EFI when using a Roots supercharger

Back when I owned Mopar Engines West we spent some serious time testing various configuration of EFI and Roots blowers. Here are some comments from our testing:

* Throttle Body:
Many people liked the look of the Enderle or BDS “bug catcher”. But these flow so much air that it’s hard to get a stable idle RPM because of butterfly sealing and inconsistent idle return. Also, the tip-in just off idle can be hard to control because the increase in airflow is dramatic with a very small change in throttle. For a street driven car at least one port should be totally blocked off.

*IAC:
Provisions should be made for a remote IAC. This can be plumbed into the back of the injector plate.

* Injectors above or below the blower:
Answer -> both. The injectors above the blower keep the blower happy and the injectors at the ports allow for better idle control and off idle tip-in. Also, running sequential injection allows for individual cylinder control which is much more important in a blown engine (see testing).

Testing:
1) We ran several tests with Roots blowers with fuel delivery only above the blower, and found that the rotation of the rotors can bias the charge causing inconsistent cylinder charge at low and mid RPMs. Also we found an airflow bias toward the front of the engine. When injecting only above the blower there’s nothing you can do about that.

2) For all-out drag engines injector location didn’t matter all that much, but in a street engine control of the low RPM torque is critical to good drivability. I have driven cars where injection was only above the blower (not by me) and the customer was very happy with the set up. However, when analyzing the exhaust with a wideband O2 verified that these engines were set up VERY rich. Far to rich, in my opinion, for a reliable street engine. Plugs were black and cylinder wash was prevalent. You can live with this in a short duration drag engine, but not on a long-term reliable street engine.

BTW: We have used street blowers with no seals, so minimal cooling requirements, and injected only below the blower with great success. Of course a blower like this doesn't create much boost but it still "looks good".