Base fuel pressure adjustments

**asshanson** · 01-07-2009

I have a question about how different base fuel pressure affects the need for global injector compensation, in this case I'm tuning with DSMLink.

Consider the following situation hypothetical, I haven't changed anything since Shane tuned my car this summer.

My 2g has 43.5psi base fuel pressure, quite a bit higher than the 37psi base for a 1g manual. Since I have more than enough injector for my setup, I could *try* and get more out of my pump by putting more load on the injectors, and lower the base to 40psi (about 8% lower).

1) Does this mean I need to raise my global injector compensation approximately 8%, or does it not work like that? If it does work, will this global compensation take care of the necessary corrections, or will it act differently under a lot of boost and need a complete retune?

2) Will lowering my fuel pressure actually let me get more room out of my pump if I ever max it out? Have 2g owners like C3L1CA who maxed out their single 255 pump tried this?

**Halon** · 01-07-2009

Quote:

Originally Posted by asshanson

2) Will lowering my fuel pressure actually let me get more room out of my pump if I ever max it out? Have 2g owners like C3L1CA who maxed out their single 255 pump tried this?

Yes. If your pump is (or nearly) maxed out, and you still have plenty of injector, lowering the Fuel Pressure will let you squeeze a tiny bit more out of your setup.

**Shane@DBPerformance** · 01-07-2009

Going from 43.5psi base to 40psi with 1600cc injectors is about a 4% change in fuel flow. But it might only be a global change of 1-3%. It might effect it differently at idle than at full throttle where deadtimes make a big difference, but a change of only 3.5psi in base is not a huge amount. Essentially, if those injectors actually flow 1600cc at 43.5psi, then at 40psi they would be rated at 1534cc.

C3L1CA · 01-07-2009

I'm cheap so I'm still on the stock regulator, so I can't play with base pressure at all.

I just picked up a boost a pump over the winter to hopefully get a little more out of the single walbro but it'd be interesting to see how much more you could get out of it your way.

**asshanson** · 01-08-2009

Quote:

Originally Posted by C3L1CA

I'm cheap so I'm still on the stock regulator, so I can't play with base pressure at all.

I just picked up a boost a pump over the winter to hopefully get a little more out of the single walbro but it'd be interesting to see how much more you could get out of it your way.

LOL, I bet that FP turbo setup wasn't cheap

Is that boost a pump always on, or is there some switch where it increases the voltage? Just curious if you're going to overflow the stocker.

Shane, I'm curious why an 8% reduction in base fuel pressure is only a 4% flow reduction. Is there some formula based on injector size? Why do deadtimes make a bigger difference at full throttle, I thought it was more of an idling thing.

I probably just need to find a good read on injectors.

**Shane@DBPerformance** · 01-08-2009

I meant that the deadtimes affected it a lot at idle, often more than anything with huge injectors, I had just worded it poorly above.

Here is the formula to find injector flow with changes in base fuel pressure. Injectors are usually rated for 43.5psi.

SquareRoot(New Fuel Pressure / Old Fuel Pressure) * Old Injector Flow in CC = New Injector Flow in CC

SQRT(40psi/43.5psi)*1600cc=1534cc

1600/1534 = 4.3%
1534/1600 = 4.1%

**asshanson** · 01-08-2009

Good info, thanks. Now I'm off to look at walbro 255 flow charts to see if the 3-5psi reduction in base pressure will affect flow very much. Assuming 27psi boost pressure, my total would be anywhere from 64psi (lowered to 1g base of 37psi) to 70.5psi (stock base).

**Halon** · 01-08-2009

http://www.roadraceengineering.com/f...pflowrates.htm

Not sure if you remember that old link, but may help a little in your research. According to it with the 255HP, a 15psi difference in Fuel Pressure looked to change the flow by about 20 Liter/Hour. So lowering your FP 3psi would net you an extra 4 Liter/hour from the pump.

**asshanson** · 01-08-2009

http://www.max-boost.co.uk/max-boost...ensowalbro.gif

On this chart it looks like only a 10lph increase for lowering the base pressure 6 or 7psi. It looks like upping the voltage increases the flow 40lph at 70psi, and an increase of over 50lph at 64psi, seems like a MUCH better option.

That's between a 15-20% increase, I'm interested to see how well the boost a pump works for C3L1CA.