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Fuel Mileage/Runtime Myths and Truths

Fuel Mileage/Runtime Myths and Truths

Old 11-30-2010, 08:56 PM
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Default Fuel Mileage/Runtime Myths and Truths

Hi all,

I'm trying to learn a bit more about the factors that affect fuel mileage. I've done a lot of searching and reading, and it seems there are lots of opinions out there. There are also a lot of posts to the tune of "it depends" without deeper explanations of the factors.

Some of the dependencies of fuel mileage as I understand are listed below, any corrections, opinions, or additional information would be appreciated. I'm trying to get this straight in my head.....

tune - can't be fat
driver - a smooth finger is preferred, any other tips out there to drive for max mileage?
track conditions - loose = less runtime, high traction = more runtime?
track design - long straights = less runtime, tight and tech = more runtime
plug - hotter = ?, colder = ? or is this an equation of having the "right" plug for the engine
humidity - ?
ambient temperature - ?
fuel - I'm hearing 25% is the best mix for runtime?
carb restrictor - smaller = less air/less fuel/more runtime, larger = more air, more fuel, less runtime
clutch - is a later or early engagement better for runtime?
epa - I've read that reducing EPA is a bad practice since it changes the tune
flywheel - I've seen a lot of steel flywheels being used these days to soften bottom, does this have an impact on runtime?

Thanks!
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Old 11-30-2010, 09:12 PM
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I didnt read the whole thing but basically tune and driving smooth is best run time. What gives you good run time is a perfectly tuned top needle on the carb, normaly leaning the top will give you mileage but you want to run at a safe/good tune with great power band. Also a good tunned pipe that is specified for that engine will help. If your pinned when you crash for a marsahl and just hit full throttle every turn then that will effect mileage to. I dont see clutches being a factor unless they are tired and slip.
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Old 11-30-2010, 09:15 PM
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Tune - Lean = More Run-Time / Rich = Less Run-Time - FACT

Driver - Obviously if you are easy on the trigger you will get more runtime, but I'm not very easy on the trigger and have no problem getting 12-13 Minutes in my truggy and 10+ Minutes in buggy.

Track conditions - More wheel spin = more wasted power = more wasted fuel

Track design - High-Speed Tracks use more fuel since you are usually full throttle most of the time and Low-Speed Tracks take more finesse so you usually get better fuel mileage.

Plug - Depends on the weather...to be honest, I rarely have to change what plug I am using. Only when there are severe changes in weather do I change plugs.

Humidity - As far as I know, as long as your tune is correct humidity won't effect it (I could be wrong)

Ambient Temperature - Same deal as humidity (I could be wrong though)

Fuel - A lot of people like 30% - I'm one of them. Never tested run-time between fuels.

Restrictor - I have tested restrictors back to back and noticed absolutely no difference in run-time between a 6mm and a 9mm. Only difference is the bigger restrictors seem to be smoother...

Clutch - Softer clutch is better on fuel mileage, so earlier engagement.

EPA - Well...I set mine so that the slide is opening about 95% of the way. You can keep turning your EPA up, but you won't gain any power...just waste fuel.

Flywheel - Heavier flywheels stay spooled up...I prefer lightweight flywheels though, personal preference.

I'm sure more people will chime in and add/correct me.
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Old 11-30-2010, 09:44 PM
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Originally Posted by CJ Weaver View Post
Tune - Lean = More Run-Time / Rich = Less Run-Time - FACT

Driver - Obviously if you are easy on the trigger you will get more runtime, but I'm not very easy on the trigger and have no problem getting 12-13 Minutes in my truggy and 10+ Minutes in buggy.

Track conditions - More wheel spin = more wasted power = more wasted fuel

Track design - High-Speed Tracks use more fuel since you are usually full throttle most of the time and Low-Speed Tracks take more finesse so you usually get better fuel mileage.

Plug - Depends on the weather...to be honest, I rarely have to change what plug I am using. Only when there are severe changes in weather do I change plugs.

Humidity - As far as I know, as long as your tune is correct humidity won't effect it (I could be wrong)

Ambient Temperature - Same deal as humidity (I could be wrong though)

Fuel - A lot of people like 30% - I'm one of them. Never tested run-time between fuels.

Restrictor - I have tested restrictors back to back and noticed absolutely no difference in run-time between a 6mm and a 9mm. Only difference is the bigger restrictors seem to be smoother...


Clutch - Softer clutch is better on fuel mileage, so earlier engagement.

EPA - Well...I set mine so that the slide is opening about 95% of the way. You can keep turning your EPA up, but you won't gain any power...just waste fuel.

Flywheel - Heavier flywheels stay spooled up...I prefer lightweight flywheels though, personal preference.

I'm sure more people will chime in and add/correct me.
just to add, fuel is more about shimming and what your engine is setup for. i get better run time with 30% and a min more run time switching from byrons 30/11 to O'donnell race 30%. but there could be other factors than just fuel there...

restrictors just depend on driving style, track and engine... Alpha engine i have, the 8mm got 1 min less than the 7mm but 6.5mm got 1.5 min less than 7mm... OD engine the 7mm got about a min less than 8mm...it depends on the three factors i stated IMO and from experience. only way you can find out is to try different ones yourself.

just what i think and have experienced.
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Old 11-30-2010, 09:54 PM
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Most of what CJ said is pretty accurate.

Humidity is a factor however. Higher humidity hurts run times.

Ambient temp can be a factor as well. I find that all of my engines get better run time in cool air. It seems kinda backwards because in theory colder air contains more oxygen so it should need a richer tune to compensate, but I've found that fuel mileage and power are better in cool air. It could just be that cool air makes more power so you don't need as much throttle and that compensates for a richer tune. I really don't know how to explain it, but that's what I've found. I could be crazy.

I run 20% fuel exclusively. I have no desire for any more power than what it can provide when properly tuned, and I do get better run times as a result.

Carb restrictors can make a difference in fuel mileage, but only if you use them properly. Often times if you go to a smaller restrictor you will be forced to pull on it more and negate any run time advantage. As CJ said, larger restrictors make for a smoother powerband. Smaller restrictors will limit the top end some but the overall powerband is more aggressive.

Softer clutches and heavier flywheels will help control wheel spin on snappy engines which in turn helps run time. However this is engine and application specific. If your engine can't pull the softer clutch or heavier flywheel, you will be on the gas more and kill run time. Having the proper clutch setup for your application is crucial.

Driving with a smooth finger is one of the biggest factors behind run times. Other ways to drive for fuel mileage is to limit throttle in the air. Taking jumps so that you don't need to correct with the throttle will save a bit of fuel. Not a lot but some.
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Old 12-01-2010, 04:59 AM
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This is great info and explanation, thanks for the responses.

What about gearing? Theoretically higher gearing/larger bell on a engine with very solid bottom would have the crank turning less, but I'm gathering too high would have the driver compensating and wasting fuel trying to get it spooled up. (similar to too soft a clutch, or a heavy flywheel in the wrong application)
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Old 12-01-2010, 05:23 AM
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Awesome thread, should be a stick IMO.

There are a lot of good points there....yes A LOT

Wich is why I say there are many factors for run time,

There is no one magic thing,
you need to have the rite mixture/combination of all the things above for each thing to help the other.

Like all the stars have to be aligned etc etc.

It helps to know your gear well and what one thing will do for the other to also work .......


I know what I'm trying to say but it's not coming out rite, can't find the rite words.

I think you get the gist.
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Old 12-01-2010, 06:24 AM
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One very key factor that nobody has mentioned is the car. Having a light car and an efficient drive train helps with run time. Lightened axles, drive cups, as well as ceramic bearings, anything you can do to minimize the effort that the engine uses to get the car up to speed. The same driver on the same track, same motor, fuel, etc..but driving a different car, or a more efficient car of the same brand will get more mileage right away. IMO one of the most important things, and it doesn't even come from the engine, fuel or tune.

As far as the engine is concerned. You will get better mileage running the bottom lean and the top rich, considering the track isn't very high speed where there are wide open stretches everywhere. An engine that seems lean because it screams down the straight but blows lots of smoke out of every corner will not get very good mileage, but run cooler. An engine that is very crisp and peppy out of corners but runs a little rich down the straights will run a little warmer but get better mileage. Most guys I have seen that get great run time spend lots of time tuning the LSN to a specific point, usually to where it doesn't lean bog, but it's just 1-2 hours away from lean bogging, then by running the top a little rich you still get enough lubrication.
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Old 12-01-2010, 12:30 PM
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My top 3ish:

#1 Driver: smooth and knows how to control wheel spin.

#2 Tires: the correct tires to negate wheelspin.

#2.1 Engine: tuned properly. Each engine is different. (Sometimes a little rich will help control wheel spin as a trade for better economy)

#3 Gearing: gear as high (numerically lower FDR) as you can. You will need to test this with YOUR car and YOUR engine tune.
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Old 12-01-2010, 02:46 PM
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Lift out of the throttle sooner allowing your momentum to carry you in to the turn then power out of the turn.
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Old 12-01-2010, 03:13 PM
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Originally Posted by Brandon Melton View Post
One very key factor that nobody has mentioned is the car. Having a light car and an efficient drive train helps with run time. Lightened axles, drive cups, as well as ceramic bearings, anything you can do to minimize the effort that the engine uses to get the car up to speed. The same driver on the same track, same motor, fuel, etc..but driving a different car, or a more efficient car of the same brand will get more mileage right away. IMO one of the most important things, and it doesn't even come from the engine, fuel or tune.

As far as the engine is concerned. You will get better mileage running the bottom lean and the top rich, considering the track isn't very high speed where there are wide open stretches everywhere. An engine that seems lean because it screams down the straight but blows lots of smoke out of every corner will not get very good mileage, but run cooler. An engine that is very crisp and peppy out of corners but runs a little rich down the straights will run a little warmer but get better mileage. Most guys I have seen that get great run time spend lots of time tuning the LSN to a specific point, usually to where it doesn't lean bog, but it's just 1-2 hours away from lean bogging, then by running the top a little rich you still get enough lubrication.
Brandon is smart



On another note , there is a ton of great info in many of these threads and it just takes a little time to cipher through it all but in the end it will be well worth it for the guys who are looking to gain more knowledge in this department .

I would like to thank the creators of rctech and those who help keep it going in the proper direction when it gets a little off from time to time
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Old 12-01-2010, 03:15 PM
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Originally Posted by houston View Post
Brandon is smart
I agree 100%
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Old 12-01-2010, 04:08 PM
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After watching the Worlds, I wonder what engine temps those guys were running. I assume most were going for mileage and most seemed to have very little smoke except a small bit exiting corners. The smoke trails that I saw seemed much less than your typical race (especially a club race). I believe a comment was made about Hara's first run in the final, relating to his heavy smoke trail. At his first pitstop, the commentators mentioned his engine was being leaned and indeed his smoke trail thinned. I suppose the DVD will show better, but it sure seemed that the straightway runs were virtually smokeless.

Somewhere it was mentioned that mileage comes from a very lean bottom and a somewhat richer HS for cooling. It sure didn't seem that way to me. How did the Worlds tuners seemingly run so lean?
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Old 12-01-2010, 04:25 PM
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You can only go so lean on a engine at the HSN and i personally would doubt they were running excessively lean on the top. A long final you'd want reliability instead of a increased risk of blowing a plug. Plus i don't think they looked that 'lean', you can never tell anyway by watching the videos by looking for smoke. A lean engine would bog anyways be it on the straight or exiting a turn. Seriously, don't go tuning by temp. . . . worst way to go ever. You'll knacker a engine trying to get that 'magical' temp reading of 250F in a 2*C weather conditions.
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Old 12-01-2010, 05:11 PM
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Default little smoke trail...

if i'm not mistaken (which is rare, but it does happen...) the reason for the lack of smoke in the worlds was due to lack of oil in the fuel....i know when i made the change from byrons race 30/11 to purple sidewinder (30/6? or 30/7? nobody has been able to tell me for sure...just don't use the sidewinder unless your motor is 99% completely broken in due to the lack of oil!!!) the smoke trail went from nice and smooth to almost non-existant...and i had to lean a couple hours on the bottom and 1 on the top of my B5....so there's ya another milage tip....a low oil fuel will yeild slightly better run times due to being able to lean it more...
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