Does Boost Blow the motor?
Originally Posted by crcain
Great points and I totally agree. I think where people took exception, and I guess where I was totally off base, is I thought at a boost pressure of about 14 psi, the stock turbo would be plenty efficient, and not adding to increased charge temps much more than a 35r. But I think I've underestimated the power increases possible when you do the two things a larger turbo does. I do understand the ideal gas law, and do know for every degree increase in charge temp, you get a decrease in density.
I'm starting to think that, where I've gone wrong, is in a forced induction application, relieve back pressure can result in massive gains. Which is why there have been claims in this thread of making like 500+ hp on at less than 20 psi. I don't know.
I'm starting to think that, where I've gone wrong, is in a forced induction application, relieve back pressure can result in massive gains. Which is why there have been claims in this thread of making like 500+ hp on at less than 20 psi. I don't know.
Scorke
Scorke
I think where you got confused was that a turbo whether it be stock or larger is good for a certain amount of air flow or CFM (cubic feet per minute) and that can be reached at different pressures. You can max out a stock turbo at 10psi if you have an engine large enough to process it's max air flow at that pressure. This is why V8s seem to make so much power at such low boost pressures, they just have the capability to process more air.
Last edited by fre; Feb 14, 2010 at 09:01 PM.
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You have the right idea. You just have to be careful when you assume all engines are created equal. A built engine can process quite a bit more air than a stocker. I have to shove 33+psi into my stock block with hks 272s to reach my 131 trap speed, where a built up engine could probably do it with 25psi (of course assuming the same gt35r is used).
the turbo on my 8v VW at 24 psi that gets me 320 whp
Turbonetics t3/to4b "S" trim .63 stage 2 hot side
would get me 100 more hp on an evo engine at the same pressure....as the VW head flows NOTHING
Now back to the OP.....
the tune beiong good and not melting things....
stock rods dont like higher than designed engine speeds...rod cap bolts streach and break...or the rod will snap under the piston
and tq will break them in half as well...this differs from one engine to another
a 1.8t 20v VW engine will break rods at 350 WTQ....an older 8v or 16v engine will stay together to 475 whp or so.....not exceding foctory rpm limits mind you
If this is a matter of word games I apologise. I feel I have adequately explained the physics involved, used real world examples, and even the math in one case. Backpressure between two turbochargers is not the reason a GT35R makes more power than a stock turbo. I guess the easiest way to visualise this is why does a 2.0l vs a 4.0l always make less power if everything else is the same? Displacement. The larger motor MOVES more air, just as the larger compressor MOVES more air. You seem to be coming at the problem from the wrong end.
VE does not apply to anything but the engine, UNLESS you are using it to express the relationship between air mass ingested by the turbo and airmass compressed for engine use. Even then that is not the term used. This is where thermal efficiency and lbs/min are the correct and industry accepted terms.
Last edited by JohnBradley; Feb 14, 2010 at 10:32 PM.
No it doesnt, thats what I have been trying to say. I can have the same backpressure amount and still make more power. AIR MASS as expressed in lbs/min is independent of backpressure. Lbs/min is directly related to compressor size and backpressure only comes into play when you are at the far right of the compressor map looking for every last bit out of a motor (more or less).
If this is a matter of word games I apologise. I feel I have adequately explained the physics involved, used real world examples, and even the math in one case. Backpressure between two turbochargers is not the reason a GT35R makes more power than a stock turbo. I guess the easiest way to visualise this is why does a 2.0l vs a 4.0l always make less power if everything else is the same? Displacement. The larger motor MOVES more air, just as the larger compressor MOVES more air. You seem to be coming at the problem from the wrong end.
VE does not apply to anything but the engine, UNLESS you are using it to express the relationship between air mass ingested by the turbo and airmass compressed for engine use. Even then that is not the term used. This is where thermal efficiency and lbs/min are the correct and industry accepted terms.
If this is a matter of word games I apologise. I feel I have adequately explained the physics involved, used real world examples, and even the math in one case. Backpressure between two turbochargers is not the reason a GT35R makes more power than a stock turbo. I guess the easiest way to visualise this is why does a 2.0l vs a 4.0l always make less power if everything else is the same? Displacement. The larger motor MOVES more air, just as the larger compressor MOVES more air. You seem to be coming at the problem from the wrong end.
VE does not apply to anything but the engine, UNLESS you are using it to express the relationship between air mass ingested by the turbo and airmass compressed for engine use. Even then that is not the term used. This is where thermal efficiency and lbs/min are the correct and industry accepted terms.
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Well guys, I know this debate has been taxing. 
But I guess we'll have to agree to disagree.
IMO, an engine is an air pump, and if you have back pressure on one side, and boost pressure on the other side, and IAT is fixed, it doesn't matter if you have a 90mm compressor or a 50mm compressor, both air pumps will pump the same amount (lb/min) of air (provided the compressors are in their effciency and flow range).
The straw analogy makes no sense because IC, TB, plenum, and port size is constant.
What is this mystery reason beyond back pressure and IAT that makes more power? You keep saying mass air, but how does a pump flow more air, if boost is the same, and the pump is unchanged, and charge density is the same?
But I guess we'll have to agree to disagree. IMO, an engine is an air pump, and if you have back pressure on one side, and boost pressure on the other side, and IAT is fixed, it doesn't matter if you have a 90mm compressor or a 50mm compressor, both air pumps will pump the same amount (lb/min) of air (provided the compressors are in their effciency and flow range).
The straw analogy makes no sense because IC, TB, plenum, and port size is constant.
What is this mystery reason beyond back pressure and IAT that makes more power? You keep saying mass air, but how does a pump flow more air, if boost is the same, and the pump is unchanged, and charge density is the same?
You have the right idea. You just have to be careful when you assume all engines are created equal. A built engine can process quite a bit more air than a stocker. I have to shove 33+psi into my stock block with hks 272s to reach my 131 trap speed, where a built up engine could probably do it with 25psi (of course assuming the same gt35r is used).
I think where you got confused was that a turbo whether it be stock or larger is good for a certain amount of air flow or CFM (cubic feet per minute) and that can be reached at different pressures. You can max out a stock turbo at 10psi if you have an engine large enough to process it's max air flow at that pressure. This is why V8s seem to make so much power at such low boost pressures, they just have the capability to process more air.
I think where you got confused was that a turbo whether it be stock or larger is good for a certain amount of air flow or CFM (cubic feet per minute) and that can be reached at different pressures. You can max out a stock turbo at 10psi if you have an engine large enough to process it's max air flow at that pressure. This is why V8s seem to make so much power at such low boost pressures, they just have the capability to process more air.
and I don't remember reading in earlier posts, but there is another inherent cause that enables the larger compressor to produce more power with all other variables being constant. It has everything to do with flow. If you can move that air without adding as much heat, you automaticaly increase mass flow. Since cooler air is denser, you get more of it in at the same PR. This is part of the reason big turbos can move a lot of air at low boost, in addition to having a good flowing setup to go with it.
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and I don't remember reading in earlier posts, but there is another inherent cause that enables the larger compressor to produce more power with all other variables being constant. It has everything to do with flow. If you can move that air without adding as much heat, you automaticaly increase mass flow. Since cooler air is denser, you get more of it in at the same PR. This is part of the reason big turbos can move a lot of air at low boost, in addition to having a good flowing setup to go with it.
What is in question, is whether there is another factor.
Again, this is all considering both turbos are operating within their efficiency range. Not too much lb/min and not too much pressure ratio.
A turbo has limitations both moving to the right of the compressor map (mass airflow), and moving upward (pressure ratio). Some turbos are more efficient at moving less volume at higher pressures (e.g. T67), others are more efficient at moving greater volumes at lower pressures (e.g. 60-1).
If we manipulate the turbo to increase pressure in the line, we increase the rate of mass airflow by increasing the pressure.
If we manipulate the turbo to maintain a constant pressure in the line (as per common boost control methods), and we increase the size of the orifice, we increase the rate of mass airflow without increasing the pressure. This increases the load (torque) without any change at the boost guage. This is what would happen if we could bolt a Honda F20C head onto our block.
Anytime we increase mass airflow either by increasing the pressure or increasing the size of the orifice (VE), we place additional demands on the compressor, and we tend to increase backpressure, which if inadequately controlled, reduces the pressure differential across the orifice, which reduces mass airflow (VE).
This hypothetical situation assumes our IC is 100% efficient, so IAT is not a factor.
but working within the efficiency range will obviously vary depending on multipe aspects that would most likely be independent of each other. You cant really compare two unlike turbos. Can a graph/equation calculate the CFM according to actual size and dimensions, sure, but to what degree of accuracy? 10-20%. I think that has been proven numerous times on numerous platforms. Plus, I thought your arguement had fixed ratios and dimensions minus compressors?? Anyway, I hope you find your answers. And there was definitely a lot of good info in this thread.
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A turbo 'sees' an engine is an air restriction - like an air line with a boost pill in it. The size of the orifice in the 'pill' is the mechanical VE of the engine, which as you know, lies in the head/cams, all else outside the engine being equal.
A turbo has limitations both moving to the right of the compressor map (mass airflow), and moving upward (pressure ratio). Some turbos are more efficient at moving less volume at higher pressures (e.g. T67), others are more efficient at moving greater volumes at lower pressures (e.g. 60-1).
If we manipulate the turbo to increase pressure in the line, we increase the rate of mass airflow by increasing the pressure.
If we manipulate the turbo to maintain a constant pressure in the line (as per common boost control methods), and we increase the size of the orifice, we increase the rate of mass airflow without increasing the pressure. This increases the load (torque) without any change at the boost guage. This is what would happen if we could bolt a Honda F20C head onto our block.
Anytime we increase mass airflow either by increasing the pressure or increasing the size of the orifice (VE), we place additional demands on the compressor, and we tend to increase backpressure, which if inadequately controlled, reduces the pressure differential across the orifice, which reduces mass airflow (VE).
This hypothetical situation assumes our IC is 100% efficient, so IAT is not a factor.
A turbo has limitations both moving to the right of the compressor map (mass airflow), and moving upward (pressure ratio). Some turbos are more efficient at moving less volume at higher pressures (e.g. T67), others are more efficient at moving greater volumes at lower pressures (e.g. 60-1).
If we manipulate the turbo to increase pressure in the line, we increase the rate of mass airflow by increasing the pressure.
If we manipulate the turbo to maintain a constant pressure in the line (as per common boost control methods), and we increase the size of the orifice, we increase the rate of mass airflow without increasing the pressure. This increases the load (torque) without any change at the boost guage. This is what would happen if we could bolt a Honda F20C head onto our block.
Anytime we increase mass airflow either by increasing the pressure or increasing the size of the orifice (VE), we place additional demands on the compressor, and we tend to increase backpressure, which if inadequately controlled, reduces the pressure differential across the orifice, which reduces mass airflow (VE).
This hypothetical situation assumes our IC is 100% efficient, so IAT is not a factor.
However, I do understand the fundamentals of what you describe, although you did enlighten me a bit with the notion that back pressure rises as a smaller turbo has to work harder to maintain boost / mass air flow for a high VE engine. I always thought of it as a small or large turbine housing choking the exhaust energy. But I can see you need to consider turbine speed and resistance I guess as well when thinking about back pressure.
With that said, do you agree that with two turbos both in their efficiency range, the only reason the larger turbo makes more power is because of a reduction in back pressure and cooler IATs?