Lima Delta Posted January 1, 2022 Share Posted January 1, 2022 How far down in the bore are your pistons? If they aren't pretty much at deck height you won't get to the ideal "quench" with any gasket. That's because you need about .040" between the piston and the head to have good quench. Summit says you should be at "0.035 - 0.045 in." So with even the thinnest gasket you mentioned of .039" you can't get there w/o being zero-decked. Hi Gary, I can't remember the source now, but I have been using 0.060" as (let me rephrase) maximum quench - which I guess is different from the "ideal" zone. I'm no expert. To answer your question deck clearance in my case is 0.032". So, .032+.047=.079 vs .032+.027=.059 (just under the "max") All that said, I still wonder at the importance of getting an ideal quench in lower compression engines. When I was shopping for pistons there wasn't a single piston I looked at that, when combined with stock rods and a stock deck, achieved an "acceptable" quench without boosting the compression to something close to 10:1. I know it's not unusual to mill the deck to get close to zero clearance for a good quench... but if this is so important for an 8.5:1 engine, why didn't Ford design the block that way in the first place? Link to comment Share on other sites More sharing options...
Gary Lewis Posted January 1, 2022 Share Posted January 1, 2022 Hi Gary, I can't remember the source now, but I have been using 0.060" as (let me rephrase) maximum quench - which I guess is different from the "ideal" zone. I'm no expert. To answer your question deck clearance in my case is 0.032". So, .032+.047=.079 vs .032+.027=.059 (just under the "max") All that said, I still wonder at the importance of getting an ideal quench in lower compression engines. When I was shopping for pistons there wasn't a single piston I looked at that, when combined with stock rods and a stock deck, achieved an "acceptable" quench without boosting the compression to something close to 10:1. I know it's not unusual to mill the deck to get close to zero clearance for a good quench... but if this is so important for an 8.5:1 engine, why didn't Ford design the block that way in the first place? My understanding is that "effective quench" is when the gases in the combustion chamber are squished so tightly that they move rapidly and both cause effective mixing of the fuel and air and prevent detonation. But that by .060" clearance you don't have "effective quench". Motor Trend says "It's not uncommon for old-school, low-compression, smog-motor, stockers to have over 0.060-inch quench at TDC. We've even seen some '70s-era Chrysler 440 engines with the piston 0.150 inch or more down in the hole at TDC. Originally, this was done to lower compression and slow the burn rate in an effort to reduce detonation. Current theory is that this is counterproductive, producing more unburned gases and such a slow burn that the detonation tendency actually increases. Don't run more than 0.060-inch quench, even if you have a need to run a low compression ratio." But to answer your question of "but if this is so important for an 8.5:1 engine, why didn't Ford design the block that way in the first place", I don't know. My guess is that it wasn't that important to them. So, what should you do? I don't think it really matters when you get very far past .040". With your deck height of .032" you aren't going to get to .040" regardless of what gasket you use. I say don't worry about it. Link to comment Share on other sites More sharing options...
Lima Delta Posted January 1, 2022 Share Posted January 1, 2022 So, what should you do? I don't think it really matters when you get very far past .040". With your deck height of .032" you aren't going to get to .040" regardless of what gasket you use. I say don't worry about it. Yeah, that's my take as well. I might be more concerned about it if I was shooting for something like 9 or 9.5:1 because the increased compression would push combustion closer to detonation anyway (esp on 89 octane) and having a good quench would mitigate that. But to get good quench with one of these engines, looks like it's going to cost some money in modifications. I have about 310k kms on the original 5.8 HO in my truck now. By the book it has 8.3:1 compression and probably doesn't have a good quench, yet I tow with it all the time and only recently have been getting some slight pinging if I'm on the gas too hard while climbing this one hill near my house. Link to comment Share on other sites More sharing options...
Gary Lewis Posted January 1, 2022 Share Posted January 1, 2022 So, what should you do? I don't think it really matters when you get very far past .040". With your deck height of .032" you aren't going to get to .040" regardless of what gasket you use. I say don't worry about it. Yeah, that's my take as well. I might be more concerned about it if I was shooting for something like 9 or 9.5:1 because the increased compression would push combustion closer to detonation anyway (esp on 89 octane) and having a good quench would mitigate that. But to get good quench with one of these engines, looks like it's going to cost some money in modifications. I have about 310k kms on the original 5.8 HO in my truck now. By the book it has 8.3:1 compression and probably doesn't have a good quench, yet I tow with it all the time and only recently have been getting some slight pinging if I'm on the gas too hard while climbing this one hill near my house. Yep, you should be fine. And I agree that it would take a lot of mods to get to effective quench. Link to comment Share on other sites More sharing options...
Rembrant Posted January 1, 2022 Author Share Posted January 1, 2022 So, what should you do? I don't think it really matters when you get very far past .040". With your deck height of .032" you aren't going to get to .040" regardless of what gasket you use. I say don't worry about it. Yeah, that's my take as well. It's one of those things that a guy can really get dragged into a rabbit hole with online. People often refer to something as being superior, or better, or ideal whatever. I always have to ask the question, what is the unit of measure in determining that something is better? I have checked several 302's now, and from the factory the piston to deck heights on them were in the 0.014"-0.015" range, with roughly 0.040" compressed thickness factory head gaskets. That puts them in the 0.055" range for quench. I just assembled my little 5.0/302 today, and it is basically zero-decked now. I installed the stock spec Fel-Pro head gaskets that are supposed to be around 0.047" compressed. I suppose it is still not 'ideal', but it is better than it was originally. I asked the question originally because somebody had asked me online elsewhere, and it got me wondering. I suppose I could have ordered the other gaskets that were 0.040" thick (and twice as much money), but who in the heck would know the difference? Not me, not the truck, nor would anybody else that ever drove it know...lol. Link to comment Share on other sites More sharing options...
mat in tn Posted January 1, 2022 Share Posted January 1, 2022 So, what should you do? I don't think it really matters when you get very far past .040". With your deck height of .032" you aren't going to get to .040" regardless of what gasket you use. I say don't worry about it. Yeah, that's my take as well. It's one of those things that a guy can really get dragged into a rabbit hole with online. People often refer to something as being superior, or better, or ideal whatever. I always have to ask the question, what is the unit of measure in determining that something is better? I have checked several 302's now, and from the factory the piston to deck heights on them were in the 0.014"-0.015" range, with roughly 0.040" compressed thickness factory head gaskets. That puts them in the 0.055" range for quench. I just assembled my little 5.0/302 today, and it is basically zero-decked now. I installed the stock spec Fel-Pro head gaskets that are supposed to be around 0.047" compressed. I suppose it is still not 'ideal', but it is better than it was originally. I asked the question originally because somebody had asked me online elsewhere, and it got me wondering. I suppose I could have ordered the other gaskets that were 0.040" thick (and twice as much money), but who in the heck would know the difference? Not me, not the truck, nor would anybody else that ever drove it know...lol. ive been reading alone and enjoying the thoughts and theories. we all have some. i have my own. having just finished another build i decided to put it in my flareside . im in the habit of "trying things". building my own parts etc. this 5.0 got bored .020 over and decked. pistons are approx.003 above the deck. heads milled also and gaskets are .044 thickness. ive now put 1500 miles on it and will do a compression check soon. zero pinging and i run 87 as a rule. power is great and mpg went up a little too Link to comment Share on other sites More sharing options...
Gary Lewis Posted January 1, 2022 Share Posted January 1, 2022 ive been reading alone and enjoying the thoughts and theories. we all have some. i have my own. having just finished another build i decided to put it in my flareside . im in the habit of "trying things". building my own parts etc. this 5.0 got bored .020 over and decked. pistons are approx.003 above the deck. heads milled also and gaskets are .044 thickness. ive now put 1500 miles on it and will do a compression check soon. zero pinging and i run 87 as a rule. power is great and mpg went up a little too So you have .041" of clearance, or quench, and it seems to be working. Cool! Link to comment Share on other sites More sharing options...
mat in tn Posted January 2, 2022 Share Posted January 2, 2022 So you have .041" of clearance, or quench, and it seems to be working. Cool! working great! my only complaint is the dang rev limiter engine pulls like a freight train til it hits that . one hiccup and shifts Link to comment Share on other sites More sharing options...
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