ZEE'S IS-F Exhausts
09-20-09, 10:07 AM
#16
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Backpressure will increase not decrease. That's why bigger pipes don't necessarily perform better. Atmospheric pressure increases at the exhaust port with larger tubing - it does not decrease as so many would like to believe - and that's what causes the power loss. If it truly did reduce "backpressure," it would be better period.
I'll be happy to discuss the physics of this, but the reality is most guys don't understand beans about pressure, volume, and exhaust flows; including many who claim to be tuners.
I'll be happy to discuss the physics of this, but the reality is most guys don't understand beans about pressure, volume, and exhaust flows; including many who claim to be tuners.
Wouldn't the amount of pressure decrease as the amount of volume is increased? The way im looking at it is that if the exhaust has the ability to flow better, less pressure is going to be evident.
09-20-09, 10:41 AM
#17
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Backpressure will increase not decrease. That's why bigger pipes don't necessarily perform better. Atmospheric pressure increases at the exhaust port with larger tubing - it does not decrease as so many would like to believe - and that's what causes the power loss. If it truly did reduce "backpressure," it would be better period.....
Exhaust pipes that are increased in size passed the point of valuable performance, will gain you nothing. .
Especially on an NA motor...
But I am sure you know what I meant.
Joe Z
09-20-09, 02:02 PM
#18
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The engine can breathe only soo much... I think once you hit a certain size it doesnt matter anymore because only soo much exhaust will be flowing through the pipes..
Once you get a turbo or s/c I'm sure it will help more because more exhaust will need to be flowing...
09-20-09, 02:50 PM
#19
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So when you want to move a gas from one place to another, you have to set conditions conducive to movement. All gases move from high pressure areas to low pressure areas and to move a gas, you have to create those conditions.
Exhaust systems are designed to create those conditions. A couple of fundamentals - any tube containing air at atmospheric pressure has 14.7 psia along it's length, but the total pressure it presents to movement has to include the cross-sectional area of the tube. So a tube with a 2" diameter presents pi*r^2*14.7 psia of pressure, or 46.2 pounds of total pressure resisting any flow attempting to displace the air. If you increase the size of the tube to 3", the resistance increases to 103.9 pounds.
What this means is, the "backpressure" is not decreased with a larger tube, it actually INCREASES because the area of the tube is larger and there is more force to overcome before the air will move down the tube. So just arbitrarily sticking a larger tube on an exhaust port will very likely decrease performance at lower rpm and smaller throttle openings because the pressure differential between the cylinder at blowdown and the exhaust is smaller and more of the energy in the exhaust will be used to push out the spent gases rather than continue pushing the piston down. Conversely, a smaller tube will have less force to overcome and will perform better at lower rpm and smaller throttle openings (also it will be much more thermally efficient, but that's a different discussion) while the smaller tube will not support the same maximum flow the larger one will.
The key to all of this is to decide where you want to make power. John Lingenfelter published a book on the small block Chevy engines addressing this issue right out of the gate because so many junior tuners don't understand the whole tuning thing is a big trade-off. The first thing you have to decide is where in the rpm range you want to make power. Then you tune for best torque over the range. Exhaust tuning is just one of the things you can do to move best torque in the rpm range.
There's a whole lot more to this, but what I've covered here is the essentials for why saying a bigger exhaust reduces backpressure is just plain wrong.
09-21-09, 01:03 PM
#25
Former Vendor
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Looking only at the ratio of power to price in regards to an exhaust would be akin to shopping for a car based on power to price. The Shelby GT500 makes more power for less money than the IS-F. Does that mean that it's better?
Materials, build quality and exclusivity matter just as much when choosing an exhaust as a car. This exhaust is not for everyone, but I think it fits the ethos of the IS-F quite well.
Saying larger piping increases back pressure is not a totally correct statement just like saying larger piping reduces back pressure is not a totally correct statement. Without getting into an in depth discussion, blow through a big straw and a small straw and you'll get a very real demonstration of flow and back pressure on a smaller scale.
ZEE's exhaust is sized appropriately for the IS-F and I think the dyno charts illustrate that well, with gains throughout the rev range.
-Patrick
Materials, build quality and exclusivity matter just as much when choosing an exhaust as a car. This exhaust is not for everyone, but I think it fits the ethos of the IS-F quite well.
Saying larger piping increases back pressure is not a totally correct statement just like saying larger piping reduces back pressure is not a totally correct statement. Without getting into an in depth discussion, blow through a big straw and a small straw and you'll get a very real demonstration of flow and back pressure on a smaller scale.
ZEE's exhaust is sized appropriately for the IS-F and I think the dyno charts illustrate that well, with gains throughout the rev range.
-Patrick
09-21-09, 11:17 PM
#26
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What I see on the dyno chart is an exhaust without cats out-performing an exhaust with cats. Tube sizing doesn't move dyno results the way we're seeing it here. What they've achieved is similar results to a lot of exhausts we've already seen.
They've done good work, but attributing the results to sizing would be like putting a pea in the large straw and leaving it out of the small straw and saying the small straw is actually better because it's smaller when it's better because it doesn't have a pea stuffed in it.
They've done good work, but attributing the results to sizing would be like putting a pea in the large straw and leaving it out of the small straw and saying the small straw is actually better because it's smaller when it's better because it doesn't have a pea stuffed in it.
09-22-09, 01:51 PM
#27
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Wow, it looks like someone else understands the notion of pulse flow and charge velocity.
If it were possible to "freeze" the exhaust system at a moment in time and measure the pressure density all along its length from the exhaust valves back to the tail pipe, you'd find areas of high pressure and low pressure. The idea behind exhaust design is to land the high pressure pulses right behind the low pressure pulses. We want lots of velocity when the exhaust valve is open, especially at that critical point where the intake and exhaust valve are both open (overlap) to encourage cylinder scavenging.
In smog controlled cars, that has to be carefully done so the cylinder is clean, but excessive amounts of unburned HC don't come out the tail pipe.
In the old days, the idea was to pump as absolutely as much air-fuel through the engine as possible.
Lots of tradeoffs, lots of clever engineering, flow dynamics and differential equations behind it.
If it were possible to "freeze" the exhaust system at a moment in time and measure the pressure density all along its length from the exhaust valves back to the tail pipe, you'd find areas of high pressure and low pressure. The idea behind exhaust design is to land the high pressure pulses right behind the low pressure pulses. We want lots of velocity when the exhaust valve is open, especially at that critical point where the intake and exhaust valve are both open (overlap) to encourage cylinder scavenging.
In smog controlled cars, that has to be carefully done so the cylinder is clean, but excessive amounts of unburned HC don't come out the tail pipe.
In the old days, the idea was to pump as absolutely as much air-fuel through the engine as possible.
Lots of tradeoffs, lots of clever engineering, flow dynamics and differential equations behind it.
09-22-09, 04:30 PM
#28
Former Vendor
Thread Starter
They've done good work, but attributing the results to sizing would be like putting a pea in the large straw and leaving it out of the small straw and saying the small straw is actually better because it's smaller when it's better because it doesn't have a pea stuffed in it.
-Patrick
09-23-09, 01:02 PM
#29
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This is a true statement and I can see how my comments may have been interpreted. I simply meant to state that the dyno results indicate that the ZEE's exhaust isn't sized improperly as some have seemed to suggest. It's obviously not creating undue backpressure in any area of the rev range.
-Patrick
-Patrick
I just re-read the whole thread to make sure.
Joe Z
