4.1:1 to 4.5:1. If 0.150" can be removed, 4.7:1.
How does one go about milling a head? Can it simply be done by squaring up a head on a mill table and milling it with a flycutter? I have heard that T heads bend under the pressure if not supported in the middle.
My dad had 3/16" taken off a low head that we ran on our '15 roadster for years without trouble. This would have been about 1974. That head was on the car until about 1995. At that point I found water in the oil when I overheated on a tour near Lake Tahoe, and the head gasket was blown. I changed it, then overheated again a couple of days later. When I returned from the tour I pulled the head and found a crack between the #2 & #3 cylinders that was about 3"long. I changed the head and never had any more problems. The head was put into service as a great wheel chock after that.
Milling a head won't cause overheating problems. In fact raising the compression ratio generally lowers the amount of heat transfered to the coolant.
A good high performance head is more than a higher compression ratio though. It also has better breathing and improved combustion.
That said, our 1916 currently has a low head which I believe was milled to flatten it, not sure exactly how much. It also has a Stipe 250 cam and NH carburetor, and will run 40+ mph on level ground with no problems. I haven't run faster due to lack of confidence in the wheels.
All interesting responses...... In the relative "age" scheme of things, a low head having thinner casting due to rust in the water jacketing is a major concern, maybe more than on a high head. Compression pressure means little unless directed by combustion head/cylinder shape. Original Ford low heads may give higher cylinder compression, but combustion chamber shape won't give more "umph"
We have a '26 with a "Z" head, and working on another keeping with stock parts. Plan on using the low head.
I'd agree with Seth. You can't mill enough off any head to increase CR much more than a few tenths of a point. More can be gained by increasing the bore and/or stroke than milling a head (safely).
Forgot to add:
If you really want spark plug popin' compression, make your own head.
True - the early low head (until about '12?) was 4.5:1 CR. High compression pistons would bring that 4.5 up to about 5:1 - assuming that rare and pricey early low head has not been machined so much that the HC pistons can't be used.
With today's fuel, I'd be willing to bet you'd feel the difference in "umph" with a 20-25% boost in CR, regardless of the combustion chamber.
Bob, I have tried both style heads (low style and Z head). A friend of mine loaned me his low head to use, and i did feel a small difference over the standard high head. However, when I put the Z head on, the difference was very noticeable. I probably added 5 to 8 MPH on the top end, and now I can pull all but the most ornery hills in high gear.
IMHO, I think the key is more in the combustion chamber, not so much the height of the head. I would think you can't shave enough off of the low head off to bring the compression ratio up to match the Z head. And with the rarity and therefore value of the low heads (and especially since this one is not mine!) I would not be tempted to try shaving it down.
Thanks Bob, I'll not expect too much from the low head after driving the '26 with the "Z" head.