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turbogrill

Fuel pressure rated 50-59 psi?

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Hi,

 

The 2006 Mazda FSM specifies that the fuel pressure should be 50-59 psi. Why the large range? Wouldn't that cause some cars to run lean vs rich?

 

Or will the fuel trims adjust for this after a while?  How does that work after a tune?

 

When I did my megasquirt installation the fuel pressure would work as a multiplier for the fuel map, so changing the fuel pressure would directly change the tune 

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Injectors determine the amount of fuel going into the cylinder, not the pressure at the rail. 

 

Do NC's have a return line coming off the regulator?

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1 minute ago, turbogrill said:

Hi,

 

The 2006 Mazda FSM specifies that the fuel pressure should be 50-59 psi. Why the large range? Wouldn't that cause some cars to run lean vs rich?

 

Or will the fuel trims adjust for this after a while?  How does that work after a tune?

 

When I did my megasquirt installation the fuel pressure would work as a multiplier for the fuel map, so changing the fuel pressure would directly change the tune 

Generically speaking, older cars have a vacuum controlled pressure regulator, newer stuff has a PWM control on the fuel pump, this is to (among other things)enrichen under load thus a acceptable range. When road testing for a drivibility issue, fuel pressure is monitored to see if it rises or drops @ WOT, yes fuel trims make up the difference for part throttle.

 

For your purposes just go with a non variable set pressure.

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26 minutes ago, Wink said:

Injectors determine the amount of fuel going into the cylinder, not the pressure at the rail. 

 

Do NC's have a return line coming off the regulator?

 

The regulator is in the tank, so it regulates to let say 55 psi to a return less fuel system. Pulse width from the ECU determines the amount of fuel.

 

 

25 minutes ago, Team Infiniti said:

Generically speaking, older cars have a vacuum controlled pressure regulator, newer stuff has a PWM control on the fuel pump, this is to (among other things)enrichen under load thus a acceptable range. When road testing for a drivibility issue, fuel pressure is monitored to see if it rises or drops @ WOT, yes fuel trims make up the difference for part throttle.

 

For your purposes just go with a non variable set pressure.

 

My car doesn't do PWM, the regulator doesn't use a vacuum reference either.

 

Since the ECU doesn't know the fuel pressure the AFR would be pretty inaccurate until the trim values are dailed in. 

 

Seems to be a standard Bosch regulator that is 58psi, seems good (there will be a slight pressure drop as well, it should be 50-59 psi at the rail)

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18 hours ago, turbogrill said:

 

The regulator is in the tank, so it regulates to let say 55 psi to a return less fuel system. Pulse width from the ECU determines the amount of fuel.

 

 

 

My car doesn't do PWM, the regulator doesn't use a vacuum reference either.

 

Since the ECU doesn't know the fuel pressure the AFR would be pretty inaccurate until the trim values are dailed in. 

 

Seems to be a standard Bosch regulator that is 58psi, seems good (there will be a slight pressure drop as well, it should be 50-59 psi at the rail)

 

The neon uses an in take reg as well. For those cars there will be no reference to vacuum or boost, and the ecu has the VE table tuned to compensate for this. They are "non reference", and would require a FPR not be hooked to the engine vacuum. 

 

If you look up the flowrate change of an injector vs pressure you will realize it takes a reasonable amount of pressure to change your afr by .5 (ratio, not lambda). When the stock engine is in open loop (wide open) it is most likely tuned rich enough that several psi error band still results in a richer than ideal mixture. Some cars will trim even the open loop map slightly when it notices the closed loop trim is off, so there is some chance a slight pressure change would be "compensated" for in your stock ecu either way. I wouldn't loose much sleep over a handful of psi. 

 

If you are doing aftermarket ecu there is some debate that making a non reference car reference will make it easier to tune (less change needed in the VE table), but I personally would stick with OEM stuff for durability and tune around it. 

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The difference in fuel delivered varies as the square root of the delivery pressure. Tuning WOT with fuel pressure is crude but works, and these days there are better options.

 

59 psi would result in 8.6% more fuel injected than 50 psi. That would change a 13.5:1 AFR to 12.5:1. One thing to note is going the opposite way in delivery pressure because of a failing fuel pump or clogged up filter can get you into the danger zone fairly quickly.

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3 hours ago, Black Magic said:

 

The neon uses an in take reg as well. For those cars there will be no reference to vacuum or boost, and the ecu has the VE table tuned to compensate for this. They are "non reference", and would require a FPR not be hooked to the engine vacuum. 

 

If you look up the flowrate change of an injector vs pressure you will realize it takes a reasonable amount of pressure to change your afr by .5 (ratio, not lambda). When the stock engine is in open loop (wide open) it is most likely tuned rich enough that several psi error band still results in a richer than ideal mixture. Some cars will trim even the open loop map slightly when it notices the closed loop trim is off, so there is some chance a slight pressure change would be "compensated" for in your stock ecu either way. I wouldn't loose much sleep over a handful of psi. 

 

If you are doing aftermarket ecu there is some debate that making a non reference car reference will make it easier to tune (less change needed in the VE table), but I personally would stick with OEM stuff for durability and tune around it. 

 

Thanks! Makes sense.

 

So the OEM solution seems a little less accurate than I would think. But I guess running rich at WOT doesnt really hurt power much. Just wasting fuel?

From what I can tell from our sensor we are running a lamdba of 0.83 during WOT. So that is 12.2, seems very rich. We also have pretty bad fuel economy. 

 

6.35 gallons a minute for 160 whp.

 

 

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2 hours ago, mender said:

The difference in fuel delivered varies as the square root of the delivery pressure. Tuning WOT with fuel pressure is crude but works, and these days there are better options.

 

59 psi would result in 8.6% more fuel injected than 50 psi. That would change a 13.5:1 AFR to 12.5:1. One thing to note is going the opposite way in delivery pressure because of a failing fuel pump or clogged up filter can get you into the danger zone fairly quickly.

 

Specially if you are running a turbo on your Miata!

Curious why the service manual allow such a range

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1 hour ago, turbogrill said:

 

Specially if you are running a turbo on your Miata!

Curious why the service manual allow such a range

Long and short term fuel trims will more than compensate for the variation. 

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1 hour ago, turbogrill said:

Curious why the service manual allow such a range

 

Because everything in the modern auto industry is driven by the lowest bid?

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2 hours ago, turbogrill said:

6.35 gallons a minute for 160 whp.

So two minute stints? ;)

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2 hours ago, mender said:

So two minute stints? ;)

 

Oops! 

3 hours ago, mender said:

 

Long and short term fuel trims will more than compensate for the variation. 

 

But not for WOT and open loop right? Or do moden cars use closed loop for WOT?

 

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Short term fuel trims are used by the ECU to indicate a trend, which results in an incremental change in the long term fuel trim. That adds or subtracts fuel in an attempt to reduce the short term fuel trim numbers (wants to get those close to zero). That affects the base fuel map.

 

All this happens under closed loop operation. WOT goes open loop and uses the programmed base fuel map, plus whatever the long term fuel trim says is appropriate. That's the "learn" function built into factory ECUs, as you drive around town the ECU learns the correct amount to shift the base fuel map.

 

The problem with this is that the ECU typically needs a constant 12 v supply to maintain its memory, so every time you hit the kill switch the ECU loses its mind memory.

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