Compression Ratio?

[s10sakota]

I need help understanding something about engines. I want to install a new 180 horse engine from Van's in my RV-7A.

I want 180 horse, but I also want to run auto fuel. I am told that I can do this but it depends on the compression ratio.

So I'm guessing I can change compression ratios in the O-360.

1. Changing compression ratios is done by changing the pistons, right?

2. I don't understand how this works, but does changing the compression ration change the horsepower? If I make the compression ratio suitable for auto fuel, is it still 180 hp?

3. Van's recommends I talk to someone at Lycoming. Anyone have a contact for someone at Lycoming?

Thanks!!

[s10sakota]

Oh boy, I just saw this sticky thread that answer most of my questions!

http://www.vansairforce.com/communit...ad.php?t=35765

[rv7boy]

Congratulations! You dodged the dreaded "search the archives" reply.

[AK4x4]

A possible solution is the Lycoming IO-360-L2A. It is a fuel injected 180 horse engine that is approved (by Lycoming no less) to run on premium car gas. Good luck, Russ

[WAM120RV]

Hi

I suspect that most people have avoided answering your questions is because they don't want to cause more confusion by getting tooooooo technical.

I have a Lyc 0-320 which was originally fitted with 7 to compression pistons and produced 150HP at its rated RPM. I have fitted 8.5 to 1 pistons which will give me 160 Hp..... at Rated RPM. So, yes more compression can give more HP.

What you are trying to avoid is pinking.... the fuel in your engine starting to burn before it should, another term is detonation. Pinking is very bad for an engine.

This is only part of the problem...... auto fuel have additives that may not be suitable your engine...... ie seals may get eaten by it. And in 100LL the lead acts as sort of a lubricant on the valves.......... take the lead away and you loose this effect. There was a period in time when we were doing a lot of work on valve seats in the UK when they took lead out of fuel, possibly it was the same in USA.

A number of years ago various engines were permitted to use auto fuel in the UK, but the goal posts keep moving because the petrol companies change the additives they put in there which can affect seals in carbs, etc.

So, if you are in a position to buy a 180 that is certified by Lyco for auto fuel thats the way to go, otherwise you will need to get technical to make sure you are safe.

Don't forget there is much more likelyhood of vapour lock with auto fuel.

[az_gila]

Originally Posted by AK4x4

A possible solution is the Lycoming IO-360-L2A. It is a fuel injected 180 horse engine that is approved (by Lycoming no less) to run on premium car gas. Good luck, Russ

Actually most of the Lycoming O-360 carb versions seem to be approved for Auto Fuel

http://www.lycoming.com/support/publ...fs/SI1070Q.pdf

The question may be getting fuel that meets the ASTM spec. quoted by Lycoming - see page 5/6 of the SI.

[DEWATSON]

If you mix 87 octane premium auto fuel (no corn squeezings) one to one with 100LL avaition fuel, the average octane will be 93.5. Is this considered an option to acquire the necessary octane rating for an IO 360 with 9 to 1 compression?

[BobTurner]

This is incorrect on at least 2 fronts, demonstrating why this is a tricky business.
1. Avgas uses "motor octane" only. Car gas uses the average of "motor octane" and "research octane" (R+M/2 you see on the pump). The motor octane of 87 car gas is about 83.
2. The increase in octane as you add tetraethyl lead is not linear. The first gram per gallon raises the octane more than the next gram, etc. So a 50-50 mix of 87 car gas and 100LL should give you a motor octane higher than (83+100)/2 = 91.5. I can't say how much higher.

[nineninefour]

After reading this thread, a quick Google found the page below, which is full of interesting and valuable information. After reading it, I'll not be planning to use auto gas, although I will probably work toward some no-lead solution.
http://eprinc.org/?p=300

[smokyray]

John,

I utilized MoGas in my 150HP 7.4:1 compression 0-320 RV4 for 12 years/1000 hours and continue to use it in my RVX. I have a local source for Non-Ethanol MoGas which can be found on http://pure-gas.org/ I changed engines at 1000 hours to a 170HP/9.0:1 compression Wide Deck 0-320 for better takeoff performance from my 1200' strip. I continue to use 91 octane MoGas with no issues whatsoever. BTW, My Dad (an IA) ran MoGas in our 57' C-182 for over 30 years with no problems, ever. When we overhauled the 0-470 at 2300 hours, the combustion chambers were some of the cleanest I have ever seen.

My technique has always been to use Marvel Mystery oil laced into the 91 Octane MoGas per the instructions and sump the tanks the same.. On hot days I do a longer run-up with the boost pump on to introduce cold fuel into the induction system. Other than that, standard. I have never had a problem with vapor lock, high temperature or plug foul, ever.

If you have 100LL available at an affordable price, no worries. Unfortunately, I don't. Therefore I utilize an affordable available fuel source which I have found satisfactory and likely to be around for the foreseeable future.

V/R
Smokey

Here is a good article:http://www.avweb.com/news/maint/187232-1.html


PS: For the 180HP, Superior offers a "MoGas option" and the Airflow Performance injection is approved for MoGas. With 100LL's future in question or it's cost becoming even more prohibitive, this may be a more popular option, if you like to fly.