Thornton exhaust manifolds (SBO) on a G-body

[kustomkyle]

Just wondering has anyone used these? Would be a great option for a daily driven car.

http://thorntonmusclecars.com/product/thorntons-350-jr-exhaust-manifolds

Summit etc. claim fitment for 1964-1977 only, however Thornton's website says anything up to 1988 including 307's in G-bodies.

Due to port size, I was told by a coworker they wouldn't seal against 1986+ 7A swirl port heads? I always thought it was the other way around (too small header/manifold port was no good). Here's some good pictures for comparison:

http://tech.oldsgmail.com/307_heads.php

Definitely look to be a better match to the earlier 5A head, at least due to port size/shape. But I have 7A's, and not having a header option makes using the factory Olds engine a harder option (since the factory manifolds are such a poor/restrictive design).

Any ideas? What specific head design these would work on, or any/all small block Oldsmobiles?

 

[olds307 and 403]

Yes, they will fit. A member used them on his 86 442. He needed gaskets to seal but they worked. One thing that is different is the angle of the passenger side manifold. Straight down on yours vs angled on the early cars and these manifolds. Anything is better is than those stock tubular manifolds.

 

[kustomkyle]

Thanks for the reply. I did a search and didn't find anything other than someone using them on a very modified Cutlass with a 350 Olds, but it was mentioned that there was a slightly different setup. I do plan on retaining my AIR tubes, which with an Olds doesn't seem like it would be an issue even with headers.

Any special type of gasket? Like perhaps the copper ones Thornton offers? I had bought a set of Felpro exhaust manifold gaskets for the early A5 heads (1984/1985) to use with headers, as all headers are made for the rectangular ports.

But it is good to hear I should be able to use them. I read Thornton is coming out with tubular stainless headers like they already have for the big blocks, and that would be nice too. However I'm really looking for long term durability and as little (as in no) maintenance as possible.

Any more input/experiences or links is appreciated. I stumbled upon the manifolds earlier today and was pretty discouraged to hear they supposedly wouldn't work due to port mismatch.

EDIT: Found the 1986 442 build utilizing the original swirl port engine. More modified than mine was, but it does seem like they should work fine.

https://gbodyforum.com/threads/1986-oldsmobile-442-build.46606/page-20

 

[88ss408]

You can get shorty headers for the SBO that would be better around the same price

 

[olds307 and 403]

The Sanderson shorties I have on my 350 should work, I had them on my 260 for a while, smaller 1 5/8" primary tubes. Yes, the Copper gaskets are good and very durable but even the cheap silver Felpros should work. There is an argument of how much better these manifolds are vs regular manifolds. But like any manifold, put them on and never touch again, antiseize on the nolts is a good idea. The Pypes down pipes are supposed to attach to these manifolds as well. I wouldn't buy them unless you are planning on true dual exhaust. The factory setup is dreadfully restrictive.

 

[kustomkyle]

You can get shorty headers for the SBO that would be better around the same price

Maybe, and at one point I was heavily considering the American Racing Headers $1200 stainless long tubes. However, I am going to use dual 3" catalytic converters. And, more than anything else, I just want to be "done" with it after the rebuild. Plus $1200 turns into $1500 because I would have to get a mini starter, not taking into consideration all the hardware, gaskets, and other bits and pieces that need relocated/modified.

Probably not the case with shorties, but I would still rather just have something better than what came on it, yet retain the "no maintenance".

It seems like no matter how high of quality or thick the flange, headers always leak, and very few people actually use them on something they drive everyday, year round. I might have 200 horsepower if I'm lucky, and that's fine. Driveabilty/go anywhere vs. what most people call "actual performance". Plus low 20MPG on trips.

 

[olds307 and 403]

What is your plan on the build?

 

[kustomkyle]

What is your plan on the build?

At this point basically a stock rebuild. Whether or not it will require an overbore, I'm not sure, whatever it needs. "True" dual exhaust, but retaining all the factory emissions equipment. Inspections won't allow without it, even if there are ways around it. I only see restrictions getting more stringent as time goes on, and I'm not looking to reinvent the wheel or hot rod anything other than to open up the horrible exhaust system.

I have a swirl port, roller NOS 442 cam, which was the plan all along to use someday, but I'm not sure if that's necessarily compatible without changing the computer, carburetor, gearing, torque converter, etc. That cam isn't going to just add 20 horsepower by itself I wouldn't think, as its profile was specific to work along with all the other tricks GM made to make the 442 what it was. In other words I'd like to use it, but if it's not compatible without changing a bunch of stuff, then I won't.

And no, swapping in a 350, 403, 400, 455, or a small block Chevy is not an option I'm considering unless the 307 ends up having some sort of terminal damage. And even then I'd probably just find another 307. It's a 200-4R, 2.56 rear geared car and staying that way.

 

[olds307 and 403]

I would use the cam, it is still mild but massively better than the stock cam 194/198 .400/.402 on a 109 lsa, Vin 9 roller 196/210 .435/.438 on a 109 lsa, which helps retain bottom end torque and help detonation uf anything. It should work fine even with Vin Y stuff, both are 7.9 to 1 compression, it should be worth 20 hp. The valve springs and balancer just allow more rpm. The factory cat back set up added maybe 5 hp, the 79/80 350 in the HO/442 gained that much. The rest is cam, carb and dual snorkel and maybe exhaust manifolds for the 30 hp gain. Pick up a Hurst/Olds 442 dual snorkel and run hoses to cold air. Good luck on 200 hp with the Vin Y cam.

 

[kustomkyle]

I would use the cam, it is still mild but massively better than the stock cam 194/198 .400/.402 on a 109 lsa, Vin 9 roller 196/210 .435/.438 on a 109 lsa, which helps retain bottom end torque and help detonation uf anything. It should work fine even with Vin Y stuff, both are 7.9 to 1 compression, it should be worth 20 hp. The valve springs and balancer just allow more rpm. The factory cat back set up added maybe 5 hp, the 79/80 350 in the HO/442 gained that much. The rest is cam, carb and dual snorkel and maybe exhaust manifolds for the 30 hp gain. Pick up a Hurst/Olds 442 dual snorkel and run hoses to cold air. Good luck on 200 hp with the Vin Y cam.

Thanks for all the good info.

"200 hp" isn't necessarily a goal, just a ballpark figure. Been driving the "140 hp" car for 10 years, losing cylinders along the way due to poor maintenance by the previous owner, and it's probably got less than 100 by now I'm sure. It's an oil burner. Rings are in there, but they're not doing the complete job.

Have two 1986 era dual snorkel air cleaners, one off a low mile black Utah 442 that got "cash for clunkered", and one I found locally at a car show/swap meet. Probably will use the swap meet one since the Utah one almost looks NOS.