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Old 07-01-2019, 01:27 PM
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Quench area

So, as I understand...being a novice at this, If I rock the piston downward ( pressing at 6 o'clock) and get a measurement of .017" above the deck, this would give me a quench factor(or measurement) of .036" with a head gasket thickness of .053"? From what I have read, this is too close for comfort...another unknown is how much will this piston tighten up as the engine heats up?? In this case I am referring to a 4.600" bore chevy with premium crank and rods. I would like to shoot for a quench clearance of .040"-.042" Is the .053" gasket too thin? Should I use a .057"??

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Old 07-01-2019, 01:36 PM
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P.S. If measured 1/3 down the piston, in the center above the pin, before the dome begins, (small dome, 10.8:1 compression) without forcing rock in the piston, the piston is .011" above the deck.
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Old 07-01-2019, 05:25 PM
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With the piston out of the bore +.011" at TDC measured roughly over the pin axis I would have no problem with running the ,053" gasket, most all of the rock disappears once the piston gets hot. That rock is only there because the piston is barrel shaped top to bottom. It is smaller diameter at the top to allow for it seeing more heat and expanding more, larger diameter at the skirt since less heat builds up down there and the skirt is being constantly splash oil cooled.
Piston manufacturers go to great lengths gesigning, FEA computer modeling and testing to determine shapes and material thicknesses all over the piston with the goal of having it be round and close to near actual bore size(but just a little less LOL) all over when hot.
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Old 07-01-2019, 06:12 PM
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If you stack three .015 gaskets to .045 then subtract .011 above the deck you get a quench clearance of .034. If you sub a .019 for one of the .015's you get .038. Close without spending a pile of money remachining something and you get to build your own MLS gasket, it works.

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Old 07-01-2019, 06:37 PM
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thanks for your reply...
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Old 07-17-2019, 06:39 AM
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Thanks for the reply...I don't dispute your answer, just wondering if you have ever run, or witnessed a similar combination with this bore size (4.60")As you realize, I don't want to damage a bunch of expensive parts. I am getting answers from builders that I i believe are generic opinion answers, all over the park, I found since I have been researching this that if I ask 20 builders, 10 say squish is of no benefit, 10 say it is...I personally believe it matters more in a pump gas engine...perhaps high compression engines see little to no benefit...i just want to take every precaution to stave off detonation using "gas station" 93 octane fuel...
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Old 07-17-2019, 10:56 AM
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That is true. Racing engines have so many other choices to manage that sort of thing. 100 different fuels and at different price points and stable timing and frequent refreshes and the like.
Street engines are really pushing the limits of the fuels that are readily available and forever changing so tolerances tend to gravitate toward that.
Out of the 20 engine guys you ask. 15 aren't very good.
So the question ends up being how does it effect YOUR application. That's a question hard to quantify without ABA type testing that isn't within the reach of the average hotrodder guy.

A few things come to mind, .011 outta the hole isn't a big deal. The piston will tighten up, the block will get longer, the heads and decks will move, rods will stretch at RPM, just about everything will move so I'd pick a gasket that'll get you in the .035-.045 range and worry about something else.

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Old 07-17-2019, 12:02 PM
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Quote:
Originally Posted by [email protected] View Post
Thanks for the reply...I don't dispute your answer, just wondering if you have ever run, or witnessed a similar combination with this bore size (4.60")As you realize, I don't want to damage a bunch of expensive parts. I am getting answers from builders that I i believe are generic opinion answers, all over the park, I found since I have been researching this that if I ask 20 builders, 10 say squish is of no benefit, 10 say it is...I personally believe it matters more in a pump gas engine...perhaps high compression engines see little to no benefit...i just want to take every precaution to stave off detonation using "gas station" 93 octane fuel...

You will find that different types of race engines derive a different answer for the technologies specific to the needs and conditions. Typically domes are avoided especially for long track engines where the length of races require higher reliability and better unboosted power. Here very small chambers are used with very tight squish/quench clearance with flat top pistons if burning gasoline.This keeps the chamber open for better breathing and flame travel. The high turbulence is useful in detonation and preignition tolerance while aiding the burn speed.


The same engine running on methanol would typically open up the chamber and increase the squish/quench to make space to deal with the much wetter mixture alcohols require. This can drive a domed piston to recover the compression where that has a better benefit on power output in this case than does the squish/quench effects for gasoline as alcohols bring a lot of that through sheer volume of liquid and a higher natural resistance to detonation. So this is a different problem set requiring different solutions.


Drag engines are typically short lived motors running stuff like nitro-methane usually in concert with benzene and methanol, or other crazy concoctions. This basically is running on liquid dynamite, there is so much liquid that the engine is running on the edge of hydro lock. The air is basically there to prevent that occurrence more than it is needed to support combustion as the chemistry to a great extent is bringing its own oxygen as a mono-propellant, not a lot unlike some rocket or torpedo fuels. Compression ratios are very high to some extent driven by what the fuel likes as well as cam timing that hardly seats the intake valve. The blower forcing induction against the rising piston lets you do this. This is like mechanical, chemical fantasy land on steroids. There are plenty of examples of where this edge goes one step over the line resulting in amazing explosions and crashes.



Now I'm picking the extremes as examples but the point is you have to build into sets of constraints that are bounded by specific conditions. Engine building is not a case of one size fits all situations. You can't just go if it works on a double AA fuel dragster engine I can use those techniques for my 93 octane burning street engine and I'll just be the fastest thing out there, it just won't happen like that.


Bogie
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