Injector Question - 2.4L swap

Working on a DOHC or 2.4L swap, or maybe you have some other idea and have some questions? Feel free to ask them here...
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DOHCRT
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Post by DOHCRT » Sat Jan 16, 2010 9:20 pm

Low-Speed wrote:you have 2 tables. One for spark, one for fuel. VE doesn't directly factor. The mappings on the fuel table and spark table correlate a MAP and RPM point with a fuel injector pulsewidth. Its a very simple system. The MAP indicates load, at a given RPM.
The ignition table has nothing to do with what we're talking about. VE has everything to do with what we are talking about. That's why the fuel map is sometimes referred to as the VE table. The PCM uses the table values to modify the base injector pulse-width based on what point in the table the engine is operating at. The base injector pulse-width is the amount of fuel required to produce a stoich mixture at 100%VE and a certain temperature, given a specific injector flow rate and engine displacement.

If you understand what I just wrote, your theory of Chrysler switching injector sizes between model years becomes more plausible. A switch to a different size injector would simply require a modification to the base pulse-width rather than having to modify the entire VE table. I think its worth while to investigate further into it.
The reason 25 percent is the limit regardless of open/closed loop operation, is the factory calibrates for 75% duty as the normal max on the injectors. That leaves margin. from there, it applies a percentage richening based on IAT, etc. But, the table runs a percentage.
In open loop operation, the PCM has no feedback from the O2 sensors. It has no idea what the air/fuel ratio is. Since airflow is calculated based on pre-determined values as I explained earlier, it will not provide enrichment if airflow is increased over what the stock engine normally generates. This is where a piggy-back computer can help control enrichment (as long as the MAP sensor range is not exceeded).
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Low-Speed
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Post by Low-Speed » Sun Jan 17, 2010 8:30 am

Your VE table, or fuel table, is ampped out in terms of rpm and MAP. From there, a global change in pulsewidth affects all the cells, as does a global change from IAT. Basically, the values will have to change across the board, or as you point out, the car will run rich or lean.

I don't know anyone who's dumped and hacked a chrysler pcm. But honda uses values up to 82% of the maximum pulsewidth in their stock maps. I understand there is no feedback fom the o2, but there's an 18 percent margin for cold IAT, ande any other richening factor, such as cold idle, etc.

Given that the pcm would have to be altered for injector changes, considering the motor wasn't, its sound logic to think it would work. It also shows why some only need a 58 regulator, some only need 24 injectors, and some need both.
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Post by DOHCRT » Sun Jan 17, 2010 11:22 am

Low-Speed wrote:I don't know anyone who's dumped and hacked a chrysler pcm. But honda uses values up to 82% of the maximum pulsewidth in their stock maps. I understand there is no feedback fom the o2, but there's an 18 percent margin for cold IAT, ande any other richening factor, such as cold idle, etc.
There has only been a handful of people who have ever unlocked a Chrysler PCM. Chrysler seems to think they need to protect their PCM's more than any other company. IAT is more a factor of ambient air temp than engine size. I would not expect to see any noticeable increase or decrease in IAT between a 2.4 and a 2.0, especially if the 2.4 has the 2.0 DOHC intake manifold.
Given that the pcm would have to be altered for injector changes, considering the motor wasn't, its sound logic to think it would work. It also shows why some only need a 58 regulator, some only need 24 injectors, and some need both
It certainly is worth gathering more info on. You may want to post up your injector size findings on the big .org and see if you can get any valuable input.
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