Rear mount turbo smaller A/R question (why would it hurt performance, LONG)
01-20-2006, 12:40 PM
#1
Rear mount turbo smaller A/R question (why would it hurt performance, LONG) Ok, I'm quite familiar with turbos and how they function. I've got a question for some more of the engineering types out there though.
On a turbo mounted close to the exhaust ports you need a larger a/r housing to get enough flow. This is because heat creates volume (works to a turbos benefit).
Move the turbo all the way to the back of the car and the exhaust temps plummet (less exhaust volume). The rear mount guys make up for this by going to a smaller a/r housing to create more pressure on the turbine wheel.
I'm pretty sure I've heard Jose say that running the smaller a/r limits the output of the turbo, even on a rear mount setup. My question is why?
The same number of exhaust molecules have to exit. So if there is less volume and therefore less pressure, and the molecules still need an exit, wouldn't the smaller a/r housing be equal in flow to the larger a/r housing at the hotter temperature?
As a generic example, say a Q trimmed T76 with a .96 a/r housing is capable of flowing 850 hp through it's exhaust housing when mounted close to the energy source.
Now move that same turbo 13' away from the enrgy source, the exhaust cools and becomes more dense. That means that .96 a/r should now be capable of flowing say 1050 hp (these are just examples, not factual data).
The problem there is the turbo is not capable of making that much power and the .96 a/r housing causes the dreaded lag monster to rear his ugly head like a 6spd 2jz with a t88. So we take the .96 a/r housing off and replace it with a .68 a/r housing. This housing on turbo mounted close to the energy source would choke up the flow pretty bad and probably limit it to 650 HP. But move that to the back of the car away from the mass volume of exhaust and it's now capable of supporting 850 hp. The lag monster has been slayed and high HP is still capable of being made without choking the ehxuast flow.
Thanks
BTW, My R-Trim T76 with a tiny .70 divided (morelike .64) a/r spools fast and my EGT's were only about 1250 at the header collector on 15psi.
On a turbo mounted close to the exhaust ports you need a larger a/r housing to get enough flow. This is because heat creates volume (works to a turbos benefit).
Move the turbo all the way to the back of the car and the exhaust temps plummet (less exhaust volume). The rear mount guys make up for this by going to a smaller a/r housing to create more pressure on the turbine wheel.
I'm pretty sure I've heard Jose say that running the smaller a/r limits the output of the turbo, even on a rear mount setup. My question is why?
The same number of exhaust molecules have to exit. So if there is less volume and therefore less pressure, and the molecules still need an exit, wouldn't the smaller a/r housing be equal in flow to the larger a/r housing at the hotter temperature?
As a generic example, say a Q trimmed T76 with a .96 a/r housing is capable of flowing 850 hp through it's exhaust housing when mounted close to the energy source.
Now move that same turbo 13' away from the enrgy source, the exhaust cools and becomes more dense. That means that .96 a/r should now be capable of flowing say 1050 hp (these are just examples, not factual data).
The problem there is the turbo is not capable of making that much power and the .96 a/r housing causes the dreaded lag monster to rear his ugly head like a 6spd 2jz with a t88. So we take the .96 a/r housing off and replace it with a .68 a/r housing. This housing on turbo mounted close to the energy source would choke up the flow pretty bad and probably limit it to 650 HP. But move that to the back of the car away from the mass volume of exhaust and it's now capable of supporting 850 hp. The lag monster has been slayed and high HP is still capable of being made without choking the ehxuast flow.
Thanks
BTW, My R-Trim T76 with a tiny .70 divided (morelike .64) a/r spools fast and my EGT's were only about 1250 at the header collector on 15psi.
01-20-2006, 01:06 PM
#3
Originally Posted by 2000 Tran Zam
makes sense to me, but i didnt know that the A/R controls power output, just thought it controls pressure within the turbo.
01-20-2006, 04:37 PM
#4
TECH Apprentice
Join Date: Jan 2004
Location: Frisco TX (Dallas Area)
Posts: 337
Likes: 0
Received 0 Likes
on
0 Posts
I'm no expert and don't have a definitive answer. I also think that there are more considerations than mentioned (I sure don't have a complete list). But here's some info for consideration.
When you reduce the turbine's A/R you are doing the same thing as putting your thumb over a water hose to squirt the water further/faster/harder. The more you squeeze the opening, the higher the pressure in the hose (exhaust system). What's happening is that the more boost you make (or try to) causes increased exhaust backpressure. Sometimes as much as 3 times the intake pressure you're getting. That chokes the engine (fights boost) and requires a significantly different cam to compensate for the exhaust overdriving the intake.
Yeah, I know, water is incompressible and air isn't, but the analogy gets the idea across. My point is that the exhaust restriction experienced vs the boost realized is a limiting factor in a given setup, front or rear. I hope that makes some kind of sense.
When you reduce the turbine's A/R you are doing the same thing as putting your thumb over a water hose to squirt the water further/faster/harder. The more you squeeze the opening, the higher the pressure in the hose (exhaust system). What's happening is that the more boost you make (or try to) causes increased exhaust backpressure. Sometimes as much as 3 times the intake pressure you're getting. That chokes the engine (fights boost) and requires a significantly different cam to compensate for the exhaust overdriving the intake.
Yeah, I know, water is incompressible and air isn't, but the analogy gets the idea across. My point is that the exhaust restriction experienced vs the boost realized is a limiting factor in a given setup, front or rear. I hope that makes some kind of sense.
01-20-2006, 04:53 PM
#5
9 Second Club
iTrader: (2)
Join Date: Jun 2005
Location: Reston, VA
Posts: 2,034
Likes: 0
Received 0 Likes
on
0 Posts
Zombie... thanks man... great post. This is exactly what I am trying to understand.
If heat is the advantage or drawback for a turbo application, wouldnt it be in the benifit to "warm up" a turbo to a specific operating temp to make targeted boost?
With a smaller AR ratio, but a larger turbo housing wouldnt you see spooling advantages to include higher spool RPM's due to the increased pressure over the wider AR?
I am a turbo noob, looking forward to the first day of school.
If heat is the advantage or drawback for a turbo application, wouldnt it be in the benifit to "warm up" a turbo to a specific operating temp to make targeted boost?
With a smaller AR ratio, but a larger turbo housing wouldnt you see spooling advantages to include higher spool RPM's due to the increased pressure over the wider AR?
I am a turbo noob, looking forward to the first day of school.
01-20-2006, 06:00 PM
#6
TeeKay, you are missing my point. The smaller a/r shouldn't be a restriction in the back because the exhaust is more dense and therefore able to flow through a smaller opening. You just use the smaller a/r to increase the pressure to the same amount that the larger a/r housing does when the turbo is closer to the energy source.
I have a program that calculates pipe flow based off of length, gas type, pipe material and gas temp. It tells me that I can flow more cold air through a smaller diameter than hot air.
I'm just trying to find out if i'm missing some info here based off of what Jose is said. I hope he sees this post and comments.
I have a program that calculates pipe flow based off of length, gas type, pipe material and gas temp. It tells me that I can flow more cold air through a smaller diameter than hot air.
I'm just trying to find out if i'm missing some info here based off of what Jose is said. I hope he sees this post and comments.
01-20-2006, 11:04 PM
#7
LS1TECH Sponsor
iTrader: (2)
Join Date: Aug 2002
Location: Huntsville, AL
Posts: 2,725
Likes: 0
Received 0 Likes
on
0 Posts
First of all, I didn't sit down and do the math, so if it doesn't make sense, sorry....lol
All the things your are stating are in fact true about pressure drop, volume, and density since thats the way standard auto exhaust work. The one thing your failing to see is, its the amount of temp drop across the turbine wheel is where power is extracted. The higher the turbine wheel blade inlet to post turbine wheel heat ratio is the more power is made and quicker the spool up. This theory is best shown with diesels. What happens to diesels when the EGT is not hot enough....no spool
. Call me on this. I am not allowed to post the info we do have. But you and I have discussed alot of this before and, this all comes from the engineers and manufactures that we work with.
Also by the theory your using (which is standard exhaust operation) what happens to your pressure source (ala your engine) when you neck the A/R down (anywhere in the flow path)?? Remember the A/R means smaller scroll area, but it also means you are now bypassing less air around the blade face and forcing it through. With a front mount setup the heat generated gives more then enough exhaust gas energy to spin the turbine and you can get away with bypassing more exhaust gas around the turbine for less backpressure, so you have greater manipulation of the ratio.....this is the key and holy grail to this. In essence if I could attach the turbo to the exhaust runners directly this would be the place to do it. I could then run an insanely huge A/R to bypass larger amounts of gas around the blade, but still have enough energy, pressure, volume and velocity to power it more efficiently. Also the basic operations of the (engine/exhaust) you speak is like we all know air moves into the combustion chamber, oxygen in the air is lit, and then it is expelled to make room for the next combustion cycle. The combustion that happens we all know is what makes power. But it also adds heat which increases the volume of the combusted air. Remember ~30-40% of the energy from the combustion process exits the exhaust pipe. The way it exits is heat. Their is also another form of energy in your exhaust and that is knetic energy (velocity). But remember, if the air wasn't heated in the combustion process, then this kenetic energy would be (Inlet = outlet). So with that being said as the heat drops and other mentioned variables decrease how are you going to add more energy to a closed system then is available?? Also you still have to factor in flow restrictions and parasitic drag through 13ft of pipe. Bernoulli (however you spell that clowns name) sums it up quite well in his equation.
Jose
All the things your are stating are in fact true about pressure drop, volume, and density since thats the way standard auto exhaust work. The one thing your failing to see is, its the amount of temp drop across the turbine wheel is where power is extracted. The higher the turbine wheel blade inlet to post turbine wheel heat ratio is the more power is made and quicker the spool up. This theory is best shown with diesels. What happens to diesels when the EGT is not hot enough....no spool
. Call me on this. I am not allowed to post the info we do have. But you and I have discussed alot of this before and, this all comes from the engineers and manufactures that we work with.Also by the theory your using (which is standard exhaust operation) what happens to your pressure source (ala your engine) when you neck the A/R down (anywhere in the flow path)?? Remember the A/R means smaller scroll area, but it also means you are now bypassing less air around the blade face and forcing it through. With a front mount setup the heat generated gives more then enough exhaust gas energy to spin the turbine and you can get away with bypassing more exhaust gas around the turbine for less backpressure, so you have greater manipulation of the ratio.....this is the key and holy grail to this. In essence if I could attach the turbo to the exhaust runners directly this would be the place to do it. I could then run an insanely huge A/R to bypass larger amounts of gas around the blade, but still have enough energy, pressure, volume and velocity to power it more efficiently. Also the basic operations of the (engine/exhaust) you speak is like we all know air moves into the combustion chamber, oxygen in the air is lit, and then it is expelled to make room for the next combustion cycle. The combustion that happens we all know is what makes power. But it also adds heat which increases the volume of the combusted air. Remember ~30-40% of the energy from the combustion process exits the exhaust pipe. The way it exits is heat. Their is also another form of energy in your exhaust and that is knetic energy (velocity). But remember, if the air wasn't heated in the combustion process, then this kenetic energy would be (Inlet = outlet). So with that being said as the heat drops and other mentioned variables decrease how are you going to add more energy to a closed system then is available?? Also you still have to factor in flow restrictions and parasitic drag through 13ft of pipe. Bernoulli (however you spell that clowns name) sums it up quite well in his equation.
Jose
Last edited by JZ 97 SS 1500; 01-20-2006 at 11:10 PM.
Trending Topics
01-21-2006, 02:19 AM
#8
Originally Posted by JZ 97 SS 1500
The one thing your failing to see is, its the amount of temp drop across the turbine wheel is where power is extracted. The higher the turbine wheel blade inlet to post turbine wheel heat ratio is the more power is made and quicker the spool up.
Jose
Jose
So you are saying that the delta of the inlet/outlet temp is affecting the effeciency of the system. Makes sense I guess. The greater the delta the more energy that was imposed on the turbine wheel, aka turn the heat into energy.
Do you know roughly how much that affects overall system effeciency (SWAG is ok with me, just curious on ball park). I'm just guessing from seeing different dyno graphs that if you average 1000-6000 rpms it's probably in the 8-10% range. I know it affects spoolup some as i've noticed on my car compared to other similarly equiped t76 cars. If it is around 10% then it's not too bad, but thats 500 average hp vs 550 hp, I can live with that. The gap would close up quite a bit if just measuring used power band in drag racing 4000-600 rpms. Seems to all come back to how fast you can get it spooled.
I'll probably give you a ring just to clearify a few points, I knew there was some piece I was missing.
Am I on the right track?
01-21-2006, 05:51 AM
#9
Originally Posted by Zombie
As a generic example, say a Q trimmed T76 with a .96 a/r housing is capable of flowing 850 hp through it's exhaust housing when mounted close to the energy source.
Now move that same turbo 13' away from the enrgy source, the exhaust cools and becomes more dense. That means that .96 a/r should now be capable of flowing say 1050 hp (these are just examples, not factual data).
Now move that same turbo 13' away from the enrgy source, the exhaust cools and becomes more dense. That means that .96 a/r should now be capable of flowing say 1050 hp (these are just examples, not factual data).
your main objective is spinning the turbine. so moving it back you can push more exhaust gasses out of your engine, but since the exhaust gas isnt as fast (or actually having more energy) the turbines arnt spinning as fast, so say you up the rpms more to get the gas energy output in the back to equal what the gas energy output in the front you've ecentailly moved your power curve on your engine over at a higher rpm making the engine less effiecent.
01-21-2006, 08:01 AM
#10
Let me take a shot at this. Having all the fluid mechanics, thermodynamics, and other unpronounceable courses as an undergrad in engineering a lifetime ago [BUT: have not worked as an engineer since '79], I have "bench raced" this rear turbo setup from last spring when I first heard about it. I do NOT know much textbook info about turbos...so very simplistically:
When you first go full throttle, the exhaust plumbing and turbo are quite cool. I am amazed at idle the turbo on my goat won't fry you. So, a tremendous amount of heat loss [=energy loss] occurs all the way throughout the system, and the lag in response is noticeable. The smaller A/R does help it get spinning faster, and in a second or two, everything along the way is nice and toasty. In the 2nd-to-3rd shift on my car, the boost gauge is back at 7 psi instantly. At this point, the exhaust and turbo are at a steady-state level of functioning...but then the smaller A/R rears its head. More kg of air moving through the engine = higher pressure drop across the exhaust housing...more back pressure.
It really isn't the distance involved nearly as much as the heat loss. Jose mentions the energy loss due to drag from the several feet of exhaust plumbing; I suspect that the frictional drag doesn't lose nearly as much of the energy as thermal conductivity across the pipe. This is why I have previously speculated about the potential benefit of insulation [heat wrap] from the engine through the turbo. If you could stop ALL of the energy loss there, the only difference [front mount vs. rear mount] would be related to the distance flowed. That would be frictional loss and time-delay related to the volume of the plumbing [which really is miniscule: a 3" pipe x 120" =848 cu in = ~.5 cu ft. Even at 100 cfm airflow, it only takes .02 sec for that volume to move through the pipe].
Certainly in our GTO's which suffer from heat soak underhood, there is a performance benefit to keeping all of the heat out of the engine compartment [as well as potentially improved longevity of other components]. Also, there is heat loss from the charged air pipe under the car which is beneficial. Obviously it doesn't lose as much heat as the exhaust side, as the exhaust has a much larger delta-T compared to ambient temps.
All of this makes me [once again] think about the benefit of shorty headers, no cats, and a bunch of heat wrap down the entire exhaust and turbo. For my particular setup, I have the .81 housing on now [GT40-67 turbo] but have the .96 sitting around. The LS1 cars seemed to build boost too slowly with the larger housing. The LS2 might be slightly better, but I haven’t gotten around to it yet. Also, I am still running at 7psi; RWTD in Mobile is ready to complete the tune at high boost [12+] when I can get back down there. Obviously there will be more problems with the system choking at that boost…may try to swap the housing for a run.
When you first go full throttle, the exhaust plumbing and turbo are quite cool. I am amazed at idle the turbo on my goat won't fry you. So, a tremendous amount of heat loss [=energy loss] occurs all the way throughout the system, and the lag in response is noticeable. The smaller A/R does help it get spinning faster, and in a second or two, everything along the way is nice and toasty. In the 2nd-to-3rd shift on my car, the boost gauge is back at 7 psi instantly. At this point, the exhaust and turbo are at a steady-state level of functioning...but then the smaller A/R rears its head. More kg of air moving through the engine = higher pressure drop across the exhaust housing...more back pressure.
It really isn't the distance involved nearly as much as the heat loss. Jose mentions the energy loss due to drag from the several feet of exhaust plumbing; I suspect that the frictional drag doesn't lose nearly as much of the energy as thermal conductivity across the pipe. This is why I have previously speculated about the potential benefit of insulation [heat wrap] from the engine through the turbo. If you could stop ALL of the energy loss there, the only difference [front mount vs. rear mount] would be related to the distance flowed. That would be frictional loss and time-delay related to the volume of the plumbing [which really is miniscule: a 3" pipe x 120" =848 cu in = ~.5 cu ft. Even at 100 cfm airflow, it only takes .02 sec for that volume to move through the pipe].
Certainly in our GTO's which suffer from heat soak underhood, there is a performance benefit to keeping all of the heat out of the engine compartment [as well as potentially improved longevity of other components]. Also, there is heat loss from the charged air pipe under the car which is beneficial. Obviously it doesn't lose as much heat as the exhaust side, as the exhaust has a much larger delta-T compared to ambient temps.
All of this makes me [once again] think about the benefit of shorty headers, no cats, and a bunch of heat wrap down the entire exhaust and turbo. For my particular setup, I have the .81 housing on now [GT40-67 turbo] but have the .96 sitting around. The LS1 cars seemed to build boost too slowly with the larger housing. The LS2 might be slightly better, but I haven’t gotten around to it yet. Also, I am still running at 7psi; RWTD in Mobile is ready to complete the tune at high boost [12+] when I can get back down there. Obviously there will be more problems with the system choking at that boost…may try to swap the housing for a run.
01-21-2006, 10:15 AM
#11
Banned
iTrader: (2)
Join Date: Jun 2005
Location: OKC OK
Posts: 238
Likes: 0
Received 0 Likes
on
0 Posts
ok explain this one to me then.
i bought a used turbo from billy's son at texas rear turbos
he had the turbine clipped 7.5 deg and its a 60-1 turbo
.70 housing on the cool side and .70 split tang on the hot side.
its has the same power output that my gt 67 turbo i bought
from sts at 5psi. The sts gt 67 is a .60 on the cool side and a .81 on the
hot side which the larger 67 size wheel and both have the same
p trim on the hot side and they both dynoed up at 5psi almost exactly
the same! But with the clipped turbine I could stall the car and get it
making boost from a standstill but with the gt 67 it will not make boost
while doing a stall at the start line..... with my 3500 stall..
since sts systems use less psi for the most part than conventional
turbos kits. bringin heat back to the turbo or shortening the over all
length really is going to do much of anything noticeable
untill you try making lets say 8 psi and up for the most part
it just doesnt matter what yah use . I do think my gt 67
might have more potential but the 60-1 will probably do just
as well at low boost settings and make boost at the start line
it was only 1 psi I could hold it at but it was still boost
which cuts out lagg almost to nothing. and with a clipped turbine
you get more flow just like with a larger housing less overall
efficency but more flow.. maybe efficency at these low boost levels
doesnt mean alot and low means more because its acting more like
a supercharger.
i bought a used turbo from billy's son at texas rear turbos
he had the turbine clipped 7.5 deg and its a 60-1 turbo
.70 housing on the cool side and .70 split tang on the hot side.
its has the same power output that my gt 67 turbo i bought
from sts at 5psi. The sts gt 67 is a .60 on the cool side and a .81 on the
hot side which the larger 67 size wheel and both have the same
p trim on the hot side and they both dynoed up at 5psi almost exactly
the same! But with the clipped turbine I could stall the car and get it
making boost from a standstill but with the gt 67 it will not make boost
while doing a stall at the start line..... with my 3500 stall..
since sts systems use less psi for the most part than conventional
turbos kits. bringin heat back to the turbo or shortening the over all
length really is going to do much of anything noticeable
untill you try making lets say 8 psi and up for the most part
it just doesnt matter what yah use . I do think my gt 67
might have more potential but the 60-1 will probably do just
as well at low boost settings and make boost at the start line
it was only 1 psi I could hold it at but it was still boost
which cuts out lagg almost to nothing. and with a clipped turbine
you get more flow just like with a larger housing less overall
efficency but more flow.. maybe efficency at these low boost levels
doesnt mean alot and low means more because its acting more like
a supercharger.
Last edited by 98Ztwentyeight; 01-21-2006 at 10:55 AM.
01-26-2006, 01:07 PM
#13
9 Second Club
iTrader: (2)
Join Date: Jun 2005
Location: Reston, VA
Posts: 2,034
Likes: 0
Received 0 Likes
on
0 Posts
Ok, as I am learning from all of your posts… (Takes a lot, I am a computer engineer, not a physicist) With the drop in the temperature of the exhaust as it gets further from the source, this decreases the velocity and pressure applied to the exhaust wheel side of the turbo. I also understand that decreasing the size of the A/R will increase the pressure of the exhaust (a method of compensating for the remote mount?) but with the decreased size, you also decrease the overall potential throughput of the turbo.
I visualize things… so the analogy earlier of taking a hose and putting your thumb over it was good for me. To add however, if you were to restrict the outlet side of anything, your maximum flow would only be directly proportionate to the diameter of the output and the pressure applied to the medium your flowing. Kinetic energy produced in your exhaust (velocity) does not degrade with temp does it?.
This being said… wouldn’t there be some good research in finding what the most responsive A/R, Wheel, and housing sizes are that would be optimized for the reduced exhaust temp, increased velocity, and any other arguable difference of a remote mount setup?
Many of you are the ones that not long ago fought the same battle with the folks that said one turbo cant touch the performance of two…
I visualize things… so the analogy earlier of taking a hose and putting your thumb over it was good for me. To add however, if you were to restrict the outlet side of anything, your maximum flow would only be directly proportionate to the diameter of the output and the pressure applied to the medium your flowing. Kinetic energy produced in your exhaust (velocity) does not degrade with temp does it?.
This being said… wouldn’t there be some good research in finding what the most responsive A/R, Wheel, and housing sizes are that would be optimized for the reduced exhaust temp, increased velocity, and any other arguable difference of a remote mount setup?
Many of you are the ones that not long ago fought the same battle with the folks that said one turbo cant touch the performance of two…
01-27-2006, 12:05 AM
#16
Completely disregarding the amount that the smaller AR will choke the turbo's ability to produce boost, how much does it really choke overall exhaust flow? Since the wastegate bypasses extra exhaust, does the smaller AR also choke the exhaust flow? Or would the wastegate need to be so big that it would cause surge? I am just trying to figure out a reasonable way to balance spool time vs peak flow and intake pressure vs exhaust pressure. It seems that as long as the cam has zero overlap, it should be possible to create a mathematical formula in which to enter these variables once displacement and approximate VE of an engine are considered, along with the appropriate turbo map. I'm sure that with any cam overlap the variables would go through the roof, but a simplistic approximate formula should be doable.
01-27-2006, 08:08 AM
#17
I continue to wait for someone to get some REAL numbers that look at this issue of exhaust temp and the possible benefit of SERIOUS insulation. As I noted above, the loss of heat is really the only downside other than some SLIGHT frictional loss...and actually, that frictional loss is into MORE heat.
If we can just keep that exhaust tract HOT and insulate the turbo, I think we could potentially see the best of both worlds: less underhood heat/easier install/charged air cooling from running in air stream under the car AND good spool time/good power.
If we can just keep that exhaust tract HOT and insulate the turbo, I think we could potentially see the best of both worlds: less underhood heat/easier install/charged air cooling from running in air stream under the car AND good spool time/good power.
01-31-2006, 10:59 AM
#19
TECH Resident
Join Date: Mar 2002
Location: North Las Vegas, NV
Posts: 782
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Slowhawk
PurEvl's car had a .68 a/r housing on his T70 STS setup.We pushed 700rwhp at 14lbs and it had alot left. There's my math
Exactly. Theory is nice, but doesn't always work
