FPR Vacuum

Thumper · 01-23-2011, 04:32 PM

Ok I'm sure that this has been covered 100 times before, but I tried searching and came up with nothing. I've removed the rats nest on my S4 NA now where do i hook the fpr vacuum hose? Can I just connect it to the DC? Also what are you guys doing with the line running from the charcoal canister?

RETed · 01-24-2011, 01:53 AM

FPR vacuum line can go anywhere *after* the throttle body - it has to see vacuum at idle, or else it'll run rich (at idle).

I still have my charcoal hooked up for emissions sake.

-Ted

NoDOHC · 01-24-2011, 11:39 PM

The charcoal canister line typically runs to the port near the oil fill on the engine. I could get you a picture, but my one car is far from stock and the other is covered in snow.

The vacuum bias line on the FPR is required to keep the pressure that the injectors see constant (linear injection quantity at all manifold pressures). Because the bottom of the injectors see manifold vacuum, the effective pressure seen by the injector is rail pressure + manifold vacuum. If the rail pressure remains constant, you will see a very rich idle (9 psi vacuum + 42 psi fuel pressure = 51psi). SQRT(51/42) = 1.10 = 10% increase in fueling at idle.

As Ted said, anywhere after the throttle plate is good. I connect mine as close to the actual injectors as possible, but it doesn't really make a difference.

01-23-2011, 04:32 PM	#1
Thumper Backyard Mechanic Join Date: Dec 2010 iTrader: (0) Posts: 50 Rep Power: 14	FPR Vacuum Ok I'm sure that this has been covered 100 times before, but I tried searching and came up with nothing. I've removed the rats nest on my S4 NA now where do i hook the fpr vacuum hose? Can I just connect it to the DC? Also what are you guys doing with the line running from the charcoal canister?

01-24-2011, 11:39 PM	#3
NoDOHC The quest for more torque Join Date: Sep 2008 Location: Sheboygan, Wisconsin iTrader: (1) Posts: 855 Rep Power: 17	The charcoal canister line typically runs to the port near the oil fill on the engine. I could get you a picture, but my one car is far from stock and the other is covered in snow. The vacuum bias line on the FPR is required to keep the pressure that the injectors see constant (linear injection quantity at all manifold pressures). Because the bottom of the injectors see manifold vacuum, the effective pressure seen by the injector is rail pressure + manifold vacuum. If the rail pressure remains constant, you will see a very rich idle (9 psi vacuum + 42 psi fuel pressure = 51psi). SQRT(51/42) = 1.10 = 10% increase in fueling at idle. As Ted said, anywhere after the throttle plate is good. I connect mine as close to the actual injectors as possible, but it doesn't really make a difference. __________________ 1986 GXL ('87 4-port NA - Haltech E8, LS2 Coils. Defined Autoworks Headers, Dual 2.5" Exhaust (Dual Superflow, dBX mufflers) 1991 Coupe (KYB AGX Shocks, Eibach lowering springs, RB exhaust, Stock and Automatic)