Stock exhaust back pressure

[Kjewer1]

I don't see how dropping 14 psi of back pressure could not help. I'm going to try a cutout on this downpipe. I bought the cheap one on purpose so I wouldn't feel bad modifying it.

Offroad downpipe, stock catback - 8 psi exhaust pressure, 40 psi turbine pressure

In this configuration the car ran a 12.1 at 112 on a shitty 1.73 the other night. With a more typical 1.63 it would dip into the 11s. I thought this might give some hope to people that want to keep the stock catback. I expect the open cutout will still drop 1-2 tenths, but we'll find out soon.

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[5pointOh]

Backpressure= Good on N/A cars.
Backpressure= bad on turbo cars, the less the better. (not sure with s/c since its not part of the exhaust, but I'd imagine that its good for s/c)

 

[Kjewer1]

I disconnected the stock catback again today and it made seemingly no difference. Same ET and MPH. It was probably ten degrees warmer than Wednesday night, so there may have been a slight gain, but it was slight. I may still install the cutout so I can do back to back tests on the same day, just out of curiosity. Considering this, and how quiet the stock catback is, I might just keep it.

I did get my 11.9, but that was from a small change to the tune and another 1.59 sixty.

 

[ypena02]

I disconnected the stock catback again today and it made seemingly no difference. Same ET and MPH. It was probably ten degrees warmer than Wednesday night, so there may have been a slight gain, but it was slight. I may still install the cutout so I can do back to back tests on the same day, just out of curiosity. Considering this, and how quiet the stock catback is, I might just keep it.

I did get my 11.9, but that was from a small change to the tune and another 1.59 sixty.

congrats on the 11sec pass, what's your tire setup?

 

[Kjewer1]

MT ET streets. Got a couple of 1.59s today with them (best on the stock tires was 1.71).

 

[ypena02]

MT ET streets. Got a couple of 1.59s today with them (best on the stock tires was 1.71).

both impressive! what psi and launch rpm worked best on the stock tires?

 

[Kjewer1]

No specific rpm or boost, just a matter of timing the throttle and brake pressure. Trying to build any meaningful boost on the line just blew the tires off. On the MTs it will take all of the boost it can make, but that doesn't always result in the best 60 foot. I got one of the two 1.59s that way (left at 25-26 psi and ~3400 rpm), the other leaving just like the stock tire strategy. More than one way to skin that cat apparently.

 

[Rick B]

It seems to be similar in capacity to an EVO3 16g, obviously, but if these back pressure numbers are right, the turbine side is a vast improvement. When I ran 10.31 at ~130 on an EVO3 turbo in my Talon, airflow was 42 lbs/min at 19-20 psi at redline, but back pressure was 40-80 psi depending on what I was doing. It ran the best at ~45 psi back pressure. That would have been with 0 psi at the turbine outlet of course, the "exhaust" was a single 3" bend. So that shows the difference in turbine expansion ratios. Also noteworthy, no difference whatsoever between that 3" bend and a 2.5". 3" is always the standard, and there's nothing wrong with going bigger, but I've been able to put 103 lbs/min through a 3" downpipe for reference.

How and where did you measure back pressure. I don't exactly understand.
I am new to turbo charging in cars. I'm thinking that it's exhaust gases going to the turbo to spin it up, but i'm not sure. I know I should know this stuff from school but it's been like 45 years since i graduated. BE (ng).

 

[Kjewer1]

Post #37 has pics and locations.

Looking back through this thread, WTF ever happened to that transbrake?

 

[Rick B]

Got it thanks. 11's on this car is nice. I have to know: What tune and suspension setup to get those great 60' times.
Transbrake, yea, I looked for a thread about it with no joy.

 

[Kjewer1]

The suspension is basically stock. Kait bought a bunch of stuff for it that I never installed. I think we did the vertical links and that's it. We were both getting low 1.7s on the stock tires. The 11.9 pass was with a 1.59 if I remember correctly, MT drag radials.

 

[Rick B]

Thank you sir. What tune are you running.

 

[Kjewer1]

My own, on a Cobb AP.

 

[Heardcuh]

So as far as the math for flow goes would a 3" open Downpipe off the turbo be best because it has no backpressure?

 

[tw557]

The way I see it, any exhaust modifications or intake for that matter will not make ANY difference at all in performance other than possible a very small fraction of a second in response time of the power. As far as I can tell the power is completely PCM controlled and limited. At every single rpm and load the amount of boost is limited my the blowoff valve or wastegate. Any thing to free up the exhaust flow will just be wasted thru the wastegate. All the tunes prove this. Evidently all our stack parts are making more then enough power but they are limited by the PCM. All the tune does is allow the stock parts to be less limited and use the power they already can make.

Now normally aspirated cars are a whole different story.

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