Cooling your AeroVee Heads
Posted: Mon Sep 27, 2021 12:30 am
by karmarepair
Head cooling is THE limiting factor in longevity of your engine. PARTICULARLY the area around the exhaust valves. While the oil can and does carry away a lot of heat from the heads, you need airflow through the fins to carry off more of it, or else your exhaust valve clearance goes away, until you run out of adjuster threads, the head snaps off inside the cylinder or the valves or seats burn and you lose compression.
Leaving aside leaks in baffling for a moment there are two big issues I see with AeroVee installations.
- No “heat deflectors” plugging the gap in between the heads. See here for a discussion of that https://bobhooversblog.blogspot.com/2006/12/vw-head-baffles.html. I’ll post later about how I dealt with this on my heads.
- Lack of air passage around the exhaust valves. I’m going to elaborate on that here.
This is what a bare VW cylinder head should look like (from Bob “Veeduber” Hoovers Blog):
http://bp1.blogger.com/_JU6RC7jJfRc/Rq8HjOwE0lI/AAAAAAAAAt8/R_4vb8FG88w/s1600-h/blog_head.jpg
And take a look at this head comparison for “Dune Buggies and Hot VWs” http://www.cfiamerica.com/images/pdf/Ho … _Heads.pdf
Now look at your heads. You can see by the comparison that not all heads are shot full of holes around the exhaust port like the stockers. Some are cast solid - which you can get away with on a drag strip running nitromethane.
It’s easier to fix this on a bare head, but at least on the forward cylinders, it can be done on an assembled engine, with 6 inch aircraft drills, chainsaw files, and patience. Veeduber talks about using a pneumatic riffler or a jig saw or hand saw. http://bobhooversblog.blogspot.com/2007 … s-101.html
Once I figure out how to post pictures, I’ll show you my work in progress.
Re: Cooling your AeroVee Heads
Posted: Mon Sep 27, 2021 12:40 am
by karmarepair
OK, this is the Starboard forward head, minimally worked on - I think I’ve already drilled one of the web I.E. there was only one, partially blocked, passage for air in this area!
https://drive.google.com/file/d/1gCb4SjWlMly4Pa4W_ZiDIBYCtcpPYs9R/view?usp=sharing
And here is the Port forward head, with a little more work done:
https://drive.google.com/file/d/1Bz-6JnIfI0wLOWphBGhgshH6RussGL7g/view?usp=sharing
That dark area to the lower left has, since this pic was taken, been drilled out. There is another head stud under the dark area just below the head stud you can see, that makes it hard to drill that area out. To the far right, and out of the frame to the right of the spark plug, are other passage I’ve either opened up or drilled through the existing casting flashing.
Re: Cooling your AeroVee Heads
Posted: Mon Sep 27, 2021 11:06 pm
by mike.smith
Just as a point of reference, I have flown 500 hours behind my AeroVee. No modifications to anything from the original build from the kit, including the super tin, baffles and cylinder fins. No cooling issues even in the dead of summer. I borescope my valves at every annual, and so far they look as they should, with no burns or abnormal conditions. There is nothing inherently wrong with the construction and cooling of the stock AeroVee. If someone wants to make improvements, fine, but as a general rule it’s not necessary, and may just add time and money.
A VW will run hot during the first 25-50 hours. That’s normal. After that is should settle in. If not, then you can look for problems to solve.
My 2 cents.
Re: Cooling your AeroVee Heads
Posted: Tue Sep 28, 2021 12:54 am
by karmarepair
mike.smith wrote:Just as a point of reference, I have flown 500 hours behind my AeroVee. No modifications to anything from the original build from the kit, including the super tin, baffles and cylinder fins. No cooling issues even in the dead of summer. I borescope my valves at every annual, and so far they look as they should, with no burns or abnormal conditions. There is nothing inherently wrong with the construction and cooling of the stock AeroVee. If someone wants to make improvements, fine, but as a general rule it’s not necessary, and may just add time and money.
A VW will run hot during the first 25-50 hours. That’s normal. After that is should settle in. If not, then you can look for problems to solve.
My 2 cents.
All good EXPERIENCE BASED observations/opinions. My engine/plane has not flown, yours has.
AeroVee changed head suppliers several times. When did you buy your engine? Can you tell who built your heads (usually there is identifying information cast into the rocker boxes, which you can see when you pull your valve cover to check the valve clearances)?
Re: Cooling your AeroVee Heads
Posted: Tue Sep 28, 2021 1:41 am
by karmarepair
So, I learned more about my heads today when I pulled the valve covers.
First, they are CB Performance 044s. CURRENT specs (my engine is 20 years old, so it MIGHT not have all this kit):
- 12mm 3/4" Reach Spark Plug Holes
- 40mm x 35.5mm SUPERGRIP™ 214N Single Groove S/S Valves with Stellite Tips
- SUPERGRIP™ Chromoly Valve Spring Retainers
- SUPERGRIP™ Single Groove Valve Locks
- Performance 4 Angle Valve Job (30°, 45°, 60° & 75°) on seats
- Manganese Bronze Racing Valve Guides
- Tungsten Carbide Alloy Valve Seats
- Single Hi-Rev Chrome Silicon Valve Springs
My heads are also fitted with “Elephant Foot” valve adjusters (GOOD) and ratio rockers (which means stock stamped steel valve covers won’t fit).
More as I learn more.
Re: Cooling your AeroVee Heads
Posted: Tue Sep 28, 2021 7:28 pm
by mike.smith
My airframe is 7 years old, so the engine is about 9 years old. I originally had the nickasil cylinders, but quickly had to replace those. Since then all has been fine. Nothing wrong with cleaning up if there is slag in some of the head openings, but even that I never did (didn’t know to even think of it at the time). All has been running fine, so never went back and did anything.
Re: Cooling your AeroVee Heads
Posted: Wed Sep 29, 2021 7:43 am
by GraemeSmith
I keep a spare head “ready to go”. In case I get an exhaust valve problem. Two are EMPI and one I am not sure. Whatever was supplied around 2014 to the builder.
I’ve done some porting and polishing, some rebaffling, checked the area of the exit slot, cleaned up fins, and I agree. Keeping No 3 cooler is harder.
But I don’t think it is all about airflow. I think another factor is mixture. Simple naturally aspirated engines are notorious for not producing even mixtures across all cylinders. Aircraft pitch, air swirling into the carb differently due to airflow across the cowl, turbulence bumps producing different G loads on the mixture. All make it tough to get an even mixture.
With one head (despite porting it some more but not over porting it - I hope) - when the head is in the No 3 position - it runs leaner (EGT) and so hotter than all the others. I’ve juggled the length of the intake tubes on left and right side of the engine and managed to even the mixture across all 4 a bit - but it’s not perfect. A different head in the same position - different result.
I also tried the Great Planes inlet manifolds so see if I could change things. Frankly - as I said at the time - what was supplied was junk and they made no real difference.
So yes - getting cooling air to the fins is part of the story. But I think getting even mixture is another part.
I suspect the folks experimenting with fuel injection will find it much easier to deliver consistent mixtures and so get more even EGT and CHT’s.
Re: Cooling your AeroVee Heads
Posted: Wed Sep 29, 2021 9:39 am
by pappas
I have been posting my trials and tribulations with CHT’s for my turbo for some time. I just replaced both MOFOCO heads with EMPI’s supplied by Sonex. 3 angle cut valve job on these and the casting is much more substantial. My #3 went from my hottest cylinder to my coolest right away. #1 and #3 are my coolest cylinders #4 is close behind and #2 is 25 deg hottest but still not more than about 365 or so in cruise.
The EMPI’s had almost all of the flashing between the fins already removed when I got them. I am not a fan of the MOFOCO’s after switching to the EMPI’s.
Re: Cooling your AeroVee Heads
Posted: Fri Oct 01, 2021 6:55 am
by skyrangernz
I have been battling temperatures ever since I installed the Aerovee screw-type senders that are attached to the head as close to the sparkplug as possible. I did this because I spent a fortune fixing or buying the old fashioned ring under the plug senders as I kept breaking them when I removed/installed the plugs. The ring CHT’s are consistently 100 F hotter than the Aerovee probes. At full power and 3200 rpm, I get 325 F on the Aerovee probe and 425 F on the ring under the plug probe. I am at a loss as to which one to believe as 400 degrees is the published limit.
Photos show temperatures. Photos were taken after a 5 minute run at 2000 rpm and then at shutdown. Engine top overhauled, piston/rings/barrels/Mofoco heads. Engine not yet run in, only a 13-minute test flight that was terminated when the ring probes wouldn’t get below 425 F.
Thanks in anticipation.
Wayne
Re: Cooling your AeroVee Heads
Posted: Fri Oct 01, 2021 10:15 am
by daleandee
skyrangernz wrote:The ring CHT’s are consistently 100 F hotter than the Aerovee probes. At full power and 3200 rpm, I get 325 F on the Aerovee probe and 425 F on the ring under the plug probe. I am at a loss as to which one to believe as 400 degrees is the published limit.
Wayne,
Many moons ago a few of us (Charlie Radford & I were two of the guys) were known as the “Hot-Head Club” as we continuously battled temps on the VW conversion. At the time Charlie did exactly what you are doing i.e. running the plug & a separate fin mounted probes. IIRC, his testing found something like 30ºF difference between the probes. Quite a lot of the accuracy of the fin probe depends on exactly where it is mounted and how much air blows over it.
The original specifications for the head temps (still currently used), as given by Sonex at that time, were for the “under the plug mounted” CHT probes. Using the later model probes gives a bit of advantage to those that want to see lower head temps. On my current engine (Corvair) I use the probes under the plugs as I believe that is where the most accurate and consistent readings will be found. YMMV but I doubt it. Seeing 425ºF during break-in on a VW is not uncommon.
VW engines run hot at the power levels required of them in this application. That is not to say the engine won’t work but attention to cooling set-up is paramount and flying the plane in a manner to aid cooling (step climbing is one example) is required. Do all you possibly can to keep the little engine as cool as possible, and perhaps have another set of heads on standby.
The temps will get lower after break-in. The best I could do (many years ago) with my VW on a warm day was 385ºF in cruise on the hottest cylinder. I wasn’t real happy with that but I did put 175 hours on it before selling it and building the plane I have now.
Re: Cooling your AeroVee Heads
Posted: Fri Oct 01, 2021 10:20 am
by Scott Todd
You have to think about whats going on. That spark plug is firing hundreds of times a second generating explosions in the thousands of degree range. Look up combustion temperatures if you like. But you can look at the EGT for a simple analogy. It is running around 1300F. So there is 1300 degree air feeding the head underside around the spark plug hole. A large gradient occurs as the heat travels thru the head and the spark plug threaded portion to the outer surface near the plug. Measuring the temperature there and assuming the entire head is that temperature is not very accurate. For example, if you countersunk the probe half way down next to the plug, you might read 600-700. That would be considered unacceptable and ground the engine. That’s why Sonex recommends the probes screwed onto the head near the plug but not against it. This has been established thru years of testing. If you put the probes where they suggest and limit the temperatures to what they suggest, you’ll be just fine. Its pretty easy to pull the heads at annual or after 100 hours and take them to a VW shop. Those guys will tell you exactly whats going on inside there. Your temperatures look really good. Keep flying!
Re: Cooling your AeroVee Heads
Posted: Fri Oct 01, 2021 6:17 pm
by Area 51%
Scott Todd wrote:That spark plug is firing hundreds of times a second
Actually, the plug fires 50 times a second at 3000 RPM. And out of those, only 25 are responsible for an explosion. Still damn hot though.
I too installed the probes under the plugs initially, and fretted over the same 425+ degree reading till I tried the recommended probe location. Didn’t get a 100 degree difference, but proved to myself the CHTs were within limits if not still running on the high side. Extra cooling exits (Van’s louvers) brought everything (CHTs and oil temp) down an additional 20-30 degrees.
Re: Cooling your AeroVee Heads
Posted: Sat Oct 02, 2021 10:39 am
by Scott Todd
I was too lazy to do the math. 11 year of college does that 
I can’t say it enough, exit area is almost always the answer.
Re: Cooling your AeroVee Heads
Posted: Sat Oct 02, 2021 12:26 pm
by karmarepair
mike.smith wrote:A VW will run hot during the first 25-50 hours.
I can understand the oil temps being high, as various bearings and the rings find their Happy Place during Break In (which I plan to do ON THE GROUND, using a “hood” to direct prop blast through the existing baffling), but the HEADS? What physical mechanism would cause the HEADS to run hot?
Re: Cooling your AeroVee Heads
Posted: Sat Oct 02, 2021 2:49 pm
by AlexZ
Don’t do break-in on the ground!!. You’ll ruin your heads in no time. The prop doesn’t deliver enough air. You need airspeed to get enough air through the cowl by generating low pressure at the cowl exit. Keep ground running to a minimum. Adjust the Aerocarb as described in the manual and go flying.
The heads run hot because the original design was only 45HP and forced cooling. There’s simply not enough material to store the heat generated at full power during takeoff.
Secondly a new engine has tight tolerances. Breaking in requires more power which results in more heat.
Re: Cooling your AeroVee Heads
Posted: Sun Oct 03, 2021 11:48 am
by karmarepair
AlexZ wrote:Don’t do break-in on the ground!!. You’ll ruin your heads in no time. The prop doesn’t deliver enough air.
This is true for the stock cowl, assuming the upper cowl is ON. That is not my plan. I’ll be using a big hood over the top of the engine to force cooling air down. LIke this:
https://i2.wp.com/flycorvair.net/wp-content/uploads/2017/02/img_1189.jpg?ssl=1
This is an engine I did not build myself, and I’m not comfortable with the risk of the Sonex recommended procedure. If I have any problems, I want them to manifest ON THE GROUND. I do not suggest anyone else follow my path, but this IS My Path.
You need airspeed to get enough air through the cowl by generating low pressure at the cowl exit.
William Wynne’s setup does not have a “lower cowl”. But you make a good point. This installation is well instrumented, with 4 CHTs (under the spark plugs, which I don’t like, but I’m going to keep them there for now) and if the temps get too high, I’ll shut down, and revise my plan.
Keep ground running to a minimum. Adjust the Aerocarb as described in the manual and go flying.
You’re not wrong, and for most projects/builders, this is the right answer. It’s obviously worked for a lot of airplanes and engines.
The heads run hot because the original design was only 45HP and forced cooling.
Precisely why attention to detail in this area is important. The heads that were fitted to my engine would not, without cleanup, cool as well as stock.
There’s simply not enough material to store the heat generated at full power during takeoff.
This might be true, although I’d argue I don’t WANT to store heat, at all, for the transient condition you describe. I’ve read enough reports of engine that could not maintain CRUISE power settings, plus lots of exhaust valve troubles. Carrying the heat away is what I want to be doing.
Secondly a new engine has tight tolerances. Breaking in requires more power which results in more heat.
I can see hot BARRELS due to ring seating. I can see hot OIL as heat from tight rod and journal bearings gets dumped to the oil. I’m skeptical of your explanation that it’s the required power to turn the engine that makes the difference at break-in. The horsepower the prop absorbs is far in excess of the frictional and pumping losses within the engine.
Re: Cooling your AeroVee Heads
Posted: Sun Oct 03, 2021 11:50 am
by karmarepair
AlexZ wrote:Don’t do break-in on the ground!!. You’ll ruin your heads in no time. The prop doesn’t deliver enough air.
This is true for the stock cowl, assuming the upper cowl is ON. That is not my plan. I’ll be using a big hood over the top of the engine to force cooling air down. LIke this:
https://i2.wp.com/flycorvair.net/wp-content/uploads/2017/02/img_1189.jpg?ssl=1
This is an engine I did not build myself, and I’m not comfortable with the risk of the Sonex recommended procedure. If I have any problems, I want them to manifest ON THE GROUND. I do not suggest anyone else follow my path, but this IS My Path.
You need airspeed to get enough air through the cowl by generating low pressure at the cowl exit.
William Wynne’s setup does not have a “lower cowl”. But you make a good point. This installation is well instrumented, with 4 CHTs (under the spark plugs, which I don’t like, but I’m going to keep them there for now) and if the temps get too high, I’ll shut down, and revise my plan.
Keep ground running to a minimum. Adjust the Aerocarb as described in the manual and go flying.
You’re not wrong, and for most projects/builders, this is the right answer. It’s obviously worked for a lot of airplanes and engines.
The heads run hot because the original design was only 45HP and forced cooling.
Precisely why attention to detail in this area is important. The heads that were fitted to my engine would not, without cleanup, cool as well as stock.
There’s simply not enough material to store the heat generated at full power during takeoff.
This might be true, although I’d argue I don’t WANT to store heat, at all, for the transient condition you describe. I’ve read enough reports of engine that could not maintain CRUISE power settings, plus lots of exhaust valve troubles. Carrying the heat away is what I want to be doing.
Secondly a new engine has tight tolerances. Breaking in requires more power which results in more heat.
I can see hot BARRELS due to ring seating. I can see hot OIL as heat from tight rod and journal bearings gets dumped to the oil. I’m skeptical of your explanation that it’s the required power to turn the engine that makes the difference at break-in. The horsepower the prop absorbs is far in excess of the frictional and pumping losses within the engine.
Re: Cooling your AeroVee Heads
Posted: Sun Oct 03, 2021 12:42 pm
by Rynoth
For what it’s worth, if you go to 1hr 4 minutes in this old Aerovee Turbo Presentation by Sonex, you can see a picture of the shroud setup they used for ground testing.
https://youtu.be/T759DF_E8Ys?t=3840
Re: Cooling your AeroVee Heads
Posted: Mon Oct 04, 2021 5:29 am
by AlexZ
It are basically simple thermal principles.
- The heads don’t have enough cooling capacity at full power. So during takeoff and climb out the heat builds up in the heads. After climb out at level flight it takes quit some time to get to ‘normal’ temps. I have graphs of temps building up while doing consecutive takeoffs at MTOW in short time. The next takeoff the temps started 10+ degrees Celsius higher. Downwind and final at low power where not long enough in time to get rid of the excess heat. We ruined (loose exhaust valve seat) a head doing that.
- During break-in there’s more friction and that creates heat. It takes power to overcome friction. Let’s say the extra friction requires only 2 HP. All this will become heat and results in higher temps.
Re: Cooling your AeroVee Heads
Posted: Tue Oct 05, 2021 1:03 am
by karmarepair
Rynoth wrote:For what it’s worth, if you go to 1hr 4 minutes in this old Aerovee Turbo Presentation by Sonex, you can see a picture of the shroud setup they used for ground testing.
In my opinion, that shroud doesn’t go far enough out on the heads.
