Simple question about AFPR's

[QuickWE4]

Why is it necessary to remove the vacuum line when making adjustments to your AFPR?

I notice that everyone mentions that their pressure is xxpsi with "line off" and xxpsi "line on, car running and warm".

What's the purpose of the line-off measurement? If you want to know your pressure while the car's running, why should I care about what it is with the vac line off?

I know, I should probably know this by now, but I had no problem setting my fp with the line on and car running. Just wondering if there's something I'm missing here.

Rich

 

[tazgn]

I think it's just so everyone is talking ont he same page. The amount of vacuum each car makes is different and with the line on, this varies FP as well. So I think it's just the 'standard' that everyone uses. Of course this is just what I have ASSumed over the years, and never actually asked this question. Am I right??

Derrick

 

[Turbo 6 Justin]

thats exactly it. Line off is also known as base fuel pressure. when the car is running under vacuum it pulls the FP down from the base setting, under boost it obviousely raises in a 1:1 ratio with boost PSI. it is hard to determine true fuel pressure with the vac pulling it down however as there may be a cam involved (less vac) or a vac leak (again less vac) even the same car at a slightly diffferent idle doe to temp or iac will be different. there are just too many variables so we all set the base with the line off. It also works to leave the line on and just use the fuel pump jumper wire that way you don't have to constantly disrupt the fairly fragile rubber elbow

 

[SilverSleeper]

True, But...
Testing the pressue by just jumpering the fuel pump isnt a great idea either.
With our cars not running, it is using battery voltage to feed the pump. With our car running, it is getting the alternator voltage which is actually higher than battery voltage. This would cause the pump to move more fuel. I dont know of anyone that does it that way for reasons stated. Our fuel pumps are sensitive to voltage fluctuations (thats why we hot wire them off the alternator).
I broke my fpr line once while checking it. I just bought a new line from kirbans and havent had a problem with it since. The old hard vac lines will break if you arent careful.

 

[2QUIK6]

Another reason for checking with "line off" is that you are mainly tuning your motor for boost conditions, with the line off you are simulating what the pressure should be just before boost starts to climb at 0 vacuum. Upon boost, you should have an additional lb of fuel pressure per lb of boost.
So if you have 43 # of fp with line off, then at 17 #s of boost you should have 60 #s of fp.
This would be true of any car no matter what vacuum amount they are pulling if the static fp is set at 43.

 

[Turbo 6 Justin]

silvers now you have me thinking. My thoughts are the FPR is mechanical, a certain fuel pressure pushes on a diaphram and the diaphram pushes on a spring once the pressure overcomes the spring it bleads the rest of the fuel back to the tank. I agree that a fuel pump will run more volume and pressure at more volts, that is why I have a hotwire like everyone else and a homemade volt booster. but at the levels we are talking 40-50psi base I don't think it matters if the pump sees 12.7V or 14.5. Keep in mind I am not arguing with you here, I keep going back and forth on this in my own mind. In your scenerio if you had an old stock pump and put a 340 in the fuel pressure base would change. I do not believe this would happen. some people put a hot wire kit in and say they think they are running rich but I believe this is more of a volume thing not a pressure thing, if it does happen.

I say if you switched pumps (unless going up to a really big pump that was overpowering the return line) the base FP would stay exactly the same. It would allow for more volume at that same pressure if the injectors need it but that is it. Silver or anyone else have any thoughts on this?

 

[azgn]

you got it right.....

 

[Turbo 6 Justin]

who is? I am going through every auto theory book I have here trying to convince myself. I am about to go throw a different pump in to see if it matters, maybe I'll just have a beer instead?

 

[azgn]

pressure is a function of the regulator....a different fuel pump will not affect pressure unless the volume supplied overwhelms the return line (the fuel pump always pumps the same......the volume supplied at a given pressure will vary...increased voltage to the pump will change the amount of fuel pumped....like volt boosters & hot wires...)

 

[SilverSleeper]

My bad....
I was thinking volume not psi.

 

[QuickWE4]

Hey guys...

Thanks for the feedback. I had done a search on the subject but never found anybody asking the "why" part of the equation.

I set mine with the line on precisely because of things like vacuum, voltage, etc. I thought it was a better, real-world indication of my "real" fuel pressure.

However, I did notice one thing. It was easier to lower pressure rather than raise it. In other words, dropping it was very linear - a small turn on the top of the FPR and it dropped smoothly. Raising it was a different story, it seemed like I had to turn it more and it wasn't quite as linear. I'd shut the car off, and start it again and it took a min for it to stabilize, etc.

Anyway, appreciate the info.

Regards,
Rich