OS 105 on 30% Nitro - extra shims necessary ?

[chosen69]

I realize that the manual says to include an extra .1mm or .2mm.
However, I live at 4000' MSL and it feels like I'm taking away a lot of power with the extra shims (I have .2mm extra installed right now for a .4mm total), eventhough it might make for easier tuning. The engine runs great but it doens't produce as much power as I expected.
I'm wondering if I will hurt the motor by trying to run it with only the default 0.2mm head gasket on 30% nitro ? Anyone have any experience with the OS105 on 30% and only the default .2mm shim/head gasket installed ?

[automagp68]

Does anyone know the answer to this as I'm trying to figure out what shim to put in mine also

????

[ticedoff8]

What are you comparing the power output too?
Did you run a test comparing 15% against 30% at 4000'?
I would expect at least 30% lower power than at sea-level. And an even bigger loss on hotter days.

Adding more nitro isn't going to help too much - and it will definitely make the engine run hotter.
Most everyone is running the 1.05 & the 1.20 with 15% nitro with great results. They have shown that running these engines with 30% nitro makes them run much hotter with little usable increase in power.
Adding the shims will reduce the heat build up due to the higher nitro, but the higher nitro isn't going to make up for the altitude.
Adding more shims isn't going to hurt anything - it lowers the compression and lower compression results in lower combustion pressures with less chance of preignition (ping).
Unless you try to lean it too much in the expectation of gaining some power due to the 30% + the shims. You will burn up the engine and think it was due to the shims.

[automagp68]

It seems that people are more prone to run 30% at our altitude

Almost everyone i know that flys here in colorado or Nm is running 30%

The questions is not a justification of which is better 15 or 30. The question is for the people who are running 30% in a 105 what head shim configuration are they finding to works the best?

does the 4mm shim lower the compression by to much making it feeling like a 91?

Is the 3mm total shim setup still to high and causing pinging?

I think that is the two questions here not a debate on which fuel is better or what percent nitro is better

[chosen69]

Here is what I came up with after trying most possible combinations and a lot of expereince with the OS 91 HZ (and its respective power output):

If you're at 4000'+ MSL like me:

• use 30% instead of 15%, just like automagp mentioned. 15% will give you noticeably less power at this altitude.
• the total of .4mm of shims that was recommended makes for a smooth running engine, but it takes away quite some power at this altitude. I went in steps down to the default .2mm, which did neither hurt the engine but finally produced the power that I expected in comparison to the 91HZ. It did make tuning a little harder in terms of you having to be pretty precise with your adjustments. I would recommed to use a temperature telemtry setup like I did for the tuning phase, so you know at any time whats going on temp-wise.
• for this engine (HZR) it is imperative to make sure that there are NO air leaks anywhere that would deplete the pressure built up in the system. Especially the Trex700N tank with it's blind plug is prone for that ! I JB-welded the blind plug into the tank after finding that I lost some pressure through it. You should be looking for airleaks if you find the engine running inconsistent i.e. great in the beginning and significantly leaning out towards the end. I even dropped the header tank and went to a fuel magnet, to eliminate possible airleaks and was rewarded with a very powerful performance.

sorry for the long write up - just my experience.

[marc8090]

I don't have a 105 but have run 91's since they came out and have played with head shims for years on numerous engines. The shims are .1mm and .2mm. I believe the engine comes with the .1mm shim installed from the factory. When you want to use 30% nitro you would REMOVE that shim and install the .2mm instead. Having both installed would lower the compression ratio alot at sea level, nevermind 4000 ft. Now, that would be the starting point at sea level. At 4000 ft the air is thinner so you may be able to run 30% with the thin shim. You can always try it and see if you can get it tuned or not. I would start decreasing the total amount of shim, see if the power increases without signs of too much compression, and go from there.

[chosen69]

The OS105 has a single .2mm shim installed by default.

[automagp68]

I just loaded mine with the 4mm total shim

Going to see what happens. Im going to let my buddy fly it who has extensive expereince with the 91 and see if he thinks its flying like a 91 or if he thinks it feels like a 105 and go from there.
If i can get it to the 3mm shim with out blowing something up that is where i would like to be