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Fuel Pressure


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Hi all,

I was wondering what everyone elses fuel pressure gauge reads at idle, at cruise, and when under extreme torment?

 

Trav.

I have a 3.6 Kenne Bell 800HP Super Snake: idle is at 30 on start up and goes up to 40 after about 20 seconds; cruise is 40-50 depending on load; and up to 100 at WOT.

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Thanks for the info. Mine is roughly the same. I thought I read somewhere that the pressure should be higher at cruise and idle but maybe not!!

 

Trav.

 

The old PFI systems with a fuel return used to run right around 50 psi at idle and right around 100 psi when you pinch the return (aka "deadhead" the pump).

 

Understand that when you get boost your fuel pressure must go up 1 psi for every 1 psi of boost you add, PLUS 1 psi for every 2" Hg (vacuum) you lose, PLUS the additional amount of fuel needed to run a richer AFR for power.

 

So for instance let's say you are running at idle. You're running a 14.7:1 AFR (which is realistic for running in Closed Loop operation) at 18" Hg manifold vacuum and 40 psi of fuel pressure.

 

When you give it WOT you instantly loose the vacuum (18" Hg = 9 psi) and the boost will rise to (again, "let's say") 20 psi. 40+9+20=69 psi. Your AFR will need to be richened up considerably for power from 14.7:1 to ("let's say") 10.0:1 so you'll need more FP to push more fuel through the injector too.

 

And at 10.0:1 vs 14.7:1, that's quite a bit of additional fuel you need which is why you'll see the 100psi reported (PLUS a very wide injector pulse width).

 

That gave us a easy way to check a fuel pressure regulator on a Naturally Aspirated car. Connect a FP gauge and a Vac gauge, note vacuum and fuel pressure at idle, whack the throttle and note vac. vs. FP. FP goes up 1 psi for every 2" Hg lost. If not, bad regulator or pump (deadhead the pump and see if you get 100psi to eliminate the pump).

 

Now with fuel pressure being electronically controlled, that's mostly just trivia....

 

 

Phill

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  • 2 years later...

I have a 3.6 Kenne Bell 800HP Super Snake: idle is at 30 on start up and goes up to 40 after about 20 seconds; cruise is 40-50 depending on load; and up to 100 at WOT.

Hi,

 

I also have 3.6 KB with these gauges http://www.shelbystore.com/product-p/KIT-G.htm

 

The 3.6 supercharger is however too big to fit the fuel pressure adapter, which is probably meant for the 2.8KB, so it does not read the fuel pressure. How did you solved it?

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