2.5" exhaust

[keyserscott]

I heard that I had to have a 2.5" exhaust for any turbo kit. I have read conflicting things though. Flyin Miata ran a test in which a 2.5" was no more effective than a 2.25", check it out fmprotege.com. I would hate to think that the 550$ on a Thermal 2.25" were a waste if I was going turbo.

 

[Keith@FP]

If you retain the stock cats, as we did, a 2.5" will not have any benefit over a 2.25" exhaust at 6 psi. If you remove one or more of your cats - which will cause all sorts of fun with check engine lights - you can make use of a 2.5".

Basically, bigger is better - but there's a point where you get to be big ENOUGH. After that, there's no benefit in a larger pipe.

Keith

 

[keyserscott]

Okay, that's interesting. Kind of diminished returns then? From your sight, you have a turbo exhaust in the works. What would be the differences with that? Of course that's out of the question for me since i'm happy with the Thermal. And I see you did your dynoes with a 2.25 Thermal. have you been getting any figures on an install $$$. I've heard a wide range quoted.

 

[BluMp3Turbo]

Actually bigger is better but if you get bigger cats like maybe from a diesel truck or even a high performance cat it will still work and you will get more whp

 

[BluMp3Turbo]

sorry I ment it for a full 3" cat back!

 

[I-Am-Chris]

So has anyone tested 3 Inch yet. Why is everyone on the 2.5 inch boat?

 

[Keith@FP]

Again, you only need to go as big as necessary. 3" wouldn't get you any performance gain over 2.5" if the 2.5" was already flowing all you need. Well, it might impress the ladies more. But that's about it.

Since the cats are the biggest restriction, removing or replacing them with higher flow would be the next step.

The 2.5" was our prototype turbo exhaust. Given the current results, I don't think it's going to be our top priority Install cost on a 2.25" catback would be 1 hour ($65) at our shop.

Keith

 

[Traveler]

Hey Keith, did you guys try to open up the stock first cat outlet at all? I've opened mine up from the stock 1.72" to 2.05 and made a custom 2.25" cat forward. I'm thinking of trying to get a 2.5" downpipe made and go 2.5" between the cats. I'd have to open up the primary cat outlet to 2.38" (ID of a 2.5" pipe for the rest of you out there) to make it work. Anyone tried this yet?

 

[Minus]

hey on a spoolin kit, with its 2.5 down pipe, would it not be wise to continue on back the car with a 2.5 all the way back?

 

[Keith@FP]

We are still running with stock cats on our car, although there's a test pipe on the way for us to dyno for the front pipe. Even just running different tubing to a stock cat might help.

Keith

 

[keyserscott]

BEGI/Spoolin

I guess my question now would be if Spoolin has told me a 2.5" is required and BEGI has not, what's the reasoning? Is it because of differences with both of their systems? I guess my main gripe is that I would hate to toss my brand new Thermal 2.25" if I don't have to. Also, if Spoolin is saying 2.5" is the way to go and BEGI is not, who's right? Just want to get some opinions.

 

[1sty]

Maybe Spoolin replaces the first cat.

Keith, with the offroad down pipe with the CEL still be getting thrown all the time?

Whats are chances of getting a high flow cat worked into the mix?

 

[Keith@FP]

IF you are running the stock cats, going larger than 2.25" won't do you any good. The FM kit keeps the stock cats for emissions reasons.

IF you modify or remove the stock cats, then it's quite likely that you will benefit from something bigger. The Spool kit tosses one of the cats, so in this case it's probably true that 2.5" would have a benefit.

We're not BEGI, by the way...

If the ECU determines that the cat is not working, it will trigger the CEL. If the cat isn't there, it's not working It's possible that we'll come out with a high-flow cat. Probably not a bad idea.

Keith

 

[1sty]

Well you have my credit card number so just send the thing when you get it working .

 

[keyserscott]

Sorry about the BEGI confusion Keith. I was thinking of it at the same time. I guess going with the Spoolin kit might be an issue then for anyone with a 2.25 exhaust. Too bad, it's a good system, but I can't drop a brand new 2.25. Unless someone knows a way around this...Spoolin? anyone?

 

[70]

Um, buy the FM kit...And you'll be set. retain the stock cats and all will be well with the 2.25"...


And I was thinking along the same lines as what Traveler mentioned. The first cat and that stupid tiny connecting pipe is the first restriction when dealing with the stock exhaust.

Terry's kit does away with the first cat, and the down pipe is larger than the stock connecting pipe. So that is why he gets better flow in the exhaust. For now at any rate...

 

[keyserscott]

I see you have the FM turbo. Do you like it so far? Install it yourself? I'm looking to get a qoute on an install. I see you haven't fullt optimized it yet. Having problems?

 

[BluMp3Turbo]

Listen all you really need is one cat and if anything a high flow cat... with that you can go up to 2.5 w/ no problem this way you can use a different downpipe which is better becuz of better flow. Meaning more whp!!!

 

[Keith@FP]

Listen all you really need is one cat and if anything a high flow cat...

The People's Republic of California begs to differ. If the car came with two cats, you cannot take one off and stay emissions-legal. Actually, I believe it's a federal matter - you can't remove cats. Yes, you might still pass a sniffer test. Probably will - the front cat is primarily there for startup emissions - but don't get caught.

whp is not the only goal for some people.

Keith

 

[1sty]

Can the first cat be removed and not screw up the car performance or throw CELs?