Going fast with the stock cat-back set-up?

[Fred 86 GN]

I know this is "all wrong" in many folks minds but...

How fast have you gone with the stock cat-back exhaust from the factory? I am wondering if it is a big restriction or not. The "propaganda" that has brainwashed all of us is that the stock pipes are too small and the cross-over muffler is terrible for flow. Any before and after dragstrip mph numbers to show that switching to an aftermarket cat-back system made a big difference?

The stock pipes are 2.25" in diameter. Two 2.25" pipes have 7.95sq.in. of area while the 2.5" diameter pipe feeding the turbo has only 4.91sq.in. of area and the 3" diameter THDP feeding the cat-back has only 7.07sq.in. of area. So...how can the dual 2.25" pipes be a restriction? Especially as the exhaust gases cool as they flow through them?

Thanks,

 

[UNGN]

I kept the stock "Y" and oval section after the cat but threw away the rest and had my muffler shop bend me up some non mandrel bent 2.5" duals with ultraflow mufflers and I picked up almost 4 mph.

Your result may vary

 

[Fred 86 GN]

Wow! 4mph is like 40hp! If the stock pipes and cross-flow muffler are that restrictive in my testing I guess I will stick with the Hooker cat-back and wear ear plugs on the highway!

 

[Mr URL]

If I remember right you can get around the low 12's area with the stock CAT back setup and a test pipe. Removing any restrictions after the turbo definately helps. Least the stock headers where done right from the factory! On the other hand I've seen people run low 11's through the stock down pipe, stock intercooler, and a ported stock exhaust elbow in the TFS class.

 

[jojo]

Hooker Cat Back w/Aerochamber straight thru muffler design baby!! with test pipe in place of the cat and 3"DP without the stock elbow. Either the Mease or the THDP, now your looking at a whole new no restriction beast. If you really want to throw the icing on the cake go with the V-4 while your at it thats what im doing, and expecting about 60Hp. Seam to be having a restriction lower performance with anything over 20psi of boost with my current combo, stock dp, not so hot exhaust with cat installed, and the stock IC.

jojo

 

[Fred 86 GN]

JoJo -- what you described is what I have now. I am sure it flows well. Not 60hp worth, but well. For as fast as I want to go -- high-11's -- the stock muffler/pipes might be good enough.

 

[jojo]

Oh sorry Fred didnt see your little sig down there, i was getting excited at the idea of what im going to add in the next couple of weeks. I guess while im at it did you see a big improvement with the V-4 and 3"DP with the hooker and test pipe?? im expecting some real suprise with that combo all at once.

thanks

jojo

 

[UNGN]

Originally posted by Fred 86 GN
Wow! 4mph is like 40hp! If the stock pipes and cross-flow muffler are that restrictive in my testing I guess I will stick with the Hooker cat-back and wear ear plugs on the highway!

With ultraflows, you won't need earplugs on the highway unless your kid turns the stereo up too loud.

If that's too loud, which I can't imagine, put an ultraflow where the cat is.

The easyiest way to test the restriction is unbolt the cat flange and bolt the top two polts of the down pipe to the bottom two bolt holes of the cat back. We used to do this on our mustangs years ago.

 

[Rayk]

You asked about how restrictive the stock cat back system is and the answer is that with a basic recipe car it is not very restrictive at all. My first T/R was a 38,000 mile '87 Limited which I did only the basic recipe mods to. K&N, valve springs, XP Pump and hot wire, adj regulator, 160 stat, yada, yada. It had the stock turbo, stock downpipe, stock intercooler and stock cross flow single muffler. This car went 12.6s @ 106 MPH with just a Red's 100 chip and a 50/50 mix of race fuel and pump gas & recap drag radials. Back then Red Armstrong told me to leave the cat back system on unless I wanted to change turbos and he was right. It was a super smooth and super quiet ride.

 

[UNGN]

I gained 4 mph in a stock turbo, stock injector, stock intercooler car. From 12.7's@106 to 12.2's@110.5. I still kick myself for not ditching the stock pipes sooner. All those close races that wouldn't have been close.

Keep the stock exhaust if you want. If you think you can run 11's with it, more power to you. I'd stick with the 2.5" duals, myself.

 

[blackbuick87]

I am going to get rid of my stock exhaust because I do not like the sound of it. It sounds like a Lumina when I am not on the turbo. I thought the Aerochambers did not have the "drone" at hwy speeds?

 

[BLACK6PACK]

I picked up over 5 mph and .5 seconds in the 1/4 with a fairly stock setup. That thing is RESTRICTIVE!!! I did exactly what UNGN mentioned, unbolted the catback from the downpipe and dropped it down so it dumped right out of the downpipe. I went from 13.8 to 13.2+ and from 98 mph to 104mph. I had to raise the fuel pressure 4 lbs just so it wouldn't lean out.

 

[Fred 86 GN]

Thanks for the information!

In the near future...

I am hoping to run mid-12's through the stock exhaust and cross-flow muffler then open the dump on the THDP and run high-11's. Something like 12.6 at 110mph through the pipes and 11.9 at 115mph through the dump. Run the nice quiet stock exhaust when driving around and open the dump at the track.

Sound like a plan?

BTW, yes, the Aerochamber mufflers do drone some at 70mph on the highway. Not the worst drone I have ever heard, but more than I like. The worst aspect is the vibration I feel from the exhaust. Thus the reason for this post and my upcoming experimentation. I'll post the results.

 

[QuickWrench]

Who carries a cutouts and test pipes for the stock downpipe these days?