Compression Ratios, Fuels and Power Output


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


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.


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.


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.


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.


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.


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.



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.


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.


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.



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.


Woody in the Grand Teton National Park WY


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.



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



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.



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.



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.


About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at and in more than 50 magazine articles.

13 Responses to Compression Ratios, Fuels and Power Output

  1. Carl Orton says:

    While I intend to run nothing except 100LL (2700 / 110 heads / flat tops), I’d like to be pragmatic with an eye to the future. Have you given any thought on the upcoming 100LL replacements like 94UL? Any recommendations on what (if anything) we should do if we build for 100LL?

    • Carl Orton says:

      Meant to say “if we build for 100LL but it gets phased-out for the new unleaded formulations.”

      • Carl,
        My understanding is that all the new formulations of replacements for 100LL will work in it’s place with the exception of supercharged engines. I am sure that any replacement that is promoted will have to work in all naturally aspirated GA engines.

  2. Jeff Moores says:

    Good stuff. More helpful than the drama this week. The only reason I wish to switch to the 3L with 95HP heads is the lower compression ratio. I would like to be able to use auto gas; even mixed 50/50 would be ok. I’m not a cheapskate but the price of 100LL here is $3.50 a litre. I fill up two 45 gallon drums at a time at the airport and that costs $1400.00. Sometimes they don’t have any. I fly a lot when the weather is good and I have burned a full drum in a weekend !!

    Great info on the different combinations; I think I’m on the right track. I have a new billet crankshaft in my workshop I received last year just waiting for a crankcase, cylinders, and pistons 🙂


  3. softoolsmith says:

    What compression ratio would I get with 3000cc jugs, dished pistons and 140hp heads? I only ask because my core came with 140hp heads…


    • Dave,

      The compression ratio would be between 9.5 and 10:1, as the 140hp chambers have less volume than a 95 or 110. This is really impractical, and the larger valves in these heads are net hp losers until they reach high rpm. They also require custom exhaust, as the stacks are a different size. We ran these experiments years ago, but the real value of such tests is reinforcing the concept that 95s or 110s are the best choice. Dan Weseman has about 80 pairs of core heads I sold him when he took the lead on the head project. When you are ready he will probably be able to set you up with a rebuilt set without you having to find another pair in for sale.

  4. William (Bill) Mills says:


    Have you done any research using 110 octane auto racing fuel as produced by Sunoco?

    Bill Mills

    • Bill,

      There are a number of companies that make race fuel, for a sample get a look at this company:
      For practical purposes as far as Corvairs go, the base fuels are roughly the equivalent of 100LL, and in places where 100LL is high priced, some of these race fuels don’t seem so expensive. But in reality, you have to contend with availability, and half the airports in the US have 100LL and none of them have race fuel. The only real advantage of race fuel is that some blends get 100 octane ratings without lead, and they burn very clean compared to 100LL. Personally, just for operational flexibility, I would not build an engine for myself that required the use of 100LL. Comparatively, if you had an ultra high compression 2700 that had to have 100LL vs an 8.3:1 3,000 cc engine that ran on 90 octane car gas, they might make the same power, and the 3,000 would cost $1,200 more in parts to build, but if Boat gas is $3/gal. and 100LL is $6/gal, the difference in engine costs is made up in 100 hours.

  5. Mark Miller says:

    William- I started my (your) engine yesterday, ran great on both sides of ignition and started on both sides. I used 100 LL. What compression and heads are on my 2700? Can I use auto gas?

    • Mark,

      I don’t build engines with Compression over 9:1 unless I specifically discuss it with the builder in advance. I highly suggest doing the whole 40 hours on 100LL, with Decalin fuel treatment in it (aircraft spruce) before slowly introducing auto fuel into your program.

  6. Dave Hoehn says:

    I must admit that I hadn’t paid much attention to my heads beyond the fact that I had acceptable heads and that they were finally back from Falcon. After reading this post, I cross-referenced the casting number in the Junkyard Primer and discovered that I have 110 HP heads. You may recall from our conversations at CC32 that retaining the option to use mogas has always been one of my objectives. While perusing the Clarke’s catalog, I noted that they offer .042” and .052” head gaskets as well as the standard .032” gaskets. A little number crunching indicates that increasing the head gasket thickness by .010” will reduce the compression ratio by approximately .25. The catalog also mentions stacking 2 head gaskets to adjust for a resurfaced head. Stacking a .042” gasket on a .032” gasket should drop my compression ratio from 9~9.5:1 to 8~8.5:1 necessary to burn ethanol mixed mogas. What are your thoughts on using such an approach?

    Dave Hoehn

    P.S. I haven’t had a chance to pull one of the exhaust valves yet. Trying to figure out the best way to compress the spring and release the valve keeper without messing up any of the machined surfaces.

    • Dave,
      A number of people have successfully run 92 octane fuel with compression ratios around 9.25 to 1. It should be fairly easy to get your combination down to a good number without resorting to stacking gaskets. Also note in my story about Oversquare the role that proper mixture plays in detonation resistance.

      I have spoken with Dan about getting a little more info out on this and ID’ing valves.

      To be clear, the only person I a sure put a clarks std valve in 2 sets of heads was Roy. I never saw mark use those valves. The ones he did use that got tired in 200 hrs were installed before he started putting rotators on. Your heads being done in the last few years will have rotators on the exhausts, and these will greatly extend the life of the OK valves. The valves that Dan is using are undercut and swirl polished, which are easy visual identifying features that will allow you to see which ones you have.

  7. Mike Crowley says:


    I enjoyed my first Corvair College in San Marcos last year, and have just shipped my core to the Wesemans for machining and head refurbishing. I have been rereading this article to help me decide on the best compression for my application. I am planning on the 3.0L build to install on the Savannah I’m building. I’d like to run car gas locally and 100LL on cross country flights, and we have some hot summer flying here in Midland, Tx. What would you recommend? You mentioned the difference in power in terms of cruise speed, how about take off and climb performance? This is probably obvious but I sure appreciate your insight as I’m rapidly approaching a decision point.

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