shaved head
- Thread starter briggs33
- Start date
To get a useable number regarding the head volume [on these low compression engines] I use a hypodermic syringe. They are marked in cubic centimeters [cc] but that can easily be converted to cubic inches. Leave the valves and spark plug in the head. Turn it upside down and get it as level as possible. Repeatedly fill the syringe with rubbing alcohol [available in the grocery store] and squirt it into the head chamber. When full you'll have the number of cc's that it contains. There are some high zoot ways to get the volume but its not necessary unless you are on the leading edge of spark engine technology....we ain't. Then measure the thickness of the head gasket and determine its cut-out area. Add that little bit of combustion space to the head volume. Then get the bore and stroke info and determine that "swept" volume of the pistion. Divide the little number into the big number and you have the mechanical compression ratio. Then fool around with the math and determine how much you need to take off the head to get the higher ratio.
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I know it is very common to shave the head but doing so tends to impeed flow and will require even more machine work like valve releifs and deck work
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I have found that if you ignore trying to raise compression and focus on flow you will be better off but keep in mind that porting has its limits too if you go to large you will loose velocity so so sum this all up I would recomend just cleaning up the ports without getting to crazy finish is important to you dont want a mirror finish in your intake ports a slightly matte finish helps atomize the fuel air charge
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of course I could be completely wrong about all of this go with your gut feeling
do you have a ohv engine or a flat head? my comments are in reference to a flathead
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I have found that if you ignore trying to raise compression and focus on flow you will be better off but keep in mind that porting has its limits too if you go to large you will loose velocity so so sum this all up I would recomend just cleaning up the ports without getting to crazy finish is important to you dont want a mirror finish in your intake ports a slightly matte finish helps atomize the fuel air charge
.
of course I could be completely wrong about all of this go with your gut feeling
do you have a ohv engine or a flat head? my comments are in reference to a flathead
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You are dead right about getting the combustion chamber roof too close to the valves [on a flathead] when the head is milled. Flathead motors are a giant compromise between better breathing and high compression ratio. As I recall Ford flatmotors had to have a lot of work done to relieve around the valves so that flow was not too badly impeded if a lot of milling was done. My belief is that for the really low compression engines we are dealing with the cheapest way to make horsepower, without the work necessary to increase the rev limit or increase flow, is to increase the compression ratio.
If you really, really want an accurate chamber volume, get a piece of 1/8 or 1/4" Plexiglass or polycarbonate(Lexan) sheet big enough to cover the chamber. Drill a 1/16 or 1/8" hole in a spot near the edge of the chamber. Spread a thin layer of grease or petroleum jelly around the chamber edge to seal the plastic, position the fill hole right at the edge of the chamber, and start filling it with one of those syringes. Congratulations, you've just build a chamber cc'ing tool that would look right at home next to Oldsalt's degree wheel. :wink:
Regarding milling, many say that going too far will kill the flow on a stock head. Increasing displacement seems to be the best and unfortunately most expensive way to raise compression on a flathead.
Regarding milling, many say that going too far will kill the flow on a stock head. Increasing displacement seems to be the best and unfortunately most expensive way to raise compression on a flathead.
You should be able to shave off a large amount if needed. But i can't tell you how much. It's best to simply measure the height of the valves at the highest opening possition. (usually at the port side of the block)
Then measure how deep the head combustion chamber is. Now you can mill off the difference, or mill the difference plus half the head gasket thickness.
For best performance it make more power if you fully ported the ports, and cut and blended the eyebrow area. Then do the mill job.
Then measure how deep the head combustion chamber is. Now you can mill off the difference, or mill the difference plus half the head gasket thickness.
For best performance it make more power if you fully ported the ports, and cut and blended the eyebrow area. Then do the mill job.