Is bigger better? (downpipes)
05-07-2008, 03:29 AM
#1
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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
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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05-07-2008, 06:09 AM
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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.
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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05-07-2008, 08:57 AM
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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.
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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05-07-2008, 11:05 AM
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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.
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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05-07-2008, 11:14 AM
#6
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.
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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05-07-2008, 03:45 PM
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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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05-08-2008, 11:47 AM
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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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05-08-2008, 08:29 PM
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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.
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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