Question for Engines Expert Paul McBride: I want to install 10:1 pistons in my Lycoming IO-360-A3B6D engine (angle valve), but I am not sure of the initial timing that I should use.
I understand Lycoming moved its recommended timing to 20° from the initial 25°. Does 18° with 10:1 pistons sound like a good start?
Paul Roberts
Answer: Paul, I’m assuming this engine is installed in an experimental aircraft? That being the case, I’d suggest you stick with the 20° BTC timing.
Several of the IO-360 series engines have had the timing reset from 25° BTC to 20° BTC in accordance with Lycoming Service Instruction 1325A. However, there is no IO-360-A3B6D mentioned in that publication. The reason for the timing change on the models mentioned in that publication was for improved performance.
That being said, and taking into consideration that you’ll be using 10:1 compression ratio pistons, I think you should use the 20° BTC setting for your application. I base this on the HIO-360 D series, which is used in a helicopter, and it also uses the 10:1 compression ratio pistons, but a higher engine rpm than your engine. I think using this setting will avoid the potential for detonation.
Read this very carefully before taking any action: Remember, you’ll be taking more horsepower out of your engine with the increased compression ratio, therefore you’ve got to increase the engine fuel flow amount to support the additional horsepower.
Failure to do this could result in severe damage to the engine and possibly cause engine failure.
Somone have Lycoming Service Instruction 1325A? I go to Lycomings website for SB’s, SI’s, etc and they don’t even have.
Is listed in their index, but good luck finding it.
I have been flying an experimental with an O-360 and E-Mags. The electric ignition changes timing automatically much like a car, each mag has an alternator that will not only run each other but also a radio. It is one of the smoothest and torqueiest engines I have ever flown behind.
This engine already has 8.7:1 compression. So, how much HP increase can be expected ?
From what I’ve been able to research, 1 point in compression can yield about 3% increase in HP.
So, with 10:1 pistons, it might get 8-10 more HP vs the original 200….maybe not worth the complete top overhaul ?
Maybe bolt on a turbocharger and get sea level HP at altitude.?
I think the only accurate determination for fuel flow and timing would be monitoring adjustments on a dyno. Even that is subject to operational altitudes which GA aircraft are not equipped to compensate for accurately, short of CHT is climbing so richen mixture.
Everything else, as with a lot of aviation, is a large degree of assumption and, like speed mods, individual interpretation.
An electronic engine monitor system the measures fuel flow to all clyinders, temps, get and vibration would be good ideas. Since the engine is no longer certified talking to Garmin or other makers makes sense.
Raising the compression alone (and retarding the timing for peak torque) increases the fuel efficiency of a given engine. It lowers the specific fuel consumption from around .5lb/HP/hr to approximately .45 lb/HP/hr, a more than 10% increase in efficiency. You will burn less fuel for the same power. Raising the CR also increases the maximum power to the point where you can burn approximately the same amount of fuel as before at the increased power. CR is the swept volume (360in^3)+ the compressed volume (the volume of the combustion chambers) divided by the swept volume. Raising the CR only reduces the compressed volume. The total volume, SV+CV is slightly reduced, thus there is slightly less volume for air and fuel so no extra fuel is needed with an increase in CR. The extra efficiency and power comes from the increase in BMEP which is the pressure in the cylinder prior to combustion. At a higher pressure the molecules are closer together and that causes a faster flame front in the combustion chamber (along with the increased turbulence) which requires a reduced ignition advance.
Thx for the great explanation!
I had noticed the issue in the article also and was going to mention the compression / fuel point.
I believe u are quite correct!