Well, after reading the responses today to my fuel pressure post I decided to do science project when I got home.

With the GDS spring only installed my fuel pressure was 102 psi.

I removed the spring and cut 5 full coils off the spring and put it back in with a BB. With KO and EO, I had a 90 psi reading.

Took the BB out and put the modified GDS spring back in and got a 78 psi reading. I left it there. Took it for a test drive and when I returned and checked the gauge, it was still reading 78 psi. With the pressure drop through the filter and the lines and the pressure drop at normal driving RPM, I figure I am probably at about 65 psi to 70 psi actual pressure at the injectors. Probably could cut 1 to 2 more coils off to get it between 60 psi to 70 psi at the gauge. This is where a fuel pressure gauge really comes in handy. You can grind the spring down to get the exact fuel pressure you feel cofortable with. IMO, to read actual injector fuel rail pressure, you would have to remove one of the fuel rail plugs in the back of the head and attach a fuel pressure line and gauge.

On the GDS site their spring write up states that the spring will increase the fuel pressure to between 90 and 110 psi with the average at 95 psi. I would not have installed the spring if I was aware of the recommended Ford max fuel pressure of 80 psi. I would have done the BB mod with the OEM spring.

I sent an email to the GDS tech support people asking if they knew of the max recommended Ford pressure of 80 psi and what data they had to support running at the 95 psi to 105 psi pressure. If I get a response from them, I will post it.

Question: does anyone know if Ford read the pressure at the filter as we do or at the head fuel rail port? One of these days when I have absolutely nothing else to do, I may do that little science project.

I did notice one thing. There was a noticeable lose in throttle response by reducing the fuel pressure. Don't get me wrong. It still goes when I push the long one on the right but it did it a lot better at the higher fuel pressure.

DSMMH

 

Excessively high pressure could reduce your MPG.

 

I agree especially when that high pressure is applied by your right foot.

 

There should only be a very minor difference between the fuel pressure at the filter vs the fuel rail port... I would be very surprised if you could even notice a difference on a gauge. I would be interested to see the results of your experiment....

 

All of Ford's literature directs the fuel pressure measurement to be at the fuel filter test port. All of their specifications on fuel supply are in reference to the pressure at this point (unless you are trying to troubleshoot a bad HFCM, plugged suction line, etc).

 

This is reply I got from Rich Gillette at GDS. "teppler" got the same answer.

Ed. The navistar engineer that helped us design the spring said the fuel system will handle 120 psi. We have sold over 3000 spring with no problems.
You can cut a coil off the spring to drop the pres. Thanks Rich.

From reading about fuel pressure on other sites, the concensus is the higher the fuel pressure, the larger the decrease in fuel mileage. So when I was at 102 psi, I know why my mileage dropped, my oil level did not increase and my throttle response was a lot better.

Also, I read on another site where someone actually measured fuel pressure at the fuel filter and the back of the head. Here is his post.

Before I put a cross connect tube and fuel pressure gauge on the back of the heads, I measured(iirc) 48-40ish at the pump(HFCM), 45-38ish at the filter housing and 35-25 at the back of the heads(idle-WOT). Is that good pressure? I didn't think so. Is it normal? Unfortunately I believe it is.

NOTE: he has #12 injectors installed (whatever that means)

Does anyone know the Ford TSB number recommending 80 psi max fuel pressure?

I'm also trying to find the Navastar fuel pressure info.

DSMMH
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sorry but not all of fords test suggest takeing readings from the fuel bowl.
refer to the econoline book and you will find a spec for testing at the back of a cyclinder head. the spec is also less.

 

Good point - I do not have an econoline and do not think of them much when I post (also, none of the folks that are in this post have an econoline).

How much less is it??