Proof of larger DP/ Fluid Dynamics
 

Okay, I've done some research, and I'm sure that it can be debated until nights end. I don't want people giving their opinions or what they've heard. I just want proof. Proof as in dyno charts/numbers or the word from someone who has done this.

I want to know if you have a pretty sizeable setup that's pushing 500+whp that a 4" downpipe with a 3" testpipe/catback would yield a gain of more power than just a full 3" exhaust.

I'm asking this because I'm questioning if the bottleneck created downstream from the turbo would cause a loss in power. Or if the extra 4 feet of larger diameter piping allowed the turbo to spool faster. Something.

Re: Proof of larger DP/ Fluid Dynamics
 

A 4" downpipe is cool, but if you have ever been under your car and looked at your oil filter, you would realize that there is no way in hell to get a 4" pipe through there without seriously sacrificing ground clearance.
Also, exhaust gases expand, so should your piping diameter.

Re: Proof of larger DP/ Fluid Dynamics
 

With out a cat, your gases don't expand. Other way, they are cooling.

Re: Proof of larger DP/ Fluid Dynamics
 

Thats what I meant. They are cooling, and you are gaining density because of it. In order to "get it all out" you need to open up the pipe. You loose even more velocity, but that is counter-acted against because you have a turbo. In the case of a turbocharged setup, the less (or sometimes the larger diameter) exhaust you have, the better. This is of course within reason.
Good catch on my wording!

Back to his question:
I remember reading a long time ago, on a "unmentionable" board a "certain well know member" switched his "vehicle" from a 3" catback to a 4" catback and saw something like "50hp/xxx 1/4 ET gains".
If someone looks up the "archives" on that certain "board" I bet the specifics can be found.

Re: Proof of larger DP/ Fluid Dynamics
 

If you're talking about Shep, he switched to the same 3.5" Buschur system I have a fewy years ago. Then he went to a shorty pipe out the front bumper last year. I've only heard of three actual 4" systems on DSM's. Trust me, the 3.5" is plenty huge.

I've never seen actual back to back comparisons above 3" systems. But rough guesstimation by some NABR big hitters says a true 3" system isn't a restriction until at least 550 whp.

Re: Proof of larger DP/ Fluid Dynamics
 

I don't think it would even be possible for a 4" downpipe unless you were able to move the turbo over towards the tranny a ways or start with a smaller 02 housing.

Re: Proof of larger DP/ Fluid Dynamics
 

Oh there are ways my friend.

There are ways of doing this.


Back to the question. It's not about a FULL 4" exhaust. It's questioning if the 4" to 3" reduction under the car is going to cause a bottle neck that would cause a loss in effientcy or if it actually has positive results. Because obviously the best exhaust on a turboed car is no exhaust.

Re: Proof of larger DP/ Fluid Dynamics
 

I wasn't going to drop names ;)
I forgot if it was a 3.5" or a 4", I couldn't remember.
I know I'll need a 4" catback.

Re: Proof of larger DP/ Fluid Dynamics
 

To make a long story short, going to a smaller diameter somewhere downstream in the system is going to be a restriction. If you're doing a 4" downpipe, why the hell would you go down to a 3.5" catback? Run 4" all the way, the best exhaust is no exhaust.

Re: Proof of larger DP/ Fluid Dynamics
 

i say go 4" all the way back....until the muffler. then find a muffler with a 4" inlet and a 0.5" outlet

surely you will see the benefits of this across the board

Re: Proof of larger DP/ Fluid Dynamics
 

Yes yes yes. I agree with all of you, but this isn't a "Why not go 4" all the way back" question. I drew up a scenerio. I would like answers, not suggestions. I believe the post that started this thread indicated that.

Sport Compact Car's September 2004 issue has WOTM's Supra featured. It states in the article that he had a 3" exhaust and was running 10.2 in the quarter. He bolted a 4" downpipe and midpipe and immediately ran 9.9. Has anyone else done this and seen this result?

Re: Proof of larger DP/ Fluid Dynamics
 

I have seen people pick up 3 tenth in the quarter by changing nothing.

WOTM used to fight about something on Supraforums a lot about downpipe size, but I think Sound Performance proved him wrong a few times.

Re: Proof of larger DP/ Fluid Dynamics
 

Larger is usually better up to a point, but remember that your turbo is your biggest restriction in your exhaust system. :cool:

Re: Proof of larger DP/ Fluid Dynamics
 

Yes, but technically you want it to be the restriction.

Re: Proof of larger DP/ Fluid Dynamics
 

A turbo is a restriction to the engine itself, but since the turbo makes positive pressure, it's technically not a restriction. The less restriction after the turbo is best.

You're clearly asking this question in the wrong place, there's only about 5 DSM'rs in the country that'd have a solid answer for this. You're asking the 16G crowd here, thereby wasting yours and our time.

Re: Proof of larger DP/ Fluid Dynamics
 

Huh? The entire exhaust "track" of an automobile, whether it is turbocharged or not, is one big pressure differential. It is that pressure differential which causes the exhaust to flow because of your tailpipe being at atmospheric pressure and the exhaust exiting through your exhaust valves is at a much higher pressure. Since gases flow from high pressure to lower pressure, that's why automobile exhaust flows the way it does.

I wish I had a better handle on my fluid dynamics because I do not believe the crude calculations associated with the original question in this thread to be very complex.

Re: Proof of larger DP/ Fluid Dynamics
 

I think you're wrong on both ends here. From what I can see, there are about a half of a dozen (at least) running GT35+ series or SCM61 in this forum.

I wouldn't know why a turbo wouldn't be a restriction.
1. Generally the pressure in the exhaust side is about double of what is in the compressed side.
2. The combustion chamber divides the intake and exhaust tracts. More pressure is caused by the explosion (expansion).
3. The turbines are spinning, but they're creating some backpressure. Which is why after the turbo, the best exhaust system is no exhaust system.

Re: Proof of larger DP/ Fluid Dynamics
 

By "16G crowd" I don't think Steve was referring to the turbos people are running. Maybe I'm the only one who laughed after reading that ;)

Re: Proof of larger DP/ Fluid Dynamics
 

No, I chuckled as well.

Re: Proof of larger DP/ Fluid Dynamics
 

I cried. :(

Re: Proof of larger DP/ Fluid Dynamics
 

I'll answer yours and Jakey's response since it's the same answer.

You're both forgetting that the turbo is a restriction to the exhaust pulses, and if the compressor side of the turbo didn't feed back into the intake as a feedback loop, you'd have a plain old restriction. It's a little different that the turbo creates positive pressure going into the combustion chamber, therefore creates higher cylinder pressures. The more pressure the turbo makes, the more of a "restriction" the turbine side of the turbo is. I just can't justify calling it a restriction though since the turbo itself creates more cylinder pressure and exhaust pressure than a NA engine would. I guess it's all semantics and maybe the engineering world would disagree. But I don't see something that adds more than it takes away to technically be a restriction overall.

And please, a GT35 or SCM61? Neither are capable of much more than 600 whp, and besides Shane and Cher, I haven't seen anyone here over that amount anyways. Just because half a dozen people have those turbos, doesn't mean they're all maxing out their 3" exhaust. You're looking for people at that power range or higher who have also done back to back comparisons with either timeslips or by measuring exhaust backpressure after changing exhausts. Do you really think you're going to get multiple good answers here?

Re: Proof of larger DP/ Fluid Dynamics
 

Think velocity. For one thing you would need a smooth transition to maximize gains. For another, you are allowing the gases to expand to fill a 4" diameter pipe and then "compressing" them back down into a 3" pipe. Regardless of how you look at it that will take energy, and I think you would agree that it would be a waste of energy. So, you MAY see gains from an ass-backwards setup like that, but they will be very inefficient gains.

I actually had a 2.5" downpipe on the stock catback. This setup was basically to keep the car quiet on the street. The catback was to be removed at the track. Did I see gains over running the stock DP? Well, yes. Not significant gains though, where I would say "this is a decent upgrade for poor kids like me" and probably due in part to the amount of exhaust leaking where they met. If you think you need that size piping, then run it all the way back.

So from my theory based perspective and my real world experience, I think you should just get a whistle tip. They go woo woo.

Re: Proof of larger DP/ Fluid Dynamics
 

Technically it just keep exponentically expanding pressures across the board. You say pot-a-to, I say po-tat-to. I believe it is a restriction, because if you measure pressures before and after the turbine, there is a difference, no matter what's going on in the intake tract.

True it would take a helleva lot of power to max out a 3" exhaust. I'm not trying to see huge gains. Just gains or not. I'm questioning if it helps on spool time. I really don't need multiple good answers. Just one, since I understand that not many have a 4" DP. I know of 2. I'm not really willing to ask these people, since I really don't like them.