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Old 12-07-2010, 05:02 PM   #1
cris well
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tbi on older model motor

i have a cammed-up 305 with 601 truck heads (one of the desirable casts) and shorty headers that i wanna put in my 90 5.7 tbi truck.

i'm not sure of the cam specs because i bought another truck with this motor already re-built in it--it lopes pretty good tho. i never drove the other truck--just cranked it up and let it run in the yard.

could i make the tbi intake work on the older model heads or should i use the single plane street dominator on it now with a tbi adapter?

i'm also looking in to the obd1 upgrade kits where you burn your own chips--however i'd like to try it with the stock ecm at first.

any suggestions?
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Old 12-07-2010, 06:28 PM   #2
ChevyTech
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Re: tbi on older model motor

The TBI system used on these trucks will not tolerate much of a cam change. If the cam change is enough to lower the vacuum, a custom chip (prom) is needed to make it run well.

If it lopes it will not run well on a stock chip / tune.
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If it is modified from what came stock from the factory, let us know that too.
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Old 12-07-2010, 09:26 PM   #3
bwood
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Re: tbi on older model motor

How old is your "new" engine? If it is an older 2 piece rear main seal engine, you will have to get a flexplate/flywheel to match it. The engine in your 90 has a 1 piece rear main seal, the bolt circle for the flexplate/flywheel is different. You will also have to install the knock sensor in the passenger side of the block (in the block drain). The intake will fit physically, but the two mounting holes in the middle of the intake on each side have a different bolt angle. I know people put older intakes on the TBI heads by modifying the holes in question, I don't see why you couldn't go the other way. If I remember correctly, the TBI bolts in the middle are at something like 72 degrees (not sure on the angle) and the older heads are all perpendicular to the mounting surface.

And like ChevyTech said, anything too "lopey" will run poorly with the stock tune/chip.
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Last edited by bwood; 12-07-2010 at 09:28 PM.
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Old 12-07-2010, 09:47 PM   #4
cris well
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Re: tbi on older model motor

it's a 2 piece rear main truck motor (early 80's). i have the flexplate that's on it now (bwood) but you're saying it won't work with the 700r4 torque converter?

the knock sensor you're talking about is it hard to change? can i use it out of the 350 in the truck now?

yea i know about folks using the older intakes on the newer center-bolt heads but i've never seen anyone go the other way either.
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Old 12-08-2010, 10:13 PM   #5
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Re: tbi on older model motor

Its not a matter of it fitting to the trans, its the bolt circle on the crank that different between the two engines. The TBI (1 piece) flexplate won't bolt to the older (2 piece) crank. The torque converter holes are the same.

I can't remember if the knock sensor for a 305 is different than the 350? ChevyTech might be able to answer that one. There are different kinds for different engines, although they may all fit in the same hole. As far as installation, it just threads into the block drain on the passenger side, nothing complicated.
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Old 12-09-2010, 12:14 PM   #6
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Re: tbi on older model motor

Quote:
Originally Posted by bwood View Post
...
I can't remember if the knock sensor for a 305 is different than the 350? ChevyTech might be able to answer that one. There are different kinds for different engines, although they may all fit in the same hole. As far as installation, it just threads into the block drain on the passenger side, nothing complicated.
Be careful on what you follow because there is a lot of mis-information on this issue.

It is my understanding that the knock sensor has to match the ESC module.

If you swap engines, but keep all the electrical parts with the vehicle, it will not be a problem.

That being said here is how people get into problems.

A vehicle with a 305 may, and often does, come from the factory with a different ESC module & knock sensor then the same model that has a 350 engine. This is especially true when it comes to Tuned Port Injection vehicles, such as the Camaro.

When the switch was made to electronically shift transmissions with a PCM, instead of an ECM, the ESC is built into the PCM instead of having a separate module. On those cars a PCM swap is usually done so the Knock sensor needs to be swapped to match the ESC which is in the PCM. This is where the statement that the knock sensor must be swapped originates from and people donít realize that the model year, vehicle and the exact parts being used makes a difference in which knock sensor to use.

On some years and models the same knock sensor is used with a 305 and a 350. Other years they are different with different ESC modules.

Here is another example of why it is so important people give more complete information when asking a question. A 1993 1500 with an automatic transmission uses an 8192 BAUD rate PCM. A 1993 (and some later model year TBI trucks) with a 5 speed manual transmission used a 160 Baud rate ECM, like the older TBI trucks use, with a separate ESC module.

If someone tells you to swap the knock sensor without asking several questions, they may be wrong.

I have held my opinions to myself on this topic because I am not positive, as I have not done ALL the research on it myself, and I donít trust anyone elseís work, but this is what I believe is true.
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Old 12-09-2010, 01:56 PM   #7
cris well
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Re: tbi on older model motor

it's a 1990 silverado standard cab step-side with the original 350 and 700r4--all stock TBI and ECM.


it's a good running motor but has about 290,000 miles and is using and leaking some oil now.


the 305 is an early 80's truck motor that has been newly rebuilt but i don't know any of the internals of it. i know it has 601 heads with a single plane intake, shorty headers, and a moderate cam.
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Old 02-19-2011, 11:25 PM   #8
cris well
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Re: tbi on older model motor

i'm just getting around to the swap again and want to clarify some things.

ChevyTech says if i use the knock sensor and the esc off of the 350 in the truck now i should be ok as long as the 305 holds enough vaccum.

if i recall, the 305 held about the same or maybe a little more vaccum (around 17 to 19) at idle. I checked the 350 yesterday and it held 17.

what about the intake? would i be better off sticking with the single plane street dominator with a TBI adapter or should i stick with the stock TBI intake off the 350?
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