It has a cat, without it I lost 20hp even after a tune from the dyno guys. . Its an after market free-flow cat. No netting, just straight through.

The sensor is mounted 18" before the cat after the Y pipe.

 

Makes no sense that you lost HP without a cat. I have never seen an engine that enjoyed the restriction of a cat.

 

That right there would tell me something is wrong. Was it the same dyno or a different one?
I have been tuning cars for years and that is the first time I have heard that.
I personally would not run a cat on an engine set up like that. Depending on how rich the engine is, it could easily be starting to clog already. Plus if it is cruising rich like all holleys do out of the box it will run lower egts which will also cause issues with the cat as they have a minimum operating temp.

 

It needs richened up. Cat removed freed up exhaust flow and likely improved scavenging, but motor was already lean to begin with. Might have drawn in more air charge but carbs not feeding it. Its kinda strange tho as usually a stronger signal on the carb richens mixture and you jet down to get air fuel back in line, if carb is sized right.

I'd keep playing with the tune.

 

My money is on this as well. Putting a cat in slowed the velocity of air exiting motor which contaminated the intake charge and richened the motor.

 

There appears to be a lot of input here, from getting a new intake, new exhaust, potentially the wrong Carb, placement of O2 sensor, etc etc. While all is very informative (as it should) I have a lot of things to accomplish.

First off:

1) I need to forget about these #'s
2) Go to the track & let those numbers do the talking
3) Try a different dyno, the one I took my 350 to and squeezed more than 40hp out of after tune.

Stand by for results in the up-coming weeks...........Thanks again!

 

Update:

I got a set of hooker longtube headers, 2210hkr. The slp shorties I had on it only had a 1-5/8 diameter primary tubes, which is smaller than the exhaust ports on the head.

In springtime im going to complete the system, trying for a single 4" back with no cat.

Secondly, a flow test was done on the fuel pump. How long does it take to fill a one gallon bucket? 40 seconds. A new pump is on order, as Holley tech support said this is a problem. It has flow but registers zero pressure. That test was done, bypassing the return line.

I hope when finished it brings some results !