Torque x rpm/5252 = hp

As an example of how this can be used, a Lycoming O-320 would have 320 foot pounds of torque. At 2700 rpm, then

hp = 320 x 2700/5252 = 165 hp

The rated hp of this engine is either 150 or 160 depending on compression ratio. If the O-320 were a higher compression auto engine it would probably produce around 165 hp.

Where this technique is useful is in answering questions such as Hobart Jones asks in the Engine Q&A section of the September issue. Hobart wanted to know if a 2.7 liter Subaru could produce 100 hp at 2800-3000 rpm. To answer this requires one more formula,

cubic inches = liters x 61

A 2.7 liter engine x 61 cubic inches per liter = 165 cubic inches. Then

165x 3000 / 5252 = 94 hp

close to his goal. By contrast, a 2.2 liter Legacy engine at 3000 rpm would produce

(2.3 x 61) x 3000 / 5252 = 77 hp

The Legacy is simply not large enough to produce the required hp at the desired rpm. Note however that with a reduction drive the Legacy could provide far more than 100 hp when operated at higher rpm, in fact it is rated at 130 hp by Subaru.

Several years ago a kitplane manufacturer was using a 1.8 liter Honda engine for power. The engine was rated at 125 hp at 4500 rpm. The performance was disappointing and eventually the manufacturer decided to specify conventional aircraft engines rated at 135-150 hp which performed considerably better than the extra 10-25 hp should have.

To solve this puzzle we can estimate the Honda was producing

(1.8 x 61) x 4500 / 5252 = 94 hp

considerably less than rated and in line with its actual performance.

In answer to Jim Delaney’s question regarding horsepower and thrust, 65 hp is 65 hp no matter what engine produces it assuming the manufacturers are honest in their rating. Where differences in thrust occur is when the propeller is turning at different speeds. A direct drive Rotax of 65 hp turning a propeller at 6500 rpm will not produce nearly the thrust of a 65 hp Continental turning 2300 rpm.. This Rotax with a reduction drive will swing a large, slow turning and efficient propeller and match or exceed the Continental’s thrust.

Regarding using a Volkswagen with a reduction drive, it should easily match the Continental’s performance but may need extra help in cooling as the increased hp produced by revving the engine beyond direct drive speeds will produce more heat.

Incidentally, I have a project consisting of a TriPacer airframe modified similar to a V-6 STOL which will have a Mazda 13B rotary engine for power. I would like to hear from anyone else with a similar project. Based on the performance of my father’s Mazda 12A rotary powered Wag Aero Cuby, I think it will make a nice airplane with less weight and a little less hp than the Ford V-6 for much less money.

Sincerely - Guy Bony, Prineville, OR

Q.  Dear Engine Q&A - In the September issue, Jim Delaney of Coralville, Iowa had questions about the horsepower ratings of several engines and their relative ability to produce thrust. Thrust is directly related to horsepower and propeller design. The propeller must be designed to produce its maximum thrust at the same rpm that the engine is designed to produce its maximum horsepower (work). This is not exactly a true statement, as there are propeller options, depending on what you’re after - climb performance, speed or economy cruise. It takes "x" amount of hp to turn a propeller and produce "y" amount of thrust. It makes no difference who built the unit producing the hp (65 Subaru or VW hp does exactly the same amount of work producing thrust as 65 Continental hp). The difference between engines (and the question we need to answer) is, at what rpm is that hp produced? In general, the engine with the greater cubic inch displacement will produce maximum hp at a lower crankshaft speed than a smaller displacement engine. In this case, the Continental 65 may displace 250 cubic inches and produce maximum hp (65) at 2700 crankshaft rpm while the 65 hp Subaru or VW may displace 2000 cc (122 cubic inches) and produce maximum hp (65) at 4000 crankshaft rpm . A propeller designed to produce maximum thrust from 65 horsepower at 4000 rpm would look considerably different than one designed to produce the same thrust at 2700 rpm. This may explain your observation, "by looking at the prop it’s obvious they don’t produce the same thrust," which is probably true if they’re all spun at the same rpm. The propeller designed to produce maximum thrust at 2700 rpm could be used on the Subaru or VW (65 hp at 4000 rpm) engine with a 32.5% reduction gear, reducing the prop speed to 2700 rpm, the thrust will be the same (discounting reduction gear power losses). In the end, 65 hp is 65 hp, and contrary to your statement, Jim, in scientific terms, it really is horsepower that takes your flying machine aloft.
- Gale Schmidt, Oak Harbor, WA

Q.  Dear Mr. Bronson - I have enjoyed the Engine Q&A articles in EXPERIMENTER and now I have one of my own. I am completing a Volksplane with a VW 1600 cc engine. I followed the plans for engine conversion according to Mr. Ackerman recommended by Mr. Evans in the plans. I purchased a longblock in kit form and assembled the engine myself. This did not represent any problem, and I did not see anything which would lead me to expect the results. Initial oil pressure using the supplied standard oil pump would rise to about 40 psi when the engine started, then began to drop as the oil heated. At about 160¡F. the pressure would be down to about 20 psi at 200 rpm and was near 0 psi at idle. I contacted the suppliers of the kit and they recommended a heavy duty oil pump, which has about 1-1/2 the width for the pump as compared to the original. This pump resulted in a slightly higher oil pressure initially, about 45 psi followed by slightly higher pressures at lower speed, about 3 psi at idle with 160¡ F. oil. I then contacted the suppliers again and they suggested changing the oil pressure regulator springs and pistons, which I did. The net result is still no more than 25 psi at 2000 rpm and about 5 psi at idle.

My problem is twofold: One, what is normal for a VW engine used in aircraft?; and secondly, what possible problem could cause my low pressure? I am satisfied with the engine performance other than the oil pressure. It will turn a 54 x 30 prop 3470 rpm static, which I believe is ample power for this aircraft. Now will it continue this or wear very rapidly from low oil pressure?

Thanks for your interest in our problems. - John Wiebelt, Mercedes, TX 

A.  Dear John - Thanks for your letter. You may not have any problem at all. Oil pressure is influenced by many things, so I will list my thoughts on this matter. Your temperatures are nice and low, rpm is good, 3 lbs. at idle will protect bearings, and 25 lbs. full power hot is okay too. Probably, you have bearing clearance which is a little loose so pressure cannot build up to the regulator setting. I use 10W-30 Syntec oil in my plane, and have run similar readings since it was new. At 100+ hours, performance is perfect. Actually, oil pressure in the high (normal) range just adds power robbing drag on the engine and promotes wear of the timing gears, pump and drive tangs, as well as "frothing" of the oil, and generating excess heat. Log your readings and monitor oil change to oil change pressures. Also, inspect old oil and filter for metal particles. If nothing changes, I would consider everything A-okay.

I might add that you could check your building style against my 61-page narrative ($75) on "How I Built a Smooth 1/2 VW." If you see something you did miss, you can correct it. - Bill Bronson

Q.  Dear Mr. Bronson - A few months ago I purchased a 1600 cc HAPI converted VW engine, complete with mount and cowling, it has no fuel pump, stock distributor, the exciter coil and charging coil of a motorcycle, a direct drive starter that will barely turn it over. I looked all over for a serial number and I could not find one anywhere. I was told it was a type three engine. I understand Mosler bought out HAPI. I wrote them a nice letter requesting information; their ad said to send $15.00 so I did. I received my money order back from Christine Areo Engines and all I got was a cheap flier on their own products. So I will ask you:

1. What do you know about these engines?
2. Do they have a forged crank and pistons?
3. What should I do to it to make it more reliable?
4. Do they have any major faults?
5. How is the ignition suppose to be wired?
6. How is the alternator to be wired?

I am an aircraft mechanic with 40 years experience. I have overhauled lots of different kinds of engines. One more question, what is the difference between Type I and Type III and what parts are interchangeable?

Yours sincerely - Cliff Stacey

A.  Dear Cliff - Thanks for your letter. It sounds to me like you have an engine of mixed origin. I assume you bought the engine cheaply enough, so my suggestion will be given from the safety perspective. If you completely disassemble the engine, you can positively identify the components, and correct any defects, which will address your first four questions. Any VW engine converted for aircraft use can be built to expected reliability standards.

For simplicity, I would install a magneto which would eliminate the need for an alternator and battery, thus saving weight and increasing reliability. (No battery, less chance for post-crash fires, etc.) This will address questions 5 and 6.

I hope this is helpful to you. - Bill

Q.  Good Day Don (Bouchard) - I read your articles in EXPERIMENTER every month. This letter is in response to your answer to Russell Abrahams, England, on 2 plugs per cylinder in EA-81 engines.

In 1990 I bought a Kitfox III. Before the kit was delivered, I read in EXPERIMENTER articles how "bad" the 582 was. I had just bought one! I decided to install an EA-81 with the AMAX redrive - sold the new 582 in the box. First flew the Kitfox in April of 1992. It has run perfectly ever since (have only had to change the oil and filter).

I understand Russell’s concern for two plugs per cylinder. I’ve sent pictures of my EA-81 heads with two plugs per cylinder to Russell. My EA-81 runs so well that the heads with dual plugs have set for two years in a box on the shelf. Someday I will put them on. By the way, I used a Ford Bronco carburetor - 6 cylinder 170 CID. - Jack Bally, Dixon, IL

Q.  Dear Don (Bouchard) - I am not an EAA member, but was recently loaned some issues of the EAA EXPERIMENTER by an EAA member that I know after talking to him about Subaru engines. I am writing to you in hopes that you might have an answer for a question that I have regarding the lowest horsepower pre-1992 liquid-cooled opposed-cylinder 1600 cc Subaru four-cylinder engine.

Specifically, my question is how many horsepower does this engine put out at 2400 engine rpm?

Thank you very much. - D. A. Dowless, Philomath, Oregon

A.  Dear D.A. - Nice receiving your letter. You were asking about the 1600 cc, it puts out 67 hp at 5200 rpm; torque range is 81 ft.lbs. at 2400 rpm. At 2400 rpms, you probably putting out around 40 hp because of not getting into the power range. Now if you attach a redrive at 1.860:1 reduction or higher, you get better performance. If you decide to use direct drive, you are probably would use a prop 48 x 26 or 24 pitch and could turn it up to 4000 rpm. That should work for a small aircraft.

I hope I’ve answered your questions. - Don

Q.  Dear Don - Your Q&A column is timely. I’ve been in the process of putting together an EA-81 for my Avid Flyer for about the last three months and am about to put the juice to the starter for the first time.

My question concerns EGT probe placement (distance from the head) and what EGT operating temperatures I should be looking at. Thanks for the help.

Sincerely - Tony Latham, North Fork, Idaho

A. Dear Tony - On the EGT probe placement, come down six inches from the base of the plate attached to the engine. Are you using one or two probes? If you use one, find out which cylinder is the leanest and put the probe on that side. Sometimes the EGT gauge does not read correct; it will read higher or lower on one side than the other. It shouldn’t go over 1200 degrees average. If you have anymore questions, please call or write. - Don

Q. Dear Mary - By unwanted ground simulation with a VW Dune Buggy, special fuel tank and two in-line gasocolators, I experienced engine failure from unknown causes. Does this accident verbiage sound familiar? Further, with a good mechanic friend we really trouble-shot the VW engine before giving up to a garage with a hoist.

The cause? Both gascolators were clogged with rust/silt, and water had eroded the carburetor body and clogged jets and displaced fuel. Please note the filter elements are centrally located in the gascolator thus hiding 100% the contaminants while allowing a clear water/fuel mixture residual to give the impression all was well. For starters, shouldn’t the contaminant buildup be completely visible for inspection purposes without breaking the fuel line and risking reassembly problems? (See Tony Bingelis, SA,June ‘94, p. 71 on this last point.)

Then, why should it be necessary to remove such as a sediment bowl when a good drain screw, when safetied, will do the job? I’d hope the EAA membership might check their fuel systems for these inadequacies and exercise preventative measures, including development of safe and sensible gascolators.

All the best - Norman Benedict, Santa Maria, California

 
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