changing nitro %, need to add head shim?

[vgibbens]

I have a Novarossi RX21 P5 engine in my Mayhem. I've decided to race it this week and will be making the move to 30% nitro from 20%. I know I'll have to richen the engine a little, but will I have to add a head shim? I believe the manual that came with the motor said that 25% was recommended.

 

[powder_king]

you shouldn't have to change the shims i think on most motors you are alowed to go up or down 5% nitro content before having to reshim so you should be fin. try it if it starts to detonate add a shim

 

[CorradoPsi]

the P5 comes ready for 30%, also, you will be leaning it out, not richening it up. you can start right were your settings are now (assuming its tuned for 20%) and just lean out as nessessary. if you decide to go back to 20%, richen it up again before ever starting it.

 

[crzy-svg-usr]

you dont richen when increasing nitro %, with the higher % nitro you need less air for combustion thus a richer setting, this is why 30% runs cooler than 20% because in fact the engine is richer then 20% but since it has 10% more nitro it runs faster then 20 at the richer setting

 

[vgibbens]

I'm confused now, I'm looking through my Revo manual in the section about engine tuning. It is stating that when you increase nitro % that you richen the mixture.

 

[vgibbens]

this is what caused my question. It is a quote from the engine tuning article that's in the general nitro talk forum http://www.rcnitro.com/rn/articles/super_tune2.asp :

"Adding up to 10 percent more nitro than is typical produces more power, but you have to know how to adjust your engine to accommodate the extra nitro. Fuel-mixture settings need to be slightly richer when nitro content is increased. Also, you may have to increase head clearance by adding an extra head shim. The extra fuel introduced into the combustion chamber increases compression by adding non-compressible matter; this also increases cylinder pressure during the combustion process, which may cause detonation. Detonation occurs when the fuel explodes instead of burning, and that can cause internal engine damage. The extra head shim will likely prevent detonation when fuel with higher nitro content is used."

 

[CorradoPsi]

thats a pretty good article, though i did find a few mistakes in it. put the 30% in your car and see whos right. will it need leaned out or richened?

 

[mikeburgin]

the reason people run higher nitro is to increase octane levels, octane not only gives you more bang it also lets your motor run cooler, therefore letting you run your motor leaner to give more power, why would you run more nitro if you have to run richer and loose performance ????????????
once you start getting over 30% then nitro starts working the other way

 

[niggle]

the reason people run higher nitro is to increase octane levels, octane not only gives you more bang it also lets your motor run cooler, therefore letting you run your motor leaner to give more power, why would you run more nitro if you have to run richer and loose performance ????????????
once you start getting over 30% then nitro starts working the other way

Not sure I agree. You can't directly compare alcohol based fuels to petroleum based fuels. Glow fuel does not contain any octane or heptane so you can't equate RON ratings.

Nitromethane is an oxidising agent, not an octane booster (anti-knock agent). It performs an equivalent function to laughing gas (NOS) in a petrol engine. i.e. it introduces a higher volume of oxygen in the combustion chamber without resorting to forced induction and this allows you to burn more fuel (a richer mixture) and therefore effectively increases the volumetric efficiency and thus the compression ratio. More fuel = more energy if burnt at roughly the correct air/fuel ratio and more compression = more torque if you can avoid detonation.

With a methanol fueled engine the air/fuel ratio is less important. Basically the engine will burn as much methanol as the oxygen in the combustion chamber will support. The rest disappears out of the exhaust unburnt.

The reason that R/C 2-strokes run "better" on a leaner mixture is the reduction in the windage (drag) caused by the lubricating oil in the fuel mixture in the crankcase. Knife-edging the conrod and using a racing blend with less oil content helps to reduce windage. Ultimately, performance is dictated by how little oil you can get away with before the engine overheats. These engines don't produce much more torque on a leaner mixture, they just rev higher. Power increases with engine speed as long as the torque does not drop off. A higher revving engine means a faster top speed.

Cooling is primarily provided by the evaporation of the atomised methanol during the induction phase and by cool air passing over the crankcase and cylinder head fins. Some heat is also absorbed by the unburnt oil and carried out of the engine.

Increasing the nitromethane content of glow fuel also helps smooth running at idle speeds, where the LSN metering is very approximate due to low air speeds through the carb venturi.

In summary, in two identical engines with identical carburettor needle settings, one running on 30% nitro content and the other running on 20% nitro content, the engine running on the higher nitro fuel will have better idle and torque characteristics (and therefore a higher power output) due to the superior volumetric efficiency and compression ratio. Maximum engine speed may also be slightly higher as the higher torque will help to overcome windage losses.

That's my 2c worth. Healthy discussion and alternative theories are most welcome.

 

[Diver6127]

DAMN, did you do a dissertation on fuel and the additives they put in them? That wore me out! Almost like trying to pass chemistry all over again. Brrrrrrr, gave me chills thinking about chem class!

 

[mikeburgin]

ok i gotto agrre with your um yeah report
but most of these guys dont understand the long winded report on methanol
so to put in octane is easiest
as far as the cooling goes oil is a huge part of it not just a minimal, also aids in compression ETC

 

[vgibbens]

Well, I ran the 30% yesterday leaving the needles where they were set for 20%. There was no smoke. I had to richen it up about a half turn on the top and bottom to get it to run lower than 280°. I must say I like the way the 30% makes the lower end more responsive.

 

[mikeburgin]

yeah um ooops sorry i did set you wrong on that, u do have to richen up not lean out, my bad

 

[low10s]

thank you i made a statement on another post about having to richen the needle on 30%, wasn't going to get in a cat fight about it LOL, but you do have to richen it. on both my Rb's it was a full turn on th hsn and a 1/4 turn on the lsn. i tried leaving mine alone at first, lean bog from hell and a little on the warm side. yea the 30% really wakes it up.

 

[CorradoPsi]

very strange, what exact fuels are you guys switching from and to? i remember switching from 20% about a year ago on all my engines, old fuel was odonnell and new fuel was odonnell race 30% all my engines needed leaned out. temps were under 200* on all of them, and so rich that they spit raw fuel.

 

[low10s]

o donnel for me too. i guess the area, and if you added a shim or not. on my RB i did as Rody suggested and shimmed it before i ever ran it, even though alot of people run the 30% with stock shimms i did not want any probs.

 

[vgibbens]

I went from Traxxas 20% to Sidewinder Race 30%

 

[CorradoPsi]

vgibbens, maybe that is why, traxxas is a cool running fuel and sidewinder is known to run hot. if you feel like it, next time try picking up a gallon of traxxas 30% to see how it runs. id be interested to find out.

theoretically when you go up in nitro you need to lean out to maintain the same A/F ratio. this is because the fuel you are adding in the same volume has more "fuel" in it, the extra 10% of nitro. so to maintain AIR TO NITRO ratio you need to reduce that amount of fuel comming in by leaning out the engine.

however if the oil package causes hotter running, i could see a need to richen up.

 

[Rolex]

Any time you change the nitro content in your fuel, you also need to do a partial break in to get the engine 'used to' the change.

 

[CorradoPsi]

that is more to clean out the old fuels residue and build up than anything else rolex. just taking it easy on it for the first tank or 2 is enough.