changing nitro %, need to add head shim?

[niggle]

theoretically when you go up in nitro you need to lean out to maintain the same A/F ratio. this is because the nitro actually is carring O2 in with it, in a denser amount than the ambient air. so to maintain A/F you need to reduce that amount of O2 comming in by leaning out the engine.

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

Leaning out the mixture will not reduce the oxygen content, it will increase it. To reduce the oxygen content (or increase the fuel content) you would need to choke the carburettor (not possible on most glow engines) or richen the needles.

Your experiences in retuning when switching to a higher nitro content kinda flies in the face of theory, but then the theory assumes that the methanol and oil content stays pretty much the same. Common sense dictates that if the nitro content increases by 10% then the methanol and/or oil content has got to reduce by 10% and changes in either will necessitate changes in carb needle settings (usually richer). It could also be that the 30% fuel that you used had a higher oil content than the 20% fuel and you could get away with running leaner.

I think the bottom line here is that if you switch to a higher nitro content, then some significant needle twiddling will be required. If the engine is producing more power on the new fuel then it is likely to produce more heat so pay particular attention to the head temperature and smoke trail and retune accordingly.

 

[CorradoPsi]

Leaning out the mixture will not reduce the oxygen content, it will increase it. To reduce the oxygen content (or increase the fuel content) you would need to choke the carburettor (not possible on most glow engines) or richen the needles.

Your experiences in retuning when switching to a higher nitro content kinda flies in the face of theory, but then the theory assumes that the methanol and oil content stays pretty much the same. Common sense dictates that if the nitro content increases by 10% then the methanol and/or oil content has got to reduce by 10% and changes in either will necessitate changes in carb needle settings (usually richer). It could also be that the 30% fuel that you used had a higher oil content than the 20% fuel and you could get away with running leaner.

I think the bottom line here is that if you switch to a higher nitro content, then some significant needle twiddling will be required. If the engine is producing more power on the new fuel then it is likely to produce more heat so pay particular attention to the head temperature and smoke trail and retune accordingly.

ok I edited my post that you quoted. i was sleepy and spewing bad info at the time. the reason you have to lean out with higher nitro content fuels is because while your flowing the same volume of fuel, the fuel that you are feeding your engine is more potent by 50% in this case. the nitro is what is making your power, and while you havent turned the needles at all, you have essentially richened the mixture by making it more nitro rich. so to maintain your air to nitro ratio (what actually makes the power) you need to lean the engine out.

what i have in itallics, i agree with whole heartedly. any time you make a change in fuel, you need to retune. period.

just for the record, when i switched from 20% to 30% it was from odonnel race blend 20% to odonnel race blend 30%. identical oil packages, the only thing that changed was nitro content went up 10% and methanol content went down 10%.

 

[Sgt Blamo]

OK, so If I switch from 20% Trinity Monster w/12% oil,to trinity 30% w/same oil content, will my engine run hotter and will it wear out sooner? Is it worth the extra cost? will I have less tuning issues w/ 30%?

 

[CorradoPsi]

if you make a straight switch like that, temps should go down and require you to lean out the engine. i find 30% easier to tune than 20% because there is a broader "window" of happy tune. if you tune properly, you shouldnt notice any accelerated wear

 

[Sgt Blamo]

Ah,thankyou very much,ah! (saaid like elvis),
Looks like I'll be switching to the 30%. What are your thoughts on the Crono RS3, as that thing screams @ only 210-210 degrees.

 

[niggle]

ok I edited my post that you quoted. i was sleepy and spewing bad info at the time. the reason you have to lean out with higher nitro content fuels is because while your flowing the same volume of fuel, the fuel that you are feeding your engine is more potent by 50% in this case. the nitro is what is making your power, and while you havent turned the needles at all, you have essentially richened the mixture by making it more nitro rich. so to maintain your air to nitro ratio (what actually makes the power) you need to lean the engine out.

I see where you're coming from now. Nitromethane is a fuel as well as an oxygen provider. You need an A/F ratio of 1.7:1 for complete combustion of nitromethane so an increase in nitro of 10% should require an increase in air of 17%, which implies a 17% leaner mixture. However you then have to take into consideration the air that is no longer required because you now have 10% less methanol, so you will probably not need to lean out the full 17%.

Increasing the nitromethane content will provide a cooling effect but this will be offset by extra heat produced by the more potent fuel and the increased compression. Finally, if the oil package changes, this might affect the running temperature of the engine. If you are tuning for temperature, then these last two factors will determine which way you need to adjust your needles and you may need to go either way.

If the predominant effect in your setup is mainly down to the increase in air required for the extra nitro then yes, you will need to lean the mixture, although I'm not sure how many turns of the HSN an increase in the A/F ratio of up to 17% equates to.

I guess we're lucky that here in the UK we all use 25% nitro content in our buggy fuel and don't have to worry about retuning in this respect, except when the air temperature varies by 30 or more degrees from day to day (which it invariably does at this time of year)!

 

[niggle]

OK, so If I switch from 20% Trinity Monster w/12% oil,to trinity 30% w/same oil content, will my engine run hotter and will it wear out sooner? Is it worth the extra cost? will I have less tuning issues w/ 30%?

You should have better compression and thus more torque with 30% and in theory there will be more strain on the conrod and its bushings. In practice, the piston and sleeve will be worn out long before the conrod bushing.

 

[mikeburgin]

You should have better compression and thus more torque with 30% and in theory there will be more strain on the conrod and its bushings. In practice, the piston and sleeve will be worn out long before the conrod bushing.

Niggle are u like a nitro methanol engineer or something?????

u obviously know abit about fuels

 

[niggle]

Niggle are u like a nitro methanol engineer or something?????

u obviously know abit about fuels

I am now...

Seriously tho', I was a very enthusiastic 4-stroke 1:1 tuner/modder and then mortages, marriage and kids (not necessarily in that order) put a stop to all that.

It's not practical at the moment to fill the garage with a 1:1 scale race car project (we need 2 practical road cars, there's no room/budget for a 3rd [race] car, so I'm keeping myself occupied with 1:8 stuff.

A lot of the 4-stroke stuff is relevant to 2-stroke tuning as well. A race tuned 4-stroke with a lot of cam duration and overlap benefits from pulse tuning in the same way that a 2-stroke exploits tuned pipe lengths. NOS in a petrol/gasoline engine is like nitro in a 2-stroke glow engine. Gas speed, venturi effects, ignition timing, port/valve timing are common principles in both 2- and 4- strokes.

 

[Kevinb]

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?

Because the nitro content is higher the methanol content is lower so you have to richen the settings to get the correct amount of methanol to ignite.
Also nitro bursts into oxygen, this is why you need more fuel, in nitromethane the fuel is methanol. To get more fuel you richen it some.
20% to 30% is about a 3/4 turn on the high speed and 1/4 turn rich on the low speed. Mid range usually isn't an issue.
Doesn't matter if the engine is reshimed or not when bumping up the nitro content you must go richer.
Most engines run fine up to 33% without shimmying up but one thing to think about is pipe length. Higher nitro content requires slightly longer pipe length to accommodate the faster sound wave.