This is meant to be a simplified explanation of how Intercoolers impact turbo system operation and clear up some of the confusion around “pressure drop” and turbo lag. As such, I am leaving out the fancy math and a lot of details. Armchair engineers - please refrain from trying to turn this into an overly complex technical discussion that’ll lose the average reader; this is already longer than I wanted it. The drawings and explanations won’t cover every setup and technology. I’m leaving out stuff like octane impacts, meth injection, etc. to keep this discussion focused.
The basics:
Below is a simplified drawing of a turbo motor setup. The general concept is that exhaust spins a turbine, which turns a compressor, which raises flow and pressure going to the engine, but adds heat. The IC cools the charge air to counteract this. As engine RPM rises, the engine consumes more air, resulting in increased mass air flow, but reducing pressure. Counteracting this is more exhaust flow available to the turbine, which lets the compressor push more air and raise pressure. Which effect wins depends on the turbo design, but a good rule of thumb is the following:
-with small turbos, the motor outpaces the turbo as revs increase and the pressure drops as the compressor fails to keep up with the motor (think of the IS20 top end performance, where boost pressure and torque drop off at high rpm)
-with big turbos, the turbo flow outpaces the motor as revs increase and the pressure continues to increase up to redline (check out some BT dyno charts and you’ll see torque still rising near redline)
So how does an IC impact how much power is made?
P2 and T2 in the drawing (pressure and temperature at the engine intake) are what determines how much power can be made. The lower the temperature, the less likely pre-ignition (knock) will be. This allows for more timing advance and leaner air/fuel mix; more efficient power. The second impact is the ratio os P2/T2, which indicates higher mass air flow (I’ll skip the ideal gas law explanation). More air allows burning more fuel and higher engine output (torque). Lower temperature and pressure also allow the compressor to push more air with less resistance (reduced wastegate demand). The GTI / R ECU uses target torque values and takes P2 and T2 into account. If it can make the same power with less pressure due to lower temperature, it will. This is why you may see lower pressure after installing an intercooler and why a tune will help you get the most out of it. If you’re already getting a lot of KR in warm weather due to high intake temps, a better IC will net you more power on the same tune. The drop between T1 and T2 tells you how well your IC is getting rid of heat.
So what is “pressure drop” and why is everyone worried about it?
Any fluid (and yes, air is a fluid) that flows through a pipe, will have pressure loss due to friction as it moves through the pipe. The IC is a heat exchanger, but it is also part of the piping and will create a pressure drop as the air moves through it. This could be measured as the pressure drop between P1 and P2 at a steady flow rate. A high pressure drop is bad because it creates more resistance to flow for the turbo to overcome. This means your turbo will have to work harder or you’ll see reduced power as a result. Most aftermarket bar and plate ICs will not create much, if any, more resistance to flow than the small stock IC and there is no additional piping, so the difference in “pressure drop” should be negligible. In fact, many may flow better and reduce the pressure drop. Most FMICs are smaller than the stock location replacements, but the shape is more favorable for flow, so again, minimal change or maybe even an improvement. The additional piping required will add some flow resistance, but if sized properly, should be more than offset by the improved cooling. It’s nearly impossible to tell exactly how much pressure drop you created or removed when changing an intercooler because of the competing temperature and pressure effects, but you can get a pretty good idea of how it’s working with a good logging software running traces on engine RPM, Charge Air Temp (T1), IAT (T2), Intake Manifold Pressure (P2), and WG demand.
Turbo Lag
One last note - adding volume to the system (larger IC, more piping) can introduce some turbo lag, as the compressor has to fill that extra space with air to raise the system pressure (P2). The turbos in our cars (IS20 and IS38) spool very quickly though and if the flow resistance (“pressure drop”) is improved, it may offset it. Most people have reported not noticing a difference, which seems about right for our platform. It would be more noticeable in a car where you went from a short run, top mount IC to a long run, FMIC. So, you will add turbo lag, but you will not likely notice it.
Hope this helps!
The basics:
Below is a simplified drawing of a turbo motor setup. The general concept is that exhaust spins a turbine, which turns a compressor, which raises flow and pressure going to the engine, but adds heat. The IC cools the charge air to counteract this. As engine RPM rises, the engine consumes more air, resulting in increased mass air flow, but reducing pressure. Counteracting this is more exhaust flow available to the turbine, which lets the compressor push more air and raise pressure. Which effect wins depends on the turbo design, but a good rule of thumb is the following:
-with small turbos, the motor outpaces the turbo as revs increase and the pressure drops as the compressor fails to keep up with the motor (think of the IS20 top end performance, where boost pressure and torque drop off at high rpm)
-with big turbos, the turbo flow outpaces the motor as revs increase and the pressure continues to increase up to redline (check out some BT dyno charts and you’ll see torque still rising near redline)
So how does an IC impact how much power is made?
P2 and T2 in the drawing (pressure and temperature at the engine intake) are what determines how much power can be made. The lower the temperature, the less likely pre-ignition (knock) will be. This allows for more timing advance and leaner air/fuel mix; more efficient power. The second impact is the ratio os P2/T2, which indicates higher mass air flow (I’ll skip the ideal gas law explanation). More air allows burning more fuel and higher engine output (torque). Lower temperature and pressure also allow the compressor to push more air with less resistance (reduced wastegate demand). The GTI / R ECU uses target torque values and takes P2 and T2 into account. If it can make the same power with less pressure due to lower temperature, it will. This is why you may see lower pressure after installing an intercooler and why a tune will help you get the most out of it. If you’re already getting a lot of KR in warm weather due to high intake temps, a better IC will net you more power on the same tune. The drop between T1 and T2 tells you how well your IC is getting rid of heat.
So what is “pressure drop” and why is everyone worried about it?
Any fluid (and yes, air is a fluid) that flows through a pipe, will have pressure loss due to friction as it moves through the pipe. The IC is a heat exchanger, but it is also part of the piping and will create a pressure drop as the air moves through it. This could be measured as the pressure drop between P1 and P2 at a steady flow rate. A high pressure drop is bad because it creates more resistance to flow for the turbo to overcome. This means your turbo will have to work harder or you’ll see reduced power as a result. Most aftermarket bar and plate ICs will not create much, if any, more resistance to flow than the small stock IC and there is no additional piping, so the difference in “pressure drop” should be negligible. In fact, many may flow better and reduce the pressure drop. Most FMICs are smaller than the stock location replacements, but the shape is more favorable for flow, so again, minimal change or maybe even an improvement. The additional piping required will add some flow resistance, but if sized properly, should be more than offset by the improved cooling. It’s nearly impossible to tell exactly how much pressure drop you created or removed when changing an intercooler because of the competing temperature and pressure effects, but you can get a pretty good idea of how it’s working with a good logging software running traces on engine RPM, Charge Air Temp (T1), IAT (T2), Intake Manifold Pressure (P2), and WG demand.
Turbo Lag
One last note - adding volume to the system (larger IC, more piping) can introduce some turbo lag, as the compressor has to fill that extra space with air to raise the system pressure (P2). The turbos in our cars (IS20 and IS38) spool very quickly though and if the flow resistance (“pressure drop”) is improved, it may offset it. Most people have reported not noticing a difference, which seems about right for our platform. It would be more noticeable in a car where you went from a short run, top mount IC to a long run, FMIC. So, you will add turbo lag, but you will not likely notice it.
Hope this helps!
