We get a lot of inquiries regarding the stock fuel pressure regulator and when its time to upgrade to an aftermarket, adjustable FPR. So we decided to do some testing to show the capabilities of the stock FPR.

We started with a stock Evo 8 fuel pump and then installed a Buschur Racing 'double pumper' fuel kit.

The factory power supply for the fuel pump is a two-stage system. At idle and low engine loads the voltage sent to the fuel pump is lower (roughly 9Vdc). Once engine load exceeds a certain value (generally once the turbo starts adding boost) the power supply raises voltage. This is done for one reason, to prevent the fuel pump from overrunning the FPR's ability to regulate the fuel pressure at idle and cruise conditions.

The test vehicle is a 2003 Evo 8 with stock cams at stock idle.



As can be seen by the results, the stock FPR is capable of regulating fuel pressure at idle with a Walbro 255LPH fuel pump exactly the same as the stock fuel pump when the stock power supply is used. Even when the single walbro fuel pump was rewired for direct battery voltage, idle fuel pressure only went up 3psi over the stock fuel pump. This equates to about 4% more fuel flow at idle than stock.

What was interesting to see was when we tested both double pumper fuel pumps at their maximum voltage with no boost pressure being applied to the stock FPR. The FPR instantly jumps to 74psi! This is why it can be tricky to tune the fiuel maps during the transition from the single pump to dual pumps.

The most important data is the FPR's ability to regulate fuel pressure while boosting. We tested the FPR by supplying pressure to the FPR with an air regulator while monitoring the pressure being applied with the map sensor on our dyno. This allowed us to get accurate readings without having to do WOT pulls with the vehicle. We also monitored the battery voltage to ensure consistent results.

The one problem we found (if you want to consider it a problem really) is that the double pumper setup would jump to 74psi immediately. If you are running 10 psi or 26 psi of boost, you're gonna get 74psi of fuel pressure. This is only a drastic jump once the HOBB switch engages at a preset boost level, below that threshold and the system has the same results as a single walbro 255LPH on the stock wiring harness. Once we exceeded 26psi of boost, the FPR started regulating fuel pressure as expected. All this means is the fuel pressure isn't a predictable linear curve below 25psi. With that said, most people running a double pumper system wouldn't be running less than 25psi so its a non-issue for almost anyone using that setup. Still, good info to know for the data-junkies out there.

Conclusion:
The stock FPR is capable of handling the job on just about any setup while boosting.

The only time the stock FPR will be a problem is when running some higher flowing fuel pumps at idle, like the new Walbro 400LPH fuel pump and such. We will be testing the DW300 and Aeromotive 340 pumps in the near future to see how they affect the FPR at idle.

Walbro 255LPH with stock wiring: OK
Walbro 255LPH with re-wire kit: OK
BR Double Pumper Kit: OK
300LPH+ pumps: should be tested


I hope this was interesting data for some.

 

Wow, this is some great data! Thanks for testing and sharing.

 

Most if not all double pump setups use the hobbs switch on the second pump so it is only on when the boost is above a certain point to prevent this. No one runs two pumps at 14v at idle.

 

Yes, we all know that its not an IDLE issue but it does overrun the FPR drastically once the Hobb switch engages. Even if it turned on at 10psi, it would flow 74psi of fuel pressure. Thanks for your wisdom though.

-Jamie

 

You are welcome for my wisdom. Hahahahaha! I didnt see the last bit of your post where it said it really isnt an issue.

 

This is some wonderful info when you look at it, price wise . After buying the aftermarket FPR and new single pump you might as well get a double pump I think the difference in price is just around $100 est, and you get two pumps IMO.

 

I thought Ted B has proven that with the siphon tube drilled out you will not have overrun problems at idle.
He has a double pumper running rull time (both pumps) but i'm not sure if he has both pumps switching 9v to 12v or not.

Jamie, in your testing did you try returning the fuel to another vessel (bucket etc) to eliminate the siphon tube/return line as the problem rather than the stock FPR?

Cheers, Mike

 

Haha, it's all good.

 

This test was for drop-in solutions. We used the stock return line in this testing. Seeing as a double pumper kit with a HOBB switch setup easily solves the issue, I see no reason to need to modify anything else really.

 

Just for additional data, I just installed a 4' length of fuel line directly to the return line on the fuel rail and had it dump into a fuel container. With both pumps activated, fuel pressure still jumped to 74PSI immediately. I also tested a single walbro pump re-wired at idle with the vacuum line connected and disconnected, there was no difference as well. So this means that modifying the 'h' siphon tube would be a waste of time and might affect the ability to pull fuel from the passengers side of the tank like its supposed to.

 

awesome testing, thank you

Whats your flavour of aftermarket FPR for the Evo?

 

My stock regulator failed after a little less than a year of be subjected to constant fuel pressure spikes from the double pumper.

When the hobbs switch kicked in I would get an immediate rich AFR swing.

I have since replaced my factory regulator with an Aeromotive unit and all is good now.

 

Jamie, be careful about letting the fpr overrun at idle on pumps that have rewired pumps with no low voltage circuit. The higher pressure at idle and possibly cruise will mess with fuel trims.

 

Interesting test. The one piece missing is that there is a lot more fuel flowing through the injectors at 10 psi than there is at idle. This will significantly reduce the load on the fpr. You would not necessarily see 74 psi as the lowest fuel pressure. As flow through the injectors increases, you'd likely see a drop in fuel pressure.

 

Very interesting info, keep up the good work.