Hot weather effects

So hot air is less dense. It's summer now. Bad air. Does a bigger carb help this situation? Air is less dense, but a bigger carb will get more of it in, right??

Brian

Dont the master tuners have any input on this question?

No master tuner here. Everybody at race is in the same boat. (no pun). I would deal with the pipe and prop, not the carb. OMHO

Longer pipe and less prop is a good way to deal with the hot weather. What is the main cause of poor performance the hot or the humidity? Less nitro or more?

P THOMAS said:

Longer pipe and less prop is a good way to deal with the hot weather. What is the main cause of poor performance the hot or the humidity? Less nitro or more?

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ok i'll jump in.the problem is the lack of o2 molecules. humidity is just more water and less o2 to burn.in my drag car when it is more humid we can put a touch more timing in to start the burn cycle earlier but with a 2 stroke you are kind of limited.Ray and Phil are correct with pipe change/adj and less prop.you could also depending on motor and mods,you could play with glow plug heat ranges.most times more nitro does not do much and in so cases makes it worse,and bigger carb is not the answer.i really is a matter of trying different thinks until you find what it likes and keep notes of what it likes in hot weather and cold so you can race all year round consistantly.

We always notice a huge difference when we go south, the Nats in Hunstville this year was a good example.

I had to pull stuff back about 5 steps to get it to run (prop down, pipe out, lean down) but so did everyone. One thing I've noticed that has a huge effect is how much water is in the air. I never looked at that before just measuring density with air pressure and temp (water doesn't effect the density reading much). Since I got my little Kestral unit I've been recording the water grains per pound of air and when it gets above 100 or so things go south fast. I saw 120 one afternoon in Huntsville. Typically here on a humid day we might see 75. I might try running a hotter plug the next time I see those conditions.

A bigger carb? Maybe if you were a little under sized to begin with...

In the cooler months engines will generally gain good power due to being able to richen the needle and in hotter months the opposite.

Cooler weather short and fat, hot weather long and lean.

When there is less air, such as hot days or high altitude, a smaller carb diameter will increase the velocity of the air flowing by the spray bar. A bigger diameter carb means slower moving air, less vacuum to draw fuel. Bigger carbs work better with dense air, such as sea level. Smaller carbs work better with lighter air, such as higher altitudes.

Ten days ago I raced in Colorado at 5000 ft altitude. Light air produced much less power. To run meant a lot smaller prop and a longer pipe than I normally run at 3500 ft altitude.

Al Hobbs

Don't discount compression ratio and plug heat range in temperature and air density changes. The hotter plug will build more cylinder pressure earlier than a colder plug.

In general terms, elevated temperature and humidity require a leaner fuel setting, because there's less burnable oxygen in the air. Lower barometric pressures

behave the same way. The same follows for higher altitudes, as Al illustrated.

It's good to have a hot weather and a cooler weather tune-up, and know which direction to go in between. Recording notes is an imperative for doing this consistently.

The other guys are right- pipe length, propping, and carb flow. It's all relevant in terms of tune-up variables.

Learning to read plugs correctly and finding a plug manufacturer who has fairly consistent plugs across the heat ranges helps, too.

Below is a general guide line...Hope this helps some.

TW

Higher air temperature

Lean

Lower air temperature

Rich

Higher humidity

Lean

Lower humidity

Rich

Higher barometric pressure

Rich

Lower barometric pressure

Lean

Higher altitude

Lean

Lower altitude

Rich

Higher nitro content

Rich

Lower nitro content

Lean

Higher oil content

Lean

Lower oil content

Rich

Hotter glow plug

Rich

Colder glow plug

Lean

There is a point where at high altitude and hot temperatures that you will actually need to richen the mixture. Typically, as air becomes less dense you need to lean the mixture. But, as the air becomes even less dense, it becomes more difficult to draw fuel through the spray bar. At that point you will actually need to begin opening up the needle valve to allow the fuel to flow more easily. This not a problem if you never race at high altitudes.

When its hot and no air I would back off the Nitro.If your running 60% I would go down to 45% this wil give you more Alc to burn.Smaller props and longer pipes.

Dave

Hmmm,I have had a bit of luck bumping the nitro %. I would assume that is because the nitro carrieds some oxygen molecules that perhaps somewhat offsets the less dense air coming through the carb. The higher % nitro fuel seems to affect the drivability though. Not much throttle response. The midrange rpm's seem mushy.

Does alcohol carry oxygen in it? Maybe I've been going the wrong way.

I always think my boat is just running like a turd in hot weather but when I look around the other boats are not as fast either.

A bigger carb will only allow more air in the engine if the carb is what is the limiting factor on how much volume the engine can draw. The carb bore also affects the ability to tune the engine, and to transition from lower rpm to higher rpm, so too much carb can be a bad thing.

Think of less dense is the same you'd look at a smaller displacement engine. That is effectively what has happened.

Huntsville, at this years Internatts was the true test of everyones tuning skills!! However Stu with his 99 MPH passes seemed to be prepared for the weather along with others like Ron Zaker. Once I removed my modded disk, put the stock EVO carb on and pulled my pipe out on my EVO 45 things started to work. Knowing the science behind our engines and boats helps but being there is the real test!! Tim

Here's a cool little calculator. You can play with the numbers and see how different factors affect relative air density.

https://wahiduddin.net/calc/calc_da.htm

+1 on Chilli's post. I use that chart anytime I travel. Altitude above sea level and humidity are bigger factors. 500 feet makes a difference. Here in Florida we have heat and humidity but low altitude compensates, Density altitude and power available go hand in hand.

As for carb it's more about atomizing fuel properly than size. We have air to fuel ratios very low compared to gasoline motors. Maybe 2-3 to 1 versus 12-14 to one. More air not mixed with fuel is not going to make power.

Mic

Chilli said:

Here's a cool little calculator. You can play with the numbers and see how different factors affect relative air density.

https://wahiduddin.net/calc/calc_da.htm

Click to expand...

Used that for years to verify stuff in the field. If you know the identifier of the nearest airport you can go to the ADDS site and get a history for up to 36 hrs and plug in that info: http://www.aviationweather.gov/metar Elevation can be had from Google Earth.

Notice how changing the dew point from low to high effects the density altitude very little, but the amount of water in the air sure effects how our little motors run!