High flow cat- +20whp?
 

There is a credible rotary shop near where I live, who sells a high flow cat for the FD. They claim 20 more whp with their cat. I was wondering what some of you guys with aftermarket cats have gotten in power. Is this claim too much?

Depending on the other mods in place, that's reasonable.

Dave

It all depends on boost and other mods but generally I think that is a bit optimistic. I have a before and after dyno from when I put mine on. It's on my site.

But will the high-flow cat allow you to pass CA emissions? If not, there is no reason to fork out extra $$ for a high flow.

The oem cat has a design flaw that can be easily modified. The problem is that the flanges to the DP, and CB are welded on the inside. The weld reduces the interior diameter of the pipe from 2 1/2" to about 2 1/8". Lets do the math:

Old opening area in square inches: 3.14* 1.0625 *1.0625 = 3.548
New opening area in square inches: 3.14* 1.25* 1.25 = 4.906

By increasing the openings to a full 2 1/2 inches, we get openings 38% larger. That is significant. After doing this mod, I was able to feel a slight increase on the old "butt dyno". It wasn't huge, but it was noticeable. I wonder how many rwhp are needed to feel a difference? I noticed more of a difference from this than when I installed my RB CB.

To do the mod:

Remove the cat. Have the flanges re welded all along the outside. Make sure there are no gaps in the new weld. Then carefully grind down the inside weld so that it is smooth. Reinstall the cat. Thats it :)

^ I like the idea...if only I could weld. I didn't know the stock cat was only 2.5" diameter. That kinda sucks

On a close to stock car, I doubt you would gain more than a few rwhp with a hi-flow cat. You should still be able to pass Cali emissions with the hi-flow.

I'd guess you'd need a 10-15 HP difference to feel it in a car of this weight & HP (2900 lb & 300 HP). In my 1200-lb, 150-HP racecar, I can feel a 5-7 HP increase.

Dave

I always imagine I feel a difference after an oil change. :D

HAHAHA

If you remove the cat, a welder should be able to weld it in a few minutes, at a low cost. My neighbor welded mine for free. Ok, I did give him some beer!!!

Dave,

I would be pleasantly surprised if I were to have made that kind a of gain from this. Perhaps I have an "almost hi-flow cat" now ;)

My friend grinded the welds on the stock cat on his FD.

Personally, I'm going w/ a metal cat (which is the brand name).
If you search on ebay you can find them for a decent price. They're supposed to flow a lot for a cat, and pass emissions just fine on a rotary.
-Ben Martin

with mods you can gain around that much but on a stock car NO! id go with a 3in but an exhaust i know which makes alot of power is the aGreddy titanium and its very very loud. 3in exhaust. And it doesnt past smog legal.

Well, I think the RB catback is supposed to give ~10 HP compared to stock, and you said the cat weld grinding gave more than that did, so 10-15 should be in the ballpark...

Dave

I believe it....only if the old cat is clogged :D

Thanks Dave

Not to butt in,, but whats the power and flow difference between a silenced and unsilenced midpipe??

I've seen dynos of the stock cat vs a Hi-flow cat vs midpipe... many years back. The difference was 6 rwhp between the cats. Going to a midpipe was ~20 rwhp if memory serves me correctly. The cat made more power under 4700 but the midpipe made more past the 4700 rpm point.

None.

Based on actual flow bench work that I have done, I can tell you that most high-flow cats have no advantage over the stock cat, even with its smaller 2 1/2" pipe and restrictive weld beads. This is because the stock cat has substantially more core area than the higher flow cats.

The most restrictive parts of a stock FD exhaust system in order are: 1) the metal precat followed by 2) the main [cat-back] muffler. The stock main cat is 3rd on the list, and finally at 4th is the stock 2 1/2" tubing.

Meh, just move to a state without emissions... ;)

Interesting stuff!! Perhaps people that are installing high flow cats are getting big improvement because their OEM cat is clogged, or partially clogged.

I have a properly working stock main cat that is not clogged or plugged. I only use it to pass ca emissions. I usually run a small 3 inch round aftermarket cat(they call it high flow), and there is a noticeable difference betwee the two cats. With the small aftermarket cat, the car defienitely picks up faster and the boost is higher. This probably has more to do with the 3inch piping that the aftermarket cat is welding in than the cat itself. I think my aftermarket cat might actually be able to pass smog, but i never tried.

When my FD was stock with just downpipe and high flow cat it dyno'd about 217rwhp. Pretty much stock numbers but that was at about 6000 feet elevation. When I added the Racing Beat exhaust to finish opening things up it made a huge difference! Adding back the stock cat for emissions testing also made a huge difference, 20HP to the wheels would not shock me at all. It was like night and day.

My point is that just a high flow cat wont do much of anything for you. Any one part will only really make a big difference if it is the single remaining restriction in your system.

I believe I read somewhere that a high flow cat added 9hp which I think is realistic.
As far as the stock cat goes... It weighs 26 lbs., my new Random Technology from Gotham weighs NINE.
Adding 9hp, losing 17lbs and still being able to pass emmisions is...priceless.