A
Anonymous
Guest
Hey y'all --
Awhile ago I asked if anyone on the forum knew how to map sub-maximum turbo pressure vs CFM off of a turbo map. A few of you were also interested, but nobody on the forum had much of an idea how to do it. Well, I talked with a couple of automotive turbo engineers with years of application experience, and here's the synopsis of what they said about turbo maps. I also talked with them about intercooler functions, and they are included as well....
Q1: How to calculate from a turbo map how a turbo will respond as RPM and CFM change with shifting?
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This is a long tough calculation and is as much dependent on the turbine as the compressor, which also means it depends on the intake and exhaust system. Intakes and exhausts are not parts of the turbo map, so you already are in trouble.
One would start with the flow needed from the compressor at specific shaft speeds, all on the typical map. Then the estimated power to make that boost and flow, must come from the power of the turbine. This could be estimated by the exhaust gas energy and the percent of total power output available from the engine to drive the turbo -- but you can't just calculate that; you need to measure it on the actual system.
You could come up with some rough rules of thumb to make the calculations, but in the end the error range of those calculations would make the result of little practical use. Remember, though, that once you start playing with it, and develop some boost pressure ratio curves, find a system that'll allow the turbo to stay at maximum boost _after_ the shift. That'll do the trick, and you won't need to worry about the changes..
Q2: What do the 'air flow' stats of different intercoolers really mean?
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You need to find the little print to know; there are no mandated measurement standards. In general, flow numbers for intercoolers indicate the maximum CFM flow at an often UNSTATED back pressure (i.e., pressure loss). Some maufacturers have actual CFM vs pressure loss curves for their ICs (the late 80's early 90's Spearco catalogues published them), but they often do not get into sales literature or on sellers' websites.
To make it more difficult, some manufacturers' list single mzximum CFM numbers at 1 pound pressure loss, some at 2 pounds pressure loss. Some may go back and forth between product lines, making the numbers really tough to compare across manufacturers. Read the fine print. Caveat emptor.
Q3: If you look at the CFM of a 250 at > 7 pounds boost, increasing depth of an IC by one inch (same length/width) will increase capacity by 20%-30% or so but will only add something around 1/50th of a second lag -- not enough to worry about in a street car. So why not go with the bigger IC and get the lowest pressure loss possible?
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Cooling efficiency is not just area and metal. Recent studies suggest that heat transfer increases with turbulence, which in turn increases with air velocity. If this indeed pans out, then there is no value in a larger than necessary intercooler. The efficiency will not necessarily increase, and may well (probably will) decrease due to the lower velocities.
Efficiency is further complicated because density increases under boost, while velocity does not necessarily increase. Pressure loss increases fractionally with density, but increases as a power function of velocity -- meaning that velocity is the primary driver of both back pressure (bad) and efficiency (good).
The crux of the matter is what velocity to use. It is likely that in the next few months data will be generated using this more sophisticated understanding of IC design and function. It may well point to practical ways to improve IC efficiencies from the industry standard of 70-75% now to the upper 80% values. Yes, those will carry higher back pressure, but there are ways to deal with that as well -- it all will depend on the trade-offs.
Keep tuned in for the big changes that are coming.
--------------------------------------------------------------------------
Hope that helps the group!
--- Barrett
Awhile ago I asked if anyone on the forum knew how to map sub-maximum turbo pressure vs CFM off of a turbo map. A few of you were also interested, but nobody on the forum had much of an idea how to do it. Well, I talked with a couple of automotive turbo engineers with years of application experience, and here's the synopsis of what they said about turbo maps. I also talked with them about intercooler functions, and they are included as well....
Q1: How to calculate from a turbo map how a turbo will respond as RPM and CFM change with shifting?
----------------------------------------------------------------------------------------------------
This is a long tough calculation and is as much dependent on the turbine as the compressor, which also means it depends on the intake and exhaust system. Intakes and exhausts are not parts of the turbo map, so you already are in trouble.
One would start with the flow needed from the compressor at specific shaft speeds, all on the typical map. Then the estimated power to make that boost and flow, must come from the power of the turbine. This could be estimated by the exhaust gas energy and the percent of total power output available from the engine to drive the turbo -- but you can't just calculate that; you need to measure it on the actual system.
You could come up with some rough rules of thumb to make the calculations, but in the end the error range of those calculations would make the result of little practical use. Remember, though, that once you start playing with it, and develop some boost pressure ratio curves, find a system that'll allow the turbo to stay at maximum boost _after_ the shift. That'll do the trick, and you won't need to worry about the changes..
Q2: What do the 'air flow' stats of different intercoolers really mean?
---------------------------------------------------------------------------------
You need to find the little print to know; there are no mandated measurement standards. In general, flow numbers for intercoolers indicate the maximum CFM flow at an often UNSTATED back pressure (i.e., pressure loss). Some maufacturers have actual CFM vs pressure loss curves for their ICs (the late 80's early 90's Spearco catalogues published them), but they often do not get into sales literature or on sellers' websites.
To make it more difficult, some manufacturers' list single mzximum CFM numbers at 1 pound pressure loss, some at 2 pounds pressure loss. Some may go back and forth between product lines, making the numbers really tough to compare across manufacturers. Read the fine print. Caveat emptor.
Q3: If you look at the CFM of a 250 at > 7 pounds boost, increasing depth of an IC by one inch (same length/width) will increase capacity by 20%-30% or so but will only add something around 1/50th of a second lag -- not enough to worry about in a street car. So why not go with the bigger IC and get the lowest pressure loss possible?
-----------------------------------------------------------------------
Cooling efficiency is not just area and metal. Recent studies suggest that heat transfer increases with turbulence, which in turn increases with air velocity. If this indeed pans out, then there is no value in a larger than necessary intercooler. The efficiency will not necessarily increase, and may well (probably will) decrease due to the lower velocities.
Efficiency is further complicated because density increases under boost, while velocity does not necessarily increase. Pressure loss increases fractionally with density, but increases as a power function of velocity -- meaning that velocity is the primary driver of both back pressure (bad) and efficiency (good).
The crux of the matter is what velocity to use. It is likely that in the next few months data will be generated using this more sophisticated understanding of IC design and function. It may well point to practical ways to improve IC efficiencies from the industry standard of 70-75% now to the upper 80% values. Yes, those will carry higher back pressure, but there are ways to deal with that as well -- it all will depend on the trade-offs.
Keep tuned in for the big changes that are coming.
--------------------------------------------------------------------------
Hope that helps the group!
--- Barrett
