Squish!

Silver95bird

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As a thought, how much do the 4.6L's and 5.4L's vary typically from cylinder to cylinder in their height stackups at the deck face? The tallest one becomes the limiting factor.
 

OLD H2S

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My build on a 3V aluminum block showed only .001 change between all the piston deck heights, but the deck height was -.015-.016 down in the hole. Next the last set of Fel Pro head gaskets I got came with both .050 type in the box so they have people that can not read. I have gone through a lot of head gaskets in the last 6 months.
 

96blak54

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Wow...thats .066" squish. Gonna get the block decked?
 

OLD H2S

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Yeah, go look at the threads on it. Cut the block .012 and the PI heads .080. Cracked heads, going to look at my spare set of NPI's today.
 

joe65

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i read through this from the beginning and my head is spinning a little. lol. I can grasp the concept and i can get how some people are so intrigued by the processes involved in the ICE. But, i'm so skeptical now especially after reading some thoughts by other members here, and have to think that its still about getting close and being lucky.. What makes me say that is the whole concept of being hundreds or thousandths of an inch in accuracy. Anyways, this is amazing how far we have come to understand about the process and squeezing power out of every possible process/setup in the engine.

then, here we are heading towards the basic electric motor in cars that produces large amounts of power with amazingly fewer parts.
 

96blak54

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I believe the electric motor efficiency has been solved for some time now. Its battery demands and the life of them through the demands that has to be solved while manufacturing them safely, plus recycling, and not being a danger to humans in a crazy scenario like wrecks.

The internal combustion engine has far greater interest. Take this thread for instance. Its simple break down into how fuel reacts in the cylinder and its a lame explanation.
 

Notthenow

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Hey 96Blak54, I know this thread is old now, but as it ran so many years it seems like one that will continue on. Just as confirmation, with stock pistons and rods, do you feel .30 clearance for piston to head is safe? Also, what do you think is the maximum varience per cylinder for compression that is acceptable? Is that varience or varience in squish more important?
Thanks for your insite.
Mike
 

96blak54

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This is what i replied about achieving the squish distance.

In a perfect world, machining tolerances of the engine components would be dead on and i would be able to inform you what could be used with certainty, but since its not a perfect world and machined components have a tolerance range to be in when manufactured it would be wrong of me to inform you how close you can get with no problems. And getting the squish distance to where the piston literally kisses the head is idea. Especially when the stakes are high in drag racing....such as a strict naturally aspirated class racing.

Keep in mind the factory set squish distance tolerance is safe and anything under that is pioneered by the user. However i can say with certainty that a few thousands under what your stock scenario is, could be safe. But if you really want to get to that "kissing the head" squish distance, its gonna have to take some development with multiple head gasket changes to figure out what works with your build.

In other words, getting intimate with your engine, one person might discover a .032" head gasket thickness to achieve the kiss whereas you may discover a .036" head gasket thickness to achieve the kiss. It would be wrong of me to claim what works without knowing the actual measurements of your engine, even if its an untouched stock engine. Discovering the kiss scenario takes running the engine hard a few times, pulling the heads off and coming to the conclusion after a few head gasket changes. And it might take $300 worth of head gaskets and possible machine work, decking the block to achieve desired results.

And this all of course is coming down to dog eat dog naturally aspirated racing where racing classes are strict.

If you are just re-freshing an engine for a street driver, a few thousands under stock is safe. If the engine has had either machine work or has aftermarket pistons, the squish distance needs to be measured out and calculated, then ran hard a few times, heads off for inspection, then the adjustments of head gasket thicknesses come into play.
 
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96blak54

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My thoughts about maximum variance in compression check?

Long as the cylinders are close in compression check, run it! So many variables alter compression checking. Poorly seated valves, ring seal, engine temperature when checking, battery voltage, fuel or other liquids in the chamber.

Here is a copy/paste suggestion from popular hotroding
"If you find one or more cylinders are below the 10 percent range of the others, squirt a little oil down the cylinders and try again. If the pressure comes up significantly, the problem may be worn piston rings. If the pressure does not come up on retest, the issue may be with the valves or the head gaskets."

So with not knowing your compression checked numbers, lets say most were roughly 150psi and a few cylinders touched 135psi. Ten percent of 150 is 135 and considered acceptable. I feel 10percent is to tight for a high mileage engine especially for a modular with pi heads knowing how a high mileage pi headed exhaust valve looks. I have yet seen one with a good machined seal. The exhaust valve seal is pitted and beat up looking. Also engine temp needs to be hot when checking compression especially modulars. Cold lashers can keep the valves from sealing completely. This could be only .005". Its not a complete seal, but will raise a compression gauge.

You also asked what is more important, compression deviations or squish kiss.

Pursuing squish means unbolting heads. Try to figure out why your engines compression numbers are different before unbolting them. This way you can potentially correct the problem....it could be bad valve sealing where a simple valve lapping will bring it back to a good seal.
 

Notthenow

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Thank you Blak, I appreciate the time you have taken and thoroughness of both of your replies! I thought there would be a simple mechanical range to measure and so I did not give enough details of why I asked both questions.
So, the long winded, detailed explanation, first, I asked about squish because I am in the middle of a pi head install on my 98 which has 160k on it, but did a good compression test before tear down. I am also installing Comp 262AH cams. I ported the heads meticulously going by port to port measurements. I know a flow bench is the best way, but I the heads were $120 and I wanted to learn. I installed bronze guides and re-cut the seats in 3 angles using carbide cutters. I installed new stock exhaust and titanium intake to reduce the margin for help with ptv, as well as Comp springs and new seals. Then I cc'd a chamber on each for reference. Right bank 42, left 44. I'm assuming one head was cut in its lifetime (heads are off a police car in a u pick).
This got me thinking about compression ratio and juggling gasket sizes. That led me to your post on quish and checking piston to head clearance.
My block varies from bank to bank and within the banks themselves. I am scratching my head to balance compression ratio and squish without a trip to the machine shop.
Sooo, after all that....stock piston in the hole should be .012 and stock gasket compressed should be .036. Total of .048 factory quish. In your other reply you mention a few thousands under should be safe, so roughly .045 as a target. My block measures .006 for the shallowest piston (maybe decked before, although stock pistons).
I am looking at Mahle head gaskets because they offer a plus .010 (it is actually .043 compressed per their tech line) that is cheap compared to Cometic. The problem is then in balancing the compression ratios per bank dues to the 2cc difference. The calculators show up to .2 difference between banks and I wonder if that matters in a mostly stock engine (about 10.4 to 10.6). Maybe that is common and no one realizes since most don't measure?
And that is the origination of my two part question of safe squish and compression ratio variance between cylinders.
Sorry for the book, but I hope the background helps as I am trying to be careful without be paralyzed by numbers. There are numbers out there for Chevy, but with the small bore of the Ford mod I wanted to not assume.
 

garrittpwl

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So going to revive an old thread here but I've been freaking out in my head. I'm literally writing this as I file fit my rings lol. I've been talking for some time about building a 5.4 and have gotten to the assembly stage (probably too late to ask these questions but it is what it is). Current build is forged rotating assembly, flat top pistons, PI heads and comp xe278ah cams. Per what I've read, pump gas doesnt like above 8.5:1 dynamic. per the calculators out there, my engine will have 8.9:1 dynamic. Should I be worried about this?
 

96blak54

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No. Pump gas will be fine.

What pistons are you going with?

What rods are you using?
 
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garrittpwl

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No. Pump gas will be fine.

What pistons are you going with?

What rods are you using?
I have these pistons, Mahle Powerpak 930256252

And these rods, Callies Compstar 24106
 

96blak54

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I see nothing wrong with those parts. The rods are a tad heavier than stock but to have that peace of mind that they wont break goes a long way. Might feel a slight vibration on the low rpm's... should be fine. Wont be the 1st engine with vibration, wont be the last.

Are the comp cams off the shelf?...not reground?
 
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garrittpwl

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I see nothing wrong with those parts. The rods are a tad heavier than stock but to have that peace of mind that they wont break goes a long way. Might feel a slight vibration on the low rpm's... should be fine. Wont be the 1st engine with vibration, wont be the last.

Are the comp cams off the shelf?...not reground?
I had the rotating assembly balanced and they added a bit of weight to the crank to make up for it plus a fluid dampener which should help a little too
 

ttocs

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I had the rotating assembly balanced and they added a bit of weight to the crank to make up for it plus a fluid dampener which should help a little too
just curious - should it be "damper" or "dampener" since we are dealing with fluids?
 

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