With more fins, good baffling plus a head designed to reduce heat uptake by the exhaust valve stem, it seems 75 reliable continuous HP should be available from an air cooled Type 1 without a problem. That's great, and it starts bumping against other limits (esp of the Mg case and bearing saddles). Water cooling might still be attractive to some folks, given the more uniform head temps possible and the packaging options. I'd think planes designed for an 80 HP Rotax 912 might be adapted readily (especially faster ones where a smaller prop wouldn't be a big problem). A Sonex with an underbelly rad scoop would look fine, and shaving some cooling drag while allowing more continuous HP (say an honest 75 HP with moderate CHTs) would be welcome.My personal preference would be water cooled heads. But that requires a whole different set of modifications - and some new airframes designed around a radiator.
Always good to go back to make sure we are actually going down the correct path. Not so sure I'm doing that but I'm so far down the road I'd kind of like to see the if the gate is open or closed at the end.
My personal preference would be water cooled heads. But that requires a whole different set of modifications - and some new airframes designed around a radiator.
Water cooled heads have been made in the past by simply TIGing on some metal around the existing top and bottom fins with some tubes to seal off around the head studs. There are such things as bolt on water pumps.
View attachment 119041 Not a stock head, but the same idea.
I too would like to see a modular approach like the 911. There just isn't enough room on the 1/2 VW head unless you basically copy the Mosler's in post #158. For a 4 cylinder it looks like it might be possible.
For now the 1/2 VW head set looks like a simpler path. Pic of the stock VW pushrod angles (sketch) and what could be done with individual stud mounted rockers:
View attachment 119042 If the rocker studs were screwed into a rocker carrier similar to the 911 then there would only need to be A and B heads. With a siamesed head, like the stock VW, we would still need right and left castings to fix the pushrod angles. Or just accept the lousy pushrod angles and just move the exhaust port for better exhaust valve stem cooling.
If only someone would put the 356/912 crankcase back into production............
The available VW aircraft heads are so inexpensive and so readily available that doing much work on them is cost prohibitive.While VW cyl heads are cheap, the issue is they WILL crack and they need service at short intervals, and often need to be replaced rather than renewed. The value in a modular billet or cast head system is that the front end cost is a little higher, but the long term cost of ownership and increased reliability outweighs those initial costs. Being able to replace a single head or a single rocker box is a good thing to have as well.
The available VW aircraft heads are so inexpensive and so readily available that doing much work on them is cost prohibitive.
Here's the head Sonex sells, already drilled for a second plug. $325 per side, with new valves, springs, guides, bored for 92mm cylinders, etc, ready to run. Now, I suspect aftermarket heads with better cooling are available (thinking hard about the CB Panchito heads....), but they are all relatively inexpensive, and that can make things tough for someone trying to make a better mousetrap, to be handcrafted in small numbers.
Same problem for aircraft conversions.It was also shopped around to offroad racers but the price and complexity was just too discouraging.
I suspect Hot wings is right to concentrate on heads that can appeal to the 1/2VW market.
Fins need to have a rather critical thickness to length ratio and spacing distance. The requirements change for type of airflow(pressurized and if high pressure or low pressure or free air). A good example of high pressure air cooling was used on the Corvair engine. Low pressure air cooling was used on VW's. I used to have the formulas for aiding in determining fin requirements. They were on the net some 20 years ago. Possibly someone can still find them.Sorry, but just to re-mention an idea previously brought up by BBerson many posts ago: maybe the head casting and machining ($$, time) could be simplified a lot through the use of add-on extruded AL fins. That just requires flat spaces on the head, a few tapped spots to affix the fin blocks, and the right heat transfer paste. It's easy and cheap to extrude very fine fins (compared to casting them, cleaning out flash, etc), and stock heat sink extrusions are widely available (experimentation will resolve suitable mix of fin thickness, spacing, length, etc). It also allows easy customization for different possible airflow directions. Want bigger fins near the exhaust port? Bolt 'em on.
There might even be folks that would use the flat pads as a basis for oil or glycol cooling of the heads. I think air and AL heat sinks make more sense but it is another option, and it would open up other packaging possibilities.
Again, sorry to step back, but as you are now deep into the details of prototyping I didn't want a possibly good idea to be overlooked.
Right. The fins have to be right dimensions, at the right spacing, to optimize heat transfer to the passing air. There are a lot of variables, but we know a couple of things:Fins need to have a rather critical thickness to length ratio and spacing distance. The requirements change for type of airflow(pressurized and if high pressure or low pressure or free air). A good example of high pressure air cooling was used on the Corvair engine. Low pressure air cooling was used on VW's. I used to have the formulas for aiding in determining fin requirements. They were on the net some 20 years ago. Possibly someone can still find them.
Just adding length to fins may not accomplish much.
Increasing number of fins by reducing space between the fins can compromise the air flow between the fins needed to remove the heat from the fins.
A fin that is too thin for length will get cold easily but will not have sufficient thickness to conduct much heat to the edge of the fin. A good demonstration of this is to remove a 2 inch wide strip from a roll of kitchen aluminum foil. Crease it lightly lengthwise so it can be held by hand from one end and remain rigid. The opposite end can be heated with a torch or stove burner to the point of melting yet the hand held end remain cool enough to hold onto with bare fingers.
Look at that Jab head for inspiration?. Just machine flat pads on the outside of your head wherever it is convenient, screw on a block of extruded fins as thick and tall as you want, and bore down into it to make clearance needed for any bolts, spark plugs, exhaust flanges, etc you need. No flash to clear out, no fragile fins on your casting, etc.Fin thickness and spacing? I'm using monkey see, monkey do. Limit is probably going to be what I can mold/cast.
I usually get the answers to questions that is troubling me doing the middle of the night when I wake up with the answer. I'll try that shower thingI thought about the idea of different housings for ground bound. The more I thought about it the less appealing the idea became. Most (all?) of the non-stock VW heads have been aimed at high flow race use.
Valves and ports for 100 efficient Hp @ 3600 rpm (very optimistic) probably don't work so well at 6000+.
What is the best engineering tool available? Lots of people would say Excel.
For me it is a shower. I seem to get an inspiration to solve a nasty problem after only a couple of minutes in the shower. The whole idea of utilizing the 911 style modular heads was about getting to the point yesterday that I was considering giving up on the idea. I just couldn't find enough area for the bolts needed to keep everything together. I think I may have figured out how to bolt a rocker carrier to a single head, or a pair, during my last shower.
New problem - My CAD computer has taken to just simply shunting down at random. Nothing hot, but I suspect a power supply.
Fin thickness and spacing? I'm using monkey see, monkey do. Limit is probably going to be what I can mold/cast.
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