Will a throttle body and intake off of a 91 350 fit on an 86 305? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-20-2010, 10:58 AM
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Will a throttle body and intake off of a 91 350 fit on an 86 305?

I have a bad 350 in my 91 silverado. I got a free 305 from father-in law, it only has 50k on it. It came out of his 86 gmc pickup. It doesnt look like the intake will fit on the 305 block. can anyone give me some words of advice? thank you very much.
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Old 08-20-2010, 12:40 PM
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It will swap, it may run rich on the 305 because the 350 TBI has bigger injectors than the 305 TBI would have had. An adjustable fuel pressure regulator may be enough to get it to a good usable point.
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Old 08-20-2010, 01:05 PM
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well I think the intake from the 350 is more flat, the bolts on the intake from the 305 look like a different angle. the 350 still runs so I dont want to take it off until I do the swap. that is a good point about running rich thank you for that. I guess I should say I dont know it the 350 intake will bolt on to the 305 block and heads? I will take what ever advice anyone has thanks
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Old 08-20-2010, 01:42 PM
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If the bolts on the 305 intake area at a different angle(I am assuming you are talking about the center pairs) then the 305 must not have 1986 heads on it or is not an 1986 engine. All SBC heads 86-95 have the same bolt angle, prior to that(55-85) is different(all are the same angle, centers not different) and 96-up have the vertical Vortec pattern(only 8 bolts, not 12)
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Old 08-20-2010, 08:02 PM
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So the 305 is out of an 86 gmc, it could still have some things from 85. That may be one of them, just depends on what time of year the truck was made.
Ok now lets say the block is 85', is there a way to get my intake 91' onto that block? would my heads from the 350 fit on the 305? Sorry Im just dont know technicalities on engines, I can diagnose and fix them but dont have alot of experience with what you can mix and match? Thank you though.
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Old 08-20-2010, 08:17 PM
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Quote:
Originally Posted by redkramer
So the 305 is out of an 86 gmc, it could still have some things from 85. That may be one of them, just depends on what time of year the truck was made.
Ok now lets say the block is 85', is there a way to get my intake 91' onto that block? would my heads from the 350 fit on the 305? Sorry Im just dont know technicalities on engines, I can diagnose and fix them but dont have alot of experience with what you can mix and match? Thank you though.
The heads will fit fine- but the combustion chambers are different sizes. The 350 heads on a 305 engine will lower the compression ratio.

Is that a problem? If the original 305 heads were 58cc, and the 350 heads are 64cc swirl ports, the difference will be about .5:1. With the 64cc heads the CR will be about 8.15:1.

The intake can be modified (center bolt holes elongated, using special washers to compensate for the change in angle) to fit the OEM intake onto the older style heads, I THINK. There are adapters for putting a TBI throttle body onto a 4-bbl. intake, so you could use a Performer (as an example) made for the 305 heads, mount an adapter, then mount the 350's TBI throttle body to that.
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Old 08-20-2010, 08:34 PM
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I think the 350 intake is more flat meaning the bolts are more vertical
on the 305 where the intake sit on the heads that is more vertical and the bolts are more horizontal. sound right? thats my major problem I wouldnt be able to put a gasket on. thank you
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Old 08-20-2010, 08:41 PM
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They make aftermarket adapters as stated earlier, on the 454 throttle body GM used an adapter and put it straight to the 4bbl intake but it is ugly!
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Old 08-20-2010, 08:55 PM
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Quote:
Originally Posted by redkramer
I think the 350 intake is more flat meaning the bolts are more vertical
on the 305 where the intake sit on the heads that is more vertical and the bolts are more horizontal. sound right? thats my major problem I wouldnt be able to put a gasket on. thank you
Other than the mentioned year differences to just the angle of the four center bolts between years, there is no difference to 305 or 350 intake bolt pattern. Intake may be shaped slightly different in hieigth of TBI mount or runners, but bolts and pattern are the same. If there is no different angle between the center bolts and the end bolts between the two engines heads, they will interchange. 305's with angle inner bolts will accept 350 manifold with angled inner bolts, regardless of if the intake looks different. All that matters are that the bolt angles all match. 305's with all straight angled bolts will accept 350 manifold with all straight angled bolts.

Look a little closer at what you have, specifically just the four center bolts, if angles match there they will interchange. 305 or 350 manifolds are not bigger or smaller than each other physically.
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Old 08-20-2010, 09:04 PM
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ok thank you very much. I will look tomorrow if I can I will take some pictures.
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Old 08-20-2010, 09:23 PM
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The two center bolts on each side for a '86 are at different angles than that of a '91.

I have heard of people modifying the intakes to fit...never tried it myself.
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Old 08-20-2010, 09:25 PM
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91 intake to 86 heads

I think most of 86 heads have the later angle on the 4 center bolts, I have a quad intake in the shop that has the later pattern which off an 86. Either way you can order an inexpensive kit to make it work. We make our own; it takes about 45 minutes to cut four pieces of small pipe on the angle you need and then cut out the holes a bit and go. I have 993 heads on a later model TPI car. It's easy
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Old 08-20-2010, 09:31 PM
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Quote:
Originally Posted by GMC boogie
The two center bolts on each side for a '86 are at different angles than that of a '91.

I have heard of people modifying the intakes to fit...never tried it myself.
86 was first year for the angled bolt pattern on the 4 center bolts, same year 1-piece rear main seal was added also.
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Old 08-21-2010, 07:55 AM
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Quote:
Originally Posted by ericnova72
86 was first year for the angled bolt pattern on the 4 center bolts, same year 1-piece rear main seal was added also.
I agree with you on the 1-piece seal but the '86 C10 I had with the factory 305 came with the old style intake bolt angles and perimeter bolt valve covers.

'87 for sure had the later angled intake bolts and centerbolt valve covers.
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Old 08-21-2010, 11:22 AM
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Quote:
Originally Posted by GMC boogie
I agree with you on the 1-piece seal but the '86 C10 I had with the factory 305 came with the old style intake bolt angles and perimeter bolt valve covers.

'87 for sure had the later angled intake bolts and centerbolt valve covers.
You may well be right, being this is a changover year and they may have been using up inventory,... and the trucks didn't get FI until '88?? may depend on hp rating also??

Like I noted, all he has to do is look at the bolt angles of the 4 center bolts to know if the swap will work, and even that can be overcome with a die grinder at assembly as others have said.
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