Fuel Pressure Regulator Vacuum Line - Needed on NA Engine?

Fullthrottleguy said:
Looks like ALOT  of "opinions" here. Most have the answer I'll add the color if you care...

What I can tell you is doing this wrong or changing this CAN destroy your engine.  Not opinion, just fact and the recommendation of various manufacturers

From an ECM/tuning aspect, you should always whether Boosted or NA always set fuel pressure with the line disconnected. Full stop doing it another way will provide incorrect fuel pressure numbers

In an EFI engine fuel pressure determines how rich or lean and engine runs.  depending on the cam and compression the Vacuum at idle can be 10-25 inches of suctions and part throttle at 5 ish and 0 at WOT 

the vacuum pulls on the diaphragm of the regulator and will change the fuel pressure  - you don't ever want fuel pressure changing as you drive - you want the Fuel map in the ECM to control how munch fuel the engine gets. - not the vacuum ( well thats how a carburetor works actually).

Imagine with the line on at idle your fuel pressure is sitting at 43.5 because your tuner set it there (like he/she should with the line off)   now its 38Lbs with line on with the line on and 20 inches of vacuum car runs at 16:1 AFR (its way lean and ecM if it has compensation or learn (basically all modern ECMs) is throwing in fuel to bring it to the 13.8 or so the tuner set - you just F'd his/her/they/them:-) fuel map. -  Now you romp the gas vacuum goes to zero your fuel pressure goes to ~ 48LBs - this messes with your AFR. and the ECM starts pulling fuel out like a Mofo  and your tune is all messed and you blame the tuner for black smoke and loss of power , a hot running idling engine and an overall crappy tune  - just because of a little vacuum line and believe me its now the first place I look, but it took a while to get to that conclusion.

When I get an NA car with line on in to me to help tune - I drop a BB in line so I don't have argue with the person and tune the car the right way. 

 - A little more info.  - The manufacturers say the port on NA applications should be open to atmosphere.  If you attach to vacuum the Fuel pressure will be reduced at Idle and part throttle as vacuum will pull on the diaphragm.

This will affect the full map and tuning on the car.   This also affects the spray pattern of the injectors.  Most injectors are made ti see 43.5lbs MINIMUM to have good spray pattern (aka smooth idle). and can support upward of 100lbs   (boost referenced). but they usually work best between 43.5 and 60lbs 

If the regulator had the line attached to vacuum and car was tuned that way leave it - removing vacuum will change the fuel map and vice versa - adding the line  after the tune will change the fuel map 

The easiest way to burn down and engine is to mess with the tune 

While I do 90% + boosted cars, on NA cars I leave the port open  and with run a "snail" turned piece of tubing pointed down and away from the headers or buy a screw in air filter  (stone filter) as JALPER does and usually thats if the regulator is away from the engine

On a boosted engine running a line is a MUST  - you have to boost reference the fuel -  think of it this way  - if my fuel is at 60Lbs and I add 10Lbs of boost I have effectively reduced my fuel pressure to 50lbs and I seriously messed with the tune.  So I set the fuel pressure with the line off  and adjust the fuel map with the line on  - and I have full control  - 

The only bad thing on boost referenced regulators or regulators with line attached or a line at all is if you burst the diaphragm  (ethanol can do that on older Areomotives's the port  shoots gas out like a small fire hose  - so watch where you mount it if open and it connected to the engine. it pours fuel into the manifold and can hydro lock the engine (ASK ME HOW I KNOW :-( )

As said regulator are almost all 1:1 rising boost to pressure, Then there are FMU'as and thats a whole different game 10 to1 or 20.1   .. I am not going there ...here

But to the OPs specific,  that 13105 is an awesome NA regulator its my goto, but be careful where you buy it - aeromotive stuff is 100%  being knocked off and the knock offs are very unreliable 

https://aeromotiveinc.com/buyer-beware-genuine-aeromotive/

I hope that long explanation helps.  take it or leave it FWIW  
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Hednjry said:
Well this is clear as mud  :blury:
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Basically: if the motor was tuned with it plugged in, it should stay there. If it wasn’t, it shouldn’t ever be connected.

If you’re starting from scratch, go unplugged, though having it on the manifold makes everything nice and sealed. 

 
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