I am running a TPI unit from Affordable Fuel Injection based on a 1990 speed density unit. I have been chasing my tail with a part throttle miss for almost a year now with no success. I checked my fuel pressure last week and it was 32 at idle (vacuum connected) and 37 with the vacuum disconnected. I taped the guage to the windshield and the fuel pressure was steady between 32 and 37 during all driving conditions. My question is from most of the posts on this website, fuel pressure for these TPI units seems to run around 45 PSI. I called the guys at AFI and they said they run their systems at a lower pressure and that my readings are normal. Would there be any advantage to buying an adjustable FPR and cranking it up to 45 PSI? And why would their system run at 10 PSI lower than a stock GM system? I just assumed all of these TPI units would run the same. Thanks in advance for any help!

 

Buy a FPR and try it and see what happens,
fuel pressure seems low. Did you ask them why
they run a lower fuel pressure?

 

I emailed AFI and here was their reply :

We use 19 & 24 lb. Injectors and the stock injectors are 17 & 22 lb. injectors. Fuel pressure is a result of a lot of initial items that are taken into consideration. We just use this pressure for design as that is what allows the injectors to flow at the rate we design to.

I thought stock injectors for a 305 were 19 lbs. Any thoughts?

 

What size injectors did they use in your car. Did they also
do a prom for you, if so what injector constant did they use?

 

I have 19 lb injectors in my car. In the prom, the injector flow rate is 19.8. I assume that is what the injector constant is. I compared the prom they sent me with a stock 1990 305 TPI and it also has an injector flow rate of 19.8.

 

Can you datalog and see what's
going on

 

Also did your car ever run right or did it have
the problem when you first got it back from them

 

I have had the problem ever since I first bolted on the system. I have done dozens of datalogs and have the car pretty well tuned. The BLMs are all with 124-133 and the car starts right up and idles at 800rpm as indicated in the chip. The car pulls well at WOT but has a persistent miss at part throttle cruising. I have replaced every component of the ignition system over the last year. I have played with timing and fuel curves with no success. I am running out of ideas, but this fuel pressure question has me confused. From reading other posts, it sounds like correct fuel pressure is pretty critical to these TPI units. If I am running 10 to 15 PSI too low, then that could have been my problem all along. I am guessing too low of fuel pressure would cause poor fuel atomization and suffering drivability. I might just go ahead and buy an AFPR ,crank it up to 45 PSI, and see what happens. Thoughts?

 

For a 305, the stock injectors are 19lb, the 22lb injectors are for the 350. Fuel pressure should be around 43.5psi
What application are they referring to for stock 17lb injectors????

 

AFI said that stock is 17 lb. Every post I have found on these boards say 19. Would fuel pressure that is too low cause a part throttle miss? I just want to make sure before I shell out $70 on an AFPR.

 

So correct me if i am wrong, you have a stock 305? With 19lb injectors?
Your pressure is at 37psi? If that is the case you are running lean because 19lb injectors at 37psi are equivalent to running 17lb injectors.

 

That is correct. I am running 19 lb injectors. With the vacuum disconnected, my fuel pressure is 37 PSI. Connected its arounnd 32. Driving around at part throttle cruise, fuel pressure hovered in the low 30s the entire time.

 

southbay is right, crank up your pressure. Get an AFPR and try 45psi and go from there

 

Just get one and try it ask them what they did
to lower the pressure, did they install a weaker
spring or what

 

I just ordered an AFPR. I will let you know (fingers crossed).

 

Keep us posted!!

 

Well did you get it fixed, if so what was the problem

 

People ask questions and you try to help and
they don't respond, don't you hate that

 

Sorry for the delay. Just got the AFPR installed yesterday. It was on back order from Summit for 2 weeks. I cranked up the WOT fuel pressure to 50PSI. With the vacuum line attatched FP is at about 43PSI. Quite a difference. Car starts quicker, the throttle response is better, and the part throttle miss is almost gone (still there but barely noticable). I am burning some new chips today to compensate for the greater fuel flow I am now getting. Can't believe I have been running around with 31PSI for the last year! Anyway, thanks for all the help from everyone on this thread and the guys at SouthBay fuel injectors.

 

Great news

 

Glad things worked out for you

 

I'm curious, are you running an egr with this system?

 

No EGR. No pollution stuff at all.