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Old 08-02-2010, 09:51 PM   #16
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Originally Posted by PowerHouse View Post
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.
X2!!!...J/k that was very informative.
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Old 08-02-2010, 10:00 PM   #17
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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.
My thinking is the same way. I'm running large venturi's and 20% nitro with awesome power and fuel mileage. Truggy I'm running a 9mm insert in my speed and getting between 12 and 14 minutes on a tank. Buggy I'm running an 8 and getting between 9 and 12 minutes on a tank. The larger inserts provide a smoother powerband as well.
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Old 08-02-2010, 11:30 PM   #18
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Hi Niklas, old chap

Look at it this way:

Nitromethan needs A LOT more oxygen to explode compared to methanol.

An engine will suck X-amount of oxygen at eg. 30.000 RPM no matter what fuel you are using. To make the right mixture with high nitro percent you will need to add more fuel/richer setting and visa versa.

See you at the National in Skive/DK?
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Old 08-03-2010, 12:42 AM   #19
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Originally Posted by PowerHouse View Post
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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Old 08-03-2010, 02:02 AM   #20
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See you at the National in Skive/DK?
Unfortunately no. I promised my sponsor to help out at the Greve birthday-event.
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Old 08-03-2010, 04:41 PM   #21
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How would oil content factor into it, such as in Byron's fuels? I would think that when going from 12% oil to 16% oil in 20% nitro fuel, the tune would need to be a bit more rich to get a bit more fuel for the given oxygen available (since there's less Meth in the fuel), and would therefore run cooler because of a much larger amount of oil going through the engine. An I close to the right mindset on this?
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Old 08-03-2010, 08:26 PM   #22
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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.
That's why you'll see racers inducing gas into the supercharger of our alcohol dragsters...to get some heat in the motor...we'd like more heat....but we gotta' run what the class dictates.
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Old 08-03-2010, 08:32 PM   #23
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How would oil content factor into it, such as in Byron's fuels? I would think that when going from 12% oil to 16% oil in 20% nitro fuel, the tune would need to be a bit more rich to get a bit more fuel for the given oxygen available (since there's less Meth in the fuel), and would therefore run cooler because of a much larger amount of oil going through the engine. An I close to the right mindset on this?
Not sure about the tuning differences with oil content, but going any higher than 12% oil is only hurting you. Oils today are so good that any more than that will just cause problems. Too much oil will rob power and make the engine run hot.
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Old 08-03-2010, 08:50 PM   #24
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Not sure about the tuning differences with oil content, but going any higher than 12% oil is only hurting you. Oils today are so good that any more than that will just cause problems. Too much oil will rob power and make the engine run hot.
Gotcha, thanks for the reply.
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Old 08-03-2010, 11:01 PM   #25
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Not sure about the tuning differences with oil content, but going any higher than 12% oil is only hurting you. Oils today are so good that any more than that will just cause problems. Too much oil will rob power and make the engine run hot.
Hi.

You need more oil in the fuel, if you are running low nitro % and visa versa. The fuel economi are much better with less nitro (leaner HSN). Now you'll have less fuel through the engine = less oil to lubricate.
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Old 08-03-2010, 11:50 PM   #26
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I've seen this debate many times. Fuel mileage comes down to driver control and being smooth. I've ran both % never really seen big different in setting or mileage

Heres another "if" it was that big of a deal the pro's would all run 20% most run 25 or 30%. They are always looking for advantage. H*ll then go to 10% or less nitro.

Onroad guys less nitro to tame the engine down to be smoother not to have that power hit like a switch being turned on so it don't break traction "spin tires"
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Old 08-04-2010, 02:21 AM   #27
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Heres another "if" it was that big of a deal the pro's would all run 20% most run 25 or 30%. They are always looking for advantage. H*ll then go to 10% or less nitro.
Tried it. BIG load up with half throttle if you are running under 10-12% nitro.
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Old 08-04-2010, 07:17 AM   #28
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Onroad guys less nitro to tame the engine down to be smoother not to have that power hit like a switch being turned on so it don't break traction "spin tires"
Sorry but not true. If I set up for 16% and get a good run in, I can switch to 25% and go .1 faster. Then run out of fuel at 4:45. If I make adjustments to try and make time on 25 (smoother driving, smaller venturi, softer clutch, etc.) I end up going .3 slower.

We in 1/8th onroad ALWAYS want more power but lower nitro gives us better mileage and decreases power LESS than other methods.

I suspect the reason offroad pros don't go to less nitro is that low nitro could be less reliable for offroad. High nitro will run cooler, give better throttle response and be less likely to flame out and vapor lock.
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Old 08-04-2010, 07:36 AM   #29
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here is a good link showing the differences pound for pound Nitromethane has far less BTU then either petrol or methanol...but you can burn 2.5 times more nitromethane then you can methanol so you make more actual power

http://www.smokemup.com/tech/fuels.php#nitro
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Old 08-04-2010, 11:13 AM   #30
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Sorry but not true. If I set up for 16% and get a good run in, I can switch to 25% and go .1 faster. Then run out of fuel at 4:45. If I make adjustments to try and make time on 25 (smoother driving, smaller venturi, softer clutch, etc.) I end up going .3 slower.

We in 1/8th onroad ALWAYS want more power but lower nitro gives us better mileage and decreases power LESS than other methods.

I suspect the reason offroad pros don't go to less nitro is that low nitro could be less reliable for offroad. High nitro will run cooler, give better throttle response and be less likely to flame out and vapor lock.
Isn't that more or less what I was saying!!!!!!!!!!!!!!!!
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