Temperature Balancing by Turning/Twisting the Carb

[lutorm]

Just to close the loop on this discussion, we finally got our Aerovee with the custom intake plenum flying again. I haven't flown it myself but my co-owner/builder says it's running super well. There is still some unevenness in the CHTs, but it's now possible to climb at full throttle without any of the CHTs hitting 400F, and the cylinders aren't even broken in yet.

I don't yet know to what extent the temperature difference now is mixture or cooling, we need to do some dedicated tests once we know it won't come apart on us.

Best of all, the intake plenum doesn't get warmer than 150F, and that's only after shutdown, so there's no danger of melting the 3-d printed plastic!

[bvolcko38]

Do tell! I am having the same problem with my Aerovee with stock intake. Please tell me more. I have not seen your posts on this list before. I have a very hot #1 and hot #3 cylinder. If I richer the mixture enough to to have the temps acceptable on 1 then it starts to stumble rich on the front cylinders.

[lutorm]

Read back through this thread for a description if what I did.

[Onex107]

We need to hear more about your design and build. All the details. don't assume we heard it before. Please.

[Bryan Cotton]

If you read back through this thread lutorm provides links to his blog on the topic. It is there! Just need to go back a page or two.

[mike.smith]

Do tell! I am having the same problem with my Aerovee with stock intake. Please tell me more. I have not seen your posts on this list before. I have a very hot #1 and hot #3 cylinder. If I richer the mixture enough to to have the temps acceptable on 1 then it starts to stumble rich on the front cylinders.

There are lots of potential reasons. My own experiences and solutions start here on my Kitlog site:
http://www.mykitlog.com/users/display_l ... 932&row=11
Item #6 is something I no longer think is/was an issue and I run my oil right at the full mark.
The other thing I didn't note there (I'll have to update it) is that I ran for too long with the #2 needle. Once I switched to the 2.5 needle and did a little trial and error tuning, that changed a LOT of what was going wrong. Not everyone will experience proper metering with the same needle number, but you should be willing to try others. I've had my current AeroInjector setup now for nearly 4 years and I've not made any adjustments to it even going between 100 deg summers and zero degree winters.

[lutorm]

We need to hear more about your design and build. All the details. don't assume we heard it before. Please.

Yeah, it's all in the links I give in this thread. I'm planning to write it up for the Sonex Foundation newsletter, but I want to get a solid chunk of flight data first.

[lutorm]

One interesting thing about using a plenum as opposed to the stock intake pipe is that the larger intake volume also holds a large amount of fuel. In steady-state this has no effect on the operation of the engine, but it does when throttling up or down. When the throttle is opened, you immediately get more air but it takes a while for the plenum to get "filled up", leading to an initial lean condition. (This is why carburetors and fuel-injection systems have an "accelerator pump" function, to momentarily add more fuel and avoid this initial lean spike.) Conversely, when the throttle is closed, there is too much fuel in the plenum for the new airflow and the engine momentarily runs rich. (The latter is very noticeable with the my plenum, when quickly closing the throttle from full power, you get a distinct rich misfire as the rpms come down.)

Anyway, I was just reading about the "Aerocarb burps" and realized that the fuel contained in the plenum probably also makes this effect much less severe. With the stock intake, an air bubble in the fuel line will lead to a lean spike, but with the plenum the effect gets "smoothed out" as the engine continues to run on the fuel that's already present.

[N190YX]

Interesting reading. I would add that with any aircraft engine, rapid movements of the throttle should be avoided, the throttle should be moved slowly enough that the engine RPM will continuously match the throttle position, both on the ground and in flight. I shudder when I hear pilots jockeying the throttle around in ground operations, their flight instructors should have taught them to be gentle with the throttle movement. This is critically important in larger engines with weights on the ends of the connecting rod caps, but, being blunt, it is poor technique to jerk the throttle around on any aircraft engine at any time. Please be kind to your engines and operate the throttle smoothly!

[lutorm]

... it is poor technique to jerk the throttle around on any aircraft engine at any time. Please be kind to your engines and operate the throttle smoothly!

I agree. (Although it is, by definition, not possible to move the throttle at all if you want it to always match the RPM...) What I mean by "quickly" in this case is closing the throttle from full power to idle over maybe 4s. Surely any car-mounted VW experiences throttle changes much quicker than that.

With the plenum you get worse transient response than with the stock intake. To not get a noticeable mixture excursion, I think you'd need to go from full to idle over maybe 15s. I doubt many people have the patience for that... ;-) Now, opening the throttle you have to be more careful, precisely because if you do it too fast you'll get a lean stumble and the engine won't respond.