[Tim M]

Likely not the most popular option, but I plan to remain with OEM exhaust manifolds/cats on my LT1 with a LT4 supercharger. Various reasons to include concerns over German inspection and sound (car is bit too loud in the cramped neighborhoods!)

Nonetheless, oldman posted an image (which I've reposted-thanks oldman!) of opening up the OEM exhaust (lead-in to the main cats). This is interesting as my tuner (Jannetty) commented reducing the output restriction would make more power (obviously), but also reduce tendency for knock (reducing boost as well) which I actually run a can of octane booster during events to mitigate.

Wondered if oldman or anyone else has more details/images of performing the process.

I realize at 650 CHP, each manifold/pipe is accommodating 325 CHP thru a tube that would be considered a pea-shooter on the 60's muscle car dual exhaust. Assume any gain in this area would be multiplied.

[LT1ornothing]

In your situation with the strict emissions compliance it may be worth exploring the porting option, those primary catalytic are an enormous restriction at your current settings, i guess you would pick up an extra 30whp at least without the catalytics on your current set up, and more with longtube headers of course. Tough emissions regulations there, few options.

[laynlo15]

I don't think it will help. If it does that would be cool.

[oldman]

I also grind away the O2 boss inside the pipe and space the 9)2 sensor out away from seat so there is far less projection into pipe. Both sides intake to pipe flange can be ground smooth, there is a an abrupt flange to pipe transition. The passenger side is easy, the transition and the 02 boss, only the driver”s side cat needs to be cut apart.

[Tim M]

oldman: do you have any additional images - helps tremendously with the German translation!

And how did you space the O2 sensors out? Washers?

Thanks.

[oldman]

I cut spark plug anti-fouler, but I also have used washers, I have also sectioned 02 bungs and welded them on the outside of the factory bung.

I can picture the stock stuff off the car, but the modified one is on the car so I'll wait till I try 2" headers...

[oldman]

here is looking down the driver's cat:
1) the pipe suck in the cast flange is a major restriction and needs to be ground to a taper.
2) you can look down and see the 3/8 smash just before the 02.
3) The 02 boss is quite large and the the 02 itself can be spaced away. I may actually put my next mod into the the large round area just before the cat brick so there is nothing at the entrance of cat pipe (which is where most of the restriction is).

You can plainly see that 2 and 3 are additive in their effects on flow. i.e. 2 is just before the 02 which of course was on purpose probably to get flow velocity up to heat the 02 up faster.

I estimate:
1) pipe transition blocks 15% of the flow
2) smash bend 15 to 20% of the flow (2.1" ID ideal pipe)
3) 02 boss and 02 sensor blocks 15% of the total flow. Even CA cat delete pipes suffer from this one.

[oldman]

crush pipe before O2 driver's side. I section the pipe, expand and re-weld.

[Tim M]

oldman: This is great stuff! Appreciate you posting the images - amazing 325 HP passes thru that restriction. Seems like small changes can return pretty good dividends for those sticking with the OEM parts.

Thanks!