3.2 intake/brake booster on EFI conversion

sjf911 · sjf911's Garage

I am just finalizing the details of the 3.2 intake manifold based EFI conversion of my '83 SC and have a question relating to the brake booster vacuum line setup.

My system will be MS-II driven and normally aspirated (MAP/speed density). There will not be an MAF, rather an aftermarket airfilter will be attached to the rubber elbow above the throttle body.
My concern is in leaving the booster line setup as is (attached to both the left hand manifold and the rubber boot/elbow) may lead to excessive air bypass of the throttle plate through the brake booster vacuum line (down through the venturi and back into the manifold leaning out cyl's #1-3 and effecting idle).
It appears others have simply attached the booster vacuum line to the left hand manifold nipple directly (presumably with a check valve) and plugged the elbow nipple. Is this reasonable? Is there some performance issue at stake? Why is the factory solution so complicated?

crashr · 08-02-2009, 01:38 PM

I think this thread is related to an issue I am currently having with a hunting idle. If I am not using the brake booster is there anything that needs to be done to make sure there is not to much of a vacuum on the air box that would effect the idle? It appears that if I remove this line from my system that it idles perfectly.

David · David's Garage

As long as there are no vacuum leaks on your brake line, there shouldn't be a problem. Just connect to the brake booster on the left side of the Carrera manifold.

cmonref · 08-02-2009, 03:37 PM

I don't think there is any airflow thru the brake booster line -- it just provides negative pressure to the brake. Maybe there is slight flow when the brakes are actually operated. David is right -- no leak, no problem. A leak between the manifold and the booster would display classic intake manifold leak symptoms. Here's one leak source that John Walker highlighted a while back.

vacuum leak you probably didn't ever think of

crashr · 08-02-2009, 04:21 PM

So if I am not using the brake boost line because I don't have vacuum assisted brakes what is the point of the venturi tube?

My understanding of a venturi is that the difference in pressure or flow through the main passage vents through the secondary channel. In this case wouldn't that secondary vacuum channel be this hose:

I don't understand why a venturi tube was used to create a vacuum from this line back up to the intake? Is there ever no vacuum on this line?

All I do know is if I block off this return line (ie. remove the pictured device above and block both sides off) my car will idle perfect but with it installed it hunts teribly.

gsmith660 · 08-02-2009, 04:55 PM

Brake booster just needs to go to the left side manifold connection on an early car based on other megasqurirt installs as well as mine even though its a turbo I get plenty of vac. from that connection as well as my boost gauge and my boost controller. I have seen guys take the elbow off and put a cone filter straight on the throtle body so you can get rid of all that stuff. When I was going to use the elbow for a N/A application that was where I was going to tap my AAR and temp. sensor into like it is now.