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Old 11-06-2001, 02:04 PM   #1
Dan_E
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Question Larger headers (Benefit or Drawback)?

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Most of the headers use larger diameter pipe from the head to the collector. While this is good on a NA motor, It is not "desirable" on a turbo motor, especially ours, with the turbo sitting 2 miles downstream from the exhaust valve. The velocity of the gasses slow down in the larger dia pipe, which in turns causes more lag because it can not spool the turbo up quick enough.
Posted October 2000 by Andre Vandenberg

With the past year of R&D, have these ideals changed at all? Would a turbo setup on the EJ25 benefit from an aftermarket header (larger piping) at all? Is there a happy medium? Or is it just best to stick with the stock headers. It just seems to me that better flow is better! ...Then again, too much flow can be a problem?!?!

So with that said, where do we stand today?
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Old 11-06-2001, 02:37 PM   #2
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larger piping is OK, to a extent... You don't want the piping to be TO big, it WILL rob power and increase turbo spool time, the optimal setup would be an equal length header about as large as the up-pipe (2.25" in my case). Apex'i says a gradual increase helps increase velocity (increase as if the pipe were 2" and expanded to 2.25" a few inches downstream.)

Ahead of the turbo, you want velocity, behind the turbo, you want the gasses to get the hell outta the way.
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Old 11-06-2001, 02:53 PM   #3
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Bigger will increase flow and decrease velocity. To an extent. I will bet you, come spring time my turbo spools faster than yours Joel....borla headers take the turbo litmus test.
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Old 11-06-2001, 03:18 PM   #4
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Shawn, bigger will increase flow and decrease velocity, period. Ahead of the turbo you want higher velocity. Even tho a say 2.5" header will flow more gas, it won't be going as fast as say a 2.25", especially with the turbine in the way. The gas will have more room to expand out and fill the header, and won't spin the turbine blade as fast. With a turbo, you want a good compromise between the two.

Like I said, velocity is key ahead of the turbo, high flow is key after it.

Come spring you can bet all you want, but just a header doesn't dictate how fast the turbo spools, piping behind the turbo, the turbo itself, boost control, lubrication, the muffler, even the engine and it's base power output all have a say in turbo spool.
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Old 11-06-2001, 03:22 PM   #5
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equal length header would help
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Old 11-06-2001, 06:32 PM   #6
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What about heat? Will a larger diameter header dissipate heat better by moving the gasses away from the engine faster?
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Old 11-06-2001, 06:41 PM   #7
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Quote:
Like I said, velocity is key ahead of the turbo, high flow is key after it.
Actually, velocity is key before and after. You want the gasses out of the exhaust as fast as possible, so they don't cool down, slow down, and give backpressure. Yes, this means high flow, but so does high velocity before the turbo.


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Old 11-06-2001, 06:42 PM   #8
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Quote:
Originally posted by Dan_E
What about heat? Will a larger diameter header dissipate heat better by moving the gasses away from the engine faster?
Yes it will dissapate more heat. No it will not dissapate heat because it's moving faster, it'll dissapate more because it's moving slower.

Shawn - The Borlas aren't the best idea for a turbo system, strictly based upon the fact that they're nowhere near equal length.
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Old 11-06-2001, 10:22 PM   #9
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What about exhaust gas pulse? Wont bigger tubes help to smooth out the flow and aid in a happy life for the turbo?
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Old 11-06-2001, 10:25 PM   #10
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The only way to "smooth" out the pulse in this case is to slow part of it down. Slowing down the exhaust leads to poor performance.


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Old 11-07-2001, 01:03 PM   #11
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Should I go back to the crappy stock manifolds then? I figures the header would help spool up and top end by moving the exhaust gasses quicker.....
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Old 11-07-2001, 01:08 PM   #12
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Larger = slower. They remove gasses from the engine faster, but at a slower velocity. Velocity is what spins the turbo.
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Old 11-07-2001, 01:34 PM   #13
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Velocity is what spins the turbo

Really? I though it was a little squirrel that ran around in a cage to try to get away from the heat?
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Old 11-07-2001, 01:37 PM   #14
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Quote:
Originally posted by STiShawn
Velocity is what spins the turbo

Really? I though it was a little squirrel that ran around in a cage to try to get away from the heat?
[EDIT]

Last edited by Jewbaru; 11-07-2001 at 01:48 PM.
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Old 11-07-2001, 02:32 PM   #15
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You want your exhaust gas to be "as hot as possible" at the turbo because heat is energy, and the more energy the exhaust gas has the better it can turn the turbine blades. Larger pipes give a larger volume, allowing the gas to expand more before the turbo, and expansion consumes energy from the gas.

A gradual increase in header size is also good as you collect the gasses. So at each outlet port you'd want a small pipe. After each collector you'd increase pipe size because now the header is accomodating gas from more cylinders.
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Old 11-07-2001, 03:39 PM   #16
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i dont think it's just heat and velocity that spin the turbo. it's pressure. To create pressure on one end of the turbo (intake) you have to add at least the same amount on the other (exhaust). And i'd think a larger diameter will be less pressure. More header pressure will rob some power from the engine... but will get the turbo to spin easier.
did that make any sense?
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Old 11-07-2001, 04:10 PM   #17
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Quote:
Originally posted by giulio
i dont think it's just heat and velocity that spin the turbo. it's pressure. To create pressure on one end of the turbo (intake) you have to add at least the same amount on the other (exhaust). And i'd think a larger diameter will be less pressure. More header pressure will rob some power from the engine... but will get the turbo to spin easier.
did that make any sense?
It's all the same thing. Hotter air = higher pressure, right?

And just remember the ideal gas equation: PV=nRT. So if you increase V, then P must decrease at a given nRT. So adding length or diameter to the header decreases the pressure by increasing the volume. With less pressure differential across the turbine you'll spool up slower.

But also remember that the more pressure in the exhaust header when the intake port opens will promote more exhaust to get sucked back into the cylinder before the exhaust valve closes. Unless your intake manifold is at a higher pressure than your exhaust, which I don't think is usually the case.
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Old 11-07-2001, 04:33 PM   #18
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Joel, you're right about velocity spinning the turbo, but what you're not thinking about is that a certain diameter pipe is only going to flow so much with the created pressure from the exhaust gasses exiting the engine.
YES - before the turbo velocity is important, but afterwards, it doesn't matter. There's a fine line between piping being too big and cutting down on velocity and being too small and creating too much backpressure. You can't just assume that YOUR setup or YOUR idea is the correct one b/c it verywell may not be.
Also, after experimenting with unequal and equal length headers on my 96 eclipse GSX, there is not much difference as long as the pulses between the cylinders do not interfere. Spool up was the same either way and if anything was a little QUICKER with the unequal length header.

Chad
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Old 11-07-2001, 04:39 PM   #19
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Chad, maybe you want to read a little closer before you directly respond to my post.

Quote:
Ahead of the turbo, you want velocity, behind the turbo, you want the gasses to get the hell outta the way.
I know there is a line between good velocity and too much backpressure and low velocity and very little backpressure. It's a VERY FINE LINE however.

Besides it's not a matter of ideas. It's simple, the expanding gasses spin the turbine, the faster they're moving, the sooner the turbo will spool. If you give them too much room to roam, they're going to waste energy filling in the volume of the header.
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Old 11-07-2001, 04:49 PM   #20
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Quote:
There's a fine line between piping being too big and cutting down on velocity and being too small and creating too much backpressure.
Any backpressure is too much, please drive through.


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Old 11-07-2001, 04:58 PM   #21
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Quote:
It's simple, the expanding gasses spin the turbine, the faster they're moving, the sooner the turbo will spool. If you give them too much room to roam, they're going to waste energy filling in the volume of the header.
EXACTLY!
So you're telling me that you know the exact amount of volume to create optimum velocity while maintaining low to non-existant backpressure? I think not. Too small diameter piping can also "choke" the whole system. Velocity is not a problem as long as things as flowing well on the intake side unless you're planning on running a 5inch exhaust system on a 4 banger.

CHad
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Old 11-07-2001, 08:21 PM   #22
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You guys rock... Talk about information overload! THATS what I like too see! Just like flow, too much is better than not enough
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Old 11-07-2001, 10:48 PM   #23
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Quote:
Originally posted by RacerinLSL

EXACTLY!
So you're telling me that you know the exact amount of volume to create optimum velocity while maintaining low to non-existant backpressure? I think not. Too small diameter piping can also "choke" the whole system. Velocity is not a problem as long as things as flowing well on the intake side unless you're planning on running a 5inch exhaust system on a 4 banger.

CHad
ok, what the hell are you talking about Chad? It sounds like you're arguing for no reason, and you aren't helping.

I'm not going to bother, I made my point already.

The question "is it still 'bad' practice to go with large headers on a turbo car?" The answer, yes, to a certain extent. Too large = bad.

:monkey:

Oh, Ben
Quote:
Any backpressure is too much, please drive through.
Don't belive everything SCC tells you.
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Old 11-07-2001, 11:13 PM   #24
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Thumbs down

man, I'm in a crappy mood...

Anyway, Chad, the builders of the turbokits for our cars picked a diameter for the up-pipe with backpressure, and velocity in mind (in the form of spool time). We want our headers to be as close as possible to the diameter of the up pipe to help keep a smooth flow. I don't know the best size for the up pipe, never said I did, but having headers that are the same diameter will yeild the best velocity/backpressure combination.

Small headers to larger up-pipe will decrease velocity from the gasses expanding outwards, but according to APEX'i that helps scavenging. Large headers to small up-pipe will cause a molecule jam.

BTW, sorry if I'm sounding like a jerk, I'm just really pissed off right now (girl problems)
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Old 11-07-2001, 11:38 PM   #25
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OK, if you run 7psi of boost your roughly putting 50% the air into the engine, right? So the stock header can't be the best solution for this set up [I know no one is saying there the best] so after market headers should help in gaining maximum HP shouldn't they? They will slow down turbo spool up a little, but wont you gain top end power in the trade off? I was under the impression that are N/A headers do make an [for lack of a better term] off beat exhaust pulse. And making the header runners bigger will slow down the gas velocity and help a little in smoothing it out. Now I don't know any of this for shore and equal length turbo headers would do a much better job, but it should help right?
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