190 vs 255 and FPR stuff [Kevin, proceed to enlighten!]

ok, i have been told more than a few times that a 255 does not need a FPR, and more recently Joe said he has some of the same info. now, being a healthy poster on this site, i heavily believe every keystroke from Kevin, and would never doubt his infinite wisdom in the mysterious ways of the DSM and 4g63 [blah blah blah ;)]. that said, please enlighten us as to why exactly an FPR is needed so that i can go back to the people that are telling me what they are telling me and i will have some hard info to the contrary. mainly what you should be looking for in data-logs and such, and the mechanics of it.

thanks :)

LMAO, sweet thread. Alright, because I'm lazy, heres a cut and paste from the other thread. Its a good idea to move this discussion here since its getting a bit off topic over there... I Wish I had seen this thread first :D

Any 255 does in fact need a AFPR. I have the datalogs to prove it. The 190 does as well but in a very minor way. You can certainly run it without a AFPR. So what you read on Talk is correct, but you can much more easily get away on a stock reg with the 190 than with the 255.

A bit more info on the 255 vs 255 HP. The HP version is capable of flowing more at High Pressure, hence the name. At idle, there is no high pressure involved, so there is little difference between them as far as AFPR goes. Any 255 will require one.

Of course some people say "my car is fine with the stock reg and 255," but that just tells me they dont have a logger, dont know what they are looking at, or dont care about getting the car to right as intended in closed loop. Which is fine. Just beware

Its all a matter of how you define "need."


I personally would do the FPR upgrade for a 255, but let it slide on the 190.



Again, it depends on your definition of "need." Will the car run without an AFPR and a 255? Absolutely. Will WOT performance be affected? Not at all. So its no wonder the "fast guys" say you dont "need" one

Think of the high base pressure you get from a sotck reg and a 255 as boost creep on your turbo. You can rightly call it fuel pressure creep. I used to get boost creep to 18 psi on my 14b. The car ran well with 18 psi, so was anything wrong with that? You could say no, because it ran fine, or you could say yes, because I had limited control over boost, and the WG wasnt doing its job. Same thing with fuel pressure creep. The issue is that more often than not, the fuel pressure creeps enough that your fuel trims are completely bolixed and closed loop runs like shit, and economy goes down the shitter. If you are tuning logger/afc style, you will jsut adjust your low trims to try to make up for it. If you are running DSMlink however, the problem is so obvious, it'll drive you nuts. Trust me

So again, it all depends on how you look at it, and what your goals and concerns are. For a low HP daily driver I dont think its worth running around with banjaxed trims 99% of the time. For a track car that spends most of its life at WOT, who cares. For street cars I personaly suggest using a FPR for the 255, or if thats not in the budget, and you are sure you arent going to go over say 120-125 mpg traps, just go with the 190 and drive it on. I've seen it do 124 mph, very capable pump. And the fuel pressure creep is minimal. Without DSMlink you would be unlikely to notice it.

I hope that helps. If not, let me know. Its one of those subjects where there is no "right" answer though, and whatever opinion you take, you cant be right or wrong. One should be sure he understands all of the implications either way though before running off at the mouth, as I see so often on the internet forums...


As I said there, with DSMlink you can see all kinds of stuff you can see otherwise. I never would have thought my 190 was over running the reg (a little) with the pocketlogger and AFC combo. But upgrade to DSMlink, and you can see it. With the 255 it was just pissing me off. Now, I did the FPR before the return line, but I wonder how well it would work with an upgraded return line... Nah, that reg is tiny. Just like with boost creep, it can be the WG valve itself, or the O2 housing passage down stream...

The problem with finding this stuff with regular loggers is that if you have to take out fuel on your low maps, you dont know if its FPR overrun, or just some quirk in your setup. But odds are if you have to go leaner than the standard correction for your injectors (old injector (450) divided by new injector, minus one = some percentage) and you have a bigger than stock pump but stock FPR, you're getting fuel pressure creep, FPR over run, whatever you want to call it.

And again, there is no right answer, since it depends on what you mean by "needing" an AFPR. I've given up the arguement, but still try to make sure people that take either side fully understand what it is they are actually debating. Its not wether or not you need an AFPR with a bigger pump, its wether or not you give a shit about the resuting condition :D

that is exactly what i was looking for!


mark it another one for the 'read this before you post: FAQ' ??

please do!

I fully agree, however I also should state you can run reasonably well without an aftermarket one with a big pump. As I said in another post:

"RE: Fuel pumps....Years back I ran the Mazda Denso big FP( flows 250 LPH@43PSI 12 volts) with stock injectors, stock regulator on my 14b 1g Talon, no electronic controllers either, just my personally hacked MAF.

I ran 22 psi boost on race gas for consistent 12.4@110 times. The also car ran great around town on 93 pump gas at 18- 20 psi. I ran like this for 2 years and 40,000 miles plus many track runs with no problems. So I'm not convinced you need aftermarket regulation on a big pump to run reasonably well, certainly it would be good and the better thing to do however."

Now that's not to say it wouldn't have run better with one, but...I had no problems whatsoever and power was great from idle to redline in all gears.

point being: how much $ do you want to spend, acheving accepatable tunning results.

Let me restate what I said earlier. Running a large pup and stock reg has nothing to do with WOT performance. :) Its about closed loop operation, which you dont care about on a race car but probably do on a daily driver. ;)

Lets say with a 255 and stock reg your base fuel pressure is 55 psi, and we will assume this is a 2g that requires 43 psi of base fuel pressure. At 20 inches of vacuum, you should be at 33 psi base fuel pressure with teh hose on (1 in/hg = .49 psi). Instead you are at 55 psi becaseu the reg cant bypass enough fuel to maintain set pressure (sound familiar? same thing as boost creep). You will be flowing at least 10-15% too much fuel, and the 2g ECU has 12.5% range on its long term trims.

As you raise boost fuel pressure should rise with it as well. Just like above, where it should drop with manifold vacuum. If you are at 5 psi boost, and base is 43 psi, you should see 48 psi. You will still have that 55 psi. Not until you reach 12 psi of boost will you be getting the correct fuel delivery rate. At 13 psi of boost you'll see 56 psi fuel rail pressure, and it will now rise properly (1 psi fuel pressure to 1 psi boost pressure, 1:1) to your set boost. So at anything over 12 psi everything is normal. Below that point (whatever it may be on the car in question) it will be too rich once the ECU runs out of range with the trims.

In my personal expereince datalogging and watching fuel pressure, a 190 will overflow the reg but not enough to put the ECU out of its range. The correct AFR can still be maintained by the ECU. A 255 on the other hand will cause you to run rich. With DSMlink this is painfully obvious, it jumps right out at you. On a pocketlogger or similar its harfder to see and you will just compensate for it in your tuning.

So like I said before, yes it will work with a stock reg, its just a matter of how you want it to run... In closed loop. Open loop/WOT is unaffected.

this just gets better and better...

Jake! Joe! one for the books...