I have a comp 268 and an extra set of heads I plan to port/polish and mill 0.020"-0.030" off.

Does anyone know exactly how much I can mill the heads to bump up the CR without valves/pistons getting too friendly?

Is there a way to check or has anyone been down this road before?

.020 will work for sure, and probably .030. Piston to valve clearance should be checked.

You should definitely check the clearance. It's easily done with modeling clay. Clean the piston then cut a couple of strips of clay about 3/16-inch thick and put it where you think the valves will hit. Spray the valves with some WD40 or something so they dont stick to the clay. Put the head back on with the gasket, install the valve train. Here's the tricky part -> since your engine has hydraulic lifters, use a piece of wood dowel to replace the lifter. Why? The lifter will not be at its full extension with you just rolling the engine over by hand. Now just rotate the engine through two complete rotations slowly. If the engine doesn't want to rotate completely through, don't force it.
Now remove the head and check the clay depth with your gauge at the deepest impression point.
Thats it. I haven't done it in years and I'm sure that someone will chime in with some more pointers but this gives you the general idea.

Thanks guys! I have built engines, just never got into anything like this yet. So ANY info is much appreciated!

I'd love to know the answer to this but at some point where is trade off for an aggressive tune vs. a "stock based" tune and .8 more compression? Folks coming to me with milled heads in trucks seem to really spark knock. It's more of a factor with stock pistons than some forged ones. Guess it's the extra load from a truck vs. the milled heads in an LX.

I've tuned a few truck folks with 10.5 to 10.7 compression and stock pistons and both ended up with timing curves less than stock.

Build your engine for the octane you're running with. 165 to 175 cyl pressure is good for 93 octane. And the heavier your truck is, the more load it produces and more prone to spark knock. I've got a guy I've tuned that has 205 cyl pressure in his truck and he runs 93 and 101 octane mix just to run a stock timing curve.

And one guy has about 200 cyl pressure and runs about 6 degree's less than stock timing with 93 octane and has commented that it's being held back at WOT. I'm thinking...it's because the timing is so retarded.

I'm starting to think this head milling idea was not a good one ..it was good in theory though! Anyone want a set of stock 5.7 heads?? haha, i'll probably just end up porting and polishing them ..maybe 0.020"

Hemifever, do you primarily just do tuning, or do you go into the engine also?

SuperJay, I go into the engine on my own stuff but for 99.99999999% of the tuning I do it's just "tuning". I offer assistance with engine parts. I can tell you what cam "base grind" surge and which ones are pcm friendly. That sort of stuff.

So they come to me with engine configs and ask to get it running good. I know there are a lot of factors that could assist with the spark knock and head milling. I know that cyl pressure WILL add overall power. A pt of compression is generally good for a solid 15 hp. Compression, cylinder pressure...it's all good but not for your 5200# daily driver that rarely see's a drag strip.

I have dyno'd different timing levels on some strokers and going from 28 degrees to 22 degree's was a 20 hp drop on one stroker I worked with. So when somebody has a built "non stroker" and is running 15 to 17 timing at WOT I feel that the compression increase is worse off than stock compression and higher timing.

I'm just putting together patterns that I see from engine configs and talking out loud here. I'm by no means telling folks NOT to mill heads. I just wonder what's better for the 99% of the folks that use their heavy trucks as daily drivers....stock compression and an aggressive timing/fuel curve or higher compression with stock or less than stock timing curve and richer a/f for cooling down the cylinders.

You make a good point.

And in many cases bigger or more is not better.
It looks good in calculation, but is not practical for everyday operation.

Overcammed and overcarbed are the two problems seen most often in the car hobby.

I agree on the compression ratio issues.

I see so many backyard engine builders trying to run over 10.5:1 with iron heads and having the same problems on pump gas.

I know Jay sees this a lot in his business. I see it with the street crowd in our area.

There are more poor running cars than I can count sometimes.

That cam that only pulls 5hg of vaccum is a real
lot of fun on a street car.

hi..I am one of the guys with over 10:1 c0ompression that has problems with 268 cam

I had the head milled .030 then bigger inlet valve which futher increase the compression...

My guess is that I am about 10.4 :1.

cant run more tha 22 degress of advance and still get 5 degrees of Kr at some points.

The engine runs fine and the idle is now good thanks to hemifever,.

One question how thick are stock gaskets a I plan to reduce the compression a bit...I think that I may get more power with 10:1 with more advance than with 10.5 :1 ish ...and with 22 degrees and 5 degrees KR.

quote:

Originally posted by SouthernComfortHemi:
hi..I am one of the guys with over 10:1 c0ompression that has problems with 268 cam

I had the head milled .030 then bigger inlet valve which futher increase the compression...

My guess is that I am about 10.4 :1.

Before you do anything, try pulling out your power wire and see what you get for KR.

cant run more tha 22 degress of advance and still get 5 degrees of Kr at some points.

The engine runs fine and the idle is now good thanks to hemifever,.

One question how thick are stock gaskets a I plan to reduce the compression a bit...I think that I may get more power with 10:1 with more advance than with 10.5 :1 ish ...and with 22 degrees and 5 degrees KR.

Normally taking off .020-.030 from the head surface, won't induce knocking. Assuming the info I'm getting from u tuners is correct, then it beckons the question, why. Most aluminum headed motors have no trouble running 10.5 on 91-93. Hell, the 09 5.7 is running 10.5. How do they do it? Is the knock sensor, too sensitive? What's different in our Hemi heads, verses a conventional V-8. The way I see it, only three things. Our valves are arranged vertically, opposed to horizontally. I see no reason that would induce knock. Second, we have quench areas on both sides of the head chamber. Reducing that area, "could" have something to do with it. Third, two spark pugs. Defiantly different than the others. If I were an energetic young gear-head, (which I'm not any more), I would take 8 used spark-plugs, close the gap to 0, and install them, one per cylinder. "IF", the colliding flame fronts are a source for knocking, this would cure it. If not, ya only wasted a couple of hours. I would also try doing away with the thermostat, and use restrictions of different hole sizes. (would not be good for emissions, or warm up, but would keep the heads cooler). Has anyone tried using pistons, to bring the C/R to 10.5, verses milling the head, to get 10.5? How does the 09 5.7 get 10.5? Piston, head cc's, deck height? Experiments, will sooner or later yield results.

The 09's heads have closed chambers, no quench area.

My question out another way is;

Which gives more power 10.5 : 1 compression with 5 degrees of KR and 22 degrees of spark advance.

OR

10:1 compression with no KR and 22 degrees of spark advance.???

Or for that matter stock compression with 28 degress of spark advance with no KR.

Only the dyno knows.