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Old 11-04-2013, 03:46 AM   #1
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Exclamation How much can block/heads be shaved?

I bought forged .020 internals from someone for cheap, i will be putting them in my 3400 block with 3500 heads and top end, this being said the block will have to be bored out for the bigger pistons, but since i have a 3500 top end my compression goes from 9.5:1 to 9.15:1 so how much could i have the block and or heads shaved to raise my compression back up some with out causing issues?
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Old 11-05-2013, 07:52 AM   #2
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Well, over boring it will raise the compression back a little bit, especially if you use 3400 gaskets. I had mine over bored 1mm (.04") while keeping stock size 3400 gaskets and heads, so that bumped compression a bit, plus I had the one set of heads milled but only .002" to make sure they were flat.

I'm not sure what the limits are, but I don't think I would have the block decked at all, because the pistons normally come up past the deck height slightly and into the head gasket bore. Without getting pistons made to a specific height, I'd think you'd run a higher risk of piston to head contact by doing that. The 3500 bore is 2mm larger than the 3400 bore, but I don't think the combustion chamber diameter is that big. It's usually quite a bit smaller, so I think you could use a 3400 head gasket still with a 3500 head. You'd have to line them up and check of course. But that with a half mil over bore would bring compression back up a little. I wouldn't be able to tell how much though without CC numbers for the head and piston to plug in a calculator.
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Old 11-05-2013, 09:15 AM   #3
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Do not mill the heads or deck the block any more than is needed to get the surfaces flat.
Pistons come out of the block by .020", and combine that with a .060" compressed thickness head gasket, you only have .040" to play with.
Shaving your heads .020" will only net you 0.1 higher static compression. Not worth it.
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Old 11-05-2013, 11:40 AM   #4
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Do not mill the heads or deck the block any more than is needed to get the surfaces flat.
Pistons come out of the block by .020", and combine that with a .060" compressed thickness head gasket, you only have .040" to play with.
Shaving your heads .020" will only net you 0.1 higher static compression. Not worth it.
I think your numbers are off by quite a bit Matt. Based off relative combustion chamber cc's between stock heads and ones ported to stage 2 and then milled .015", we're seeing a compression difference of more than .6, and that is with us removing material from the combustion chamber (which would decrease compression). I've never taken more than 20 off the heads, so that is as far as I would recommend going.

Going with thinner headgaskets will get you more of a compression change per thickness reduced compared to milling because you are decreasing the entire cross-sectional area of the ID of the gasket, not just the area at the bottom of the combustion chamber, but there is a gain in compression to be had by milling the heads.
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Old 11-05-2013, 12:38 PM   #5
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You are correct; I was mistaken. Momentary brain-fart.
Still, I don't think going up 1/2 a point in compression makes a huge difference.
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Old 11-05-2013, 03:00 PM   #6
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Yes, you could also go with thinner gaskets to up compression. Since you have to go with special order MLS gaskets to do it though, and they don't seal up as easy, I would still have the head decked at least a tiny bit to make sure they are flat and smooth. But as stated, just be careful how thin you go. You need to know exactly how far your new pistons are going to come up above deck height, and a good engine builder will even take into account tolerance reduction from thermal expansion of the pistons. Beyond my skills in calculating, but you get the idea.
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Old 11-06-2013, 10:24 AM   #7
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Quote:
Originally Posted by MilzyZ34 View Post
I think your numbers are off by quite a bit Matt. Based off relative combustion chamber cc's between stock heads and ones ported to stage 2 and then milled .015", we're seeing a compression difference of more than .6, and that is with us removing material from the combustion chamber (which would decrease compression). I've never taken more than 20 off the heads, so that is as far as I would recommend going.

Going with thinner headgaskets will get you more of a compression change per thickness reduced compared to milling because you are decreasing the entire cross-sectional area of the ID of the gasket, not just the area at the bottom of the combustion chamber, but there is a gain in compression to be had by milling the heads.
Info is much appreciated, i will not be going with MLS hg's since i already have new gm graphite ones and i did not mention i will be pushing around 9-10psi of boost so i will be having the block and heads shaved no matter what since the block needs bored..

But if i had my heads ported well and shaved a TOTAL of .015" off of the block+heads how much power would i stand to gain from the .6 increase in compression?
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Old 11-06-2013, 03:04 PM   #8
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Quote:
Originally Posted by Slinky View Post
Info is much appreciated, i will not be going with MLS hg's since i already have new gm graphite ones and i did not mention i will be pushing around 9-10psi of boost so i will be having the block and heads shaved no matter what since the block needs bored..

But if i had my heads ported well and shaved a TOTAL of .015" off of the block+heads how much power would i stand to gain from the .6 increase in compression?
any material taken off the block will be more effective at increasing compression than taking that material off the heads. If you're doing it this way, and want to take off as much as possible, I would do .005" off the heads and the rest off the block. Keep in mind, this will have an effect on pushrod length if you are using an aftermarket cam with longer pushrods than stock.

As far as power per compression difference, 10hp per 1.0:1 compression difference seems to be a good rule of thumb. It's going to vary with volumetric efficiency, but ballpark numbers should be pretty close. On a boosted setup, it just means you'll need less boost to make the same power.
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Old 11-06-2013, 05:57 PM   #9
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Are you suggesting to take .010" off the block? That would only leave .030" piston to head clearance.

With a .020" overbore and using 3400 gaskets you will have just over 9.5:1 compression.

For every .010" you take off the block or head gasket (max compression raise), you will gain right about .25:1 compression - just calculated a 3400 using a compression calculator in Dynomation. Taking .020" off the head will net a little less since the chamber area is smaller than the actual bore. The 60v6 compression calculator is also a fairly accurate guide, and even will show you all the stock values:

http://60degreev6.com/content.php/101-CR-Calc

Without going into complicated formulas, the general consensus of theoretical increase in power is generally around 3% with diminishing returns, per one point of compression. So, if you plan on being around the 300hp mark, you are only talking 9-ish hp if you were to get your engine to 10.5:1 vs the 9.5 it's going to be without decking either the heads or block.

Albeit it's a 3800, but one of our clients just made 540whp with 9:1 and 15psi with our heads/cam, in the end how much power are you wanting to make? I would worry about just getting the block deck square/flat by taking the minimum, same with the heads and concentrate more on the balancing, head/cam package and charger choice. You will barely notice whole point in compression, and with the .020" over, you are only .1:1 down from a stock 3400.
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Old 11-06-2013, 06:41 PM   #10
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Thanx for the info,Yes I know I was just trying to squeeze every bit I can get out of it, and 540 hp is crazy
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Old 11-06-2013, 07:34 PM   #11
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I agree with John. Take off the minimum amount needed to make sure things are flat and that's it. You can up the compression a little by using 3400 gaskets with those heads and the overbore. I wouldn't be trying to make a lot of power by taking too much off the block and heads, 'cause it's just not going to make that big a difference and you risk damaging your engine if things get too close. It's much easier to make power by upping boost, and in that case lower compression is better for you anyway.
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Old 11-06-2013, 08:10 PM   #12
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Thanx for the info,Yes I know I was just trying to squeeze every bit I can get out of it, and 540 hp is crazy
Yes, it is. His old setup/motor went flat 11's at just over 400whp. The fuel system and management is safe for over 1000hp, turbo is capable to boost into the 30's. He never got a clean run in this season, hopefully he will be able to make a solid pass in the spring.

Here is a link to one of his dyno pulls if you are interested...

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Old 11-07-2013, 09:44 AM   #13
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Are you suggesting to take .010" off the block? That would only leave .030" piston to head clearance.

With a .020" overbore and using 3400 gaskets you will have just over 9.5:1 compression.

For every .010" you take off the block or head gasket (max compression raise), you will gain right about .25:1 compression - just calculated a 3400 using a compression calculator in Dynomation. Taking .020" off the head will net a little less since the chamber area is smaller than the actual bore. The 60v6 compression calculator is also a fairly accurate guide, and even will show you all the stock values:

http://60degreev6.com/content.php/101-CR-Calc

Without going into complicated formulas, the general consensus of theoretical increase in power is generally around 3% with diminishing returns, per one point of compression. So, if you plan on being around the 300hp mark, you are only talking 9-ish hp if you were to get your engine to 10.5:1 vs the 9.5 it's going to be without decking either the heads or block.

Albeit it's a 3800, but one of our clients just made 540whp with 9:1 and 15psi with our heads/cam, in the end how much power are you wanting to make? I would worry about just getting the block deck square/flat by taking the minimum, same with the heads and concentrate more on the balancing, head/cam package and charger choice. You will barely notice whole point in compression, and with the .020" over, you are only .1:1 down from a stock 3400.

whether you're milling the heads or decking the block or going with thinner headgaskets, the cylinder head is still getting closer to the pistons. We do .015" off the heads all the time, so it would stand to reason that taking 5 off the heads and 10 off the block would give you the same resulting piston to head clearance, but would give you more of a compression difference compared to taking all the material off the heads. The person who started this thread wanted to know the max numbers the parts could be milled, and that's what I gave him, but I will agree that if it were my call I would only machine as little as needed (.005 is probably perfect) because you can always make up the difference in power by adding a little boost.
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Old 11-07-2013, 10:37 AM   #14
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whether you're milling the heads or decking the block or going with thinner headgaskets, the cylinder head is still getting closer to the pistons. We do .015" off the heads all the time, so it would stand to reason that taking 5 off the heads and 10 off the block would give you the same resulting piston to head clearance, but would give you more of a compression difference compared to taking all the material off the heads. The person who started this thread wanted to know the max numbers the parts could be milled, and that's what I gave him, but I will agree that if it were my call I would only machine as little as needed (.005 is probably perfect) because you can always make up the difference in power by adding a little boost.
Think about what you are saying. The pistons are .020" above deck and have a .040" clearance to the head. If you take .010" off the block, you are lowering the head deck by .010" - the pistons will now stock out .030" and you will lower the quench from .040" to .030", provided a .060" gasket is used. If you take it off the head, the pistons will still stick .020" out of the block no matter how much you take off the heads

The piston to head clearance is dictated by the block deck height, has nothing to do with what's shaved off the head. If we were discussing VALVE to piston clearance, your statement holds true, but we are talking about piston to HEAD clearance, which is a very respectable .040" in stock configuration.
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Old 11-07-2013, 12:42 PM   #15
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Originally Posted by ForcedFirebird View Post
Think about what you are saying. The pistons are .020" above deck and have a .040" clearance to the head. If you take .010" off the block, you are lowering the head deck by .010" - the pistons will now stock out .030" and you will lower the quench from .040" to .030", provided a .060" gasket is used. If you take it off the head, the pistons will still stick .020" out of the block no matter how much you take off the heads

The piston to head clearance is dictated by the block deck height, has nothing to do with what's shaved off the head. If we were discussing VALVE to piston clearance, your statement holds true, but we are talking about piston to HEAD clearance, which is a very respectable .040" in stock configuration.
sorry I was thinking valve to piston clearance for some reason, my bad.

It sounds like all of us are saying the same thing though ... if you are going boosted, why mill everything to the max? keep it safe and if you want more power, just turn the knob a little.
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Old 01-28-2014, 03:49 AM   #16
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If i made 10hp more with 10:1 vs 9:1 with no boost how much more power would i make with 10:1 @10psi vs 9:1 @10psi ??
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Old 01-28-2014, 07:47 AM   #17
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You would probably make more but it depends on several factors. Mostly whether you could run that high CR with boost and not get detonation. If you get detonation the PCM will pull timing and you'll lose power. It is possible to run boost and higher CR's, but it takes some special preparations and sometimes a specific engine design (like direct injection and special combustion chamber design) to run boost with a significantly higher CR.

You'd have to start looking at dynamic compression instead of static compression... see how much air/fuel you are cramming into the cylinders and asking it to compress, and how close it is to the limit for the fuel you are using. It would be easy to do with E85 or race gas. Might be harder with 93 octane pump gas. You'd have to run some numbers. But just as a guess, since plenty of people have run 10-14 psi on these engines at 9.5 or 9.6:1, I'd think you'd be able to get away with 10:1, as long as you can tune the ignition timing well enough. If it doesn't work, you can always back the boost down a few psi. Or run alky injection.
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Old 01-28-2014, 08:23 AM   #18
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You would probably make more but it depends on several factors. Mostly whether you could run that high CR with boost and not get detonation. If you get detonation the PCM will pull timing and you'll lose power. It is possible to run boost and higher CR's, but it takes some special preparations and sometimes a specific engine design (like direct injection and special combustion chamber design) to run boost with a significantly higher CR.

You'd have to start looking at dynamic compression instead of static compression... see how much air/fuel you are cramming into the cylinders and asking it to compress, and how close it is to the limit for the fuel you are using. It would be easy to do with E85 or race gas. Might be harder with 93 octane pump gas. You'd have to run some numbers. But just as a guess, since plenty of people have run 10-14 psi on these engines at 9.5 or 9.6:1, I'd think you'd be able to get away with 10:1, as long as you can tune the ignition timing well enough. If it doesn't work, you can always back the boost down a few psi. Or run alky injection.
My iat temps are extremely low with my aftercooler to help with the KR and Well my engine is forged rods/pistons, but i was told by ben at wot-tech that 10.5-11:1 would be the best cr for 10-14psi in a forged engine, i was just wondering how much more power is crated with boost factored in..
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