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I've got a 92 Z28 Convertible with the LB9 engine, 5sp manual and 308 rear gear. It hasn't run in about 15 years. An since I know it needs new valve guide seals plus some other gaskets. I plan to at least tear it down if not pull it. I want to know if the larger manifolds are a direct bolt up swap? If so can my air tubes also be used? Also what Y pipe and cat would I need? Please don't tell me to use headers. As keeping the stock look is a priority. I just want to do some basic upgrades that won't look like I'm trying to build a race car.
AIR plumbing is the same, everything else is different. You'd need the manifolds, spacer, Ypipe, cat, etc. Probably not worth doing unless you can find all the stuff to do an N10 swap.
Thanks for the info Drew. I figured since I found the manifolds for $100. An I needed to tear it apart and replace part of the exhaust anyway. I would see if they would bolt up and be do able. I'm not sure yet if my cat is any good.
Originally Posted by TransamGTA350View Post
There were only 2 different manifolds for the V8's throughout the years, but they were different depending on what motor and gearing package they came on. Here is the breakdown.
The manifolds from the single 3" cat and the dual 2.25" cats are interchangeable. On the passenger side manifold, some will have the heat riser butterfly valve, some will have a spacer, and some will just have a donut gasket. You most likely have the wrong manifold for your application. If you have the larger manifolds, then it may be worth it to get the larger y-pipe for some more performance.
I found out recently that the 305 and 350 TPI's use the same exhaust manifolds...save your money unless you have something else in mind?
Sorta right... but a convertible wouldn't be a G92 car, therefore OP has the TBI manifolds. I'd only do it if I had the stuff laying around or could get it dirt cheap. It's not a huge difference unless going from the tiny single to dual cats, even then if you're going to spend money doing it, headers would make more sense. I surely wouldn't give more than $50 for a set of used manifolds.
Originally Posted by TransamGTA350View Post
There were only 2 different manifolds for the V8's throughout the years, but they were different depending on what motor and gearing package they came on. Here is the breakdown.
The manifolds from the single 3" cat and the dual 2.25" cats are interchangeable. On the passenger side manifold, some will have the heat riser butterfly valve, some will have a spacer, and some will just have a donut gasket. You most likely have the wrong manifold for your application. If you have the larger manifolds, then it may be worth it to get the larger y-pipe for some more performance.
I was in the manifold thread where the above was printed. I asked WHO the author was, with no response. WHO reported the above info and how do they know it to be correct?
See where it says "Originally Posted by TransamGTA350View Post". That's a link to the source. It doesn't really matter who looked it up that time. Anyone can look up the part numbers in the GM catalog and see what manifolds were used where. You can look yourself, or take his word for it, or my word for it. Personally, I'd find a copy of the Parts and Illustrations catalog, even if it's a PDF that you might find somewhere on the History subforum, and verify it for myself before I spent any money, but that's just me.
Drew, so your are saying the printed data comes from the GM Parts Catalog? I did not see any TGO poster acknowledging GM as the source. Even by clicking the aforementioned "LINK". And viewing "TransamGTA350"'s post of 12-25-13
I am not an English Major, but it is a general procedure, when quoting information to acknowledge the source of said information. It lends credibility and reliability to the information.
I was in the manifold thread where the above was printed. I asked WHO the author was, with no response. WHO reported the above info and how do they know it to be correct?
My post back then was based on my own experience with these cars since 1994 and research with others on this forum 15 or so years ago. Years and years of questions, debates and discussions. It was posted back then to be helpful to the person asking the question.
To the OP, I have done this same manifold swap on 2 different cars about 20 years ago. One was an 83’ with an LG4 and the other was a 92’ with an LO3. It required the entire exhaust to be swapped (manifolds, y-pipe, converter, cat-back). As Drew stated, it’s only worth doing if the manifolds and Y-pipe are free or almost free, which was the case for both of the ones I did. It did make a noticeable difference in power though.
"My post back then was based on my own experience with these cars since 1994 and research with others on this forum 15 or so years ago. Years and years of questions, debates and discussions. It was posted back then to be helpful to the person asking the question.
To the OP, I have done this same manifold swap on 2 different cars about 20 years ago. One was an 83’ with an LG4 and the other was a 92’ with an LO3. It required the entire exhaust to be swapped (manifolds, y-pipe, converter, cat-back). As Drew stated, it’s only worth doing if the manifolds and Y-pipe are free or almost free, which was the case for both of the ones I did. It did make a noticeable difference in power though."
Accompanied your post, it would have explained how your information was derived. Thus allowing the reader to better evaluate it's value and accuracy.
"My post back then was based on my own experience with these cars since 1994 and research with others on this forum 15 or so years ago. Years and years of questions, debates and discussions. It was posted back then to be helpful to the person asking the question.
To the OP, I have done this same manifold swap on 2 different cars about 20 years ago. One was an 83’ with an LG4 and the other was a 92’ with an LO3. It required the entire exhaust to be swapped (manifolds, y-pipe, converter, cat-back). As Drew stated, it’s only worth doing if the manifolds and Y-pipe are free or almost free, which was the case for both of the ones I did. It did make a noticeable difference in power though."
Accompanied your post, it would have explained how your information was derived. Thus allowing the reader to better evaluate it's value and accuracy.
There is no need. It’s not a published research paper and there is no reason to cite my “sources” on every response to every post. Many on this forum have been here a long time, have knowledge and are just trying to help people out. You just asked me where/how I got the information and I politely answered you and you respond by criticizing a post I made 17 years ago? Real nice. Use the info, don’t use it, or look it up yourself. Good luck whatever you choose to do.
Last edited by TransamGTA350; May 8, 2020 at 08:41 PM.
This is getting out of hand. As all I am saying, when posting facts, recommendations, etc., the poster should provide SOME IDEA, of how the facts were gathered or why an opinion is such.
I don't know about anyone else, but when I got into these cars they were all over the place. I worked until midnight at the time, so I'd spend the midnight to 2am time slot driving around to all the car lots in town to see what was new. It wasn't every night, but close. If anything caught my eye, I'd check it out, before work the next day. I've probably looked over a thousand thirdgens in the dark of night, and driven all kinds of them during the day. From behind the wheel, you figure out really fast that V6s, CFI, TBI, and 305 TPI auto cars aren't worth getting up early to test drive before work. Sometimes you make an exception, like the 91 Trans Am 305 auto I drove one time. Beautiful car, but slower than ****. First thing I did was look underneath, single cat and TBI/V6/LG4 intermediate pipe. So then I looked over the papers, well damn, 305 autos without the performance package have the TBI exhaust... That's gay. Never bothered looking at another late thirdgen without dual cats. I mean that's pretty much the GO/NO GO test for thirdgens. I really only get excited about 89-92s, and pretty much only the LB9 5spd and L98 cars, because everything else feels like they're chained to a God damned tree.
You just sorta pick up on these things, and go looking for plausible reasons that one car could stomp a mud hole in another. He figured it out from asking questions and comparing notes, I've seen it in person, read it in threads, and verified the ones that interest me in the catalog. I personally believe in trust, but verify. Because if I'm spending my money, I want something more concrete than someone else's opinion. Fortunately we've all got this huge resource, more complete than any public library, right at our fingertips and indexed with multiple search engines. It's never been easier to look something up.
That's one of my photos. The middle manifolds are TBI garbage, the outer are 91 Z28 L98 manifolds. The driver's manifolds are similar, the outlet on the passengers manifold is longer and a different angle on the HO manifolds.
That's one of my photos. The middle manifolds are TBI garbage, the outer are 91 Z28 L98 manifolds. The driver's manifolds are similar, the outlet on the passengers manifold is longer and a different angle on the HO manifolds.
HO Manifolds being the 91 Z28 manifolds, in the middle? If that's correct, are they also known as L98 manifolds?
HO Manifolds being the 91 Z28 manifolds, in the middle? If that's correct, are they also known as L98 manifolds?
In the photo the top and bottom are the L98 (HO) manifolds. The middle 2 are the TBI low output manifolds.
There were only 2 manifold types for all thirdgen v8’s and this picture shows the difference. In 1982, only the small outlet manifolds came on all v8’s and were used in all years for the lower output v8’s. In 1983, the large outlet manifolds were introduced for the 305HO and in later years also used on the 350TPI L98 and some 305 TPI LB9.
All thirdgen V8’s had either of these manifolds (Low output or High Output).
You can call them whatever you like. L98s used them, but so did a bunch of 305 TPI cars, L69 cars, and the ZZ3/ZZ4 conversion package that GMPP sold for upgrading LG4/L69 thirdgens. What all those engines have in common is they're HO - High Output V8s. There's also that little detail that the cast spacer usually used with the passenger side manifolds has HO cast right into it.
I call them HO manifolds, you can call them "bigger" manifolds, or Big Rhonda manifolds, etc. Sorry if it comes across as defensive, but every time it comes up, some smart *** has to come along and say the only "HO" engine is the L69. Anyway....
The absolute top and bottom manifolds came off a 91 L98 Z28. The middle manifolds are LG4/L03 parts from some pile of scrap I liberated. The larger manifolds alone really aren't worth much as an upgrade. It's the much less restrictive Y-pipe and the DUAL CATS that make the difference. You need the manifolds, the larger spacer or donut, and the Y-pipe to get the benefit, and honestly unless you can get the dual cats it's really not worth messing with. Especially when you consider that the stock dual cats are 30 years old, they liked to crack and clog when they were new, all the aftermarket dual cat setups are apparently crap, AND the hangers and related hardware are all completely obsolete. There's virtually no reason that a person should do the manifold swap at this point in time. It's a mod that's really not relevant anymore.
You can call them whatever you like. L98s used them, but so did a bunch of 305 TPI cars, L69 cars, and the ZZ3/ZZ4 conversion package that GMPP sold for upgrading LG4/L69 thirdgens. What all those engines have in common is they're HO - High Output V8s. There's also that little detail that the cast spacer usually used with the passenger side manifolds has HO cast right into it.
I call them HO manifolds, you can call them "bigger" manifolds, or Big Rhonda manifolds, etc. Sorry if it comes across as defensive, but every time it comes up, some smart *** has to come along and say the only "HO" engine is the L69. Anyway....
The absolute top and bottom manifolds came off a 91 L98 Z28. The middle manifolds are LG4/L03 parts from some pile of scrap I liberated. The larger manifolds alone really aren't worth much as an upgrade. It's the much less restrictive Y-pipe and the DUAL CATS that make the difference. You need the manifolds, the larger spacer or donut, and the Y-pipe to get the benefit, and honestly unless you can get the dual cats it's really not worth messing with. Especially when you consider that the stock dual cats are 30 years old, they liked to crack and clog when they were new, all the aftermarket dual cat setups are apparently crap, AND the hangers and related hardware are all completely obsolete. There's virtually no reason that a person should do the manifold swap at this point in time. It's a mod that's really not relevant anymore.
What's wrong with a single 3-inch high-flow cat? Even with dual cats, they're still flowing into a single exhaust pipe...
In the photo the top and bottom are the L98 (HO) manifolds. The middle 2 are the TBI low output manifolds.
There were only 2 manifold types for all thirdgen v8’s and this picture shows the difference. In 1982, only the small outlet manifolds came on all v8’s and were used in all years for the lower output v8’s. In 1983, the large outlet manifolds were introduced for the 305HO and in later years also used on the 350TPI L98 and some 305 TPI LB9.
All thirdgen V8’s had either of these manifolds (Low output or High Output).
I don't know about the '82's but in '83 the Crossfire cars got the HO manifolds, as a matter of fact, my '83 Crossfire TA, my '87 LB9 TA and my '87 Formula 350 (all WS6 cars) all had the bigger HO manifolds on them.
I don't know why this thing needs to look stock, but I agree that it's not worth it, headers are cheaper than getting a whole head to cat factory setup, make it easier to work on, sound better and make MUCH more power. I'm not sure you'll get them cheap used, not for $50, we used to throw them out, but when I pulled them off my '87 a few years ago I was surprised, I was suggesting I might have a nice set and before I even listed them on a for sale site I had a few offers, all for multiple times what I thought they'd be worth, and people were willing to drive way out of their way to get them first, and I'm not even in CA with crazy emissions laws (here these cars are old enough that they're not required to be tested)
What's wrong with a single 3-inch high-flow cat? Even with dual cats, they're still flowing into a single exhaust pipe...
Nothing. Except that the T junction in the stock HO Y-pipe and the catalytic converter are the choke points. The dual cat Y-pipe doesn't have a T junction, the pipes are relatively smooth into the cats, individual cats flow better than a single, and the junction behind the cats is smoother than the T in the single cat Y-pipe.
If you are trying to get the most power out of stock stuff, the dual cats are where it's at. The manifolds are bad, but it's the Y-pipe that's really the problem with the single cat setup.
I've got one of the Magnaflow Y-pipes in the attic. It's slated to go on the Iroc, but then I've got a set of headers for the Iroc too. Haven't decided if I want to mess with the headers or the Y-pipe, it's not like I really drive the car, or that either change would make much difference. It's kind of a big waste of time, it's a TPI 350, it's not going to win any races. LOL
I've quoted TransamGTA350's above post many times and find it completely correct. It encapsulates all the 3rd gen V8 exhausts in one concise, handy post.
Ive got the HO manifolds and Magnaflow y-pipe but I also Hooker headers and haven't made a decision on which one to use if I ever get around to my swap. Like Drew said Im not going to win races with a stock L98 engine anyways.
There's actually different manifolds for 1986 and earlier V8s and 1987 and later V8s. So there are at least 4 different pairs: 82-86 small, 83-86 large, 87-92 small and 87-92 large.
The difference is due to the new heads introduced in 1987 with the center-bolt valve covers.The valve cover mounting surface is wider on the 87 and later heads.
If you have the 87 and later heads, you need to use the later exhaust manifolds. 14094063 (LH) and 14094064 (RH) are the later manifolds with the large outlets.
If you try to bolt early manifolds to late heads, you will likely have problems....something to keep in mind when buying used manifolds.
The only reason I started this post was because my 88 iroc had the same motor and trans as my 92 convertible. But the iroc ran better. It had the larger manifolds and exhaust. Also I want to keep the stock look. After reading these replies and searching the internet the swap doesn't seem very practical. Since I can only find the manifolds. An without a spacer and y pipe I can't even figure out a cost.
Im hoping the larger manifolds and well designed y-pipe from magnaflow isn't much of a power robber over a hooker header and y-pipe system. I like the idea of not having to tighten headers and be watching for leaks and etc. This will be the first time ive ever put manifolds back on a car ive always went with headers but this car aint for racing.
I also just wanted a little boost in hp without using headers. But I couldn't find all the needed parts to even figure out if the cost was worth it to me. I did find some aftermarket y pipes that mite work but no spacer.
hw4mike, the bottom line is if you have the smaller manifolds used on the TBI cars, then replacing them with HO's found on the L98 and some LB9s will improve that restriction. BUT while they will bolt right up to the block, they will NOT fit the Y pipe as the Y pipes are different.
Further, there are bigger restrictions in the air flow than the manifolds such as the cat, the intake, etc. They all work together as a package. That's why if you put Dyno Don headers on a stock TBI, you won't notice much difference, but if you put them on a supercharged TBI with free flowing exhaust and stock TBI iron manifolds, oh yes.
I changed the 2" manifolds on my former '83 CFI with a 2.25" pair from an L-69. As has been mentioned, this requires a new Y-pipe for them to line up. I had my shop do a whole new y-pipe and 3" IP. It had more power, but all of that has to be seen through the lens of the 1980's. Did I gain 10 hp? 15? More? Back in the day that was a significant bump.
hw4mike, the bottom line is if you have the smaller manifolds used on the TBI cars, then replacing them with HO's found on the L98 and some LB9s will improve that restriction. BUT while they will bolt right up to the block, they will NOT fit the Y pipe as the Y pipes are different.
Further, there are bigger restrictions in the air flow than the manifolds such as the cat, the intake, etc. They all work together as a package. That's why if you put Dyno Don headers on a stock TBI, you won't notice much difference, but if you put them on a supercharged TBI with free flowing exhaust and stock TBI iron manifolds, oh yes.
This is true. For a mostly stock motor, the HO manifolds, dual cat y-pipe or Magnaflow single cat y-pipe, high flow cat and 3" cat back will make a difference and headers won't offer that much further improvement. Match it with a higher flow air intake and it's a reasonable low cost performance improvement. If you are doing a more aggressive cam, higher flow heads, etc., then the manifolds become a bigger restriction.
My brother had a 92' RS with a 305TBI/auto years ago. This was back in our broke college days, so any performance improvements had to be done on the cheap. We did the HO manifold swap with a stock 3" y-pipe (free from a friend who swapped to headers), Magnaflow 3" cat and Flowmaster 3" cat back. We swapped the restrictive air filter assembly to an open element, bumped the base ignition timing from 0 BTDC to 6 BTDC with 93 octane and swapped the entire rear end from the original 2.73 to a 3.73 from an 84' T/A that we got for next to nothing. It was a considerable amount of wrenching time, but minimal money and it made a big difference in the fun factor of driving the car. We did some before an after 1/4 mile runs and stock was mid-high 16's and after was mid-low 15's. So, it went from really slow to less slow, but was more fun to drive and sounded good.
Last edited by TransamGTA350; May 13, 2020 at 09:43 AM.
Other than exhaust I can tell you putting a 3.23-3.73 gear + posi rear, a mild torque converter "maybe 2400" such as an S10 converter, and shift kit will make your car feel like a different car all together. All of these changes will go over well when or if you do a 350 swap or any improvements later on.
Totay, I finally got under the hood of my 1989 IROC Z Vert, 305 TPI / auto. The left side exhaust manifold has a 14094063 cast into it. I guess that is the "better" manifold?
You don't exactly need the spacer. You can use a typical exhaust donut. Likewise you can reuse most of the stock Y-pipe for smaller manifolds, just have your exhaust guy cut the pipe for the passenger's side and fab a new one to fit at the altered angle. Been there, done that, probably would not bother doing it again. The Y-pipe is the worst part of the system, without changing it to a larger Y-pipe and getting rid of the T junction, the manifolds aren't going to make much difference.
You know, it used to make a lot more sense to do the HO manifold swap. They were everywhere, and dirt cheap. You could find them in the junk pile or the junkyard, and headers were $600 for a good set, cheaper headers didn't fit, would rust out, and generally fall apart. Now there are 6 or 7 decent sets of headers available, in stainless or ceramic coated, and they're CHEAP. You can get Chinese Ebay headers for a couple hundred bucks. You can get Hookers, Hedmans, used SLPs, used Edelbrocks, Dyno Don's... I mean I've got a set of barely used stainless coated SLPs on the shelf and a new coated set of Hooker 2460s, and didn't give hardly anything for them.
Totay, I finally got under the hood of my 1989 IROC Z Vert, 305 TPI / auto. The left side exhaust manifold has a 14094063 cast into it. I guess that is the "better" manifold?
Yes and that would be correct for a 1989 TPI motor. You should also have the 3” single cat y-pipe.
That manifold casting number is the same as my 1989 350TPI w/dual cats.
Totay, I finally got under the hood of my 1989 IROC Z Vert, 305 TPI / auto. The left side exhaust manifold has a 14094063 cast into it. I guess that is the "better" manifold?
Yes! That's the good one.
That Magnaflow Y-pipe looks pretty nice...
i used the magnaflow y pipe fits perfect with the spacer,used a universal magnaflow cat and scored a 3 inch pypes system a friend had that he never used.
I bought my exhaust spacer from someone on this website that sells parts. Sometimes you see them on E-Bay. Magnaflow y-pipes are easy to find everywhere from Jegs to E-Bay, mine was less than $200
Thanks for all the good info. As for why this car needs to look stock. There were only 266 convertible Z28's with the Z03 option. There were even less with the tpi and 5sp manual (numbers not available). So for me keeping everything as original looking as possible is important. I know it's **** but that's my problem. I also know it's an 80's car and will never be really fast. An like I said my 88 iroc had a little more pep and better sound. Since gm didn't allow the same options on the convertible as the coupe. I wanted to see if I could find some bolt on items that won't change the stock look. That will make this car breath and exhale better. What I'm asking for may not be possible but you can't fault a guy for trying. Also money is not and issue. I've owned this car since new and it's my fault it's in the shape it's in. Now that I'm retired this car is going back on the street regardless what it cost me.
If anyone know were I can find the cast iron spacer for the right side exhaust manifold let me know.
If the only part you can't find is the spacer, just use a donut. Some later thirdgens used what is basically a steel donut. Your car might already have some version of the spacer on it. It's just mild steel or cast iron, bore it out, hone it, run a sanding drum inside it, cut out the EFE flapper, whatever - it's not difficult.
You could hit up " Beth's 91 Camaro " on this board. She scraps third gens and she ships anywhere. I think you will need the spacer if you go with the Magnaflow y pipe. I just had it done 6 months ago and it all fit to perfection. I too have a 91 with the LO exhaust, well had the crappy exhaust. I went with the HO manifolds (for all the same reasons as you except my vert is 1 of 3200!) the Magna flow y pipe, a custom made 3" test pipe (where the cat should be) and a hooker competition 3" cat back with their Aero Chamber muffler. What a big difference in power and sound! I think I got a good 25hp increase, it was that noticeable of a power and audible difference. The cost for all the parts including the manifolds I got from a member here was $725.
