Thursday, April 10, 2014

MEL Engine Series - Combustion Chamber & Piston Design

This post is about MEL combustion chamber & piston design. It provides information to all MEL owners who need a new set of pistons. The topic at hand is why they should buy OEM stepdesign pistons (not to be confused with a pop-up design) & where they should buy OEM design pistons.

What is the difference with other engine designs? The difference is that the combustion chamber is defined by piston dome design and not the combustion chamber design in the cylinder head itself, as it is in most other designs. Each cylinder features an angle wedge combustion chamber at the top of the cylinder bores. Put differently; the MEL cylinder bore is sliced off at an angle. Therefore when you bolt on flat head surface with valves also flush with the flat surface, what you have is a valve opening into a cylinder bore, rather than into a combustion chamber in the head. The MEL cylinder bore is therefore the combustion chamber. The combustion chambers-in-block are formed by casting the top of each cylinder bank on a ten degree angle to the piston. Since the two surfaces of the combustion chamber (cylinder wall and bottom of head) are precision machined, and the third surface (top of piston) is smoothly cast, closer tolerances are maintained, resulting in better regulation of compression ratio.

The turbulence-top swirls the fuel mixture in a controlled direction at high speed assuring virtually complete combustion for finer performance of the engine. As the piston reaches the top of the compression stroke, the turbulence-top step drives into the narrowing wedge of the combustion chamber. The forced pressure jets the fuel-air mixture at a high velocity across the spark plug electrode, giving optimum combustion (source: 1958 Mercury Maintenance Manual).

Why is replacement piston design so important for the proper operation of this engine design? If you change one of these components by using flat top pistons you have changed the most important thing of this engine. You will get less performance and bad combustion. Flat top pistons are not designed specifically for the unique combustion chamber design of the MEL. Without the proper piston, you will not realize the power and efficiency of the design. Whether one decides to use this replacement piston is entirely his choice, it is important for the user to understand what is being lost. 

Due to the unique engine design of the MEL with its plain heads and the 10 degrees chamfered block deck the piston dome is the most important and only feature for the combustion chamber design. Simply put, the flat top pistons offered by some parts suppliers will negate the design of the engine, resulting in nothing more than an air pump as the actual combustion chamber is defined by the dome of the piston. The piston step is on the "high" side to force the mixture into the pocket. The cylinder block deck is actually chamfered 10 degrees to complete the chamber configuration. The head surface may be flat or have actual chambers depending on year and design series of the engine). 

The early heads are flat surfaced; later designs had a small pocket to decrease CR (and slightly different piston crown configurations). The original (featured in late 59) MEL 8.5 to 1 pistons have the necessary wedge/squish area. The special engineered design of the chamber squish the gases to the sparkplug and give a miss detonation free fire with out any ping. Flat top pistons will result in lower compression. If you desire reduced compression (modern fuel has lower octane) with the correct designed piston tops, you need to find a lower compression ratio set as used in the 60/62 430 2V (also 60 383 2V). On a side note; the MEL pistons are specified L or R due to piston pin offset. The straight surfaces of the combustion chamber walls eliminate deposit forming.

In sum: the generic piston dome shape will result in more than just CR loss, it will defeat the original design of the actual combustion chamber. To have this engine type function as designed and properly, the original piston dome design (or slight variant) must be retained or the engine will simply become an air pump.

Generic flat top pistons:

Original OEM replacement pistons are not available unless one finds a NOS set. There is just not enough demand to mass produce them. Correct replacement pistons are available but expensive. This is a WISECO forged piston, recently installed in an EDSEL 410CI E-475 engine.

Owner Ted E. comments:
For conventional measurements, I simply summed half the stroke, the rod length, the piston pin location, and the distance (deck clearance) from the top of the piston to the deck as measured at the very top of the bore. This is how I calculated the wrist pin location for the Edsel 410. I'd have to get out the trig formulas to actually calculate the distance from the top of the piston to the actual deck at the center of the bore but that measurement wasn't needed to actually order the pistons. All I needed off the deck was the actual angle measurement so that the dome would be parallel with both the deck and head surface.

Compression ratio was lowered by simply making the valve reliefs deeper. Ended up with 9.25:1 compression ratio. Block was cut 0.030 and pin height for the pistons was right at 2.000". Calculated deck height at the centre of bore before cutting anything off the decks was 10.482". That's figuring on it having the 90° decks and not the additional 10° that's added on. Final bore size is 4.225". I ended up with the piston at 0.007" in the hole at the top of the bore. Beats me if it's a high or low deck block as the manual I was looking at didn't give any deck height specs in that regard. All I can tell you is that both '58 410 blocks I was working on had very similar deck heights after dry fitting both of them with the same checking assemblies.

When dealing with some of the orphan engines, I like to deal with Brian at Wiseco. He's also a MEL aficionado which helped immensely in this particular case as He's quite familar with the quirkiness of the dome design. These pistons cost comparably the same as other custom pistons I've had done and although I don't have the paperwork in front of me, they were considerably less than $1000 for this particular set. Although I had considered a dish in the piston in the spark plug area of the piston, Wiseco's idea on lowering the compression ratio by simply making the valve reliefs deeper was a much simpler way to accomplish the same thing. As with any custom piston, these pistons will not have a part number that will be generic due to the specific dimensions I specified. I supplied the bore size, wrist pin size and location, ring specifications, combustion chamber volumes, etc. I additionally supplied the rod length, stroke, and various measurements with the cut down 383 piston to that Wiseco could double check my figures.

Below is another set of Wiseco pistons for a 1959 Colony Park with MEL 430, owned by Shelby#18.

Below are replacement MEL 462 pistons from Egge (which I ordered from Lincoln Land). They are the correct shaped pistons. Whether they are an exact copy of the OEM design is questionable but they are correct. As far as I know, it's easier to find 1966-1968 MEL 462 pistons than it is to find 1957-1960 MEL 430 and 1961-1965 MEL 430 pistons.

Another OEM after market piston:

Below is a notching tool for plug electrode clearance. There is also a similar tool(s) to fly-cut the piston for valve clearance(s).

Between '62 and '63 a few changes were made to the 430 that was used in Lincoln Continentals. Note the difference is piston design. Piston dome shape was reconfigured in 1963 to a new swirl design that included piston dome configuration and chambers in the heads. The 63/65 4V LINC 430 was the only one of the MEL/LINC series to use a chamber in the head itself.

These photos were taken by member BenderJ. He found 4 OEM .030" 1963 pistons on "a-very-well-known-auction-site" (the 4 pistons are from one side only).




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