I am building a big block 468 for my trans am and need to raise my compression from 10:1 to 10.5 -11::1 to fit the size of the camshaft. I am using Edelbrock performer Rpm rectangular port heads p# 60559 disquised a GM series w/ 118 cc combustion chambers. From using a calculator online Ive figured that milling my head about .025, with a straight cut no fancy angle milling, will give me compresssion around 10.6:1.
I want to know are there any drawbacks to milling a head? Im only using a piston w/ a 23cc dome but does milling my head bring the valve and piston closer? is this a problem? Ive also read that i will need to mill the intake port side of my intake and the bottom of it. Why and how hard is this to do, Can i do it w/ the same mill im using for my heads? and i dont really understand what people are talking about when they talk about milling the intake i cant get a good visual picture of what needs to be cut and where. Anybody know of any good pictures or websites that explain it more? what happens if i dont mill the intake ?
thanks

 

You've heard all of the issues. You need to check piston to valve clearance, especially if you use a high-lift cam. Think what happens when you mill the heads - the intake mount bolts are moved farther down, along with the ports. If you don't mill the intake, the end seals will be squished out when you bolt down the intake. The intake will be held up higher by the block and ports not align quite right. When you mill the ends of the intake, that makes the "V" of the intake ports too wide for where the heads are. So, a little off of the ends, and a little off of the sides. Kind of like a hair cut (except the intake won't grow back).

.025" isn't a lot, so it probably won't be a big issue. But, any competent machine shop should be able to figure out how much to take off of the intake based on how much is taken off of the heads.

Depending upon how far down the pistons are in the hole, it might make more sense to deck the block instead. This can make for more effective quench to reduce the tendancy for detonation.

 

ya my block is already 0-decked and im going to be using a .040 head gasket so quench is already near perfect. I didnt realize i needed more compression till i had already built my shortblock and another 750$ set of pistons is out of the question. my cam has .595 lift w/ 294/294 duration and the pistons only have a 23cc dome so the valve shouldnt be open far enough to hit the piston when the pistons at TDC if im only milling .025 off? Im using a 454 stroker motor as a comparison where with the 4.25 stroke the piston moves up .040 in the bore and at TDC is .04 higher into the combustion chamber and a lot of times use larger domed pistons and have larger cams with lots of duration w/o having an opening intake valve hit the piston so since my motor is using a 4.00 stroke and has a smallish dome and im only milling .025 off this shouldn't be a problem? or am i confusing concepts, i read that in a book a little back about the increased stroke. How do i check PTV clearance, do u think i need to?

 

use playdoh to check piston to valve clearance. Good idea to check it now, before you mill. put playdoh on a piston, bolt the head on with 4 bolts or so, and rotate the motor over a few times. Then measure the indent in the playdoh, see how much is left. It'll be easy to eyeball once you see it.
Then imagine everything down an extra .025", and figure if it'll be doable.

You're doing the machining at home? You have a big enough mill and flycutter to cut the heads and keep it flat? I usually see 12" diameter flycutters used on this sorta thing...

 

IMO,your going to a bunch of trouble for very little gain.I've ran that exact same cam,which is a Comp 294S,if you doubt me here,at 10.2:1,with excellent results.Just my 2 pennies,but just run it as is,save yourself the trouble,and be absolutely thrilled with the way a bigblock 3rd gen runs,even though you may lose 10hp from being just a fuzz shy of the "ideal" compression ratio.Just means that much better chance of getting by on junk gas.

 

Unless you have access to high octane gas (more than 93-94 octane) you're making more trouble than its worth. As long as your at or near 10:1 that cam will work fine. Raising the cr will just move you too close to the point of auto ignition ( detonation). You won't get the power increase theory suggests. Look what the Pro engine builders build their custom street motors too. (not ones used in magazine dyno contests). You'll see most are 9.5:1 especially on big bore motors like the BBC. Concentrate on breathing improvements rather than trying to get every ounce of compression ratio. Much more power to be had, without the reliability penalty.
Normally the 294s does not cause valve to piston clearance problems in BBC's. But since you have 0 decked the block and are hell bent on milling the heads, your VTP clearance will be reduced and deserves checking before running the motor. I use the .100" down from 0 valve lash method.
I set the intake and exhaust valve to exactly 0 lash on #1 cylinder, then using a dial indicator on the valve retainer, I further tighten down the rocker nut to open the valve +.100" on both the intake and exhaust. then SLOWLY rotate the crankshaft BY HAND 2 complete revolutions. if the valves do not contact the piston or retainer smack the valveguide tops while rotating the crank around BY HAND Your motor has at least the minimum required valve train interference clearance. Its more then .100". To ensure the camshaft/valve are properly oriented for adjustment, rotate the crank till the intake valve just closes {on seat}. Set the exhaust valve lash. Now rotate the crank till the exhaust valve just begines to open, set the intake valve lash.
Advancing or retarding the camshaft will effect the in motor valve to piston clearance. Can be used to gain clearance in a borderline clearance situation.

 

ok well im rethinking milling my head and i can see how its gonna give me more of a headache than help. So well see but i think im scrapping the idea