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Compression ratio limit for auto fuel ?

Larry DeCamp

Well Known Member
It sounds like a 320 "BUMPED" is a good tradeoff to 360 for a light FP RV4. Does anyone know of a fact based limit for compression ratio and still use auto fuel. let's talk in terms pump numbers like 87/89/91/93.
 
Difficult question

The answer will be based on a number of variables and I am not getting into it.
8.5:1 works for me on 91E10 as tested in all phases of flight that I can think of.
If you limit yourself to partial operation on 91 octane, you might be ok with
A compression ratio higher than 8.5:1.
Among things to consider when contemplating the use of low octane fuel is operating temperature, power settings etc.
My short answer to your question would be to stay away from high compression cylinders if you plan on using mogas. You are simply reducing the detonation margin that is already reduced by using gasoline with octane ratings lower than
100LL.
 
My 8.5:1 O-360 Pitts said min 91/96.

I have a RV-6 O-320 with H2AD pistons at 9:1

So far so good running 50:50 100LL and 90REC local ethanol free Mogas.


Easy ratio to mix, I use Decalin TCP in the AVGAS, clean plugs, no fouling. Carb jetted right (flows over 13.5 gph wide open down low std day) and mags set at 26 degrees BTDC.


Probably a belt and suspenders approach.

Avgas is $3.90, 90REC no ethanol mogas $2.60 near home, but it's not really the savings, it's avoiding excess leading.

Vividia VA-400 borescope after 200 hrs looked good yeaterday.
 
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I've run a stock 160 hp engine for over a decade on 91 E-free mogas. Wouldn't hesitate to use E-gas if my plane wasn't so old that I worry about the tank sealant, and the old, non E-proof diaphragms in the mech fuel pump.

Higher compression would make me less comfortable.

Charlie
 
In my experience, there are auto fuel STC's for some certified planes with 8.5:1 compression. Many more at 7.low:1. The STC for a Cherokee 180 (O-360 w/ 8.5:1) requires two electric pumps with a switch for either/or as well as the engine-driven pump, presumably because of the lack of head pressure due to being a low-wing airplane.
 
Long road to Dallas

As has been stated above this subject can be muddied depending on what you know and how you use it. I think it is best to just tell you our rule of tomb.
We run 9:1's in an IO-360. This is because if we can we like to go CX a little higher than most others do. If we use 100LL we watch our EGT's and CHT's and if we are getting warm like in the summer we will state below 75% power.
If we are using 50:50 mix of 93 "no/E" and 100LL we try to stay below 75% except if we have to, Like T.O.. Our fuel system is all new age seals and Pro seal so we are not afraid to run 82-83/15% E, But we do it with good caution and prefer to stay under 60% in the summer heat and 70% in the winter months. I hope that the FAA and the fuel makers will be getting the new no lead fuel in service in the next couple of years so our lead deposits will go away, but until then we will try to play it cool so we don't melt a piston or valve with an experimental engine. We should not, I am used to running with a lot higher BMEP in the long ago past. Hope this helps, Yours, R.E.A. III #80888

P.S. on older engines it is not a bad idea to use some leaded fuel to help lube the seats and stems. With a new engine even one with good alloy valves, seats and guides it is good to run some lead to help the valve train in the first 20-50 hours of that new engines life.
 
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8.5 is probably a good figure for 91 octane Mogas. With some programmable EIs like the CPI, you have the flexibility of easily varying ignition timing for octane used to get best performance possible.
 
I have 8.7:1 on my IO360, running 91E10 for the last 60-ish hours. During my testing I tried hard to induce detonation just to find out where and if that would start, and only one time was I successful in doing so, and it was a case of beating on the engine hard to make it happen - not something you would typically do without knowing it.

If you have full engine monitoring and 8.5:1, you are not going to get it to detonate on 91E10 without at least a couple parameters being well outside the normal operating range, IMHO.
 
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I have 8.7:1 on my IO360, running 91E10 for the last 60-ish hours. During my testing I tried hard to induce detonation just to find out where and if that would start, and only one time was I successful in doing so, and it was a case of beating on the engine hard to make it happen - not something you would typically do without knowing it.

If you have full engine monitoring and 8.5:1, you are not going to get it to detonate on 91E10 without at least a couple parameters being well outside the normal operating range, IMHO.

Lycoming detonation studies would seem to indicate otherwise with timing at 24 degrees BTDC.

I am wondering how you are measuring detonation? Are you able to hear it above the exhaust noise?
 
Lycoming detonation studies would seem to indicate otherwise with timing at 24 degrees BTDC.

I am wondering how you are measuring detonation? Are you able to hear it above the exhaust noise?

No, I was watching the CHT's like a hawk and saw two cylinders take off like a rocket, I made the logical assumption that I was seeing early detonation. I've got my timing set at 25 BTDC.
 
I have 8.7:1 on my IO360, running 91E10 for the last 60-ish hours. During my testing I tried hard to induce detonation just to find out where and if that would start, and only one time was I successful in doing so, and it was a case of beating on the engine hard to make it happen - not something you would typically do without knowing it.

So what were the operating conditions which would induce detonation? And that's a parallel valve IO-360?

(Note, there is some decent data for the angle valve, but I'd love to see a parallel valve detonation study.)
 
So what were the operating conditions which would induce detonation? And that's a parallel valve IO-360?

(Note, there is some decent data for the angle valve, but I'd love to see a parallel valve detonation study.)

It's been a while, and I didn't capture an engine datalog of it (really really wish I had) but my memory of it was during the early testing of my airplane on 91E10. It was a hot summer day, mid-90's on the ground, I was putting the engine into full-power climbs one after another at low altitude (max temps, max MAP, max RPM etc) and progressively leaning the mix on successive climbs to tickle the dragons tail and try to induce detonation to see where it might begin. I was starting near ground level 3000 MSL, with a DA of probably 6000-ish, and on a run with the mixture set just a hair LOP I saw my #4 CHT go from 390 to 440 in about 10-12 seconds and #2 was coming up through 425 at the same time. Cylinder #1 and #3 were in the high 300's but did not exceed 400, I do remember that. That was close enough for me, I can't say for certainty that I saw a detonation event but I know I saw something close enough to it for my taste.

I have never seen a repeat of anything like this during a full-power climb, even extended into the mid-teens, as long as I stay nicely rich during the climb.

And yes, this is a parallel valve IO360, Bendix injection with balanced nozzles, Superior cold-air sump, James cowl and plenum, CS prop, one Slick Mag and one Lightspeed Plasma I.
 
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You are playing with fire!

No, I was watching the CHT's like a hawk and saw two cylinders take off like a rocket, I made the logical assumption that I was seeing early detonation. I've got my timing set at 25 BTDC.

What you have seen must have been very light detonation. I have seen one totally destroyed engine with a hole in one piston and some more with damaged pistons and connection rod bearings. It just took 2 seconds from the first indication of detonation until reaching the kill switch. This was on a well-equipped engine test bench. I would never ever experiment with detonation margins in the air.
 
And yes, this is a parallel valve IO360, Bendix injection with balanced nozzles, Superior cold-air sump, James cowl and plenum, CS prop, one Slick Mag and one Lightspeed Plasma I.

And what is the fuel pump situation, are you using standard engine driven pump with electric pump in series?

I suspect new mechanical pumps do use ethanol compatible materials but no manufacturer will verify it.
 
Not in writing, but I got verbal affirmation from Tempest several years ago, when discussing using E-mogas in experimental a/c.

1st hand info for me; 2nd hand info for you. But all it took was a phone call...
 
And what is the fuel pump situation, are you using standard engine driven pump with electric pump in series?

I suspect new mechanical pumps do use ethanol compatible materials but no manufacturer will verify it.

No mechanical pump, I run dual Airflow Performance electric pumps pulling from the wing root and sending pressurized fuel FWF. Hot starts can be interesting sometimes with the high vapor pressure fuel, but I've had zero problems with vapor lock operating in west Texas heat.
 
What you have seen must have been very light detonation. I have seen one totally destroyed engine with a hole in one piston and some more with damaged pistons and connection rod bearings. It just took 2 seconds from the first indication of detonation until reaching the kill switch. This was on a well-equipped engine test bench. I would never ever experiment with detonation margins in the air.

Detonation won't destroy a Lycoming piston or damage the rod bearings in 2 seconds but pre-ignition will. The 2 things are quite different as are the failure modes. Detonation almost always breaks the rings and ring lands first, pre-ignition almost always puts a hole in the piston crown first and/or melts off the ground electrodes on the spark plugs. Pre-ignition is a very fast mechanism to failure- usually just a few seconds.

I agree that ground running with proper instrumentation is a better place to test detonation margins.
 
With some programmable EIs like the CPI, you have the flexibility of easily varying ignition timing for octane used to get best performance possible.

This.

Also, EFI and knock sensing and the advancing engine management technology that is starting to migrate from the automotive world into the aviation world.

The new cars out there are often 10:1 or higher compression, new Lexus V8 is 12.3. Smart electronics make it possible--------that and more effective cooling ability.

When this thread started, I wondered if anyone would get into the cooling aspect of the increased compression-----guess I need to open that door.
 
This.

Also, EFI and knock sensing and the advancing engine management technology that is starting to migrate from the automotive world into the aviation world.

The new cars out there are often 10:1 or higher compression, new Lexus V8 is 12.3. Smart electronics make it possible--------that and more effective cooling ability.

When this thread started, I wondered if anyone would get into the cooling aspect of the increased compression-----guess I need to open that door.

Would you believe that some cars are now running 14 to 1 CRs on regular pump gas. My Suzuki bike is 12.8 to 1 and also happily runs on 87 octane with no knock sensors and port injection. It's all in the chamber designs, valve timing and computer controlled ignition to make it happen safely. We can do some of the same things with Lycomings at least on the ignition fronts with EIs.
 
I have a Mazda6 I bought last year that runs 13:1, and they are 14:1 overseas. Apparently they solved the heat problem with extra long exhaust headers. The thing drives and runs great, although sounds kinda funny when it's cold. Runs fine on regular car gas with ethanol.

Chris
 
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Automotive High Compression

Would you believe that some cars are now running 14 to 1 CRs on regular pump gas. My Suzuki bike is 12.8 to 1 and also happily runs on 87 octane with no knock sensors and port injection. It's all in the chamber designs, valve timing and computer controlled ignition to make it happen safely. We can do some of the same things with Lycomings at least on the ignition fronts with EIs.

Liquid cooling (better temperature control), variable valve timing, small(er) cylinder bore sizes, and better intake charge distribution (4 valve cylinders help here) are some of the things beyond knock sensors that help modern auto engines survive with such high compression ratios. For this reason, I urge caution to those who believe that the compression on a Lycoming can be taken up to automotive levels and still be run safely, especially on MoGas.

Skylor
RV-8
 
Liquid cooling (better temperature control), variable valve timing, small(er) cylinder bore sizes, and better intake charge distribution (4 valve cylinders help here) are some of the things beyond knock sensors that help modern auto engines survive with such high compression ratios. For this reason, I urge caution to those who believe that the compression on a Lycoming can be taken up to automotive levels and still be run safely, especially on MoGas.

Skylor
RV-8

I don't think anyone here thinks they'll be running 14 to 1 or anything close to that in a Lycoming on Mogas. I earlier suggested 8.5 was a good target. Even then you won't be running 25 degrees of timing at SL WOT. The thing is, programmable EI will allow you to sacrifice only a little power at SL WOT while still giving you decent cruise power and economy at altitude. Fixed timing with mags won't allow you to do that as well.
 
I don't think anyone here thinks they'll be running 14 to 1 or anything close to that in a Lycoming on Mogas. I earlier suggested 8.5 was a good target. Even then you won't be running 25 degrees of timing at SL WOT. The thing is, programmable EI will allow you to sacrifice only a little power at SL WOT while still giving you decent cruise power and economy at altitude. Fixed timing with mags won't allow you to do that as well.

I think the ballpark figure Ross is giving pretty much agrees with my experience.

I have operated the parallel valve IO-540 in the RV-10 with 93 octane NON-Ethanol car gas for about 2000 gallons with 9.0:1 compression ratio and electronic ignition varying from about 20 deg BTDC at full power (2700 RPM) to around 31 degrees timing at 2400 RPM full throttle and ~10,000 DA.

No problems with detonation when operated ROP at higher than 70% power and LOP when 70% or lower.

My ignition system is capable of in-flight manual adjustment and I was capable of causing 1 or 2 cyl's to start what appeared like a thermal run-away by advancing the timing from the curve stated above at high power settings and if ~100-150 ROP (which is not really where I operate the engine anyway)

Main problem I had with car gas is lower Read vapor pressure which caused slight vapor lock issues when very hot ambient temperatures and high fuel flow without the boost pump running.

Since I do not want the airplane to be dependent on the boost pump and I didn't feel like re doing the fuel system to provide lower suction pressure at the engine pump, I switched back to 100LL.
 
I have operated the parallel valve IO-540 in the RV-10 with 93 octane NON-Ethanol car gas for about 2000 gallons with 9.0:1 compression ratio and electronic ignition varying from about 20 deg BTDC at full power (2700 RPM) to around 31 degrees timing at 2400 RPM full throttle and ~10,000 DA.

Sent you a PM, couple questions about your ignition.
 
Lycoming SI 1070 link update

Hi All,

I'd been looking for Service Instruction 1070 to find the approved fuel types for Lycoming engines, but all old links to the Lycoming website appear to have been broken and don't redirect you to the latest version of this SI.
I've subsequently found the latest version. It can be located on the Lycoming website using "1070" in their search function, but not via Google searches.

https://www.lycoming.com/sites/default/files/SI1070AB Specified Fuels.pdf

I just thought I should update this because I've been asked couple of questions lately regarding high octane mogas and acceptable compression ratios.

Cheers,

Tom.
RV-7 with a IO-360M1B 8.5:1 that loves drinking 98 RON mogas.
 
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