K45 info

Intlwaters

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MarkDanielsSr

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Feb 10, 2009
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What is the consensus on carbs on the new re-released K45 engine? I fitted one this evening with an OS 7F, got it together and thought i would ask for advice. Going into light 38 inch surface drive mono, should let it wind up. What carb bore is proving successful ? Using a wide band pipe with 3/8 diameter exhaust stinger, again, thinking let it breathe.
 

Jeff Lutz

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Jan 2, 2005
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What is the consensus on carbs on the new re-released K45 engine? I fitted one this evening with an OS 7F, got it together and thought i would ask for advice. Going into light 38 inch surface drive mono, should let it wind up. What carb bore is proving successful ? Using a wide band pipe with 3/8 diameter exhaust stinger, again, thinking let it breathe.
I think it’s safe to say that .390”-.410“ would be the best all around .45 bore sizes. I ran .335 bores on Picco 45’s back in the day (briefly) and went to .355 then .375 . Started getting fast at .390“ plus. Jeff Lutz
 

Al Hobbs

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I was fortunate to spend a lot of time with Jim Allen, Rod Geraghty, Howard Power, Alessandro Rossi, and Ginnaty Kalistratov. Those people were involved in engine design and manufacturing. In my opinion, they were the most intelligent RC engine people I ever met.

Howard Power was a boat racer and a Physics Professor for the University of California. Howard also wrote a monthly article for RC Modeler. In one article, he explained the physics of a model engine carburetor. It was difficult to get through the article withou falling into a comatose state.

Basically, the amount of fuel charge going into the engine is limited by the engine's bore and stroke. A good tuned pipe boosts the amount, but not much.

I does not matter how much nitro content is in the fuel. You can't put more fuel in the engine than the engine can hold.

Whatever the carb bore is, you can't get more fuel into the engine by making the carb bore bigger.

It is the air, drawn into the carburetor, that creates a vacuum and draws fuel through the spray bar. The air, moving fast, creates the vacuum. A bigger bore means slower moving air and less vaccum. That is why big carb bores at higher altitudes have so much trouble. Thin air, moving slowly cannot draw enough fuel for the engine to run well.

A big bore carburetor can work well at sea level and low altitudes. However, if you want to run above 1000 feet altitude, a big bore carb will cause some trouble.

However, a medium sized carb bore will work well at all altitudes. The air, moving quickly through the carb will draw all the fuel you need, unless your spray bar is too small.

From what I learned from, Jim Allen, Howard Power, Rod Geraghty, Alessandro Rossi, Gennaty Kalistratov, putting a carb with a 0.40 bore or larger causes more trouble than it helps. I recommend a carb bore at least 0.335" to 0.395". These will work well at sea level and higher altitudes.

Al Hobbs
 

Terry Keeley

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Isn't the 7F 12mm (0.472")? If so there might be a restrictor for them, I ran a 9B on my purple head CMB's with one that brought it from 12 to 11mm (0.433).

With the low end metering it ran well but I don't run at high altitude.
 

Jeff Lutz

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Jan 2, 2005
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871
Mark, some good points have been discussed. Although a small carb has more suction on the spray bar, when running a large bore you will have to open up the needle more. The difference in the flow number will vary for many reasons. But, when running a small bore carb the engine is working harder to get the fuel in. Read that again and think about it. It’s true you can only get so much fuel and air into a engine of a given size,then you have to break down other details. One is port sizes in the sleeve. Considering you can only get in so much air per stroke, the port sizes in the sleeve start to matter over most factors in the design of our small nitro engines. After looking at the sleeve of a K45 at the Voodoo race, I would say it’s a relatively small size port layout. Im not saying that is good or bad and in some cases it’s easier to burn the fuel when they are small . Mac 45’s which sold like hotcakes came with .470” bore carbs. I consider .375-.410 medium bore carbs. if your going to run the K45 stock a .375” bore may just be ideal. Jeff Lutz
 

Grimracer

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Dec 27, 2001
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Yea.. nobody understands why they put that on the motor.. I spoke in depth with them about this and they just could not grasp proper bore size . That and that silly flywheel and water cooled glow plug.. and well.. it was the death of the motor. (having said this I have a few setup correctly and they are great)

I am sure I have a few of the carbs in my motor box and now thanks to Al a motor to use them on!

Grim
 

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