HFC fitment...2.5" or 3"?

bcfromfl

I'd like to start moving ahead with my planned mods to my car, but am having difficulty getting an answer to a basic question. I'm starting first with the exhaust, because everything else seems to build off of that first. Next clutch, then s/c, fuel, and tune.

I'm fairly certain I'm going with Garageline's X/Mid-pipe combo (2.5" diameter), and having a shop (haven't found one yet) weld in the cats into the X-Pipe. Both mil-spec and GESI come in 2.5" and 3" sizes. Which one would give the best fit and weld strength? The 2.5" would be a butt-weld, while the 3" would slip over the X-pipe and perhaps allow for a stronger weld. Also, the 3" would let the exhaust gases slightly longer to pass over the metal element and convert more fuel, as well as perhaps offer less restriction overall.

Once I get all this put together, it will allow me to evaluate how the stock Nismo muffler sounds, and either revise my plan or keep as is.

Thanks!

bcfromfl

OK...over the past few weeks, I've learned more about cats than I thought I'd ever need to, by talking with manufacturers, Seb, and a local custom fabricator. Hopefully, someone else will find this thread and not have to delay their exhaust like I had to, waiting for responses, etc.

There are catalytic converters, and catalytic converters. The quality is directly reflected by price. Several manufacturers out there market "HFCs" -- but they are ONLY APPROPRIATE for NA applications!!

In my application, since I'm using the Garageline X and Mid-pipe combo, it's a 2.5" exhaust. I decided on going with GESI cats, but, as stated above, was stuck between the 2.5" or 3" model. It gets more complicated than that. GESI's 2.5" model is designated "High Output," but limited in the amount of heat and pressure it can withstand. The exhaust fabricator I spoke with said that he would prefer working with the 2.5" cat. However, I double-checked with GESI, and they were not confident that the 2.5" HO cat would be able to withstand the demands of forced induction, and recommended I go with the "Ultra High Output" 3" cat instead. (I just called the fabricator again to see if he could work with it, and he said it would be OK.)

I don't know how to link a photo, but the placement of the cats is at the back-end of the X-pipe, just ahead of the flange to the mid-pipe.

BTW, I ordered the Motordyne ART pipes. Knock on wood...all these parts better work nice together!!

Quicksilvers

If you know for sure you are going forced induction in the future you will benefit from 3” catalytic converters if you have emissions testing and/or inspection to pass. With a supercharger if that is the route you decide to go catless would be your best option to see the best gains. There are some moderately prices catalytic converters or “HFC” for our cars that are good quality and will last. Why are you Choicing this exhaust set up??

bcfromfl

Well, generally speaking, HFCs are a direct bolt-on in the position directly behind the headers. There is just too much heat and pressure from forced induction for a cat here. Those who are going forced induction have a few choices of high-performance cat "blanks" that are welded into the exhaust further downstream. There are several out there that CLAIM to be robust enough for this, but when you take a closer look, there are really only a couple of choices. Interestingly enough, the amount of cells is not the deciding factor when considering how well the cat flows.

You are right...a 3" cat will be more efficient in burning remaining fuel than a 2.5" one, but there are potential issues welding the larger one into a 2.5" pipe. I won't know for certain until I get the parts and can examine inside and outside diameters. What will probably happen is the fabricator will "build up" the outside of the X-pipe after he cuts it somehow, to get a tight fit on each side.

There are two reasons I'm going with cats: first, I want a car that doesn't stink, and second, it will act as a secondary baffle of sorts to help cut down on excessive noise. I'm hoping to retain the factory Nismo muffler for this same reason.

In the U.S., cats are required on race vehicles, and a properly-designed high performance cat does not reduce horsepower by a significant margin.

bcfromfl

I'm adding a postscript to this thread, in case anyone wants to try to follow the path that I've started. One of the frustrating things about this messageboard is that sometimes folks won't report back what they've discovered as they try to do certain modifications, or make parts fit.

Unfortunately, I had a bit of bad luck trying to get my exhaust installed. The fabricator I initially contacted and who agreed to do the work, went out of business before I could get the parts together. (Perhaps in retrospect, that was a good thing!) I thought he was the only fabricator in the area, but after a bit of investigating, I found a full-service speed shop not too far from my home that doesn't advertise. He agreed to do the work, as well as anything else I can't do myself as I work towards supercharging my car. After a few minutes of speaking with him, I could tell he really knew his business, as well as caring for his customers' cars. (For example, he just purchased a new hydraulic lift with rubber guards on the arms to protect the pinch welds on cars.)

As it turns out, the GESI 3" ultra high output cats will not work with the Garageline X-pipe. There simply is not enough room against the undercarriage of the car. We briefly discussed the possibility of placing them at the front of the mid-pipes, but dismissed that location as being too far back and not having enough heat to operate them. He recommended finishing the install catless, and having me pick up the car and drive it a few miles to see how I felt about it. Some observations:

• The resonated Motordyne test pipes coupled with the factory muffler go far in moderating sound levels.

• I noticed beforehand that the factory cats passed quite a bit of raw fuel under certain conditions. By comparison, without the cats wasn't terribly different.

• He suggested that as time passed, the sound will get less as the insides of the pipes get coated with a layer of soot.

I drove it a few miles today, and quite honestly, there isn't a lot of difference now from stock. At normal speeds and engine load I can still carry on a quiet conversation with a passenger. At low RPMs and engine load there is a buzz/rumble through the floorboards and under the seat that wasn't there previously, but it passes as the RPMs climb. Plus, it revs noticeably easier. I'm going to have to retrain my clutching.

There is the hiss that others have reported with resonated test pipes as they fill with exhaust gas. Something to get used to, but not noticeable when driving with the windows up. There is a light but infrequent "burble" from the factory muffler when revving.

So, hopefully I won't have any major issues returning the cats to Vibrant. I'll also have to return the pricey Z1 metal gaskets I purchased for the test pipes. They did not fit well and leaked! The Motordyne test pipes come with a two-piece gasket with metal rings that sealed perfectly against the headers, and the Garageline system came with an extra pair that sealed perfectly between the test pipes and X-pipe.

If you want or need cats on a forced induction car, your only option is the full Fast Intentions system with mil-spec cats welded into the rear of their X-pipe. The Fast Intentions system will not mate up against any other system, so, if you don't like how theirs sounds, you are out of luck. I personally don't like how any aftermarket system sounds, but that's just me.

Now, hopefully, I can move beyond the exhaust to the next project: a stage 3 clutch and the RJM pedal!

ZontheRocks

Great follow up!

Quicksilvers

Good information and follow up! The best of luck on your clutch selection for your set up.

Jayhovah

Thanks for the followup bc.. I am sure many have struck out on this path you will save many more from repeating it!

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bcfromfl

Thanks for everyone's thoughts! Z1 would not take the gaskets back since the installer tried them on the car, so I just posted them in the Classifieds for free...plus $15 shipping.

bcfromfl

I put some more miles on the car today, and observed an unexpected benefit. The oil takes much, much longer to reach operating temperatures, and once there, runs noticeably cooler...by maybe 5-10 degrees or so. That will be important once the s/c is running.