Finally, almost 2 months after the initial intake experiment that indicated the need for a complete redesign of the AeroVee intake, we now have some data!

After assembling everything on the engine yesterday, I test ran the engine today and the results, while not entirely perfect, are very promising. A full throttle run shows that the mixture is much more even between the front and back cylinders than it was before.

Here is the updated version of the EGT plot I made after the first experiment. It’s a bit busy, so let’s go through it step by step. The three runs are plotted with different lines, today’s plenum test in solid, the stock intake in dashed, and the 3d-printed elbow in dot-dashed. Times are aligned such that at times before 0, the mixture is full rich and at time 0 the mixture is leaned until the engine starts running rough.

The EGTs of cylinders #2 (front) and #4 (back), in degrees C, are plotted in blue and green. The red lines are fuel flow and black ones engine RPM, scaled to fit on the same plot.

The fuel flows at full rich, just before time 0, are very comparable for all three runs. The engine RPM is also very similar, although the solid line is slightly higher than the others. Static RPM with the plenum was about 3220 while the others were around 3150, so that’s a good sign; whatever else is going on, the engine is making more power than it used to.

Looking at the EGTs, the blue and green solid lines, it’s obvious that they are closer together than they were before. At time 0, the EGTs are 675C and 700C while the stock intake run had them at 550C and 720C. The behavior when leaning is also very different; with the stock intake the #4 EGT (dashed green line) dropped while the #2 (dashed blue line) rose. With the plenum, they were almost unaffected, although there’s some indication that the green line dropped and the blue line rose slightly.

All in all, these data indicate that the #2 and #4 cylinders now run a much closer mixture, but also that that mixture is quite lean since both cylinders appear to be running near peak EGT with the mixture at full rich. The #4 cylinder appears to be about the same mixture as before, since its EGT is the same, while #2 is much leaner. Yet fuel flow is the same. How is this possible?

I think the answer is that the engine makes more power so, even though the fuel flow is about the same as with the stock intake, the fact that the engine is running at a higher RPM means that its pumping more air. More air, same amount of fuel means a leaner mixture. It’s interesting that, apparently, the more even mixture distribution means overall more power than before even though the cylinders appear to be running quite lean. (That also likely means it’s possible to pick up a bit more power by tuning the richening up the mixture, when I get around to that.)

So why would the carb not give more fuel with more airflow? I’m speculating a bit here, but the conventional wisdom in carburetor circles appears to be that a more pulsing airflow will make a carburetor give more fuel compared to the same average airflow without pulses. Something about the pulsing airflow giving higher maximum air velocities through the carb will tend to draw out more fuel. I don’t know whether this applies to the AeroCarb with its dirt-simple construction, but it’s at least plausible.

That’s the upside. The downside is that it won’t idle. I could not get it to run stably at an RPM less than 1600. Anything less and it would stumble and surge. After fiddling a while I found a throttle position where the RPM would oscillate like clockwork between 1100 and 1600 with a period of about 2.5 seconds. While doing so, the EGTs for the front cylinders also dropped out, so it was really only running on 2 cylinders. This was leaned a bit, at full rich all four EGTs were at least alive but it would still not idle stably.

This kind of surging idle is a textbook symptom of an intake leak. It doesn’t matter much at full throttle, since the pressure in the intake manifold is close to atmospheric, but at idle there’s a substantial vacuum in the intake and any leak means the engine will draw in air without a corresponding amount of fuel. The fact that the front cylinders both dropped out might mean that the leak is where the plenum halves are joined together. If the joint on the front side leaked, it might predominantly pull that air into the runners to the front cylinders and make them run lean.

When I joined the plenum, I just put a small bead of silicone RTV on the halves and put them together. It’s quite possible this was not entirely sealed, especially since the plenum flexes quite noticeably from the engine vacuum. During the test run I had the GoPro mounted on the firewall pointed at the plenum and intake runners, and you can actually see how the sides collapse slightly when the engine starts:

Look at the rear side of the plenum, to the lower left in the image, when the engine starts. I’ve included a few cuts back and forth between engine running and engine off to make it more obvious.

The static stress simulation I made in Fusion 360 to make sure the design was strong enough indicated that the sides of the plenum would flex about 1 mm under ~50kPa vacuum, but that did not take into account the fact that the wall is not solid but has a honeycomb infill, the exact material characteristics of Alloy 910 (I used Nylon 6), or the fact that the joint along the center will tend to pry itself apart as the sides flex inward and the edges of the joint rotate. (I’m reminded of the reason the SRB joints on the Space Shuttle leaked, leading to the Challenger disaster. It was a similar effect, the pressure on the inside of the SRB casing made the joint flex and unloaded the O-rings, letting gas blow by… but I digress.)

I will try reassembling the plenum halves using more sealant, and try “Aviation Form-A-Gasket” instead of RTV (which is not really fuel-resistant). Aviation Form-A-Gasket is very tacky and never really solidifies, so hopefully that will keep a seal even with some flexure of the joint. If worst comes to worst, I can glue the two halves together, because I’ve established that it is indeed possible to extricate the assembled plenum through the spaces in the engine mount.

So that’s where things stand as of now. I’m going to order the stainless tubing for making the permanent runners, and re-seal the plenum halves and see if it indeed is an intake leak. At least now we have a running engine to work with.