Pre-cat bypass vs. secondary cat bypass

[harryrcb]

Originally Posted by Johnny Danger

If I am understanding things correctly, its seems as though you have gone completely catless. If that is the case, then you have eliminated virtually all of the back pressure from your exhaust system. That would explain why you have lost so much low end to mid range torque. I would suggest putting the oem headers/pre-cats back on, and eliminate the secondary cats. By doing so, you will regain some essential back pressure, at the same time open up the flow of exhaust further down stream and shed some unnecessary weight.

yup completely catless. I did get a chance to take it on the road today with those venturis removed from the muffler and there is a definite improvement . I will take your advice and try putting in the original primary cat and see what happens, I wont be able to get to it till next week, will let you know what happened.

[Idaho Red Rocket 3]

Hello Harryrcb,

What happened ?

[harryrcb]

Originally Posted by Idaho Red Rocket 3

Hello Harryrcb,

What happened ?

Sorry about not getting back to you. putting the secondary cats back did NOTHING. so after some thinking about what's going on and remeasuring everything, I realized the headers were 2" pipe at the flange and the mid pipes were 1 3/4" so I made new pipes with 2" dia and replaced the 1 3/4 " catless mid pipes. I was thrilled, got all my low end power back , increased my mid range torque considerably which showed up at the track went from 2:38 to 2:33 at Sebring. The muffler inlet pipes are still at 1 3/4" so I'm wondering what would happen if I replace those with 2" . This will be a little more difficult unless I make up a whole new muffler system.
The back pressure theory is not incorrect, I know if you go with too large a diameter pipe you loose all back pressure and lots of power, but these stock pipes are far from too large. I know some race guy who use 2 1/2" exhaust pipes with no muffler and gain hp and torque.
Around christmas time I put my stock s muffler back on because the Borla is realy loud and I went to visit my mother so I wanted it toned down, well the extra back pressure definitely reduce the power and it sounded strange, more like stock although throatier but not good.
anyway I'm working on another project now since I daubt I can get anymore hp and torque out of this engine. It was at 310 up from 256 before the pipe change but have not had a chance to run it on the dyno. This engine might be for sale soon if interested pm me.