How wide does the unmachined piston deck have to be to give effective quench? I have my eye on some dished pistons & there is the land around the perimeter of the machined dish thats in the center/middle. Any thoughts? Thank you for your time.

The quench area of the piston will be the high, or machined part...the area that is directly opposite to the flat spot in the combustion chamber, opposite the valves. This is the area that will usually be responsible for pre-ignition. Bringing the piston closer to the head in this area (.035-.055") pushes the A/F mixture out into the path of the spark plug. In certain applications, the quench dome needs to be so high that they dish the non-quench area of the piston to bring the CR of the piston back down to a street fuel-friendly level.

And that's a really roundabout way of saying based on what you wrote, I'm not sure I understand the question...

You normally use a flat top piston that sits even with the deck surface of the block to get effective quench. The head gasket thickness then provides the clearance with the closed side of the chamber opposite the plug. As 4speeds4me stated to reduce compression they will put a D-shaped dish in the piston under the plug side of the chamber. To raise compression they will put a D-shaped dome in the same area.

You can get some quench effect with open chamber heads by using a domed piston the extends into the chamber but its not as effective as the flat top closed chamber design.

From your description it sounds like you have standard dished pistons. These will provide very little if anything in the way of quench.

page 73 of the book
"The Internal Combustion Engine"
by CF Taylor and son
has the pair of important graphs
on quench clearance and squish area.

To give you a rough idea
at 2000 rpm
on an engine with a 2.75 inch stroke
and 10 to 1 compression ratio
a piston design with 0% squish area
needed roughly 95 octane fuel
to keep from detonating
and needed
about 29 degrees btdc timing
to make best power

In contrast
the same engine, rpm, CR and conditions
with a piston design giving 50% squish area
only needed 75 octane fuel
and 14 btdc degrees of ignition timing for best power

This was with a 'Very Tight'
0.0065 times bore quench clearance.

That worked out to 0.018 inches on the 2.75 inch diameter bore engine in the test

It would be 0.026 inches on a 4.00 inch bore engine

the actual graphs show the effects using compression ratios from 4 to as much as 16,
and with 0, 10, 20, 30, 40 and 50 percent squish areas

there used to be an article
on the Edelbrock website
by Hugh McCandless about a 360V8 with aluminum Edelbrock cylinder heads
where he points out that he set the quench clearance at 0.026 inches for the 4.00 bore

the old web address for the article was
http://www.edelbrock.com/automotive/stories/hpm907/index.html

but unfortunately it seems to have disappeared
and the Google sponsored Wayback machine does not seem to have a cached copy

http://web.archive.org/collections/web.html

Perhaps someone knows a web link to it?

I think the title was 'Feeling Light Headed'

.026 !! thats too tight. am i wrong? heres a good write up http://www.chevyhiperformance.com/techarticles/94138_piston_head_clearance_guide/index.html

Thanks guys for your time. With this fantasy mockup I would have ~.035" piston to head distance but with these particular pistons(that have a dish) there would not be enough AREA in the flat raised circular perimeter surrounding the dish so I will scratch those pistons & move onward.

The example was at 2000 rpm, or they would have kissed the pistons.

I actually made an Autocad drawing of the exact shape of the quench area of my heads (302s) and had the pistons made to that drawing.

I have found the same thing as the testing did. Way less detonation and way less timing (about 60%) than open chamber.

I think .035 is too tight, even for a steel rod engine. I run .038 , and I think that is on the edge, but I only go to 5500rpm. Once you get to about .040 everything has to be perfect, and you need to preassemble and check every cylinder for piston height in the bore. Even with very careful machining, and redone rods, I had almost .004 variance hole to hole. I did rod repositioning and got rid of most of it, but still had to offset bush a couple of pin bushings to get to under .001 variation.

The central dish is the best design piston you can get, bar none. The issue is the wedge cylinder heads offset the valves and plug, so effective quench with a standard central dish is marginal at best. The best design that exploits the effects of squish and quench is an off set dish, or quench dish, which is what the Mopar pistons from everybody look like. With twisted wedge heads, or a centrally located chamber (I believe some of the W and P series small block heads have this) the central dish with the quench band will make more power.

if my memory is not playing tricks on me,
David Vizard did a set of tests once
where he also doubted that clearances as tight as
0.026 inches were absolutely necessary
and he found something like
anything tighter than 0.060
worked equally good for him

never the less
the original article calls for 0.0065 times the bore diameter

certainly the majority of 1994+ Magnum 360s are much looser clearance than that, even though the piston crown clearly is meant to take advantage of squish.

another 'practical ' consideration is
"If there is clearance, how much of that clearance will be closed up with carbon film deposits in a short amount of running time ? "

Diamond did some pistons for me w/ a small "dish" which "mirrors" the chamber of my Eddy 84cc heads. My thought is as the piston nears TDC the mix will "roll" into the chamber near the plug. These fit at 0 deck which gives me 10.2:1 comp ratio and .039 quench. So far runs perfect and easily swallows pump swill w/38 total timing. 470" low deck street toy. Hope that helps?

Dcuda--you may want to try backing off on the total timing on that engine. When I got good quench and port velocity I was able to take out almost 10* and get better economy and performance. Your Eddies have bigger ports than my 302's, but they still have good velocity compared to a lot of the iron heads.

Dgc has it pretty well pegged. His example of the advance needed for magnums comapared to open chamber heads is mainly port design and velocity IMO, as the factory setup was not very good at getting quench tight and consistent. On my 302's I found I needed even less timing, in the range of 28 to 30* maximum, based an exhaust temp and fuel consumption off the efi logs. There is also very little difference in the advance needed for cruise or power, which would be an indicator of very good charge mix and turbulence.

I have had the scanner on a couple of later model cars (not high perf) and found they also run very low advance compared to what we are used to. My Escort was at only 25* at 60 mph (2300 rpm) cruise.