Exhaust Size vs HP & TQ
How straight the exhaust is and how good the mufflers are will make a big difference in pressure drop. I'd have to grab a book again to verify, but a single 3" diameter, 90 degree, 3" radius bend (typical of what's used in exhaust work) has the same pressure drop as like 8' of straight 3" tube.
A louvered 3" resonator will KILL flow. Even a perforated one won't be good for it.
If you want a good 3" exhaust that can make 600+ HP, you have to keep bends to an absolute minimum. Your welded joints need to be well aligned. Your flanges need to promote good alignment (v-bands for example). Last but not least, the muffler needs to flow well.
A louvered 3" resonator will KILL flow. Even a perforated one won't be good for it.
If you want a good 3" exhaust that can make 600+ HP, you have to keep bends to an absolute minimum. Your welded joints need to be well aligned. Your flanges need to promote good alignment (v-bands for example). Last but not least, the muffler needs to flow well.
Unless you have a dyno chart to prove this... that's total and absolute BS. That's not how a turbine operates and you know it.
You may have gained much more up top...making your car feel like it lost midrange, when in fact it remained the same. boost for boost you'll make more power everywhere... PERIOD
You may have gained much more up top...making your car feel like it lost midrange, when in fact it remained the same. boost for boost you'll make more power everywhere... PERIOD
there are people going very fast on 2.5 inch exhaust. there was guy over on tuners maxing out his 16g at 455whp on e-85. with 475tq if I remember correctly. there was a turbo honda many years ago in one of the mags running mid tens with 550whp. he had a 2.5 inch exhaust.
the effect as I said earlier is hard to capture as the tubing size has to be exact for the setup. I am completely aware of benefits of getting back pressure to a minimum. (the bigger is better thing) But what you need to understand is to grab this concept is in order to see the benefits of scavenging in a turbo exhaust it has to be achieved within the window of except-able exhaust back pressure.
If i co over 3" it will be 4" as i dont see why not other than noise, clearnce isnt a big issue as im at stock ride height. I was thinking of the vibrant 4"inlet/outlet part # 1091, Would it be a good choice? Or should i go with there race series muffler that will provide almost no sound reduction or just straight pipe no muffler?
A lot of questions like this can be answered by picking up and reading a book on fluid dynamics. Not only do you gain the knowledge for life, but you don't have to go on other's people opinions, that may or may not be true.
Just as 03whitegsr mentioned, bends come into play. Also, transition angles of smaller to larger diameter pipe (too big of an angle hurts flow), muffler and or resonator design, etc.
For example, just because a muffler is a 3" straight through design doesn't mean it's going to flow like a 3" pipe. The perforations or muffler design create a turbulence at the boundary, which impedes a smooth airflow, increasing pressure in the upstream flow. The same is true with bends and why they act like a longer section of pipe.
The point is there are several factors in creating a perfect exhaust pipe, but sometimes we are limited due to other constraints, such as sound levels, space restrictions, costs, etc.
Just as 03whitegsr mentioned, bends come into play. Also, transition angles of smaller to larger diameter pipe (too big of an angle hurts flow), muffler and or resonator design, etc.
For example, just because a muffler is a 3" straight through design doesn't mean it's going to flow like a 3" pipe. The perforations or muffler design create a turbulence at the boundary, which impedes a smooth airflow, increasing pressure in the upstream flow. The same is true with bends and why they act like a longer section of pipe.
The point is there are several factors in creating a perfect exhaust pipe, but sometimes we are limited due to other constraints, such as sound levels, space restrictions, costs, etc.
I have 30 years turbo charging experience to your 2. It doesnt surprise me to get a response from you like this. I dont have any dyno proof to post. Nor do I care to try and convince you so take it however you like. It is very common knowledge that exhausts are a series of pulses. The N/A crowd takes serious note of this as they need to squeeze every once of HP from their exhaust system . They are full aware of the need to size the exhaust pipe to exact diameter to maximize the scavenging effect of pulses. Turbo charged F1 cars also use this plus some other unique things we dont at our level of play. I simply have seen the back to back results of this scavenging that you have not. it is that simple whether you care to believe it or not. If you think turbo charging eliminates exhaust pulses you only have to go start your car and put your hand over the exhaust tip. what do you feel?
there are people going very fast on 2.5 inch exhaust. there was guy over on tuners maxing out his 16g at 455whp on e-85. with 475tq if I remember correctly. there was a turbo honda many years ago in one of the mags running mid tens with 550whp. he had a 2.5 inch exhaust.
the effect as I said earlier is hard to capture as the tubing size has to be exact for the setup. I am completely aware of benefits of getting back pressure to a minimum. (the bigger is better thing) But what you need to understand is to grab this concept is in order to see the benefits of scavenging in a turbo exhaust it has to be achieved within the window of except-able exhaust back pressure.
there are people going very fast on 2.5 inch exhaust. there was guy over on tuners maxing out his 16g at 455whp on e-85. with 475tq if I remember correctly. there was a turbo honda many years ago in one of the mags running mid tens with 550whp. he had a 2.5 inch exhaust.
the effect as I said earlier is hard to capture as the tubing size has to be exact for the setup. I am completely aware of benefits of getting back pressure to a minimum. (the bigger is better thing) But what you need to understand is to grab this concept is in order to see the benefits of scavenging in a turbo exhaust it has to be achieved within the window of except-able exhaust back pressure.
Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.
FWIW, the most common engine modeling software packages used by OEM suppliers are all based around the idea of a turbocharger acting like a double nozzle system. What this means to everybody?
A LARGE amount of the pulse energy gets reflects back from the turbine from the change in cross-sectional area...SOME of it makes it through. YES, some scavenging can take place post turbo, but it's MUCH Less significant then in an N/A application.
However, the area ratio of that nozzle (model of the turbine) comes into play. The less of a reduction in area the turbo can be modeled as, the less it will reflect the pulses and the more likely scavengine post turbo can happen. This is where a lot of the technology mentioned in turbo F1 comes into play. But let's bring it back to our world, we don't make 1500HP on a 1.5L.
Compare this to let's say a GT42 with a big *** turbine and housing (low nozzle ratio) to a stock 16G (relatively high nozzle ratio). IMO, the only setup you'd be interested in increasing mid-range torque would be the 16G. Let's face it, no mater how fast you get that 42 to spool, you're still more interested in all out HP, other wise you'd be using a different turbo.
Now, proper scavenging will have a much more pronounced effect on the GT42 because it offers much less resistance to the exhaust pulses. This can be seen VERY clearly on the dyno too. I've personally seen a 2.5L subaru make over 630 WHP on 18 PSI, and it souned N/A out the exhaust. I'm sure a high level of scavenging was taking place as the intake, cam package, and exhaust manifold were all setup to promote exceptional cylinder fill in the 5500-7500 RPM range. The 16g on the other hand just posses as a large reflector to those pulses. YES some portion of the pulse makes it through and can be optimized. Yes, it's possible to see increase in mid range (I've seen it honestly). But it's no where near the gains seen in N/A applications and often comes at the expense of top end power.
Just for the record, this was probably 8 years ago and I could be recalling specifics a little incorectly. I remember thinking though, "who cares about the 15 ft-lbs below 4000 RPM it made, god damn subaru guys." I use to talk to a couple of the guys at Cobb tuning. They were developing two exhaust systems for the WRX. A 2.5" and a 3". Sure enough, the 2.5" exhuast showed mid-range gains over the 3". I believe this comparison even got printed in like sport compact car. The 3" picked up a bunch of top end over the 2.5" though on a sub 300HP car.
A LARGE amount of the pulse energy gets reflects back from the turbine from the change in cross-sectional area...SOME of it makes it through. YES, some scavenging can take place post turbo, but it's MUCH Less significant then in an N/A application.
However, the area ratio of that nozzle (model of the turbine) comes into play. The less of a reduction in area the turbo can be modeled as, the less it will reflect the pulses and the more likely scavengine post turbo can happen. This is where a lot of the technology mentioned in turbo F1 comes into play. But let's bring it back to our world, we don't make 1500HP on a 1.5L.
Compare this to let's say a GT42 with a big *** turbine and housing (low nozzle ratio) to a stock 16G (relatively high nozzle ratio). IMO, the only setup you'd be interested in increasing mid-range torque would be the 16G. Let's face it, no mater how fast you get that 42 to spool, you're still more interested in all out HP, other wise you'd be using a different turbo.
Now, proper scavenging will have a much more pronounced effect on the GT42 because it offers much less resistance to the exhaust pulses. This can be seen VERY clearly on the dyno too. I've personally seen a 2.5L subaru make over 630 WHP on 18 PSI, and it souned N/A out the exhaust. I'm sure a high level of scavenging was taking place as the intake, cam package, and exhaust manifold were all setup to promote exceptional cylinder fill in the 5500-7500 RPM range. The 16g on the other hand just posses as a large reflector to those pulses. YES some portion of the pulse makes it through and can be optimized. Yes, it's possible to see increase in mid range (I've seen it honestly). But it's no where near the gains seen in N/A applications and often comes at the expense of top end power.
Just for the record, this was probably 8 years ago and I could be recalling specifics a little incorectly. I remember thinking though, "who cares about the 15 ft-lbs below 4000 RPM it made, god damn subaru guys." I use to talk to a couple of the guys at Cobb tuning. They were developing two exhaust systems for the WRX. A 2.5" and a 3". Sure enough, the 2.5" exhuast showed mid-range gains over the 3". I believe this comparison even got printed in like sport compact car. The 3" picked up a bunch of top end over the 2.5" though on a sub 300HP car.
Last edited by 03whitegsr; Dec 16, 2009 at 12:43 PM.
A lot of questions like this can be answered by picking up and reading a book on fluid dynamics. Not only do you gain the knowledge for life, but you don't have to go on other's people opinions, that may or may not be true.
Just as 03whitegsr mentioned, bends come into play. Also, transition angles of smaller to larger diameter pipe (too big of an angle hurts flow), muffler and or resonator design, etc.
For example, just because a muffler is a 3" straight through design doesn't mean it's going to flow like a 3" pipe. The perforations or muffler design create a turbulence at the boundary, which impedes a smooth airflow, increasing pressure in the upstream flow. The same is true with bends and why they act like a longer section of pipe.
The point is there are several factors in creating a perfect exhaust pipe, but sometimes we are limited due to other constraints, such as sound levels, space restrictions, costs, etc.
Just as 03whitegsr mentioned, bends come into play. Also, transition angles of smaller to larger diameter pipe (too big of an angle hurts flow), muffler and or resonator design, etc.
For example, just because a muffler is a 3" straight through design doesn't mean it's going to flow like a 3" pipe. The perforations or muffler design create a turbulence at the boundary, which impedes a smooth airflow, increasing pressure in the upstream flow. The same is true with bends and why they act like a longer section of pipe.
The point is there are several factors in creating a perfect exhaust pipe, but sometimes we are limited due to other constraints, such as sound levels, space restrictions, costs, etc.
This topic has intrigued me and made me recall some of my fluid dynamics lessons in college.
I believe the "best" exhaust for our cars is one that tapers.
The idea is to keep the exhaust air hot because the hotter the air is the faster the exhaust travels. In addition, hotter air is less dense and therefore moves faster. Smaller exhausts move air very quickly at the cost of increased back pressure becasue it condenses the volume. Since bends, resonators, mufflers, and the inside of the pipes themselves all slow the flow considerably the point of a tapered exhaust comes clear to me.
If the turbo exhaust outlet is say 2.5" and we immediatly jump to a 3" or even a 4" downpipe we have a huge pressure drop that occurs very quickly. This pressure drop also rapidly decreases laminar flow, increases turbulence, and reduces air speed.
By tapering the exhaust in a manner that slowly and smoothly goes from 2.5" to 3" and heck even on to 4" we eliminate the turbulence and keep laminar flow as smooth as possible. Taper out too fast and you lose pressure, heat, and speed; too slow and you build back pressure which increase turbulence and therefore decreases laminar flow.
Another point is that as the exhaust travels through the pipe it loses heat, pressure, and speed. By having the exhaust taper we allow the frictional forces the pipe causes on the airflow to be reduced therefore helping keep the speed and volume of the exhaust flow higher.
SO....to taper the exhaust in a way that eliminates turbulence, reduces back pressure and allows constantly decreasing volumetric laminar flow is the ideal situation. Of course make it as straight as possible and without resonators to enhance laminar flow.
As for the issue of noise....Simply wrapping the downpipe and mid pipe with header wrap and using the high temp silicon spray will greatly reduce the sound, especially in the cabin. In addition, the wrap helps to hold the heat in the exhaust and therefore assists in keeping velocity higher.
In all, there is no exactly perfect exhaust size. You need to build the exhaust to suit your specific car and it's power. Exhaust building, as in most things we do with our cars, is all about compromise. What you gain in one area, you will lose in another.
Paul
I believe the "best" exhaust for our cars is one that tapers.
The idea is to keep the exhaust air hot because the hotter the air is the faster the exhaust travels. In addition, hotter air is less dense and therefore moves faster. Smaller exhausts move air very quickly at the cost of increased back pressure becasue it condenses the volume. Since bends, resonators, mufflers, and the inside of the pipes themselves all slow the flow considerably the point of a tapered exhaust comes clear to me.
If the turbo exhaust outlet is say 2.5" and we immediatly jump to a 3" or even a 4" downpipe we have a huge pressure drop that occurs very quickly. This pressure drop also rapidly decreases laminar flow, increases turbulence, and reduces air speed.
By tapering the exhaust in a manner that slowly and smoothly goes from 2.5" to 3" and heck even on to 4" we eliminate the turbulence and keep laminar flow as smooth as possible. Taper out too fast and you lose pressure, heat, and speed; too slow and you build back pressure which increase turbulence and therefore decreases laminar flow.
Another point is that as the exhaust travels through the pipe it loses heat, pressure, and speed. By having the exhaust taper we allow the frictional forces the pipe causes on the airflow to be reduced therefore helping keep the speed and volume of the exhaust flow higher.
SO....to taper the exhaust in a way that eliminates turbulence, reduces back pressure and allows constantly decreasing volumetric laminar flow is the ideal situation. Of course make it as straight as possible and without resonators to enhance laminar flow.
As for the issue of noise....Simply wrapping the downpipe and mid pipe with header wrap and using the high temp silicon spray will greatly reduce the sound, especially in the cabin. In addition, the wrap helps to hold the heat in the exhaust and therefore assists in keeping velocity higher.
In all, there is no exactly perfect exhaust size. You need to build the exhaust to suit your specific car and it's power. Exhaust building, as in most things we do with our cars, is all about compromise. What you gain in one area, you will lose in another.
Paul
Here's the deal... call up a Honeywell Engineer, BorgWarner, or Holset Engineer and ask them. You're looking to maximize the difference in ethalpy across the turbine. Basically the larger the pressure and temperature differential across the turbine, the more energy you can harness... this is done by lowering the pressure and temperature post turbine. Physics always wins.
Last edited by R/TErnie; Dec 17, 2009 at 12:13 AM.