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These things are awful restrictive on the outlets. If you install them the way they come, you are strangling your engine
the as cast inner diameter is only 1.87” 😳
i cut them to 2.200” all the way. They probably have enough material to go 2.4 or 2.5” but these are going on a stock type build with 2 1/4 down pipes so I didn’t go any bigger
Last edited by CANADIANOLDS; May 23rd, 2023 at 01:31 AM.
I just went through this a few months ago with these exact manifolds on my 403 build. They really shocked me as to how restrictive they where on the outlet ends - just exactly what you show. Using a borescope, I examined the insides of the manifolds to find them otherwise in good shape. Use your common sense here, just like porting a cylinder head. Get busy with a host of various carbide bits, especially some overlength bits, and hog them out - WAY out. Let your fingers do the talking, feeling the contours and profiles inside as you work with them, also using your fingers as a basic measure of wall thickness during your mods. I ended up taking a massive amount of metal out before finding my happiness, making sure that I had felt that nowhere did I allow wall thickness to go less than approximately 5/16". I would say you are off to a good start, but keep going to smooth out and contour everything you can reach leading up to the nice work you have already done so far.
After that, I cleaned the fire out of them, then coated them with this:
Git 'er done - these are great manifolds in my opinion. Their big brother W30 manifolds are the same way - port them accordingly, and they also flow very very well too.
they don’t have a true full divider between the centre ports because it doesn’t go down as far as the factory one on W30 manifolds and it doesn’t hook towards the exit… the centre one only goes part way and ends at the bottom of the opening
the rear port exist just dumps open into the manifold..not good for flow
ill post pics of them compared to a factory split flow big block manifold.
you don’t need no bore scope to see inside these things, they are wide open, you can see everything
Last edited by CANADIANOLDS; May 23rd, 2023 at 07:02 PM.
The W30 repro Thorntons that I have do have the internal dividers just like the factory W30 manifolds - I know, I have them. These also received the full outlet treatment and ran exceptionally well on my 488 stroker.
Thank you for posting on this. Is it fair to assume these porting mods correct an "as cast" deficiency in the aftermarket pieces, making them the equivalent of the factory W30 manifolds? Or is there any reason to believe they are better, and therefore somewhere between the factory W30 manifolds and headers?
Thank you for posting on this. Is it fair to assume these porting mods correct an "as cast" deficiency in the aftermarket pieces, making them the equivalent of the factory W30 manifolds? Or is there any reason to believe they are better, and therefore somewhere between the factory W30 manifolds and headers?
I have more pics to post but I can’t for some reason..I’ll try later
they definitely need to be worked on at the exits or they won’t even flow as good as stock SB manifolds.
no, these are not equivalent to split flow bb ones. My pics will explain better when I can load them
Thank you for posting on this. Is it fair to assume these porting mods correct an "as cast" deficiency in the aftermarket pieces, making them the equivalent of the factory W30 manifolds? Or is there any reason to believe they are better, and therefore somewhere between the factory W30 manifolds and headers?
These are small block manifolds and don't really compare to the big block manifolds. Also there's no such thing as a W30 exhaust manifold. The same exhaust manifold was used on any A-body equipped 455 and a W-30
May 23rd, 2023, 01:29 AM
Yeppers
