Gutted pre cats?
11-19-2010, 10:49 PM
#1
Gutted pre cats?
I know I'll probably get bashed for this, but this is what the forum is for. I am currently jobless and want to do sum performance mods to my car, but can't afford it at the moment. My question is, if i were to gut my pre cats on my 5.5 gen would there be any benefits? I mean they would be sort of like headers besides the weight differences. I know some would think it would be crazy to go through all that trouble to just put the factory stuff back on, but i have nothing but time on my hands at the moment.Any suggestions appreciated.
11-19-2010, 11:04 PM
#3
Unless you weld some pipe into the gutted cat, the turbulence of the air flowing into a larger chamber and back into the rest of the exhaust might actually make performance worse.
11-20-2010, 05:27 AM
#6
Senior Member
Join Date: Dec 2005
Location: Leicester, Ma
Posts: 1,124
I experimented with a pipe slipped into the rear one but it makes it sound like a hole in the exhaust for me and i didn't notice a difference from a pipe or not.
My advice is if you do attempt this soak the bolt and nuts with wd40 and get yourself a propane torch or if you have access to a shop torch you will prob need it to heat the stubborn nuts up to break the rust loose.
PS.. YOu will need to use spark plug foulers on the o2 secondaries to keep from getting a CEL
Last edited by jeff5347; 11-20-2010 at 05:33 AM.
11-20-2010, 05:32 AM
#7
yea i highly doubt that "turbulance" would be a factor as with the stock cats there would be more turbulance seen as though its more restrictive than the gutted cats... i say go for it... there is noticeable gains as there was an altima around here with gutted cats giving ppl the business... it was a 5spd alti that iirc ran 13.8 with gutted cats
11-20-2010, 06:17 AM
#8
I had mine gutted i noticed a little difference in performance, surprisingly enough I never ran foulers and never got a code... but yeah like the said soak the bolts and torch I broke 2 bolts when doing mine..
11-20-2010, 07:59 AM
#9
Thanks for the response guys. My mods as of now are GAB with a k&N filter flow master 40 series muffler,vias delete, NWP spacers, and i already gutted the rear cat. Will it be any benefits if i got a straight pipe welded vs just gutting them? Any idea about how much HP this might add 3-5 maybe? Whats another good DIY home made cold air intake that's better then the GAB design?
Last edited by dnice02; 11-20-2010 at 08:44 AM.
11-20-2010, 08:00 AM
#10
^^Disagree. I gutted mine about a month ago. I actually feel a difference. Like Gh said it prob wont be as big as headers (dont know how much since my max has never worn headers), but i can feel a gain. The sound will be louder, but not by much. The front cat is easy to gut, it is just ceramic and you can break it out with a screwdriver and then clean it so nothing goes back to the engine. The rear one is more of a PITA but you can do it. The rear cat is a steel mesh that took me 2 hrs to get out. I had to use a drill with different bits, hack saw, hammer chisel, ect.
I experimented with a pipe slipped into the rear one but it makes it sound like a hole in the exhaust for me and i didn't notice a difference from a pipe or not.
My advice is if you do attempt this soak the bolt and nuts with wd40 and get yourself a propane torch or if you have access to a shop torch you will prob need it to heat the stubborn nuts up to break the rust loose.
PS.. YOu will need to use spark plug foulers on the o2 secondaries to keep from getting a CEL
I experimented with a pipe slipped into the rear one but it makes it sound like a hole in the exhaust for me and i didn't notice a difference from a pipe or not.
My advice is if you do attempt this soak the bolt and nuts with wd40 and get yourself a propane torch or if you have access to a shop torch you will prob need it to heat the stubborn nuts up to break the rust loose.
PS.. YOu will need to use spark plug foulers on the o2 secondaries to keep from getting a CEL
11-20-2010, 08:10 AM
#11
Senior Member
Join Date: Dec 2005
Location: Leicester, Ma
Posts: 1,124
i have a test pipe where the main cat is behind the flex pipe and both precats are gutted, i would just get a test pipe off ebay fro 30 bucks so at least in an inspection if they need to do a visual they can see the cat under there. They aren't gonna check the precats
11-20-2010, 02:42 PM
#16
I know I'll probably get bashed for this, but this is what the forum is for. I am currently jobless and want to do sum performance mods to my car, but can't afford it at the moment. My question is, if i were to gut my pre cats on my 5.5 gen would there be any benefits? I mean they would be sort of like headers besides the weight differences. I know some would think it would be crazy to go through all that trouble to just put the factory stuff back on, but i have nothing but time on my hands at the moment.Any suggestions appreciated.
yea i highly doubt that "turbulance" would be a factor as with the stock cats there would be more turbulance seen as though its more restrictive than the gutted cats... i say go for it... there is noticeable gains as there was an altima around here with gutted cats giving ppl the business... it was a 5spd alti that iirc ran 13.8 with gutted cats
13.6 to be exact and kevin007 (r.i.p) did the same thing to his 6th geno.m.g stfu about your cattman purchases....no one cares
11-20-2010, 04:12 PM
#19
dont ever get caust asking that question again... it not a good look
i got more than you 
... i got 9.4633
i got more than you ... i got 9.4633
11-20-2010, 09:18 PM
#20
Depends on how bad your precats are. If they are seriously clogged, you could pick up 30 hp easy. If they are factory fresh, probably less than 10. When I replaced my precats with low mileage used ones, my 45-85 time dropped by nearly 3 seconds.
11-20-2010, 09:24 PM
#21
Disagree all you want but exhaust flowing into gutted cats and expanding before flowing into the rest of the exhaust won't make as much of a gain as keeping the pipe diameter constant all the way through. I believe you did get gains (especially if your cats were in as bad shape as mine were) but welding straight pipes into them would have maximized the gains. Not sure why if you did that to one of them it sounded bad, if there weren't any leaks there shouldn't have been any change in sound. 
Just seems to me that if I were to go to the trouble of gutting my cats, that putting straight pipe in them would give me the best results, especially in light of the work to remove the cats, gut them and re-install them...along with running O2 sims or the non-fouler bungs so I can pass my yearly emissions inspection.
Just seems to me that if I were to go to the trouble of gutting my cats, that putting straight pipe in them would give me the best results, especially in light of the work to remove the cats, gut them and re-install them...along with running O2 sims or the non-fouler bungs so I can pass my yearly emissions inspection.
11-20-2010, 10:02 PM
#22
Senior Member
Join Date: Dec 2005
Location: Leicester, Ma
Posts: 1,124
^^ agree to a point. I agree that going from a small pipe to a large area then back to a small pipe reduces the velocity, but i honestly didnt notice a difference from the fully gutted or when i had a pipe slipped in. I used a 2" piece of pipe and tacked it in but the tacks broke and it made a horrendous rattle/vibration at 2k. But it def does change the tone. With the fully gutted it has a nice deep tone but when the pipe slipped it it made the tone higher a bit and made it sound like it had a hole in the exhaust. I may try it again and honestly i had removed and reinstalled the rear cat over 3 times trying things that the removal now only takes me 10 minutes. I have it down pat.. hahaha. Once i can get access to the welder i will fully weld it next time so it will stay put. If i had the money i woulda dyno'd before gutting and fter gutting t o put all the "does it make power" ?s to rest.
11-20-2010, 10:45 PM
#23
Senior Member
Join Date: Oct 2009
Location: Montgomeryville, PA
Posts: 394
Thanks for the response guys. My mods as of now are GAB with a k&N filter flow master 40 series muffler,vias delete, NWP spacers, and i already gutted the rear cat. Will it be any benefits if i got a straight pipe welded vs just gutting them? Any idea about how much HP this might add 3-5 maybe? Whats another good DIY home made cold air intake that's better then the GAB design?
nvm. Just saw this
Disagree all you want but exhaust flowing into gutted cats and expanding before flowing into the rest of the exhaust won't make as much of a gain as keeping the pipe diameter constant all the way through. I believe you did get gains (especially if your cats were in as bad shape as mine were) but welding straight pipes into them would have maximized the gains. Not sure why if you did that to one of them it sounded bad, if there weren't any leaks there shouldn't have been any change in sound.
Just seems to me that if I were to go to the trouble of gutting my cats, that putting straight pipe in them would give me the best results, especially in light of the work to remove the cats, gut them and re-install them...along with running O2 sims or the non-fouler bungs so I can pass my yearly emissions inspection.
Just seems to me that if I were to go to the trouble of gutting my cats, that putting straight pipe in them would give me the best results, especially in light of the work to remove the cats, gut them and re-install them...along with running O2 sims or the non-fouler bungs so I can pass my yearly emissions inspection.
Last edited by Ninety9gle; 11-20-2010 at 10:52 PM.
11-21-2010, 07:52 AM
#26
Scott here is why I disagree with the gain thing... the exhaust flow from stock was already shrinking to go through the collector then expanding into restrictive cats. The two major restriction is of coarse the. Shrinkage and the cats, so removing or gutting the cats doesn't really change how the flow of gas comes from the motor to the collectors moreso just cut out the second part of restriction which is the cats which is where the gain is... if there was any exhaust turbulance it would be in stock form more than with gutted cats... what the OP can try is when he gets the headers off try to put a uni bit to it to try and open it out at least an inch then he could possible free up another 5whp (not accurate number of coarse).. but you catch my drift
11-21-2010, 09:07 AM
#27
I'm guessing you meant to post this in the "Srry for doing this..." thread. Here's the link for page 6 where the pic is posted. Just a heads up: I only glanced through the thread, but I think the picture was posted in direct reference to the 2.5" dual tip he was talking about just a couple posts prior to this one and not the 3". 
But going through the thread does show a colossal failure on CustomMaxima's part. I kinda pity them.
But going through the thread does show a colossal failure on CustomMaxima's part. I kinda pity them.
11-21-2010, 10:27 AM
#28
Scott here is why I disagree with the gain thing... the exhaust flow from stock was already shrinking to go through the collector then expanding into restrictive cats. The two major restriction is of coarse the. Shrinkage and the cats, so removing or gutting the cats doesn't really change how the flow of gas comes from the motor to the collectors moreso just cut out the second part of restriction which is the cats which is where the gain is... if there was any exhaust turbulance it would be in stock form more than with gutted cats... what the OP can try is when he gets the headers off try to put a uni bit to it to try and open it out at least an inch then he could possible free up another 5whp (not accurate number of coarse).. but you catch my drift
11-21-2010, 12:01 PM
#29
Scott here is why I disagree with the gain thing... the exhaust flow from stock was already shrinking to go through the collector then expanding into restrictive cats. The two major restriction is of coarse the. Shrinkage and the cats, so removing or gutting the cats doesn't really change how the flow of gas comes from the motor to the collectors moreso just cut out the second part of restriction which is the cats which is where the gain is... if there was any exhaust turbulance it would be in stock form more than with gutted cats... what the OP can try is when he gets the headers off try to put a uni bit to it to try and open it out at least an inch then he could possible free up another 5whp (not accurate number of coarse).. but you catch my drift
I understand 100% that maintaining smooth linear runners, especially in the headers is vital to maximize the engines performance. But with as much material as there is even in a brand spankin new cat, it's apparent that some gains can be found if all that material wasnt literally just sitting in the exhaust stream blocking airflow.
I personally was running hotshot headers --> stock main cat --> stock b pipe--> cattman muffler. I later gutted the cat out and deffinately noticed an increase in performance. I wish I could give exact dyno proven numbers, but I cant which means my input is invalid for those that want physical data. But my point is that if I felt such an increase from removing a restriction so far down the exhaust stream, there are deffinately gains to be had from removing the same type of restriction closer to the engine. I'm not claiming any miraculous power gains at all, but in theory, there is some hp to be had from this.
11-21-2010, 12:22 PM
#31
That's not a bad idea, but that's some hard stuff man. It would take one hell of a uni bit to bore that out. What else could possibly be used to bore it? Would the o2 bung be in the way? It looks like you don't have much room to play with on that one side.
11-21-2010, 01:38 PM
#39