Compression Ratios, Fuels and Power Output

Builders:

Here are three topics that are related. Although the conversion manual covers this in some detail, I will put a short summary here.

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We have 3 popular displacement s for Corvairs 2700, 2850 and 3000 cc (read more: Sources: Choosing a displacement.)  The latter two are made with a very special dish in the piston to lower the static compression, but keep the ‘quench area tight. On any of these displacements you can either put lower compression 95 hp heads, or you can put higher compression 110 hp heads. Right there you have six combinations with different compression ratios, but it is also possible to build engine with high or lower compression, but those six are the popular ones, and having the option allows Corvairs to suit different builders purposes.

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The two basic rules are: The higher the compression, the more power the engine will make….and There is a limit to how much compression you can use with car gas. The commentary here is general, but it comes running engines on our own planes from 7.7:1 compression (1998 2700 engine in our Pietenpol) to 11:1 compression (2005 3100 engine in our 601XL) I write this as a guideline, if you have a specific application, feel free to ask in the comments section.

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Basically any of the three displacements with 95 hp heads will have compression ratios from 8:1 to 8.5:1. Engines built with 110 hp heads will have ratios from 9.0:1 to 9.5:1. The variation is mostly in the machining done to the head gasket area, and the actual gasket thickness.

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The First question is what is the lowest Octane fuel you will ever use in your plane? In the answer is “I will never put anything but 100LL avgas in my plane” then you can use any compression ratio you like up to 11:1. Your engine will make about 5% more power for each whole point you raise the compression. But….you can never, not once, ever, run the engine on car gas, even 93 octane car gas if the compression is much over 9.5:1. 100LL is great fuel. yes engine can be detonated on it, but this is done by leaning the motor out far too much or not having the timing set correctly. Our 11:1 compression engine flew more than 600 hours on two different airframes The first 200hr was 11:1, we dropped it slightly to 10.5:1 for the rest of its time) It never detonated at all, and it never saw a drop of car gas either. 100LL when running slightly rich has a comparative octane of nearly 120.  Keep in mind that many people swear they will use nothing else, but later after the 40 hrs. are flown off, some people start getting cheap, and the are tempted to run car gas in a 9.5:1. They might get away with it under some conditions, but sooner or later, they will pay.

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Let’s say you are going to run ethanol free 93, or some mixture of this and 100LL, how high is smart to go? You could run up to 9.3:1 and get away with it, as long as you don’t excessively lean it. But what is the benefit of running on the ragged edge? If your engine is built with a ratio of 9.0:1, there will be hardly any measurable performance difference, but it will have a large increase in resistance to detonation.

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What about running 91 or 92 Octane car gas? Then it seems prudent to shoot for the lower range offered by using 95 hp heads. I have never been interested in speculation on what “should work”, I am much more interested in builders developing enough judgment to understand they are far better off with set ups that have a greater margin of safety than a slight performance edge.

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Examples:

Woody Harris , who’s plane is pictured below, started flying with a 9:5 to 1 compression 2700. (Flat top pistons and 110 hp heads). Because he was planning on switching to a Turbocharged installation, he went to an 8.25:1 2,850. (dished pistons and 95 heads). Woody had enough fun flying around, that he didn’t get to turbocharging. Woody has long reported that the power output between the high compression 2700 was about the same as the low compression 2850. This isn’t a surprise. Woody mostly flies on 100LL, but if he or anyone else was planning on running 91-92 octane fuel, they would be vastly better off with the lower compression 2850.

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My own 3,000 cc engine: Is set up at 8.3:1 compression (dished pistons and 95 heads) Although it might make 6 to 8 more hp if the compression was raised to 9.5:1, I don’t care because I am not running 100LL, my choice is to run ethanol free boat gas, which here in Florida is 90 octane and sells for about 10 cents a gallon more than 93 with ethanol. This week that is $2.80 a gallon. This is a very clean burning fuel and  it stores for a long time. On a cross country the engine will not care if it drinks some 100LL, again the compression ratio is determined by the lowest octane you will use, not the highest.  A few more hp isn’t going to make the Wagabond into a speed demon, I am after absolute long term reliability and being able to run any fuel available.

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Ron Lendon’s 2,850: Ron built a clone of Woody’s 2,850 engine with dished pistons and 95 hp heads. Recently he changed to flat top pistons and 110hp heads. This changed his compression ratio from 8:1 to 9.5:1. Yes, this will make more power, and it is OK because Ron says that he only runs 100LL . In short, he didn’t start with the highest performance option for the fuel is was going to use. Ron has worked for GM in their enginnering department for decades, so perhaps like most people who saw fuel prices in 2009, he might have been thinking about auto fuel then. But it pays to plan around the fuel you will eventually use. To keep things in perspective, I am sure that a 601 with a low compression engine and wheel pants met a 601 with high compression and no pants, the one with wheel pants might be faster.

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In the above photo, Woody Harris’ 2,850cc Zenith 601B sits at the end of the ramp in North Carolina at First Flight Airport with the Wright Brothers Monument in the background. Woody’s home airport is in California. He has nearly 500 hours on the plane without issues. read more:Woody’s 2,850cc Corvair/601XL hits 400 hours.

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Woody in the Grand Teton National Park WY

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Lest anyone think that low compression engines don’t make good power, above, Woody flying over Grand Teton. He often flies around the Sierras, and has flown to the highest and lowest airport in California in the same day.

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Above, a drop forged, made in the USA piston for the Corvair. The  displacement of this piston is 2,850 cc. read more: Turbocharging Corvair Flight engines Pt. #2

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2005 photo of our 601XL in front of our Edgewater hangar. The engine is a 3100 cc with 140 hp heads, oversized exhaust and 11:1 compression. Because it was a tail wheel and low drag it was fast. With wheel pants and the right prop turning 3,500 rpm, this plane could exceed 145 mph at sea level. People asked about weight, but at the 601’s low wing loading, it is slightly faster when loaded. They are good planes, but other than demonstration purposes, anyone really concerned about getting the last mph out of a 601 probably picked the wrong plane. It beauty is in utility, not speed. Note the size of the inlets: Here we have the most powerful Corvair engine that builders have heard of, yet it cooled itself just fine in hot Florida with 4.75″ inlets and a front alternator. It is a myth that this installation needs giant inlets to cool itself.

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Same plane, at sun n fun 2006. Sensenich prop was faster, but didn’t climb as well. I could have built the same engine for the plane we have today, but instead I chose something on the other end of the compression scale because I don’t wish to be tied to 100LL forever. Take your pick, what ever makes sense to you.

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Above, Dr. Andy Elliott, of Mesa AZ with the same engine on his 601XL. The photo was taken at Oshkosh, so it is safe to say the plane flew without issue. Andy flew the engine another 400 hours. His state has the highest summer temps of anywhere in the US, and yet the high performance engine cooled through the same size inlets. Andy’s plane could do nearly 140 mph. The power was a factor, but aerodynamics matter more and are cheaper. Before selling the engine to Andy, I reduced the compression slightly, but he still knew to always run 100LL.

-ww.

An Internet drama in a teapot.

Builders:

A little internet drama is a guilty pleasure of many aircraft builders. Submitted for your approval, a little drama that ran this week; It has a cast, a plot and a twist. Fun, but in the end it is only drama, and like a dozen other dramas before, it entertains, but doesn’t advance your plane toward flying.

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Cast:

Mark from Falcon Heads, Roy from Roy’s Garage, and 601XL builder/pilot Ron Lendon.

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Background:

At the end of last year, I privately told Mark and Roy that I was no longer going to have them at Corvair Colleges nor in my booth at Oshkosh. After nearly 10 years of being their single most vocal supporter, I was tired of Mark not making heads and Roy telling people his work was “Technically Correct” with the implication that people choosing other suppliers were making a mistake. To retain some portion of builders, they decided Mark would come up with a special set of magic head mods for $500, and Roy would run people’s engines on his dyno with promises of further power increases. To sell this to people, they enlisted Ron Lendon to put it on his plane, and then tell people what an improvement it was. The broke the ‘story’ on the “Corvaircraft” discussion group, a venue where I am not allowed to participate.

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It all sounded pretty good to people who like a good drama/conspiracy theory. Mark and Roy had “discovered” the dirty secret of Corvairs: The way we tell people to build them (Just as Mark and Roy have done for years) is terribly down on power. They claimed to have raised Ron’s power output by something like 24%. Ron followed up with a detailed flight report that showed his plane to now run 116-117 mph, a large improvement. Roy then comes in with some graphs showing Ron’s plane now makes 100 HP.

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Sounds great, except:

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Several well known and trusted 601XL pilots with 2700 cc Corvairs chime in to say that their planes are that fast already. Lynn Dingfelder and Phil Maxson, who have both been flying for years, point out that their planes do 115 mph, and Ken Pavlou’s will break 120, in the same configuration as Ron Lendon’s in spite of Ron’s engine being a 2,850 cc. The logical observation is that these unmodified airplanes have the same output as Ron’s now does, which Roy’s dyno said to make 100HP. Most people concluded that the test validated my long standing power output claims, because there is little variation in 601XL airframes, so the same speed  = the same power.

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Points to understand:

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Roy claimed to have previously tested a 2700 cc engine and the output was only 82-83HP. His contention was that all 2700s built to my suggestions had that power output. Clearly that wasn’t so, based on the other pilots reports compared to Ron’s ‘modified’ 2850.

I have little doubt that Ron’s plane had an improvement. He had been plagued by engine problems in his first years of operation, mostly caused by his adamant use of a obscure carb of a 65HP engine. In spite of working for GM for decades and having significant flying time, Ron  missed that his engine was running lean enough to damage itself bad enough to need a rebuild. His plane was never a particularly good performer by 601 standards. He got another Carb, much closer to correct, but still didn’t recognize it was running lean.

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There was no ‘before’ run made on Ron’s engine, just an after. Although he mentioned that his plane now has flat top pistons, he didn’t mention that the heads were changes from 95 to 110 high compression ones,  the valve size increased, and again the carb was made richer. because it was never tested, there is no before and after, but judging from performance, his plane does run much better, but evidently not significantly better than other 2700cc 601XL’s. Changing the compression from 8;1 to 9.5:1 and making the carb a lot richer could account for the improvement alone. Perhaps the other ‘modifications’ have negligible or negative effect.

A great claim was made that Roy’s dyno was scientific because it used a data program called “Labview”. A guy who got it from his work traded it to Roy for a discount. Same guy claimed “This is basically the same software & hardware that is used on a $50-$100K dyno.” I tend to disagree because you can go on National Instruments website and see they sell the Labview dyno soft ware brand new for $1,290. I don’t think having  that software makes Roy’s dyno the equal tool as a $100,000 dyno.

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No mention was made of correcting the dyno runs to standard atmosphere. Without this, there is no comparisons between engines, even ones run a few hours apart, far less weeks or months apart.

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Above, Roy and I running an engine I built in 2014 on his dyno. Several people chimed in on Corvaircraft to praise Roy for his testing, even though they have no experience with dynos. Does this look like a $100,000 piece of equipment? On the day in the photo we could not get a test more than a few seconds long, and Roy had to manually manipulate the controls, there was no real data from this. I am sure it is better now, but this isn’t the “technically Correct” infallible tool that some people suspect.  If you would like to read a dozen stories of practical testing spanning 10 years, look here: Testing and Data Collection reference page

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Above, a 2008 dyno calibration run in my yard in Florida. Notice Kevin and I are wearing jackets. We’re waiting just before sunset for a rare weather phenomena to occur: a perfect standard day of 59F 50% relative humidity and a pressure of 29.92. Any time you read a dyno report and it says “corrected horsepower,” they’re making a calculation, sometimes accurate and sometimes not, to adjust for their test conditions not being at standard atmosphere. Because we live in Florida near sea level, there have actually been three occasions the past years when these conditions were met during daylight hours on testing days.

Our dyno relied on the super accurate optical Prop Tach for the rpm measurement and it will only reliably pick this up in daylight. A few minutes after the photo above was taken, we made a dyno run which required no correction. By testing the same engine later in the week, we reconfirmed our correction factors for this particular dynomometer and we retained accurate measurements all year round. If you want to read the whole story, it is here: Dyno testing Corvairs, 2008 Any dyno run that doesn’t reference a correction to the ‘ICAO Standard Atmosphere’ has no meaning, and there is a significant difference between  the reliability of a calculated correction and a measured one, as we are doing above. People get excited hearing about ‘software packages’ but in reality the value of the tests relies on basic things like atmospheric corrections. 

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Above, Ron Lendon, running his engine on my stand at Corvair College #17, having a good moment. Yesterday he said this on Corvaircraft: ” I even heard WW say to Dan W. that he would fly the engine I just built at CC17 to the Bahamas. But I don’t here him saying that now, no he is heaping his opinion on people he called friends because they are behaving as he did several years ago. “  A big part of why Ron’s engine ran great on my stand is that my stand has the recommended carb, a MA-3SPA. He promptly went home and bolted the incorrect carb on his plane, because it was cheap, starting a long series of issues. Oddly, the people Ron is championing today, Ron and Mark, supplied him with parts and service that he was previously angry about. As for his evaluation of the behavior of ‘friends’, perhaps he can review the definition of “ingrate.”

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As for Mark or Roy being able to claim ignorance of the output of engines they happily built and sold to people, I submit the photo above: Marks EFI 2,700cc Corvair in 2007, on my dyno, right in front of mark’s shop in WI. He certainly didn’t think this motor, nor a carbureted one was 82hp that day. You can also see that Mark was present in the calibration story above. Roy had also flown as a passenger in Lynn Dingfelder’s 601XL and saw what a good running plane, with a stock 2700cc ww engine could do. Before making his claims this week he understood that his ‘modified’ engine in Ron’s plane was no more powerful than Lynns.

 My testing was absolutely satisfactory for Mark and Roy to sell heads and engines to people for years, but somehow they have suddenly ‘discovered’ that none of these engines worked, coinciding with them becoming unwelcome at events I am hosting. Think it over.

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That concludes todays entertainment programing. I am headed back out to the shop to prep for the next Corvair College, I suggest builders intrested in progress do the same.

-ww.