1/3 Scale Ford 289 Hi-Po

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  • If you do that Dave, see if you can use the miniature coils Roy used to use. The physical size is great if trying to hide it and the little buggers will jump a half inch gap.
That's the plan.
For the uninitiated - Be careful testing any coil with an excessive spark gap. At some point the coil will breakdown inside and destroy it or limit it's output. Full size or model same problem.
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 anyway). Their designs were mainly influenced by their need to coexist with the coolant returns in a very crowded manifold that I wanted to look like the cast original but be machinable from bar stock.

The carburetor's cosmetics are based upon a Holley 4 bbl, but it's really a simple air bleed carb. Diameters of .180" and .270" are being used for the Venturi and throat, respectively. The throat is smoothy blended into the atmosphere above the Venturi and into the plenum below it.
The bowls are functional float-less reservoirs for a return-type fuel loop that will be driven by an electric fuel pump. The current plan is to not use the additional reservoir volume that was machined into the manifold just below the carburetor.

The volume needed for the reservoirs helps set the height of the Venturi above the floor of the plenum and becomes part of the 'rise' of the manifold. A high rise manifold can add top end power to a full-size engine, but it's maybe not such a good thing on a scaled model. Since a portion of the rise comes from the carb adapter, some of it may be removed later.

The elongated Cobra style air cleaner that's still planned for the engine will hide the carburetor's cosmetics, and might make cold starts a little more difficult. I've found that a 'thumb in the throat' of a carburetor will pre-wet a complex manifold during cranking and can aid an initial cold start. If there's an air cleaner in the way, it's best removed for quicker first starts. Incorporating a choke into the 289's carb is certainly possible, but accessing its linkage in the tiny available space looks tricky. I may try to incorporate a 'thumb' into the air clearer assembly.

The photos include renderings of the carb's current design as well as cross-sectional views of the induction system. I've been out of pocket for the past couple weeks and am anxious to start making chips again, and so a few still missing features will be filled in along the way. - Terry

First question. In regard to your air bleed circuit. Have you had luck with your previous builds using this design? This is what I'm referring to. I design my air bleed port to be fully open only when the throttle barrel is almost completely closed. I only use the air bleed circuit for just idle or low speed, no transition. I do like your shutter plate design for regulating the amount of air being supplied.
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
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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

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George, While I know less than is useful about venturi design.... I am learning from this discussion.
But a question...
After the throttle valve, there is a length of passage that is parallel bore, before enlarging into a tapered section.
"Why the length of parallel bore?"
My guess (totally unfounded) would be to have a longer tapered expansion and short parallel bore? - maybe only 1/2 to 1 x bore diameter? - But I have no idea as to "why", or what effect it may have on the carburettor. So may be completely irrelevant?
Thanks, (just learning!).
Incidentally, Posts #264 to #268 were some of best I have come across. = "2 guys sharing knowledge" - and checking the work so mistakes are rectified before cutting metal. BRILLIANT! - Post of the month! Keep up the good work guys. :D
The most important part is the venturi itself. The reduced diameter increases the air velocity and thereby reduces the pressure at that point. (Vacuum) The reduced air pressure is lower than the atmospheric pressure so fuel is drawn through the jet. The amount of fuel is controlled by the needle valve but only at wide open throttle. As the throttle barrel closes the air passage gets smaller than the venturi itself creating basically a secondary venturi with a much greater vacuum signal. The greater negative pressure then draws more fuel through the needle jet (if the needle remains in the same position) To compensate for the increase in fuel (too rich) a port called the air bleed starts to be uncovered as the throttle barrel is rotated to the idle position . This reduces the vacuum signal and therefore draws less fuel. The conventional air bleed port has its own control needle which adjusts the amount of bleed air supplied. Terry has an idea I hadn't seen before. It's a sliding shutter valve to control the bleed air. Once the proper adjustment is made it never needs to be moved. These carbs are very simple but effective. I have tried almost every type of carb on my engines and have had the most luck with the air bleed type.
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
This is an over simplification but here's an intake manifold explanation. Older carbureted engines used a dual plane manifold. This was a manifold where the runners were on two planes. Each plane fed by one or two barrels depending on the carburetor (2 bbl or 4 bbl) Each plane fed 4 cylinders on a V8 engine. The runners were designed for a calculated velocity that allowed the engine to perform well throughout its working rpm range. Small single runners provide good velocity for the average engine. We're not going to get into cylinder heads and camshafts. High performance engines that operated higher in the rpm range can benefit from single plane manifolds. These are designed with a plenum or large central cavity below the carburetor. From this cavity individual runners go to each cylinder in a single plane. This type of manifold generally works better at higher rpm's where the air/fuel can be drawn from this reservoir. Like I said previously this is a simplified explanation. There's so many other factors involved
Perhaps counterintuitively, a sudden or rapid expansion is 'lossy' while a rather rapid contraction isn't. The bell-mouthed entry to a carb is efficient, and a shallow taper entry not as good. But the reverse is true on exit from the venturi. A taper of about 1 in 6 in dameter, or 10° included makes the most efficient use of Bernoulli's theorem in recovering pressure in the inlet tract. Any steeper and the flow starts to break away from the surface. Again, rather oddly, If you do not have enough length available to reach the plenum diameter, it is better to have a sharp expansion after the 10° portion than to increase the taper to fit the available length. A hatchback with a small spoiler likely has a lower drag coeffient than a so-called fastback.

In a similar type of Westbury designed carb, the bell-mouth closes right down to the choke diameter, the throttle barrel has a parallel bore to half way, and is tapered to match the exit (reamed in situ) from there.

Wanting to try a newly made 10° taper venturi reamer, I found an offcut of 1/2" ally bar and drilled 5/32" through, and reamed it taper out to 1/4" bore at the end. Job's a good 'un. There was a short length of the parallel bore left, and I had a bright idea. I decided to see what happened if I put a bell-mouth at the other end. Anyone who puts the piece to their mouth is suprised how much easier it is to blow through it from the bell-mouth than from the taper. The sound is also completely different.
Incidentally, I have considered a Morse Taper reamer for a "crude" venturi expansion reamer, or an easi-out - broken stud remover - that has a 15 degree taper and enough hard reverse tapered cutting edges to ream tapered holes when applied "cutting at slow speed on a workpiece. But it really need re-grinding at the required 9~10 degrees as per Bernoulli / De Laval?
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 of 20 degrees. After reducing the inlet angle to 20 degrees (my original seat of the pants angle was a whopping 44 degrees) the outlet angle was reduced to 16 degrees followed by a short kick-out to match the plenum. I also have to correct my use of the term 'carb throat' which should refer to the narrowest diameter of the Venturi. The throat's length which in my first design was essentially zero, was also changed to be equal to its diameter (.180"). - Terry

Thanks Terry,
It is all clear to me now! - Looks great! - Just what I expected.
The final "kick-out" to match the manifold will create some turbulence to mix the fuel and air more than laminar flow does. - As per Charles' "it is better to have a sharp expansion after the 10° portion than to increase the taper to fit the available length. A hatchback with a small spoiler likely has a lower drag coefficent than a so-called fastback.". This should help ensure a more even combustion too.
For gas jets inducing air at the beginning of a venturi, the "choke" (Throat) diameter is about 2/3rds of the main passage diameter. (Ideal 43% of main passage CSA). Is there a similar "rule" for the "ideal" carb throat? (I am interested re: My Moto Guzzi...).
That looks better to me, but we seem to have our wires a little crossed. I was indeed talking about the exit taper, and I did mean 10° included. For the entry side, I was trying to suggest that you do not actually need to have any a taper atall if you have a bell mouth and the inlet path is short. I had another look at my old uni fluid mechanics textbook (B S Massey). A wide taper entry, of 30° to 60° included, has a fairly small loss coefficient, while a bell-mouthed entry to a parallel pipe is virtually lossles if the rounding radius is greater than 0.14 pipe diameters. I am only talking about flow losses - mixing may be another matter.
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Terry, curious about your carb throat, I have a 7.5mm RC carb on my V12 with .938" bores, started with a 6.5mm but could not get high RPM at WOT, and believe that even larger would benefit (but since I'm running with a wood prop don't actually want to see how much RPMs I can get out of it). Your 4.5mm throat is maybe undersize?, I guess we'll know shortly. Pete.

(sorry for mixing units, but RC carbs seem to always be metric)

also wondering if I've got my "physics of scaling" right. firstly if you aim for the same venturi velocity (assuming you need that for fuel suction and atomization, which might not be the case, just assuming here). secondly if you aim for the same HP per displacement you'll be running at higher RPM inversely proportional to the scale factor. so the rate of flow per second is (1/3)^3 (displacement) x 3 (rpm) = (1/3)^2, so thats the scale factor for the area of the venturi, take its square-root for the scale factor for the diameter and get sqrt((1/3)^2) = 1/3. IE linear scaling for the venturi throat.

so your venturi may be "correctly" scaled, but my larger-than-scale "works", I'm puzzled ☹️ ??? (it may be the case that at V12 the rule-of-thumb that you base your carb on single cylinder bore breaks down, it stands to reason that that must be the case eventually)
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 running model engines. I didn't participate in the discussion because up until then I'd been using RC carbs in my engines and had nothing to contribute. But after studying the results contributed by others my take away was an equation for Venturi diameter that I used on my last three engines:

D = K * sqrt(c*n)

where D is Venturi diameter in mm
K is a constant .65 to .90
c is cylinder displacement in cc
n is top rpm/2000 (for a 4-cycle engine)

The results from this equation come out significantly smaller than what was in the RC carbs that I had been using in my early engines which also seemed to run well. However, the equation seemed to work for others' air bleeds and so with some trepidation I eventually started making my own air bleeds in place of RC carbs.

This equation is what I used to come up with the .180" diameter for the 289's Venturi. I should also add that in my earlier engines I gave no consideration to optimum Venturi inlet and outlet angles, as everything was seat-of-the-pants including the throat length which was essentially zero on all of them.

The included angle of the outlet taper in my last revision was 16 degrees which is within the recommended range that I came across in my reading. It can be easily reduced to bring it within Charles' recommendation, but I'm wondering if these Venturi optimizations really apply to a rotating throttle valve in a carb. The theory seems to have been derived for optimum results at wide open throttle, but I wonder how much still applies at part throttle when the inlet and outlet geometries have changed so radically.

I'm happy to try something new and am very interested to see if these optimizations will work better than my previous ad-hoc approach. I guess I really should go back and replace one of my working carbs with one with these optimizations, but I can't seem to find the energy. - Terry

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