Argess
Posts: 409
Joined: 1/12/2006
From: Pleasantville,
NS, CANADA
Status: offline
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I posted this in another forum and it seemed to generate interest, so I thought I'd re-post it here.
OMG, you are all saying, but I did figure out a mod that worked for me, and thought I’d share it with you.
First of all, I addressed this issue a while back when I was thinking of the dynamics of “how fast the tank can pressurize” with respect to a full tank vs. an almost empty one. My final thought on this was to decrease the pressure line restriction, and this mean shortening the pressure line as much as possible. I have tried this in 1/8 Buggies and it works great. It also reduced the problem in my Savage 25 to an acceptable level, but you can only make it “so short”.
Second of all, is the actual “head pressure” change as the fuel level drops….a more ”static” evaluation, as the level doesn’t drop quickly during engine operation. Although muffler pressure reduces the percentage change that the HSN/Carb “sees”, there still can be an issue with this dependant on the engine you use. (I really didn’t have much of a problem with the Nitro-Star 25 after I shortened the pressure line.)
Anyway, apparently some engines seem to react differently to this Savage fuel tank issue, and with our newly installed OS 30VG, the ½ tank, lean problem became more noticeable.
I was reading with some interest on the Uniflow system, but it had drawbacks. The worst one was fuel coming back and draining out the muffler, or with a 1-way check valve in the pressure line, the engine flooding when stopped for some reason. So I decided to try to improve it.
Please forgive my bad sketch, but referring to it, here goes:
Engine at a fixed or accelerating RPM:
1) Good exhaust pressure at “A” and “B” and “D”
2) Reduced pressure by fuel “head” at “C”
3) One-way Valve closed as pressure at “A” is greater than at “C”
Engine deceleration:
1) Pressure is reduced at “A”
2) One-Way Valve is open as pressure at “A” is now less than “C”
3) Exhaust gas flows more quickly from “C” to “A” (than fuel from “B” to “A”) so tank pressure is reduced before any of the pressure line fills with fuel.
So I tried it tonight. Absolutely no variation in performance or temperature from a full tank to an empty one. No fuel coming out exhaust. No flooding of engine. Success!!!
Also attached is a picture of the installation. Not very “clean” as I wasn’t sure it would work.
I really didn’t want to start any sort of argument about one mod being better or worse than another, but I am hoping that someone else will try this and let me know how they made out.
You need:
1) some extra fuel tubing
2) a barbed, thread in fuel fitting
3) a one-way valve
4) a T-Connector
Nipple was a 8-32. I drilled a hole smaller than you would if tapping metal and only partially tapped the hole (tapered tap not all the way through). Nipple started getting snug long before gasket was compressed. Very tight. Same concept as a nylon loc-nut.
This may not be the best place for the nipple, as it and the pressure line are kind of exposed. Below is a pic of an alternate arrangement that would do the same thing. Basically a nipple (say a 10-32 for 1/8 hose) drilled out to accept a brass fuel tube, and the two soldered together. Tube length cut so end is about 1/8" from bottom of tank.
There is one draw-back to this mod. You get no warning when you are about to run out of fuel, so it's possible that you might be making a screaming long distance pass when it runs out. I sort of liked the warning signs of it getting lean so I could be taking it easy when it ran out of fuel.
An up-side, is that the engine warms up very fast as it isn't running with a rich mixture for the first part of the tank. This helps reduce wear.
Here's the P/Ns I used:
T-fitting: Robart 221 http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXES20&P=7
Nipple: Du-Bro 539 http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXD623&P=ML
One-Way Valve: Yamada YS0405 this was an expensive unit, but all the LHS had. I suggest this instead: Fourmost 138 http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXG850&P=ML
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All Forums >> RC Cars, Buggies, Trucks, Tanks and more >> RC Monster Trucks >> HPI Monster Truck Forum >> Yet Another Savage Fuel Tank Mod ... 
Yet Another Savage Fuel Tank Mod ... - 
) and it looks pretty interesting. I was thinking about using a Mugen tank, but if this works then I may opt for it. Good find!
Is that remotely close to what you meant? 
Tom.
