Which Thornton exhaust would you choose?? Headers OR Exhaust Manifolds...

[olds307 and 403]

I saw that but the shorties are already a compromised design for fit. Flattening a very short tube has to block flow. Olds small block manifolds are terrible. The Thornton manifolds are only a slight improvement as cast. One member on here spent some grind time to actually make them the best they could possibly be. I noticed a good amount up top from 260 manifolds capped off with 1 3/4" dual exhaust to Sanderson shorties with a 2.5" X pipe exhaust. Was a good percentage the 2.5" dual exhaust, probably. Another reason to get the Ram Air Restorations true 2.5" mandrel bent down pipes and open up the manifold outlets to match the 2.5" opening.

 

[pontiacgp]

I other thing I forgot to mention that I like is the headers have a ball and socket collector which uses no gasket and is not prone to leaking. I have those collectors on my Hedmans

 

[JAMCAR223]

Thanks for the responses guys!! I appreciate it. I was already going to buy the manifolds, but wanted some validation. Well you guys gave it to me. Although I agree the 1/2" flanges on the headers would be less likely to leak, my motor probably runs out of steam before I could take advantage of them. I also agree that the castings on the manifolds are not the best looking. I can fix that, but I'll have to see how porous the finish is. I don't want smooth flat spots in the finish. We'll see. I'll be ordering the exhaust manifolds, and hoping for a reliable leak-free seal.

 

[oldsmobile joe]

Thanks for the responses guys!! I appreciate it. I was already going to buy the manifolds, but wanted some validation. Well you guys gave it to me. Although I agree the 1/2" flanges on the headers would be less likely to leak, my motor probably runs out of steam before I could take advantage of them. I also agree that the castings on the manifolds are not the best looking. I can fix that, but I'll have to see how porous the finish is. I don't want smooth flat spots in the finish. We'll see. I'll be ordering the exhaust manifolds, and hoping for a reliable leak-free seal.

I have their manifolds, but the w30 ones on my 455 calais. Fit like a charm. I did a little porting on the outlets. Their small block manifolds look great.

 

[pontiacgp]

Thanks for the responses guys!! I appreciate it. I was already going to buy the manifolds, but wanted some validation. Well you guys gave it to me. Although I agree the 1/2" flanges on the headers would be less likely to leak, my motor probably runs out of steam before I could take advantage of them. I also agree that the castings on the manifolds are not the best looking. I can fix that, but I'll have to see how porous the finish is. I don't want smooth flat spots in the finish. We'll see. I'll be ordering the exhaust manifolds, and hoping for a reliable leak-free seal.

there goes my commission.... 😢

 

[olds307 and 403]

I have their manifolds, but the w30 ones on my 455 calais. Fit like a charm. I did a little porting on the outlets. Their small block manifolds look great.

I think Thornton did the best they could with the space provided. Unfortunately, due to space they aren't as good as the Big Block ones, not completely divided inside or a as large. I was talking about the interior casting needing material removed to get the maximum flow possible, supposedly a lot of casting boogers. I haven't heard of any leak issues with their manifolds except on 7A 307 heads, which have small, goofy exhaust ports. Their shorties seal the exact same way as the manifolds where the exhaust pipe connects, no gasket, which is nice.

 

[Max Headroom]

You seem to have already made your decision but I wanted to mention a couple of things:

• I would find out how thick the manifold castings are and compare that to the wall thickness and outside dimensions of the headers. I would imagine that the manifolds are more than a little bit thicker to prevent heat cracking. If they are, some of that additional thickness will almost surely be to the inside of the manifold, making the exhaust passage narrower. This will reduce flow capacity.
• The manifold will be combining the exhaust of all of the cylinders on one side into one exit "body". This is going to crap on the reverse pulses in the exhaust and impede exhaust clearing at moderate to high rpms. Not exactly back pressure but just like 50 people trying to get out of
• I would ask them for flow rates for both pieces and compare them for total flow. I would bet the headers flow better. The question is, how much.
• Remember that, dents, flats spots or whatever, the headers have individual runners that keep the exhaust gas confined to a single cylinder. Studies show that this greatly improves exhaust gas scavenging and helps improve incoming air/fuel charge density. I realize that the headers don't have a collector but having the exhaust travel even a few inches in a separate pipe will provide some benefit, especially if that pipe has more flow than the manifold due to a larger inner diameter.

Just a thought.

 

[JAMCAR223]

You seem to have already made your decision but I wanted to mention a couple of things:

• I would find out how thick the manifold castings are and compare that to the wall thickness and outside dimensions of the headers. I would imagine that the manifolds are more than a little bit thicker to prevent heat cracking. If they are, some of that additional thickness will almost surely be to the inside of the manifold, making the exhaust passage narrower. This will reduce flow capacity.
• The manifold will be combining the exhaust of all of the cylinders on one side into one exit "body". This is going to crap on the reverse pulses in the exhaust and impede exhaust clearing at moderate to high rpms. Not exactly back pressure but just like 50 people trying to get out of
• I would ask them for flow rates for both pieces and compare them for total flow. I would bet the headers flow better. The question is, how much.
• Remember that, dents, flats spots or whatever, the headers have individual runners that keep the exhaust gas confined to a single cylinder. Studies show that this greatly improves exhaust gas scavenging and helps improve incoming air/fuel charge density. I realize that the headers don't have a collector but having the exhaust travel even a few inches in a separate pipe will provide some benefit, especially if that pipe has more flow than the manifold due to a larger inner diameter.

Just a thought.

Good points. Thanks for that!

 

[olds307 and 403]

Thornton first designed the Small block header with 1 5/8" primaries and a 2.25" OD collector to work with stock 2.25" down pipes. One member of Classic Olds had interference at the oil filter adapter and one other pipe on the other side, so two spots that needed flattened, not good. He then used Pypes down pipes which have a necked down 2.25" opening into a 2.5" pipe. He went from full length headers that had the usual interference issues. He noticed a big power drop. Thornton redesigned the headers, basically made them to replace the awful log manifolds they first designed for the 455. They flattened one tube so they would also fit the 260 through 403. They have 1 3/4" primaries and 2.5" exit. Maybe with the flat pipe is less of a deal with larger tubes, maybe still provides adequate flow. The manifolds have a 2.25" exit and just don't the the space inside for individual runners like the W30 manifolds which still give up 30 hp over good, full length headers. I researched this quite throughly. Since I am doing up my 70, my choice is Chinese Stainless steel full length headers, which have thick flanges, are well made and fit but will hang low, being a BBO design, they say they fit both. They technically do but hang below the crossmember. My car sits high in the front, I plan on putting a floor jack on the collector to bend it up as much as possible, once installed. Tough choice.