I had a 305 TPI in my 87 Z28. I pulled the motor and placed a 350 (of the same year) in it's place. Everything is stock, no mods yet. Do I need to remove the ABUN PROM and replace it with an ABWN PROM?

 

Yes. You also need to be sure you've used the detonation sensor from the 350, and injectors sized for the 350.

 

Yeah, what he said. And then start learning to tune your PROMs yourself. Its fun.

 

Vander

I'm in the process of obtaining injectors for a 350. I do know the 305 injectors aren't up to par.

Let me be more clear as not to confuse. I pulled the 305 and put a 350 crate motor in all the wiring and induction on the 350 is from the 305. I know the 305 injectors won't work. Tell me, how would leaving the PROM from the 305 hinder the performance?

92 zzz28,

I'd like to learn how to tune my own PROM. That would be the next step. you seem to know what I'm thinking. First i'd like to just get the thing to run consistantly

Thanks

 

Buy a stock GM Prom for a 350 from the Dealer, to get up and running. It will cost you about $60.00. Then you can get a custom chip.

 

First, the detonation sensor for the 305 is designed to be attached to a case with a lot thicker cooling jackets due to the smaller bores. The sensor is therefore much more sensitive to be able to detect the knock through the additional muffling of the case. This will cause it to "read" much smaller detonation events on your 350, and create unnecessary spark retard when installed on a 350, limiting both power and fuel mileage.

Second, the injectors for a 305 are rated to flow about 19 pounds of fuel per hour at a maximum 80% duty cycle. The injectors for a 350 are rated at about 22 pounds per hour. The larger (by about 15%) displacement of the 350 requires more fuel flow capacity at all operating ranges to maintain a correct fuel mixture. The PROM is programmed to use 22 PPH injectors, and will try to vary the injector pulse widths based on what that size injector is expected to flow. Even with the feedback from the oxygen sensor in closed loop mode, the system will always tend to be too lean.
Third, the spark advance and injector offset tables in the PROM are also different for a 350 due to the different characteristics (bore/stroke ratio) of the engine as compared to a 305. The 350 may run with a 305 PROM and injectors, but not at all near the level it should. Timing and mixture on acceleration are not going to be what is even close to optimal for a 350.

 

I think I might be in the same boat ! With the exception that I have the CC Q-Jet set up. Would this also require that I get a prom for the 350 rather than the 305 ??? My 87 T/A seems to run fine as is. Could I be missing some power because of this ?

 

Vander,

Got it! The third part about timming and mixture is where i am at right now. When I accelerate I noticed it falls on it's face then a sec an a hlf later picks up but doesn't quite got the punch it should. You sold me! I am gonna have to get myself a PROM for a 350 as well as 350 injectors

Thanks :yourock:


I'll let you know how things turn out.



Now Can someone direct me where to buy a stock PROM for an 87 350 TPI. I Checked dealer. They want about $100 for a new PROM. Any Cheaper place with reliability?

 

His name is VADER!!! You know, like the evil dark jedi from Star Wars who was hell bent on total conquest of the galaxy!!!

 

Look Here

Think they have them for about $50-60