ghoonk

Okay guys, need some advice regarding risking rate fuel pressure regulators, e.g. HKS fuel pressure regulator, SARD fuel pressure regulator.

When installing an adjustable fuel pressure regulator (FPR), many people make the mistake of setting the fuel pressure with the vacuum line on the FPR still connected. From what I now understand, the correct way is to set it at a fuel pressure high enough to get the engine running, then disconnect the vacuum hose on the FPR, THEN set the fuel pressure to the 'correct'/desired level before reconnecting the vacuum hose. This will typically cause the fuel pressure to then drop.

For the purpose of this question, let's say we set the fuel pressure to 2.8 bar (40.6 psi)with the vacuum line disconnected.

Now, according to a number of online references from FPR manufacturers, this means that the fuel pressure at WOT is 2.8 bar, and the idling fuel pressure should be lower. In many cases, this would be irrelevant since we should only be concerned with fuel pressure under throttle.

Now, with the understanding that

a. fuel pressure should be set in accordance with the target peak boost level (i.e. to provide the right level of fuel pressure at max boost), and

b. this seems to be consistent with the description of the purpose of the differential pressure warning lamp in the Defi Link Control Unit II,

what is the 'correct' fuel pressure to set (with the vacuum hose off) if I am running 1.5/1.6 (21.7-23.2 psi) bars of peak boost, assuming that I am using stock fuel injectors and a Walbro fuel pump?

Precision

iTrader: (1)

Don't quote me on this but I believe that stock fuel pressure is 43.6 PSI. So set you FP to that with the vacuum hose unhooked as you said. The reason for the hose if for manifold pressure reference. So under low pressure (vacuum) the fuel pressure should drop. However high pressure over 1 Bar (when manifold absolute pressure is higher than 100KPA or 0 PSI(atmosphere))your FPR will add one pound of fuel pressure for every one pound of boost that the turbo generates.

sparky

iTrader: (1)

Why did you install an adjustable FPR in the first place? At any rate, you should keep track of your AFRīs(especially at WOT)after making any changes to fuel pressure at idle. To me this would seem like a senseless mod on most Evos. I am assuming that you have a fuel pressure gauge in your car as well as an AFR gauge.

ghoonk

Yes, I have an AFR gauge. The stock FPR was starting to show inconsistent AFRs in my datalogs and testing showed that it was going out the door. Facing a choice of getting another stock one against using an adjustable one that I already had lying around, I had the shop install this one, along with a new fuel pressure gauge.

I had the fuel pressure set to 2.8 bars (40.6 psi) with the vacuum hose connected (*doh!*) and it peaks at ~4.4 bars (~63 psi) on full boost, and somehow I had a feeling that I didn't do something right. That's why I went back online to check on the correct way to install and set fuel pressure on an adjustable rising rate FPR and realised that I needed to disconnect the vacuum hose before setting it.

Thanks for the quick and informative replies, I really appreciate it tons. Looks like what I'll need to do now is to set the fuel pressure at 43.5psi (any idea why 43.5 psi and not 50 psi or 40 psi?) and focus on the tune

sparky

iTrader: (1)

In the old days we used to tune Buick GNīs by adjusting the FP. Lean was always faster as long as the engine didnīt knock. On race gas we would lean them out to 36-37 PSI. On pump gas we ran a bit richer(about 38-40PSI), The Buicks came 42.5 PSI fuel pressure stock.

50 PSI at idle is way rich. At lower engine RPMīs and idle the regulator would be returning a lot of fuel back to the tank. The higher fuel pressures tend to screw up the fuel trims(old terminology) as well. I hope this helps. Maybe you can get more Evo specific info from some of the other guys though.