calteg

I highly, highly, highly doubt any exhaust shop is going to go anywhere near your cats.

PPEHeaders

iTrader: (4)

Just some food for thought:

The stock manifolds have very short runners which is not good for performance. It can cause exhaust gas reversions where exhaust from one cylinder under high pressure flows into another cylinder under low pressure. The 4-2-1 design is an attempt to minimize this but it seems the tube lengths are still to short to prevent reversion between cylinders. Short runners are necessary in order to keep the cat close to the cylinder for fast heat up and efficiency.

The outlet of the stock ISF manifolds are very small. ~2.125" OD and are the same size as the outlets of the stock IS250 manifolds. Again, not ideal for making top end power. (the stock RCF manifolds outlets are significantly larger at over 2.5") This is obviously a major flaw of the stock ISF manifolds that was corrected on the newer cars.

While gutting the cats will increase power a little, adding a straight pipe to match the outlet would be 2.125" and still too small.

To improve the stock design, you would need to remove the manifolds cut off the cat and bottom flange, expand the last merge section to 2.5" and then run a straight 2.5" tube to replace the cat and outlet with a new 2.5" flange. This is a lot of work and cost to still end up with very short runners and possible reversion issues.

Also if you want to go back to stock, you would have to find used cats, which are not legal for sale through junk yards, or purchase crazy expensive stock cats.

With a good tune, our headers should produce even more increase in performance over gutted OEM manifolds.

Lastly, we can sell the headers without the O2 conditioner (cheaper) for anyone purchasing a tune since the conditioner wouldn't be needed.

MileHIFcar

^^^^^Thanks for that info!! Very informative for people now that they have the ability to tune for headers! And should be beneficial to obtain the most from aftermarket headers.

XutvJet

Don't forget that the RCF's bigger cams and revised intake manifold come into play here, necessitating the need for larger manifolds. One should really do the calcs and flow bench testing before determining that the bigger outlet is needed.


With the exhaust systems, especially in naturally aspirated cars, bigger isn't always better. In some cases, it can neuter low and mid-range power. For example, an impressive gain of 50whp from 5800-6300rpms will not be realized in acceleration testing if 20whp is lost from 2000-5000rpms. Secondly, even if no low/mid range power is lost, a 10 to 50whp gain spread out over a mere 500rpms will not result in a significant improvement in acceleration. That's why many in the "know" state that for performance to increase, the entire power curve must be elevated, not just a small portion of it.

lobuxracer

iTrader: (2)

One of my favorite misunderstandings in exhaust systems - big tubes somehow have less back pressure, so they must be better. Nothing could be further from the truth, and a simple physics equation will completely clarify this.

Pressure in whatever units you like is measured by taking atmospheric pressure and multiplying by the cross-sectional area of the tube you are using. Do not confuse this with units per square measure (like PSI), because the amount of force you will need to move the column of air in the tube is only secondarily related to this measure. The mass of the air you need to move increases with every increase in size, so the energy required to displace this mass increases with every increase in size. It's exactly the opposite of what you've heard for years from misguided sources, but the reality is, it takes MORE energy to drive exhaust out of a big pipe than it does out of a small pipe. The only time it gets complicated is when you approach the maximum flow for the diameter tube you are using. Even then, there's a lot to be said for a smaller tube retaining more heat (less surface area to dissipate heat) and getting the exhaust out faster because less heat energy (the thing that actually moves the gas down the pipe) is lost.

4-2-1 isn't a compensation for reversion, it's a way of tying cylinders 180 degrees out of phase together before you hit the main collector. We did this on motorcycle engines with long tubes back in the 80's and 90's to improve mid-range torque, and it worked marvelously on four cylinder bikes. It was a lot more hassle, but the results made it worth the effort over the simple 4-1 designs.

I am not suggesting anyone take their OEM exhaust and slice it all up, just suggesting the bulge remaining after the catalyst is removed does two things really well - first, it dramatically slows velocity, second, it dissipates more heat than a tube of constant diameter. Stuffing a tube inside the gutted cat might improve things a bit.

And I'm in the camp, if you haven't tested it, you don't know if it's any good or not. I remember years ago thinking I could look at something and make a determination about how well it would flow and how well it would work in service. A lot of time studying gas behavior has changed that opinion completely. Sure, there are a whole lot of reasons why a properly designed and built header should out perform anything else, but just like there's one really fast line around a corner, there's another line you might be forced to take to pass someone, and it's not that much slower...

ISFwpg

Worth the watch if you're not sure how exhaust works.

imdrax

Sorry to bump an old thread, but i am gutting the primaries this week and wondering if anyone had a long term update on the mechanical o2 antifouler/extenders. i have a set that i used to use on my m3. And whether or not the catr material or steel wool is needed? either way i will be trying it out but figured i would ask here to see what has worked best before i slap them on to save some time.

flowrider

iTrader: (9)

^^^^No longer true. Electronic conditioners are really not being used much any more. They have been replaced with tunable bung fittings like this:



I've had experience with both. The mechanical solution is superior IMHO

Lou

Jwconeil

With an available tune, it’s the only real fix for the issue and adds power. Everything else is counter intuitive.

steeven001

You've had luck with the mini catted spacers working all year round?

I've used these on a few vehicles with no problems but they don't work on the ISF.

I currently have RRRacing o2 conditioner but it throws codes in the winter so I'm constantly having to adjust it.

ahnajjar

I too am looking to gut my cats and i currently have the rr tune. this may be a stupid question but do i keep the o2 sensors that are currently there or remove them when i gut the cats? also if i end up breaking the o2 sensor while gutting, will this cause a problem or not considering i have the tune installed right now? Thanks and if there is a video on how to do it then plz someone link it!

lobuxracer

iTrader: (2)

FWIW, anyone outside of California planning to gut the cats needs to reconsider. You can sell your OEM exhaust intact to a recycler for a decent chunk of change vs. shattering the substrate and tossing all those precious metals in a waste stream. Take the money from that sale and apply it to headers with decent aftermarket cats and have the best possible option AND still meet EPA smog requirements. This also means your car won't stink and our kids won't have to breathe the unquestionably detrimental crap coming out of your tailpipe.

If you still want to gut the cats, you need to have a talented person weld in a tube to maintain velocity. The bulge from the exhaust where the substrate formerly lived will cause a significant velocity drop in the exhaust and will also create unnecessary turbulence. Putting a tube inside the shell will correct this issue and get you the most possible horsepower from a gutted set up. It's not mandatory, but if you really want to get the most for the effort, it's worth the work.

Jwconeil

X2. My stock cats sold to a scrapper for 850. Would be silly not to sell them and just buy headers, instead of destroying valuable materials for subpar performance.

AlOtaBblGm

^^^ What they said

1A1

Great point. Also, high flow cats work very well and cost just a few HP, quiet down the car a bit (if you care about that) and won't stink up the garage.

Steve