BlackGT Posted December 10, 2008 Share Posted December 10, 2008 It does not seem like many in the Subaru community increase the fuel pressure to get more fuel into the engine at a given IDC. Is there a reason why not? If I'm at 95% IDC and I want a little headroom for safety's sake, shouldn't I be able to adjust the fuel pressure up 10% and then adjust the injector scaling factor? I realize that the increase in pressure is probably not a 1:1 relationship with flow, but the tuning options we have does support increasing fuel pressure, unless I am missing something.... your thoughts? Jeremy Link to comment Share on other sites More sharing options...
LittleBlueGT Posted December 10, 2008 Share Posted December 10, 2008 I have the flow per pressure chart somewhere at home for my cleaned and flow tested stock injectors. Full tune of 68HTA, KSTech 73 MAF, Racer X FMIC and ID1000s................by the DataLog Mafia!!! Link to comment Share on other sites More sharing options...
rao Posted December 10, 2008 Share Posted December 10, 2008 You need to have a fuel pump that can support the higher flow at the higher pressure. The increase is based on this formula: New injector flow rate = the square root of the new pressure divided by the presure at the rated flow all multiplied by the rated flow Read more here with some calculators http://www.rceng.com/technical.aspx The bottoom line is that it is not the best way to go about it. Besides, according to LittleBlueGT himself you can run 110%-120% duty cycle with no problems at all Rob IF YOU CARE ABOUT YOUR CAR YOU SHOULD NEVER DRIVE IT Link to comment Share on other sites More sharing options...
Infamous1 Posted December 10, 2008 Share Posted December 10, 2008 Besides, according to LittleBlueGT himself you can run 110%-120% duty cycle with no problems at all Save the headache, get a pump and injectors Link to comment Share on other sites More sharing options...
rao Posted December 10, 2008 Share Posted December 10, 2008 Duty cycle is like age - it's just a number Rob IF YOU CARE ABOUT YOUR CAR YOU SHOULD NEVER DRIVE IT Link to comment Share on other sites More sharing options...
BlackGT Posted December 10, 2008 Author Share Posted December 10, 2008 Save the headache, get a pump and injectors Why?, LGT's come from the factoty with a pump and injectors, thats like walking up to a lopey idling car at the drag strip and asking if "that thing got a cam in it?" And what headache? Measure FP, Pull off the FPR, squeeze in vice until small amount of plastic deformation, place back on car and measure FP again. Repeat until target is achieved.... Actually I think it is even easier, they are adjustable from the factory, to what range I do not know. Modding and tuning these cars is all about headaches!, why else would we put on FMIC's and 3" turbo inlets and Walbro fuel pumps and 18x8" wheels where there is obviously no room for any of these? The LGT in question already has a pump and injectors, if the only negative is a headache, then I am so there! Link to comment Share on other sites More sharing options...
BlackGT Posted December 10, 2008 Author Share Posted December 10, 2008 I have the flow per pressure chart somewhere at home for my cleaned and flow tested stock injectors. Genius! If you ever make it to Detroit the beer is on me! So my injector flow sheet has the same thing, and it says my injector suggested fuel pressure operating range is up to 80psi. I plan on running 22 psi (boost) max. So, even using 75 psi FP as a maximum for the injectors, I should theoretically be able to safely set my fuel pressure at a base of 53 psi instead of the lowly stock 43.5 psi. Jeremy Link to comment Share on other sites More sharing options...
BlackGT Posted December 10, 2008 Author Share Posted December 10, 2008 You need to have a fuel pump that can support the higher flow at the higher pressure. The increase is based on this formula: New injector flow rate = the square root of the new pressure divided by the presure at the rated flow all multiplied by the rated flow Read more here with some calculators http://www.rceng.com/technical.aspx The bottoom line is that it is not the best way to go about it. Besides, according to LittleBlueGT himself you can run 110%-120% duty cycle with no problems at all More genius, More beer when you all make it to Detroit! Seriously, there's good housing deals here, the vacancy sign is lit in Michigan... It seems that my injector report uses the calculation that rao presented. I do not believe it is an actual flow measurement at other pressures, but just a calc. Link to comment Share on other sites More sharing options...
rao Posted December 11, 2008 Share Posted December 11, 2008 Let us know how it works out Rob IF YOU CARE ABOUT YOUR CAR YOU SHOULD NEVER DRIVE IT Link to comment Share on other sites More sharing options...
BlackGT Posted December 12, 2008 Author Share Posted December 12, 2008 Alright, last night did not work out as planned. I found what I thought was our fuel pressure regulators, they may not have been pressure regulators, it was dark and 15 degrees and I did not dig around too much... Anyway, there is one on each fuel rail, right in between the fuel injectors, are these the fuel pressure regulators, or some sort of pressure damper? So I popped the plastic cover off them and turned the adjustment screw clockwise 2 turns each, while watching the immediate fuel trims, nothing happened so I screwed them in a bit more. Nothing else happened so I returned them to the stock setting. Jeremy Link to comment Share on other sites More sharing options...
rao Posted December 12, 2008 Share Posted December 12, 2008 Those are dampers Just put a C-clamp on the return line Rob IF YOU CARE ABOUT YOUR CAR YOU SHOULD NEVER DRIVE IT Link to comment Share on other sites More sharing options...
BlackGT Posted December 12, 2008 Author Share Posted December 12, 2008 Yeah, after there was no change in pressure I figured those were dampers... So, smart guy:), where is the FPR? Link to comment Share on other sites More sharing options...
rao Posted December 12, 2008 Share Posted December 12, 2008 Follow the return line to the shock tower and then commence your scientific experiments...... Of, and don't let the fact that the fuel pump runs at 33% voltage at idle throw you Rob IF YOU CARE ABOUT YOUR CAR YOU SHOULD NEVER DRIVE IT Link to comment Share on other sites More sharing options...
testes1010 Posted December 12, 2008 Share Posted December 12, 2008 To the OP, this is more common modification on the 02-05 WRXs where stock injectors get maxed out on the stock turbo when tuned aggressively. Here's the FPR mod on Nasioc. It does work, however I wouldn't try to crush it too much to get the additional pressure and I would recommend picking up a 255lph Walbro if you are trying to get much more increase in pressure. Jeremy Link to comment Share on other sites More sharing options...
05pearl Posted December 12, 2008 Share Posted December 12, 2008 There is a critical point in fluid flow through an orifice when you reach a situation called "choked flow". At this point, it does not matter how much you turn up the pump pressure. I don't know what this point is on the fuel injectors, but I would not be surprised if this plays a significant role in the "injector duty cycle". Link to comment Share on other sites More sharing options...
testes1010 Posted December 12, 2008 Share Posted December 12, 2008 I don't know what this point is on the fuel injectors I don't know on Subaru injectors, but I know that the stock Honda injectors(~220cc/min ea) can handle +150psi w/ AF fuel rails/reg, that was enough to get a turboed Honda hatch into the 10s in 1/4mile. Link to comment Share on other sites More sharing options...
05pearl Posted December 12, 2008 Share Posted December 12, 2008 I don't know on Subaru injectors, but I know that the stock Honda injectors(~220cc/min ea) can handle +150psi w/ AF fuel rails/reg, that was enough to get a turboed Honda hatch into the 10s in 1/4mile. Why would someone who modified a car to run 10s be so cheap as to use stock injectors:confused: Link to comment Share on other sites More sharing options...
testes1010 Posted December 12, 2008 Share Posted December 12, 2008 Why would someone who modified a car to run 10s be so cheap as to use stock injectors:confused: Don't have a clue, but the car had a completely stock OEM honda long block as well. So his only expenses were the turbo manifold/turbo/fuel rail/fuel reg & some form of fuel/timing management. The car was totally gutted & the had a great driver. Link to comment Share on other sites More sharing options...
05pearl Posted December 12, 2008 Share Posted December 12, 2008 Don't have a clue, but the car had a completely stock OEM honda long block as well. So his only expenses were the turbo manifold/turbo/fuel rail/fuel reg & some form of fuel/timing management. The car was totally gutted & the had a great driver. Doesn't sound like that car will be running 10's for long Link to comment Share on other sites More sharing options...
testes1010 Posted December 12, 2008 Share Posted December 12, 2008 Doesn't sound like that car will be running 10's for long This was several years ago, the guy ended up selling it. However the car did run this way for over 2yrs until the next owner rolled it. Link to comment Share on other sites More sharing options...
BlackGT Posted December 12, 2008 Author Share Posted December 12, 2008 To the OP, this is more common modification on the 02-05 WRXs where stock injectors get maxed out on the stock turbo when tuned aggressively. Here's the FPR mod on Nasioc. It does work, however I wouldn't try to crush it too much to get the additional pressure and I would recommend picking up a 255lph Walbro if you are trying to get much more increase in pressure. Jeremy Thanks for the link, that explains exactly how I've increased fuel pressure on the other cars I've owned. I already have a Walbro, I'm just trying to get 10% less duty cycle out of the injectors so I can turn up the boost and fuel and get mo powah. I'll give it a shot and let you all know how it works. Jeremy Link to comment Share on other sites More sharing options...
BlackGT Posted December 14, 2008 Author Share Posted December 14, 2008 Well, so far I have the fuel pressure up to 51psi, and I've managed to take 8% of duty cycle out of the injectors. There seems to be no negatives, at least not yet.... I think I'll keep it here for a while. Thanks for the help everyone! Jeremy Link to comment Share on other sites More sharing options...
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