How different fuel regulators, fuel dampeners and fuel line can affect the tune.
This post is kind of long and it may be common knowledge for some but for me it was all new and I thought it might be useful for other tuners.
In the attempt to archive the best tune possible I have been messing with different set ups trying to make all the mechanical aspects as perfect as I can.
Back in August I had my Jeep (350 SBC, vortec heads, 7747 ECM with TBI) well tuned using a close to stock set up (factory steel fuel lines with short pieces of EFI rubber fuel hose connecting the tank to fittings to frame mounted steel lines and short EFI rubber fuel hose connecting the frame mounted steel line to throttle body steel lines. The only real changes to the system was a 1997 pickup in-tank fuel pump, adjustable factory style regulator and a inline fuel gauge.
This system worked well but the fuel pressure always seemed to fluctuate some. The fuel pressure could very 2 psi from one day to the next. Knowing how this can affect the tune I was always concerned about it and wanted to try something different.
I pulled the factory style regulator off of the throttle body and replaces it with a block off plate, installed a Mallory 4307M regulator
http://www.jegs.com/i/Mallory/650/43...FWxo7AodlF0A6A
re-plumbed all of engine compartment with -6 steel braided Teflon line and went with steel AN fittings. I mounted the regulator to the firewall and re-plumbed the incoming fuel to the throttle body, from the throttle body to the regulator and from the regulator to the return line. I also have a pressure gauge mounted in the regulator. Upon start up the pressure stayed rock steady when revving the motor up or even in gear with the brakes applied and loading the engine up hard the fuel pressure stays exactly where I set it without any variations. So I thought the problem was solved.
However to my confusion when I went for a test drive it ran very erratic running good for a little bit but then acting like it was starving for fuel and the BLM’s would go high in the 160 and as high as 170 range. I felt like it was a mechanical issue caused by the changes I had made. None the less decided to make some changes to the VE table just to see if it made any difference. No matter how much I changed the VE table still same problem. Therefore I was confident that it had to be some kind of mechanical issue.
To try and diagnose the problem I hook up a temporary fuel pressure gauge by installing a Tee into the fuel line right before the throttle body with a 6’ piece of EFI hose running to a small fuel pressure gauge that I placed inside so that I could monitor it while I was driving. After the install of the gauge I went on a test drive to see if there were any fuel pressure issues. The fuel pressure was still rock solid, but a strange thing happened the problem I was having went away and the BLM’s became rich now running in the in the 90’s up to about 110. I went ahead and did a few data logs and got the VE table tuned in getting the BLM’s back around 128. After getting the BLM’s back in a good safe range it ran real good with a very stable fuel pressure.
Now I was even more confused trying to figure out why when I added the temporary cockpit mounted fuel pressure gauge the problem went away. My first thought was the fittings I was using. I had a 90 degree bend right where it hooked up to the throttle body and it was one of those block style 90’s that are just drilled making a hard tight bend. I thought maybe by adding the straight tee between the two had maybe cured a turbulent flow issue fixing the problem. So I decided to order a couple of high flow 90’s to replace them with. The next day I kept thinking about it and was sure there had to be more to it. After doing some searching around on the internet I discovered that most EFI system have some type of fuel dampener engineered into them to counter act the pulses caused from the injectors opening and closing. In a TBI the factory style regulator doubles as a regulator and a dampener. So I decided maybe by adding the 6’ of EFI hose I had created a dampener and that was what solved the problem.
So yesterday when the new fitting arrived I installed them and removed the temporary mounted fuel pressure gauge. To see what happened and sure enough the problem came back. Proving it was a fuel dampener issue.
Now with my new found knowledge I decided to reinstall the factory regulator but I removed the flat disk valve off of the regulator diaphragm so that it would only act as a dampener.
Went out for another test drive and it worked, the only thing was the VE table tune that worked with the 6’ EFI hose as a dampener was now very rich again with the BLM’s in the low 90’s up to about 110. After a few logs and adjustment I have everything back in a good rage again. But I am amazed to learn how much the fuel dampener affects to tune. I haven’t finished tuning the VE table but so far it is one of the smoothest VE tables I have had.
Now that I have it all working good and have some new found knowledge the next experiment is, I am going to attempt to tune it with the vacuum reference on the regulator connected. I have spoken with Dave W. and he said he has had good results using the VRFPR on set ups like mine. It allows me to tune with lower fuel pressure at idle or low load (high vacuum) but when a load occurs cause the vacuum to lower the fuel pressure will rise allowing me to tune the upper end with higher fuel pressure were it is needed.
Stay tuned ,I will post my findings.
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