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I too am amazed that equation holds over such a wide range. The thread I referred to had several examples of running model engines where the throat diameter seemed to agree with the equation whether or not the equation was actually used or whether those diameters had been empirically determined...

Pete, The throat diameter doesn't linearly scale. You can't divide the full size Venturi diameter by the linear scaling factor to get the scaled value. The equation shows the diameter scales as the square root of the cylinder displacement. - Terry

A 650 cfm Holley 4 bbl (the largest you'd want to put on a 289) has a 1.125" Venturi. If you put 5krpm into the above equation, convert metric to imperial, and use a K of .7 you wind up with 1.02 inches for a needed Venturi. - Terry

Peter, Charles, et. al. For my last few engines I've been referencing the HMEM discussion thread that began here: Determining a carburetor throath diameter | Home Model Engine Machinist Forum It's a really good read on the subject and included practical throat data for several successful...

After the comments I did some research on Venturis that resulted in some final tweaks to the carb's design. I initially misinterpreted Charles' post about the optimum inlet angle being 10 degrees, and after some reading and measurements on a couple RC carbs, I realized he meant an included angle...

Steamchick, Good question about the throat extension. I thought it might help keep the mixture velocity up a bit before entering the plenum which felt like a good idea but I'm not 100% sure. I'm flying by the seat of my pants here, although nothing's set in stone (yet). - Terry

Don't know where my head was when I did the original air bleed. I had the throttle turning backwards as well as a few other oversights. Here's the new revision. Thanks George. Let me know if you still have any concerns. - Terry

George, I do use a bit of transition but I think you're right. I got my throttle rotating the the wrong way. I'll take another look at it before drilling any holes. Thanks - Terry

On to the carburetor ... The carburetor which is the first stage in the 289's induction system is essentially an extension of the intake manifold which has been a real PITA part of this build from day one. The plenum and runners weren't optimized for flow (I wouldn't know how to do that...

It's a reasonable question and one that I re-ask on every build. The answer is that with a CDI I don't have to deal with hiding a big coil. I've enclosed a photo showing a size comparison between a couple model engine coils and a CDI. I used the Exciter coil in a couple engines along with a TIM...

Before starting work on the carburetor, high voltage testing was performed on the distributor. All my distributors are finally checked in a test set made up of one of Roy Sholl's first generation CDI's and a bank of wide-gapped spark plugs. The plug wires were made up so they could be included...

The Hall sensor was potted (using JB Weld) inside the bracket that attaches the non-functional vacuum advance to the distributor body. A failed sensor would be tough to replace, and so a batch of brackets were made up. There's little room inside them for the sensors, and so Kapton tape was used...

Ray, Looks good. What paint are you using? - Terry

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send me your personal email ...

Mark, Yes, I have the module. It's a spare that I kept on hand since I've used so many of these in my other engines. That being said, Roy had one last PCB that he's making up for me to replace my spare. - Terry

Dave, George, Thanks for the responses. I'm planning to use one of Roy's magnum CDI modules, and so dwell shouldn't be an issue. I won't be be able to rev to 7800 rpm, but that's ok. I suspect my rpm will ultimately be limited by my intake valves. - Terry Terry

https://www.homemodelenginemachinist.com/threads/quarter-scale-merlin-v-12.24153/

The full-size engine's distributor was fully mechanical with extended-dwell dual points. The model's distributor will be electronic and based upon a Hall effect device. Trigger possibilities include eight magnets mounted on a non-ferrous disk or a single stationary magnet mounted behind an...

I sketch out all my machining steps and correct them along the way if I find issues with my original thinking or find a better way. They all go into a big 3-ring binder for that project so I can refer to them later on a future project if I'm doing something similar. More importantly though if...