Benefits of Increasing Base Fuel Pressure
 

I'm going through the process of completing the fuel system upgrade on my built 1.9L NA. I've already installed a DW200, ID1050xs, Radium rail, and a new fuel filter. I am now converting all of the stock rubber lines to braided steel and AN6 fittings as well as adding a fuelab AFPR and an ethanol sensor. My power goals are limited by my 5 speed but I'd like to have all the headroom in the world when/if I decide to throw a BMW trans in, so I am curious about whether it's worthwhile to bump base fuel pressure for a little more overhead. I am not extremely experienced with fuel injection, but in theory increasing fuel pressure will increase fuel flow through the injectors as well as improve atomization.

This thread https://www.miataturbo.net/race-prep...ressure-70094/ implies that an increase of base fuel pressure, all the way up to 70 PSI potentially (I am running <20psi of boost so that lands me close to the 90psi rail pressure that Sav mentions). My questions are:

1. When talking about 70psi base pressure, I assume this means non-vac referenced (100kPa)?
2. I may just have to experiment, but as a street car I'd like to avoid having a terrible idle. Is it still reasonable for the injectors to handle this at lower idle PW due to increased rail pressure?
3. How should I take care of re-tuning the car (MS3)? As far as I know, neither base nor sensed fuel pressure is not factored into fuel calculations in any way. Will I just need to take the time to retune my Idle VE and VE tables?

1. Yes with vacuum unplugged. This helps cars idle better, since vacuum lowers the pressure. However your 1050x injectors work wonderfully at low PWs.
2. See above. However standard measurements are usually in bar, 3, 4, or what you’re referring to, which is probably 5 bar (72.5psi)
3. If you have a fuel pressure sensor, you can add it to your fuel calculation, but if it isn’t already added, I don’t think it’ll do you any good. Rough calculation would be 43.5/70=.62, multiply your fuel table by .62, otherwise known as 38% less fuel. The rest of your dead time/ait/clt corrections may through this number off a bit, best to aim rich than lean.

that all being said, i don’t think anything more than 60psi is needed, and beyond needing a re-tune, there aren’t any down sides. Keep in mind 60psi and 150x’s are capable of ~475whp, even at 90%DC.

Just what I wanted to hear! Sounds like I just need to set my base pressure at say 60 to 70 multiply my fuel tables by 43.5/(60 to 70) and then run VEAL a few times and smooth!

Run some tests at idle first. Leaving it at current fuel pressure, multiple your idle area by 1.1. AFR should change by the same factor. So 14.7 would be 13.3. Do the same but multiply by .95, so 14.7 would be 15.4. If that works, then your dead time settings/afr gauge are decent. Then change fuel pressure to 60, multiply by .62, and your AFRs should be the same. if not, it’s probably not as a direct relationship as were assuming, and you’ll have to mess around with it a bit to get a decent starting correction factor.

Ummm, the cool kids use req fuel.

Increasing base fuel pressure provides you with no real benefits until you need more fuel. ID1050s can already kill a 5 speed nearly twice over, you will probably end up with less than 50% duty cycle on a 5-speed safe tune. You will already have to tune/retune when you increase power later with a tranny swap.

Don't do it.

Only fear I have with req fuel is that if anyone ever changes it using the calculations it will be incorrect. Also, it has less granularity being a much smaller number limited to one decimal. And yes, you could also go back and argue that ID1050xs are overkill for a 5 speed, and a built motor is overkill for a 5 speed, etc etc. I like my projects even if I won't see an ROI right away.

What you said here really doesn't make much sense, sorry.

Req fuel and VE both have 1 significant figure, so there is no "granularity" (resolution). Using req fuel gets you closer to true VE, which has advantages. There is no ROI, only inferior idle and a reduced dynamic range in your control system due to a reduction in the maximum duty cycle you use.

When you increase power later, you will retune. You leverage nothing.

Edit: clarify 1 sig fig after decimal point

"Can I increase the base fuel pressure?" "Sure, why would you want to?"

Increasing fuel pressure will decrease idle duty cycle and that will decrease idle quality. IDs are high quality injectors, so perhaps (hopefully? probably?) that decrease will be imperceptible, but it will be a decrease nonetheless.

The gains are purely theoretical. It's not going to do anything to make the car run better today, and there's some small chance that maybe it will mean a tiny bit less work in the future when you make some other changes and want more power. Odds are, though, that you'll have changed enough other stuff by then that you'll be redoing the fuel map anyway.

Perhaps it's useful as a learning exercise?

--Ian

Not trying to be a dick here, I communicate with a certain level of clarity that frequently comes off as harsh. It is highly beneficial IRL for me.

Sig figs are irrelevant in this sense: If my req fuel is 3.0 and I decrease it by the smallest possible factor (0.1), that is an overall fuel change of 3(.3333)%. Any change to fueling via that method must be made in 3% increments. If I modify a VE value of 60.0 and decrease it by the smallest possible increment (0.1), that gives me a ~0.1% change in fueling for that cell. Granted, there isn't a huge difference between reducing fuel by 60% and 61.5% for example, but the VE table would need to be retuned regardless so changing req fuel is really just an extra step right?

I agree, I am not gaining anything by doing this other than the theoretical gains from fuel atomization, which are likely negligible. My goal here isn't as concrete as making more power, I just think it would be an interesting thing to try since I've got the AFPR anyway. At least I can say I used it for something!

Also per last post, no hard feelings. I don't mind the curtness of folks on this forum, if anything it has kept me from making stupid mistakes that kinder posts may not have prevented.

TLDR for this thread: Learning exercise is a good way to put it, there are no tangible gains to be made from this in my circumstance.

Hmmm, yeah, well I can't argue with that logic, except that ultimately it is the product of req fuel and VE that is used in integer math.

Additionally, when you reduce all of the values in the VE table, you are devaluing each available increment. So let's say your max VE is 100.0 pre-scaling and 62.0 post scaling. You now have only 620 bins for your dynamic range where as before you had 1000. So with your 3.0 ms unchanged req fuel, each bin in the VE table now changes the PW by 3.0/620 or 0.005msec. Working the other direction, scaling 3.0msec and rounding equates to a req fuel of 1.9 (instead of 1.86), but we still have 1000 bins in the VE table, so 1.9/1000 or 0.002msec per bin. I don't remember off of the top of my head what the PW resolution on the MS3 is, but if it is less than 5usec, req fuel looks like a win.