Question for those w/ hi-flow cats (especially Magnaflow 59975)
Senior Member
Joined: Jan 2012
Posts: 880
Likes: 182
From: BC Canada
Posted by virtualgordon:
I just thought to suggest an answer to the question about why fitting the downstream O2 sensor-extender would provide a fix; this may be because of laminar-flow in the exhaust - the extender pulls the sensor-tip out or the main body of gas flow. The closer the sensor-tip is to the outside diameter of the pipe the slower the gas is moving (slightly more time for the reactions to have completed post-cat? Presumably the tip would also be slightly cooler due to the sensor conducting a slightly greater % of heat from a lower-concentration of heat due to the slower-moving gas at the periphery).
I just thought to suggest an answer to the question about why fitting the downstream O2 sensor-extender would provide a fix; this may be because of laminar-flow in the exhaust - the extender pulls the sensor-tip out or the main body of gas flow. The closer the sensor-tip is to the outside diameter of the pipe the slower the gas is moving (slightly more time for the reactions to have completed post-cat? Presumably the tip would also be slightly cooler due to the sensor conducting a slightly greater % of heat from a lower-concentration of heat due to the slower-moving gas at the periphery).
Junior Member
Joined: Oct 2012
Posts: 14
Likes: 7
From: UK
Agreed - supported by evidence!
Could well be... But to be more sure of passing the emissions test (as opposed to fooling the ECU), wouldn't it make more sense to relocate the downstream sensor 2" further away from the cat? That would ensure a more complete reaction, i.e. less oxygen available, i.e. a bigger voltage diff between the 2 sensors.
Senior Member
Joined: Jan 2012
Posts: 880
Likes: 182
From: BC Canada
Conclusions
I'm trying to use all the great comments on this thread to specify a future exhaust system for my STR. Originally a California car now in Canada, I hope that the ECU isn't going to give a CEL when I eventually replace the OEM cats. Here's what I conclude:
CATS: Although Magnaflow 59975 and 99675HM cats seem to be almost the same shape, size and cost, the latter seem to be more suited for OBDII-compliant cars. Neither model shows any signs of bungs for sensors, so no difference in welding costs. Downstream O2 sensor should be 2-3" away from the cat, no extender used. The 99675HM is preferred. Heat shield needed?
RESONATOR: Looks like the Magnaflow 11385 or 14385 are preferred to the 11378.
REAR EXHAUST: Prefer the Mina cat-back system.
What do you guys think of these choices? Any chance of a 20HP gain?
CATS: Although Magnaflow 59975 and 99675HM cats seem to be almost the same shape, size and cost, the latter seem to be more suited for OBDII-compliant cars. Neither model shows any signs of bungs for sensors, so no difference in welding costs. Downstream O2 sensor should be 2-3" away from the cat, no extender used. The 99675HM is preferred. Heat shield needed?
RESONATOR: Looks like the Magnaflow 11385 or 14385 are preferred to the 11378.
REAR EXHAUST: Prefer the Mina cat-back system.
What do you guys think of these choices? Any chance of a 20HP gain?
Joined: Sep 2011
Posts: 833
Likes: 199
From: Aurora, CO
I am running the 99675HM cats, no extenders, no CEL after 5+ months. I would highly recommend NOT using the 10790 x-pipe unless you like a raspy exhaust. I think 20 whp is optimistic, but 20 chp might be realistic. I didn't dyno between mods, so I can't tell you exactly how much each upgrade gave me.
Senior Member
Joined: Jul 2011
Posts: 547
Likes: 101
From: USA USA
I'm trying to use all the great comments on this thread to specify a future exhaust system for my STR. Originally a California car now in Canada, I hope that the ECU isn't going to give a CEL when I eventually replace the OEM cats. Here's what I conclude:
CATS: Although Magnaflow 59975 and 99675HM cats seem to be almost the same shape, size and cost, the latter seem to be more suited for OBDII-compliant cars. Neither model shows any signs of bungs for sensors, so no difference in welding costs. Downstream O2 sensor should be 2-3" away from the cat, no extender used. The 99675HM is preferred. Heat shield needed?
RESONATOR: Looks like the Magnaflow 11385 or 14385 are preferred to the 11378.
REAR EXHAUST: Prefer the Mina cat-back system.
What do you guys think of these choices? Any chance of a 20HP gain?
CATS: Although Magnaflow 59975 and 99675HM cats seem to be almost the same shape, size and cost, the latter seem to be more suited for OBDII-compliant cars. Neither model shows any signs of bungs for sensors, so no difference in welding costs. Downstream O2 sensor should be 2-3" away from the cat, no extender used. The 99675HM is preferred. Heat shield needed?
RESONATOR: Looks like the Magnaflow 11385 or 14385 are preferred to the 11378.
REAR EXHAUST: Prefer the Mina cat-back system.
What do you guys think of these choices? Any chance of a 20HP gain?
I would recommend the Magnaflow 11385 or the Borla 400287, with about a $100 price difference. A few members have the 11385, but havent posted any videos. The 400287 has only been done in conjunction with a catback so its hard to say how it will sound. The 400287 is what I will be going for this November if funds are available. Please do not use the muffler in my signature, which is very raspy.
My guess is you might get 8-10rwhp, but if anything you eliminate like 70 lb's, which is good for another 7hp or so. All things considered, probably 1.5 tenths at the track.
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