There is a small hose attached to each of the fuel injectors that as far as airflow is concerned is connected to the new intake manifolds runners. The fuel injection side of the hose is at manifold vacuum. Those hoses come together at the plastic tree and connect to a 3/4" I.D. hose which connects to the factory take off point for the air injection which is right past the HFM which is at atmospheric pressure.
What the air injection basically does is flow air through the fuel injector itself to try to improve the pintels fuel atomization slightly. This is of course a good thing.
Or is it?
Yes and no on a forced induction engine. When you are cruising it functions normally. Air gets taken into the hose behind the HFM, flows through the fuel injector and into the engine because the fuel injection side is at a lower pressure then the HFM side of the hose.
What happens under boost though?
Now the fuel injector side of that hose is at a higher pressure then the HFM side. Air is still flowing through the hose but in the WRONG direction. You are now most likely screwing up your fuel injectors atomization and are leaking boost out of your manifold into the start of your intake again.
At least in theory.
I just finished installing my boost gauge yesterday so I couldn't test it until now. I was talking with Jim Goodroe from Downing/Atlanta about this yesterday to see what his opinion was. They hadn't thought of this so he wanted to run it by his engineers and get back to me. I am impatient so I tested it tonight.
I measured my boost with the system in its normal configuration then I plugged off that hose completely to see what sort of an effect this has on boost.
With the hose connected I had max boost of about 7.25 psi maybe a hair less. With that hose plugged I was getting just over 8 psi of boost. Pretty nice increase for 5 minutes work.
The problem though is with those hoses plugged my idle was tending to hunt for a second or two before it settled down. Cutting off the airflow at idle through that tube probably slightly richened my mixture at idle and the DME was trying to recover.
So what we need here is a one-way valve that will let air flow into the manifold (when it is under vacuum) but will not let air flow out of the manifold under boost.
I was all set to come up with some brilliant solution to this ;) but then did a little searching and found out that early Sebring equipped Miata's had a different (though similar in theory) problem and that they had already had a brilliant solution. Best part is their solution looks like it will work perfectly for our problem. Check out the link below and read about the ISC Check Valve. I tried to find the part locally but after a couple of stores gave up and ordered the kit from R Speed which is mentioned there. I should have it next week and will know for sure if it fits the 1.9l hose properly.
For those not running Downing/Atlanta kits you might want to check where you kit gets the injectors air from. If it is from before your super/turbo charger you too have this problem. From talking to Karl I think BEGIs have the same problem and I'd bet most(ALL?) of the other kits do as well. For you turbo guys this is not going to increase boost (the waste gate is going to limit boost) but it will help with fuel atomization, might reduce lag a touch and should give you a slightly better pressure ratio.
I received my parts kit from R-Speed today and just installed it tonight.
The install is a 5 or 10 minute affair. Disconnect the air injection hose from the intake bellows side. The bellows is what runs between the HFM and the throttle body. If you want you can disconnect the hose at the fuel injector side and remove it entirely from the car to make this mod but it isn't needed. On the fuel injector side BE CAREFULL with the plastic tree leading to each injector. It breaks very easily... trust me on this.
Postion the check valve as shown below in relation to the intake side of the 3/4" hose.
You then need to cut the hose basically at both ends of the check valves body so you can insert the valve in the middle. Make sure you install the valve so air flows toward the fuel injectors. Try blowing through the valve if you do not know which way to install it.
Be sure to use the hose clamps at both sides of the check valve to prevent any leaks.
Now just re-install the hose hose back in the car. I positioned the valve and clamps as shown below to avoid any wire chaffing problems down the line.
The valve ends up being mounted very close to the bellows and is down out of the way.
Since you will be pushing a little more boost after this mod (I saw about 3/4 PSI increase) make sure you are running the highest octane gas you can find. And of course it is a good idea to make sure your mixture is proper.