Current set has the crossover, did they make ones without this? Thinking of not using my headers.

 

what car, year, engine? (I see this is in SBO, most 350s with dual exhaust used a block-off plate with the same manifolds as single exhaust cars)

 

Working on the 75 hurst, just thinking about getting a cheap set of 350 manifolds and running dual exhaust.

 

411961 LH & 417754 RH (not sure if those are casting or part numbers, but the numbers should be close)

They were used on many 260/350/455s thru the '70s.

 

I don't know those actual casting numbers you listed above, but all the sbo manifolds in the 70's had the crossover on the passenger side manifold no?

The only passenger side manifold for a sbo that I know of with no crossover BS were the #12's that came off of late 70's/early 80's Toronado's. The question then becomes, does it really matter or make a difference? I don't know, but if I were to guess I'd say not really.

 

Do they make something to cap them off then?

 

Yeah but I don't remember where. Year1 maybe? Either way, you can make your own easily enough. Just take the stock crossover pipe and cut it down close to the flange then weld it up. That's all the factory ones were like anyway.

 

An exhaust shop can fabricate a block off pretty easily. To the best of my knowledge, all SBO used the block off.
These are available aftermarket but not any cheaper than basic headers would be
http://www.ebay.com/itm/Oldsmobile-C...#ht_575wt_1141

 

Inline tube sells the block off http://www.inlinetube.com/PDF%20cata...dsmobile04.pdf

 

ALL RWD small block Olds manifolds have the crossover provision. Even the W-31s used this configuration with the cap on the passenger side manifold. Folks have adapted Toro manifolds, but these require custom bent pipes and sometimes the manifold itself needs to be cut and welded. No, BBO W/Z manifolds will NOT fit on an SBO as installed in a car.

And since someone will bring up Thornton's manifolds sooner or later, here are some thoughts:

https://classicoldsmobile.com/forums...manifolds.html

https://classicoldsmobile.com/forums...manifolds.html