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Old 08-01-2010, 03:32 PM   #1
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Default Nitro percentage vs tune

Hello.

Today at my local track we had a little debate on how nitro percentage affects the tune and how to tune when you change nitro percentage.

It doesn't really matter here in Denmark, as we are only allowed to run 25% or less, but some interesting points were made, so I want to run them by you.

The 2 main opinions were:

1. Going up in nitro percentage (like 25->30%) means you have to richen the tune. Nitro carries more air into the engine, but that also means less methanol gets to the engine. So to equal it out, you richen the tune.
So a higher nitro percentage means more power, but more fuel is used, and because of that runtime will suffer.

2. Going up in nitro percentage (like 25->30%) means you have to lean the tune. Nitro cools down the engine, and to get the same temperatur as before, you have to lean the tune.
So a higher nitro percentage means more power (because of the lean mixture), less fuel is used, and runtime is better.

Which one of these opinions are true?


// Niklas
(Sorry about my poor english :P)
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Old 08-01-2010, 03:37 PM   #2
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More nitro = richen the motor

Less nitro = lean the motor

Assuming everything else is EXACTLY the same. Change brands of fuel or other components and you may see different results.
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Old 08-01-2010, 03:43 PM   #3
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Originally Posted by Nilks View Post
1. Going up in nitro percentage (like 25->30%) means you have to richen the tune. Nitro carries more air into the engine, but that also means less methanol gets to the engine. So to equal it out, you richen the tune.
So a higher nitro percentage means more power, but more fuel is used, and because of that runtime will suffer.
This is the right opinion but for the wrong reason. Nitro does carry ox with it and therefore requires less air to burn. But unless you change the venturi, the same amount of air is going into the engine. Therefore, you need to increase the amount of fuel to get the right air/fuel ratio.

This is why methanol requires MUCH richer settings than gasoline. If you convert your dirt late model (full size oval racing car) from gas to meth, you not only need a different carb but you had better get a bigger fuel cell as well. You will burn almost twice the methanol as gas. Nitro needs even less air.
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Old 08-01-2010, 04:01 PM   #4
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This is the right opinion but for the wrong reason. Nitro does carry ox with it and therefore requires less air to burn. But unless you change the venturi, the same amount of air is going into the engine. Therefore, you need to increase the amount of fuel to get the right air/fuel ratio.
That was actually what I wanted to say, but reading it again it may came out wrong. But thank you for your reply
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Old 08-02-2010, 12:24 PM   #5
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1. Going up in nitro percentage (like 25->30%) means you have to richen the tune. Nitro carries more air into the engine, but that also means less methanol gets to the engine. So to equal it out, you richen the tune.
So a higher nitro percentage means more power, but more fuel is used, and because of that runtime will suffer.
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Old 08-02-2010, 12:30 PM   #6
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If this is true then why don't racers use a really low % of nitro in races that need more than 1 tank of fuel to complete?
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Old 08-02-2010, 12:53 PM   #7
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More Nitro increases power more than less nitro increases runtime ;-)
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Old 08-02-2010, 01:00 PM   #8
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If this is true then why don't racers use a really low % of nitro in races that need more than 1 tank of fuel to complete?
They do. 1/8th onroad pros often run 16% on big tracks to make time in qualifiers.
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Old 08-02-2010, 01:06 PM   #9
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Originally Posted by Klimpen717 View Post
1. Going up in nitro percentage (like 25->30%) means you have to richen the tune. Nitro carries more air into the engine, but that also means less methanol gets to the engine. So to equal it out, you richen the tune.
So a higher nitro percentage means more power, but more fuel is used, and because of that runtime will suffer.
Again, you are right but for the wrong reason. It has nothing to do with the methanol. By that reasoning, a motor wouldn't run at all on 100% nitro but they can. The "fuel" in our fuel isn't methanol, it's the combination of methanol AND nitro. Nitro is a "fuel" too. The difference is the required air/fuel ratio. Nitro needs less air than methanol does but the air is constant, therefore more nitro percentage needs more "fuel".
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Old 08-02-2010, 01:29 PM   #10
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More nitro needs a richer tune. That's why lower nitro content fuels get better mileage. They can be run leaner and therefore less fuel is used.
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Old 08-02-2010, 01:33 PM   #11
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Quote:
Originally Posted by wingracer View Post
Again, you are right but for the wrong reason. It has nothing to do with the methanol. By that reasoning, a motor wouldn't run at all on 100% nitro but they can. The "fuel" in our fuel isn't methanol, it's the combination of methanol AND nitro. Nitro is a "fuel" too. The difference is the required air/fuel ratio. Nitro needs less air than methanol does but the air is constant, therefore more nitro percentage needs more "fuel".
So by running a smaller venturi you can run like 30% nitro and still good gas millege
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Old 08-02-2010, 03:02 PM   #12
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So by running a smaller venturi you can run like 30% nitro and still good gas millege
Yes, though in onroad we are finding that high nitro, small venturi is not as good as lower nitro, big venturi.

16% with an 8.5mm seems to give better lap times than 7mm 25% for about the same mileage.
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Old 08-02-2010, 03:12 PM   #13
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Originally Posted by wingracer View Post
Yes, though in onroad we are finding that high nitro, small venturi is not as good as lower nitro, big venturi.

16% with an 8.5mm seems to give better lap times than 7mm 25% for about the same mileage.
Maybe because the power is to unusable?
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Old 08-02-2010, 03:14 PM   #14
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Ther is a 3rd option:
A stronger combustion with more nitro, means more pressure in exhaust and tank -> lean it out.

The only real answer is to correct that what the motor is telling you to do.
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Old 08-02-2010, 04:28 PM   #15
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You guys are all on the right track. In lamens terms yes, more nitro means richer mixture and less nitro means leaner mixture but what plays more of a role as far as how much leaner or richer it needs to go are several factors:

1) Burn Rate difference between the alcohol vs. nitro

Alcohol burns slower and cooler than nitro and therefore requires less oxygen density. Nitro burns hotter and faster so more oxygen density needs to be present. An engine running on pure alcohol without any type of heat inducing chemicals will actually burn so cool, the engine will develope frost on the outside of the engine. Nitro burns so hot and so fast that it creates incredible heat signatures and a big reason there is a mixture regardless of what it is, is help offset and control EGT and running temps of the engine. Especially with these types of engines that require a certain operating range for the metals to expand to proper tolerances under load.

2) Compression Ratio trapped vs. static

Each engine needs compression to run and depending on what nitro/alcohol ratio is being used will depend on what it can get away with for compression trapped or static. Trapped ratio is when the exhaust port is sealed and the pressure that is built up from there on is considered trapped ratio or what is commonly referred to cranking compression which is measured with a guage. Static compression is when you take the overall swept volume and divide it by the installed cc of the combustion chamber at TDC and it will come out a ratio. We will say for sake of arguement that the ratio is 20:1. That basically means that at BDC, the piston upon starting it's compression stroke will compress the swept volume 20 times it's overall volume as it reaches top dead center or taking a 200cc swept volume and compressing the molecules into a 10cc cavity what is know as the combustion chamber. In theory, the higher either ratio is, the richer or less nitro is needed to prevent pre-ignition. But you can also manipulate it. For example, you could raise the exhaust port timing reducing trapped ratio but reshim it to increase static ratio and run a higher nitro percentage which is what i do all the time. You still get the performance of the higher nitro percentage while increasing the engine's output throughout but not having to worry about the engine detonating from the trapped ratio being to high and the engine cooking off. There are so many combinations you can try and reach similar results so it more of a trial and error thing.

3) Altitude and Barometric Pressure

Getting even more complicated with this but you can't forget about air quality. The air coming into the carb creates a vortex in the throat around the LSN which creates a vacuume which helps to draw the fuel out of the nozzle or what is referred to as the spray bar. The engine will only draw so much air through the venturi depending on displacement, stroke, case volume etc and with that, will only take in so much of the oxygen needed to have the proper ratio. When you get into higher elevations and/or the barametric pressure drops, the engine will need to be leaned out and/or compression raised to compensate for the reduced oxygen density. There are some other factors such as quality of ingrediants used in the fuel etc that will also help dictate where the engine wants to be to run properly.

This is just a basic overview but there is more to it than just lean this or richen that. Hopefully this will give you guys something to think about and may even help you to further improve the engine's potential.
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