Thoughts on Boost and Spoolup
06-16-2007, 01:05 AM
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Thoughts on Boost and Spoolup
OK, I've noticed some "myths" starting to get traction on the forum, and in this piece I'm going to attempt to bust that myth, but also share with you my general thoughts on boost and spoolup.
The myth: larger injectors will cure a laggy turbo.
Thoughts on boost: Boost is a necessary evil. The least amount you need to get the airflow you require is best. Any more is inefficient.
Thoughts on spoolup/lag:
There are two completely separate components to lag (which I'll use to refer to spoolup): one is boost THRESHOLD (a steady state function), the other is BOOST CHANGE (a dynamic function).
It's helpful to think of them as relating to separate circumstances. Boost threshold relates to steady state operation, where are the variables are held constant: load, EGT, fueling, etc. Boost change is simply how quickly the turbo can adapt to a change in one of the variables that were previously held steady: load, RPM, EGT, fueling, etc.
It's possible to have a turbo that has a very low boost threshold but terrible spoolup. Imagine you have an HY35 on a bombed engine, only you've added a flywheel to the turbo's shaft. The result is something that "comes into the boost" very early, but can't transition from 10psi of boost to 30psi very quickly because of the higher rotational inertia of the flywheel.
On the other hand, you can have a turbo that has a higher boost threshold, but once it's "on the turbo" it spools very quickly. For example, some of the newer big-turbine singles are kinda slow to get to 10psi, but will go from 10-50 in a blink!
Now, onto the influence of injectors upon spoolup. First we ask, what causes spoolup?? Well, the turbine responds to mass and the amount of energy it contains (heat). I can spool a turbo with a TON of icy cold air, or I can spool it with a much lower mass of much hotter air.
Obviously, when you step on the loud pedal, the additional fuel flow increases the mass of the exhaust stream-- but by a tiny amount. The far greater impact is on temperature, and hence the energy content of the mass flow.
But there is a limit to how much energy you can add. Once I've used up all the oxygen available to burn fuel, adding more fuel will not increase the energy content of the flow stream. In fact, adding a lot more fuel past this point will actually reduce the energy level of the flow stream because of the cooling effect of the surplus fuel.
Spoolup for a given turbo then is a function of how quickly you can increase the energy content of the exhaust flow. PERIOD. As the turbo begins to spoolup, the increased inlet air mass will support more fuel burning, and hence add more energy to the exhaust flow stream. Obviously, this becomes a snowball effect where the more is burned, the more CAN be burned.
OK, this is already way too long, but the point here is that larger injectors are NOT a cure for slow spoolup. They can help-- but only to a point. Once you're past that point, more fuel will not enhance spoolup.
Anyone who's driven a Smarty on "Kung Fu" with bigger injectors can attest to this. If you just floor it, you get huge billows of smoke-- and lots of lag. But if you knock the Smarty down to #3, you get instant response and a lot less smoke, because the fuel rate is closer to that the turbo can handle.
Now, the larger the turbo, the larger an injector can be and still contribute to better spool up. But there's a finite limit to how fast a turbo will spool, no matter how big an injector is.
The last element of this picture is how FAR a turbo is spooled and how injector size contributes. If you can floor it at 20psi and get lots of smoke, then you probably are not going to get any benefit from a bigger injector. Of course, this ignores the boost-related fueling of any boxes or programmers, which is a much greater factor.
In another thread a guy has a 64mm turbo and is wondering if he should go with a 200hp injector or larger to help with spoolup. Guys, you might see a tiny improvement in RATE of boost gain, but you're not going to see any change in boost threshold, and the lag that occurs until that threshold is met will only be even smokier with the larger injector.
That said, I'd much rather have sticks a little too big than a little too small
JMO
The myth: larger injectors will cure a laggy turbo.
Thoughts on boost: Boost is a necessary evil. The least amount you need to get the airflow you require is best. Any more is inefficient.
Thoughts on spoolup/lag:
There are two completely separate components to lag (which I'll use to refer to spoolup): one is boost THRESHOLD (a steady state function), the other is BOOST CHANGE (a dynamic function).
It's helpful to think of them as relating to separate circumstances. Boost threshold relates to steady state operation, where are the variables are held constant: load, EGT, fueling, etc. Boost change is simply how quickly the turbo can adapt to a change in one of the variables that were previously held steady: load, RPM, EGT, fueling, etc.
It's possible to have a turbo that has a very low boost threshold but terrible spoolup. Imagine you have an HY35 on a bombed engine, only you've added a flywheel to the turbo's shaft. The result is something that "comes into the boost" very early, but can't transition from 10psi of boost to 30psi very quickly because of the higher rotational inertia of the flywheel.
On the other hand, you can have a turbo that has a higher boost threshold, but once it's "on the turbo" it spools very quickly. For example, some of the newer big-turbine singles are kinda slow to get to 10psi, but will go from 10-50 in a blink!
Now, onto the influence of injectors upon spoolup. First we ask, what causes spoolup?? Well, the turbine responds to mass and the amount of energy it contains (heat). I can spool a turbo with a TON of icy cold air, or I can spool it with a much lower mass of much hotter air.
Obviously, when you step on the loud pedal, the additional fuel flow increases the mass of the exhaust stream-- but by a tiny amount. The far greater impact is on temperature, and hence the energy content of the mass flow.
But there is a limit to how much energy you can add. Once I've used up all the oxygen available to burn fuel, adding more fuel will not increase the energy content of the flow stream. In fact, adding a lot more fuel past this point will actually reduce the energy level of the flow stream because of the cooling effect of the surplus fuel.
Spoolup for a given turbo then is a function of how quickly you can increase the energy content of the exhaust flow. PERIOD. As the turbo begins to spoolup, the increased inlet air mass will support more fuel burning, and hence add more energy to the exhaust flow stream. Obviously, this becomes a snowball effect where the more is burned, the more CAN be burned.
OK, this is already way too long, but the point here is that larger injectors are NOT a cure for slow spoolup. They can help-- but only to a point. Once you're past that point, more fuel will not enhance spoolup.
Anyone who's driven a Smarty on "Kung Fu" with bigger injectors can attest to this. If you just floor it, you get huge billows of smoke-- and lots of lag. But if you knock the Smarty down to #3, you get instant response and a lot less smoke, because the fuel rate is closer to that the turbo can handle.
Now, the larger the turbo, the larger an injector can be and still contribute to better spool up. But there's a finite limit to how fast a turbo will spool, no matter how big an injector is.
The last element of this picture is how FAR a turbo is spooled and how injector size contributes. If you can floor it at 20psi and get lots of smoke, then you probably are not going to get any benefit from a bigger injector. Of course, this ignores the boost-related fueling of any boxes or programmers, which is a much greater factor.
In another thread a guy has a 64mm turbo and is wondering if he should go with a 200hp injector or larger to help with spoolup. Guys, you might see a tiny improvement in RATE of boost gain, but you're not going to see any change in boost threshold, and the lag that occurs until that threshold is met will only be even smokier with the larger injector.
That said, I'd much rather have sticks a little too big than a little too small
JMO
06-16-2007, 02:57 AM
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OK, great explanation--the best I've seen, now it makes sense. 
But I have to ask, are you an engineer, mathematician, physicist? I haven't heard the words "steady state function" since I was in school, in fact your entire explanation has a very mathematical-proof like theme. For a second there I thought you were going to say that too big of an injector would violate the Uncertainty Principle and therefore invalidate all that we know about quantum mechanics. . .
By the way, I'm a physicist by education, but I've been absorbed into the systems engineering field by necessity (have to pay the bills).
But I have to ask, are you an engineer, mathematician, physicist? I haven't heard the words "steady state function" since I was in school, in fact your entire explanation has a very mathematical-proof like theme. For a second there I thought you were going to say that too big of an injector would violate the Uncertainty Principle and therefore invalidate all that we know about quantum mechanics. . .
By the way, I'm a physicist by education, but I've been absorbed into the systems engineering field by necessity (have to pay the bills).
06-16-2007, 08:49 AM
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Great read Justin!
Needs to be heard because so many members are going to bigger turbos...with varying degrees of satisfaction!
It's a fact of life... big single turbos are a compromise. With the latest compressor blade technology & lighter metalurgy, they are better than ever. But being aware of the facts you have covered above, before we buy, is important to understanding and being happy with a new/bigger turbo.
As your explanation of the physics behind it explains...don't expect to 'have it all'. I would only add this for those wondering if they made an expensive mistake by going too big on a new turbo; give yourself time to learn how to drive around the slower spoolup... especially with an automatic! It's not harder... just different.
RJ
Needs to be heard because so many members are going to bigger turbos...with varying degrees of satisfaction!
It's a fact of life... big single turbos are a compromise. With the latest compressor blade technology & lighter metalurgy, they are better than ever. But being aware of the facts you have covered above, before we buy, is important to understanding and being happy with a new/bigger turbo.
As your explanation of the physics behind it explains...don't expect to 'have it all'. I would only add this for those wondering if they made an expensive mistake by going too big on a new turbo; give yourself time to learn how to drive around the slower spoolup... especially with an automatic! It's not harder... just different.
RJ
06-16-2007, 09:22 AM
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This may be a good time to put a ? out there that occurred to me , looking at future mods , I put in a 4" exhaust , my intention was to put in a inline ex-brake , as funds allow , HP build to 350-400 , everyday drive tow Airstream
32' , mountains .
The ? comes in with the turbo mod , elbow getting bigger to match 4" ex. , my theory was something like the venturi effect , making exit hole bigger , losing resistance of exiting ex. , reducing spool up ?
This is a long way around to asking about what turbo , or parts to change out on my WCH1 original ,
I like to understand my changes not just follow recommendations .
Thanks John
32' , mountains .
The ? comes in with the turbo mod , elbow getting bigger to match 4" ex. , my theory was something like the venturi effect , making exit hole bigger , losing resistance of exiting ex. , reducing spool up ?
This is a long way around to asking about what turbo , or parts to change out on my WCH1 original ,
I like to understand my changes not just follow recommendations .
Thanks John
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06-16-2007, 02:26 PM
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lgp: with your star wheel all the way back, you're almost like a 24V with big injectors and a Smarty-- the fueling is all in your foot. NOrmally, the AFC controls most the fueling, based on boost readings. The more you wind the star wheel back, the more you are eliminating the AFC function and making fueling simply a matter of the loud pedal.
John-- not sure on your question. Your 4" exhaust is a help to your stock turbo, but the stock turbo is far more restrictive than the exhaust coming after it. There isn't really a "venturi effect" to speak of. Think of where you see Venturis: pitot tubes, manometers, carburetor boosters, etc. In each of these cases, a tube is stuck into the flowstream and and the pressure at the end of the tube decreases with flowstream velocity.
So while your exhaust is speeding up as it exits the turbo, there's no "tube" stuck into the flowstream to take advantage of it. If there was an advantage to a decrease in cross section, we could just crimp the downpipe. The exhaust only sees cross-sectional area. Changes in cross sectional area should always be 1) increasing and 2) gradual.
That why, in my opinion the SINGLE most important thing to look for an an aftermarket 4" exhaust is how gradual is the transition to the 4" diameter. Some cheap exhausts have downpipes that transition in just an inch! A better exhaust would have a transitional section that was 3-4 inches or so. The key spec here is angle: what is the angle of the wall during the transition? If it's 30º or less, then it's good to go imo.
Once you're past that transition, the 4" piping itself has little effect on flow-- any one 4" pipe is as good as another. The muffler is the second most important part, after the downpipe transition.
JMO
John-- not sure on your question. Your 4" exhaust is a help to your stock turbo, but the stock turbo is far more restrictive than the exhaust coming after it. There isn't really a "venturi effect" to speak of. Think of where you see Venturis: pitot tubes, manometers, carburetor boosters, etc. In each of these cases, a tube is stuck into the flowstream and and the pressure at the end of the tube decreases with flowstream velocity.
So while your exhaust is speeding up as it exits the turbo, there's no "tube" stuck into the flowstream to take advantage of it. If there was an advantage to a decrease in cross section, we could just crimp the downpipe. The exhaust only sees cross-sectional area. Changes in cross sectional area should always be 1) increasing and 2) gradual.
That why, in my opinion the SINGLE most important thing to look for an an aftermarket 4" exhaust is how gradual is the transition to the 4" diameter. Some cheap exhausts have downpipes that transition in just an inch! A better exhaust would have a transitional section that was 3-4 inches or so. The key spec here is angle: what is the angle of the wall during the transition? If it's 30º or less, then it's good to go imo.
Once you're past that transition, the 4" piping itself has little effect on flow-- any one 4" pipe is as good as another. The muffler is the second most important part, after the downpipe transition.
JMO
06-16-2007, 04:42 PM
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Nothing new under the sun, Justin - these myths have been around for a while!
Most people don't know why stuff works - they just know it does, and that can lead to throwing parts & $$$ at a situation as opposed to reasoning what will work best first. I think it's better to cure the problem instead of treating the symptom.
Higher quality boost produces more power than a high quantity of boost, and of course a lower maximum-HP turbine/compressor 'charger can often perform better on the street where part-throttle and lower RPM is prevalent.
There's a bunch of math involved in sizing compressor & turbine trim (by varying inducer and exducer diameters), as well as turbine housing A/Rs (compressor A/Rs don't have hardly any effect on performance).
Generally, lower trims and A/Rs will have low boost threshold, faster transient response and more low-RPM power than larger trims & A/Rs would. Of course, there are exceptions to this - fluidics can seem just as esoteric as quantum physics!
Anyway, I wouldn't dump extra fuel into a CTD to accelerate spool-up until it's VE was increased as much as practical - starting from a low BSFC baseline is best for making max "safe" power, IMO.
p.s. in regards to the post-turbine exhaust, smaller tubing diameters (~ 4") promote higher flow velocities and thus better scavenging, which maximizes the potential pressure delta (and work capability) across the turbine.
A "muffler" (which it actually isn't) like the AeroTurbine helps keep flow velocity from falling as it cools and reduces the decibel signature, which is obviously better than some of the brute-force type conventional mufflers.
Most people don't know why stuff works - they just know it does, and that can lead to throwing parts & $$$ at a situation as opposed to reasoning what will work best first. I think it's better to cure the problem instead of treating the symptom.
Higher quality boost produces more power than a high quantity of boost, and of course a lower maximum-HP turbine/compressor 'charger can often perform better on the street where part-throttle and lower RPM is prevalent.
There's a bunch of math involved in sizing compressor & turbine trim (by varying inducer and exducer diameters), as well as turbine housing A/Rs (compressor A/Rs don't have hardly any effect on performance).
Generally, lower trims and A/Rs will have low boost threshold, faster transient response and more low-RPM power than larger trims & A/Rs would. Of course, there are exceptions to this - fluidics can seem just as esoteric as quantum physics!
Anyway, I wouldn't dump extra fuel into a CTD to accelerate spool-up until it's VE was increased as much as practical - starting from a low BSFC baseline is best for making max "safe" power, IMO.
p.s. in regards to the post-turbine exhaust, smaller tubing diameters (~ 4") promote higher flow velocities and thus better scavenging, which maximizes the potential pressure delta (and work capability) across the turbine.
A "muffler" (which it actually isn't) like the AeroTurbine helps keep flow velocity from falling as it cools and reduces the decibel signature, which is obviously better than some of the brute-force type conventional mufflers.
06-17-2007, 10:57 AM
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I haven't opened up for repair or upgrade any turbos , just R&R for general repair .
But I've read many posts here & els where , changing turbines , housing - ex/in , I was under the impression that there were changes being do to housings with out changing turbines , [ I thought what was happening was in changing an ex housing they were either closing in on the turbine or opening up the area around the turbine - forcing ex gas's into or allowing some bypass ] ?
Again trying to understand before changes , in order to get peak performance out of each mod [ then keeping in mind to blend all mods-relations to each other ] .
Thanks John
But I've read many posts here & els where , changing turbines , housing - ex/in , I was under the impression that there were changes being do to housings with out changing turbines , [ I thought what was happening was in changing an ex housing they were either closing in on the turbine or opening up the area around the turbine - forcing ex gas's into or allowing some bypass ] ?
Again trying to understand before changes , in order to get peak performance out of each mod [ then keeping in mind to blend all mods-relations to each other ] .
Thanks John
06-17-2007, 11:32 AM
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Sure John - changing turbine housings to adjust the A/R ratio is a common practice to match the 'charger's pressure map to the engine's requirements... smaller A/R normally produces faster spool-up and lower-RPM boost at the expense of higher EGTs and less maximum flow (& HP potential).
The Black Magic really starts when you start trying to choose compressors and turbines based on their actual dimensions and shape instead of relying on the manufacturers' published "trims"!
The Black Magic really starts when you start trying to choose compressors and turbines based on their actual dimensions and shape instead of relying on the manufacturers' published "trims"!
06-17-2007, 11:16 PM
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06-18-2007, 02:03 AM
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Ya, if you go by what they tell you, you're only decision is to trust them-- you can't really measure and do your own analysis.
Which, as most of you know, I MUCH prefer to do-- the analysis thing.
It's honestly enough to just want to go to T-netics, Garrett, or Precision just to spit a little in the eye of these diesel shops that have made tons of money off of internet buzz without having to disclose FACTS.
For example, what is done to an S300 to make a SPS62? How is it different than an HTT ss62? WHY should I choose shop A over shop B?
I'd much rather just be able to point to a real, no-kidding compressor map and figure out how it will work for me.
JMO
Which, as most of you know, I MUCH prefer to do-- the analysis thing.
It's honestly enough to just want to go to T-netics, Garrett, or Precision just to spit a little in the eye of these diesel shops that have made tons of money off of internet buzz without having to disclose FACTS.
For example, what is done to an S300 to make a SPS62? How is it different than an HTT ss62? WHY should I choose shop A over shop B?
I'd much rather just be able to point to a real, no-kidding compressor map and figure out how it will work for me.
JMO
06-18-2007, 09:19 AM
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Honestly HOHN how many people are actually going to study the compressor maps if they are released? Its like any other type of sale, most of the buying population is going to buy what the salesman tells them too. 
. Whenever I start modding my truck I am going to go based on reviews of products I've heard based on there setups and from trucks I have rode in taking in account all the other factors (tire size 2wd vs 4wd etc..). The problem I see is that with all automotive projects, very few ppl have the money to finish a project from the start. So stuff gets changed and setups are mis-matched thus causing poor-spoolup and general unhappiness.
. Whenever I start modding my truck I am going to go based on reviews of products I've heard based on there setups and from trucks I have rode in taking in account all the other factors (tire size 2wd vs 4wd etc..). The problem I see is that with all automotive projects, very few ppl have the money to finish a project from the start. So stuff gets changed and setups are mis-matched thus causing poor-spoolup and general unhappiness.
06-18-2007, 09:23 AM
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It might be a little large, but I think I can get my Garrett to play nice with the over-fueling 12v I have. If my EGT's are right, I like my choice for now. I got a few more things to do to try and get the engine cooperating, then I'm going to a smaller housing on the 4088R to see what happens!Then TWINS!
Chris