First Drive with New 3in Downpipe - OPEN!

[TurboATM]

hell, my wastegate arm came undone from wastegate itself, so i was running only 1 psi....it was like i was towing a train...it was right as a camry pulled up beside me and schooled me...horrible. give it a try

 

[ZyklonBZombie]

i had a setup kinda like that on my old tsi fwd, it had good spool up and top end and i LOVED how loud it was .... my parents and inlaws always mistook the trashtruck for me

 

[UofACATS]

No again. The reason is, backpressure didn't cause the loss in torque. Sure, it could have been lower with the mods, but that isn't looking at the full picture.

The intake/exhaust system must be tuned to a specific length and diameter. It is all about getting as much air as possible into the cylinder. The problem is, everything is a compromise. A system tuned for producing high rpm torque might fail at producing low end torque, while the opposite may also be true. Stock systems are and always will be a compromise to provide a wide powerband. That is good when you are racing too, but top end torque might be more important, so the engine is tuned as such.

The point is, that Honda didn't lose power because all of a sudden it didn't have "backpressure," it was because the intake/exhaust simply weren't efficient until high rpm. When the exhaust valve is opened, a high pressure pulse is generated. When this pulse hits a change in area (a large chamber, end of the pipe, etc.) some of the pulse is reflected back as a "negative" pressure. Exhaust tuning involves timing that pulse (by adjusting the length of the pipe) so that it reaches the port during the overlap period, providing a pressure difference across the intake/exhaust, thereby scavenging the rest of the exhaust out of the cylinder and bringing in fresh charge. That is great and wonderful, but one length of exhaust pipe that works at a certain rpm doesn't necessarily work at others, and so you must compromise. The same thing goes for pipe diameter - a large pipe flows well but velocity is poor, a small pipe has high velocity but flow will suffer.

So, a simple look at the dyno results and one might assume that the car lost low end torque because it doesn't have backpressure, but that simply isn't taking the full system into account.

Someone get this guy some green squares. Thanks for the lesson.

 

[CrazyM]

hell, my wastegate arm came undone from wastegate itself, so i was running only 1 psi....it was like i was towing a train...it was right as a camry pulled up beside me and schooled me...horrible. give it a try

I can definately relate to that! Except mine was when a civic pulled up next to me while cruising, I dropped a gear stomped on it, and then thats when the little clip broke and the arm popped off. For a few seconds I thought I was racing a fast ass civic till I realized how slow I was accelerating and building NO boost

EDIT: Gotta love the paper clip mod I added to hold it on now

 

[meliketoball]

I don't think turbocharged engines need any backpressure.

That's why people run their o2 out the side of the bumper.

N/A is where backpressure should be applied.

 

[99 GST-R]

I drove my gst around with just the DP but it started to bug me, so I put the rest of the exhaust back on. From what I understand turbocharged engines are MUCH better off with no back pressure, while NA engines needs some. I know its a different engine, but when I was helping a buddy with his 350 we ran it around the block with just headers, yea it was loud, yea it sounded good, but it drove like crap. There was NO low end power at all, it just seemed to be all noise and no go.

 

[SpeedAddict62]

I drove my gst around with just the DP but it started to bug me, so I put the rest of the exhaust back on. From what I understand turbocharged engines are MUCH better off with no back pressure, while NA engines needs some. I know its a different engine, but when I was helping a buddy with his 350 we ran it around the block with just headers, yea it was loud, yea it sounded good, but it drove like crap. There was NO low end power at all, it just seemed to be all noise and no go.

The same argument for the reason the Honda lost power on the dyno applies to the 350. The engine didn't lose power because it lost backpressure - it lost power because the headers weren't doing a good job of scavenging exhaust gases. Taking a well tuned system off a car and replacing it with just an open header can actually hurt power, because where the tuned system was producing a nice low pressure at the exhaust port during overlap, helping flow, the open header actually has a HIGHER pressure at the port during the critical exhaust and overlap phase. So you could actually say that "backpressure" was MORE with the open header, not less. The pressure right at the exhaust port is what is critical in making power.

Again, people just assume that cars need backpressure, but it just isn't that simple. They need velocity, not backpressure, although unfortunately increasing velocity (good) also increases backpressure (bad) with a smaller pipe, hence the relationship.

How could a higher pressure on the exhaust of an engine possibly gain torque? It defies the laws of physics. Air flows from a high pressure to a low pressure. There is a high pressure in the cylinder after the compression stroke, and we are trying to get rid of it, so we try to make the pressure outside the cylinder (the exhaust) as low as we can. ALL engines could benefit from low backpressure. For NA cars, scavenging is more important, so the system is designed as such, with backpressure being the unfortunate outcome. We can't get rid of it, but we can try. Turbo cars can go with the biggest exhaust possible because scavenging isn't so important but spooling the turbo is.

 

[tkelly27]

Flowmaster, Inc.

There's some more reading, really helped me understand things in a NA motor.

The only time I've ever heard of a big exhaust causing problems on a turbo engine is in tiny engines with tiny turbos that are acting a lot like a NA engine anyway.

I think when most people say they've "lost torque" by going to a bigger exhaust it is only that they feel they have lost torque. The car will feel like it's not accelerating as hard in the lower end because it pulls so much harder once the turbo spools up.

 

[nanokpsi]

But the intake/exhaust was efficient before in the low rpms. So you can't blame it on the motor, it was in direct correlation to the exhaust modifications.

So yes, the Honda did lose power because it didn't have backpressure, you can't blame it on anything else.

That is becasue of the sizing compromise mentioned earlier. The The stock system worked better at those rpm ranges, but was restrive up top. The new system increased the VE in the upper rev ranges, but also did not work as well in the low rpm ranges. Most changes that increase VE in the upper rev ranges are also detrimental in the lower rev ranges, ie cams, intake manifolds, bigger turbine housings, etc..

 

[wishihadatalon]

Well this thread was much better than I thought it would be. Awesome backpressure talk. No backpressure = win

Maybe this could go in another thread, but what do you do on the exhaust side if you are getting huge amounts of backpressure pre turbo? Clip the turbine wheel? Port the turbine housing more on the exit side to create a larger pressure differential? More efficient exhaust manifold?(I am assuming that something like this could increase exhaust velocity and help drive past the turbine wheel?

 

[SpeedAddict62]

Well this thread was much better than I thought it would be. Awesome backpressure talk. No backpressure = win

Maybe this could go in another thread, but what do you do on the exhaust side if you are getting huge amounts of backpressure pre turbo? Clip the turbine wheel? Port the turbine housing more on the exit side to create a larger pressure differential? More efficient exhaust manifold?(I am assuming that something like this could increase exhaust velocity and help drive past the turbine wheel?

I would ASSume that the easiest way to do this would probably be to use a larger turbine housing (greater A/R). I wouldn't port the exit of the turbine housing for fear of messing up flow in that region. Maybe someone who has taken actual pressure measurements off the manifold can get in here?

 

[JusMX141]

Here's my take on all of this.

I ran an open 3" downpipe on my '92 TSi FWD for a while just to see what would happen versus the stock exhaust. This was a while ago before I knew much about these cars....I was on the stock fuel system, stock 14B turbo, at the stock boost level.

The first thing I noticed was a noticeable loss of off-boost torque. The turbo seemed to spool quicker once boost was being built, but if you were in a higher gear and out of boost, the car felt much slower going uphill than it did with the stock exhaust.

Another thing I noticed was that my bone-stock car would now hit fuel cut with the open downpipe. Why? Boost creep. I didn't know why at the time, but now I have the answer. There's literally no backpressure to cause the turbine housing to add enough turbulence to divert exhaust flow past the housing once the wastegate opens, so I could've been making 20psi. With no boost gauge in place I had no idea what the hell boost level I was running.

I believe that backpressure is a good idea in order to keep boost under control for a street car. If I had a fully-built race engine that would support 30psi on a race-built car, I wouldn't care how much it would overboost from a lack of backpressure....I'd want that exhaust to exit as much airflow as I'm shoving into it.

I feel that a 2.5" turbo-back system has adequate flow and enough restriction to control boost properly on any street-driven car under 18psi. Once you've upgraded to a turbo that generates some serious airflow or run a very high boost level on a smallish turbo, then you should consider a 3" system. If you run out and buy a catless 3" system and slap it on a 14B-powered car and try to run 12psi, you're most likely going to have issues controlling boost.

For a race car, I like the sound of a 3" full system with no muffler. It doesn't drone as much as an open downpipe, and sounds really mean under full boost. Here's a clip of my buddy Dave's car with a stock 6-bolt, SBR G60 @ 20psi, and a full catless, muffler-less 3" turbo-back exhaust:


<embed width="448" height="361" type="application/x-shockwave-flash" wmode="transparent" src="http://i239.photobucket.com/player.swf?file=http://vid239.photobucket.com/albums/ff165/jusmx141/My%20Videos/Dave1234.flv">

 

[Bluefire16]

I ran open o2 AND DID it spool quick as hell, was awesome...

 

[SpeedAddict62]

Do you have any info to back up your theory on not having enough turbulence to divert exhaust flow? That just doesn't make sense to me...

It sounds more to me like air flowing through the path of least resistance...suddently flowing through the turbine wheel isn't so bad when compared to the wastegate. I had the same issues when I first upgraded to a 3" exhaust, above 5500 rpm, I couldn't keep boost below 15 psi.

I think a better solution (and much more efficient) option would be to upgrade the wastegate at the same time the exhaust is added. An external wastegate off the collector would be best - vented to atmosphere if possible. Then you could enjoy the performance benefits of a larger exhaust and still control boost levels.

 

[nanokpsi]

I would ASSume that the easiest way to do this would probably be to use a larger turbine housing (greater A/R). I wouldn't port the exit of the turbine housing for fear of messing up flow in that region. Maybe someone who has taken actual pressure measurements off the manifold can get in here?

Correct. Porting the inlet side of the turbine housing will help a small amount, but the bigger turbine housing is the way to go. Having the turbine wheel clipped is an option as well, but I can't speak firt hand because I went straight to Garretts and never played with the small-ish mitsu stuff.

A nice turbular manifold with a well designed collector is awesome for improving VE in the upper rev ranges.

Boost creep is casued by the wastegate not being able to bypass enough exhaust gas. The bigger downpipe only aides in that department. However, It also increases VE enough that the stock wg cannot bypass enough exhaust, causing the creep. The smaller pipe is less prone to creep becuase it still restricts the motor slightly. That is why most people reported that creep wasn't as bad with a high flow cat as opposed to a test pipe.

 

[wishihadatalon]

Correct. Porting the inlet side of the turbine housing will help a small amount, but the bigger turbine housing is the way to go. Having the turbine wheel clipped is an option as well, but I can't speak firt hand because I went straight to Garretts and never played with the small-ish mitsu stuff.

A nice turbular manifold with a well designed collector is awesome for improving VE in the upper rev ranges.

Boost creep is casued by the wastegate not being able to bypass enough exhaust gas. The bigger downpipe only aides in that department. However, It also increases VE enough that the stock wg cannot bypass enough exhaust, causing the creep. The smaller pipe is less prone to creep becuase it still restricts the motor slightly. That is why most people reported that creep wasn't as bad with a high flow cat as opposed to a test pipe.

Yeah clipping the wheel is not something that is going to happen and a larger turbine housing isn't going to happen either. Right now we are going to run an fp manifold and fix any small boost leaks to make the turbo work less which should help a little. Also going to try to degree the cams to see if that helps. Also from this point on, I will be taking back pressure test to see how well thing help. Then hopefully this year the car will get an sc6176 and we can stop worrying about this stock mitsu stuff.

 

[99 GST-R]

Hmm.... After a little more research on this topic Im starting to better understand your point SpeedAddict62, I just always to the word of the local car and truck guys around here because it was simple and made some sense if you didn't think about it alot... Does anyone else have any more links to more info on this?

 

[vanilla gorilla]

You need SOME exhaust period. I run open Apex'i N1 DP on my GST....not because I think it will give me performace...I actually think the opposite. I just run open DP because I like loud, trashy sounding cars, and I'm too cheap to buy a $500 catback.

 

[SpeedAddict62]

You need SOME exhaust period. I run open Apex'i N1 DP on my GST....not because I think it will give me performace...I actually think the opposite. I just run open DP because I like loud, trashy sounding cars, and I'm too cheap to buy a $500 catback.

Right, you do need an exhaust (but not backpressure). The reason being that a well designed system can give more power than no exhaust at all. Its all about tuning.

 

[Gooberlog]

Right, you do need an exhaust (but not backpressure). The reason being that a well designed system can give more power than no exhaust at all. Its all about tuning.

I think the biggest difference in running an exhaust off the o2 would be the temperature drop. The turbo will create it's own back pressure. The exhaust might help the flow to channel the exhaust out faster. I know with certain "X pipe" exhaust designs, they are designed to alternate the exhaust pulses at the intersection and then they will actually pull the exhaust back out to the rear of the exhaust. Obviously dsm's do not use an Xpipe set up but maybe in theory this is similar? I would say at least a couple feet off of the o2 housing to direct flow, but DP to catback differences in performance i would think would be similar.