Saturday, December 16, 2006
With more than 22,000,000 air cooled engine’s-worth of experience you can bet your bippie Volkswagen knew what it was doing with regard to the cooling system. Everything you see on the engine is there for a purpose and the only thing you will gain by leaving something off is a reduction in the engine’s useful life.
One of the most critical parts on VW heads is the small air-dam located on the underside of the head between the combustion chambers. Without this air-dam in place most of the cooling air reaching the heads is lost. With the air-dam in place, the gush of cooling air is obstructed and directed to either side, bathing the underside of the combustion chambers. When the cooling air is slowed down in this manner two things happen. The air picks up more heat since it is in contact with the heads for a longer period of time. The second – and more important thing – is that the pressure of the cooling air above the heads will rise.
Didja get that? The cooling air pressure goes up with the air-dams in place.
It is that subtle increase in pressure that allows the cooling air to be forced toward the corners of the heads, out toward the exhaust valves – the hottest parts of the engine, where it is needed most.
All of the new heads I've seen in the last five years or so did not include the little air-dams on the underside between the chambers (ie, where the thermostat rod goes). They are also missing from most overhauled heads. As the Mechanic-in-Charge it is your job to ensure they are in place.
I make these out of .010" stainless steel shim-stock, secured with fine SS safety wire.
I cut the air-dam to fit with scissors (!) then drill a small hole near each corner. I drill one hole through the middle fin in the center of the area to be covered by the air-dam and thread two lengths of safety wire through the hole, then thread the wires through the four holes in the air-dam and work it down the wires until it rests flat. Then I simply cross the wires, twist them tight, snip off the ends and press the twists flat against the air-dam.
Yeah, I know – Mexican-built beetles don’t have them installed. Nor do they use a thermostat. But you’re comparing apples to oranges. The stock 1600 engine built for domestic (ie, Mexico) use is fitted with dished pistons giving it a compression ratio of only 6.6:1, allowing it to run on low octane Mexican gas. In that configuration it is DERATED to 45bhp. This engine uses multi-point FUEL INJECTION. Cooling is handled by the engine’s on-board COMPUTER which automatically reduces the injector on-time as the cylinder head temperature increases.
You can improve the cooling ability of your heads by a considerable amount if you will abrade the fins with COARSE media. (I use #80 sand.) DON'T blast anything else on the heads but DO try to get down between the fins. Mask-off the chambers, all of the studs, the exhaust ports, inlet port & sealing surface, and the entire valve gallery, taking special care to insure the gasket rail remains mirror-smooth.
Abrading the fins increases their surface area; the more area, the more heat that can be transferred to the cooling air.
Note: Normally, abrasive media is NEVER used on non-ferrous materials; heads are usually cleaned with walnut shells. But this procedure is now commonly done by the manufacturers of aircraft engines as a means of improving the head's cooling ability.