High Flow Cat = Less Responsive Than Standard Cat.

[Munkyb0y]

a couple of months ago i switched my standard 2 1/4 inch cat to an xforce highflow 3inch cat.

exhaust is 3 inch turbo back, no mufflers, no restrictions, just a cannon at the end.

standard 25det, running 9-10psi

the car feels less responsive, although i dont think it's making less power overall.

does a more free flowing exhaust trade off response for top end? or is it just my imagination.

its definately louder now, and i'm more reluctant to give it a boot full, maybe that's the problem?

[Pauly33GTS-t]

Um, if you have what I call a hi-flow cat (600-700CFM) with ur bmeter properly installed on the cat and there isnt any pieces of turbo fin stuck in it, then you should be getting an increase in response and a slight increase in power.

I mean the ideal working conditions for a turbo is no pipes running off it at all but that would be highly impractical.

What brand of 'hiflow' cat have you got?

[No Crust Racing]

This topic has been a matter of contention before. You should gain power from increasing flow but in some cases you will lose some responsiveness in the trade - by this I mean the car will not feel as quick to respond to your throttle inputs but will pull harder once it does. Is this how it feels or are you saying it just feels worse all round?

In my old car I described the sensation as feeling the engine was not as 'tight' - as a stock RB25DET feels quite tight and zippy on the throttle response but you lose some of that feeling with a full turbo back exhaust with the obvious benefit being more pulling power (especially at higher revs).

[Pauly33GTS-t]

Ie, less backpressure and a less harder working turbo (less stress on it).

[Gazee]

When i got my turbo back exh done i felt as tho it wasn't as much different low-mid rpm but after 4k it reved out higher and a bit faster... Just opened it up more i suppose

[No Crust Racing]

Correct ^_^

[fuzzy_scuz]

I got a Nismo 3" high flow from Japan

Increased kw by about 20 and improved response. Maybe it comes down to what Highlow brand you get. eg. some are designed for holden's and fords and don't operate as well in a forced induction car say compared to a HKS or a JUN.

[No Crust Racing]

Not to sound disrespectful but 20kw's flywheel or atw's sounds like a mighty gain from just a new CAT, I'd be inclined to think that might be a mistake. If you're referring to a full Nismo 3" Turbo back exhaust the numbers make more sense.

[fuzzy_scuz]

No offence I have dyno prints to prove it.

before high flow with ADR compliant restrictor 154.7 @ all 4 wheels

After Nismo high flow installed 175kw @ all 4 wheels (plus this is a high flow I improted from Japan the brand is Welidin which is NISMO)

BTW I am running a stock GTR exhaust systems.

Edited January 16, 2007 by fuzzy_scuz

[No Crust Racing]

Didn't realise you were talking GTR sorry as the car we're discussing here is RB25DET - my bad ^_^

Munkyb0y: There's a bit to be gained from using a genuine jap high flow compared to locally produced units. BATMBL has some decent group buy items also that may be worth considering. The JustJap item comes in the $699 Turbo back system so you can imagine it's quality and flow rates given a really good cat generally costs 300+ on its own. Not to say it's not better than stock but just to point out there are better options.

[Munkyb0y]

In my old car I described the sensation as feeling the engine was not as 'tight' - as a stock RB25DET feels quite tight and zippy on the throttle response but you lose some of that feeling with a full turbo back exhaust with the obvious benefit being more pulling power (especially at higher revs).

that's exactly it. doesnt feel as tight/zippy. but once it starts winding out, the power feels fine.

i'm using the xforce 3 inch cat. i know its a cheap option, but better than having the standard cat restricting the system.

Edited January 17, 2007 by Munkyb0y

[Munkyb0y]

so what's the solution?

i'm not convinced that changing to another cat will make a difference.

[No Crust Racing]

Changing to another Cat might get you some more power but as far as giving the car back it's response there are no cheap or easy ways that don't involve reducing total power output. The nature of turbo cars is essentially lag, run more boost, get a FMIC, get a full exhaust, all reduces initial response or give more lag but increased power/torque everywhere else. You could try borrowing a mates Cat to experiment with how they affect the car if you're that concerned.

[fuzzy_scuz]

There is one thing to check.....

If you are running stock ECU make sure that it is set 100% learn mode others wise it won't self regulate the changes required with the new CAT under load.

[Trust33]

once you hit full boost you should have a little more power and torque thru most if not the entire rev range.

Are you adding more boost as well? This will add to the lag effect.

[Munkyb0y]

fuzzy_scuz, what do you mean by 100% learn mode? do you mean reset the ecu?

Trust33, i didnt add anymore boost. the only change was the cat. it gained an extra ~1psi after the install.

edit: i should add that there is a unichip piggy back on the stock ecu. it was there when i got the car, i havent tuned it, but the car has always, and still does run fine, besides this recent loss of response. all in all, it's not a huge issue, but like many of you here, i'm pretty picky about my car, and i tend to notice every little thing.

Edited January 17, 2007 by Munkyb0y

[fuzzy_scuz]

You do not need to reset your ECU it is a setting within the ECU, next time you go in for a services or a tune ask the person to check when they are connected with the ECU coding console for your car.

[BOOSTD]

I would expect the car to require a re-tune, may explain were some of the power is going. I would expect it to be running richer than before....

[fuzzy_scuz]

BTW 100% learn mode is that when there are changes to your car the ECU will automatically adjust tuning, rev, power and air fuel mixture.

[No Crust Racing]

The ECU will adjust itself accordingly if you reset it then take it for a hard drive. Resetting is very easy and basically involves disconnecting the battery and pumping the brake pedal (to drain any residual power) then reconnecting the ECU and going for a hard drive. During this period the ECU will adjust timing and listen for knock and adjust timing again accordingly as well as adjust other variables as required.

Good idea to do it after new mods.