16197427 $OD, help!! have the PCM control (reduce) fuel pressure during idle mode

[camaro354]

Calling all you smart ones with regards to the PCM code:

I have an idea of using one of the computers unused electrical ports to trigger a vacuum solenoid to control the fuel pressure... basically like creating a FMU. While most agree using a vacuum referenced fuel pressure regulator is a major undertaking to tune. If we only allow the vacuum to be applied to the regulator while in idle mode with like an egr solenoid inline of the vacuum hose to the regulator, and in the $OD there is already a separate VE map for idle, I can not think of how this could not be a simple solution for rich idle due to high fuel psi requirements of higher HP engines ? With this setup you can command not variable fuel pressure but two set pressures, one for idle and one for off idle. My problem is trying to figure out the best way to achieve the electrical control via the pcm. The A.I.R, EGR, Vari manifold, etc seem like a good place to start. But basically I only want an output while the "idle flag" is set. Is this something that could be achieved with little work to the code or manipulation of existing settings ? Please chime in with your thoughts.

Working on a '94 tbi chevy 355 with the 16197427 $OD computer and 4L60E transmission.

[1project2many]

Vacuum is applied to a port fuel regulator to make sure the pressure across the injector is the same regardless of the amount of vacuum in the manifold. Applying vacuum is like making an automatic pressure lowering device. When the throttle closes, pressure drops automatically. For TBI pressure is maintained at a constant value because both ends of the injector are at atmospheric pressure. There is no need in a stock application to adjust pressure. Using a vacuum regulator on TBI is like turning the injectors into variable flow parts. As vacuum increases the injector delivery per ms of time decreases. This only makes tuning more of a challenge, not a major undertaking imo. You can calculate the change in flow rate per unit MAP then apply that correction to the VE tables. AE and DE will still need to be worked out, as always, because they are independent of the VE calculations.

A vacuum solenoid is a neat idea. I think you might see some potential transition issues but it would be interesting to try it out. Some of the BBC code used a throttle kicker that IIRC would only activate at idle. It would take some research to determine which pcm and code that was in. With luck you could switch to that mask with no additional code work needed. IMO from a coding perspective one should make a table that modifies the injector constant based on MAP then add a bit for "vacuum regulator present" to engage the table. That would allow variable pressure and a corresponding automatic change in flow. As always, the AE and DE tables would need to be corrected manually.

[camaro354]

Thanks 1project2many for the throttle kicker idea, I will have to investigate that further.

Some back story is I have a SBC 355 with edelbrock rpm 195 heads (60899), Flat top pistons, Comp cams 08-464-8 roller cam with 1.6 rockers, edlebrock rpm q-jet intake. The engine should make around 350 hp, maybe more. Presently I have the BBC tbi with stock 350 injectors (Have the BBC injectors too, been back and forth with them both). Also utilizing a Vacuum referenced regulator, but was hoping to not need it to achieve a nice idle....

I have recently fixed my adx file to report the correct duty cycle and now it confirms my injectors have been going static above 4000 rpm. Which is what I had suspected. So I have been bringing fuel psi up and adjusted injector constant to keep the inj DC in the 85% range. However I have a feeling I will end up in 30 psi range to get it the fuel it needs, which is going to make the idle PW very short. Which is well documented on the many forums. This is were my idea mentioned above came into my head. As with the vehicle in gear the inj PW is still in an acceptable range and the only problem is really during idle in neutral.

I will post more as things progress and I have more time to play with this idea, might be chasing my tail (once again) but I have so much time invested trying to get this combo to play nicely. Ultimately my fear is I should have switched to a MPI setup to make the fuel demands easier to meet with regards to available inj DC available with a MPI setup. Guess I will see here soon enough. Thank you in advnce to all of you on this forum and the help you have provided.