The standard SLC fuel system is fairly simple. A low pressure fuel pump siphons fuel from the bottom of the tank and dumps it into a swirl pot, or second, smaller tank that’s usually tall and skinny. A second high pressure fuel pump pulls from the swirl pot and feeds the engine. The purpose of the swirl pot is to keep a steady supply of fuel to the high pressure pump under all circumstances, even under high acceleration events. The tall shape of the tank ensures that any sloshing in the swirl pot happens high enough that the HP fuel pump never loses prime as it’s gravity-fed. In almost all circumstances the low pressure pump flows enough fuel that the swirl pot is basically topped off. Any excess fuel being pushed into the swirl pot is redirected back into the tank. For my LS motor, any excess fuel sent to the engine via the HP pump is also redirected back to the fuel tank.
In the system described, the fuel tank experiences a constant change in fuel volume and pressure, either due to fuel being removed, fuel being dumped back in, or by fuel vapors being generated due to changes in temperature or due to excitation (aka being shaken). How do we deal with all these changes to ensure the pressure inside the tank matches the pressure outside and doesn’t blow up or implode? The low-tech solution is to simply have an uncapped hole at the top of the tank. This allows for any excess pressure to bleed out and any decrease in pressure to be equalized by ambient air.
In my case I went a little crazy and even added a rollover valve to my vent just in case I decided to go skiing on the roof, I’ll be keeping any fuel in the tank inside the tank. The rollover valve is a pretty simple device; there’s a ball that sits such that fuel vapor inside the tank can cross-communicate with the outside world while right-side up. Roll your car and flip it upside down and the ball checks, preventing any liquid from escaping.
I’m using a Tanks Inc. rollover valve, PN VVR; it even comes with a handy little screen to keep dirt from intruding into the tank. I had mounted the valve up high, as you’re supposed to, and against the firewall of my engine compartment since this is the highest point in the car.
For a more in-depth review of my fuel system check out my YouTube video on the subject: Superlite SLC – Going THERMONUCLEAR!
When I would get back from driving my SLC my wife and daughter would complain that I smelled like gasoline – apart from the stuff running through my veins, I couldn’t deny that yes, I did smell suspiciously like I was doused in the good stuff. It never really bothered me; I had always chalked it up to being around the car after a drive. The exhaust is so rich your eyes will water just standing behind the car; and I always back into my spot in the garage so there’s a fair bit of time spent just above idle while I attempt to not crash my car while staring into my rear view monitor. After parking, the garage is pretty much engulfed in gas fumes – and I always figured my clothes were getting gassed up because I was in the presence of so much exhaust gas as I was messing around in the garage before heading back inside.
In hindsight, I think what was happening is I had fuel vapors venting through the rollover valve and collecting near the rear firewall. I suspect the rear engine compartment is a high pressure zone and that it was pushing collected fuel vapor back into the passenger compartment; my firewall seals are not 100% effective. Not enough to gas me out of the car, but enough for it to slowly permeate my clothes.
Fast forward to a few weeks ago. I’d left the SLC sitting in the garage for a few weeks while I was attending to “house things” and the next time I got in to drive the car my fuel meter read dangerously low – funny, I’d just topped it off and gone for a drive just before parking the car, there’s no way I could have run through so much gas just on that one drive. Then it occurred to me – my unsealed tank may be allowing fuel inside the tank to evaporate more quickly than when I was at sea level. Between smelling like gasoline after a drive and losing precious drops of ethanol via evaporation, I decided it was time to do something to make my venting system a little more sophisticated.
Enter IIMUch – read “2 Much”. The IIMuch VSR vent system is a black canister which was apparently designed by orcs and wizards because it claims to be able to reduce fuel odors without using steel wool or some other similar device to catch and re-liquefy aerosolized fuel vapors. However it works, I was willing to spend the $200 if it meant the only fuel being wasted was being done by my right foot!
NOTE: There is a recall for earlier IIMuch VSR vent cans. If you own one of these earlier designs, PN 100034, contact IIMuch for a free replacement with one of their latest, PN 200024.
The Tanks Inc rollover valve can be integrated in series with the IIMuch VSR by installing it into the “TANK” side of the can. The outlet side of the can features an included porous filter. I’ve relocated my discharge filter to just aft of my starter to try and get it even further away from the firewall and cabin.
On the drive immediately after installing this I was surprised that neither my wife nor my daughter said anything about me smelling like gas. Seems to be working!