With Classic Japan being a great day 2 weeks ago, I have found again the desire to do something to the HOLK.
Whilst what I really want to do will take me a long time, there are little things I can do on and off over few weeks ends. One of those is the engine bay.
As some of you know, the HOLK runs a F5A DOHC from an Alto Works.
Fuel comes directly from a Bosch High pressure pump (mounted externally underneath the petrol tank) and gets to the EPI: any surplus goes back to the pump via a return line.(no lift pump or swirl pot). The arrangement has worked well for several years, even if not ideal.
I am now thinking of installing an aftermarket fuel pressure regulator, both to improve efficency and, whilst cleaning up the engine bay, for that w#nk factor (I am already replacing all fuel lines with braided ones).
Do you guys know if the F5A Dohc has already an inbuilt fuel pressure regulator? If so, do I need to blank it off?
Should I expect an improvement in fuel economy/efficiency?
Paride
Aftermarket fuel pressure regulator - Any real gains?
Hi Paride 
Yes your F5A has a regulator bolted to the middle of the fuel rail, you would need to remove it & replace it with an adaptor to connect the fuel line to the aftermarket regulator & then return from regulator to fuel tank. The setup on the MYDBOY is a rising rate (fuel pressure rises proportional to turbo boost) which was necessary with the upgrade kit to supply enough fuel at high RPM & boost to prevent lean out. I don't really think you need all of this but be carefull of cheap units which may not work as well as the original. I would doubt that simply changing the reg would effect economy or performance unless the standard one was faulty.
Yes your F5A has a regulator bolted to the middle of the fuel rail, you would need to remove it & replace it with an adaptor to connect the fuel line to the aftermarket regulator & then return from regulator to fuel tank. The setup on the MYDBOY is a rising rate (fuel pressure rises proportional to turbo boost) which was necessary with the upgrade kit to supply enough fuel at high RPM & boost to prevent lean out. I don't really think you need all of this but be carefull of cheap units which may not work as well as the original. I would doubt that simply changing the reg would effect economy or performance unless the standard one was faulty.MYDBOY Co-Pilot
You dont need to remove the stock FPR all you do is fit the new RRFPR in the fuel return line after it. THe stock FPR maxes out and starts to return fuel but as it then goes to the new RRFPR it holds the pressure in line with your setting and compensates for boost.
You probably dont really need one unless you are pushing things to the limit but they are good insurance as they hold a more constant fuel pressure through the boost range particularly when you are boosting to higher pressures than the original factory pressure.
You probably dont really need one unless you are pushing things to the limit but they are good insurance as they hold a more constant fuel pressure through the boost range particularly when you are boosting to higher pressures than the original factory pressure.