The next piston option we are going to look at is one we developed in-house, as a purpose-built, clean sheet of paper concept, to bring advanced combustion characteristics to Corvair flight engines.
Most people who work with engines understand that the shape of the chamber in the head has a great effect on how an engine runs, particularly in challenging circumstances like aircraft. What a lot of these people miss it that at top dead center on the compression stroke, the head chamber is only half of the shape of the remaining space, the top of the piston is the other side of the equation. While a flat top piston works in many cases, it isn’t the optimal design to compliment the combustion chamber in flight engines. Flat top pistons are less expensive to make, but if you are willing to go a bit further, there are significant advantages to be had.
If you’re talking about putting the piston in a plane, the single most significant advantage is detonation resistance. If you think that you would like to run auto fuel extensively, using a piston like the designs we have in 2,850s and 3,000cc Corvairs is a very significant advantage. Regular flat top Corvair pistons can be run on auto fuel, people do it every day. Smart people doing this reduce the timing of the engine and enrichen the fuel mixture slightly, and resist the temptation to aggressively lean the engine. These things provide the margin of safety. On a 2,850 or 3,000 with our pistons, the margin of safety is provided by the shape of the piston head, and there is no reduction in performance required to have a very wide margin on decent auto fuel.
The 2,850 design predates the 3,000 piston by a year, so it is correct to see the 3,000 piston as a ‘big’ 2,850 instead of the other way around. The bore on a 2,850 is 90mm, which works out to .105″ over the stock 3.437″ (The 3,000cc bore is 92mm). The cylinder we use for the 2,850 is the full fin, thick-walled Clarks cylinder they have had for a number of years. These are new, not rebored. We have sold the 2,850 piston/rod cylinder package as a kit for a number of years. Using our numbering system, lets look at how this fits in:
Piston and rod group (1300)
1300- Piston set with wrist pins
The set comes with their own pins in the kit. Pistons are set to use either standard Corvair rods or ones bushed for floating wrist pins.
1301- Ring set
The rings that come with the kit are made by Total Seal, and they are specified by the Piston maker as the optimal ones for the piston design.
1302- Connecting rods -6-
The rods that come in the kit are standard rebuilt ones with ARP bolts. Most of the kits we have sent out have had the pistons mounted on Clarks 9203ww rods, some of the kits were delivered with upgraded rods with 12 point ARP nuts. We mount the pistons on the rods for the builder.
Cylinder group (1400)
1400- Cylinders -6-
The Cylinders in the kit are the Clark’s HD, full fin, brand new ones. Clarks has these cylinders in many sizes from STD to .060″. The 2,850 is bored .105 over and requires the boring bar to make several passes to get it his big, thus they cost more than the same cylinder with a smaller bore. A replacement 2,850 cylinder is an on the shelf item at Clarks, and has it’s own part number, C-11628ww. The Cost is $84. The six cylinders are part of the kit price.
1401- Base gaskets -6-
The base gasket that I prefer for all Corvair engines is the all copper Clark’s part number C-1180. As a part number for an individual gasket, you will need to order six of them for a complete engine. We install them in the engines dry, with nothing on them. We built engines for many years with the stock steel gaskets, but they are less forgiving than copper base gaskets. A set of six is about $42.
1402- Head gasket set
For all 2,850s the standard head gasket that we recommend is a .032” solid copper gasket. These are available from Clark’s, part number C-3946 The head gasket set is about $30.
We sell the 2,850 kit for $1,750. The gaskets to complete Groups 1300 and 1400 bring the total to $1,822. Notice that this is $827 more than a 2,700 builder will spend. What is the attraction? First the combustion chamber shape, second the pistons are even stronger than the sealed power ones, third, up to 9 more cubic inches, and fourth, that the cylinders are new. If a builder had 3 or 4 core cylinders that have chipped fins, paying the core charge from Clarks gets part of the way into the price difference. If a builder is considering leaving the option open for turbocharging later, this is a much better combination to work with than a 2,700. Below, some photos and commentary about the builders who are using 2,850 engines now.
In the next instalment we will look at an imaginary piston.-ww
Above, a drop forged, made in the USA piston for the Corvair. The displacement of this piston is 2,850 cc. Look at the dish in the head of the piston. Notice that it still has a quench area to match the one in the Corvair head. This piston is designed to allow the head gasket step in the head to be completely cut out, have a quench height of only the thickness of a .032″ head gasket, but still have less than a 9:1 compression ratio with a 110 head. With a 95 head and the quench clearance equally tight, the compression ratio is below 8.4:1. The former should be an ideal engine to run on unleaded gas or 100LL. The latter is specifically set for being run with a turbo, or with standard auto fuel. I like the concept for a number of reasons. It is the largest bore that can be used without modifying the case and heads, the way you must with a 3,000. the ready availability of Clark’s new full-fin cylinders that can take this kind of overbore eliminates any special machining to the cylinders, other than boring them .105″ over.
Above, the 601XL of our west coast man, Woody Harris. It has flown all over the country on a 2,850. Note that Woody is from northern California and the photo above is at Kitty Hawk NC. Read woody’s story at: Zenith 601XL-2,850cc, Woody Harris
Above, Jeff Cochran (on the left) the day he passed his airworthyness inspection. Jeffs 750 is powered with a 2,850. Read Jeff’s story at: New “Zenvair-750″, Jeff Cochran, 2,850cc engine, N750ZV
Above, Ron Lendon of Michigan flew in to Brodhead 2012 in his scratch built 601XL. It was originally powered with a 2,700 but he later upgraded to a 2,850.
Above, Roger Grable and his grandson Graham, with their running 2,850 at Corvair College #23. Read their story at:Corvair College #23 – 2850cc Engine, Roger Grable, CH-750 Builder
Above, Blaine Schwartz’s 2,850 engine a few minutes into its test run at Corvair College #22. Blaine is in the blue hat, his 750 is almost done. Read his story at: Schwartz Engine Runs at CC #22
Above, Clarence Dunkerley at Corvair College #21, beside his 2,850 which we later ran. The engine is now installed on his completed Cleanex.