hi, so im doing a little intelectual planning. so far im going with a autolite 4100 weiand intake, GT40s, and some headers.
my question is, if i were to mill the GT40s down the .03 or whatever the number was to bring it into combra specs, would that make me have to run primium or anything? right now i have a 73 302, so my compression is prety low. id like to be able to run regular gas still.
As 67mustang302 said it depends on a lot of things. To add to his list, piston shape, timing advance, and condition of the head chambers can also play a huge factor in to CR and which fuel octane you can run. All that said, here is some info on how things affect required octane.
octane To start with octane is the stability rating of the fuel. the higher the octane the more stable it is so that things other than spark do not detonate the mix at the wrong time. With that said a lower octane fuel burns better so premium is not "better" in spite of it name it is simply suited for a particular type of motor set up.
Cam If you have a cam with a lot of overlap designed to suck extra fuel and air in to the cyl than in "effect" you are fitting more mix in the cyl which is like upping the compression. (to head off the nitpickers, compression is a fixed ratio and increasing or decreasing the fuel air mix in the cyl does not change CR, however fitting more fuel and air in to the cyl makes the system react the same as if there was more CR)
cooling Any part of the combustion chamber, including the cyl walls, piston face, valve faces, sparkplugs, basically any part that comes in contact with the compressed fuel air mix can detonate the mix prematurely if it gets hot enough, so adequate cooling will help eliminate hot spots which in turn will reduce unwanted detonation.
Piston shape Flat top pistons create a shape where the compressed fuel mix is more stable. If you used domed pistons than the shape of the mix is less stable. So if you have two motors with 9.5:1 CR one got there used domed pistons and the other got there with flat toped pistons and a smaller chamber than the second motor will be able to run the lower octane fuel.
Timing advance. When fuel burns it expands and that’s what drives the piston. the more advance you run the sooner the mix is lit and the more expanding gasses are compressed. Like with the cam CR technically never changes but by adding more advance a greater volume of hot expanding gasses are compressed in to the chamber creating a result similar to more CR
Condition of the chamber in the heads casting marks, sharp edges and rough spots can get hotter than smother cleaner sections of the head. If little imperfections are there then they can glow read hot and detonate the fuel. This is why a motor with heads that have had a port and polish can run lower octane fuel than a motor with unmodified heads and the same CR and piston shape
Aluminum head combine several of this things including, cooling, chamber shape, and reducing hot spots.
SOOOOOO to answer your question if you are running flat top pistons, a mid range cam, no more than 10 degrees advance and end up with around 9.5:1 compression you can probably use low grade pump gas.
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In addition, I wouldn't recommend milling the heads any more than .010". More than that can cause issues with intake geometry, resulting in vacuum leaks if you don't have the intake machined to match
if his 73 bottom end is stock he has plenty of headroom to mill those gt40 heads and stay within an 87 octane range. you can take more than .010 without worrying about leaky intake gaskets, especially if your using QUALITY gaskets and not sticking that stupid piece of cork on the front and back.
well, by bottom end is staying stock until i dont have to use it as a DD and or i get more cash flow ha. every thing else is pretty much stoc right now as well. and from what ive read the 73s have pretty low comression, and that decrease had to come from somwhere, and im just thinking of reverseing a little bit of that. thanks for all the info so far though!
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how high can compression go before before more octane i... - 
