Manifold selection (single,dual) and spacers
This questions may have a lot of different opinions so I am interested in hearing from those that had tried or dynoed different setups.
There is a lot out there about single plane and dual plane manifolds used with EFI. With Fitech being a TBI does it matter which one you use? This is taking aside the basic fact that one manifold will help the lower end more than the other.
The other question I have is if anyone has experience with spacers? Have you notice a difference when adding a spacer? Open or 4-hole? Tapered? Again, I know there is a lot of information out there for carbs, but does that information changes for a unit such as Fitech?
My setup - 462 CI FE - basically a bored/stroked 428 with a Blue Thunder dual plane (428CJ clone intake). My guess is that Fitech would behave the same way a properly tuned carb would to a particular engine combo. In my case I replaced a Mighty Demon 850 w/annular boosters with the Power Adder 600 system. I "felt" no difference in overall performance - just better off-idle feel and better fuel economy.
Regarding spacer - I run a shaker so I don't have the luxury of adding spacers. Hope this helps.
My .02: if running a dual plane, make sure the divider does not come all the way up to the throttle body! You can do this by either cutting a u-shaped hole in the divider, or do what I did: add an open-hole spacer between the intake and the throttle body.
The purpose of a dual plane manifold is to provide a stronger signal to the carburetor at low engine speeds to draw in fuel and improve low speed response. At higher engine speeds air flow through the system is sufficient to draw fuel through the carb without the need for a reduced plenum volume at which point the increased plenum volume of a single plane offers greater airflow equaling more horsepower. Since fuel injection does not rely on air flow to mechanically meter fuel a dual plane design is of no benefit. There was a great episode of Engine Masters where they installed an EFI, I think Holley, on a dual plane manifold and could not get it to run right until they eliminated part of the divider as mentioned above. Unfortunately the guy I bought my GTO from mounted the FiTech on an Edelbrock ESP manifold. I may try adding an open spacer at some point just to see what happens.
Nice discussion here. Thanks. You are echoing what I have been reading. If using a dual plane you need to have an opening in the divider. In my case I have a Blue Thunder manifold that has a small opening. I am contemplating using a 1/2" open spacer to see if there is a difference. That's as much I can fit without having hood clearance issues.
Edit PS: in my case, the dual plane is running well, but there is always that urge of thinking if I may be leaving some power on the table. I guess I will have to test the spacer and see if it makes a "sit of the pant" improvement.
My car runs 98% great. In my 50 years of tinkering with cars going after that last 2% has always gotten me into trouble. Then again, at least a spacer is easy to remove. The only thing that is holding me back is the possibility of altering the throttle cable geometry and then having to deal with that. Also, FWIW, FiTech says a dual plane is perfectly fine.
I also read somewhere that the issue with the Holley EFI on Engine Masters is that the MAP sensor only reads from one side. The post went on to say that the FiTech MAP sensor receives a signal from both sides of the manifold and thus will run fine on a dual plane.
I just found an informative article comparing FiTech vs. Holley carb on single and dual plane manifolds. For some reason I was not able to paste the link here but if you google "fitech dyno" you'll see "EFI versus Carb Dyno Test" at Super Chevy.
So the dyno testing confirms what Ken Farrell said, ie. the engine will respond to the different manifolds in the same way, whether carbureted or TBI. Given the fact that the dual plane with FiTech gained 17 lb.ft. of torque while losing 7 hp over the single plane I'm going to leave my engine alone.
Excellent link. Thank you.