Proof of larger DP/ Fluid Dynamics

rst95eclipse · 03-01-2006

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.

niterydr · 03-01-2006

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.

Shotgun! · 03-01-2006

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

niterydr · 03-01-2006

Quote:

Originally Posted by Grey Wulf

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

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.

**Goat Blower** · 03-01-2006

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.

slowbubblecar · 03-01-2006

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.

rst95eclipse · 03-01-2006

Quote:

Originally Posted by niterydr

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.

Oh there are ways my friend.

Quote:

Originally Posted by slowbubblecar

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.

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.

niterydr · 03-01-2006

Quote:

Originally Posted by Goat Blower

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.

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.

**Goat Blower** · 03-02-2006

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.

Super Bleeder!! · 03-02-2006

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

rst95eclipse · 03-02-2006

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?

**Shane@DBPerformance** · 03-02-2006

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.

**1ViciousGSX** · 03-02-2006

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

rst95eclipse · 03-02-2006

Quote:

Originally Posted by 1ViciousGSX

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

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

**Goat Blower** · 03-02-2006

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.

Jakey · 03-02-2006

Quote:

Originally Posted by Goat Blower

but since the turbo makes positive pressure, it's technically not a restriction.

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.

rst95eclipse · 03-03-2006

Quote:

Originally Posted by Goat Blower

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.

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.