The 'washer trick' cooling plenum modification.

JackinMichigan · #1 04-19-2018, 06:03 AM

I want to knock a few degrees of my #6 cylinder head temps, and I've heard tell of a quick-and-dirty plenum modification called the 'washer trick' which apparently entails fitting a flat washer on the back side of the plenum to space it out away from the cylinder head.

Looking at and all around my #6 cylinder I'm not seeing a place where that can be done. The #5 cylinder has a handy plenum attachment screw right on the back surface, but I'm finding no such screw on the #6 cylinder.

I've already constructed a plenum bypass duct for the #5 cylinder, which I just completed yesterday and I'll find out this weekend how well it works, and I want to make a quick mod to the #6 cylinder while I still have everything torn apart. If I can just knock 10 degrees off it I will call it good.

BillL · #2 04-19-2018, 06:23 AM

I have the Lycoming heads and they have flashing restricting the through fins just under the spark plug. I cleaned them out some and got 10F off #4. I just measured this yesterday. If you don't have flashing, then . . . well . . . . never mind.

Process:
I took a 6" #40 drill bit, a 10" (minimum) length of 3/32 piano wire, and a Grobet Swiss round file ( "0" cut) for tools. Take the drill and put the first hole in the end of the narrow slot. Then stick the 3/32" paint wire in that hole and angle your drill bit to drill the second hole as close as possible. Repeat to the end of the slot. Then put the drill bit in an end hole put some side pressure on it, and drill while stroking the bit and wait as it nibbles its way to the end. Then, take the tiny file and get hand cramps doing the remainder. The slot starts at about .030", and approaches .115" maximum at the end. A #30 bit is just a little too large.

I have some extra files if you want, typically ~$30 each, I'll send you one for $10 + usps shipping.

Toobuilder · #3 04-19-2018, 06:54 AM

Keep in mind the "washer trick" only works because it creates a path around the zero fin depth area -emulating the built in "duct" formed when a siamesed cylinder is present. #6 cylinder already has that duct up against the baffle so more spacing is not required nor helpful. If there is no casting flash present, then I'd look at leaks in the intercylinder baffle(s) and/or fabricate some upper (top side) fin wraps to get more of the fin area working.

TShort · #4 04-19-2018, 07:27 AM

The loss of the pics in those other threads is killing me.
I had them bookmarked from before, now I wish I had taken screenshots or saved the images...

JackinMichigan · #5 04-19-2018, 07:35 AM

Quote:

Originally Posted by Toobuilder

Keep in mind the "washer trick" only works because it creates a path around the zero fin depth area -emulating the built in "duct" formed when a siamesed cylinder is present.

Can you please elaborate on this? I'm afraid I have no idea what the above statement means.

David Paule · #6 04-19-2018, 08:38 AM

Have a look at this thread.

Dave

rocketman1988 · #7 04-19-2018, 08:43 AM

The short story is the washer trick only works on cylinder #5 (or 3 on a four cylinder engine.

BillL · #8 04-19-2018, 09:07 AM

Quote:

Originally Posted by JackinMichigan

Can you please elaborate on this? I'm afraid I have no idea what the above statement means.

Looking at a parallel valve head from the rocker end, the left side has the head fins nearly non-existent. The right side is full depth, nearly an inch. Since all head/jugs are the same, this ends up on the back of #1,3,5 heads an front of the #2,4,6 heads. If it is between heads there is no issue as the shared open area of the adjacent head provides air flow, if it is on the front (#2) or rear (#3 or #5) then the baffles must accommodate.

Here is a picture, some heads are worse than this one. Are we all clear now?

Toobuilder · #9 04-19-2018, 09:08 AM

Quote:

Originally Posted by JackinMichigan

Can you please elaborate on this? I'm afraid I have no idea what the above statement means.

DanH had some great visuals on this but they are gone. I'll try to illustrate with words:

The Lycoming cylinder cooling fins are not symmetrical with regard to the cylinder bore. On the exhaust side to the head the fins are deep even at the narrowest portion at the "equator". The intake side however has much lower overall cooling requirements and consequently the cooling fins are minimal. So much so that the fin depth is actually "zero" at the equator for a section of fins. This section of fins happens to be the ones that have the lower wrap though, so airflow is required. With the cylinders siamesed, there is a duct formed by the generous fin depth of the adjacent cylinder, so the lower wraps of each cylinder are both fed by this single shared duct. When you place a flat baffle against the intake side fins however, this shared duct is gone and the lower wrap is effectively starved completely.

Therefore, the "washer trick" opens a small slot (the thickness of the washer) to allow some air to reach the lower wrap. Done poorly, there is a large spill across the whole back of the head and though it will move some air where needed, you waste a bunch of Deltap doing nothing. The "bypass duct" is designed to emulate the lost duct area of the adjacent cylinder while still maintaining a good Deltap. In other words, it's providing the cooling as Lycoming intended.