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thedoctorD

An odd phenomenon that I discovered when fixing my truck

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First, some background. I own an 84 Chevy S10 with a 2.8l V6. I've been fighting the stupid Verajet II carb since the day I bought it. It's been fairly reliable, but the power was always awful, it idled terribly, and didn't get nearly the gas mileage that I think a 1900lb vehicle should get, regardless the style of induction.

Now, I am a fairly capable mechanic, and as such, I was not content to simply let this continue. No sir'ee, I had to fix it, so I went to my carb box, selected a suitable carb, (an autolite 2150 seemed appropriate) and proceeded to tear that Verajet off in a hurry. Much to my dismay, the Verajet has a very unique bolt pattern that didnt match any of the carbs in my box. Problem. I needed an adapter, so off to Oreilleys I went, purchased a universal 2bbl adapter, and went to slap it on. No dice. Not a single pattern fit.

"Hmm," says I. Looks like I'll have to make one. My MIG welder was out of wire, so steel was not an option, and my TIG welder was loaned out, so aluminum was also not an option. But wait! A wild idea emerged! Sitting right on the spool of my 3d printer, a fresh roll of Shore 95a TPU.

Experiment time. Most Chevy's run at about 210 degrees Fahrenheit on the water gauge, or a bit under 100deg Celsius, and my printer will only start effectively melting the TPU at around 230-240deg Celsius, but I knew little about how it would handle that kind of temperature for any period of time, so I took a previously failed print, stuck it on a piece of aluminum foil, and threw it in the oven at 250 degrees F for an hour.

Upon removal, the print had held it's shape, and though it was noticeably more rubbery, it did not melt or collapse, and it returned to it's previous consistency as it cooled.

This might work!

So I set about designing an adapter and printing it. 16 hours later, I have a spacer. As a strength test, I set the new carb on top of the spacer, and the spacer on the floor, and pushed it around. it seemed fine enough, but I wanted to take no chances, so I coated the outside with a few layers of fiberglass resin, and supported the upper bolt plate with a pillar of JBWeld on either side, then siliconed everything to ensure I wouldnt have any vacuum leaks.

A few bolts, some cursing, and an improperly utilized cotter pin later, and my truck has been carb swapped! First fire went great. I didnt touch a single adjustment screw and she's purring at a nice, smooth 800rpm. Revs beautifully.

NOW HERE'S THE INTERESTING PART:

Carb spacers are usually made of something solid and inflexible, like metal, and sometimes hardwood. This one is basically rubber. As the engine revs, the carb will rise almost 1/2 inch above the position in which it sits at idle, stretching the spacer, while still maintaining a seal on both gasket surfaces, thereby increasing intake runner length automatically, and proportional to engine RPM unloaded (its difficult to see when driving as I still have a hood.)

Now, my current theory as to why this happens is this: A venturi is essentially a circular wing, and uses the same principle as an airfoil to achieve a different result. In both cases, air is forced to split and travel at different rates to reach the same end point, which forces velocity along the wing edge to increase, and the pressure to decrease. This effect also occurs along the bottom air filter plate on a standard dish filter, which in my case, is bolted to the carb. I think the engine vacuum is drawing the air past the bottom dish FAST ENOUGH TO GENERATE LIFT, and because the carb is mounted on a stretchy plate, it's actually trying to fly!

I am currently in the process of designing Spacer/adapter V2, to try to take better advantage of this phenomenon, or really, just see if running an adaptable intake runner length makes any noticible changes in performance.

This was quite an exciting discovery, and I'd appreciate your thoughts. I will try to get a video up when I have the daylight.

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