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Old 12-07-2007, 07:51 PM   #1
ravenhil
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Question Fuel pressure and its effect on injector performance

Okay this has been bugging me for some time now.

I read from multiple sources that the stock fuel pressure for the WRX (regardless of model or year of manufacture, I suppose) is 2.7 bar.

My first assumption
Stock WRX injectors are scaled to spray 420cc/min at 2.7 bar of fuel pressure.

I also read that it is possible to increase the amount of flow from the same injectors by increasing fuel pressure.

My second assumption
The square root of (new pressure/old pressure) * stock flow rating = new flow rating

So finaly we come to what bothers me.

I read that injector latency, dead time, lag time, whatever, is increased when the fuel pressure increases. However, I cannot find any materials supporting tis argument by means of calculations. I suppose its a long shot, but most automotive calculations helps to minimise the range of lucky guesses. I suppose that fuel pressure CAN increase to a point that one acually get declining returns due to increasing injector latency.

So the real question is, what is the maximum peak fuel pressure that one can go without affecting latency too much?
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Old 12-08-2007, 01:43 AM   #2
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Yes, upping the fuel pressure will make them behave like larger injectors but with a better atomisation of the fuel into the mixture. The flow rate will be roughly equal to your formula but check with a wideband.

You need to have a Walbro 255 pump if you are going to be doing this though IMO.

I run mine at 3.5 bar static.
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Old 12-08-2007, 11:23 AM   #3
ravenhil
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Well that is one way, that or checking the IDC. Any idea how is IDC calculated? I made up a formula but its way off from the datalog.
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Old 12-08-2007, 05:46 PM   #4
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IDC is not a real ecu parameter. It's calculated by the logger. You can tell by looking at the logger defs.

I would just log IPW.
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Old 12-09-2007, 12:18 AM   #5
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okay, so how do you calculate idc?

ipw/(2*rpm/60*1000) ?
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Old 12-09-2007, 01:12 AM   #6
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Unfortunately, enginuity.org is down. There was a whole thread on this topic. Actually, there were several threads, but one was fairly recent.

IDC = rpm x IPW / 1200

Here's the logger definition.


<parameter id="P201" name="Injector Duty Cycle" desc="">
<depends>
<ref parameter="P8"/> <!-- Engine Speed -->
<ref parameter="P21"/> <!-- Fuel Inj. #1 Pulse Width -->
</depends>
<conversions>
<conversion units="%" expr="(P8*P21)/1200" format="0.00"/>
</conversions>
</parameter>
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Old 12-10-2007, 01:18 AM   #7
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Is the WRX/STi fuel pressure regulator a static 2.7 bar or is it relative to manifold pressure?
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Old 12-10-2007, 10:16 AM   #8
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Rasing rate of 1:1 = 1psi fuel pressure per 1psi boost pressure.
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Old 12-10-2007, 11:13 AM   #9
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Keep in mind that even with a rising rate fuel pressure regulator the static pressure seen by the injectors is always 43.5 psi. So when you are pushing 10 psi of boost, the fuel pressure is 53.5, but the pressure in the intake is 10 psi. The net effect is that fuel pressure is still 43.5 above "atmospheric". One way to look at it is the boost pressure in the manifold is helping to open the injector--it's an artificially higher pressure atmosphere.

When you increase the fuel pressure the solenoid in the injector has to work harder to open under the higher base pressure. Because there is more force holding the injector shut, the latency lengthens and heat generated in the injector increases. Most people don't increase base fuel pressure significantly because you will burn out the injectors more quickly and the fact that you only get the square root of the difference in flow.
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Old 12-10-2007, 11:17 AM   #10
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On declining returns, you bet: it's exponentially declining, and if you include the latency penalty, which you have to make up for in pulse width, it's even worse.
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Old 12-12-2007, 11:42 AM   #11
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So how bad is it? I mean, is there any formulas to work out on the declining returns? Say if I tweak it up to 3 bar or even 3.2 bar fuel pressure, will I need to change latency values?
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Old 12-13-2007, 01:42 PM   #12
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There is no set formula. Each fuel pump behaves differently.

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Old 01-20-2008, 04:30 PM   #13
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So when all is said and done(thanks for the graph Mickey) when should one think about upgrading the fpr? Coming home soon and looking into getting this done. Thanks for the info guys! Big help!
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Old 03-10-2008, 11:00 AM   #14
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Man when Chewbacca talks, you just gotta listen!
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Old 03-21-2008, 03:02 AM   #15
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So no change to latency then? Or at least no clear reference at this point...
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Old 07-01-2008, 07:27 AM   #16
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The general recommendation is most injectors do no behave reliably over 72psi. Above that base fuel pressure they can sometimes fail to open fully or over heat and die on you. I have run injectors from 35 - 60 psi with no problems.

As posted above fuel pumps may not deliver enough flow at those pressures so you need to figure your total fuel system capacity as well as the injector flow.

Larry
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