My GN - Defying the laws of backpressure?

[BlackMetal]

Just got back from a Sunday drive in my GN and something has me stumped.

Prior to leaving my driveway I cleaned out the inside of my car and found the cap and bolts for my dump pipe that I've left wide open for the past few months. I figured for a kick I'd close the dump and remind myself what it's like to have a quiet exhaust.

During my cruise I unexpectedly ended up on the same road as a buddy of mine from work in his Bronco. He's always wanted to see my GN but I never feel like bringing it to work, so when we ended up first in line at a red light I figured I'd give him a show. The light turned green and to my surprise I spun my M/T drag radials off the line and gave them a nice healthy chirp going into 2nd as well. If you had talked to me two hours ago, I would have told you I've never spun the M/Ts with my car and that it's probably impossible to do so. The car pulled like a mule and didn't seem as sluggish as it has in recent memory. These past few weeks I've actually been thinking that my trans is slipping because of the way the engine would be screaming with all its might and the car just didn't seem to be going anywhere fast.

The only other variable in this situation is that today is 70 degrees, but the other times I've taken my car out this summer haven't exactly been 90 degree scorchers either, so I highly doubt the better performance was just that I was out on a cooler day. I've had the car out during similarly cool days, open dump pipe, and it didn't perform like this. I understand heat and humidity are a huge deal when it comes to overall turbo engine performance but you'll have a hard time convincing me that the weather is really responsible for what I noticed, and closing the dump pipe on the same day was just a complete coincidence.

Is it just me, or does this defy the rule that backpressure is evil on a turbo car? I'd expect WORSE performance when running through my full exhaust, not better. Does anyone have an explanation for this?

 

[Silver 'T' Top]

With my dump open at the track vs, corked up I see a +.2 and +3-3.5 MPH difference.

 

[johnnyttype]

last week i ran at the track with dump closed and open. i gained a ****ping .02 seconds in the 1/4 and .2mph. i was quite suprised! i left the dump open simply for the fact the it sounded wicked and it was a head turner!

 

[BlackBandit]

Something interesting about exhaust gases is that a muffler can actually increase flow by regulating harmonics.

 

[Jerryl]

I researched the Back Pressure thingy a while back.
Here is an EXCELLENT post I had located with some expert feedback
http://www.turbobuick.com/forums/tu...st-vs-back-pressure-2.html?highlight=pressure

 

[BlackMetal]

With my dump open at the track vs, corked up I see a +.2 and +3-3.5 MPH difference.

Yeah, I understand most guys do benefit from it, that's why I'm confused about my car responding better to a closed exhaust.

last week i ran at the track with dump closed and open. i gained a ****ping .02 seconds in the 1/4 and .2mph. i was quite suprised! i left the dump open simply for the fact the it sounded wicked and it was a head turner!

That's mainly why I've left mine open for the last few months, I liked the sound and having an obnoxiously loud car. But if I have to choose between a loud exhaust or a stronger performing car, I'll keep it quiet.

Something interesting about exhaust gases is that a muffler can actually increase flow by regulating harmonics.

I'm starting to think it must be something along those lines. The fact that everything feels/sounds "smoother" with the exhaust closed and that the car takes off like a rocket makes me think exhaust flow might actually be improved with the dump closed. I know on N/A engines you have to worry about things like header length to make sure the exhaust is "tuned" to the engine so the pressure waves in the exhaust help to scavenge the cylinders of exhaust gases. That shouldn't be an issue on a car with a turbo in the exhaust stream, but maybe something along the same lines is happening here, where having my exhaust closed is actually providing a better exhaust flow for some reason, be it pressure waves, harmonics, whatever. The Hooker Aerochamber mufflers were advertised as flowing a certain percentage better than an open pipe alone, so maybe it wasn't BS.

I researched the Back Pressure thingy a while back.
Here is an EXCELLENT post I had located with some expert feedback
http://www.turbobuick.com/forums/tu...st-vs-back-pressure-2.html?highlight=pressure

Thank you. I will have to take a read through that and see if it helps me figure out what it going on.

 

[Turbo1dr]

What kind of exhaust system do you have on the car?

 

[BlackMetal]

What kind of exhaust system do you have on the car?

2.5" dual mandrel bent with Hooker AeroChamber mufflers. Currently it just dumps after the mufflers, I never got around to finding/buying tail pipes.

 

[Turbo1dr]

The way you can tell if you are getting more power with the dump closed is to measure exhaust back pressure with a pressure gauge ie your boost gauge. Take a reading with it open and closed.

 

[QUIP'6']

i went 11.6 through cheap $10 hookers wonder if it helped or hurt me but i think 11.6 on a ta-49 stock heads is pushing it. so it might of helped. when i run my test pipe off my car seems to flatten out in power with it on it pulls till each shift

 

[BlackMetal]

i went 11.6 through cheap $10 hookers wonder if it helped or hurt me but i think 11.6 on a ta-49 stock heads is pushing it. so it might of helped. when i run my test pipe off my car seems to flatten out in power with it on it pulls till each shift

That is very close to how I would describe it. With the dump open the engine seemed to bog down at a certain point and just lose it's power until it went into the next gear. Today with the exhaust closed the car was quiet as a mouse but just kept pulling and pulling and I could hardly believe it.

 

[QUIP'6']

That is very close to how I would describe it. With the dump open the engine seemed to bog down at a certain point and just lose it's power until it went into the next gear. Today with the exhaust closed the car was quiet as a mouse but just kept pulling and pulling and I could hardly believe it.

maybe it changes cylinder pressure by losing it

 

[vader87]

When I had my TE44 combo, it would run mega-rich without the cap. To the point of not running! As soon as I capped it, it cleared up. I too was running 009 injectors. Now that I have my other stuff and a TT chip, it runs great without the cap. I thought it was kinda strange also.

 

[UNGN]

A mandrel bent 2.5" exhaust with aerochambers won't create significant backpressure.

If there is a performance drop, it wouldn't be enough to measure with a butt dyno.

I have a non mandrel bent 2.5" exhaust and can't see a difference in ET's or mph capped or uncapped running mid 11's, up to 118 mph so far.

Put the stock exhaust back on and re-try your test.

 

[Brer Rabbit]

It was the weather..

It was probably cooler than 70 that day, the last 70 degree day we had was last Sunday when the Bears lost to the chargers. Even my Saturn is flying in the cooler weather .

 

[VadersV6]

The exhaust's natural direction is going to be straight. It comes down the downpipe and mostly flies right past the open dump. Its easier to go straight with a little backpressure than slam on the brakes and veer left to go out the dump. Exhaust manufacturers make exhaust systems out of steel because the steel retains the heat better. And the hotter the pipes are, the faster the exhaust moves. More heat=faster moving gas.
What a dump is doing is "dumping" a bunch of cold air into the exhaust stream causing it to stall out. It doesnt really know what to do. It can slam on the brakes and turn left out of the dump, or it can go straight through the muffler. Its going to go through the muffler for the most part, but it wont be easy given the fact the open gaping hole in the exhaust just dumped a bunch of cold air on the charge. Im personally not a big believer in dumps.

 

[Chuck Leeper]

My experience, on the local chassis dyno.
Dump closed, 715RWHP.
Dump open 718RWHP.
Dynomax muffs, 2 3/4" pipes, 3" DP, ATR headers.
Back PSI, measured at the base of the turbo was 40# w/ boost at 26#. Saw negligible diff, open, vs closed. Still less than 2X boost.

 

[cooled85T]

Maybe with the open dump you cant hear tire spin. In my opinion most performance mufflers would be designed to suck or force exhaust gases out by making smoother flow, maybe this is where the gain is coming from.

 

[SloGN]

The exhaust's natural direction is going to be straight. It comes down the downpipe and mostly flies right past the open dump. Its easier to go straight with a little backpressure than slam on the brakes and veer left to go out the dump. Exhaust manufacturers make exhaust systems out of steel because the steel retains the heat better. And the hotter the pipes are, the faster the exhaust moves. More heat=faster moving gas.
What a dump is doing is "dumping" a bunch of cold air into the exhaust stream causing it to stall out. It doesnt really know what to do. It can slam on the brakes and turn left out of the dump, or it can go straight through the muffler. Its going to go through the muffler for the most part, but it wont be easy given the fact the open gaping hole in the exhaust just dumped a bunch of cold air on the charge. Im personally not a big believer in dumps.

VADER if you will read the thread boost-VS backpressure you will find that i conducted some tests using my car and a pressure gauge in the EGR ports. i found some very interesting things when doing these tests.

 

[TurboTR]

My guess is the tuneup improved (it went back closer to the "right" a/f ratio) with the closed dump.

TurboTR