The idea is to make both pistons fire at the same time (a new camshaft). Now some questions come to mind . . . a little late.

Is the lack of the counterweight going to make the engine shake even if both pistons fire at the same time? Is it worth it to make the pistons fire at the same time? Is it possible to balance the engine by putting smaller weights at the flywheel (more diameter than the crankshaft)? The flywheel is a light stamping. Is the propeller going to help (as a flywheel) idling at 1800-2000 rpm?

I’ll highly appreciate your time and knowledge of the subject in answering my questions and any other comments you may add.

Regards - Javier Aristizabal, Medellin, Colombia

A. Dear Mr. Aristizabal - In replying to your letter about modifying a BMW engine, let me first point out that I am a writer and not an engineer. In doing the articles for EXPERIMENTER, I find as much useful information as I can in whatever books and magazines seem to be likely sources, then combine and condense this background into reasonably informative and readable articles. I am not a mechanical engineer. The questions you ask about your BMW modification can only be answered by an engineer having a good knowledge of the mathematics and physics involved. In your case, a familiarity with the internal parts of the BMW motorcycle engine would also be necessary. I do not personally know any person with this kind of training. Based on what I know about outboard motors, I can say that in the case of two-cycle horizontally opposed power heads, more torque is developed if two pistons transmit power to a crankshaft at the same time. But, at the same time, somewhat more torque vibration will result. In the case of your engine, which is of the four-stroke type, there will be power strokes only every other revolution . . . so only an engineer familiar with the mathematics of engines could tell how that would affect power output.

Knowing nothing of the inner details of the BMW engine, I can say nothing about the effects of removing or adding counterweights. I would say that the basis of your power problem is the high altitude of Medellin; reduced atmospheric pressure will affect any engine’s power and will require carburetor adjustments also to maintain proper mixture and burning.

We’ll run your letter in our Engine Q&A column and see if any of our readers can provide you with assistance.

In choosing to use the BMW engine you have ventured into a technical wilderness in which you are very much on your own resources. If I were living at a place 5,000 feet above sea level, I would have chosen a Rotax engine. By doing so, I would gain the benefit of the engineering knowledge possessed by Kolb, Rotax and the various propeller makers. Also, if I had decided to use a BMW engine and found the performance to be lacking, before undertaking a major rebuild of the engine I would investigate the carburetor to see if a leaner mixture might be needed to maintain proper air/fuel ratio at higher altitude than most ultralight owners operate. And, I would have asked competent propeller designers if they could suggest a propeller design able to coax the best performance out of the plane and engine combination.

I am sorry that I cannot be of more help with you on the BMW engine and hope that the suggestions I have given will lead to an acceptable solution to the problem.

Sincerely - Bob Whittier

Q. Dear Bill (Bronson) - The Engine Q&A column in the EXPERIMENTER magazine says you are the man to contact with questions regarding VW engines.

I have the empennage and wing kits completed for a miniMAX and now must decide on the engine, which determines the design of fuselage kit to order. I tend to favor the four-cycle engine, which comes from experience with temperamental two-cycle engines. I seem to be able to keep the four-cycle yard tractor, go-cart, lawn mower and old pickup going, but always have to take the chainsaw, string trimmer and motorbikes to an experienced two-cycle technician.

In discussing engine selection with persons having some knowledge and experience in light aircraft operation, I keep hearing positive attributes of the two-cycle. More thrust on take off, faster climb, shorter take off, etc. Also, more and more talk about improved reliability of the two cycle in the past few years.

What I am trying to understand is why a two cycle running 6400 rpm, producing 40 hp, driving propeller "A" through a 2:1 reduction unit reportedly will outperform (i.e., provide more thrust) than a four cycle rated at 40 hp at 3200 rpm and driving direct the same propeller? At least this seems to be the indication I’m getting. If both horsepower ratings are correct, each engine would be able to sustain 550 foot pounds x 40 of work per second. Not counting speed reducer loss on the two cycle, both engines would turn the prop at the same rpm and therefore provide the same thrust. Am I right in my understanding? I would be grateful for any help in understanding the differences in reported performance by the two types of engines.

I enjoyed the article in the May ‘94 EXPERIMENTER about you and your plane "Fourstar." I am also intrigued by Randy Sackman’s miniMAX in the picture on page 23, which appears to be sporting a half-VW engine. I would like to contact Randy and have a conversation with him about his plane and engine selection.

Thanks for any help you might provide in an engine selection. - Bill Lockin, Benton, Illinois

A. Dear Bill - When I built my airplane (of my own design),I opted for a "firewall" configuration. The reason being that the four hard points could be used to adapt any engine change by simply fabricating a new mount. The new mount can be longer or shorter to allow correct CG with different engine weights.

The attributes of two cycles are well proven, and with proper care, the reliability should not be a problem. In my narrative on how I built my engine, I go into the reasons to chose one over the other. It is a subject too lengthy for this format, but there are clear benefits to either choice.

Drawing conclusions from an apparent relation between horsepower and torque gets most folks in trouble. Propellers are everything (almost everything, that is). Gear reductions allow swinging larger and more efficient props, hence better climb. I am working on a simple reduction for 1/2 VWs which will help a lot in the climb department.

Randy’s miniMAX is a fine piece of work and, by the way, has a firewall mount. - Bill

Q. Dear Mr. Bronson - I got your name from the EXPERIMENTER and I have some questions I’d like you to answer.

First, I own a Volksplane II which has a 1843 cc engine in it, at least that is what I think it has in it. I might be wrong on the size, but it is supposed to put out about 60 hp. I bought the plane from the builder but didn’t get a whole lot of information about it from him. The plane flies just fine, but on take off it seem like it is a little low on power. I only get about 2600 rpm out of it. I have read that other VW engines are getting up to 3400 rpm. Is my prop too big? I believe it’s a 60 x 30 wooden prop. Should I be using aviation gas or regular auto gas? I have tried both and there doesn’t seem to be any difference. Could you advise me as to which I should use. The plane has only about 50 hours on it and the engine was built just for that plane. Should I be using a shorter prop?

Sincerely - Richard Bitz, Manvel, South Dakota

A. Dear Richard - Yours is an easy one. The traditional starting point for propellers on any variant of VW engines has been 54 inches. This has to do with the fact longer diameters can load the prop early in the engine’s power curve. As you know, you can’t get up into the power range, so it "staggers out." Good used props abound, and you might borrow one with the idea of buying it if you like it. 54 x 37 might work just fine. Let everyone know how you made out. - Bill

Q. Dear Mr. Bronson - I am currently working on a Volksplane VP-1 and am working on the engine installation (Volkswagen 1835 cc). I’m scratching my head at this time as to the carburetor. My copy of the Ackerman Volkswagen conversion manual calls out a Solex 32 PNN-1 side-draft carb mounted above the engine on a converted intake system. I am familiar with the Zenith carb mounted under the engine, but using this setup would necessitate a larger "bump" or cut out in the cowl as the carb will protrude below the lower fuselage surface. Do you have any suggestions as to workable carbs for the Volkswagen engine? I have a motorcycle carb that looks like it could work as it is side-draft and the venturi is about as large as the intake tube where the carb would mount. Your suggestions will be greatly appreciated.

Sincerely - Keith Schindler, Denton, Texas

A. Dear Keith - My 1/2 VW conversion’s carburetor is mounted on the manifold(s) which locate the carb (Zenith replacement carb for Model A Ford engines) below the lowest point on the engine. This is a time-proven strategy which is supposed to prevent fire should the carburetor leak gasoline. Obviously, if it is above a hot engine, trouble could develop. Also, engine heat can cause percolation in top mounts. The best way to try an unknown carburetor is to get the CFM (cubic feet/min.) rating from the manufacturer and compare it to the rating of commonly used carburetors. While no guarantee, you will get some good clues. - Bill

Q. Dear Engine Q&A - While attending Oshkosh ‘95 I saw an amphibian seaplane kit that was powered by a 350 Chevy marine engine. The kit makers (Adventure Air - Berryville, Arkansas) anticipated that the engine would produce 333 hp and produce 1100 static lbs of thrust. It had a Holley carburetor, and Northwest Aero belt drive reduction gear. They also planned to use a 6-bladed Warp Drive prop. Wow!

This sounds incredible to me, and I am very interested in this combination. I was wondering if you can elaborate on this setup. Your opinion is very important to me and I am anxiously awaiting your reply in the EXPERIMENTER magazine.

Respectfully, Harry Muse, Minneapolis, MN

A. Dear Mr. Muse - Yeh, that’s a big engine and a lot of horsepower, which no doubt is needed for an amphibian the size of the Adventure Air. Usually we don’t get into "stuff" that big, however we can advise this. Budd Davisson recently had the occasion to fly the Adventure Air Amphibian with the 200 hp Lycoming IO-360 engine (his review will appear in an upcoming issue of SPORT AVIATION), and he felt the airplane was underpowered with that engine. The Adventure Air folks are hoping to mount the big Chevy engine on the airplane this spring and begin test flying. Thus far they have not flown the airplane with this engine, so basically the facts remain to be seen. Watch EAA magazines later this summer as we’ll follow up when the test flying with the Chevy engine is completed. In the meantime, the best guess is no one knows for sure how it’ll perform. The proof will be in the pudding, as they say. - M.J.

Q. Dear Don (Bouchard) - I look forward to the monthly "Engine Q&A" section. It is interesting and highly informative. Your knowledge and thoughtfulness have opened many doors to men and women who recently have only dreamed of flying.

My father-in-law and I are planning to assemble a kit aircraft (see specifications included). The manufacturer sets maximum allowable engine weight at 250 lbs. using an EA-81 with redrive. I would like to use a Legacy engine EJ-22 without redrive. Use of this engine would supply 450-475 pounds of thrust at lower rpm (probably 2800-3200 rpm)? What type of prop would be required with the EJ-22 engine?

If we use the EJ-22 redrive engine which weighs 275 lbs. wet, what must we do to bring the center of gravity back to its original position? Are there any other penalties or problems inherent to the use of this engine?

Finally, if you please, I would like names and addresses of manufacturers of kits and blueprints for the Legacy EJ-22 equipped aircraft. If the EJ-22 engine will not work with this kit, I would still like plans for using the EA-81 engine.

Thanks for your courtesy and helpfulness. If you need additional information, please feel free to contact me. - Niaz Mian, Arlington, Washington

A. Dear Niaz - It was nice receiving your letter. The ARV Griffen looks to be a real nice project. The reason the company is recommending the EA-81 with redrive is because of its weight and size it will fit in their plane very well. I know at times bigger is not really better. The EA-81 with redrive will weigh in about 182 lbs. Using a redrive a 2.2:1 or lower ratio, with a three-blade, 72-inch ground adjustable prop, you should get 465-475 lbs.of thrust easily. Using the Legacy for this aircraft would take a lot of modification to make it work properly. With a straight drive in a Legacy, a 54-inch ground adjustable would allow you to turn the engine up around 3000 rpm or more depending on tip speed. If you use an EJ-22 you will have to contact the company you buy the aircraft from to give you the information on what to do. The November 1994 issue of EXPERIMENTER will give you a list of manufacturers of different engines.

Thanks for your letter - Don

Q. Dear Don - I just obtained a Subaru engine designated as an EA-71 (not 81), and I would like to know if it would be suitable for use in an aircraft like the Coot A or some such. This car was an ‘83 GL, four-door sedan.

Best regards - Charles S. Tulloss, Falmouth, Virginia

A. Dear Charles - Regarding your project for the Coot A, I believe they call for a horsepower range from 100-225. The engine to use for this project would be a 2.2 or the new 2.5 Legacy engine with redrive. You need a lot of power with this aircraft to get it off the water. It’s a beautiful plane, Charles, and I hope I’ve helped you. - Don

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