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Old 05-07-2008, 04:29 AM   #1
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Default Is bigger better? (downpipes)

I have a greddy kit, but this question really pertains to any turbo. I've got a lame 2" stock downpipe mated to greddy's lame 2" half-downpipe. Needs to go. But if it's going to go and something is going to replace it, I want to do it right. Whatever right is. I want power, and the absolute fastest spool I can get. I do not want to lose torque. Mid-range is more than an important factor it is a necessity. Competing in autocross and road race.
Most of the pre-fab downpipes out there are 2.5" diameter. I've seen a few guys who've gotten someone to fab up a 3"er. I'm sure the less restriction, the faster flow = the faster spool. yes?
On non-turbo cars, bigger is not better. Backpressure must be maintained - I ran an open header on my car once for grins. Ssssllllow.
For a turbo car I understand less restriction is good for more outright power. But do you still lose that mid-range by reducing backpressure?

I am working on some sort of race-day exhaust dump. Either something stupid complicated like an electronically controlled cut-off switch directly after the DP before the cat, or a simple turn-down pipe on a flange to bolt to the dp and remove the exhaust at the event.

Let's discuss what the best option for a competition car that needs low end grunt would be. Should I have something fabbed up in 3"?

Oh, and if I have a different exhaust setup for race days, I will have a separate tune optimized for that to switch to with MS for that setup.
-Ryan
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Old 05-07-2008, 07:09 AM   #2
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First off, no you dont need any backpressure.. you could go 4" everything if you could fit it. The gains would be slight, as the difference between 2.5" and 3" isnt huge- id guess diminishing returns would make a 4" dp/exhaust barely an upgrade from a 3".. if even. The reason you can go as big as you want post turbo is because the turbine itself is all the backpressure the motor needs.

A full 3" downpipe/exhaust system would be optimal.

But.. since you have the Greddy, there is practically no possible way to get a 3" downpipe to fit, because of firewall/brake cylinder/heater hoses. Maybe its possible, but I've never seen anyone with such a downpipe on a Greddy kit.

If i were you i would just buy a 2.5" downpipe- Turbotony, RM, or Begi and an Enthuza 3" exhaust and call it a day. I don't think you will get enough low end from a 3" downpipe for it to be worth the costs anyways.
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Old 05-07-2008, 09:57 AM   #3
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the key is flow; high pressure travels to low pressure. hotter gases travel faster. larger pipes make it easier to flow gases, but also lower temps and lower velocity. backpressure is evil.


now with that said. If you bolt a 3" DP to a turbo with a 2" opening, you may see adverse effects. The gases will cool instantly as they expand into a huge void. This may actually create backpressure as the turbo has to push against the slow gas filling up the void.

like wise, smaller pipes and restrictions such as the cat make it harder for gases to move out of the exhaust quickly. The velocity and EGTs may be greater, but what happens is as they travel they expand and slow and backup. Again, your turbo has to waste energy pushing against the slowed gases. Notice a theme? Backpressure is evil.

A good 2.5" DP from turbotony or alike would be a good choice. The bell-mouth style outlet has plenty of room to mix and flow into the 2.5" pipe without being excessive.
This will keep the exhaust moving out of the turbo with a matching port size, the back pressure should drop more, and since the area behind the turbo wants to keep moving rapidly, it's a great way to increase turbo performance. Gradually stepping up from here with slightly decrease velocity, however, it will help increase the flow. In all honesty matching a 2.5" catback here wouldn't be detrimental vs. a 3", I'm not sure one would really notice a difference, however the cross-sectional flow area is something like 70% larger....so the key with the exhaust is zero restrictions.
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Old 05-07-2008, 11:17 AM   #4
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I'm running a 6" downpipe straight up out of my hood, it's gonna be awesome.

Not really, my new downpipe will start with 2.25" tubing and expand via a cone transition to 3" for the rest of the exhaust.
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Old 05-07-2008, 12:05 PM   #5
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Quote:
Originally Posted by ThePass View Post
But do you still lose that mid-range by reducing backpressure?
This "backpressure = torque" thing really disturbs me...

All else being equal, increasing pipe diameter decreases exhaust velocity through the pipe. Same volume of gas per unit time, so increasing pipe volume means the exhaust isn't being squirted quite as hard through the system, thus it slows down.

Now, what this does is do decrease the scavenging effect of the exhaust system somewhat. Racers learned a long time ago that for maximum power, keeping the exhaust velocity high increases the cylinder filling efficiency slightly by causing the tail end of the exhaust pulse to give a slight "tug" on the cylinder prior to the exhaust valve closing.

Now, it's important to note the distinction here between a restrictive exhaust and one that keeps velocity up. A nice mandrel-bent system with an open muffler and reasonable pipe diameters is going to optimal in terms of scavenging efficiency. On the other hand, one with too-small pipe, crush bends, a shitty muffler, etc., will produce loads of backpressure without doing squat for scavenging.

Throwing a turbo into the mix screws everything up. The turbo itself is a pretty horrible restriction, to the point that attempting to use exhaust velocity downstream for the purpose of scavenging is thought to be pretty moot. Corky's stated view on the matter (and it makes sense to me) is that for a turbo car, the most optimum exhaust system after the turbo is none at all. IOW- the turbine wheel has already messed up the exhaust stream, don't add insult to injury with anything less than a perfectly free-flowing pipe behind it.
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Old 05-07-2008, 12:14 PM   #6
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Personally... I go as big as is doable immediately. I have had 3" directly off the turbine housing for my last two setups.

Less pressure after the wheel means less pressure fighting against the wheel.

As Joe said... the optimum turbo exhaust is none at all.

I find 3" is the ideal compromise for this... in weight/cost/fitment.
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Old 05-07-2008, 03:49 PM   #7
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3" is probably optimal. anything can be done as far as making it fit. i've made some pretty snakey downpipes for different cars. you are only limited by the imagination of your fab guy.
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Old 05-07-2008, 04:45 PM   #8
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What type of shop can do custom downpipes/does that normally cost an arm and a leg? I know my local muffler/exhaust shop is nowhere near capable of this sort of job...
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Old 05-07-2008, 05:24 PM   #9
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Any fabricator can make one, muffler shops usually dont have mandrel benders.
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Old 05-07-2008, 06:11 PM   #10
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Perfect timing, I'm gonna get a 3" downpipe/mid pipe etc fabbed up by a friend for COST soon
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Old 05-07-2008, 07:11 PM   #11
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^ I hate you.
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Old 05-08-2008, 04:14 AM   #12
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3", all things equal, would obviously be better, but I'll take a straight shot 2.5 dp over a severely kinked 3" dp any day. It's not just about volume, but also velocity.
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Old 05-08-2008, 10:28 AM   #13
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2.5 to 2.75




cat 2.75 to 3.0





catback 3.0 to the tip
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Old 05-08-2008, 12:47 PM   #14
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Quote:
Originally Posted by Zabac View Post
Any fabricator can make one, muffler shops usually dont have mandrel benders.
True, but good muffler shops (non-chain stores, owned by a fellow who has at least one racecar parked out back) will usually have a huge variety of mandrel-bends on hand, including ultra-tight-radius "donut" bends (basically a complete, seamless 360 torus) that can be cut to damn near any angle imaginable.
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Old 05-08-2008, 12:53 PM   #15
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Emailed Turbo Tony, no reply yet. Considering just gambling with buying the RM one.
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Old 05-08-2008, 08:53 PM   #16
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Quote:
Originally Posted by Braineack View Post


2.5 to 2.75




cat 2.75 to 3.0





catback 3.0 to the tip
That set up is unprotected sex right there!
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Old 05-08-2008, 09:18 PM   #17
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Quote:
Originally Posted by warhead78 View Post
3", all things equal, would obviously be better, but I'll take a straight shot 2.5 dp over a severely kinked 3" dp any day. It's not just about volume, but also velocity.
read through this thread again... velocity is not going to help in an exhaust system with a turbo at the front of it.
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Old 05-08-2008, 09:29 PM   #18
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it sure it, but it's the gradual decrease in velocity that's key


think of the opposite of intake runners...they accelerate the gases through a smaller section area. With the exhaust, it's the best to follow the opposite, keeping velocity and EGTs up so the turbine has less work to do, but only for a short section, then increase the cross-section area to increase the flow so when the gases cool they have room to expand and slow and be scavenged out into a low pressure area.
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Old 05-08-2008, 09:38 PM   #19
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This is exactly what corky told me on the phone a couple days ago. You don't want the gas coming right out into a huge hole and cooling down. Maybe if you have no exhaust that's okay, but not if it's still got same way to travel. Gradual increase to 3" is best.
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Old 05-08-2008, 10:54 PM   #20
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I wonder how loud that Magnaflow box is....
What about those mid section things that allow exhaust to 'vent' straight out at a certain PSI level ? Anyone try them ?
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