Hi everyone, new to this forum so first sorry for my english, I´m from Sweden so my english isn´t prefect.

I just got the plans for scratch building a CH 701 and I cant wait to start.

Right now I am trying to collect all the information about everything I can and have a question about engings.

I really like the autoconversion engines and I´m aware of the + and - with those engines.

But there is one thing I cant understand and that´s when someone is warning my for the high RPM an Autoconversion have.

So my question is why is it harder on the engine to drive it 1 hour @ 6000rpm then 2 hour @ 3000 rpm or 4 hours @ 1500 rpm like many cars do on the highway? of course as long as the engine temp is ok.

I have not find any good answers to that questions yet and maybe someone here knows a little bit more about engine engineering than I do

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All about stress.   Vibration is a factor, but at higher rpm settings, the loads on every part of the engine are greater.  The fatigue on the internal parts is much greater.  Best to look at the Torque / Power curve for the engine.  That will tell the story.   

Airplanes don't have transmissions that can downshift when going uphill.   Big difference.   

Google "Torque Power RPM".   Here is a good link I found helpful.    


We have seen that auto engines successfully converted for aircraft use are carefully selected to be suited for continuous high RPM operation in order to get the thing to fly and stay that way until we want to come down. 

Mr. Moore has pretty much nailed it -- at higher RPMs, stress starts to go way up -- hotrod engine builders often talk in terms of piston speed rather than just RPMs;

Here's another link that sums it up rather well... https://www.musclecardiy.com/performance/understanding-piston-speed...

Hope this is helpful

The force at higher RPM settings is greater due to higher rate of fuel burn.   Think bigger explosions -- more power, more thrust, more speed available.     

LSA aircraft have a max speed limit set by rule -- Maximum speed in level flight with maximum continuous power (Vh): 138 mph (120 knots).  

Higher power available is why if weight and balance is acceptable, higher horsepower engines are preferred for a given aircraft.   Then in order to reduce engine stress and also lower the cost of fuel burned in flight, we have an RPM setting called "Economy Cruise", typically 75% of max rpm. 

Watch this engineer and owner for 20 years of a Long EZ discuss his conversion from a Lycoming to a V8 auto engine.  All these factors in a real world application are discussed.  For 150 pounds extra weight, huge speed AND fuel economy benefit.  He discusses why he went to a water cooled auto engine.  


There is also a beneficial effect at the lowered RPM in propeller efficiency.   So that is why propeller pitch is given a great deal of attention.   Getting the thrust nicely balanced for take off at max rpm while getting good cruise speed is a key final step you will come to at the final stage of your building project.   

Here is a very typical case where an well established auto conversion engine is being tested for suitability on a new aircraft type.   You see the RPM vs Power discussion and vibration being very clearly addressed.   


While the Rotax 912 is popular in the Zenith 701, Viking's 90 seems like a good alternative.  

Hello Loren..

Since this thread is all about Auto Conversions and how they apply, here is a bit of info.   I just had a very good conversation with Deane Philip who owns a CH-701 in New Zealand.  A few years back, Deane installed the Viking 130 HP with 20 lbs of lead in the tail.   Serious STOL operator.   50 feet for take off Deane said is easy !!

Deane has flown the Viking 90 and had only positive comments - only saying it is louder.   His mission is all about STOL and reliability in rough terrain.  Hence the Viking 130.   So we see proof proper of the comment I made just earlier about weight vs Horsepower.   

How is the Alaska spring looking for your CH750?  Or does spring come in May for you?  LOL. 

Hello Michael.  How is our far east buddy doing these days?  Spring has finally arrived for us here in Anchorage, although we're still likely to see a little snow up through mid-May.  

I didn't recall that Deane had also flown the 701 with the Viking 90.  Unless into extreme STOL, seems to me the 701 would probably be best with the Rotax 912 or Viking 90.  

Samuel is still working on Phase 1 and very much enjoys flying the 750 STOL with the Viking 130.  Our builders page has numerous Utube showing take-off, landings, climbing, and stalls.  He's been collecting lots of data, but has not done any short take-off or landings.  His typical take-offs have been around 200 ft.

Trying to not hijack the original thread, but should mention the Viking 90, a Mitsubishi 3 cyl auto conversion, is on an SP-30 in Deland FL.   Although, I did not clarify with Deane, that is probably the aircraft that he flew with the Viking 90.   The SP-30 is a CH-701 derivative produced in Russia and used extensively for Agricultural spray work.  

I saw Samuel's data page on climb rates and power settings etc.   Good stuff.

Happy flying.   Hope we have been helpful to Jonas with his original question. 

Thanks for all the info, have not read all but I think it answers some questions.

I understand that higher RPM means more stress to the engine but what I meant with my original question was if an engine is designed for a car then driving it on top gear on the motorway at about 2500-3000 RPM is not a problem, even hour after hour. So that same engine in an airplane when double the RPM, how much more stress is it? is it double or is even four times higher?

I think the Rotax 912 is the best options but I like to think outside the box. But not so much that I´m willing to risk my life :).

I see now even snowmobile and motorcycle engines are used in airplanes and those engines have even higher RPM.

Hi Jonas..

Thinking those are math / engineering numbers that are not easily available.   Maybe someone here can give more specifics.   But I am not sure how this helps.

What we are familiar with is the TBO.   You saw in the video on V8 auto conversion that, because he races with the faster engine, valve jobs are frequent.   But not an overhaul of the engine.  "No appreciable wear."   So for most auto conversions there are no fixed TBO figures given.  Think years and years.   Rotax has a published TBO.  

So really the answer to your question rests with how you operate and maintain the engine.  That makes all the difference.  Even the best engine can be run into early failure by a poor operator and bad maintenance.   

We are fortunate to have lots of good engine choices now including auto conversions at less than half the price of a Rotax.   This forum discusses eight different engines.  All are well proven, safe and reliable or they would not be here.   Up to you.

Yes I agree to this, This is what I think also. It all about how you maintain and operate the engine. 

Of course there may be less maintenance on a factory built aircraft engine.

Thats why I asked this when many people say stay away from engines that are not developed for aircrafts.

I will probably go with the Rotax in the end most because I think it´s the easiest engine to buy here. But all the engines discussed here are interesting.


"Thats why I asked this when many people say stay away from engines that are not developed for aircrafts." -- Many people have minds that are closed to new ideas. These people would never condone using an engine out of an automobile, no matter how much evidence they are presented with.

Hello Ken...

Some folks won't eat yogurt no matter how much evidence they are presented with.   Ask me about Brussels Sprouts.  Yuck.  Engine choices are somewhat like that.   

Air cooled aircraft engines as with auto engine conversions to aircraft use are all based on 116 year old design features.

What has changed over more than a century??   Here is a great article from FLYING magazine.  


Really I think budget should be the major deciding factor.   If a Rotax is in your budget go for it but I would say get the fuel injected version.   My previous airplane had the HKS 700E with those same dual Bing carburetors.   If you like the standard Rotax and those Bings --  have a good and experienced Rotax A&P.  Enough said.   Why then Rotax over UL Power or Jabiru?   All three are equally well proven designs.

Those that raise the glower of doom and loss of life and limb saying auto conversions are a safety risk are simply not dealing with facts.   BUT there is serious cause for caution.

Here are four articles that cover the topic very well.   









Powerplant failures in E-AB aircraft are a serious problem and all too often in the early phase of testing.  But the problem is NOT the engine.   It is the installation and that rests solely with the builder in the E-AB world. 

Installation means fuel, air, and electricity need to be done right.   

The EAA now recommends comprehensive on the ground testing of those systems before first flight.   

Kudos to those brave test pilots who will then step up and into our E-AB's for the crucial leap into real flight.


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