Maybe this should go in Q&A...
But since it's a Restoration item, and most of the resellers lurk here... I'll start here.


My question is this... (based on what FAST guys are doing with them)

Are all of the current Repo HP Manifolds "Exact" reproductions internally?

Here is why I ask...
I have spent years in the boating world, where cast iron exhaust manifolds are the norm.
However over the years several aftermarket companies, like GLM, began to manufacture improved replacements
These replacements incorporated internal casting changes that were far superior to the original designs, yet looked exactly the same externally. (Some even cast them in Aluminum) Eventually the original manufacturers also implemented the same internal changes allowing for improved exhaust flow across the board.

From what I've read, the FAST guys have figured out how to make the stock exhaust "Flow Better".

So, my question is...
Has anyone incorporated any of those "changes" into the internal casting of the new Repo HP Manifolds?
If not, why not?
I would think there would be a strong market for anything with improved flow, just like the marine counterparts.

Just wondering.
thx

Just a guess, but I would think the f.A.S.T. guys are using the Extrude hone process of liquid media being run thru the ports/passades until they are smooth and hogged out as desired

Yup - They use Extrude-Hone. I am sure the makers of the manifolds wont pay up for precision castings because of the volume as compared to the Chevy stuff.

Im buildin a FAST car now and bought a set of the repop manifolds which are n the process of getting extrude honed out.going to hog the hell out of them as well as a intake manifold. i didnt want to ruin a good set of original manifolds.

Just finished this build.

Stock untouched manifolds. "Mild" FAST build.

596 HP. 607 lb/ft trq.

Point is EH is a surface preparation. It does nothing in the way of "hogging out". Little of the parent metal is removed. And after a short time of run in, the manifold gets a coating of surface rust and carbon buildup, so any gains in surface prep quickly vanish. Then add to that fact that you still need to run it through a stcck type 2.5" exhaust system. Better than nothing. perhaps, but don't expect much.,,,

One of the top FAST racers claimed there was very little HP gain from Extrude Honed exhaust manifolds.

Oops, I hear it works well on intake manifolds and heads?

who's repoping the HP manifolds? I haven't seen them advertised anywhere. I'm looking to can the headers on my GTX and go back to stock manifolds. The HP's breath just as good as headers with a full exhurst system and don't leak.

Year One makes the HP manifolds

I would suspect that most of any gains in the manifolds flow would be in the matchin of the portd to the exhaust ports on the heads and also at the exhaust pipe flange as there is a bit more metal on the flange end. About $500-600 for a 3 HP gain seems like a waist. I think you woud gain more by hand porting the exhaust manifolds and Extrud Honing the intake.