TonyG

iTrader: (5)

I did a little test today, comparing my current exhaust (3” dp, 3” testpipe, stock catback), against an open downpipe (disconnected testpipe). The reason I did this was to evaluate whether I should get an e-cutout (I want to keep my quiet stock catback). Below are the results. This was at Mason Dixon today, with great air (~45 degrees, low humidity, -700ft DA). The 330' timer was not functioning right


Best ET passes
(DP Connected…DP open)
60'......1.683......1.754
330'.....0.000.....0.000
1/8......7.789......7.772
MPH....86.19 ....87.71
1000...10.180...10.127
1/4......12.199......12.114
MPH.....111.61…114.74

Best MPH passes
(DP Connected…DP open)
60'......1.825......1.913
330'.....0.000.....5.342
1/8......8.023......8.099
MPH....85.78 ....87.01
1000...10.430...10.470
1/4......12.455......12.461
MPH.....112.80…116.56


Comparing the best MPH passes, the open DP was worth ~3.8 MPH (edit: 2.5mph -see below), with most of the gains occurring on the back end of the track (notice the 27-29mph gain from 1/8 to 1/4). I was running with 109 octane race gas and my peak boost was at 25 psi for all passes. The peak boost wasn't affected by the open dp (surprisingly).

I was really hoping to hit 11.9x today, but it didn’t happen. The 12.1 with the stock catback really surprised me.

Smogrunner

iTrader: (5)

Nice thread. Very interesting.

HassounEVO

iTrader: (4)

Interesting... I'm sure people running more boost, bigger turbos etc. would see even better gains. Looks like a cut-out is worth it.

NIevo

iTrader: (10)

I wonder how much of the mph change is from the dp removed and how much is from the different '60 times? Interesting results as that much of a change in traps should have brought about better ET's

mhgsx

iTrader: (6)

So basically your test highlights how restrictive the stock cat-back is, especially when you transition from 3" to 2 3/8" with no taper.

TonyG

iTrader: (5)

-NIevo, mph difference had little or nothing to do with 60ft (it usually doesn't). Compare the change in mph from 1/8 to 1/4 for the open dp vs connected.

-mhqsx, yeah, it shows the restrictiveness of the stock catback, but there's a lot more to it than the numbers show. Launching with the open DP was amazing - the spool up was instant. Wheelspin became an issue and the shift to 2nd came so fast that I screwed up my first run because it caught me offguard.

NIevo

iTrader: (10)

While I agree a change in '60 times doesnt make as big a difference as you saw it does make a difference. I have plenty of slips to show it.

I still wonder why with such a huge trap difference your ET's werent better? Was traction that big of an issue after disconnecting?

Ang Wen Yan

+1000000

TonyG

iTrader: (5)

OK, I wasn't sure what you meant before but now I think I understand your question.
You're talking about the effect of a little wheelspin off the line, which hurts your 60' and ET, but helps your trap speed a little, right?

You can see it in my slips: if you look at both open DP runs, you'll see the higher trap speed with the higher 60' time> 1.754->114.7 mph, 1.913->116.56mph.

You can see it with the connected DP runs too: 1.683->111.61mph, 1.825->112.8mph


Well, if you take the average of those two connected DP runs, you get 1.754->112.2mph.
Coincidentally, I ran a 1.754 60' in one of my open DP runs, and trapped 114.7. So comparing apples-apples now, it looks like the actual gain was (114.7-112.2)=2.5 mph, not really 3.8 mph as I originally said. This makes a lot more sense, because I just dynoed open DP vs. connected DP and saw a 24 whp difference.

Good call!

NIevo

iTrader: (10)

Have you ever dyno'd with an aftermarket cat-back? I wonder how much those of us with a full 3" TB would gain? 28whp is pretty good!

VRamirez2004

iTrader: (6)

Good info! I think I might try the same!

TonyG

iTrader: (5)

No, the only dyno pulls I did were with the stock CB, and the open DP. The gain with the open DP was consistently 24 whp, and mostly occurred at the top end (5000-7500).

Rnm

iTrader: (4)

Would be nice if you had an aftermarket catback to compare with open downpipe to see if there are any gains from that. Its not surprising to see your results that really show how restrictive the stock catback really is.

fimotorsports

iTrader: (92)

did your boost level go up with the stock cat back removed?

TonyG

iTrader: (5)

Since I don't have one, I'll leave that for someone else to try out.

It's a very simple test -make a few passes, take off your testpipe (four bolts), make a few more passes. Done