305TPI to 350TPI swap problem - Third Generation F-Body Message Boards

87SnowFlake · 06-20-2009, 07:36 PM

I have a 87 IROC that had a 305 LB9 with TPI and it had knock when i bought the car. So i swapped the 305 for a 350 and used all the stock 305 equipment. At the moment the car starts up and runs fine, just a little rough/high idle at times. I am having a problem with accelerating. when i try to accelerate the engine breaks up and miss fires and i have to let off the gas to let itself recoupe. I am using the 305 injectors and everything perataining to the computer is 305 equipment. I did have a problem with the air temp sensor wire. I could not find the "jumper wire" and i traced the wires from the computer into the engine bay and found that the wire had a two prong end on it and it was plugged into another wire lead that had the receiving end . So i cut the receiving end off and crimped it to the pig tail that i bought for the air temp sensor. So inturn i mad a jumper wire for the air temp sensor. BUt now i don't have anything hooked to where i cut off the receiving connector, so i am not sure if that is the cause of my acceleration problem or not. I have already set the timing and all my vacuum lines are hooked up, so i am at a loss for what i can do.

This ad is not displayed to registered members.
Register your free account today and become a member on ThirdGen!

92RS(real slow) · 06-20-2009, 08:29 PM

you need a knock sensor for a 350 the 305 one wont work that should fix your problem hopefully, and it wouldn't hurt to get some injectors from 350, and prom also

87SnowFlake · 06-20-2009, 08:52 PM

Is that a definitive answer or a possible guess?

ModPog84z28 · 06-21-2009, 03:02 AM

yes, get the knock sensor, your timing is being thrown off by it, especially when you try to accelerate. also get at least 21# injectors, 24# might be better. the 19# injectors you are running are waaaay to small and arent giving you enough fuel, which is giving you a lean condition and most likely causing knock, which then the timing is thrown off and the problem worsened.

the prom can be fixed by getting a stock 350 chip or an aftermarket chip. id go aftermarket, cuz after you fix the knock sensor and injectors, thatll free up another 40 hp.

so, prom, injectors, knock sensor. that and a lil tuning of the fuel pressure and timing should get it running good and respectable.

87SnowFlake · 06-21-2009, 02:07 PM

Ok i will try replacing the knock sensor and see what happens with that. Should i seriously consider replacing the injectors and the prom, or just see how she accelerates with the sensor replacement first?

ModPog84z28 · 06-22-2009, 07:39 PM

Quote:

Originally Posted by 87SnowFlake

Ok i will try replacing the knock sensor and see what happens with that. Should i seriously consider replacing the injectors and the prom, or just see how she accelerates with the sensor replacement first?

yes you should seriously consider replacing your injectors and prom. unless you only want your engine to last about 5k miles and drive like crap. your injectors are TOO small for a 350. and your computer is trying to use 305 logistics to run and engine that is far larger than the computer thinks. i would actually consider the prom and injectors your biggest issues. without the injectors, your acceleration will suck, bad. itll hiccup and sputter. (these problems are just compounded by the wrong chip and sensor.)

not too mention, your already running way lean, which will toast your motor in a hurry. seriously, do the injectors, prom, and sensor if you want your motor too last. its better to fork the money now, rather than replacing the engine in a month cuz you didnt get the proper parts.

you can usually find proms for 350s on ebay for not too much, injectors as well. total cost for all that stuff should be about 300 bux. 150 max for injectors, 100 for a prom (thats a performance aftermarket one) and 50 for the sensor from an auto parts store.

five7kid · 06-23-2009, 02:26 PM

That's way over-stated for an MAF system. Injectors don't need to be bigger until at least WOT, and even then, 22 lb 350 injector size is just for margin, not a necessity (fellow 3rd gen bud here had a ZZ4 with all-aftermarket TPI hardware, ZZ9 cam, extrude-honed plenum/runners/base/heads, etc. - it ran better with 19 lb injectors than it did with 24 lb). Ditto the PROM - with MAF, it's using mass air flow information more than displacement-driven assumptions (SD is a different matter).

Not having proper air temp information is a different story. Get that working.

Rod Behring · 06-29-2009, 03:16 PM

I second what five7kid says, although I do believe you need the 350 knock sensor. If you dont have an adjustable fuel pressure regulator that might help as well. Raising the fuel pressure (not more than 10%) will make the injectors seem a little bigger and that should easily feed a MAF L98.