chevy43

How would you go about putting one in? I was thinking if I put one in 180 deg from the carb that would get pressure only and no suckback. That really might help give even fuel pressure but I guess it would still be lower at idle.

combatpigg

My Feedback: (3)

Chevy, I just made one for my AME last weekend. I took a tiny drill bit out of my Harbor Freight $9.99 numerical drill index. So tiny it had to be chucked into a pin vise collet, then I drilled a hole dead center in the back plate. I then cut off a hunk of 3/32" brass tubing, about 1/2" long. Next step is to clean the inside of the backplate and scratch it up real good, same with the brass tubing. I also round the mouth of the tubings' O.D. on a rubber wheel, so the fuel line doesn't tend to cut itself on a blunt, sharp edge. Now, clamp a T pin in your bench vise sticking straight up and lower the back plate down [pin going through the hole] onto the vise. Pass the brass tube down the T pin, let it rest on the backplate and go mix up a batch of JB WELD. You know how the rest of the story goes, goop it and let it set up. You will need a in line check valve to complete the deal. This is a full throttle only set up.

chevy43

Good tip on the with the T pin!

I wonder if it would work without the check valve for a throttled engine?

combatpigg

My Feedback: (3)

Don't know.......crankcase pressure would have no positive pressure build up without the check valve, the positive and negative pulses would cancel. Exhaust pressure is different, 90% of the pressure waves are going towards the atmosphere, and it is obviously the way to go if you want throttle. Whatever ideas you have can't hurt to try.

chevy43

Yea I was trying to figure out the net pressure too. I came to the conclusion that the pressure spike higher than it dips down but I don't know.. At idle it might actually be negative.

That was why I was thinking about a pressure tap oposite the carb but I guess that would be the same as a check valve and have too much pressure.

I may have to do some more expirements with slight grinding of the bottom side of the carb barrel to get the fuel draw I want.

Toad

Crank pressure does build positive pressure, look at the timming of the crank when you rotate it. there is far more "positive" time than "negative" time. The tank builds alot of pressure off this. If I forget to vent my tank as soon as I get to my plane on landing, I will have a flooded engine to clear out before the next run.
It is quite simple to visulize, when the piston is going up in compression stroke, the case is open via the intake port of the crank, the area of that port is alot larger than the area of the pressure tap hole. there is very little, pressure bleeding back thru this hole from the tank in the time of this cycle, now on the down stroke when the case is pressurized, every thing is sealed off except the pressure hole and it being very small, some case pressure is used to pressure the tank, and with so many quick strokes, the effect is very positive for the tank A build up of pressure untill it equals close to case pressure.
I have used crank pressure for yrs, preferring it over a bladder for racing because I can get more consistant runs from it.
It depends on the application, for c/l combat, a bladder is better, for rc pylon, pressure from the case has proven to be easier andmore consistant for me. For the AP, the norvel pressure backplate works great. I found on plates I had to make that a .008-.012 hole is about right. .020 works but getting excessive
it sure doesnt take much pressure to get a consistant needle, the nice thing about crank pressure is you still get the same pressure at the bottom of the fuel suply in the tank.
it works for me. There is positive preasure build up without a check valve. VERY POSITIVE.
I dont use a check valve at all, I havent seen any of the racers out here use one either.


**edited cause I can never type right the first time, or the second...

combatpigg

My Feedback: (3)

I'll give it a try then. I will solder the brass tube shut and drill a .008" hole [if that drill index goes that low] and try it without a check valve. The only crankcase pressure set ups I've seen were with check valves, so that is what I based my knowledge on. Yea, the intake is open 1/3 of the time, so the pulses from the piston would be mostly positive. Just make sure not to fill the tank up to the vent line or raw fuel will get sucked into the engine via the back plate.

ZAGNUT

My Feedback: (1)

the pressure line restriction and no check valve is very important if you're using a carb, otherwise you get instant flooding when throttling back.

old and cheap way of making power with K&B outboards was to run crankcase pressure to the tank (no check valve), toss the carb and mount an NVA in the carb screw holes and use an exhaust throttle. the big gaping intake hole let extra, uneeded fuel spit out all over the engine when throttling back and the case pressure was enough to feed the engine despite no real venturi. works good on plane engines too.



dave