Does Boost Blow the motor?
I would agree that larger turbos definitely flow more air and part of that equation is definitely the size of the compressor wheel. This is why at higher rpms stock turbos fall off so bad where larger turbos can keep the boost up. Higher rpms = more potential air flow to the engine.
There are two ways to increase air flow on a compressor leaving the turbine out of the equation:
1) Increase the speed of the compressor
2) Increase the size of the compressor
Now could a stock turbo theoretically flow the same air? Sure, but it would have to spin so much faster it would be way out of it's efficiency.
My damn 35r is starting to behave like a stock turbo, hitting 33psi, bleeding down to 28psi. Still trying to figure out the culprit, but it might just be an old bearing (75k).
As far as running so much boost you blow up your engine, boost is only one variable. Size of the turbo, timing, a/f ratio, engine compression are all other important variables you have to fill in the values for as well.
Last edited by fre; Feb 12, 2010 at 02:52 PM.
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As I said guys, I might have been wrong about the notion of a "nominal" difference. But I still would like to see a 35R blow away an exact same setup car with a stock turbo boost for boost. A 35r peaking at 28 psi and dropping to 22 psi I still think is not gonna be all that hot. Yeah sure we have some dyno sheets of cars making 6000 whp at 20 psi... but somehow I don't believe it.
That said, I think my theory is correct. JohnBradley, I read your post, and agree with everything you said, minus two points which are exactly about what we are debating.
1. Your point that exhaust back pressure does not effect VE. How can you say this when it is often talked about how N/A engines can achieve over 100% VE because of perfect header design? I think that tackles that point.
2. That a stock turbo at 22 psi is out of "flow"... meaning it cannot supply enough mass air to the engine. If this was the case, how is there 22 psi of resistance measure at the intake manifold?
Bottom line, I haven't heard anyone explain to me why a bigger turbo makes more power besides less exhaust back pressure and a more efficient compressor. (less heated charge air, and can run higher boost pressures efficiently).
I'm open to learn but I don't feel anyone is addressing my question or incorrect understanding.
That said, I think my theory is correct. JohnBradley, I read your post, and agree with everything you said, minus two points which are exactly about what we are debating.
1. Your point that exhaust back pressure does not effect VE. How can you say this when it is often talked about how N/A engines can achieve over 100% VE because of perfect header design? I think that tackles that point.
2. That a stock turbo at 22 psi is out of "flow"... meaning it cannot supply enough mass air to the engine. If this was the case, how is there 22 psi of resistance measure at the intake manifold?
Bottom line, I haven't heard anyone explain to me why a bigger turbo makes more power besides less exhaust back pressure and a more efficient compressor. (less heated charge air, and can run higher boost pressures efficiently).
I'm open to learn but I don't feel anyone is addressing my question or incorrect understanding.
As I said guys, I might have been wrong about the notion of a "nominal" difference. But I still would like to see a 35R blow away an exact same setup car with a stock turbo boost for boost. A 35r peaking at 28 psi and dropping to 22 psi I still think is not gonna be all that hot. Yeah sure we have some dyno sheets of cars making 6000 whp at 20 psi... but somehow I don't believe it.
That said, I think my theory is correct. JohnBradley, I read your post, and agree with everything you said, minus two points which are exactly about what we are debating.
1. Your point that exhaust back pressure does not effect VE. How can you say this when it is often talked about how N/A engines can achieve over 100% VE because of perfect header design? I think that tackles that point.
2. That a stock turbo at 22 psi is out of "flow"... meaning it cannot supply enough mass air to the engine. If this was the case, how is there 22 psi of resistance measure at the intake manifold?
Bottom line, I haven't heard anyone explain to me why a bigger turbo makes more power besides less exhaust back pressure and a more efficient compressor. (less heated charge air, and can run higher boost pressures efficiently).
I'm open to learn but I don't feel anyone is addressing my question or incorrect understanding.
That said, I think my theory is correct. JohnBradley, I read your post, and agree with everything you said, minus two points which are exactly about what we are debating.
1. Your point that exhaust back pressure does not effect VE. How can you say this when it is often talked about how N/A engines can achieve over 100% VE because of perfect header design? I think that tackles that point.
2. That a stock turbo at 22 psi is out of "flow"... meaning it cannot supply enough mass air to the engine. If this was the case, how is there 22 psi of resistance measure at the intake manifold?
Bottom line, I haven't heard anyone explain to me why a bigger turbo makes more power besides less exhaust back pressure and a more efficient compressor. (less heated charge air, and can run higher boost pressures efficiently).
I'm open to learn but I don't feel anyone is addressing my question or incorrect understanding.
a ballon has "boost pressure" but doesnt flow.
a hurricane has pressure AND flow.
Turbo cars have back pressure inherently though it would be cool if we didnt. The ideal ratio is 1:1 or less and no more than 1.5:1 (this exhaust:boost). This will lower EGT and allow better cylinder filling with less contamination. Its what we do after the turbo that is important. Again, ideal is ZERO so as little exhaust bottleneck as possible. Either way huge or barely non-existent.
2. That a stock turbo at 22 psi is out of "flow"... meaning it cannot supply enough mass air to the engine. If this was the case, how is there 22 psi of resistance measure at the intake manifold?
Bottom line, I haven't heard anyone explain to me why a bigger turbo makes more power besides less exhaust back pressure and a more efficient compressor. (less heated charge air, and can run higher boost pressures efficiently).
Simple fact is I do not have a boost controller capable of doing what you are asking nor do I intend to get one. Math however is math, repeatable results for torque and power at one level can be used versus any other turbo.
i.e. turbo A on combo A = X power vs boost, turbo B on combo A = Y power vs boost
I'm open to learn but I don't feel anyone is addressing my question or incorrect understanding.
All motor (NA) cars want ZERO backpressure. They use tuned length headers for what is called Helmholtz frequency tuning. What it does it accelerate pulses leaving the cylinder which in turn speed up intake pulses and achieve cylinder filling in excess of what would be capable on an otherwise "stock" exhaust. Looking at an F1 car its oobvious they dont have anything other than the header
Turbo cars have back pressure inherently though it would be cool if we didnt. The ideal ratio is 1:1 or less and no more than 1.5:1 (this exhaust:boost). This will lower EGT and allow better cylinder filling with less contamination. Its what we do after the turbo that is important. Again, ideal is ZERO so as little exhaust bottleneck as possible. Either way huge or barely non-existent.
This is what I was talking about earlier. Dont think of the engine as anything other than boost assisted all motor. The engine has the inherent VE and can only process so much air. What happens on a small turbine is the backpressure inside the turbine has also exceeded the aforementioned ratio and is starting to drop the ENGINEs VE. A larger turbine helps, but as I pointed out we have tested this and leaving the same compressor and increasing the flow through the hotside is not the whole answer. The Compressor wheel is not only more efficient thermally it can physically move more mass of air. I spose that while I am loathe to use the term in this context we can say "volume". Not to be confused with VE which its ability to make that volume.
If I had a sophisticated enough boost controller I can simulate exactly what you are describing but I can paint the picture if you like. My car at 28psi makes 622whp and 520 something ft lbs. At 23psi (the lowest it will run) it makes 570/446. If I simulated your proposed boost plot I would see the torque peak from the high boost and drop to the power level I make at low boost. Still straight murdering a stock turbo at the same boost curve. This is a matter of semantics. I have run my car at pretty much every boost level a stock turbo does on pumpgas and can interpolate the numbers anyway you want me to.
Simple fact is I do not have a boost controller capable of doing what you are asking nor do I intend to get one. Math however is math, repeatable results for torque and power at one level can be used versus any other turbo.
i.e. turbo A on combo A = X power vs boost, turbo B on combo A = Y power vs boost
I think some of this is difference in terms. Like my grandpa used to say we are agreeing in radically different ways. The mental image that we have respectively is probably the same but putting thought to type is not.
Turbo cars have back pressure inherently though it would be cool if we didnt. The ideal ratio is 1:1 or less and no more than 1.5:1 (this exhaust:boost). This will lower EGT and allow better cylinder filling with less contamination. Its what we do after the turbo that is important. Again, ideal is ZERO so as little exhaust bottleneck as possible. Either way huge or barely non-existent.
This is what I was talking about earlier. Dont think of the engine as anything other than boost assisted all motor. The engine has the inherent VE and can only process so much air. What happens on a small turbine is the backpressure inside the turbine has also exceeded the aforementioned ratio and is starting to drop the ENGINEs VE. A larger turbine helps, but as I pointed out we have tested this and leaving the same compressor and increasing the flow through the hotside is not the whole answer. The Compressor wheel is not only more efficient thermally it can physically move more mass of air. I spose that while I am loathe to use the term in this context we can say "volume". Not to be confused with VE which its ability to make that volume.
If I had a sophisticated enough boost controller I can simulate exactly what you are describing but I can paint the picture if you like. My car at 28psi makes 622whp and 520 something ft lbs. At 23psi (the lowest it will run) it makes 570/446. If I simulated your proposed boost plot I would see the torque peak from the high boost and drop to the power level I make at low boost. Still straight murdering a stock turbo at the same boost curve. This is a matter of semantics. I have run my car at pretty much every boost level a stock turbo does on pumpgas and can interpolate the numbers anyway you want me to.
Simple fact is I do not have a boost controller capable of doing what you are asking nor do I intend to get one. Math however is math, repeatable results for torque and power at one level can be used versus any other turbo.
i.e. turbo A on combo A = X power vs boost, turbo B on combo A = Y power vs boost
I think some of this is difference in terms. Like my grandpa used to say we are agreeing in radically different ways. The mental image that we have respectively is probably the same but putting thought to type is not.
JB > Much smarter than Mike @ AWD..
I must dumb it down so i can understand myself..
http://en.wikipedia.org/wiki/Superch...f_supercharger
Okay I am posting this link for one reason. It gets away from what we are used to using as terms (Turbo, CFM, VE, etc.) and completely explains with math my post prior to #50. We are not concerned with how the forced induction pressure is generated or at what temperature. I could build a turbine powered roots supercharger if I was so inclined but alas I am not. CID/rev is the same concept as going from a turbo that moves 42 lbs/min to one that is 100 lbs/min. I know it is not the same on paper, but it is most certainly the same exact outcome.
Okay I am posting this link for one reason. It gets away from what we are used to using as terms (Turbo, CFM, VE, etc.) and completely explains with math my post prior to #50. We are not concerned with how the forced induction pressure is generated or at what temperature. I could build a turbine powered roots supercharger if I was so inclined but alas I am not. CID/rev is the same concept as going from a turbo that moves 42 lbs/min to one that is 100 lbs/min. I know it is not the same on paper, but it is most certainly the same exact outcome.
Mike, I am not smarter than you, I am just smart differently 
I know that when I get down your way I want to hang, race, and party Hopefully I am not too smart we cant do that...LOL
I know that when I get down your way I want to hang, race, and party
Hopefully I am not too smart we cant do that...LOL
EvoRed is a 60-1 or equivalent
EvoBlack is a 3582HTA or equivalent
My feeling, and I am not alone, is that a turbo needs to be thought of in lbs/min. The old rule of thumb was 10whp per lb/min on a dynojet. I dont think of boost or CFM, I think of what it should max out at on good fuel and go from there. So if I make the power I want at 35psi, awesome. If it takes me 45-46 then well thats time for a bigger turbo...
Stock- 42-44 top out 425-439whp (typical build)
Red-54-56 lbs per minute seem to max out 546-565 (on normal builds)
Black- 60-65 seem to max out around 620 or so.
The exceptions would be cars like Lucas' that made 675 because of fuel and compression and hidden tricks, and the 691 that Mike did because of how that combo is engineered. I believe 9sec9 did 680 or something but I have heard various explanations having to do with fuel in that. I am sure it has more or less the right combo for maxxing it out though.
EvoBlack is a 3582HTA or equivalent
My feeling, and I am not alone, is that a turbo needs to be thought of in lbs/min. The old rule of thumb was 10whp per lb/min on a dynojet. I dont think of boost or CFM, I think of what it should max out at on good fuel and go from there. So if I make the power I want at 35psi, awesome. If it takes me 45-46 then well thats time for a bigger turbo...
Stock- 42-44 top out 425-439whp (typical build)
Red-54-56 lbs per minute seem to max out 546-565 (on normal builds)
Black- 60-65 seem to max out around 620 or so.
The exceptions would be cars like Lucas' that made 675 because of fuel and compression and hidden tricks, and the 691 that Mike did because of how that combo is engineered. I believe 9sec9 did 680 or something but I have heard various explanations having to do with fuel in that. I am sure it has more or less the right combo for maxxing it out though.
Lbs/min is compressor. If the turbo is/was engineered right everything else should match it to begin with. The things that will vary are spool which means overall spread of the powerband. The turbo will still make the same power ultimately but it might require a little more to get it there. I am sure the rest of it does matter more than I am seeming to give it credit. I dont worry about it and size off the compressor by itself and where I want the car is only turbine and I generally get a car where I want it. E85 helps us cheat though as the mass of exhaust is denser and needs less room to flow. It typically means that we dont get penalised as fast like you would on a car running gas as far as backpressure ratio.
