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Discussion Starter · #1 · (Edited)
I have a 2J Racing 3-inch diameter catback replacement exhaust sitting in my garage waiting for install. However, I had a conversation with a fellow Fiesta ST owner yesterday that left me with some questions.

He suggested adding a 3-inch ID, free flowing exhaust could cause some low RPM "flat spots" in power delivery and result in spiking of AF ratio (to above 14.5). In a somewhat cryptic note from Cobb awhile ago, Ian suggested that a 3-inch ID catback could cause some variability in performance.

Anybody here with a larger diameter catback experience any of that? It was claimed that this was one reason why Cobb was slow to release its exhaust products and tune for the Fiesta ST. Apparently, according to my new Fiesta ST friend, this little car likes back pressure. This appears to be less of a problem at WOT.

With that in mind, I'm wondering if adding the small diameter CP-E adapter plate/gasket that they provide with their downpipe product would help.

I bought the 2J Racing unit for it's nice laid back sound effects, but am a little wary of any potential performance glitches.

Any thoughts folks?
 

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Well, a more free flowing exhaust could in fact alter A/F Ratios and in fact spike them above 14.5 (this does not count against 0% Deceleration, which mine spikes to about 44%) However the easier it is to breathe the easier and more prominent WOT will be, But we don't in fact drive our cars at WOT 100% of the time, i would imagine that the cat would provide enough BP for the turbo to be happy, though, I have a 2.75" Borla on my car and it's pretty happy, i do notice a little lag from a standstill, though.
 
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Discussion Starter · #3 ·
If have the stock downpipe, having "too much flow" is not going to be an issue for you AT ALL. The down pipe is easily the most important part of the system on these cars. So bolting a big catback onto your choked stock downpipe is prob just going to make more noise than power with the stock turbo..


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That's what I thought (which is fine). Thanks. In the short term, I'm mostly just looking for adjusted sound effects. Future? We'll see, but that will involve more bolt parts, tuning, etc. More $$$.
 

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That's what I thought (which is fine). Thanks. In the short term, I'm mostly just looking for adjusted sound effects. Future? We'll see, but that will involve more bolt parts, tuning, etc. More $$$.
I bought the same catbcack for sound mostly but also ended up buying the catted DP thinking I wouldnt get a CEL, so when I get my catback this week hopefully, I will install it to the stock DP till Joe or CoBB releases the tune to erase the CEL.

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Discussion Starter · #5 ·
I bought the same catbcack for sound mostly but also ended up buying the catted DP thinking I wouldnt get a CEL, so when I get my catback this week hopefully, I will install it to the stock DP till Joe or CoBB releases the tune to erase the CEL.

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Hello...so, sorry I've not followed your build, but you first got the high flow cat, yes? That resulted in a CEL? The catback from 2J Racing is coming after the replacement cat? Just wanting to understand sequence. I'm hoping the catback won't cause a CEL. Seems highly unlikely. Thanks.
 

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Hello...so, sorry I've not followed your build, but you first got the high flow cat, yes? That resulted in a CEL? The catback from 2J Racing is coming after the replacement cat? Just wanting to understand sequence. I'm hoping the catback won't cause a CEL. Seems highly unlikely. Thanks.
With cat-back alone, you will not get any CEL. On turbo cars, you really want as little restriction on the "hot" side after the turbo as possible for best performance.

The DP is easily the best bang for your buck modification for these cars, its really wakes things up nicely. The stock DP is utter crap the amount of restriction on an already tiny turbo.

In a nut shell, with a 3" cat-back exhaust, you will be fine, it will be even better if you add an 2JR DP mated up to your 2JR cat-back exhaust.
 

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I bought the same catbcack for sound mostly but also ended up buying the catted DP thinking I wouldnt get a CEL, so when I get my catback this week hopefully, I will install it to the stock DP till Joe or CoBB releases the tune to erase the CEL.

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No i have no installed my catted dp yet, but by what others are saying, i will get a CEL and the defouler that was supplied may screw with the ecu and fuel delivery so I didnt install it yet. Waiting for someone 2JR or COBB to release a tune to remedy that. I should be receiving my catback this week (hopefully). If so it will go on without the dp.

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Discussion Starter · #8 ·
I traded a little email with Ben Bruno at Custom Performance Engineering on this topic and he basically confirmed the "more flow is better with a turbo application" thinking. He did acknowledge some potential for low-end bogging. Ben kindly provided a link to a nice layman-speak article about fluid dynamics associated with a pulsing fluid flow through a pipe (which is what an exhaust situation best resembles). The article debunks the "backpressure" argument and presents an explanation of the scavenging effect associated with the pulsing flow.

Anyway, here it is: Exhaust: The straight scoop on backpressure

Enjoy.
 

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I traded a little email with Ben Bruno at Custom Performance Engineering on this topic and he basically confirmed the "more flow is better with a turbo application" thinking. He did acknowledge some potential for low-end bogging. Ben kindly provided a link to a nice layman-speak article about fluid dynamics associated with a pulsing fluid flow through a pipe (which is what an exhaust situation best resembles). The article debunks the "backpressure" argument and presents an explanation of the scavenging effect associated with the pulsing flow.

Anyway, here it is: Exhaust: The straight scoop on backpressure

Enjoy.
This:
The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.
 
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