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3" vs 4"

6K views 94 replies 35 participants last post by  ddd4114 
#1 ·
Is upgrading to 4" DP/Exhaust worth the added cost?
Set-up will eventually be composed of a Vitara Y8 + Custom Top Mount + T3/T04E 57 Trim.
 
#35 ·
99EJ6T-

Nope. You've got some of it about the turbo, but you're showing a lack of understanding of exhaust gas flow in an exhaust system. Heck, I don't know as much as I should, so, anyone who thinks I'm coming off like someone who knows everything can go suck a big fat one. I just happen to read enough to get the basics.

There is going to be little to no "tumble" in an exhaust system post turbo, unless the exhaust tubing itself is made very terribly with bad fitments and welding, or it is designed completely wrong. Manipulating volume of the exhaust is critical and key to producing as much power as possible in ANY exhaust system because of the effects on exhaust velocity. Going straight to 3" or 4" right from the outlet of the turbo is just plain silly. Transitions are key in all places where pressure changes occur, especially ones that occur rapidly, like going from low throttle to high throttle.

For example, I actually plan on using a section of 4" piping for my next turbo build. It will transition at a steeper than normal angle from the outlet size to 4", where I'll have about a liner foot or so of piping, which will transition very gradually down to 3" as it turns under the pan, continuing at 3" until after a flex and high-flow spun core cat, where it will neck down very gradually to 2.5" before the center-tunnel mounted muffler, with one 90" to the passenger side, then a turndown in front of the passenger rear tire. I neck it down after the cat, because the cat is the first real restriction, and because of the thermal effects on the exhaust gases after the cat. They'll be cool enough to be reduced in volume post-cat, and slightly decreasing the volume of the tubing will help keep the velocity up. I'm choosing this design because it will provide a very large "transient reservoir" to deal with near instantaneous throttle changes encountered when driving in the real world. Throttle changes lead to wide changes in exhaust gas volumes in very short periods of time. Creating a nozzle shape on the outlet of the turbine with enough volume to accommodate the fast increase in the volume in those transitions will lead to better total engine performance, at least, that is what I am theorizing.

Quite frankly, none of this would really matter in most cases, and for most peoples' builds. Since I am not worrying about maximum power, but throttle response and torque production as low and fast as possible with a larger than usual turbo (for the purpose of throttle response, anyway) with a larger turbine A/R, this is why I have gone through the trouble of learning how manipulation of the volume of exhaust tubing leads to different effects. And, also for the record, I could be completely wrong. But, i don't think I am. *shrugs* We'll find out in a decade or three when I actually have the time and money to do any of this.
 
#39 ·
It's doing going to be doing barrel rolls all the way down the exhaust. The middle of the volume will have the most velocity, and there will be vortices formed in the layer of flow closest to the walls of the exhaust, but, except at the outset of the initial change in flow from the turbo, the gasses will flow in a much more laminar manner. Transitions help minimize separation, reducing turbulence when moving from a smaller volume to a larger volume especially. At least from what I understand of the stuff going on in exhaust systems.
 
#44 ·
What? I think I'm misunderstanding you. Harmonics are done having any major impact after the turbo. Post turbo, it no longer moves in pulses, so bigger is better, with diminishing returns.
 
#50 ·
If gases are going to "back up" and affect the turbo, it's going to happen shortly after the turbo. You wouldn't see much of a difference between 2.5" from the turbo back compared to half way back opening to 3".
 
#53 ·
Bigger diameter is louder. No matter what. You don't want to use anything that changes direction of flow, like a chambered muffler. Not sure what you mean by a High Flow Silencer. Resonators will help quiet it down some, but turbos act as a restriction which will make it pretty quiet as well. A resonator, High Flow Cat, and 3" straight thru muffler would result in a pretty quiet boosted ride.
 
#52 · (Edited)
But can't you 'play' with the pressure differential, by using expansion chambers and then reducing it back down (Beave has described it before, not sure if in this thread)?

Like in the example Jarhead posted, what is the point of the 3" to 4" to 3" chamber?
Let the gas come out, expand (drops pressure), then as it cools, it becomes denser and you can go down to a 3" again?

If you run a expansion chamber of sorts (like described above/Jarhead's picture), is there a minimum size/length of it? Like I plan to run a top mount, so have it come out of the turbo, expand up to 4", then near the oil pan, taper back down to 3". Is that enough distance/volume? Is there a way to calculate this?

Anyone got any good articles/books on the subject they recommend?

Ha! Turns out that this thread (Page 2) is where 'Beave talked about it a little.
 
#55 ·
But can't you 'play' with the pressure differential, by using expansion chambers and then reducing it back down (Beave has described it before, not sure if in this thread)?

Like in the example Jarhead posted, what is the point of the 3" to 4" to 3" chamber?
Let the gas come out, expand (drops pressure), then as it cools, it becomes denser and you can go down to a 3" again?
You would have a beneficial effect that would make the turbo spool faster AT FIRST. The expansion chamber would fill up and equalize pressure with the rest of the exhaust system pretty quickly. If that makes sense.


If you run a expansion chamber of sorts (like described above/Jarhead's picture), is there a minimum size/length of it? Like I plan to run a top mount, so have it come out of the turbo, expand up to 4", then near the oil pan, taper back down to 3". Is that enough distance/volume? Is there a way to calculate this?

Anyone got any good articles/books on the subject they recommend?
I really don't think you would see more than 50-100 rpm's quicker spool. In my opinion, it's not worth the hassle.
 
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#58 ·
Jegs, Summit, probably ebay. You can even run glass packed mufflers in line. They are a shit-ton cheaper than resonators, but will get louder over time. I wouldn't use Borla unless the price was right, only because I find their products to be expensive.
 
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#60 ·
The more expansions and contractions you have, the more energy you'll lose in the system. I'm not saying there is no benefit, but it's a good rule of thumb. Unless you're planning to perform (or steal) R&D work, just stick with a constant diameter. N/A race engines are a slightly different story because they don't have huge dampers (catalytic converters and/or turbines) in the system.

I agree with Atreidies that at steady-state, a chamber would be of little help. There is still going to be a pressure loss across the rest of the tubing. All else being equal, bigger is better with diminishing returns.
 
#63 ·
It doesn't have to be under (back) pressure where there's gas stuck waiting to go into the turbo.
You can have a manifold with good wastegate priority (not 90% of the manifolds on the market) that simply bleeds any extra pressure. Or you can design a manifold to use a large turbine wheel and good manifold design, and have relatively little 'back pressure'. From what I understand, the Hytech (and the F1 turbo designs it's based off of) works with a combination of the above two.

But remember, you are not always in boost/making full boost. There's a RPM period where you are hardly making any power (from the turbo) and the turbo is 'hardly' spinning, and it's more like a NA engine. I would imagine that you can pick up power there using scavenging/make the engine more effecient. And if you make the engine accelerate faster through that RPM range, you get into boost sooner.

Most people don't care, though. They would rather just turn up the boost.

That's what I understand of it.
But I'm still noob status.
 
#65 ·
A pulse wave will have a high pressure front and a low pressure rear relative to the medium through which it travels.
 
#66 ·
But it is less effective with a turbo, partly due to the pulses hitting the turbine and going back upstream. This is why I would like to see real world testing of a turbo manifold with anti reversion chambers on the primaries.
 
#71 ·
Engines on the street are rarely steady state. Expansion chambers help minimize transient times to full spool/power, especially after drastic changes, such as shifting. (Including even if you have the balls and drivetrain to FTS with 300+HP . . . )

For 95% of people in the Honda world, 3" straight is much more than enough from the turbo to the tip. It's simpler, so, keep it that way.

I will always argue that a street driven vehicle show absolutely have a catalytic converter, but that isn't a terribly popular opinion. *shrugs* Whatevs.
 
#74 ·
Engines on the street are rarely steady state. Expansion chambers help minimize transient times to full spool/power, especially after drastic changes, such as shifting. (Including even if you have the balls and drivetrain to FTS with 300+HP . . . )
You're right that the engine would not be at steady-state when the expansion chamber would come into play, but I'm still skeptical that - after the turbine - it would be of much help for transient response. I guess if you're trying to win a race, it might give you a slight edge.

We are almost always talking about ID, not OD.
Huh? Exhaust systems are made from tubing.
 
#75 ·
Inside diameter of the resonator the dude above was talking about.

Racing? no. Street driven vehicle trying to spool a turbo that is likely oversized for the engine in order to get flat torque to carry from 3500-8500RPM. I'd rather fight with getting a larger turbo to spool than have everything fall flat up top.
 
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