This is a basic outline to help build an effective intercooling set up. Most of the data is found in the book Maximum boost by Corky Bell, so think of this as a cliff notes in a forum friendly format. Maximum boost is a must own book for anyone who wants to understand the aspects of their turbo set up and how to build it better and covers these things in much more detail. [ISBN# 978-0-8376-0160-1]
Factors helpful in choosing an effective intercooler and setting up an equally effective supporting system (intercooler piping, air ducting, ect).
- Intercooler efficiency.
- Approximate boost lag time.
- Gain in density of intake charge.
- Boost pressure loss.
- Supporting systems.
Calculating the efficiency of an intercooler
The idea here is to compare the amount of heat removed by the intercooler. The temperature rise through the compressor is compressor outlet temperature (Tco) minus the ambient temperature (Ta).
Temperature rise = Tco - Ta
Heat removed by the intercooler is the Temperature difference between the air exiting the compressor (Tco) and the air exiting the intercooler (Tio).
Temperature removed = Tco - Tio
Intercooler efficiency (Ei) is then the temperature removed divided by the temperature rise.
Ei =(Tco - Tio)/(Tco - Ta)
Example: Let Ta = 80°F, Tco = 250°F and Tio = 110°F
Ei =(250-110)/(250-80) = 0.824 = 82.4%
Intercooler efficiency = 82.4%
Approximate boost lag time
Judgment of the volumes relationship to lag can be made by dividing the internal volume by the flow rate through the system at the RPM at which throttle is applied and multiplied by 2. The factor of 2 results from the approximate doubling of air flow through the system when going from cruise to boost.
Approximate lag time given by:
Time = Volume/(Flow Rate) x 2
Example: Let volume of intake = 500 cu in and flow rate = 150 cfm at a cruise speed of approximately 2,000 RPM.
Time = (500 〖in〗^3)/(150 〖ft〗^3/min) x (60 sec/min)/(1728 〖in〗^3/〖ft〗^3 ) x 2 = 0.23 sec
Boost lag time = 0.23 sec
Gain in density of intake charge
Suppose a turbo has a compressor discharge temperature of 200°F above atmospheric temperature. That is about 740°F on an 80°F day. Zero degrees absolute is about 460°F; add 80°F to get 540°F; 200°F above that temperature is therefore, 740°F absolute. If we insert a 60% efficient intercooler into the system, we would remove 0.6 x 200°F = 120°F from the system, leaving a gain of just 80°F rather than 200°F or an absolute of 540°F + 80°F = 620°F. The density change can then be determined by the ratio of the original absolute temperature to the final absolute temperature.
Density change = (origonal absolute temperature)/(final absolute temperature) – 1
As per the example temperatures above:
Density change = (540+200)/(540+80) – 1 = 0.19 = 19%
Therefore this intercooler will yield a gain of about 19%. All the other things remaining equal, one would expect a similar gain in power. This unfortunately doesn't come about, because of pressure losses caused by the aerodynamic drag inside the intercooler.
Boost pressure loss
The corresponding power loss due to boost pressure loss can be estimated by calculating the ratio of absolute pressure with the intercooler to that without the intercooler and subtracting from 100%.
Example: If 2psi out of 10psi are lost due to intercooler drag
Power loss = 1 - (14.7+8)/(14.7+10 ) = 0.08 = 8%
This indicates the flow losses through the intercooler amount to 8%
(which makes your 19% gain above an actual 11% gain)
Supporting systems
This is just a brief touch on supporting systems/factors (might go into these more in depth later). Other things to consider when setting up your intercooler are things such as intercooler location. Outside the engine bay and in front of the radiator is always best. Mounting your intercooler behind the radiator will add ~40°F to the air temperature. Intercooler air ducting can show improvement up to ~20%. Duct should be at least ¼ the frontal area of the intercooler and seal all edges, corners and joints of your ducting so you give the air nowhere to go but threw the intercooler. Intercooler tubing is also an important note; the space availability in the engine bay can make it difficult to choose effectively routed tubing. Some things to note when running tubing: Bigger isn't always better, 2.5in tubing can flow 600cfm without unreasonable drag. Larger tubes add volume to the intercooler system. The more smooth tubes (no couplers) and gentle bends the better.
Other small notes:
- Air flow meter should be as close to the throttle body as possible to lessen the time it takes for the low pressure pulses from opening the throttle to reach the air flow meter and register a response for new load conditions. The farther away it is the more it will affect your throttle response.
- Increasing intercooler core thickness does not proportionally increase heat transfer capability. The thicker the core is the harder it is to get the ambient air to pass through the core and increases the drag coefficient of the intercooler.
- Turning up the boost is not a fix for boost pressure loss threw an inefficient intercooler. If boost is increased so will power, but so is your heat which will reduce the charge density. "Removing lost power by turning up the boost is in part an exercise in chasing ones tail. " a more efficient intercooler is a far more effective solution.
- Reducer couplers should not exceed a cone angle of 15° or they will cause disruption of the air boundary layer, increasing drag.
- Every time air flow must turn 90° a 1% of flow loss will occur (three 30° bends are equal to 90° and will equal a 1% flow loss)
Oct 26, 2013, 4:01 pm · #1