How can I measure the supply fuel pressure and what actual fuel pressure the injectors are seeing for my 92 RS 305 TBI?

 

Yeah, that's tough. GM never put a convenient schrader valve fitting in the fuel supply to screw in a gague like they did with the TPI engines. In the past I have tapped into the hardline on the supply side and patched in a low pressure (carburetor) gague kit, but that was just for diagnostic purposes and not at all pretty. I'm sure someboedy somewhere makes a bolt-on kit to do this, but I don't recall running across one.

Whatever pressure you're seeing in the (larger) supply line near the TB is what the injectors are seeing. Should be around 12-14 PSI for a stock TBI system of the era.

 

This is what I thought as well, but after speaking with NOS, I was told the supply pressure will be different than what the injectors see. It all started when installing my TBI nitrous kit, part # 05153NOS. In the instructions it says the fuel pressure should be between 15-20 flowing psi, which made no sense to me since tbi pressure is between 9-13 psi. I was then told by NOS that the supply pressure, which comes from tapping into the larger, supply line, is different than what the injectors see. I always thought it would be the same?

 

I'm not claiming to be an expert, but I think that's wrong. I can't see how the supply pressure is going to be different from the pressure that the injectors see.

Here's my train of thought on it, at least. If you just follow the path of fuel, it makes sense. Simplified, it travels from the fuel pump, through the fuel filter, into the fuel meter body, to the injectors, through the fuel pressure regulator, and then back to the tank (I verified this with my 91 Camaro service manual, just to be sure). Now, since the FP regulator is located after the injectors, then that means that the fuel all the way from the outlet of the fuel pump to the FP regulator is going to be at the same pressure. So, no matter where you install a FP gauge on the 3/8 inlet FP hose, it's going to read the same pressure, which includes the pressure that the injectors are at.

If the FP regulator was located before the injectors, then I think that there would be a difference between supply and injector pressure, but that's not the case, with TBI at least.

Does this make sense to anyone else, or am I just overthinking?

 

It makes perfect sense to me, but that's not I was told after speaking with 2 people from NOS. Are you absolutely sure that the FPR is after the injectors?

 

Unless my GM service manual is wrong, than yes. There's a diagram that shows the fuel flow through the TBI unit, and it clearly shows the path (in the inlet line, to the injector, out of the injector, to the FPR, out of the FPR, and out the return line). I just wish I had a scanner so I could post the diagram, but I don't have one right now. Let me see what I can do.

 

Ben, I'm with you on this one. I'm not sure how the injectors could be seeing a HIGHER fuel pressure than what's in the lines. There's no secondary "pump" inisde the throttle body that could bump up the pressure to the injectors that I'm aware of. All the diagnostic work I've done on these systems I've always gone off of fuel line pressure just about where it goes into the TB and then made sure it was in spec.

Not sure why a nitrous kit would change this, unless it was just bumping up pressure to the injectors to add the extra fuel flow to go with the nitrous. But all the TBI nitrous kits I've seen have been wet-flow style and just tap into the same fuel line that feeds the TBI without altering it's pressure.

 

Here's the picture of the diagram from my 91 Camaro service manual. It's a sloppy picture, but you should be able to see how the fuel flows through it.

http://memimage.cardomain.net/member...8_133_full.jpg

I can get a better picture if someone can just tell me how to convert a PDF to a .jpg, because I'm definitely not the most computer-literate person.

 

Damon, NOS is claiming that the injectors will see LESS pressure than what is in the lines, meaning the pressure in the supply line should be 15-20 psi, while the pressure at the injectors should be 9-13 psi. From what I understand they should be the same correct?

 

From what I understand, NOS is retarded. The supply line pressure and the injector pressure are the same. Period. The only two things in the fuel system that create a pressure drop are the regulator, and the fuel filter. And if the filter's clean that one will be negligible.


Here's the kicker. Ford's older TBI systems DO run on about 18psi. In other words, your instructions would be perfect if you drove a ThunderTurd. Makes ya wonder just how much time they actually spend on test-mule installs, doesn't it? Also makes ya wonder if they're just feeding you a line of crap to get you off the phone when you call for a simple clarification.....


Of course, that's just my humble opinion.

 

I emailed them and never recieved a reply.

 

The ford TBI systems run at pressures in the high 30's

 

Hey guys let me help you out. Your supply line does flow at a higher rate. Gm put the regulator presure at 13 psi and solder over the hole to adjust the pressure. The fuel does flow through the regulator then in to the injectors. How do I know this ? I cut one up to find out how to modify it to bump fuel pressure for my 355 tbi. So what are you trying to do with fuel pressure?

 

Thanks for trying to help, but the regulator is indeed after the injectors.

 

The inlet feeds a chamber around the injector that feeds the injector. The regulator blocks the outlet from the chamber to the return line. When the pressure gets over the set pressure of the regulator it opens and bypasses fuel back to the tank.

The fuel pressure in the line right before the TB will be roughly the same as what the injector sees (well, slight change due to very slight frictional losses).

I’m either not following what NOS is saying or what this debate is about.