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post #1 of 8 (permalink) Old 01-02-2008, 10:51 AM Thread Starter
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Fuel Pressure Regulators

If I'm not mistaken, our FPRs are vacuum actuated, aren't they?

So... For a N/A motor, more vacuum would warrant more fuel pressure...

And a Forced induction motor would be the opposite...

Do I need a staticly set FPR?

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post #2 of 8 (permalink) Old 01-02-2008, 11:00 AM
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The fuel pressure regulator with vacuum (idle or cruising speed) is at about 35psi and without vacuum (WOT or under a load) is at about 45psi.

vacuum changes the spring pressure inside the regulator which regulates the fuel, so I would assume that pressure (turbo or supercharger) would bring the fuel pressure up beyond 45 to compensate for the boost.

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post #3 of 8 (permalink) Old 01-02-2008, 11:40 AM
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The vacuum on the FPR is for reference. Whatever amount of vacuum you have going to the FPR is the same as what you have at the bottom of the injector. Fuel pressure with the vacuum hose off is the same as fuel pressure at WOT, which is 45psi. When you have vacuum in the intake manifold, it will suck the fuel out of the injector when it opens, so if you didn't lower the fuel pressure to compensate for the manifold vacuum, then the injector would be flowing more fuel under vacuum than it does under load. Basically the vacuum reference allows the pressure differential between the top and bottom of the injector to always be the same, so the injector always flows the same amount of fuel per hour, so the computer only needs to keep track of how long to open the injector for a fixed amount of fuel, which is much easier to calculate than having tables to compensate for all the possible load levels.

Basically the answer to your question is to leave your FPR alone. Under vacuum it will lower the fuel pressure, and under boost it will raise the fuel pressure, and this is exactly what you want it to do. If you are going boosted, make sure you have injectors that flow enough for the power you are making, and have the tune account for them. In some supercharging kits, they include a FPR that will increase fuel pressure by 5psi for every 1psi of boost pressure in the intake, which will work, but isn't the best way to do it.

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post #4 of 8 (permalink) Old 01-02-2008, 12:05 PM Thread Starter
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Very good post! Thanks for the information!

Don't worry, I've got 42lb'ers and a tune comming for this stage... I think in 09 I may have to revise that though.

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post #5 of 8 (permalink) Old 01-02-2008, 01:16 PM
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so when do you plan on getting this supercharger installed? I'd be more than willing to help. I'd love to see that cat w/ some serious forced induction power.

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Red Green, is that you? Might consider looking into fabbing an intake setup that isn't so, well, ghetto.
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post #6 of 8 (permalink) Old 01-02-2008, 01:35 PM Thread Starter
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I'm hoping to be able to take a weekend and a couple days off this spring when it gets warm. I'm a skinny f*er so I don't do well in the cold.

The pulley on the vortec is somewhere in the 3" range... supposed to be like 14 psi? LOL!

My only intercooler is methanol for now (this season) so I'm going to limit the rpms to 6k. I've made a good design for a frontmount when its time to goto the next level though.

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post #7 of 8 (permalink) Old 01-04-2008, 09:18 PM
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The pulley on the vortec is somewhere in the 3" range... supposed to be like 14 psi? LOL!

My only intercooler is methanol for now (this season) so I'm going to limit the rpms to 6k. I've made a good design for a frontmount when its time to goto the next level though.
Is this going to be on stock internals? You might consider running a larger blower pulley to drop the boost off a bit, if it's currently pullied for 14 psi.

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post #8 of 8 (permalink) Old 01-06-2008, 05:27 PM Thread Starter
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Nah, It's completely forged. Stainless valves, 9:1 compression, file to fit rings, Good (Don't remember which brand) head gaskets, all ARP bolts....

I've still got a PI intake and plenum on it; as well as stock heads, so that should limit the actual cylinder pressure as well.

The tune is going to be VERY conservative, and add/remove timing based on air charge temp after the methanol does its work.

Overall I'm not too worried about it. I'll be getting plenty of belt slip I'm sure. I've got plans drawn up for a FMIC in the near future too (Once my wallet catches up with me).

Thanks for the concern though

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