My GN - Defying the laws of backpressure?

[BlackMetal]

Maybe with the open dump you cant hear tire spin.

Haha I knew when I posted this thread someone would say "maybe you just can't hear the tires with the dump open", but I'm sure I would have noticed it at least once if my tires had been chirping at all over the past few months while the dump was open. And tire squealing aside, the car was pulling harder in general, not just at launch.

When I had my TE44 combo, it would run mega-rich without the cap. To the point of not running!

Yeah, way back when I first installed the dump pipe and had a few less mods on the car, I wanted to show a buddy how loud the car was with the open dump, took it out on the main road and jammed on the gas and the car actually ended up stalling out. So I have a long history of the car actually running worse with the dump open.

The exhaust's natural direction is going to be straight. It comes down the downpipe and mostly flies right past the open dump. 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.

The ideas you present make sense and could very well be the explanation for what I'm experiencing.

 

[Chuck Leeper]

Yep..

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.

The "dump" theory was used on the Mopar factory S/S cars of the 60's. If one looks at the system design, they will see that the exhaust from the behind the dump is actually " J hooked" into the side of the pipe w/ the dump cap on it."
This follows the theory quoted, in that the exh stream tends to flow in a straight line. Mopar engineers knew this, and had the system designed for max efficiency w/ the dumps open. The cars even had a x-flow single muff on them. Maybe, a definate "hint" that the dumps are primary, and "quiet" was ??.
Given the success of these cars, maybe a TR following that design would show better results??

 

[BlackMetal]

The "dump" theory was used on the Mopar factory S/S cars of the 60's. If one looks at the system design, they will see that the exhaust from the behind the dump is actually " J hooked" into the side of the pipe w/ the dump cap on it."
This follows the theory quoted, in that the exh stream tends to flow in a straight line. Mopar engineers knew this, and had the system designed for max efficiency w/ the dumps open. The cars even had a x-flow single muff on them. Maybe, a definate "hint" that the dumps are primary, and "quiet" was ??.
Given the success of these cars, maybe a TR following that design would show better results??

Yeah I've seen what you're referring to on cars with custom exhaust before, where the dump is straight in-line with the exhaust flow and the full exhaust system bends off to the side. I'm sure it is more efficient that way, instead of the way these dump pipes are designed for the GNs.

Many good ideas have been suggested in this thread, now it doesn't seem so strange that the car was running better.

 

[ijames]

Or make up a half-cylinder with the end beveled that could slide up inside the dump to seal off the muffler path so now the exhaust sees a fairly smooth 45 degree bend to the outside world instead of the Y. Bend the end over on the outside and hold it in place with a bolt or two through the dump flange. Be an interesting experiement for someone at the track or dyno, since it could be installed in minutes so back-to-back passes would be easy.

 

[VadersV6]

Another thing I meant to bring up is that even the full time drag cars with open long tube headers, usually run an 18 inch collector extension, and it always improves times. Why? One pulse draws out the next one, but only at narrow rpm ranges at around 3 points from idle to redline. Its helmoltz resonance. One exhaust pulse flies out down the pipe. The exhaust valve closes and for a moment the pulse doesnt have a vent, so it stretches, stops and bounces back toward the valve. Then it compresses and bounces back toward the tailpipe. Then the exhaust valve opens and it now has a vent and it can release with additional force if its timed right. But at other rpm ranges its timed wrong and actually is non conducive to power. The same is used in intake manifold design and has a dramatic effect on the power curve. I did alot of work with intake manifolds for 99-01 cobras and was able to get a power curve that no one out there could achieve and I used this resonance to the fullest to get there. Theres alot more to it than that but thats the gist of it. If the vent is too early on in the exhaust tract (open header), then the pulse vents and cant be used to draw out the next pulse. Formula 1 had CRAZY header designs in the 60's that made use of this principle to an extreme level. I dont know if this is happening at all in the downpipe of a turbo car, but its possible. One thing is that a downpipe can be too big and cant get enough velocity out of the gas to draw out the next pulse. But in the case of these cars, I think the fact that the dump isnt inline with the gas direction and that its dumping a bunch of cold air into the stream is what keeps you from seeing any benefit.

 

[scottyb]

It's the temperature drop. I can feel it in the butt dyno.

 

[SloGN]

One thing is that a downpipe can be too big and cant get enough velocity out of the gas to draw out the next pulse. But in the case of these cars, I think the fact that the dump isnt inline with the gas direction and that its dumping a bunch of cold air into the stream is what keeps you from seeing any benefit.

i agree with you.

Alowing the velocity to slow or stall out in the large pipe at the top it takes more energy to speed that air back up to continue out and down the rest of the pipe/system.


I posted some results in the boost-vs-bacpressure thread on this and had actual data to prove that something was happening when doing this because my car yeilded no gains when uncapping the dump. the only gains were i couldn't hear after my 1/8 mile blast.

Btu when conducting this test i took the dump pipe completely off and yeilded a gain by just using the open downpipe. I also show a drop in the backpressure preturbine as well.

 

[Roc87]

My GN seemed to run better with no exhaust after the downpipe as compared with the dump open. Seems to be related to the exhaust flow. I've been wanting to take it to a shop and have the dump fliped around and the cat-back attached to where the dump pipe usually is, and have the electric cut out where the cat-back usually is. Only trouble is most shops won't touch it without a cat:frown:

 

[Turbo1dr]

Yeah I've seen what you're referring to on cars with custom exhaust before, where the dump is straight in-line with the exhaust flow and the full exhaust system bends off to the side. I'm sure it is more efficient that way, instead of the way these dump pipes are designed for the GNs.

I was talking to a friend this past weekend about that. Except I'm going to use a factory crossflow for the diesel G body cars and make an automatic dump when boost rises. This system would be VERY quiet and allow an open dump only when needed. You could use a very light small tube system since there's no need to have a full performance exhaust system.

 

[Jerryl]

I was talking to a friend this past weekend about that. Except I'm going to use a factory crossflow for the diesel G body cars and make an automatic dump when boost rises. This system would be VERY quiet and allow an open dump only when needed. You could use a very light small tube system since there's no need to have a full performance exhaust system.

Man, you always come up with the coolest ideas!

 

[VadersV6]

My GN seemed to run better with no exhaust after the downpipe as compared with the dump open.

I told a story on here a couple months back about my friend in high school, bobby quarnstrom (who started solo motorsports..off road truck co.)...When we were sophomores in high school I changed the cam in his datsun truck...it was a 1600 and he wanted an 1800 cam...basically no power gain but whatever floated his boat...anyway he cut the tailpipe and muffler off while I was doing this. We took the truck for a ride when I was done and it was loud as hell. His girlfriend was in the truck with us and screaming saying the truck was so much faster now that it scared her.....the truck wasnt faster AT ALL...just sounded faster.

 

[Roc87]

Well, what I meant, was that the exhaust flow was going straight out thru the downpipe, no dump pipe/cat-back exhaust under the car at all, so no open dump pipe for the gas-flow to divert through at the 45° angle.

I bet that Datsun sounded horrible

 

[machinegun]

Here is what I found. With my dump open, either the reverberation or may spool up speed causes my car to setoff the knock sensor. This causes the ECM to back off the timing causing you got it less power. So in my case the open dump has done nothing to improve and in some cases has hurt my performance. Hope this helps.

 

[VadersV6]

Here is what I found. With my dump open, either the reverberation or may spool up speed causes my car to setoff the knock sensor. This causes the ECM to back off the timing causing you got it less power. So in my case the open dump has done nothing to improve and in some cases has hurt my performance. Hope this helps.

Something about that doesnt add up. The knock sensor is designed to pick up a small frequency range, and I wouldnt think the open exhaust would be anywhere near that range. The noise may have been causing something else to vibrate around that range, but not the exhaust itself. I dont remember there being too many people complaining of knock sensors going off from open dumps.

 

[SloGN]

Something about that doesnt add up. The knock sensor is designed to pick up a small frequency range, and I wouldnt think the open exhaust would be anywhere near that range. The noise may have been causing something else to vibrate around that range, but not the exhaust itself. I dont remember there being too many people complaining of knock sensors going off from open dumps.

I have never had that problem before.

 

[johnnyttype]

Here is what I found. With my dump open, either the reverberation or may spool up speed causes my car to setoff the knock sensor. This causes the ECM to back off the timing causing you got it less power. So in my case the open dump has done nothing to improve and in some cases has hurt my performance. Hope this helps.

could it be that when u open the dump, ur max boost rises just like when installing a new intercooler? ive heard of that happening before, the rise in pressure causes the detonation from lack of octane or tune is off for that pressure.

 

[Turbo1dr]

Anyway you look at it, the engine must not have ANY backpressure at all after the turbo. If you tune the car with the exhaust closed and then go to the dragstrip and open the dump then you may have just caused your tune to be wayyy off! Tune the car like you run it at the dragstrip.


If ALL the exhaust is getting out without any backpressure showing on the gauge (either capped or uncapped) the car should run at peak performance. This is how the car should be tuned....NO BACKPRESSURE.

If you open the dump and it's causing a performance decrease then it should reflect a change in backpressure to the engine causing a drop in engine performance. (It could need more/less fuel now)

If you close the dump and it causes a performance decrease then it should reflect a change in backpressure to the engine causing a drop in engine performance. (It could need more/less fuel now)

If you have backpressure with the dump OPEN then you have BOTH a restrictive exhaust AND dump that isn't designed correct. If this is the case then you need to buy some better exhaust or unhook it entirely from the downpipe.


I've said this many times...a simple backpressure test is all that's needed to confirm that BOTH the exhaust or dump is causing problems. The best place IMO to check is at the inlet of the test pipe or in front of the catalytic converter.

 

[SloGN]

Anyway you look at it, the engine must not have ANY backpressure at all after the turbo. If you tune the car with the exhaust closed and then go to the dragstrip and open the dump then you may have just caused your tune to be wayyy off! Tune the car like you run it at the dragstrip.


If ALL the exhaust is getting out without any backpressure showing on the gauge (either capped or uncapped) the car should run at peak performance. This is how the car should be tuned....NO BACKPRESSURE.

If you open the dump and it's causing a performance decrease then it should reflect a change in backpressure to the engine causing a drop in engine performance. (It could need more/less fuel now)

If you close the dump and it causes a performance decrease then it should reflect a change in backpressure to the engine causing a drop in engine performance. (It could need more/less fuel now)

If you have backpressure with the dump OPEN then you have BOTH a restrictive exhaust AND dump that isn't designed correct. If this is the case then you need to buy some better exhaust or unhook it entirely from the downpipe.


I've said this many times...a simple backpressure test is all that's needed to confirm that BOTH the exhaust or dump is causing problems. The best place IMO to check is at the inlet of the test pipe or in front of the catalytic converter.

Vernon

i really think that the best place to check back pressure is on the EGR port or for the guys with out the egr port in the heads just under the turbine housing. any backpressure the you relieve with show up pre-turbine. this also gives you how much backpressure to boost your @ also. From what i have found that lowering the backpressure preturbine is like adding that much boost to the intake manifold.

my 2 cents anyhow but what do i know

 

[Turbo1dr]

Vernon

i really think that the best place to check back pressure is on the EGR port

I was referring to exhaust system back pressure only, not pre-turbo. Measuring at the EGR port should also show an increase or decrease in backpressure. The only problem with measuring at the EGR port is you will have to have a pressure gauge that measures up to 60psi. Measuring at the bottom of the downpipe you will only need a gauge showing less than 5psi. That way, if the gauge moves any at all...you will see it. A turbo boost gauge can also be used and EVERYONE has one of those...it's just a matter of hooking it up to the exhaust system and making a WOT run to verify. So simple...yet...overlooked.

If you don't want to unhook your boost gauge to measure backpressure, then hook a line up to the exhaust system and tape a balloon on the end of the line. If the balloon blows up during a WOT blast...you have too much backpressure.