Stock cat back restrictive?

[spyderturbo007]

Everything I read says it is, but i'm not so sure.

I was previously running about 33lbs/min with the stock cat back at 20psi on an EVO III 16g. The stock cat back was coupled with a 2.5" DP and 2.5" high flow cat.

A couple of weeks ago, I upgraded to a Greddy EVO 2 cat back, HyperDrive C1 Clutch and a CoolingMist MeOH injection. With the boost set at 20psi I made a couple of pulls and the airflow was.........you guessed it, right about 33lbs/min. It varied a little bit, but nothing appreciable.

So, I'm really thinking that the stock cat back isn't really a restriction since I don't see any other bottle necks in my system.

EDIT -> Oh, and there aren't any boost leaks.

 

[DSMunknown]

I thought you were supposed to start a poll, Nate.


Any chance you know the ID of the OEM B-pipe? This is interesting news, although (of course) I'm highly skeptical of the numbers. I'm wondering if the B-pipe was creating such a neckdown and turbulence that it was relaying numbers that led you to improper assumptions.

Of course, everyone will jump in and say "Well, theoretically, this is unsound." I would concur with them, so let's just see if you may have overlooked something in your preliminary results, shall we?

 

[DSM4me44]

I have a full 3" exhuast. But whats wierd is that my car pulls hard and drives better when i have my muffler silencer in. I had to have a few buddies drive it to believe me.

 

[MelloEclipse]

i'm not sure it's as much restrictive...but i know a 3" exhaust is going to help with spool up time, and that's something we all love, but then again...it's up to you...

 

[PieEyedPiper]

I know the butt dyno is a terrible tool, but spyderturbo007, did you feel or notice any difference?
Spool up?
Response?
Neck Snapping?

I just find it quite curious that your airflow didnt change. Did you also install you water injection kit and tune for it? This might throw off your airflow maybe?
Are you using an SAFC? That, I believe, will make any air flow calculations incorrect.

 

[daren_p]

Now when you were comparing results, were the before & after done one the same day & was the test with similar test conditions? Changes in temp & barometric pressure etc. can squew test results & being we wouldn't be talking about huge differences here, the atmospheric conditions could easily make up the difference.

I believe the stock cat back is abit of a restriction at the muffler but I think the biggest restrictions in the stock setup would be the factory downpipe with the flex section & the factory cat. Being you already had these upgraded with the first results, I wouldn't expect to see gains the same as someone replacing everything from the turbo back but would still expect some gains, maybe 1 or 2 lb/min at the most.

Another issue may be also that you did only upgrade to a 2.5" system all the way back. Most people think while 2.5 to a 3" system isn't all the much bigger, when you do the math & work out the area of each diameter you will find the 3" is actually a good amount larger then the 2.5" system, ~4.9 vs 7.1.

 

[spyderturbo007]

Yeah, I always try to log my runs with similar environmental conditions, but to answer your question, they were done about 7 days apart. Similar ambient temperature, but I didn't check the pressure.

I back calculated airflow from the logger so the SAFC's manipulation of the airflow isn't the problem.

As for the butt dyno, nothing noticable. Since I haven't tuned for the WI I did unhook it for a pull and noticed a difference there, but nothing associated with the exhaust. I finally got rid of that little timing pull in 3rd gear that I could never tune out. Damn, SMIC heat soak. But with the MeOH, I don't have to worry about that anymore. Now I'm worrried that I'll lift the head when I start cranking down on the MBC.

I really thought I would see an increase in flow just based on the shear size of the Greddy exhaust over the stock. Oh, well, now I have some real world numbers showing that the cat back isn't a restriction at 33lbs/min. 33lbs/min with my mods just seem low at 20psi. I was guessing I would be around 35 or 36lbs/min at 20psi.

Oh......Anthony.....Clean out your PM inbox.

 

[daren_p]

I really thought I would see an increase in flow just based on the shear size of the Greddy exhaust over the stock. Oh, well, now I have some real world numbers showing that the cat back isn't a restriction at 33lbs/min. 33lbs/min with my mods just seem low at 20psi. I was guessing I would be around 35 or 36lbs/min at 20psi.

33lb/min for 20 psi & your mods doesn't look low to me. The 2g stock intake mani & head are pretty restrictive at the top end & this is were you get you peak airflow reading from. Now add to that your also on stock cams, I would say your actually not to bad at all especially running a SMIC.

This brings me to another point. Did you compare your airflow readings through the whole pull? Being your stock top end does pose a restriction, maybe your peak flow numbers were fairly similar but did you notice gains during the rest of the rpm range?

For comparison at 20 psi I usually see ~38 lb/min but thats running a better flowing exhaust side FP3052 & also FP1X cams. I also see this airflow right around 6300 rpms & then it basically flatlines. With a better flowing top end setup I would see that airflow continue to increase on the way to the redline.

 

[DSMunknown]

I believe the stock cat back is abit of a restriction at the muffler but I think the biggest restrictions in the stock setup would be the factory downpipe with the flex section & the factory cat. Being you already had these upgraded with the first results, I wouldn't expect to see gains the same as someone replacing everything from the turbo back but would still expect some gains, maybe 1 or 2 lb/min at the most.

I'll have to remember to post up my before-and-after results when I (finally) bolt on my RRE "Evo Style" long DP, unported Evo III O2 housing, and Thermal exhaust. I'm also thinking about going to a local shop and dynoing the car before I do the install, and then make a visit after as well. It'll be interesting to see the results, at least.

Another issue may be also that you did only upgrade to a 2.5" system all the way back. Most people think while 2.5 to a 3" system isn't all the much bigger, when you do the math & work out the area of each diameter you will find the 3" is actually a good amount larger then the 2.5" system, ~4.9 vs 7.1.

Good to know.

 

[DSMunknown]

Oh......Anthony.....Clean out your PM inbox.

All set.

 

[spyderturbo007]

33lb/min for 20 psi & your mods doesn't look low to me. The 2g stock intake mani & head are pretty restrictive at the top end & this is were you get you peak airflow reading from. Now add to that your also on stock cams, I would say your actually not to bad at all especially running a SMIC.

This brings me to another point. Did you compare your airflow readings through the whole pull? Being your stock top end does pose a restriction, maybe your peak flow numbers were fairly similar but did you notice gains during the rest of the rpm range?

For comparison at 20 psi I usually see ~38 lb/min but thats running a better flowing exhaust side FP3052 & also FP1X cams. I also see this airflow right around 6300 rpms & then it basically flatlines. With a better flowing top end setup I would see that airflow continue to increase on the way to the redline.

That's a really good point, Daren and something I hadn't thought about. I'll have to see if I can pull up some old logs and compare the airflow. I'm just used to looking at peak airflow.

 

[calreef18]

I upgraded from the stock cat-back to the 3" ebay exhaust and my car feels much better on the butt dyno, I'll try and get a log and compare the two.