blau928

Hello everyone,

Does anyone know the sensitivity of the LH Idle control pulse width signal to the fuel injectors? 1ms, 3ms, 5ms..? In other words, the signal from the LH Brain adjust the fuel injector pulse width in increments of how many milliseconds?

John S, this may be up your alley..

The other Rich Andrade as well..


Thanks,

John Speake

The incremental size is more like 0.025mSec..............the accuracy of the pulse has to be with 2% or so, so for a nominal 2mS pulse, that is 0.025ms.

blau928

Hi John,

Thanks for the answer.. Is it possible to remap the idle loop for 36lb/hr injectors..?

Rich9928p

In reality, the idle speed is maintained by the idle speed control system. The idle speed valve receives ground pulses of around 100 Hz from the LH to control the amount of valve opening. To increase idle speed, a bit more air is "leaked" into the manifold, and to slow down the idle the bleed air is reduced.

John Speake

Hi Rich9928sp,
The problem is that the idle system and the O2 loop will fight each other if the injector pulse isn't near enough the correct width to set the mixture value for the desired idle speed..............

Rich,
Yes, but what fuel pressure do you want to run at idle ?

Rich9928p

I think we all are in agreement. Porsche created a fuel system to work within a specific fuel pressure range and pass smog requirements. If you increase fuel pressure you're moving into an area that may not be able to be controlled by the idle speed and fuel injection pulse width.

With any of the electonic controlled systems, changing one item may have unintended consequences on other areas of the system. Experimentation may provide the answer. Certainly Porsche hasn't provided any information about how things work outside the intended area of operation.

The question is why put in higher flowing fuel injectors? Unless sufficient air flows through the cylinders to match the additional fuel flow, there is no benefit.

- Much less restrictive intake manifold
- Changing to a stroker
- Supercharging or turbo charging
- Higher compression

are modifications that could require more fuel flow than the stock system is designed to provide.

How to get more fuel flow and pass smog tests and also enjoy good performance in all the cold-start, hot-start, slow driving and full acceleration modes requires further system modification or changing to a different injection and spark control system.

blau928

Hi John and Rich,

Thanks for the notes.. I am curious about interpolating the control data points for 36lb/hr injectors, or possibly larger such as 42lb/hr. Yes, this is for smog purposes at idle. I will be supercharging my car, and will be flowing approximately 1000 cfm at WOT. At idle, I have to calculate, but I think twice the NA amount due to the blower..

Twin Screw Autorotor...

If I can remap the idle parameters to pass emmissions, it may be really great. I am sure I can do it with a MOTEC or Electromotive, but I want to keep the system stock for now...

Thanks again,

John Speake

QUOTE: At idle, I have to calculate, but I think twice the NA amount due to the blower..

Hi Richard,
You won't need twice the injector open time, because the s/c doesn't have a high loading at idle ?

Also, a very large part of the injector pulse is just to get the injector open. The "useful" on time is much shorter. So if you needed to increase the idle pulse by (say) 20% , then that would mean an increase of only 5-10% of the total pulse length.

regards

2V4V

As Richard said, your fuel need at idle should remain unchanged if the blower is the only mod you are concerned with fueling. Unless you are doing something *really* radical, you should have no boost at idle. The other airflow mods - exhaust, heads, cams and what not, will have some effect fueling needs at idle -- mainly because of their effect on MAP and Ve, not because of actual performance under load.

Idle tests are pathetically easy to pass. If you run cats and programmable EFI, *almost* anything can be tuned to pass an idling tailpipe sniffer. Rolling road is a different story...

Also, as Richard said, at idle a good chunk of the total idle pulsewidth is just popping the injector open. 928s tend to have ~stoich idle pulsewidths in the ~2 ms range.(Lower end of 2 for L, up a bit for most LH).This of course presumes you are using the factory '2 squirts per putt" pattern, and have injectors that are basically sized correctly for a NA engine. L-Jet idle pulsewidth is a bit lower than most would like to tune to because it has injectors sized for ~325 HP at the crank.

The interesting part of that ~2.XX ms pulse is that around 1 ms of that time is time required to get the pintle off it's seat and fully open. So, when you adjust your pulsewidth, you are really only adjusting the remaining 1.XX ms of it - the rest of the time is required by the injector to get fully 'spritzing'.

And, of that ~1 ms, running about 25% PWM seems to work very well. At least, on the low-z injectors.

Greg