Passenger exhaust manifold question on my 86 350 with newer gen heads - The 1947 - Present Chevrolet & GMC Truck Message Board Network

lbrowne · 03-29-2015, 03:15 PM

I've got to get around to fixing this soon. A few years ago I put a set of rebuilt heads on my 86 that apparently came from a early 90s 350.

Truck runs like a top without issue, but one thing I never fixed was that on the passenger side head - the original exhaust manifold lines up except for the very rear bolt hole by the firewall. All the manifold bolts are straight across, but if I recall correctly, the rear bolt hole in the head is lower than the rest.

So obviously I get exhaust noise.

Not sure if this makes sense? I'll try to get a pic posted.

Zane M · 03-29-2015, 08:12 PM

I'm sure someone with much more knowledge will chime in and help, but my simplistic way of thinking is to get manifolds that match the heads. Early '90s trucks are plentiful in the wrecking yards.

lbrowne · 03-29-2015, 09:01 PM

I crawled underneath and the issue is, the original manifold's flange extends past where the hole in the head is. Is this going to mess up my options for headers? Would a new gen manifold bolt right in and connect up to my y-pipe without issue?

Zane M · 03-29-2015, 10:54 PM

The output of the "newer" manifolds vs. the "older" ones is the same, so it won't affect how it matches up to your Y pipe.
As far as headers?? I'm not sure, but if you wanted to be really safe, you would probably need to find something that would fit a '91 Suburban or Blazer. Those models had the "TBI" heads, and would fit in your existing frame rails without question.

Axle · 03-30-2015, 11:18 AM

Sounds like you got an early head on your truck since it lacks the 2nd threaded hole at #8.
Some time around 79/80 they changed up manifolds and ended up moving the mounting hole outward, probably coincides when they went from under the spark plug logs to above the spark plug logs.

If you don't mind old style under the spark plug manifolds, grab a set of an earlier truck, or camaro, or nova since the mounting holes are "standard" for that time period. I run the old manifolds from my '73 Nova on my '83 truck, no fitment issues, and will eventually move them to my '81.

I don't think you will find an over the sparkplug manifold with the old style spacing at #8. I may be wrong on that, never had to look for that, so a later head may be your only option if you are set on the manifold style you have.

Alex.

lbrowne · 03-30-2015, 02:46 PM

Quote:

Originally Posted by Axle

Sounds like you got an early head on your truck since it lacks the 2nd threaded hole at #8.
Some time around 79/80 they changed up manifolds and ended up moving the mounting hole outward, probably coincides when they went from under the spark plug logs to above the spark plug logs.

If you don't mind old style under the spark plug manifolds, grab a set of an earlier truck, or camaro, or nova since the mounting holes are "standard" for that time period. I run the old manifolds from my '73 Nova on my '83 truck, no fitment issues, and will eventually move them to my '81.

I don't think you will find an over the sparkplug manifold with the old style spacing at #8. I may be wrong on that, never had to look for that, so a later head may be your only option if you are set on the manifold style you have.

Alex.

The guy I bought the heads from said they're from a early 90s 350, so that would be a TBI. Wasn't hard to tell he was into chevy trucks. He basically kept a motor that was totally rebuilt but went with higher flowing heads and sold me these. I'd be surprised if they were earlier style heads - is there a casting number on them that I could verify you think?

Here is a pic with a drill bit stuck through the rear hole:

So if I wanted to get headers, just get ones for a TBI squarebody suburban and I should be good then. At this point shorties might be easier to bolt in and not change anything else, I already have a new custom flopro cat back on her.

With longtubes, even though they're cheaper, I'd have to get a y-pipe as well and then have it welding into the cat back.

jetmech85 · 03-30-2015, 04:03 PM

Do you have center bolt valve covers? If so then, yes you have '87 or newer heads. Just from the little bit of valve cover I see in your pic it looks like you don't. I have a pair of TBI haeds and they have the bolt hole you need even though the hole was not used it was still there from the factory. Earlier heads did not have a hole there. Headers will bolt to your heads with no problem. I have some '70s era heads I can get a number from if you still need it but you will have to look under your valve covers to check yours. There will be a date code there also.

lbrowne · 03-30-2015, 04:43 PM

Quote:

Originally Posted by jetmech85

Do you have center bolt valve covers? If so then, yes you have '87 or newer heads. Just from the little bit of valve cover I see in your pic it looks like you don't. I have a pair of TBI haeds and they have the bolt hole you need even though the hole was not used it was still there from the factory. Earlier heads did not have a hole there. Headers will bolt to your heads with no problem. I have some '70s era heads I can get a number from if you still need it but you will have to look under your valve covers to check yours. There will be a date code there also.

You're right, I don't have center bolt valve covers. Maybe the guy I bought the heads from didn't realize what was on the rebuilt motor he had.

A set of headers even if I select specifically for my 86 350, will bolt right on then won't they. I wish I had pic of my old heads - because even though it's been a while I swear I had two bolt holes on the rear part near the firewall on the passenger side.

jetmech85 · 03-30-2015, 05:45 PM

The headers will fit the heads. The reason the manifold won't fit is because of the design dictates the bolt would be inside manifold passage so it was moved aft. After that design change most, if not all heads have that hole wether used or not. The heads you have are probably better than the TBI heads anyway.

lbrowne · 07-01-2015, 05:38 PM

Would the exhaust manifold off a 92 350 work?

I ask because one is getting parted out in my city.

jetmech85 · 07-02-2015, 09:58 PM

I think the '86 "Y" pipe would not quite fit that manifold, the '86 passenger side had a heat riser between the manifold and pipe. Also the manifold three stud pattern is slightly bigger causing a problem when you try to bolt up the "Y" pipe. On my '86 Suburban I used '90 Suburban manifolds, "Y" pipe and the entire system for a 3" single exhaust front to rear. I had to modify the factory hanger that is bolted to the transmission mount. Bought everything except manifolds and studs from Walker. Did everything in my driveway.

03-29-2015, 03:15 PM	#1
lbrowne Registered User Join Date: Feb 2011 Location: Alberta Posts: 144	Passenger exhaust manifold question on my 86 350 with newer gen heads I've got to get around to fixing this soon. A few years ago I put a set of rebuilt heads on my 86 that apparently came from a early 90s 350. Truck runs like a top without issue, but one thing I never fixed was that on the passenger side head - the original exhaust manifold lines up except for the very rear bolt hole by the firewall. All the manifold bolts are straight across, but if I recall correctly, the rear bolt hole in the head is lower than the rest. So obviously I get exhaust noise. Not sure if this makes sense? I'll try to get a pic posted.

03-29-2015, 08:12 PM	#2
Zane M Registered User Join Date: Dec 2006 Location: SLC, UT Posts: 706	Re: Passenger exhaust manifold question on my 86 350 with newer gen heads I'm sure someone with much more knowledge will chime in and help, but my simplistic way of thinking is to get manifolds that match the heads. Early '90s trucks are plentiful in the wrecking yards.

03-29-2015, 09:01 PM	#3
lbrowne Registered User Join Date: Feb 2011 Location: Alberta Posts: 144	Re: Passenger exhaust manifold question on my 86 350 with newer gen heads I crawled underneath and the issue is, the original manifold's flange extends past where the hole in the head is. Is this going to mess up my options for headers? Would a new gen manifold bolt right in and connect up to my y-pipe without issue?

03-29-2015, 10:54 PM	#4
Zane M Registered User Join Date: Dec 2006 Location: SLC, UT Posts: 706	Re: Passenger exhaust manifold question on my 86 350 with newer gen heads The output of the "newer" manifolds vs. the "older" ones is the same, so it won't affect how it matches up to your Y pipe. As far as headers?? I'm not sure, but if you wanted to be really safe, you would probably need to find something that would fit a '91 Suburban or Blazer. Those models had the "TBI" heads, and would fit in your existing frame rails without question.

03-30-2015, 11:18 AM	#5
Axle Registered User Join Date: Jul 2011 Location: SW Ontario Posts: 1,212	Re: Passenger exhaust manifold question on my 86 350 with newer gen heads Sounds like you got an early head on your truck since it lacks the 2nd threaded hole at #8. Some time around 79/80 they changed up manifolds and ended up moving the mounting hole outward, probably coincides when they went from under the spark plug logs to above the spark plug logs. If you don't mind old style under the spark plug manifolds, grab a set of an earlier truck, or camaro, or nova since the mounting holes are "standard" for that time period. I run the old manifolds from my '73 Nova on my '83 truck, no fitment issues, and will eventually move them to my '81. I don't think you will find an over the sparkplug manifold with the old style spacing at #8. I may be wrong on that, never had to look for that, so a later head may be your only option if you are set on the manifold style you have. Alex.

03-30-2015, 04:03 PM	#7
jetmech85 Registered User Join Date: Sep 2014 Location: Texas Posts: 215	Re: Passenger exhaust manifold question on my 86 350 with newer gen heads Do you have center bolt valve covers? If so then, yes you have '87 or newer heads. Just from the little bit of valve cover I see in your pic it looks like you don't. I have a pair of TBI haeds and they have the bolt hole you need even though the hole was not used it was still there from the factory. Earlier heads did not have a hole there. Headers will bolt to your heads with no problem. I have some '70s era heads I can get a number from if you still need it but you will have to look under your valve covers to check yours. There will be a date code there also.

03-30-2015, 05:45 PM	#9
jetmech85 Registered User Join Date: Sep 2014 Location: Texas Posts: 215	Re: Passenger exhaust manifold question on my 86 350 with newer gen heads The headers will fit the heads. The reason the manifold won't fit is because of the design dictates the bolt would be inside manifold passage so it was moved aft. After that design change most, if not all heads have that hole wether used or not. The heads you have are probably better than the TBI heads anyway.

07-01-2015, 05:38 PM	#10
lbrowne Registered User Join Date: Feb 2011 Location: Alberta Posts: 144	Re: Passenger exhaust manifold question on my 86 350 with newer gen heads Would the exhaust manifold off a 92 350 work? I ask because one is getting parted out in my city.

07-02-2015, 09:58 PM	#11
jetmech85 Registered User Join Date: Sep 2014 Location: Texas Posts: 215	Re: Passenger exhaust manifold question on my 86 350 with newer gen heads I think the '86 "Y" pipe would not quite fit that manifold, the '86 passenger side had a heat riser between the manifold and pipe. Also the manifold three stud pattern is slightly bigger causing a problem when you try to bolt up the "Y" pipe. On my '86 Suburban I used '90 Suburban manifolds, "Y" pipe and the entire system for a 3" single exhaust front to rear. I had to modify the factory hanger that is bolted to the transmission mount. Bought everything except manifolds and studs from Walker. Did everything in my driveway.



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