Author Topic: 1987 GMC 305 to 350 swap - TBI and a few issues  (Read 5701 times)

Offline brandon6469

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1987 GMC 305 to 350 swap - TBI and a few issues
« on: March 07, 2015, 09:17:33 PM »
I recently rebuilt a 350 out of a 1992 and swapped it into my GMC.  I added a Comp Cams 12-256-4 and long tube headers.  I kept the stock TBI setup (including injectors) for the startup and it did ok.  It ran fine though I haven't really stepped into it as I've only put about 100 miles on it so far. I'm on a stock 305 tune with a TH400.  I'm running stock heads and pistons.  Timing was advanced about 4 degrees, fwiw. 

Last night, I changed to 350 injectors, injector pod spacer, and an adjustable fuel pressure regulator.  I set pressure about 13 psi and it ran like crap when I drove it today.  I thought that possibly the fuel pump wasn't keeping up so I turned the pressure down to 12, then 10 psi.  It got better, but was still missing power from the 305 injector setup.  I took the pressure to about 9 and it's doing pretty good.  I also got a light and moved timing to 0 deg.

At this point, I think the stock 305 tune is the issue, though it may be the fuel pump.  Any suggestions or thoughts? 

I have EBL Flash on the way so I'm hoping updating the tune is going to help get the truck running right. 





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« Last Edit: March 07, 2015, 10:46:35 PM by brandon6469 »

Offline LTZ C20

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #1 on: March 08, 2015, 01:32:04 AM »
Sounds like the 305 program in the ECM is your problem. When you start changing displacements and such without adjusting the computer it can cause running issues. Your best bet is to find a place that does programing and have them build a program for your exact setup.

I have a 350 TBI, but it's bored, aluminum heads, bigger cam, headers, dual exhuast, intake. Pretty stout engine, my ECM has a modified program for all the high performance goodies and I'm using a big block throttle body.

Changing 1 thing can have a small or huge impact on everything else.
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Offline roundhouse

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #2 on: March 08, 2015, 09:24:23 AM »
Try putting the 305 Injectors back in
Chip may be programmed to work with them. 


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Offline brandon6469

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #3 on: March 08, 2015, 08:25:19 PM »
I put the old injectors back in today.  Truck started and idles much better.  I didn't have a chance to take it down the road but I will later this week. 

My dad says replacement injectors are often crap.  Anyone ever run into that?  I have a set of used injectors off an early 90's 350 I may try at some point. 

Can you 'see' any difference between a 305 and a 350 injector?


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Offline roundhouse

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #4 on: March 08, 2015, 10:27:24 PM »
They have different color paint on them
But I can't remember which is which

If the 305 injectors run ok
Just use em



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Offline brandon6469

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #5 on: March 14, 2015, 01:09:49 PM »
I've got the 305 injectors back in and they seem to be running fine.  Roads should be dry tomorrow so I'll take it out for a test drive.  I did crank the fuel pressure on the 305's to about 13 psi.

Assuming it runs fine, I'm going to install my new ECM with EBL Flash from Dynamic EFI.  To get maximum benefit, I'll need plenty of fuel.  Can I just continue to crank the pressure on my 305's?  I've heard max pressure on the stock fuel pun with 350 injectors is 13-15 psi.  Is that limited by the pressure or a product of flow capability?  In other words, can I crank it up around 18 psi since the 305 injectors 'should' flow less than 350's? 

The 350 injectors I tried last week are from Auto Zone and I'm leery about the quality.  I'd rather use the 305's if I can get enough fuel through them.  I had an injector pod that was supposedly from a tbi 350, but the part numbers on the injectors are for a 4.3L.


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Offline VileZambonie

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #6 on: March 14, 2015, 05:40:04 PM »
The fuel map is not the same for the 350. You need to put 350 injectors and programming in for it to run good.
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Offline Captkaos

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #7 on: March 15, 2015, 01:21:36 PM »
On a TBI setup I am assuming you kept the 305 chip, just swap it out to a 350 chip and put the injectors back in.

Offline roundhouse

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #8 on: March 15, 2015, 08:37:57 PM »

On a TBI setup I am assuming you kept the 305 chip, just swap it out to a 350 chip and put the injectors back in.

I thought the chip was the same.    Just different injectors?


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Offline Captkaos

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #9 on: March 16, 2015, 01:02:01 PM »
different chip and injectors, knock sensors and ESC module.

Offline brandon6469

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #10 on: March 16, 2015, 07:44:50 PM »
I've got some GM 350 injectors and a 350 esc on the way. 

With the knock sensor...will the 305 sensor be more or less sensitive?  I'm curious.  Since I'll be programming and eventually trying to tune with EBL Flash, I'm sure it's a good idea to have the right one. 


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Offline roundhouse

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #11 on: March 16, 2015, 09:38:44 PM »

different chip and injectors, knock sensors and ESC module.

So do you have to swap all that is you pull a TBI system from a 305 and install it in a 350?

I thought all you changed was the injectors


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Offline Captkaos

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #12 on: March 17, 2015, 09:04:10 PM »
to do it right, you need all of that.

Not sure on the sensitivity, just know they aren't the same.

Offline brandon6469

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #13 on: March 21, 2015, 08:52:36 PM »
Installed GM 350 injectors today and a 350 tune via EBL Flash.  It's running good.  Laptop says I'm getting some knock, but I think that may be due to the 305 knock sensor, which I plan to change soon. 


Question - are aftermarket knock sensors ok or do I need to stick with a GM part? 

Anyone have one for sale?


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Offline VileZambonie

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Re: 1987 GMC 305 to 350 swap - TBI and a few issues
« Reply #14 on: March 21, 2015, 11:30:01 PM »
Use the correct KS by engine code. 305 and 350 are different calibrations as Capt pointed out already and use whichever one is intended to be used with that ECM.
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