help with 305 to 350 swap

1911a1 · June 14, 2004

looking for some help with my sons truck project. the truck is a '87 gmc c10 swb. that came with a 305. i blew the 305 up. so i took the 350 out of a '74 3/4 ton chevy my uncle bought new. had it rebuilt 30-10-10. lunati cam dur. @ .05 is 204/214. new vortec heads, performance product vortec intake(non egr),headman headers, a tbi adapter from a supercharger kit to adapt the tbi to the intake. hedman headers. i instructed the shop i would take care of the exhaust(they couldn't anyway) so the oxygen? sensor on the exhaust is not hooked up. also i'm still running the 305 injectors in the tbi and the 305 prom in the computer. the truck is not running very well. i'm out of money for now. but i hopping someone on here has done or knows of a similiar engine swap that can help me finnish this project. thanks for any help you can provide.

Bent Wrench · June 14, 2004

Without the O2 senser connected it will run like #&*%!

snoman · June 14, 2004

looking for some help with my sons truck project. the truck is a '87 gmc c10 swb. that came with a 305. i blew the 305 up. so i took the 350 out of a '74 3/4 ton chevy my uncle bought new. had it rebuilt 30-10-10. lunati cam dur. @ .05 is 204/214. new vortec heads, performance product vortec intake(non egr),headman headers, a tbi adapter from a supercharger kit to adapt the tbi to the intake. hedman headers. i instructed the shop i would take care of the exhaust(they couldn't anyway) so the oxygen? sensor on the exhaust is not hooked up. also i'm still running the 305 injectors in the tbi and the 305 prom in the computer. the truck is not running very well. i'm out of money for now. but i hopping someone on here has done or knows of a similiar engine swap that can help me finnish this project. thanks for any help you can provide.

And it will not either. You have to be very careful about changes to a engine with a closed loop fuel injection system is used. Yours is a classic mistake, add a cam, headers, duals a such and then you just cannot firgure out why it does not run right. If you really want to go wild, you should in a older truck with a carb as they are much easier to "tune" for this. You are going to need upgraded injectors and a custom tune at the the very least here ($$$) It would have been far simple to keep engine stock and exhaust "reasonably" stock, swapped motor and added the blower but you did not do that!

1911a1 · June 15, 2004

thanks for the replys. as for the cam it is very mild and is suposed to be compatable with the computer. it sound like the first thing i'm going to have to do is finnish the exhaust work then install 350 injectors (i have a set from a wrecking yard - if they're any good) and on ebay i can get a stage 2 prom with a 175 deg thermostat fo about 75$. what i also need to know is there any special way to set the timing on a '87 model and how to do it and any sugestions on plug gap, its set at .045 now.

Cool Zr2 · June 15, 2004

Without the O2 senser connected it will run like #&*%!

don't do anything untill you have the O2 sensor hooked up.

1911a1 · July 5, 2004

update - had the o2 sensor hooked up to the headers. when i was having that done i mentioned to the exhaust guy i thought the timing was off. he said he would set it for me. it now runs much better. when he showed me which wire he disconected to set the timing i noticed it was the wrong wire. if the cam is not stock does that mean that the stock settings cannot be used? (it says to set it to 0 deg.)also the plugs were set at .045 but i noticed that the 98 305 vortec we have at work is set at .060, the factory setting for the stock 305 ignition was .035. any thoughts on the spark plug gap would also be appreciated. i still need to get a performance chip and install 350 injectors to get most of the potential for this set up.

snoman · July 5, 2004

I would try about 6 BTDC for starters. 0 BTDC is very conservative. I run my stock 350TBI at 6 BTDC.

1911a1 · July 11, 2004

today my son and i installed a '87 350 throttle body and electronic spark controler. what a difference! when turning on to a highway and burning off it immediatly smoked the tires had to let off to keep from floating the valves! the tires are 15" 275/60. cant wait to get the performance chip and true duals hooked up. still looking for any advice from anyone that has any experience with this type of engine swap.

snoman · July 11, 2004

today my son and i installed a '87 350 throttle body and electronic spark controler. what a difference! when turning on to a highway and burning off it immediatly smoked the tires had to let off to keep from floating the valves! the tires are 15" 275/60. cant wait to get the performance chip and true duals hooked up. still looking for any advice from anyone that has any experience with this type of engine swap.

True dual will kill some low end torque and part thottle response and can hit in town milelage to. Proceed to caution here. A big single with ahigh flow muffler will give you the best of both worlds.

N900WM · July 20, 2004

snoman,

I completed a very similiar swap a few years ago on and 87 Chevy 2wd long box. It was a stock 305TBI and I blew it up. Bought a Jasper 350 and put a 350TBI on it. It ran like crap as well. Guy I worked with had the same truck only with a stock 350TBI. I barrowed his computer and she ran like a top.

So I bought a replacement computer from GM using his VIN number and the truck has been flawless.

Later on, I installed Doug Thorley headers and a cam and head package from some race shop in Arizona. Put a JET chip in it and today she is still in town (I no longer own it) with over 250,000 miles on it. It's a rusty POS now but still runs great.

facn8me · July 22, 2004

add a cross over pipe to help with low end tourqe. on the spark plug gap..... the plugs you run make a big difference in gapping. get the correct plugs for your year of heads and use the recomended gap. if you have a plug that is supposed to be set at .o35 you CANNOT change the gap to suit your needs without defeating the design of the plug. ALL spark plugs are designed to have the gap paralell between the electrode and grounding tip. if you open that gap too much you take away contact area of the spark. too close and you may not have enough sparktime to addiquetly ignite your fuel mixture. the cam should be fine, but to keep it from leaning out and causing damage those injectors and chip will fix you up.