Can you make factory 403 Olds manifolds less terrible?

[Maxthe222]

Hey gang,

You know how with the 403 Oldsmobile manifolds, the one on the right-hand side splits off into like a y-shape dual exhaust port thing that is meant cross over into the other exhaust manifold? Is there a proper way of blocking them off? And this is the thing, if you do block it off do the exhaust, do the gases just get trapped in that weird "ditch" kind of thing they are angled?

I know the factory ones really do suck, but getting proper manifolds is going to cost something like a thousand Australian dollars. I likely need to get a new exhaust and new manifolds really aren't cheap, especially with shipping and the way the Australian dollar is at the moment, so what is the best way to approach the factory Oldsmobile manifolds?

[Wallington]

The factory actually blanked it off on some early 'performance' options, and also the FWD Toronado. But others simply make a blank, or weld a cap onto the original crossover. No, it just becomes an area of gases spinning in a void while the rest rushes past. Similar to the air trapped at rear of an un-cut shaker. Or on Mythbusters checking the airflow over a ute with tailgate down or up, the trapped air was less drag over vehicle. Those clever guys at Olds exhaust....ahem.

Olds exhaust crossover cap by Aus78Formula, on Flickr

Oldsmobile crossover exhaust manifold plate2 by Aus78Formula, on Flickr

[Maxthe222]

Ok great, shame no one just reproduces them outright, but seems easy enough to fabricate I'll have to crank out a few of them, at least the poor thing will breathe a lot better

[Wallington]

There's a few out there, original plate style to a thick cast iron block type. I have several originals, but easy to make something or an exhaust place that has similar, even a clamping flange with a disc welded into it.

https://www.ebay.com.au/itm/274449550294?chn=ps&_ul=AU&norover=1&mkevt=1&mkrid=705-139619-5960-0&mkcid=2&itemid=274449550294&targetid=1333657101007&device=c&mktype=pla&googleloc=9071908&poi=&campaignid=9767741252&mkgroupid=123080143856&rlsatarget=pla-1333657101007&abcId=578876&merchantid=115488642&gclid=Cj0KCQjwmuiTBhDoARIsAPiv6L-_DUU-UuOq3TjWUSTqCP0nc56kt8BKrkGLZQqXmemA4nL0IAzK_1caApqUEALw_wcB

[Maxthe222]

Nah I wouldn't waste nice originals, i'll just buy a few off those repro ones and weld those on!

[DynoLee]

Out of stock right now, but Thornton makes small block versions of their W-30 manifolds.

https://www.thorntonmusclecars.com/product-page/thornton-s-350-jr-exhaust-manifolds

[Maxthe222]

Yup, i'd like those but at the moment, that's going to be over $1000 AUD to get here in Australia right now

[olds403]

100 % agree on the thortons.  If you have  money burning a hole in your wallet- ARH headers
I hsve ARH

https://americanracingheaders.com/collections/trans-am/products/trans-am-78-79-headers-with-the-403-small-block

[Wallington]

Mike, did you ever install a set of the Thornton SBO shorty headers? Sort of a compromise between manifolds and full headers but never any real feedback on them.

[olds403]

That motor was dyno’ed with full length headers.  The car will get the shorties.  Just because

[lugnuts]

Mike, did you ever install a set of the Thornton SBO shorty headers? Sort of a compromise between manifolds and full headers but never any real feedback on them.

Mike, did you ever install a set of the Thornton SBO shorty headers? Sort of a compromise between manifolds and full headers but never any real feedback on them.

Mike, did you ever install a set of the Thornton SBO shorty headers? Sort of a compromise between manifolds and full headers but never any real feedback on them.

Don't waste your money on the shorty headers they suck for several reasons. 1. Some of the header bolts are impossible to get to even with the engine out of the car. and in the car the only way to take the drivers side header off is to cut it off because you can not get to one of the bolts at all (its a pain to get to it with the engine out) not sure why they designed them the way they did.
2. the collector is a major restriction as it is the size of the factory downtube. these were designed to bolt into a factory exhaust system and not really designed for aftermarket systems with larger diameters.

I am getting rid of mine and looking at other options but no idea what to get.

[lugnuts]

100 % agree on the thortons.  If you have  money burning a hole in your wallet- ARH headers
I hsve ARH

https://americanracingheaders.com/collections/trans-am/products/trans-am-78-79-headers-with-the-403-small-block

how do they fit? when I talked to them they told me they were not designed for the TA but fit. wouldn't give me any more details or even pictures of them. I would kind of like to see what I'm spending 1500 on and know they fit good.

[Wallington]

X2 I have no interest in buying them but like to hear of what options work and how well. It also helps make suggestions with informed knowledge from others when the topics come up. I also find their lack of information or any performance or other gains to be more to cover up the lack of, so acting dumb is their choice instead. And I doubt that's the case.