This page is a collection of notes on the broad variety of Carbs that have flown on the Corvair, and some thought on why I choose simplicity when it is available, and the development of our intake manifolds.
Above, an overhaulled NAS-3 that went on the Pietenpol of Dave Minsink.
Below is a list of stories have written on Corvair carburetors. You can click on any color title to read the whole story:
Below are the Group numbers of our intakes and the numbers we assigned to the popular Corvair carbs. You can see how this is part of our Group numbering system by studying the complete numbering system on the “Prices” section of our main page, FlyCorvair.com.
Intakes and carburetors group (3600)
3601(S)- Standard Intake manifolds
3602(A)- Marvel MA3-SPA
3602(B)- Stromberg NAS-3
3602(C)- Ellison EFS-3A
3602(D)- Sonex AeroCarb – 38mm
3602(E)- Zenith 268
3602(F)- Rotec #3
3602(G)- 1 barrel Carter downdraft
Below are to section from my Group numbering notebook. The first is the introduction to the carb section, and the second is an outline on intake manifold options. Where most companies are just trying to get you to buy something, my goal is to have you learn about, and really understand the machine you are building. The starting point on any subset of knowledge that goes into your plane is understanding the mechanical philosophy behind the choices made by successful builders.
When it comes to carbs, I like Strombergs and MA3s because they have literally millions of hours feeding air and fuel into flight engines. I know them and trust them, and if I had any little issue with one, I have mountain of expertise to draw on, not just other people flying one, but pros at fuel system repair stations. These carbs will always be my first choice to put on a plane because they are aircraft carbs, they are not just playing the role. They are doing the job they were designed to.
My father is a lifelong military engineer who spent a lot of time working in places where the people who don’t like your project are literally going to try to kill you. He upholds that the piece of machinery that has the greatest reliability requirement is the combat firearm. In these tools, reliability is an absolute requirement. All other considerations about them – weight, accuracy, firepower, cost, etc. – all are meaningless if you ever need to use one and squeezing the trigger produces a soft noise rather than a loud one.
Notice that the requirements of aircraft carbs are very much like combat firearms. When you push the throttle in, you really want to hear a loud noise, not a soft one. If your glide path leads to a place 200 feet short of the runway threshold, and pushing the throttle in gives the undesired soft noise, you will not be comforted by thoughts of how cheap, how light, how available, easy to tune or install it was, or any other factor that made it attractive in the hangar. Reliability alone gets you back to the airport.
Consider this: The Stromberg on my wife’s plane was made just about the same time that the Soviet Red Army adopted a device called the AK-47. Sixty plus years later, both of these devices have been used in countless numbers all over the globe. Both are often criticized as outdated, inefficient, inaccurate and stone age. Notice that their continued use in the face of all criticism is justified by the same three word sentence, “It is reliable.” People who have held either one in their hands, stared at its metal parts and though about how they would need to count on it, will have some appreciation for that three word sentence. If I can teach you only one thing about experimental aircraft, let it be this: There is no characteristic more important than reliability. Anything you could get in trade for reliability isn’t worth it.
A lot of builders question the length of the intake runners on our systems. Contrary to appearance, in operation, the throttle response is nearly instantaneous. Look at any modern car; designers are going to great trouble to make the intake runners much longer, not shorter. They are after more torque in the rpm range that direct drive engines fly at. A long intake tract doesn’t mean less power, and I am not sure where that myth started, but you can take a look at things as diverse as a tunnel ram with dual quads on a V-8 and see that even 7,500 rpm drag cars benefit from longer runners. But you need not be concerned with theory, I have a lot of dyno information that compares a huge variety of induction systems, and I can assure you that a long tract with a single carb pays no penalty while offering many advantages.
Many people are yet to understand that the reason why you can go out and fly a Cessna 150 and expect it to work is that Cessna made one in 1959, made it work perfectly, and for the next 18 years produced clones of it, and had an army of mechanics making sure the clones stayed clones, and didn’t develop individual personalities. You can make this work for you on your Corvair carb of choice as long as you understand the difference between the terms “Clone” and “Replica.” Listed below are a number of carbs that have proven over time to work on a Corvair. No matter which one you choose, I strongly suggest you make your fuel system a clone of a flight proven aircraft using the same carb you select.
Pick any carb you like, and install and operate it just like a person who is successfully flying the same carb on a Corvair. And then don’t worry about what anyone else is doing. This will work every time. Here is what never works: A guy jumping from idea to idea and getting speculation and commentary to make a choice for him, having it not work because the system he builds is subtly different than others. Maybe he is not good at taking input from others, and when it is all said and done, he publicly pronounces that there is something wrong with each of the carbs, or maybe the Corvair’s intake design, or auto engines in general. Many people are actually prone to taking the second path because they are more comfortable being negative, “proving” that things won’t work, (even though they are flying on other planes), and passing negative judgments on things. It is just how some people are. I try to ignore it because teaching people about aircraft, not correcting social disorders, is the focus of my work.
You don’t need a majority of builders to like the carb you are going to use. Here is what is needed in the carb you choose: To have flown on the same airframe, with the same fuel system (i.e. gravity feed or fuel pumps); it needs to have flown more than a year and 100 hours with someone you can converse with or who shares their notes; and you need to be able to buy the carb, parts and service for it. That’s it. That constitutes a system that can be successfully cloned.
One of my favorite sayings: “Early bird gets the worm, but the second mouse gets the cheese.” If you don’t want to get your head caught in a trap, be the second mouse, build a clone. Right now, in the land of Corvairs, you can clone a system of any of the first four carbs I list. You are going to have to do some pioneering work on the other installations. Every variable you add as the first mouse has the possibility of putting a big dent on the back of your head. If the imagery isn’t appealing, you can avoid the subject entirely by building a clone.
It is my strongest recommendation not to use any type of motorcycle carb. This includes a Revflow, a Keihin, an S&S, an Altimizer, a Mikuni, a Harley-Zenith, and especially not a Bing. If I were required to list all the ways that a motorcycle or other non-aviation design carb could fail, I would have a long list. For example, the Bing throttle isn’t connected to the cable, and many CV motorcycle carbs have this “feature.” The two biggest failures that I can name is throttle systems that are operated by bicycle cables and the fact that most motorcycle carbs don’t have any way in which you can attach a serious fuel line. A piece of fish tank tubing and a hose clamp is not serious, and if it works on a Rotax 503 in a cowl-less pusher application, that doesn’t mean it will live in a sealed engine compartment in a traditional aircraft. Throw in that they have no mixture control, and often don’t fit where aircraft carbs do, and you get to a better understanding why there isn’t anyone saying how well the combination worked on the first 100 hours on his Corvair powered plane. My least favorite carb in this genre is the Bing. It has a tendency to lean out on long manifolds, and it will actually shut off if subjected to ram air. In 2012, we had a builder who insisted on using one and did $3,000 in detonation damage to his engine on the first flight. The same plane would have flown perfectly fine on a $500 Stromberg. I am sure the bystanders to this event were far more willing to see the issue as a Corvair problem than to understand that it was caused by a poor German motorcycle carb mis-applied to a proven engine. Carbs salvaged off snowmobiles, outboards, imported cars and lawn equipment are never going to have a good record on planes, and their advocacy is limited to people who wish to impress others with cleverness, but never actually impress people by going flying. Again, I don’t find it my responsibility to define all the ways that will not work for people who don’t wish to go with something proven. I spend my time trying to illustrate positive examples of how to do things that will work economically, but above all else, reliably.
3601- Intake Manifolds
The Intake manifolds that we make for Corvairs evolved slowly over time and testing. Originally we made individual manifolds out of welded sections of mild steel tubing. We tested both 1.375” and 1.5” tubing, both on the ground and in the air. After a lot of evaluation, we went with the larger size from 2001 on. In 2003, we started having the main tube of the manifolds bent by a CNC tubing bender as a single piece. This eliminated a lot of welded joints and gives the manifolds a much cleaner appearance. We looked at several different materials and selected thin wall 304 series stainless steel tubing. The primary reasons for this choice are that it is essentially immune to stress cracks when TIG welded and purged correctly, it remains clean on the inside and will not rust even if the aircraft sits for a long time in humid weather, and it is as light as an aluminum manifold because the aluminum would have to be made much thicker to have the same strength and crack resistance. After nearly 10 years of continuous production, our manifolds still have a perfect track record.
When first looking at the layout of the manifold, many people think that it will not have sharp throttle response, or the length of the runners will hurt the power output. A builder with a background in motorcycle racing confessed that he first thought of a steamship’s engine telegraph where the bridge swings a big lever on a pedestal that rings a bell in the engine room and makes a hand on a clock face point to the words “Full Ahead.” After he built his Corvair engine, he was surprised to find out that the throttle response on it was just as fast as a typical car. On aircraft, the limiting factor on how fast it can change rpm is the moment of inertia of the propeller assembly. On Corvairs, this is inherently low and the engine accelerates noticeably faster than other aircraft engines, even with a long intake tract.
I have years of dyno testing of every type of intake length and carb configuration that conclusively shows that the length of the intake run has no effect on power output. For years this was a favorite Internet debate topic among people who had never seen a Corvair turn a prop, but felt certain that the world needed to hear their impression of how it worked in their imagination. A number of these people also advocated putting the carb on top of the engine. I am going to flat out say that I have never found a single good reason to do so, and there are a number of very good safety reasons to have it on the bottom. I have seen people run every carb on top from Bings to Webers, and none of these installations worked nearly as well as even Bernard Pietenpol’s 1960s installations that featured tractor carbs mounted below the engine. I have seen more than one person plan on running an AeroCarb with a fuel pump mounted on top of a Corvair engine. Such a combination is virtually guaranteed to leak fuel onto the engine in operation. If a person is that interested in cremation, they should just find the professional service in the Yellow Pages and skip all the hassle of building a plane. I will not knowingly assist anyone who puts a carb on top of an engine or uses the leak prone stock Corvair mechanical fuel pump, and especially not in combination.
There are always “experts” who claim that individual runners to each intake will make more power, that something is wrong with the offset intake pattern on the Corvairs intake log, or that the log should be removed. These are all myths that I long ago disproved with our dyno on back to back runs. In section 3700 look at the photo of Mark Petniunas’ EFI engine running on my dyno; it has individual runners and made no more power; the offset intake patter appears on many other aircraft engines such as Rangers and Allison 1710cid V-12s (good enough for P-38s P-40s and P-51Bs, probably good enough for homebuilts). The log part of the head is an important part of the mixture distribution, and it is structurally part of the head. If you mill it off you will weaken the head and blow the head gasket because the upper row of head bolts will no longer have a stiffener. Do not listen to anyone who suggests such modifications to the heads.
We make several different manifolds for the Corvair. The most common is the 3601(S) which is the standard manifold for anyone mounting a Stromberg, MA3 or any other float type carb on their engine. This fits all the Zeniths, KRs, Tailwinds, etc. The second design is a 3601(E) which is the same manifold with the carb flange rotated 13 degrees forward. This is specifically made to serve Zenith builders who are putting a flat slide carb like an Ellison, Rotec or an AeroCarb on a tricycle landing geared airframe. The rotated carb flange provides clearance to the nose gear. The 3601(C) manifold is specifically made to fit a Corvair into a Sonex or Waiex airframe using the Wesesman’s installation components. The fourth manifold is the 3601(P) which is specifically designed to use on single seat aircraft and those with narrow upper motor mount spacing, such as some Pietenpols. If you need further guidance, look at out parts catalog at FlyCorvair.com/products.html, give us a call or send a note.
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