Gas vapor recovery and turbos

[Zmanco]

I'm sorting out some of the smaller remaining issues now that I've got the turbo install finished on my L28 and need advice on how others are handling venting the gas tank on their early S30s.

Before I added the turbo I had picked up a charcoal canister from a toyota at the picknpull and had one port connected to the fuel vent line up in the engine compartment and the other end directly to the intake manifold. This setup worked very well - I never smelled fuel in the garage and there was never any significant pressure released when I opened the gas cap.

When I installed the turbo I ran out of space to keep the canister where I had it. I was also worried about going above atmospheric pressure in the canister. So I just plugged vent line in the engine compartment and left the canister off. Of course, now sometimes when I open the gas tank there is huge release of pressure, I mean huge!

So it's time for me to mount the canister again, but I'm wondering how to plumb it into the intake manifold. I'm thinking about adding a one-way valve in line back to the intake manifold. That way as pressure builds in the tank it could vent into the intake manifold, but when boost goes above atmospheric, the valve would prevent the pressure from going back into the canister.

Does this sound realistic? And where can I find a valve like this? I figure this problem has already been solved by others so I'm hoping to not have to do too much inventing

[19752802+2]

Possibly use a PCV valve? can anyone else chime in on this idea?

[Tony D]

Get one from a Geo Metro, EVAP capacity is roughly equivalent, but it's DINKY!
And fits nicely behind the right wheel in the fender well (or under the light bucket on the same side)...

Don't need that cannister in the engine bay to have it functional!

There are check valves available, take a look how the L28ET vents the charcoal cannister and you will know all you need to know. The FSM is online for free!

BTW, you really want the tank to have a couple of inches water column when low on gas (and consequently hot as hell from recirculation)---this blanke t gas pressure keeps the fuel pump primed with sufficent NPSH to prevent cavitation in the pump inlet due to the terrible vapor pressure characteristics of today's fuels!

Sorry, I'm a geek, I think about this kind of stuff way too much!

[ozconnection]

Originally Posted by Tony D

Sorry, I'm a geek, I think about this kind of stuff way too much!

[Zmanco]

Tony, good suggestion to look at the FSM for the factory car. Attached are the relevant pages and it looks like the purge valve is in the canister itself. However, I don't have a turbo canister (and they're not very plentiful) so this path may not work for me.

Any thoughts about putting a vacuum check valve in line with a 2 port canister to prevent applying positive pressure to the canister when under boost? Or does it matter? Right now max boost is about 10 psi and I don't expect to go over 15 when I finish tuning. The toyota canister is well constructed so maybe letting the pressure go positive under boost isn't an issue? Am I making this too hard?

[m4xwellmurd3r]

That canister looks JUST like the one equipt on Z31 300zx's.
I didn't pull one out of it when I was looking for one because i wasn't sure how to hook up the extra port.

[Tony D]

It was my impression talking with some parts people that the cannisters are the same, save for the pressure valve you mention. Basically they add that check valve.

The way the cannister functions is when there is pressure in the manifold, you will block it coming to the cannister body and thereby blowing EVAP out the bottom. If you were to operate under boost forever, eventually your gas tank would loose enough fuel to cause a vacuum in the tank, and at this point the EVAP that was in the cannister would then be redirected through the two-way valve to the port back to the tank, allowing makeup air into the tank.

As long as ONLY the manifold fitting has the one-way valve in it, you should still function as designed. The turbos have a dual-check valve in the manifold section of the cap, same as on the tank manifold cap.

The tank can cause pressure, as fuel expands, and it needs to go to the cannister for charcoal adsorption. When the car starts, normally the cannister is evacuated of these fumes, and then the tank check valve closes, to stop the manifold from drawing a vacuum on the fuel tank. Once tank vacuum develops, that valve will move allowing the make up air while at the same time not permitting a vacuum leak into the manifold line.

This is far easier to explain with a piece of paper and a drawing...

Yeah, basically you add a simple check valve on the manifold line from the cannister to the manifold, so it allows manifold vacuum to suck on the cannister, and prevents pressure from boost going to the cannister, and it should work just fine.

Breakdown of cannister valving and cannister action:
Tank Connections
1)allow EVAP to the cannister when fuel is hot (Flow to cannister from tank)
2)allow fresh air to the tank when required (flow from cannister -through it's filter on the bottom from ambient air- to the tank)

Manifold Connections
1) allow EVAP from the cannister when engine starts (Flow from cannister to manifold)
2) prevent pressurization of cannister (Check Valve action from manifold to cannister)
3) prevent manifold vacuum from sucking a vacuum on fuel tank (check valve action from cannister to manifold)

You can see some of the actions contradict each other, which is why there are two valve flappers usually. And that is usually all this stuff is, rubber diaphragms and some very light springs to give a residual pressure limit.

[Zmanco]

Tony, thanks for the discussion of how the factory system works. I guess you're suggesting that the best approach is to use the factory canister.

That may indeed be the best solution, but I was hoping in true hybridz fashion, that someone had found an alternative canister that was more readily available at the pick-n-pull, or even better would be a way to use the toyota one I already have.

[Zmanco]

I spent a little time tonight with the toyota canister and I think it can work fine. The purge line is a one way valve and blowing on it to simulate boost causes it to close completely. I'm going to run with it for a while and see how it does. If it fails, then I guess I'll source a 280zx turbo one.