Exhaust Reversion, velocity, pressure Qs
01-07-2011, 12:43 PM
#1
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Exhaust Reversion, velocity, pressure Qs
I have a few questions here...
I have been doing some reading in Maximum Boost and according to Mr. Bell I don't need the 3in dp that I was thinking about running on my next setup (F22A1-350 wtq is the goal not sure what snail but leaning towards a 2.4 bisi cam). According to his chart I only need 2.25in exhaust? I can see how running a smaller exh. will increase velocity (which he states specifically on pg.132 that larger is not better kind of threw me for a loop) but how do I know the proper balance of what dia. pipe I should run as to size while not hurting velocity or "choking" the turbine up top? Would this apply to building a dump tube for a external WG as well?
Also mentioned in the book is exhaust reversion which I understand can really hurt power. On page 120 in the book it shows tabs mounted inside of the exh. manifold (right by the flange that bolts to the head) to help prevent the exhaust from flowing backwards into the cc. Is there enough level of merit to actually implement these tabs on a setup such as mine? How does a properly designed cam help this issue? (Ie more lift or less duration)
I have read that a lot people are replacing their valve springs and retainers when running higher levels of boost... Any insight as to why would be much appreciated. I do not plan on revving much past 7k on my setup because power drops off well before that.
This question is a bit of a shot in the dark but how much power can scavenged (or gained over say a log manifold) with a proper manifold that allows the exhaust pulses to hit the exhaust wheel at the proper time? Is there going to be much of a decrease in spool time if so? (Ie split housing on the hot side or properly built manifold)
And finally- how do you folks go about measuring your exhaust pressure to see if you have the proper pressure ratio vs. what is going into the intake and how do you know the proper psi you should have and when does too high of back pressure start having an effect on knock threshold?
I know answers will vary from setup to setup but any light (links to setups or reading) shed on any of these questions, I will greatly appreciate!
Thanks!
Blaze
I have been doing some reading in Maximum Boost and according to Mr. Bell I don't need the 3in dp that I was thinking about running on my next setup (F22A1-350 wtq is the goal not sure what snail but leaning towards a 2.4 bisi cam). According to his chart I only need 2.25in exhaust? I can see how running a smaller exh. will increase velocity (which he states specifically on pg.132 that larger is not better kind of threw me for a loop) but how do I know the proper balance of what dia. pipe I should run as to size while not hurting velocity or "choking" the turbine up top? Would this apply to building a dump tube for a external WG as well?
Also mentioned in the book is exhaust reversion which I understand can really hurt power. On page 120 in the book it shows tabs mounted inside of the exh. manifold (right by the flange that bolts to the head) to help prevent the exhaust from flowing backwards into the cc. Is there enough level of merit to actually implement these tabs on a setup such as mine? How does a properly designed cam help this issue? (Ie more lift or less duration)
I have read that a lot people are replacing their valve springs and retainers when running higher levels of boost... Any insight as to why would be much appreciated. I do not plan on revving much past 7k on my setup because power drops off well before that.
This question is a bit of a shot in the dark but how much power can scavenged (or gained over say a log manifold) with a proper manifold that allows the exhaust pulses to hit the exhaust wheel at the proper time? Is there going to be much of a decrease in spool time if so? (Ie split housing on the hot side or properly built manifold)
And finally- how do you folks go about measuring your exhaust pressure to see if you have the proper pressure ratio vs. what is going into the intake and how do you know the proper psi you should have and when does too high of back pressure start having an effect on knock threshold?
I know answers will vary from setup to setup but any light (links to setups or reading) shed on any of these questions, I will greatly appreciate!
Thanks!
Blaze
01-08-2011, 10:45 AM
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Re: Exhaust Reversion, velocity, pressure Qs
any turbo system is going to see better results with larger dp and exhaust system period. a turbo motor likes to breathe. Think of the internal combustion engine as an air pump which is basically what it is. In NA form it can only suck in and push out so much air and smaller diamater exhaust systems are adaquite in handling that air flow but with a turbo your not forcing a greater amoutn of air into the engine and alot more is going to come out of it. The freer flowing yoru exhaust system is the better.
That being sia dyou dont NEED to run a 3" to obtain 350hp but it just makes things easier and if its not a hastle to get it in place then why not. But what ever you decide 3" or 2.5" dp be sure the exhaust system is the same size or larger. IE 2.5" dp should mate to a 2.5" exhaust system or larger. Going back down in size will kill performance
That being sia dyou dont NEED to run a 3" to obtain 350hp but it just makes things easier and if its not a hastle to get it in place then why not. But what ever you decide 3" or 2.5" dp be sure the exhaust system is the same size or larger. IE 2.5" dp should mate to a 2.5" exhaust system or larger. Going back down in size will kill performance
01-08-2011, 12:25 PM
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Re: Exhaust Reversion, velocity, pressure Qs
Plus, that was not my exact question. My question was how do I know the proper balance of what dia. pipe I should run as to size while not hurting velocity or "choking" the turbine and causing high exhaust back pressure? What is the ideal exhaust velocity? Too fast means restriction and can too slow lead to reversion?
01-08-2011, 02:43 PM
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Re: Exhaust Reversion, velocity, pressure Qs
You are asking many good questions, but as you said the answers are going to vary for each specific setup. You can try some of the tips you mentioned, but they may be used to squeeze out the last few percent of efficiency or power and may not be absolutely necessary. Having consecutive steps in port size helps with reversion, perhaps the thickness of a fingernail around the port, with the entrance to the intake port of the head slightly larger than the exit of the intake manifold and the entry port of the exhaust manifold slightly larger than the exhaust exit of the cylinder head.
As long as the exhaust diameter is larger than the turbocharger exhaust wheel, flow should be adequate - 2.5" should be fine for 350hp. Good flowing exhaust is much more complicated than x diameter is good for x horsepower of course, but this kind of estimation can be used for simplicity. Since the exhaust cools as it travels, the tailpipe need not be as large as the manifold collector or turbine exit, but in reality most aftermarket exhausts are not designed this way.
As long as the exhaust diameter is larger than the turbocharger exhaust wheel, flow should be adequate - 2.5" should be fine for 350hp. Good flowing exhaust is much more complicated than x diameter is good for x horsepower of course, but this kind of estimation can be used for simplicity. Since the exhaust cools as it travels, the tailpipe need not be as large as the manifold collector or turbine exit, but in reality most aftermarket exhausts are not designed this way.
01-09-2011, 08:16 AM
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Re: Exhaust Reversion, velocity, pressure Qs
now i havent read maximum boost in like 6 or 7 years, but i think i remember somewhere in there hearing about the best exhaust is no exhaust, and the necessary backpressure etc is all achieved between the motor and turbine in the manifold...
think of it as this, with the smaller diameter piping and the same volume of air flowing through it, more air makes contact with the pipe itself. this will cause friction and possibly slow things down a tad. with the larger diameter piping, the air will be moving slower through it, but more air will be surrounded by other moving air rather than exhaust piping which will decrease friction. it works like a river or stream flowing, the most unobstructed flow is in the middle below the surface where the moving fluid is completely surrounded by other moving fluid..
think of it as this, with the smaller diameter piping and the same volume of air flowing through it, more air makes contact with the pipe itself. this will cause friction and possibly slow things down a tad. with the larger diameter piping, the air will be moving slower through it, but more air will be surrounded by other moving air rather than exhaust piping which will decrease friction. it works like a river or stream flowing, the most unobstructed flow is in the middle below the surface where the moving fluid is completely surrounded by other moving fluid..
01-09-2011, 10:20 AM
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Brrraaaap!
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Re: Exhaust Reversion, velocity, pressure Qs
I know that there has to be some exhaust with some resistance because when I had my old setup, I couldn't get the turbo to spool until I fab'd a down pipe. I could only hit 3 or 4 psi. Once I got a DP on that just reached under the oil pan I was hitting the wastegate psi no problem.
If there is an ideal downpipe size where you can achieve maximum velocity without choking flow how do you find out what it is?
Also, I was researching some more on why valve springs and retainers should be replaced when boosting a setup and this was all I have come up with so far- via bisimoto article from here: http://www.hondatuningmagazine.com/t...gineering.html
"This intake pressure also acts against the intake valve head, especially during compression and exhaust cycles, and hence aftermarket turbo and supercharged systems truly require valve spring and retainer upgrades for optimum performance. Since increased heat is also a by-product of forced induction, spring annealing (softening) can also occur, especially in factory springs that have seen over 50,000 or more miles of road service. "
What am I missing out on if I don't upgrade - Possible valve float if the springs soften? My last setup I was running 15psi from a T3/T4 57. trim 63. AR and the valve springs seemed to work fine... Although my next setup will be making a bit more power...
I appreciate the comments thus far- if anyone has any links or insight on the other questions I have concerning reversion cones and or exhaust pulses and power differences as well as proper exhaust system pressure.... please enlighten me!
If there is an ideal downpipe size where you can achieve maximum velocity without choking flow how do you find out what it is?
Also, I was researching some more on why valve springs and retainers should be replaced when boosting a setup and this was all I have come up with so far- via bisimoto article from here: http://www.hondatuningmagazine.com/t...gineering.html
"This intake pressure also acts against the intake valve head, especially during compression and exhaust cycles, and hence aftermarket turbo and supercharged systems truly require valve spring and retainer upgrades for optimum performance. Since increased heat is also a by-product of forced induction, spring annealing (softening) can also occur, especially in factory springs that have seen over 50,000 or more miles of road service. "
What am I missing out on if I don't upgrade - Possible valve float if the springs soften? My last setup I was running 15psi from a T3/T4 57. trim 63. AR and the valve springs seemed to work fine... Although my next setup will be making a bit more power...
I appreciate the comments thus far- if anyone has any links or insight on the other questions I have concerning reversion cones and or exhaust pulses and power differences as well as proper exhaust system pressure.... please enlighten me!
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01-09-2011, 02:34 PM
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Re: Exhaust Reversion, velocity, pressure Qs
I have a few questions here...
I have been doing some reading in Maximum Boost and according to Mr. Bell I don't need the 3in dp that I was thinking about running on my next setup (F22A1-350 wtq is the goal not sure what snail but leaning towards a 2.4 bisi cam). According to his chart I only need 2.25in exhaust? I can see how running a smaller exh. will increase velocity (which he states specifically on pg.132 that larger is not better kind of threw me for a loop) but how do I know the proper balance of what dia. pipe I should run as to size while not hurting velocity or "choking" the turbine up top? Would this apply to building a dump tube for a external WG as well?
With a turbine (turbo) ideally you want to drop exhaust manifold pressure across the turbine. So the larger the exhaust system (3" vs 2.5) you easier this will be accomplished. Now some people have packaging constraints that will not allow for a large 3" or similarly larger exhaust. so they have t make due with a smaller exhaust. all he is saying is the minimum exhaust size for a given hp number. Thinking of it this way is better imo.
Also mentioned in the book is exhaust reversion which I understand can really hurt power. On page 120 in the book it shows tabs mounted inside of the exh. manifold (right by the flange that bolts to the head) to help prevent the exhaust from flowing backwards into the cc. Is there enough level of merit to actually implement these tabs on a setup such as mine? How does a properly designed cam help this issue? (Ie more lift or less duration)
Exhaust reversion would only be an issue if your running a very poorly designed exhaust manifold (log manifold) with a large cam/cams and the increased overlap allows the exhaust to reenter the combustion chamber as you have excessive exhaust manifold pressures. this is not something one needs to worry to much about on a Honda engine if using a decent exhaust manifold at the least.
I have read that a lot people are replacing their valve springs and retainers when running higher levels of boost... Any insight as to why would be much appreciated. I do not plan on revving much past 7k on my setup because power drops off well before that.
On a Honda i have never heard the need to replace the valve springs due to high or even any boost level. It always revolves around increased rpm's and or increased load levels like endurance racing. these two cases would cause the valve springs to fail long before boost pressure would cause float. In the v8 world they seem to run into valve float when boost pressure is introduced. I can only assume that the manufactures designed a spring that is just up to the task of a stock engine to save money or wear on engine parts. And the increased pressures want to hang the valves open.
This question is a bit of a shot in the dark but how much power can scavenged (or gained over say a log manifold) with a proper manifold that allows the exhaust pulses to hit the exhaust wheel at the proper time? Is there going to be much of a decrease in spool time if so? (Ie split housing on the hot side or properly built manifold)
Look into the full-race dyno test done years ago. they claimed i be leave a 50 whp increase over a log when they switched to a ramhorn manifold. Now this is just one instance where they were pushing the edge of there manifolds flow rate and by switching to a higher/better flowing manifold they saw huge increases in power. this isn't always the case and should be looked at subjectively.
And finally- how do you folks go about measuring your exhaust pressure to see if you have the proper pressure ratio vs. what is going into the intake and how do you know the proper psi you should have and when does too high of back pressure start having an effect on knock threshold?
The easiest way i have seen is the weld a 1/8" npt egt bung in the exhaust manifold preferably close to the turbine for a more even reading. And then run either a steel or brass tube many inches away from the manifold. And then hook up a standard boost gauge to the line. now you will need a gauge that reads quite high maybe 60 psi or so. As you will see pressures as high as 2:1 or 3:1 intake manifold pressures. (ie: 20 psi = 40-60 psi exhaust)
I know answers will vary from setup to setup but any light (links to setups or reading) shed on any of these questions, I will greatly appreciate!
Thanks!
Blaze
I have been doing some reading in Maximum Boost and according to Mr. Bell I don't need the 3in dp that I was thinking about running on my next setup (F22A1-350 wtq is the goal not sure what snail but leaning towards a 2.4 bisi cam). According to his chart I only need 2.25in exhaust? I can see how running a smaller exh. will increase velocity (which he states specifically on pg.132 that larger is not better kind of threw me for a loop) but how do I know the proper balance of what dia. pipe I should run as to size while not hurting velocity or "choking" the turbine up top? Would this apply to building a dump tube for a external WG as well?
With a turbine (turbo) ideally you want to drop exhaust manifold pressure across the turbine. So the larger the exhaust system (3" vs 2.5) you easier this will be accomplished. Now some people have packaging constraints that will not allow for a large 3" or similarly larger exhaust. so they have t make due with a smaller exhaust. all he is saying is the minimum exhaust size for a given hp number. Thinking of it this way is better imo.
Also mentioned in the book is exhaust reversion which I understand can really hurt power. On page 120 in the book it shows tabs mounted inside of the exh. manifold (right by the flange that bolts to the head) to help prevent the exhaust from flowing backwards into the cc. Is there enough level of merit to actually implement these tabs on a setup such as mine? How does a properly designed cam help this issue? (Ie more lift or less duration)
Exhaust reversion would only be an issue if your running a very poorly designed exhaust manifold (log manifold) with a large cam/cams and the increased overlap allows the exhaust to reenter the combustion chamber as you have excessive exhaust manifold pressures. this is not something one needs to worry to much about on a Honda engine if using a decent exhaust manifold at the least.
I have read that a lot people are replacing their valve springs and retainers when running higher levels of boost... Any insight as to why would be much appreciated. I do not plan on revving much past 7k on my setup because power drops off well before that.
On a Honda i have never heard the need to replace the valve springs due to high or even any boost level. It always revolves around increased rpm's and or increased load levels like endurance racing. these two cases would cause the valve springs to fail long before boost pressure would cause float. In the v8 world they seem to run into valve float when boost pressure is introduced. I can only assume that the manufactures designed a spring that is just up to the task of a stock engine to save money or wear on engine parts. And the increased pressures want to hang the valves open.
This question is a bit of a shot in the dark but how much power can scavenged (or gained over say a log manifold) with a proper manifold that allows the exhaust pulses to hit the exhaust wheel at the proper time? Is there going to be much of a decrease in spool time if so? (Ie split housing on the hot side or properly built manifold)
Look into the full-race dyno test done years ago. they claimed i be leave a 50 whp increase over a log when they switched to a ramhorn manifold. Now this is just one instance where they were pushing the edge of there manifolds flow rate and by switching to a higher/better flowing manifold they saw huge increases in power. this isn't always the case and should be looked at subjectively.
And finally- how do you folks go about measuring your exhaust pressure to see if you have the proper pressure ratio vs. what is going into the intake and how do you know the proper psi you should have and when does too high of back pressure start having an effect on knock threshold?
The easiest way i have seen is the weld a 1/8" npt egt bung in the exhaust manifold preferably close to the turbine for a more even reading. And then run either a steel or brass tube many inches away from the manifold. And then hook up a standard boost gauge to the line. now you will need a gauge that reads quite high maybe 60 psi or so. As you will see pressures as high as 2:1 or 3:1 intake manifold pressures. (ie: 20 psi = 40-60 psi exhaust)
I know answers will vary from setup to setup but any light (links to setups or reading) shed on any of these questions, I will greatly appreciate!
Thanks!
Blaze
Hope this helps and can get you in the right direction.
01-09-2011, 08:34 PM
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Re: Exhaust Reversion, velocity, pressure Qs
The downpipe size requirements has a lot to do with the RPM band in which the engine is operating.
There are many cases which running a properly sized downpipe on a lower revving engine increased power across the powerband. The book Maximum Boost is based on older turbo engines when redlines rarely exceed 6000RPM. This is a very critical point because if the powerband is from 1500-5500RPM, then a larger exhaust will cause the air/fuel inside the combustion chambers to burn too quickly and not give it enough time to fully burn.
When the burn is too rapid and quick, then the engine doesn't achieve its peak efficiency. A quick reference of this is similar to comparing inductive coil pack ignition systems versus CDI ignition. Why coil packs with a longer average spark can gain more midrange torque than a big powerful yet quick spark (CDI) is due to a slower burn. Why a slower moving exhaust stream (due to a more restrictive header) can gain more low-end and midrange power versus a bigtube open header is also due to having a slower yet efficient burn. Slower burn will create more midrange power and gives more time for the air and fuel to burn properly. If the turbo engine is designed to operate at low to mid RPM ranges, then going too big of a downpipe will show no power increase or very little gains but with a loss of low-end and midrange.
On a high revving Honda, whatever power increase you see with a smaller downpipe/exhaust doesn't show at all because our turbos usually don't even make full boost within the 4200+ RPM range on most setups. On most modern turbo engines, bigger is in fact, always better because our 4-cyl engines are fast burning engines with really efficient head designs.
I managed to obtain some useful data when I went from a 3.5" DP/MP/Exhaust on my Supra TT and upgraded to a 5.0" DP/MP/Exhaust, with everything else unchanged (same turbo, manifold, boost, tune, etc..) on a 700 WHP setup. The car did not gain power from 1500-5000RPM, but the turbo spooled a bit sooner and made a peak of 65 WHP more from at 7800RPM with the 5" setup. This really shows that if my engine redline was at only 6000RPM, my big 5" exhaust will actually gain almost no power at all if my powerband was from 2000-6000RPM.
There are many cases which running a properly sized downpipe on a lower revving engine increased power across the powerband. The book Maximum Boost is based on older turbo engines when redlines rarely exceed 6000RPM. This is a very critical point because if the powerband is from 1500-5500RPM, then a larger exhaust will cause the air/fuel inside the combustion chambers to burn too quickly and not give it enough time to fully burn.
When the burn is too rapid and quick, then the engine doesn't achieve its peak efficiency. A quick reference of this is similar to comparing inductive coil pack ignition systems versus CDI ignition. Why coil packs with a longer average spark can gain more midrange torque than a big powerful yet quick spark (CDI) is due to a slower burn. Why a slower moving exhaust stream (due to a more restrictive header) can gain more low-end and midrange power versus a bigtube open header is also due to having a slower yet efficient burn. Slower burn will create more midrange power and gives more time for the air and fuel to burn properly. If the turbo engine is designed to operate at low to mid RPM ranges, then going too big of a downpipe will show no power increase or very little gains but with a loss of low-end and midrange.
On a high revving Honda, whatever power increase you see with a smaller downpipe/exhaust doesn't show at all because our turbos usually don't even make full boost within the 4200+ RPM range on most setups. On most modern turbo engines, bigger is in fact, always better because our 4-cyl engines are fast burning engines with really efficient head designs.
I managed to obtain some useful data when I went from a 3.5" DP/MP/Exhaust on my Supra TT and upgraded to a 5.0" DP/MP/Exhaust, with everything else unchanged (same turbo, manifold, boost, tune, etc..) on a 700 WHP setup. The car did not gain power from 1500-5000RPM, but the turbo spooled a bit sooner and made a peak of 65 WHP more from at 7800RPM with the 5" setup. This really shows that if my engine redline was at only 6000RPM, my big 5" exhaust will actually gain almost no power at all if my powerband was from 2000-6000RPM.
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