How much head milling = how much more compression?

[timk225]

Okay, so assuming it is a stock engine, with stock pistons, at stock specified deck height, at stock 8.4 compression ratio, how much of a difference would cutting the head 0.060" make ? That's an 0.080" cut with a 0.020" thicker head gasket.

 

[RustyRatRod]

Okay, so assuming it is a stock engine, with stock pistons, at stock specified deck height, at stock 8.4 compression ratio, how much of a difference would cutting the head 0.060" make ? That's an 0.080" cut with a 0.020" thicker head gasket.

It's not 8.4:1. That's the whole reason that .100" is pretty much the starting point for head milling. But it falls on deaf ears like a lot of other advice. These engines blueprinted down in the 7s for compression. They were rated at 8.4, but they always measure way shy. 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. I've never understood guys milling any less than .100" off because the cost is the same. .100" will get you "somewhere" in the 9:1 area. But again, 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. It's grammar school math. It's not hard, but you're sure as heck making it that way.

 

[92b]

Okay, so assuming it is a stock engine, with stock pistons, at stock specified deck height, at stock 8.4 compression ratio, how much of a difference would cutting the head 0.060" make ? That's an 0.080" cut with a 0.020" thicker head gasket.

Took less than 5 minutes to find the compression ratio on this motor today. 8.8:1

 

[RustyRatRod]

Alright. I'm out. When guys start bringing 3K buck tools in the mix and totally derailing the topic, I'm done. To the OP, I hope you get what you're looking for.

........and @92b, you made the list.

 

[92b]

Alright. I'm out. When guys start bringing #K buck tools in the mix and totally derailing the topic, I'm done. To the OP, I hope you get what you're looking for.

........and @92b, you made the list.

What list?

 

[92b]

Okay, so assuming it is a stock engine, with stock pistons, at stock specified deck height, at stock 8.4 compression ratio, how much of a difference would cutting the head 0.060" make ? That's an 0.080" cut with a 0.020" thicker head gasket.

If you want to know what your current compression ratio is and you haven't torn your engine down yet one option would be to have it whistled. I don't know for sure but I would think that someone in the Pittsburgh area would have one. Knowing your current compression ratio along with bore and stroke will be enough information to determine how many cc's you need to loose to achieve the compression ratio you are looking for. There are plenty of online calculators for doing this. Most machine shops can calculate how much you cut per cc from the head. Be sure to allow for any difference in head gaskets if using a different gasket. Another way to do this is to buy or borrow a burette, graduated cylinder, syringe or what ever else that will measure accurately and a plate of some fashon. and pour everything and calculate your current compression ratio. A third option is to cut ,100" off the head like Rusty said and hope for the best. Good luck on your project.

 

[Charrlie_S]

Okay, so assuming it is a stock engine, with stock pistons, at stock specified deck height, at stock 8.4 compression ratio, how much of a difference would cutting the head 0.060" make ? That's an 0.080" cut with a 0.020" thicker head gasket.

Where did you come up with the 8.4 CR number? The chrysler factory manuals list 8.2, and 8.4 (depending where you look). The question is how did they come up with different CR numbers, when the engine specs (that affect CR) never changed.
Just pull the head and measure what you have. Then use the formular to determine how much to machine.
PS: A slant six head needs to be milled .0066 per 1cc change

 

[timk225]

I'm not asking what my current compression is (and 8.4 is what is listed in a 1973 Dodge shop manual, I know it is probably wrong). I am asking what the difference would be with a head sitting 0.060" lower. I know this number will vary some based on what the current actual measured compression is, but it should be a fairly constant figure across the range of likely compression ratios.

Surely someone here has measured their stock /6 and has all the numbers from their engine. Plug in a 60 thousands head cut and tell me what amount of change that would have made compared to whatever their stock head was.

Something like a 0.060" head lowering should improve compression by roughly 0.3 or 0.6 or whatever. That's the type of answer I am seeking.

 

[RustyRatRod]

I'm not asking what my current compression is (and 8.4 is what is listed in a 1973 Dodge shop manual, I know it is probably wrong). I am asking what the difference would be with a head sitting 0.060" lower. I know this number will vary some based on what the current actual measured compression is, but it should be a reasonably constant figure.

Something like a 0.060" head lowering should improve compression by roughly 0.3 or 0.6 or whatever.

You're asking the same question. No one can know what the compression will be after milling, because no one knows what it is now. You must measure first to determine what your compression is now, in order to calculate what it will be after milling. THERE IS no magic answer other than you must measure. The reason is because each head will have slightly different chamber sizes, each engine will have slightly different deck clearances and so on. So you must measure to find out. Charlie came as close to an answer as anyone can by telling you "about" how many thousandths of an inch equals 1 cc. But you must STILL MEASURE in order to find out.

 

[92b]

I'm not asking what my current compression is (and 8.4 is what is listed in a 1973 Dodge shop manual, I know it is probably wrong). I am asking what the difference would be with a head sitting 0.060" lower. I know this number will vary some based on what the current actual measured compression is, but it should be a fairly constant figure across the range of likely compression ratios.

Surely someone here has measured their stock /6 and has all the numbers from their engine. Plug in a 60 thousands head cut and tell me what amount of change that would have made compared to whatever their stock head was.

Something like a 0.060" head lowering should improve compression by roughly 0.3 or 0.6 or whatever. That's the type of answer I am seeking.

You can figure out what you want to know as long as you realize it is a guesstimate. Charlie is saying to use .0066 per cc. Rusty's example comes out to about .0064 per cc. Use your bore and stroke on an online calculator and a compression number some amount less than 8.4 and more than 7. Use 9.23 cc for your .060" cut and see how much that changes the compression ratio.
As far as changing head gasket thicknesses. I don't know off the top of my head what shape the combustion chamber is or the shape of the gasket. Both head gasket and combustion chamber would have to be the same shape to cut .080" and call it .060" because it is .020" thicker. If they are not the same shape and the head gaskets are round you can calculate the difference in cc and add that to your calculation. Or if you can find the each gasket manufacturer's spec for the gaskets you can use those. Hope this makes sense.

 

[timk225]

I'll do it myself. I went online and found this website to get the formula and calculator for what the engine should be at, assuming the factory stock specs were right (0.141" piston in the hole, 3.4" bore, 0.020" gasket, 60cc chamber) . RSR Static Compression Ratio Calculator It came out to 8.31 compression.

Then I found this calculator to figure the volume of a cylinder 0.060" high and 3.4" in diameter. Volume of a Cylinder Calculator It showed 8.9 CCs.

A 0.060" DIFFERENCE (0.080" head cut and 0.020" thicker gasket) would remove 8.9 CCs from the combustion chamber.

Then I ran the first calculator with the new combustion chamber number of 51.1 CCs. I got an answer of 9.17.

Therefore the difference, again, the DIFFERENCE between 9.17 and 8.31 is 0.86 of a compression ratio.

That is what I was asking. Whatever my actual CR is, cutting the head 0.080 and using a 0.020 thicker gasket should make my CR 0.86 higher.

Now every /6 cylinder head I have either seen in person or online is an open chamber. The area that would be affected by milling up to about 80 or 100 thousandths is going to be a circle, unaffected by parts of the chamber shape. So I think my calculation is reasonably reliable.

Then to experiment a bit, I put in different numbers. Let's say my piston is 0.180" down and the head is 62 CCs. That is a 7.68 compression ratio. And then I ran the numbers with the head volume 8.9 CCs smaller (again, the volume reduction of a 0.060" head lowering), and I got 8.40 compression. With a resulting DIFFERENCE of (8.40-7.68) = 0.72 of a CR in change.

 

[92b]

I'm not asking what my current compression is (and 8.4 is what is listed in a 1973 Dodge shop manual, I know it is probably wrong). I am asking what the difference would be with a head sitting 0.060" lower. I know this number will vary some based on what the current actual measured compression is, but it should be a fairly constant figure across the range of likely compression ratios.

Surely someone here has measured their stock /6 and has all the numbers from their engine. Plug in a 60 thousands head cut and tell me what amount of change that would have made compared to whatever their stock head was.

Something like a 0.060" head lowering should improve compression by roughly 0.3 or 0.6 or whatever. That's the type of answer I am seeking.

Since Rusty has done the calculations he may be able to help with gasket thickness and cc volume.

 

[RustyRatRod]

Jesus Tap Dancing Christ, I'll do it myself. I went online and found this website to get the formula and calculator for what the engine should be at, assuming the factory stock specs were right (0.141" piston in the hole, 3.4" bore, 0.020" gasket, 60cc chamber) . RSR Static Compression Ratio Calculator It came out to 8.31 compression.

Then I found this calculator to figure the volume of a cylinder 0.060" high and 3.4" in diameter. Volume of a Cylinder Calculator It showed 8.9 CCs.

A 0.060" DIFFERENCE (0.080" head cut and 0.020" thicker gasket) would remove 8.9 CCs from the combustion chamber.

Then I ran the first calculator with the new combustion chamber number of 51.1 CCs. I got an answer of 9.17.

Therefore the difference, again, the DIFFERENCE the DIFFERENCE the DIFFERENCE between 9.17 and 8.31 is 0.86 of a compression ratio.

That is what I was asking. Whatever my actual CR is, cutting the head 0.080 and using a 0.020 thicker gasket should make my CR 0.86 higher.

Now every /6 cylinder head I have either seen in person or online is an open chamber. The area that would be affected by milling up to about 80 or 100 thousandths is going to be a circle, unaffected by parts of the chamber shape. So I think my calculation is reasonably reliable.

Then to experiment a bit, I put in different numbers. Let's say my piston is 0.180" down and the head is 62 CCs. That is a 7.68 compression ratio. And then I ran the numbers with the head volume 8.9 CCs smaller (again, the volume reduction of a 0.60" head lowering), and I got 8.40 compression. With a resulting DIFFERENCE of (8.40-7.68) = 0.72 of a CR in change.

Nobody's tap dancin, bud. We're trying to make sure you get accurate numbers. You cannot trust numbers thrown out on the internet. They will assuredly be different than what you have. If you already knew all the answers, what the heck did you need us for?

 

[timk225]

Nobody's tap dancin, bud. We're trying to make sure you get accurate numbers. You cannot trust numbers thrown out on the internet. They will assuredly be different than what you have. If you already knew all the answers, what the heck did you need us for?

I thought that someone here would likely have done an operation with similar specs like this before and would have real world measurements.

 

[92b]

I'll do it myself. I went online and found this website to get the formula and calculator for what the engine should be at, assuming the factory stock specs were right (0.141" piston in the hole, 3.4" bore, 0.020" gasket, 60cc chamber) . RSR Static Compression Ratio Calculator It came out to 8.31 compression.

Then I found this calculator to figure the volume of a cylinder 0.060" high and 3.4" in diameter. Volume of a Cylinder Calculator It showed 8.9 CCs.

A 0.060" DIFFERENCE (0.080" head cut and 0.020" thicker gasket) would remove 8.9 CCs from the combustion chamber.

Then I ran the first calculator with the new combustion chamber number of 51.1 CCs. I got an answer of 9.17.

Therefore the difference, again, the DIFFERENCE between 9.17 and 8.31 is 0.86 of a compression ratio.

That is what I was asking. Whatever my actual CR is, cutting the head 0.080 and using a 0.020 thicker gasket should make my CR 0.86 higher.

Now every /6 cylinder head I have either seen in person or online is an open chamber. The area that would be affected by milling up to about 80 or 100 thousandths is going to be a circle, unaffected by parts of the chamber shape. So I think my calculation is reasonably reliable.

Then to experiment a bit, I put in different numbers. Let's say my piston is 0.180" down and the head is 62 CCs. That is a 7.68 compression ratio. And then I ran the numbers with the head volume 8.9 CCs smaller (again, the volume reduction of a 0.060" head lowering), and I got 8.40 compression. With a resulting DIFFERENCE of (8.40-7.68) = 0.72 of a CR in change.

Nice work!

 

[92b]

I thought that someone here would likely have done an operation with similar specs like this before and would have real world measurements.

That's not unreasonable. But I think your better off having done it yourself.

 

[junkyardhero]

woof. lots of cluckin' in the hen house and everybody's laying eggs.

live by the mantra: trust but verify... or die by the consequences.

this day in age the tools and materials are available to easily help us. why try and shortcut something so important?

 

[AJ/FormS]

and then
you find out that with the stock cam, your Dynamic Compression ratio, which is the ratio that the engine actually understands, is too high for even best pumpgas, and so, you gotta pull timing out, and the engine ends up making no better hp, and the torque at Part-Throttle is down, and it now sucks gas on the hiway, because of the retarded timing.
But, I suppose, you've figured all that out, right?

 

[timk225]

and then
you find out that with the stock cam, your Dynamic Compression ratio, which is the ratio that the engine actually understands, is too high for even best pumpgas, and so, you gotta pull timing out, and the engine ends up making no better hp, and the torque at Part-Throttle is down, and it now sucks gas on the hiway, because of the retarded timing.
But, I suppose, you've figured all that out, right?

I haven't gotten that far yet. Still in the planning stages.

So far I have been running my Slant on ethanol free pump gas, which is 90 octane here. I can get 93 octane easily, which is actually a tiny bit cheaper, but it is "up to 10%" ethanol gas.

 

[Killer6]

I'll do it myself. I went online and found this website to get the formula and calculator for what the engine should be at, assuming the factory stock specs were right (0.141" piston in the hole, 3.4" bore, 0.020" gasket, 60cc chamber) . RSR Static Compression Ratio Calculator It came out to 8.31 compression.

Then I found this calculator to figure the volume of a cylinder 0.060" high and 3.4" in diameter. Volume of a Cylinder Calculator It showed 8.9 CCs.

A 0.060" DIFFERENCE (0.080" head cut and 0.020" thicker gasket) would remove 8.9 CCs from the combustion chamber.

Then I ran the first calculator with the new combustion chamber number of 51.1 CCs. I got an answer of 9.17.

Therefore the difference, again, the DIFFERENCE between 9.17 and 8.31 is 0.86 of a compression ratio.

That is what I was asking. Whatever my actual CR is, cutting the head 0.080 and using a 0.020 thicker gasket should make my CR 0.86 higher.

Now every /6 cylinder head I have either seen in person or online is an open chamber. The area that would be affected by milling up to about 80 or 100 thousandths is going to be a circle, unaffected by parts of the chamber shape. So I think my calculation is reasonably reliable.

Then to experiment a bit, I put in different numbers. Let's say my piston is 0.180" down and the head is 62 CCs. That is a 7.68 compression ratio. And then I ran the numbers with the head volume 8.9 CCs smaller (again, the volume reduction of a 0.060" head lowering), and I got 8.40 compression. With a resulting DIFFERENCE of (8.40-7.68) = 0.72 of a CR in change.

I don't think I've ever seen a factory 225 slug only .140" in the hole, where did You dig that spec. up? Don't go by NHRA class rules minimums etc., those are how much you're allowed to bring those specs to for 'stock' class packages.

 

[Mike69cuda]

Is there any correlation between the pressure you measure with a compression gauge and compression ratio?

 

[Killer6]

Is there any correlation between the pressure you measure with a compression gauge and compression ratio?

Yes, more = more & less = less, but then there is the cam..... where more = less & less = more, then there is cam timing advance where more = more & less = less........this all assumes the valves & rings are sealing tight.

 

[Killer6]

I haven't gotten that far yet. Still in the planning stages.

So far I have been running my Slant on ethanol free pump gas, which is 90 octane here. I can get 93 octane easily, which is actually a tiny bit cheaper, but it is "up to 10%" ethanol gas.

There are variations in castings, so the chamber diameters vary, it may take slightly different amounts to reach a specific volume. Basically a stock 225's pistons are ~.180" in the hole, & the compression w/a factory shim gasket is lucky to be 8:1.
"At Std. Bore Dia." every .010" removed will reduce volume a tick over 1.5cc's.
Have fun...

 

[Mike69cuda]

Yes, more = more & less = less, but then there is the cam..... where more = less & less = more, then there is cam timing advance where more = more & less = less........this all assumes the valves & rings are sealing tight.

I was curious if you could estimate compression ratio by the gauge reading. For example, does 150 psi equate to a certain compression ratio?

 

[Killer6]

I was curious if you could estimate compression ratio by the gauge reading. For example, does 150 psi equate to a certain compression ratio?

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.