Also, if it is overrunning the fuel pressure regulator, if some of that fuel that can't be returned starts going into the engine as RPMs increase, then fuel pressure would drop back to base fuel pressure. Basically, once you are at 2000 RPM some of that fuel that is overrunning the fuel pressure regulator goes out through the injectors allowing the fuel pressure to drop. I'm trying to figure this out as well. What would be nice is if someone has a fuel pressure guage in the car and is using the stock fuel pressure regulator, then we could see what really happens.

-Paul

 

If the return line is overrun, the rail pressure increases. This is not a good situation and will cause the ECU to begin leaning the A/F (complicated explanation), but AFAIK, this isn't an issue with EVOs.

 

HEY, Here is an Independent test JUST for this FPR theory

https://www.evolutionm.net/forums/sh...0&postcount=55



And a Dyno to back it up Look at the A/F's They are WAY rich across the board, they can't even get lean down low!!! Look at run #017 in teh red. Look at the 3000-3500 RPMS!!!! it is suppoes to be leaner, it can't because the Fuel Pressure is backed up and stuck . Look at how smooth the STOCK fuel pump run is.

 

My point is NOT that you will make more or less power with a Stock or Aftermarket FPR, my point is that Linear Fuel control is LOST, and that is a VERY important part of tuning.


So you lose proper fuel control and force the injectors to do the job of the FPR. IT CAN be tuned, but it is 100%.

 

What I find interesting about the dyno sheets is that the A/F ratios are pretty close with both pumps at lower RPM and only seem to deviate at higher RPM. This does not indicate that the 255 is overrunning the stock FPR, but more that the stock fuel pump can't keep fuel pressure up at higher RPMs and high boost.

-Paul

 

NO, look at run 17, it isn't even CLOSE ESPECIALLY down low. The one with the stock pump starts at 13-1 right before boost hits the Walboro starts at 10-1 , that is a HUGE difference, huge. That'll kill your spoolup for sure. The stock Fuel pump is MORE then adequate for these mods and is good till about 350WHP in fact. They are only in the 290WHP Range and is NOWHERE NEAR maxing out the stock pump, fuel is NOT an issue, fuel control is. Don't forget that this Car is FLASHED, they only added the Fuel pump after for this comparison. Stock injectors and all running the SAME tuned pulse and MAP, it's the Fuel pressure that changed man.

Again Look at run #017 and TELL me it doesn't look rich. Look at the difference in fuel for off boost (3000-3500) and then you have a turbo Spike and then it just fattens up THAT MUCH? Dude that is an increase in Base Fuel pressure across the board. It has to be as the tune did NOT change, nothing changed, when you bump the Fuel Pressure, your injectors act larger and squert more fuel, which is EXACTLY what is happening here.

 

I'm not sure that the difference is that substantial at the lower RPMs. The variation between the two runs with the stock fuel pump is greater than the difference between run 16 and the runs with Walbro. Run to run variations could account for part of the change, as runs 15 and 16 are so different. Also, the RPM at which the runs were started has a huge effect on the A/F ratios for the first 500-1000 RPM after starting the run. Regardless, I don't think the results of Al's test can be applied to your concerns about base fuel pressure.

-Paul

 

There are two possible situations here:

(1) The 255lph unit is overrunning the return line.

or

(2) The factory fuel pump cannot maintain a linear rise in pressure around peak flow rates (peak hp), and this is accounted for in the factory A/F tuning.


Don't be so certain that (2) is not the case. This wouldn't be the only car I've seen that came stock with an under designed pump.

 

Ok, stock pump BEFORE boost hits, at 13-1, so we KNOW it is not a fuel starvation issue but simply a TUNING setting to increase spool and you don't need that much fuel off boost.

Add A larger fuel pump that flows a LOT more, it drops to 10-1 and probably BELOW at idle, that is a HUGE UGE difference, can't make it much bigger.


If you read the A/F curver from start to finish, and understand that nothing was changed on the car other then a fuel pump, you would realize that Fuel Pressure is increased accross the board.

Again at the VERY end of teh run, notice that all 4 A/F just SHOOT up, you mean to tell me that the 255 is running out of fuel too? No way, and neither is the stocker, it's the fuel pressure.

 

Actually, if (1) was the case, the FP would likely be higher across the board, and this would be reflected in initial dyno runs until the ECU would begin compensating, which would show gradual leaning in subsequent runs (I've seen this).

The more I think about it, the more (2) looks to be the case.

Regardless, it illustrates why returning is required with the larger pump. In theory it shouldn't be that way, but theory doesn't account for the factory pump not being able to maintain the pressure margin at high flow rates.

 

The reason I believe it to be (2) is that the A/F ratios with the stocker and the Walbro converge around 4500 RPM and then begin to deviate. If the return line were overrun, the A/F ratios would be farther apart at lower RPM and would then converge at higher RPM as less fuel would need to be returned. This does not mean that the stock FPR isn't being overrun at lower RPM and load, it just means that I don't think the dyno sheet proves the point you are trying to make.

-Paul

 

Agreed. If the return line was being overrun, the A/F would immediately be significantly richer everywhere, especially at ranges where fuel flow is low (e.g. low rpm), because a higher percentage of fuel is being shunted toward the return line. (2) gets my vote, and it doesn't surprise me.

 

Take a look at your STOCK FPR, it is about the size of your thumb, there is NO WAY it could return all that fuel, you base fuel pressure is rasied, it has been tested to be bumped from 43.5 to 52+, that is huge, especially at idle and vaccum.

Look at the A/F curves, ESPECIALLY at the end of the run, both shoot up towards lean, that is the tune, the only difference is the Fuel pressure. Look at HOW RICH it is, IDK how you can't see it at 3000 RPM's?????


10-1 Vs. 13-1, comon now

 

Are you guys blind, look at the 3000 RPM's plot, 13-1 Vs. 10-1 can't be argued.

Also do not forget that this is a FLASHED Evo, if you look at a stock Evo's Fuel curve, you will see it hit 10-1 and below. Now if the bottom A/F's were IDENTICAL, and only he TOP was richer, I would be more inclined to believe that the Stocker can't keep up, look at how rich it is down low, how can you not see this?

 

I see what you are saying. I'm not disagreeing with you that the stock FPR is being overrun. I think it probably is at low RPM. I'm just saying that the dyno sheet does not prove that. I've found through my data logging that if I start my runs below 3000 RPM, I never get any richer than 10.8:1, but if I start them around 3500 RPM and boost builds faster, my A/F ratios drop to 10.3:1 and then recover. If the dyno sheets were started around 2000 RPM, I think we could get a much better idea of whether or not the stock FPR is being overrun.

Just to summarize, my point isn't that the stock FPR is not being overrun at lower RPM, but rather that the dyno sheet does not provide adequate evidence that it does.

-Paul