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, Wed, 24 Mar 2004 07:14:16
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Compare a lower restriction unit to a higher one:
First, the cooling efficiently of the Abbott Racing (AP) unit is higher.
e=(Ti-Ta)/(Ti-Ta) where Ti = T in, To = T out, Ta = T ambient.
Note:
That the thinner the tubes, the slower the flow and the more time to transfer heat from that flow. So a very restrictive IC could have a high effiency ratio too.
And we also want to have lots of cooling air flowing through the IC. So the fin density that transfers heat to the cooling air should not be maximized. That would increase the temperature rise of the cooling air, which is not something that we care about, and it will restrict the amount of cooling air cross flow. So the fins should be open enough to maintain cross flow.
Back to the comparision-
AP = Abbott Racing
SP = SpeedParts
The AR IC will produce cooler air out of the IC than the SP, everything else been equal, which is not the case. At any given flow rate, (the T7 system does provide for same air mass flows!), the AP will have a higher pressure drop than the SP IC. For the same mass flow, this means that the turbo compressor has to create a higher discharge pressure to get that mass flow throught the more restrictive AR IC. The turbo is thus needing to spin faster and the turbo shaft torque is also greater. This increase in turbo charger shaft HP is not free. The turbine with the AR IC is working harder. To make the turbine work harder: the wastegate is closed a bit more to increase the pressure drop across the turbine. This increases the back pressure in the exhaust manifold which could easily be 30 to 40 PSI!. That back pressure directly robs engine shaft torque, so the engine output has been reduced. So any increase in IC pressure drop directly steal engine torque. And reduction in IC pressure drop increases engine torque. Equal charge air flow rates does not mean equal torque... thats the T7 myth.
Another power robbing effect is that there is now 30-40 PSI of exhaust gases in the combustion chamber after the exhaust valve closed. During the intake stroke, with lets say 15 PSI of boost in the intake manifold, the trapped gases will tend to increase in volume. This reduces the amount of fuel that can be induced into the cylinders and so the volumetric efficiency drops with a restrictive IC. So there are now two power losses associated with the more restrictive IC. Note as well, with more trapped exhaust gases, the AF mixture goes up and the resulting octane balance may start to suck and the T7 will trim timing an perhaps boost (air mass flows).
But there is a third loss issue: the higher pressures out of the compressor to achieve a given mass flow through the restrictive AR IC also move the compressor operation to a less efficient operating state. So the compressor will heat the air more compared to the SP IC.
And fourth: this is on top if the fact that the higher pressures required to get the air through the AR IC increase the air temperature as a direct result.
A Fifth effect for a given air mass flow is that the T7 will attempt to overcome the implied drop in volumetric effiency. So the turbine is now working harder again, and again there is an increase in compressor discharge pressure, and exhaust manifold pressures.
So you can see that there are many penalties to be paid for an increase in the pressure drop across the IC. Some of these are complex and compounding.
Now consider the less restrictive SP IC. It is less efficient in one sense, but it allows greater cooling air cross flow. It has less heat to deal with be because the turbo compessor is working less hard to overcome resistance. The turbo compressor does need to work harder, for a given air mass flow rate, to increase the manifold pressure of the warmer charge air. But the less effienct IC might produce cooler charge air than a more effient unit that makes the compressor work harder! This balance changes with flow rates.
Things change at high HP. High HP implies vey high flow rates. It is at these increasing flow rates that two things are happening. The pressure drop across a given IC increases exponentially with the flow rates. And the turbo starts to run out of capacity in almost any OEM application. The compressor is undersized and starts to be very inefficient at high flows. Temperatures go up, pressures go down and the torque at high RPMs drops off. When the, efficient flow capacity of the compressor, is challenged, a restrictive IC really makes things worse! A less restritive IC will reduce the roll off of torque at high RPMs. This is where the HP is.
So there is a trade off. It is all very complex. But I am certain that an IC like the OEM unit or the AR unit that has many thin tubes that resemble a radiator is not optimal.
Too quote Corky Bell: "The single most important aspect of intercooler design is low internal pressure loss."
Look at Spearco and other performance IC's and they have big IC flow tubes. They have been thoroughly optimized and they do not look like a radiator core.
So are AR units a bad as I think they appear? Could the photo that that they post be an early prototype that does not represent the shipping product?
posted by 65.68.10...
, Wed, 24 Mar 2004 07:14:16
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