Quote:
Originally Posted by Cam
I take it your asking Tom, but my reasoning is it is cheaper, and simply reverses the problem at hand. Unless thicker gaskets are not good for some reason...
Out of curiousity, would you re-machine the pistons instead?
|
If it cannot be corrected by a few thousandths adjustment via the head gasket, then yes cutting the slugs is my method. Stacking HG thickness is fine but you have to control the TOTAL on a boxer motor. Simply stacking HG thickness to get piston clearance solves one issue but can create a whole other can of worms. Doing so forces the heads farther out from the directed OE centerline and throws the cam timing off. One can check this with a set of dial indicators and degree wheels on a motor and see the difference between a new set of heads and .015" decked heads
I've lost track of how many flat motors I've built between Porsche, Ferrari, and Subaru, but I've run into the issue with about 1/3rd of those motors. Old aircooled P-car motors you actually shim the cylinders with different thickness gaskets, and Porsche was kind enough to give the motors infinitely adjustable cam timing in the sprockets to correct for the varying dimensionality(or aggressive cams for that matter)
Most recent was a 2.6L Cosworth build for a customers LGT. New OE EJ257 case(prebuilt SB via Cosworth), decked heads(not by me). Once the belt took tension, left bank was off near 2 degrees, right bank a hair more and was uncorrectable by any available HG....Unfortunately being a Cosworth SB I had little control and had to correct the issue with eccentric timing belt idlers(There's a reason why they exist now aftermarket
) Now a degree or two on a regular motor is livable, but on a high comp motor is can mean the loss of some notable power, and cam profile can further alter the outcome.
I in no way want to force my ideals down your throats here. This is just what I do for a living and just want to give you my thoughts based upon my experience
When I build these type of motors for race purposes it's a process to say the least. But then the average porsche race motor build runs around 12-18 grand and it's 100% understandable that an Eg33 build may not have the financial backing to support every bit of work that should go into it, and that a sacrifice in one area to make up for a good chunk of money is a common route to take. It's not "wrong" or bad in any way.
One thing I forgot to add here.......In reality in a high dollar motor build all the machine work is spec'ed out and done PRIOR to buying parts(pistons, rings, bearings, etc) The pistons are then machined to fit the motor, so in the true big dollar builds it's never an issue as all dimensionality is factored in and the slug is tailored to suit that exact motor from the start. One a motor such as you EG33, this is usually never the case as you're using off the shelf components.
Side detour here since were on the subject of opinions: If the CH of the piston puts the domes above the deck height of the case enough to make contact with the combustion chamber, modifying the heads would require changes to the quench(and pending the motor and pistons involved) valve seat area, etc, etc. Without the benefit of 5 axis CNC it's substantially more difficult to machine combustion chambers 100% mirror to each other than it is for the pistons, and in a motor of this nature you want the heads perfectly CC'ed to each other. Setting up the pistons in a mill(or lathe in some cases) and removing the required material evenly if far easier and cheaper. In some case one has no choice but to modify the chambers, but this is done in accordance with pistons modifications, and then followed by a proper CC of the heads to balance them. CC'ing heads on a high comp motor is essential.
Can't say I know anyone that would modify the combustion chambers on a set of CC''ed or CNC'ed high comp heads over the pistons to achieve the goal here, unless the heads were not up to spec to begin with.
Here's a panoramic view of the main machine floor(so you don't think I'm blowing smoke up your azz )
Our cleaning room to the left(shot blasters, glass and soda machines, chemical baths, autoclave and preheat ovens, teardown rooms), Office and parts/supplies area beyond the wall to the right(actually centerd in the picture), engine dyno facility through the blue overhead. Crank machines, rod conditioners, line hones, etc to the right of the picture upstairs is the ~6,000 sq foot of core storage(heads, cranks, cams, etc), CNC, etc.(facility totals about 16,000 square feet, use to be an old mattress factory in the early 1900's-1950's) Anyone local is more than welcome to stop by for a little tour if you're in the area.
Hey, at least were all in this for the fun similar reason right
I think I've jabbered here enough for one post tonight, lol