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  #1  
Old 03-07-2021, 07:34 AM
olyolson's Avatar
olyolson olyolson is offline
 
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Default 9:5:1 or stock pistons?

Had a previous post about Lycoming vs Superior cylinders. My engine is being overhauled and there was a potential for delays getting the Lycoming cylinders but the problem was resolved as the Lycoming cylinders were shipped a couple days ago.

One of the decisions during the overhaul is whether to use the stock 8:5:1 pistons in the IO-360 M1B or upgrade to 9:5:1 pistons. The extra power would be nice but the debate is mainly over whether 100LL will or will not be available long term. There are pros and cons to both pistons and I have heard multiple justifications on both sides. According to a Lycoming service bulletin the stock M1B with 8:5:1 pistons can burn anything except 91 octane auto fuel

Don’t want to start a fuel war here, just trying to get comments on what pistons would you put in the engine. The engine is apart so right now I can go either way. Swapping pistons at a later date may be an option for some but not for me.
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  #2  
Old 03-07-2021, 07:44 AM
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Carl Froehlich Carl Froehlich is offline
 
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Default

Yep - the big decision.

I was tempted to go to higher compression for this last build (IO-360-M1B) but in the end stayed with the stock 8.5.

My reason for looking at the higher compression was to gain engine efficiency, not power. Weighting all the elements, the small gain with the higher compression pistons lost out to having the 100LL gun to my head.

While I suspect when 100LL goes away there will be some replacement, considering the tiny demand for such a novelty fuel the price will be crazy. Having the backstop of burning 94UL (e.g. Swift Fuel) or 93 mogas is a nice risk mitigation - for me at least.

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  #3  
Old 03-07-2021, 08:23 AM
Freemasm Freemasm is offline
 
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Don’t decide in a vacuum. Is your crankshaft dampened? Will your propeller be OK with the higher power pulses? Got to look at the whole system to avoid any potential expensive mistakes.
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  #4  
Old 03-07-2021, 08:50 AM
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Toobuilder Toobuilder is offline
 
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Default

What do you plan to do for ignition, because one that significantly retards at high MP is a useful feature if 100 LL goes away. My engine keeps the 8.5 because I plan to use auto fuel a whole bunch, and high heat testing last Summer proved that it can eat the regular unleaded junk from the corner gas station just fine. That said, I used the dyno time to do an ignition sweep to look at just how far one can retard timing before it really hurts power at fat mixtures, and its a bunch.

https://vansairforce.net/community/s...d.php?t=182190

Does this mean that you can run regular unleaded in your 9.5 engine? Not necessarily, but should illustrate that you are not done flying if 100LL goes away.
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  #5  
Old 03-07-2021, 09:03 AM
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olyolson olyolson is offline
 
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Default Pistons

I should mention that it is a non counterweighted crank, Electroair ignition on one side (non impulse on the other) and an MTV-12 C/S 3 blade prop. I did have extensive discussions with MT and they commented that the higher compression would put the prop closer toward the top end of its recommended normal operating range. They said it would be OK but the higher power pulses can sometimes lead to grease leaks. Even though he said the MTV-12 would be OK he recommended the MTV-9 if the higher compression pistons were used.

In the end I think the 8:5:1 is the right way to go all things considered but I am open to more discussion from those that have gone this route and decision making before me.
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Last edited by olyolson : 03-07-2021 at 09:50 AM.
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  #6  
Old 03-07-2021, 08:22 PM
David Z David Z is offline
 
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Location: Thunder Bay Ontario
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Default Heat and Wear

Higher compression is also harder on the cylinders. It will run hotter and won't last quite as long. Although, at the rate most of us fly, not sure an engine worn out at 1700 hours is a big deal compared to 2000 hours.

Depends on what you want to do too. A competition Pitts that needs lots of vertical climb capability, 10:1 is worth looking at. Relaxing long distance hauler might benefit from low compression 7:1 and the ability to burn cheaper gas.
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  #7  
Old 03-07-2021, 08:46 PM
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emsvitil emsvitil is offline
 
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Or split the difference at 9 to 1


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  #8  
Old 03-08-2021, 03:01 PM
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Bob Martin Bob Martin is offline
 
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Quote:
Originally Posted by emsvitil View Post
Or split the difference at 9 to 1


Plus one on the 9 to 1.
We did this on an overhaul plus 4 new Lycoming cylinders sent to Lycon for porting and polishing. Engine is clearly stronger with very little change from stock.
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  #9  
Old 03-09-2021, 06:38 AM
Mike Houston Mike Houston is offline
 
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Default 9.5 to 1 and mogas

I am no expert on these matters so everything i say should be taken with a pinch of salt but i have been looking into this myself as i was hoping to build a RV14 but i want to run on mogas and the IO-390exp has 8.9 to 1 pistons.

As i understand it the main issues with running mogas are

1 pre-pump plumbing. Most vapor lock issues happen here and in the lines on the engine after a hot start with mechanical injection. I assume the use of emags might help with this and appropriate pre pump configuration? happy to be corrected

2 Octane being low and ethanol in mogas, however i am in Europe and our mogas has octane 95 and 97/98 for the premium so not much of an issue. As i understand it there are a few suppliers who will sell ethanol resistant systems?

If anyone has anything to add to this our thinks i am wrong please have a go i would be most grateful
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  #10  
Old 03-09-2021, 12:58 PM
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rv8ch rv8ch is offline
 
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Quote:
Originally Posted by Mike Houston View Post
I am no expert on these matters so everything i say should be taken with a pinch of salt but i have been looking into this myself as i was hoping to build a RV14 but i want to run on mogas and the IO-390exp has 8.9 to 1 pistons.

As i understand it the main issues with running mogas are

1 pre-pump plumbing. Most vapor lock issues happen here and in the lines on the engine after a hot start with mechanical injection. I assume the use of emags might help with this and appropriate pre pump configuration? happy to be corrected

2 Octane being low and ethanol in mogas, however i am in Europe and our mogas has octane 95 and 97/98 for the premium so not much of an issue. As i understand it there are a few suppliers who will sell ethanol resistant systems?

If anyone has anything to add to this our thinks i am wrong please have a go i would be most grateful
Not sure the emags will help too much with vapor lock.

What I did for mine is to assume i might have fuel with alcohol and installed viton o-rings everywhere in the system. Viton is resistant to ethyl alcohol.

I have not yet tried mogas but I think my aircraft is ready for it. Thankfully I have UL91 at my airfield. It seems to be available in the UK:

http://www.lightaircraftassociation....tal_avgas.html

This Polish company also has a UL91 product, but I don't know how to find out where they sell it.

https://www.warteraviation.com/fuels/avgas-wa-ul-91/

The issue of vapor lock is a bit of work. There are some strategies to reduce the risk - some run 100LL on one tank, and use that for ground ops, takeoff, and landing. They switch to the mogas at altitude. Another thing that seems to help is a fuel return system, either like used in some EFI systems (Ross and Robert have competing systems) or the AFP "purge valve" system which I have.

Since I have not yet flown with mogas, and it's not that hot here, I have not had to deal with vapor lock (yet).

There are some things to take into consideration - my recommendation is to do your homework, and then do it as close as possible to the way Van's recommends. Fuel system issues have caused far too many unhappy days for builders and pilots, so any changes from "the standard" should be done only after a lot of consideration.
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