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Drilling into piston top

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savedallis1953 View Drop Down
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    Posted: 16 Mar 2019 at 7:38am
I saw where A/C claimed the "Crater" pistons create great turbulance in the combustion chamber so I was wondering if any type of disfigurement to the Top of flattop pistons would be worth the trouble as I seem to remember someone drilling into the tops of pistons to get more hp.
1953 WD, 1953 WD, WD engine with WD-45 crankshaft.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote LouSWPA Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 2019 at 10:39am
well, I have no experience with such, nor am I an engineer, so take what I am out to say with that in mind. seems to me a piston is designed and cast, or forged to withstand the forces it incurs without being destroyed. AC engineers designed their crater piston with sufficient material where it was needed so that the piston was strong enough to survive as produced. Likewise a flattop piston. if you go taking away material to 'redesign' the piston for more power it seems to me what you will succeed in doing is constructing hand grenades.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMiller Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 2019 at 11:49am
Will find most piston crowns do not have enough material to do a lot of conversion, the crater head pistons were notably thicker.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DrAllis Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 2019 at 2:00pm
Let's just say that your flat top pistons were thick enough on the top to drill holes 3/8" deep without breaking thru. Let's say you took the time to drill 30 of these shallow holes in the top of your 4 inch diameter A-C pistons. The result would be less compression, by a noticeable amount. Good luck in gaining ANY HP doing that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMiller Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 2019 at 2:34pm
Years ago we would machine valve recesses into GM Engine pistons Deeper than stock for higher lift longer durations gained us little overall beyond that of the valve timing and lift.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PaulB Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 2019 at 4:06pm
I have gained compression on a bunch of engines by drilling into the tops and using stainless steel screws to add a dome onto flat top pistons. I know of NO failures so far. Some of them have been running for over 30 years now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tbran Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 2019 at 8:46pm
We built a 226 with WD pistons for a customer who insisted on running aviation fuel. The compression was a tad too high we felt for premium or 93 octane so we had a bowl cut in the piston top =  I thought we had it done under spark plug hole about 3" diameter but my shop guys told me I was mistaken - it was from the center TO the spark plug about 1/16" deep in the flat top - we were using a Tisco kit - have no idea what vendor they got theirs from. We wanted a tad lower CR and thought it might help flame travel.  I do know that today almost all the vendors are gone.... either out of business or discontinued production - the production parts pool is very slim. 
When told "it's not the money,it's the principle", remember, it's always the money..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2019 at 7:06am
The whole concept of changing piston and combustion chamber shape, is to reduce the distance that the flame must travel in order to propogate through the charge.  Absolutely everything... and I mean everything... about  combustion chamber shape...

Let's put it this way... consider a combat firearm-  the three most critical functions that occur, are a charge coming in, the charge igniting, and the charge being removed.

A piston and cylinder are no different... but depending on many things, including intended operating speed of the engine, some factors are much more important than others.

The concept of a dished piston in a flat-topped head, is exactly the same as a flat-top piston in a hemispherical head... (except... that it's upside down).  The shape of the head compresses the mixture into an area that, instead of being flat like a pancake, is round and small like a ball.  In both cases, the fact that compression is going in three dimensions, the mixture is being jostled around good (turbulence).  This disperses fuel well, but also causes the flame DISTANCE to be very short, at least, when the cylinder is close to TDC.

In order for 'drilling holes' to have a positive effect, many other things would need to happen- your compression height, for instance, would need to be increased, so that the 'sides' of the piston edge get up, basically against the head (without contact, of course).  In order for it to yield a short flame speed, the 'holes would have to be extremely wide, and without sharp edges, so as to not wind up 'shrouding' itself.  Shrouding prevents proper discharge of spent gases... and sharp edges cause hot spots that cause detonation.

A shaped chamber means nothing when the spark plug isn't well located.  Likewise, a shaped chamber is irrelevant without having it match the motion and location of the intake and exhaust valves.  As noted above, many guys machine 'pockets' into the piston top- this is solely to provide critical clearance to intake and exhaust valves while at TDC exhaust/intake stroke, to prevent a collision under high lift.

It may help to understand this by considering the combustion chamber shape of a flathead engine... with the valves off the side, the flame path and airflow is a hideously contorted mess, with the valve area being a terrible waste-of-space.  There were a few engine designs, however, where the piston was shaped to a wedge, and the spark plug placed just off to the side of the valves, to move the entire combustion chamber area volume to a point concentrated near those valves, and the piston's TDC clearance to head was very small... thus, the charge was all crammed into the 'pocket' directly above the valves.  This worked great for getting the charge consolidated around the spark plug, and it was pretty darned turbulent, but in the long run, the engine design didn't breathe well at high RPM because the valves were basically buried in a sock drawer.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMiller Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2019 at 7:57am
That same enhanced piston design of the flatheads also caused considerable side loading on combustion, that created counter intuitive drag as the piston tried to press to the cylinder wall opposite the combustion area. Was not known as a good alteration.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 22 Mar 2019 at 7:45pm
That is EXACTLY right- it caused substantial sideload wear.  OHV designs took over for very good reason:  Simple physics.  Silly thing is that some manufacturers insisted on sticking to L-head designs, when industrial engines (like Allis) had been OHV from the git-go.  Go figure... a FARM IMPLEMENT of 1934 being manufactured with more advanced technology than an AUTOMOBILE of 1953... of course, Chevrolet ran exclusively their OHV inline six from '29... Confused
Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tbran Quote  Post ReplyReply Direct Link To This Post Posted: 23 Mar 2019 at 12:49pm
The D17 head is FLAT except for a small V opening for the plug - the pistons we had were Flat and too high compression - thus the 'milling'  not drilling of a small bowl under the plug...  again it lowered the compression just enough - we have no way of knowing if  it added or subtracted to anything - just seemed logical at the time...  We do know that the latest 226 power crater engine piston as in the 175 made as much or more hp than the overbore kits for the same engine... we went overbore and higher compression to try to gain hp...  we have gone 'overboard' on many projects in our lifetime - we have the life buoy callouses to prove it... but it has been fun... one becomes an expert usually through knowing what not to do - the school of hard knocks is the best teacher , but has the highest tuition rate.. 
When told "it's not the money,it's the principle", remember, it's always the money..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DrAllis Quote  Post ReplyReply Direct Link To This Post Posted: 23 Mar 2019 at 2:15pm
I had a wise old feller once tell me:  " Don't make all the mistakes by yourself. Learn from others mistakes too."
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2019 at 1:00am
Originally posted by tbran tbran wrote:

  the school of hard knocks is the best teacher , but has the highest tuition rate.. 


That is the eternal truth.  Life is school, but school as we know it is backwards from life.  In life, if you survive the test, you learn the lessons.  ;-)

The Allis engine is a demonstration of industrial engineering in every concieveable aspect... and here's one little piece:

When you cast parts that have to mate and seal, they need to be FLAT.  One cannot 'cast' metal to be flat, because it comes into the mold as liquid, and as it cools, it shrinks.  Thicker areas cool slower than thin, but fortunately, the presence of dense casting sand SLOWS the cooling, so shrink is relatively controlled, but after it's done, the casting may LOOK flat... but it's not.  to make it flat, it needs to be planed, or milled, or ground.  The Allis four head is flat, and the piston is dished, because it was, from a manufacturing perspective, more successful (in their realm of economics and function) to make a flat surface of it, and install the valves, etc., with the only 'shape' being that little pocket for the spark plug... than try to make a casting where the combustion chamber shape was all contorted.

Simple way to understand:  When you prepare a cylinder head for a small block V8, you have to CC EACH CHAMBER, because... they vary in CASTING.  Sometimes the core is slightly shifted, hence, some chambers exhibit a little more volume than the other.  Each one must be carefully 'worked', so as to not throw off the volume of the chambers... and of course, each piston's pin height, shaping, etc., all have to be the same.  Consider all the operations done, and realize that each machining operation is expensive in terms of manufacturing resources... and when all done, core shift may make SOME chambers' wall thicknesses different from others... that means that SOME chambers may have areas that cool faster (because the casting passes heat into the cooing flow faster), and some have 'hot spots' where they're really thick.

With the Allis engine... there's no 'bowl work'.  Just make it flat.  The critical volume is in the PISTON.  the piston starts as a blank, and the manufacturing process performs all the machining in just a few tooling setups, but because of the tool setups, piston manufacturing can be very, very repeatable.  The result, is that every chamber's volume is very consistent... every cylinder's compression pressure is consistent, and each cylinder's flame path, burn velocity, temperature and brake pressure is consistent... and it's that way with the FEWEST number of secondary machining operations, the fewest number of setups (each setup introduces possibility for error-  loss of consistency).  This is important, because when all the cylinders exhibit the same burn characteristic, they all like the same mixture, they all run the same temperature, and just one carberuator setting makes all four cylinders happy.

This is just ONE SMALL piece of a huge industrial engineering demonstration that proves... Them guys at Allis really had their $#it together.

I bet someone's already done it, but I haven't seen one... a pair of 226 blocks fitted together to make a 452ci V8... with no change in anything, I imagine that'd be in the realm of 575 ft-lbs at 900rpm or so...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DrAllis Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2019 at 7:12am
The most important benefit (to me) of the basically "flat" cylinder head  design is the fact that it never (under normal use) seems to crack. Every M Farmall I know of has a cracked cylinder head when disassembled, as does a small block Chevy.  On older AC heads the 3/8" deep spark plug hole may show a crack, but I've never replaced a head because of that. New guides, grind the valves and seats and away we go for another period of many years of service life. Combine that with wet-sleeves so easy to replace and the only real send away part is the crankshaft. Many old Dealerships kept a re-ground  or new crankshaft on hand ready to go for that item. Quick overhaul times and long service life by design !!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote savedallis1953 Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2019 at 11:08am
Great information, thanks to all. My pistons shall be used as Allis Chalmers designed.
1953 WD, 1953 WD, WD engine with WD-45 crankshaft.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2019 at 11:56am
Originally posted by DrAllis DrAllis wrote:

The most important benefit (to me) of the basically "flat" cylinder head  design is the fact that it never (under normal use) seems to crack...   ...Quick overhaul times and long service life by design !!


When I was a kid, my Grandfather's neighbor pointed out that all it took to do an in-frame rebuild on an Allis, was a patient man with a flat file, a straightedge, and some cranberry juice.

At the time, I thought he was insulting the tractor, but I kept my mouth shut and ears open.  Years later, after I'd spent plenty of time with machine tools, and even more time with files, I realized what he meant.  The engineers kept the design so that all the machining processes as uncomplicated as possible.  A good quality flat file in the hands of a talented guy can make a very flat surface.  Add a good straightedge and some indicating dye, and you can hand scrape it like a machine tool.  I eventually figured out that the neighbor used to do exactly that with valve faces on steam engines... he used the Cranberry Juice as his indicating dye... he'd take a swig, spit some on the face, and lay his straightedge on 'em... LOL

On a V engine, just doing a simple skim of the gasket surface becomes complex geometry, because it affects the geometry of the intake manifold...

And to add to the mess...  the V engine is a 'crossflow' design- intake on one side, exhaust on the other.  From an airflow perspective, performance guys prefer the crossflow, but the side effect is that one side will be substantially warmer than the other... and Dr's note about cracking is much easier to understand.

Another (kinda unrelated, but notable) things... a crossflow design has the advantage of a lower fuel/air charge temperature... this means higher charge density.  From a peak power perspective, this IS somewhat important, but fuel atomization occurs best when the fuel is warm enough to evaporate fully before getting compressed.  In a day-to-day working engine, running at anything other than full throttle, it's difficult NOT to send unburned fuel out of the exhaust, so warming the mix a little before sending it into the chamber gets that evaporation done.  Having intake and exhaust ports in close proximity seems counterintuitive for extreme performance, but for variable duty of an industrial/ag environment, especially on a cold damp day, it really helps improve operability to have them nearby.  Not only do fuel droplets not burn, MOISTURE (from the air) which has condensed into droplets (due to pressure drop coming through throttle/venturi) become a 'barrier' to flame path... and result in incomplete combustion.

Yesterday afternoon, I recognized an excellent demonstration of the effects of 'shrouding'.  I had an old can of PVC cement... it'd turned into unusable gak, but still had plenty of aromatics in that can.  I have a dumpster, which gets loaded into a compactor truck... and rather than dumping things like that into the trash, I tend to make sure that the flammables are burned off, so they don't put anyone down the line at risk.  I took off the lid, pulled out the goo'dup brush, set the can out in the driveway, lit the brush, and dipped it into the can.  Of course, the vapors caught fire, but instead of burning fast, it just had a little flame barely out of the can.  Why?  Because the fire only exists where there's both fuel and air... and since the can was full of fuel, it was only burning where air contact appeared.  Without a source of oxygen INSIDE the can, it would burn no faster.  This is just one example of  'shrouding'.
Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bill_MN Quote  Post ReplyReply Direct Link To This Post Posted: 24 Mar 2019 at 8:16pm
I got a couple of MIT textbooks on the theory of combustion engines. One of the main conclusions I've drawn: the dynamics of combustion chambers are best left to the professionals...
1951 WD #78283, 1918 Case 28x50 Thresher #76738, Case Centennial B 2x16 Plow
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 25 Mar 2019 at 9:45pm
Probably same one you have, Mine was Charles Fayette Taylor: The Interal Combustion Engine in Theory and Practice, Second Edition, Revised.  C 1960, 1966, 1985 by massachusetts Institute of Technology, 1977 (first ed) 1985 (2nd Ed) 1998 (9th printing)  M.I.T. Press, Cambridge, Mass and London, England ISBN 0-262-20051-1.  Got it (both volumes) sitting on the shelf by my desk here.

Suffice to say that while it was written by Professionals, and taugh by Professionals, the guys that learned the most, weren't the guys who studied it in a room with a chalk board in front... it was in a dusty shop with a beer fridge by the door, and waterbrake dyno in the back room.  ;-)

Most of the principles are very easy to understand, as long as they're described using a relateable context.
Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.
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