How much head milling = how much more compression?

[92b]

No, only if it's a known combo in identical condition sealing-wise, with no other changes....just chamber volume...if You knew the measured ratio before, You could then calculate the new one based on that. But again, even the method/conditions have to be identical,.....ambient temp, eng temp, battery level/cranking speed, throttle position, number of compression pulse count per test, oil pressure while cranking....bleed down rate diffs in hydraulic lifters will make identical packages register different...sometimes dramatically different.
The upshot is, there are only 2 ways to definitivly check it w/o disassembly, & unless somebody is feeling really generous with their time &/or equipment....it's costly.
The OP is planning on disassembly to improve on it anyways, all the lazy costly ways are pointless here.

I agree with this post except " Lazy, costly and pointless" unless I am misunderstanding you.

 

[slantsixdan]

What part of "you're going to have to measure" do you not understand? We cannot tell you what your compression will be because we don't know what it is now. And you cannot use 8.4 as a starting point, because that ain't what it is either. […] YOU NEED TO MEASURE what you have now and CALCULATE what the difference will be after the .100" cut. There is no magic wand to wave to tell you the answers WITHOUT MEASURING. No matter how many times you ask the same question differently the answer is YOU HAVE TO MEASURE.

⬆︎That's the answer the OP needs. Maybe not the one they want, but that's a different question.

 

[volaredon]

I'm sure some here have already done the math and CC measurements on this. I'm planning some mild mods for my 1973 Dusters' 225. Among them is to pull the head, hot tank it, give it a good port clean up job (not hogging out the ports, just smooth the rough spots), back face the valves (stock size), and mill the head to boost the compression some.

Since the stock head gasket is about 0.020" thick and the replacement Fel-Pro head gasket is 0.040", cutting the head 0.080" would effectively reduce it by 0.060" because of the thicker head gasket.

So on a stock 8:1 compression engine, what should that raise compression to? And if because of factory tolerances the engine is really at 7.5 compression, then what might the head chop raise it to?

Id be leery if that engine actually being 8:1 if it's as originally built due to tolerance stacking.
It needs to be measured out to verify. I agree though about taking an extra 0.020 off to compensate for the thicker head gasket just to get status quo. I took 0.070 off the last head I did which took it from 60cc to 50cc. (Actually measured ) It's like 6-1/2 (0.065) thousandth per cc removed. And plain old "rebuilders grade" pistons will be further from TDC than original pistons were ... I cut my block 0.030 and my pistons are STILL 0.180 deep .. I got my engine as a basket case sans pistons and rods but had I known I would have had at least 2x as much shaved off the block deck.
With 0.100" shaved between the block and head I ended up exactly at blueprint spec of 8.4:1. So don't take "advertised" specs as gospel.

 

[timk225]

In regards to cutting the block 0.030", I think what's important is to get the deck height measured, then cut it to the 10.68" that factory says it should be. However much cutting that may take. If the block is just cut 30 thousandths without actually measuring the deck height, then you don't know if you are now below the factory spec or still above it.

 

[volaredon]

I never actually measured the deck height. I took a wild azz guess as to how much to shave the block and then once I got it back and I assembled it, then measured how deep in the hole the pistons were, I was disappointed that I didn't take more but wasn't about to disassemble the short block again for another trip back to the machine shop.
Member "my68 barracuda" (he goes by another name on the /6 forum) was the first one to point out that they make many aftermarket pistons intentionally shorter than the original ones for smog reason to "guarantee" to compensate "against" any amount of a compression increase upon making the piston ever so slightly "fatter" in sizing for an overbore.
Somewhere there's a magazine article on line that tells of a hop up on a /6 In a 67 dart that had been donated to a community college in PA and it shows them first rebuilding the engine "all stock" as they got it donated in siezed condition to the school, they had run it on the schools Dyno for a baseline and then pulled the head, measured their piston height (was over 0.200 in the hole, yuck) and they did some improvement including (I don't remember what all). rebuilt the distributor and modified the advance curve, added a super 6 2bbl setup and more and then ran it on the Dyno for a before and after comparo... They had to bore the block either 0.040 or 0.060 because of the rust and pitting due to having sat long enough to seize. They'd gone down to the local Napa and gotten their standard run if the mill "rebuild kit". And that's the pistons the kit had come with...

 

[volaredon]

As I bought the engine I now have in my truck at the Indy swap meet some years ago in pieces, I wanted the super 6 setup the guy had and was told "all or nothing" as he had the rest of a /6 in his van in the parking lot that he didn't feel like unloading for the swap ... It came without the rods or pistons, they weren't to be found when he went to clean out the garage of a friend when said friend died.... Said friend had torn the engine down "who knows why"....
I got lucky and found a set of factory NOS rods still in boxes from a guy on the /6 forum. And I bought some 20-over "stock replacement" sealed power pistons from Jegs at the time.... Been 4-5 years ago now. I was definitely glad I measured the piston position before I told the machine shop how much to shave off the head. But wished I had mocked it up and measured that measurement before I assembled it. Id have definitely had more shaved off had I known ... But it's way better than it would have been if I hadn't measured anything and hadn't had anything milled off.... That was definitely a wake up call for future overhauls.
Fortunately my engine is in a truck that'll never see a 1/4 mile track but the butt Dyno definitely feels a difference before vs after.

 

[PROSTOCKTOM]

If you want to really increase the compression ratio, take it apart and have the block decked. Because your pistons are probably .150"-.156" in the hole.

Spending money to deck the heads is a waste for no more than you're going to get out of it.

Either way you'll end up needing new pushrods if you start milling and you most likely have to slot out the intake bolt holes.

Tom

 

[Newbomb Turk]

In regards to cutting the block 0.030", I think what's important is to get the deck height measured, then cut it to the 10.68" that factory says it should be. However much cutting that may take. If the block is just cut 30 thousandths without actually measuring the deck height, then you don't know if you are now below the factory spec or still above it.

Why would you cut the block to factory dimensions?

Pick the compression ratio you want and figure out how to get it.

If the engine is apart measure it and then do the math.

That means a 1/2 inch down fill on cylinder and doing a volume check on the chambers. Then you have to know gasket bore and thickness and then you can math it out.

I don’t know how much you can mill off a slant block but the first thing I would do is figure that out and get the piston as close to zero deck as you can and forget about machining it to some arbitrary factory number.

 

[volaredon]

If you want to really increase the compression ratio, take it apart and have the block decked. Because your pistons are probably .150"-.156" in the hole.

Spending money to deck the heads is a waste for no more than you're going to get out of it.

Either way you'll end up needing new pushrods if you start milling and you most likely have to slot out the intake bolt holes.

Tom

Not necessarily. Ask Rusty. He had .150 off the head and had enough adjustment in the rocker arms for original pushrods. Look at some of the posts above regarding block decking.
One interesting point brought up is whoever said to measure your actual deck height and see how it compares to spec. Lots of chances for tolerance stacking there on the assembly line...

 

[Cruisingram]

Great information. In the process of gasket port matching my heads and intake now.

 

[Charrlie_S]

If you want to really increase the compression ratio, take it apart and have the block decked. Because your pistons are probably .150"-.156" in the hole.

Spending money to deck the heads is a waste for no more than you're going to get out of it.

Either way you'll end up needing new pushrods if you start milling and you most likely have to slot out the intake bolt holes.

Tom

Tom this is a "slant six". No issue with intake or exhaust manifold alignment. Also the slant six can easilly tolerate .100 off the deck and/or .100 off the head. How would that be for a compression "bump"?

 

[Jim Kueneman]

@RustyRatRod do you have a photo of the head chamber with 0.150" taken off? Is it giving it a bit of "quench flat spot" area by then?

There was a good write up on the slant 6 dot org group a long time ago. IF you still see a round edge then approximately 0.006" to 0.008" of milling removes 1 CC of volume. As you start to get close to the roof of the chamber in areas then this formula falls apart fast and you remove more CC per mil. That is why Rusty got into trouble with 0.180" off it. 0.100 off follows this rule closely.

I myself have never seen a factory 225 with more than about 7.8-7.9 compression in pre 1968 blocks (yes I know the specs are 8.2 or something but casting variations were wide). The ones I have done the pistons are typically about 0.170-0.180" in the hole. ChatGPT hit the nail on the head

• Based on common rebuild data for the Mopar 225 cubic inch (3.7L) slant-six engine, stock pistons typically sit quite far below the deck, often ranging between 0.120" and 0.187" (or more) below the top of the block.
  Key details regarding 225 slant-six deck clearance:
  • Stock Configuration: Unmodified factory engines often have pistons sitting roughly 0.160" to 0.180" below the deck.
  • Rebuild Variations: It is not uncommon for stock replacement pistons to sit deeper in the hole, sometimes up to 0.187" down.
  • Compression Impact: This large "in-the-hole" distance is a primary reason for the relatively low compression ratios (around 7.8:1–8.5:1) in many stock, un-machined 225 engines.

 

[volaredon]

As I remember, rusty didn't get into trouble with the head being milled as far as he did, which was milled around 0.150" not 0.180". And it was to correct a previous milling error, had to go that deep, to fix that and salvage the head.

 

[volaredon]

And if I hadn't milled my block 0.030" my aftermarket pistons would have been a miserable 0.210" in the hole. Had I known that when the block was still in the machine shop I'd have had them cut another 0.030 off the block while they had it... I didn't think to have them measure to see exactly what the original deck height started out at, it may have actually been taller than blueprint spec.

 

[Dave999]

i had the reverse
took a complete catastrophe of a hemi 6 motor, huge cut to the deck, previously built by "Bodge-it and Fudge-it" and built it up.
pistons stuck up 40 thou above deck
took an 1/8 inch rim off the crown of all, so the sides sat at deck height, checked clearance between dome edge and head
even with the cut these Hypereutectic Hi CR LA 318 pistons there was enough space above top ring and the deck.. gapped them for hotter running.

I let dome sit up 40 thou in my semi-hemi poly chamber style head

high static CR 12.?:1 sensible dynamic/effective DCR 8.5:1

worked out nice in the end

a researched and informed but still potentially accidental success.

needs must when its all you have to use....

you can get away with it, with the right Carbs/Cam/Curve/Exhaust

runs fine on 5% ethanol 99 RON which is 93/94 octane gasoline in USA terms
hot day, cold day, stuck in traffic, and down the strip.

i would do it in a similar manner again. but this time my choice would be deck the block and all the rest off the head same CR with no dome to keep expense down hemi 6 head works better with 80 thou cut


Dave