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Altitude effect base fuel pressure?? Also stock 1jz FPR 1:1 rising rate??

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Okay this may end up being some stupid questions, and I may just end up waiting to tune my car until I can fully upgrade the fuel system. I’m running a 1jz vvti w/ link g4x in a Nissan 240sx. I used a gauge to check the base fuel pressure, stock fpr and stock rail with 540cc injectors and Walbro 255 pump, fuel pressure is 45 psi, from everything I’m finding stock 1JZ fuel pressure should be 42.5 (3 bar.) Would altitude have an effect on this? I live in Colorado where the altitude is 5,545ft so I know it made my map/bar pressures a lot lower than what I’d be seeing at sea level, but I’m curious if with less external pressure here it could make fuel pressure higher. Another question I have is, would the stock fuel pressure regulator be 1:1 rising rate? Just trying to figure all this out before I try tuning my car and possibly mess things up. Haha.

Hello, fuel pressure should be the same at altitude. the regulator is referenced to ambient pressure and therefore maintains constant gauge pressure not absolute. but i may stand to be corrected,

Regards Ross

Larger than standard fuel pumps, especially on most factory regulators will see a slight rise in pressure at idle/minimal fuel demand. Combine that with manufacturing tolerance/lack of adjustment/genuine calibration correction data and you are probably pretty close to where you should be. I wouldn't be concerned about it at all.

Pretty much all manifold reference regulators are 1:1 (well very close to) by design, it's much more complex and costly to design them with areas ratios significantly away from 1:1. Remember that they are still flow devices with some fixed max orifice sizes and relatively short springs so they don't maintain dead on set pressure across the entire flow range, most will have a fairly linear droop curve across the usable flow range that is generally absorbed in the fuel map unless you are running fuel pressure compensation.

Ross is correct, normally regulators and gauges are referenced to atmosphere/ambient - the easy way to tell, if it isn't marked PSIG or PSIA, is if it's zero when open to atmosphere, ie, there isn't pressure applied to it, it is atmosphere referenced.

AFAIK, all automotive regulators are ambient referenced.

They are not referenced to ambient, they are referenced to MAP. The idea is that the regulator should maintain a constant Delta P. Essentially, if the manifold vacuum is 19 inHg (-9.3 PSI), your Delta P would be = Base Fuel Pressure + 9.3 PSI. As you open the throttle body, vacuum goes down because there is less flow restriction and the fuel pressure goes up to maintain the Delta P. At WOT, you'll likely see 2 inHg (-1 PSI) so your expected fuel pressure should be = Delta P - 1 PSI.

If you're interested, the best way to think of this mechanically is to think of the fuel pressure regulator as a tug o' war. On one side is your base fuel pressure, on the other side is a spring set at the base and your manifold vacuum. When you have high manifold vacuum, you have lower fuel pressure because the manifold vacuum is tugging the regulator open against the spring (Fuel pressure + MAP - Spring Pressure). When you have low manifold vacuum, there is only the spring holding your regulator closed (Fuel pressure - Spring Pressure). Run the calculations and you'll see that when fuel pressure equals the spring pressure, the regulator is closed. This is why you normally see about a 10 PSI difference between WOT and CT operation.

As previously mentioned, the most likely deviation is due to a pump that supplies a higher flow rate than your FPR can flow at max, thus causing pressure to rise. Is your measurement with the FPR vacuum hose disconnected?

Thank you guys for responding! Everything you’re saying makes sense. I was just looking too deep into it and everything else for that matter since I’m new to tuning and trying to make sure I don’t end up blowing my car up or having a bad time lol. I do have a higher flowing fuel pump and I did take my measurements with the vacuum hose taken off. I ended up getting an adjustable FPR just so I can dial in exactly what fuel pressure I want and be able to know what my fuel pressure is with confidence so I can use that info on my ECU. Again thank you!

John is correct, the regulators are normally referenced to MAP - bit of a brain fart there, no excuses as I shouldn't have made that mistake.

Pressure guage is normally relative to ambient pressure - might be better to work with PSIA guages, in order to remove the atmospheric variation?

Hi Gord, everyone has them sometimes!