Just enough clearance so the piston doesn't hit the head under any conditions of heat and RPM, including when the bearings get a little worn or the engine over-revs accidentally. This varies with piston design: pistons that fit the bores tightly can get by with less clearance. Pistons that rock in the bores need more. As said, 0.040 is a fairly safe number. I've heard of folks going to around 0.030 successfully on certain engines. Other folks look for "witness marks" where the piston has just kissed the head, when disassembled. I think that's going too far. 0.035--0.045 is the golden area. About a spark-plug gap's worth of clearance. With 0.050 clearance, you're at the edge of not having any effective squish/quench.
Just curious to those who have answered, does the combustion chamber have any impact on the quench height?
There's two sides to this, there's the mechanics end / safe clearance wise and then there's the combustion efficiency/ power side of it. On the power side of it there are atleast 6 factors that come into play, but in general with a flat top Pistons .040" to .045" will work great!
The better idea quench for a BBB is to use pistons with an actual quench pad with the closed chamber heads with .035" to .045" gap @ TDC to reap the full benefits of quench! Using a "stock" style full diameter dished piston doesn't leave much for the quench effect to happen, not nothing but not much either.
My interpretation of ideal for the BBB is a value that maximizes power output and fuel efficiency versus likelihood of detonation on some grade of pump gas. My question arose out of the fact that I see the same ranges quoted here as I do on the Pontiac sites I go to and I wanted to pressure test the numbers being quoted. (I've asked the same question over yonder.)
That's a good exploratory start! The info above specific to Buick, such as the stock piston potentially not having much of a quench to it as well as attributes particular to the chamber shape are important to keep in mind, and other than following some proven combinations with much more detailed machining specs than normally listed on a forum...that's about as good as starting point as any. The point of diminishing returns creeps up quickly at roughly .035" with any engine type, with less temperature benefits from that specific action happening. I've read that on some engines, weird things happen with the atomized fuel mixture, prompting engineers to back that figure off some, but understand this has been extensively tested and the results quite well known. Students on their quest to either design engines or work with emissions/inductions get to "create" the formulae as they come through the chapters. I'm curious about pressure testing as a viable metric for quench in Buick 455's and the purpose or goals in mind. The right amount of quench, mixture motion, various machining specs and inlet temperatures have shown pump gas to withstand compression ratios and pressures significantly greater than what's discussed casually in forums. I would venture that if some goal was in mind to exceed the norms, a well informed plan would get you there without testing. Octane tolerance limits from temperature or engine load will usually limit things before quench distance. Inlet temp, coolant temp, piston temp and heat transfer through rings, temperature increase prior to and as a result of compression, exhaust valve temp, any incandescence, end gasses, load inducing det. or acceleration rates, vehicle weight, etc etc. Having these obstacles stacked against you will further define the paradigm in which pressure and quench can be measured. Simply put...it's kind of like wanting to know how hot it takes to burn eggs by measuring the handle to your fry pan. (Hopefully that didn't offend) Hoping to peel back more layers of onion or the specific objective the OP had in mind.
Wide disparities and great range in mileage will happen regardless of quench distance. Not sure of that's what you mean with 'fuel efficiency" or if you mean BSFC or MBT?
True that there other variables in play, but if looking at quench height as a univariate analysis I think the relationship is true.
Are you suggesting that if I gave you a starting point of a 455 that proves 25 mpg in a certain rpm range, mph range, vehicle type paradigm, etc....that you would be able to measure a difference in MPG following reduction of quench by .010" increments from .080" down to .015" ? Or if the above example was only capable of 10 MPG? I'm guessing that it's more of a BSFC metric you're testing? Are you quantifying other parameters like mixture motion (speeds) or the myriad of other things happening or excluding those for the end result only? I would have a hard time seeing the forest through the fog using just quench. Pretty sure there's going to be SAE papers available with relatable quench info. I'm innately curious to see this singular variable show some results other than when the Eye Dr. flips 40 different lenses that all sort of work pretty good and the goal is to read a chart that's 20 feet away. No negativity intended!! Hoping for more dialog
There wouldn't be ONLY the quench distance as a variable. As soon as you get some mixture motion in the chamber, you could perhaps retard the timing while maintaining or increasing power. Also, if the engine efficiency improves due to better/faster combustion, you'd have less throttle opening to maintain a given cruise speed--which might pull on the vacuum advance more. I can imagine a situation where adding mixture motion in the chamber requires a series of ignition-tuning changes. We've got similar things going on with the fuel curve.
I had to jog the memory on BSFC. I haven't heard that term since undergrad 25 years ago! Yes, that is the theoretical construct I had in my mind. I'm not sure if I've seen anyone test it. But most of the articles on quench that I've read do make note of it. I actually found this article last night in mulling over your questions: http://www.superchevy.com/how-to/engines-drivetrain/94138-piston-head-clearance-guide/ Near the end the author states: "it does offer some distinct advantages when it comes to increased engine efficiency, better fuel mileage, and driveability." I think the presumption is that the "better/more complete" combustion(if it is influenced by the quench distance) provides a smorgasbord of benefits. Whether anyone has defined those benefits well, I can't be sure. Most of what I find on the web on the topic only gives the generic huzzahs noted above.
Short of going to school to create engines, skip the magazine articles and start looking for Vo-tech books on engineering gasoline engines, SAE papers and related discussion, even read through some Speedtalk Advanced threads on the limits that can be pushed. The mags can have great info but the subjects discussed are of great magnitude requiring truncation as well as the financial motivation for the 'tests' can be seen on p. 86 (buy our cylinder heads) . I want to recall that Vizard tested quench distance on SBC's and came up with a figure like (6?) hp per every .010" . Don't quote me on that. (note; nothing mentioned about part throttle or low speed) His stuff is great for bridging the gap from OEM to the hot rod industry. The benefits are very well defined and formulated as you have queried. Other physics and happenings and their subsequent formulae being how quench is quantified, makes it impossible to isolate the actual benefit from quench distance alone. Please have an understanding that these attributes are touched on and factored mostly as a part of a much larger picture, depending on what EXACTLY the goals of the engine are, the limitations each design or major change presents, how one singular attribute is impossible to separate from many other functions, and that the specific purpose of the machine changes the needs, invalidates or negates the practicality of any combination of said phenoms, etc., etc.... (Un-garble that rambling and you'll find that it has already been said above this post quite effectively by several people. You'll also find after reading very well written responses on forums more intended for pro builders and engineers that the same powerfully minded types are here, saying the same things in an easier to read way, demonstrating full understanding of the concepts) Basically, I CAN judge how my eggs will cook by the temp of the handle...until I set it over a different burner, change pans, or type of fat used. Then I have burned or otherwise ruined eggs until I figure that out. It's better for me to just watch the eggs than fly blind. There was a lot of MIT tested examples used to take the student through the creation of the formulas used on the subject, not that we can do much but look at the aftereffects on the full throttle, high rpm dyno (see where I'm going with this?) Also I'm putting it out there that you'll have a hell of a time trying to test it and separate out other factors without access to a full lab, for the singular purpose of understanding one specific factor. Books and papers relating combustion sciences to emissions will help you create all the formulas you need...if you can measure things. At some point the light bulb will go off and you'll explore the end results, understanding the hurdles on the way. If you have the capability to test or where-withall to build things, I'll recommend re-focusing your quest to a finished product and chalk up the phenom's as part of the blueprint of it. Give quench a range, as diminishing returns will get you somewhere else at some point. When talking about 'better and more complete combustion', there might be other ways to end up with the same end result. I'll suggest the 455 chamber mimics the shape of an SBC vortec or Fast-burn, if you can find relevant reading material. A hemi chamber would only apply to limits explored beyond the extremes of the Buick's attributes. The time period that fuel injection was refining as well as emissions laws placing demands on the end result (of a marketable product intended to be profitable to manufacture) lends itself to great info on the combustion process.
Can you recommend any vo-tech books? A friend of mine attended SAM. He said the book they used the most was Engine Blueprinting. I've read it, but I'm looking for more. https://www.amazon.com/Engine-Bluep...06799285&sr=8-3&keywords=engine++blueprinting I've been thinking of buying these two books. http://rehermorrison.com/engine-book/ http://horsepowerchain.com/
Gotta source your local colleges or snoop online, I can't remember stuff from almost 30 years ago. I've skimmed excerpts from the 2 above, fantastic! Those definitely better choices for all but the person wanting to test or develop engines for a living, right? Under the premise of the comment regarding some kind of pressure testing, I suspect that isn't the final goal but am nevertheless...still intrigued. I would predict nothing new learned in the world of science but would believe potential to take any engine design beyond where it's commonly talked about.
All the fancy numbers aside lets look at the situation. 1. How can I change the distance between the flat surface of the head and the top of the piston? A) Mill the block till your pistons are at zero deck then use a 40 thou head gasket and that will be your quench distance. B) Square off the block so both sides are perpendicular to the mains and then order custom pistons to sit at zero deck, with a custom dish volume that gives the compression you want. C) Take shortcuts and leave the piston in the hole 80 thou then be let down.
