I should be ordering the Ultimate Racing high flow cat really soon. My buddy has the Tanabe dp and HKS cat back with no issues. So it should work with the HKS dp as well.

 

What size high flow cat should be used with the hks dp/cat-back combo? 65mm = 2.559". I'm thinking of just getting a 2.5" high flow cat since it comes really close to the hks dp and the cat-back inlet diameter.

 

I went with a 2.5" cat. I would have gotten 3" but they were out of stock at the time. Just go with a 3"

 

How did the 2.5" high flow cat perform over the stock cat? What advantages would there be with going to the 3" cat vs. 2.5"? I see less disruption in flow going with 2.5". I also see the 2.5" cat having a better seal.

 

Good choice. I also have the HKS downpipe mated to the HKS exhaust. It really improved the car's power on the low end. I was told this was due to the pipe's diameter. Bigger diameter downpipes actually lessen power on the low end, but make up on the top end. The only problem I have now is running rich, which I will fix with some butt tuning on the HKS Super AFR. Anyone have any specs for me to start with as a base?

 

I could tell alittle difference over the stock cat on the top end. The disruption I think would be less going with a 3" cat. The HKS down pipe is close to 2.5" but it's still larger than the 2.5" cat and creates more turbulance than with the 3" cat. The 2.5" cat still works fine. Most 3" cat-backs bolt up to the stock cat with no leaks so a 3"cat should bolt up to the down pipe with no leak probs.

 

I think the actual size of the HKS downpipe is 2.65, but a 3" high flow cat will bolt up to it. I had a buddy with the same exhaust setupas me and used a 3" Perrin cat delete without any issues too.

 

is there a special way putting the spring loaded bolt back, do you have to use a tq wrench?

 

How would a 3" cat have less disruption? 65mm = 2.559". So with a 3" cat, you'd be going from 2.559" to 3" back to 2.559" at the cat-back due to the neck-down. So wouldn't a 2.5" high flow cat give you better transition?

But regardless, it sounds like the high flow cat will work better than the stocker. My main reason for going to a high flow cat would be to keep the stocker in good shape for emissions testing in 4 years. hehe.. I'm not sure if the stocker will survive the beating I'll be putting it through.

 

i have had the hks dp for almost a year now, and no complaints- i have no immediate plans to bolt on a big turbo so it was an easy choice for me

 

I have this DP and it works great. No clearance issues.

However, I am looking for either a high flow cat or test pipe that is the same diameter of the HKS. If anyone knows where to get one let me know, thanks.

 

Okay, I just got my hks downpipe in the mail today. I have to say this thing is a work of art. The flanges are CNC machined and everything is TIG welded together.

I just measured the outside diameter and it's a tad larger than 2.5". I'm gonna measure the stock cat inside diameter when I install the dp just to see how much of a bottle neck the stock cat really is. I'll probably do some very minor porting of the cat inlet flange if there appears to be a large step at the transition.

 

Sweet man glad you like it. post some pics of the ported cat when you get it done.

 

I just installed my hks downpipe today. I measured the cat inlet diameter, and it's right around 2.5" so it matches the hks downpipe perfectly. No porting needed.