Fuel Pressure Gauge tap location?

J

Joe-Racer

New member
This is a very basic question that I 'should' know the answer to - but no matter who I ask I keep getting mixed answers. I've asked a ton of people (read: 2) So I thought I'd go public with it hoping the consensus will sway my decision.


I have a Holley FPR and a Nikki with a return line. Should I plumb the gauge into the side of the FPR or tee into the return line? After taking the reading and setting the FP, I plan on removing the gauge.

Thx in advance
 
On the incoming line under the hood. When you procure you fuel pressure gauge you NEED a gauge with minimal change in the repeatable/accuracy. For an example my gauge reads from 0 to 5 psi over the full scale of the gauge. 1st gen's non ported don't need even 5 psi to get the job done. My gauge accuracy is +/- 1.5 % of gauge full scale. My gauge full scale is 5 psi therefore my accuracy is: .015 x 5 psi = +/- .075 psi. You DO NOT want a fule gauge in the drivers compartment.............. Whn loking at gauges you NEED TO KNOW the accuracy/repeatability because many gauges have a acuracy/repeatability of +/- several % as in +/- 2 to 5 %. A good gauge has a .005 inch thick diafram within the gauge that is filled with fuel at the pressure your set to & if the friking diafram breaks you have fuel spraying around inside the gauge LEAKING OUT. You have the correct idea of not leaving the gauge in your fuel system because a gauges good accuracy/repeatable will not hold up under the riggers of racing vibrations.
 
I concur with everything Dave just said.

Put the tap between the fuel filter and the carburator. I use a a brass hose barb "T" fitting, the fuel line comes off the center of the "T" to a brass valve. The valve is secured to the non-exhaust side fender. Another length of fuel line and an Acetel tubing quick disconnect (Page 175 of the current on line Mcmaster-carr catalog--www.mcmaster.com)--this is a valved fitting that shuts off when disconnected. I have 1 male fitting on my highly accurate gauge, and a second male fitting on my fuel sample hose (kept in the glove box). This way I use the same tap for both fuel pressure checks and fuel test port.
One more word about a fuel pressure gauge in the car--I find them kind of useless. What are you going to do during a race if you see low fuel pressure? It's just one more piece of information we do NOT need to worry about when we are driving.
BTW, I also have a Holly regulator. I've only had to adjust it once--I've never seen it drift. I do carry a spare diaphram for the regulator in my spares kit.

Tak
#29 ITA Rx-7
SFR SCCA
 
David and Tak -

Thanks for the helpful information. I purchased a gauge that is .05 accurate - although the range is higher at 15psi. Locating a 0-5psi on such notice was next to impossible. I used the easiest tap method to set the FP (@ 2.5psi), which is somewhat similar to Tak's suggestion.

See below

DSC00878.jpg


DSC00884.jpg


So with this setup I'll know the MAXIMUM PRESSURE going into the Nikki. BUT... The Nikki return has an orifice in it that relieves pressure and these were set to deliver no more than between 3.75 and 4.25 psi to the needle valves, depending on the model carb.

I suppose the best way to set the fuel delivery system up is to have the incoming fuel, the return fuel, and the carb feed hoses all plugged into the regulator. The fuel is like a circulating stream under pressure that I want to tap into and feed the carb, but control the pressure to the carb. But I don't want a gauge telling me that the feed to the carb is at 4.75, meanwhile the damn carb is bleeding off pressure through the return line orifice! So I'll need to deadhead that return line. I think I've opened up a can of worms here.

I'll run a T in the return line just to check what the Nikki is doing.
 
Two points I'll add here.

I used to be of the opinion that a fuel pressure gauge was useless in a race. But I've changed my mind. If I now get a high speed miss or the engine stops I now get to look at the gauge and decide whether I have a starvation/clogged filter problem or something else. This trims significant time in the troubleshooting stage. It is true however that I don't think there is much you can do during the race in response to a low pressure reading, which used to be the reason I thought they weren't useful.

I have my gauge mounting outside the car for the reasons cited above. It turns out that you can buy an isolator for your fuel gauge. (At least you can if it's an Autometer gauge) You plumb from your fuel tap into the isolator. The isolator then connects to the gauge and transmits the pressure to the gauge via a line filled with anti-freeze. This is available from Autometer for about $100. It's now on my wish list.

I'm also using the second port on the FPR for my gauge. I confirmed that I get the same reading there as I get from a tap between the regulator and the carb.

-chuck-
 
The Nikki return has an orifice in it that relieves pressure and these were set to deliver no more than between 3.75 and 4.25 psi to the needle valves, depending on the model carb.[/b]
Click to expand...

I was told that my carb (Yaw prepped) needed bewteen 2.5 & 2.75 psi to run properly. I have a guage that reads 5 psi full scale but only hook it to the same port on the Holley FPR that the guage is attached to in the pictures when needed. The thing was too damn expensive to have it riding around under the hood.
 
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