Good or bad idea...?

[flat broke]

In the two stroke world, milling the head to increase the compression ratio is a very common practice. How does that cary over into the 4 stroke world, speficially with BBCs? Obviously its done any time heads become warped etc, but is this an acceptable practice when done soley for the purpose of raising the CR? What kind of thicknesses are typcically removed and what kind of alignment problems can this cause with the intake manifold?
Chris

[78Eliminator]

Also, think about the valves kissing the pistons.

[hbhallett]

I rently had to go through this (not by choice) but due to a damaged head which I was trying to salvage. In my case the head required about .020" of surface removal to restore it to an opearble condition. This much material removal actually resulted in a slight crescent of one of the valave actually removed during the machining process. I was actually quite fortunate in not having to formally machine the Edelbrock Performer RPM intake manifold down to make it fit although I did have to take a belt sander to it some places do eliminate the interference with the head's valve cover rail. The fastener holes did line up although being on the hairy edge. Fortunately the Edelbrock manifold had rather large through holes enabling fastening by the cap screws to the "lower" tapped holes of the heads. I think that if the heads had more material removed (i.e. .030") then I would have had to had the intake manifold machined to sit lower otherwise the cap screw would never line up with the tapped holes in the head. Good Luck in your endeavor.

[flat broke]

Good point about valve to piston clearnce. I'm just kicking around ideas, not really doing anything besides killing time in between clearing conditions on loans.
For reference, if anyone wanted to know how much to cut to raise your cr a given amount, here's the formula.
Stroke x (1/orriginal CR-1 - 1/desired CR-1) = the amount of material to be removed.
Just messin around with ideas,
Chris

[LakesOnly]

flat broke:
In the two stroke world, milling the head to increase the compression ratio is a very common practice. How does that cary over into the 4 stroke world, speficially with BBCs? What kind of thicknesses are typcically removed and what kind of alignment problems can this cause with the intake manifold?
Chris The practice of milling heads to increase compression is not uncommon in either two or four stroke applications.
In regards to this conversation, we will discuss head shaving as a performance enhancement and not as a repair.
Most every two-stroke engine utilizes a combustion chamber with a squish area around the outer circumfrence of the combustion chamber. The purpose of the squish area is for the air/fuel ratio to be forced toward the center of the combustion chamber (and toward the spark plug) by the piston. At TDC, the piston comes so close to the squish band (usually .035" or so) that the nominal remains of any a/f mixture doesn't burn in that area. Simply milling the head of a two-stroke to raise compression can be effective, but is a backyard tactic at best (after a certain point), because piston to combustion chamber (squish) clearance changes as well as cylinder head volume. Mill too far and you get piston-to-head contact, the width of the squish band changes (this affects power band), the plug can get too close to piston (affects burn), etc.
In an open chamber four-stroke application such as a Chevy, one can mill a head to gain cylinder compression. After a certain point, the intake side of the cylinder heads (or the intake manifold) must also be milled in order to get a proper port match/part mating. How many cc's the combustion chamber is reduced per every .010" milled depends on combustion chamber shape. Milling a great deal can cause other potential problems that require further evaluation, such as the intake manifold matching that was mentioned earlier, also piston-to-valve clearances, proper pushrod length, etc.
I recently read about a guy who just milled .060" off his Ford D3VE closed chamber heads and recuced the overall cylinder head volume from 98cc's to 88cc's. In his application, this could raise compression almost one full compression point. But milling this much with a D0VE head could cause problems (depending on cam/pistons, etc.), because that head design places the valves .100" closer to the pistons than the D3VE heads.
I don't know which Chevy heads you have nor the application, nor do I know the thickness of the machined surface of the combustion chambers. How far is it safe to mill? Look into your water jackets and see how thick the casting is where you want to shave. Ask others with experience with Chevy's how far you can go and what associated modifications come with doing this.
I'd though that there are so many piston and head combinations now that one can get just about whatever compression ratio they want with bolt-on parts...fun to experiment, though.
LO

[MikeF]

Any type of milling of the heads or block to increase CR would have near no improvement in rpm that a jet boat could spin an impeller.
You pretty much have to BOOST it to get any type of big rpm gain . When you run out of air getting in, it can't turn the impeller any higher.
In racing where the times are very close (F1,cart,nascar) especially when qualifying.....every little bit counts. idea

[flat broke]

it was mostly just a hypothetical since as LakesOnly mentioned, I could easily go buy a different set of pistons that would get me to a higher CR. I hadn't really heard of many people doing it so I thought I would ask.
In regard to MikeF's statement about milling the heads in an attempt to up the Rs, you're right about where I am going with it, but my resoning was a little more involved. I was looking at the feasibility of doing this to raise my static CR, so that I could run more duration without dropping my overall cylinder pressure too much. Boost you say? I sold my only ticket to that game last year. Life's a bitch in that respect because I could easily put 10lbs to the new motor with a little boost referenced timing retard and see dramatic increases over last year's attempt. But I would feel so inadequate running forced induction and still making less HP that Mike's single carb 502.
I spoke with HB earlier today and he and I agree that if we put a NOS sticker on the valve cover and then put a certain white and blue sticker over it while still alowing visibility of the NOS sticker, I should add an easy 300hp. Seriously though, I'm just looking at making some minor changes to help tie the motor package together a little more cohesively, but since we have input on the thread, I'm always down for learning more about different aspects of performance modifications.
Chris
[ September 17, 2003, 10:44 PM: Message edited by: flat broke ]

[Infomaniac]

In order to mill a head enough to really make a difference requires "Angle Milling". Mill the intake accordingly - check valve to piston clearance - put good gas in it and have fun.
We angle mill small block heads on limited sprint cars to get 13:1 compression. Rules specify flat top pistons only. Can get an iron SBC head down to about 46 cc's

[flat broke]

Info,
Are you getting 46 ccs down from 64 or 72 cc chambers? Any idea on what it would take to pull a 118 chamber down to a 106?
Chris

[cyclone]

my AFR heads started out with 121 cc chambers and were angle milled down to 110 cc's to keep the CR stock after i pulled off the GMPP heads and went to the AFR's. But the AFR heads are designed to be milled if desired.