Replace Mopar MPFI with Carb

[CJ-8_Jim]

No...You are the one that said that your 2-3psi increase in fuel pressure(flow rate) fixed your part throttle detonation. With closed loop operation, it will ALWAYS target 14.6-14.7:1. So yea, your increased injector "flow rate" would have been pulled right back out by the ECU during closed loop operations. That's how it works...... If you dropped your fuel pressure 5 psi, your closed loop fuel trims would go to the positive side instead of negative(taking away the fuel).

Ahh... there's the problem.... the definition of "flow rate" was interpreted differently by you than how i used it.

When I said "flow rate" I was referring to a "volumetric flow rate" or a "volume flow rate" as used in physics where the period of time is fixed in the denominator. That is, before considering the pulse width that the PCM regulates during closed loop operation to meter fuel to maintaining the 14.7 air/fuel ratio. That is, the same definition used describing injector flow rates (lbs/hr).

It looks like the definition of "flow rate" you have been using is the amount of fuel released after considering fuel trim (the add/minus of fuel) by the PCM. That is, after considering the impact of a shortened/lengthened the pulse width (in time) to achieve the targeted 14.7:1 air/fuel mixture ratio.

No where have I or am I saying that increased fuel pressure results in more fuel (e.g., lbs/hr, cc/minute, etc.) after regulation by the PCM.
My posts are all there.

After some cursory study last night, I suspect the basis of HESCO's claim (and the favorable experience I had at normal partial throttle conditions) is that increased pressure of the same amount of dispensed fuel (but for a shorter pulse width) leads to smaller droplets (increased surface area). Every abstract I've read so far says the same thing: increased pressure (velocity) = increased atomization.

As a result, the increased surface area contributes fewer droplets escaping unburned (end gas) which means less unburned fuel remains to ignite under heat and pressure (detonation).

Here fuel volume doesn't change.... so all discussion and examples of fuel volume are not relevant.

Again, I suspect that HESCO's claim is based on something different than what you are challenging.

I think it would be better for you to ask HESCO directly if increased atomization of fuel under higher modulated pressure why they suggest increasing fuel pressure.

Somewhere on an XJ board I remember seeing a post where someone said that HESCO told them the 258 MPI conversion works best at a minimum 40 psi. Quite honestly, I can't remember what HESCO told me... it's been a long time.

 

[mysunnshine]

Ahh... there's the problem.... the definition of "flow rate" was interpreted differently by you than how i used it.

When I said "flow rate" I was referring to a "volumetric flow rate" or a "volume flow rate" as used in physics where the period of time is fixed in the denominator. That is, before considering the pulse width that the PCM regulates during closed loop operation to meter fuel to maintaining the 14.7 air/fuel ratio. That is, the same definition used describing injector flow rates (lbs/hr).

It looks like the definition of "flow rate" you have been using is the amount of fuel released after considering fuel trim (the add/minus of fuel) by the PCM. That is, after considering the impact of a shortened/lengthened the pulse width (in time) to achieve the targeted 14.7:1 air/fuel mixture ratio.

No where have I or am I saying that increased fuel pressure results in more fuel (e.g., lbs/hr, cc/minute, etc.) after regulation by the PCM.
My posts are all there.

After some cursory study last night, I suspect the basis of HESCO's claim (and the favorable experience I had at normal partial throttle conditions) is that increased pressure of the same amount of dispensed fuel (but for a shorter pulse width) leads to smaller droplets (increased surface area). Every abstract I've read so far says the same thing: increased pressure (velocity) = increased atomization.

As a result, the increased surface area contributes fewer droplets escaping unburned (end gas) which means less unburned fuel remains to ignite under heat and pressure (detonation).

Here fuel volume doesn't change.... so all discussion and examples of fuel volume are not relevant.

Again, I suspect that HESCO's claim is based on something different than what you are challenging.

I think it would be better for you to ask HESCO directly if increased atomization of fuel under higher modulated pressure why they suggest increasing fuel pressure.

Somewhere on an XJ board I remember seeing a post where someone said that HESCO told them the 258 MPI conversion works best at a minimum 40 psi. Quite honestly, I can't remember what HESCO told me... it's been a long time.

No where have I or am I saying that increased fuel pressure results in more fuel (e.g., lbs/hr, cc/minute, etc.) after regulation by the PCM.

You're the one that said it in this post-

2) Fuel pressure. Lower the fuel pressure, the leaner the air/fuel mixture and hotter the burn. When I put an adjustable fuel pressure regulator on mine and increased the fuel pressure, I solved my pinging problem. My block was recently reman, so my displacement may have increased (who knows) aggravating the tendency to ping.

Just my 2 cents.

Jim

Come on......You're the engineer and should know logic well enough to read between the lines of the above quote. Just in that statement alone, you're comparing the relationship of fuel pressures and the effects that it will have on air/fuel ratios. Then by making the note that you have a larger displacement engine which will require more fuel also confirms your implied relationship of additional fuel supplied to the engine. Also, do you really believe that a 4%(2psi) increase in fuel pressure on the stock type injector is really going to effect atomization enough to stop audible detonation? If you have audible detonation, it's going to take a LOT more than your 4% increase in fuel pressure for better atomization. Obviously you're really good at researching the internet for answers, so maybe you can find that information to back up these claims too. You really should reference Allen W. Cline if you're going to recite his information too. (http://www.contactmagazine.com/Issue54/EngineBasics.html) So now knowing what you know now after reading his article, Don't you think that by retarding the initial timing would result in less detonation because the fuel will have a better chance to fully burn before the cylinder pressures light off the rest of the mixture?

Peace

 

[CJ-8_Jim]

Quite honestly, unlike you, I'm not comfortable positively concluding why what happens (or doesn't happen) when fuel pressure is increased on a 258 block has a 94MY HESCO MPI installed.

I do know that in that situation, I followed HESCO advice and it worked to reduce/minimize detonation. There is a correlation, but I don't know why (i.e., what the caused it).

As for quoting my "leaner air/fuel mixture" comment.... yes I said that. That was based on this EFI reseller's comment I provided to you - here it is again for your reference:
"By raising fuel pressure, more fuel is forced out the metering orifice per pulse width. Thus, someone who has performed modifications to their vehicle (i.e. low restriction exhaust, improved air induction, increased boost, etc.) may be near the flow limits of their stock fuel injectors. By increasing fuel pressure, a few more lbs/hr of fuel flow may be squeezed through the injectors to meet engine demand."
http://performancefuelsystems.com/InjectorFlowRatesatDifferentPressures-TechCorner.htm

Since I had already done a number of the improvements he referenced (and perhaps others have not), yes, I was speculating that was a contributing factor.

I still haven't ruled that out (like you have), BUT I have since RULED-IN the improved atomization at a higher fuel pressure.
Perhaps both are at play to one degree or another in my situation?

Regardless, I experienced a correlation with HESCO advice and owners of a 258 block with it's longer rods and MPI (and with/without the 4.0 head) would be best served knowing that.

I just don't understand why someone like yourself who says they do EFI "for a living" doesn't want to call HESCO to understand their point of view.

Back in the mid-1990s, one of my one-year rotations at Ford was monitoring EEC development (but not developing EECs or programming software). I do remember numerous conversations monitoring that team (and other component and vehicle development teams over the years) where optimal calibrations or design under one environment were not optimal under others. As a result many times, components were not always optimally calibrated due to the varying environmental conditions a owner may experience over the warranty life cycle. Why do I mention that?

As I'm told by a former EEC collegue, engine management/injector/etc. sophistication in 1994 is not like today. There was a lot of "slop" back then (a lot less precision). Also to meet CAFE, design and calibrations were generally errored to favor lower variable cost and F&T and improved MPG (unless the product design called for performance) based on the engineer's budget. So there could be "slop" in the 94MY design where changing any variable may lead to an improvement, but it wasn't put into production.

Finally, I also remember it was common in major redesigns that:
- What theory worked on the whiteboard, would not always work on a dyno.
- And what was experienced on a dyno, was not always replicated on the proving grounds.
- And what was experienced on the proving grounds was not always replicated when Ford would drive pre-production prototypes out to Pikes Peak and then to Death Valley and back.
At each step, variances decreased, but there were still meaningful variances. That is why all that was done.

Therefore, I favor starting with theory and then valuing real-world experience.

Peace

 

[CJ-8_Jim]

Don't you think that by retarding the initial timing would result in less detonation because the fuel will have a better chance to fully burn before the cylinder pressures light off the rest of the mixture?

I have never commented one way or the other on the value of moving the CPS.

Please don't feel that my sharing (and trying to explain) my positive experience following HESCO's advice should in anyway discredit your timing approach.

 

[Polarfire]

Ok, obviously you are both well versed in this and I'm sure you both have very valid points and even considering that this is out of my knowledge realm even I know this dead horse has been thoroughly beat.

 

[mysunnshine]

As for quoting my "leaner air/fuel mixture" comment.... yes I said that. That was based on this EFI reseller's comment I provided to you - here it is again for your reference:
"By raising fuel pressure, more fuel is forced out the metering orifice per pulse width. Thus, someone who has performed modifications to their vehicle (i.e. low restriction exhaust, improved air induction, increased boost, etc.) may be near the flow limits of their stock fuel injectors. By increasing fuel pressure, a few more lbs/hr of fuel flow may be squeezed through the injectors to meet engine demand."
http://performancefuelsystems.com/InjectorFlowRatesatDifferentPressures-TechCorner.htm

This above information does not apply because of the feedback system with the O2 sensor will negate the increase in pressure which I had already discussed. As long as it is in closed loop, the above information is not applicable.



Peace-