I've read about the "factory appearing drags" races in a few recent magazines. Some of the cars in that series are running something like 11 sec. quarter miles through factory iron exhaust manifolds. The owners mention that the manifolds are ported and extrude honed (or something), but there's not a whole lot of detail there.

Sooo, has anyone ever tried improving the flow of factory manifolds, or know anyone that has? I'm thinking maybe some restricted class racers may know of this.

 

I know of someone who has.

However, I know of no "restricted class" other than that "Factory Appearing" that require using factory manifolds.

The bud who did this did so because he was waiting to get his SLP headers "right" (several years later, he's still waiting, but that's a different story - he's got debilitating help problems). He took a die grinder to the things and opened and smoothed as far as he could reach on each manifold. The engine was a ZZ3 with CNC ported heads, Big Mouth, TPIS runners, ported plenum - all of those things were extrude-honed as an assembly. He zero-decked the block, ZZ9 cam w/1.6 RR's, etc., etc., etc. With dual cats from the dealer and Big Mouth cat-back, it ran 14.0's here with 4.10 gears & 2000 stall. At sea level, that would be 13.0's to high 12's.

 

Cool, thanks for the info!

By the way, can you explain exactly what is extrude honing? I've never seen this term clearly defined.

 

It's a paste with abrasive particles that is forced through a passage to smooth it out. The paste is a "fluid", so it's a good process to prepare passages for fluid flow (doesn't remove material where flow isn't occurring, does remove material that restricts flow, etc.). This isn't a do-it-yourself-at-home process - requires special equipment - and set-up is expensive unless they happen to have a market for what you're doing.

 

There is one other guy - has a '37 Chevy pickup, weighs 3700 pounds. He has a 383 with roots blower on it (he's an engine builder by trade), he uses rams horn manifolds that he's "prepared". Runs mid-13's up here.

Toward the end of the 2002 season, he came back to the pits after a run with the engine roaring. Seems the passenger side manifold broke off above the exhaust pipe flange where he had thinned it out a little too much. Finished the season holding the exhaust pipe up to the manifold with a wire coat hanger.

In case you're thinking this must be a hack job, at the 2002 NHRA Division V ET finals in Topeka he won "Best Engineered".