Intercoolers: Are They Worth It?

Harold Asks:

Thank you for a very informative article on turbocharging ("How to Buy Your First Turbo," May '12). Most of your technical articles are of great value, and I try to digest the information with the hope that I can apply it in some way, somehow, some day. My question is what is the importance of, and how do you size, the intercooler? I have read ads for intercooler upgrades that suggest there is more power to be had just by upgrading to a larger unit. How would I know what size to apply to various turbos?

Marlan Answers:

Because a turbocharger is fed by engine exhaust, it's best to mount the turbine (hot side) as close to the engine's exhaust manifold as possible. This allows the maximum amount of exhaust heat to enter the turbine housing, and the expansion of the hot exhaust gases helps provide additional turbine rotational impetus.

The problem is some of this turbine heat inevitably transfers to the compressor (induction or cold side). Inlet charge-air heating is not a big deal at low boost pressures (say, 5 to 7 psi). Consider 7 to 9 psi as the crossover region. But when boost hits 10 psi, charge-air heating becomes a serious problem; at this point, the charge-air requires cooling to maintain optimum system efficiency. Getting the air temperature down raises the air density in the combustion chamber, offering the potential to greatly increase engine power output because more air and fuel can be squeezed into the same volume of space. Also, a cooler charge in the combustion chamber reduces the risk of detonation—otherwise, all the hot air would require backing off the ignition lead, further reducing power.

Intercoolers may use either ambient air or liquid coolant to reduce charge-air temperature. Assuming equivalent efficiency levels, an air-to-air cooler's surface area must be much larger than that of a liquid exchanger's. But because a liquid coolant like ice water has a heat transfer coefficient into aluminum that's up to 14 times greater than air into aluminum, real-world packaging constraints preclude most air-to-air installations from approaching a liquid 'cooler's efficiency level in actual service. On the other hand, ice melts, so liquid coolers are only really effective in drag racing, land-speed, or marine use. For road racing or on the street, air-to-air designs remain more practical, assuming there's enough space to mount a sufficiently large unit. Although not the first choice for ultimate performance, in some circumstances space-constrained street cars may have to settle for a liquid cooler, with the coolant medium circulated by an electric pump through an auxiliary radiator. This compromise solution is often seen on space-constrained, late-model stockers such as the ZL1 Camaro, ZR1 Corvette, or Cadillac CTS-V.

The intercooler must be large enough to achieve a significant temperature drop while minimizing any pressure drop. Typically, turbo-system engineers try to shoot for 70-percent-or-greater intercooler efficiency and no more than a 1.0 psi boost pressure drop through the unit, although on a street car, packaging constraints may force you to accept up to a 2.5-psi pressure drop and efficiencies as low as 60 percent. There are those who claim, that within reason, pressure drop isn't a serious problem: You can simply compensate by adjusting the wastegate to increase boost. Wrong! If the wastegate is adjusted to raise boost more than about 1 psi to compensate for pressure losses, it produces a slight increase on exhaust-side turbine pressure, which transfers more heat into the compressor, further raising the temperature of the air going into the intercooler and in turn reducing package efficiency. You can chase that tail like a puppy, but you'll never catch it.

How does intercooler efficiency translate into power potential? As an example, let's take an intercooler that's 70 percent efficient for the application. Turbo engineers like to use absolute temperature numbers rather than degrees F or degrees C. Zero degrees absolute equals about 460 degrees F. If the turbo's compressor discharge temperature was 300 degrees F and the ambient temperature of the coolant medium (atmospheric air or liquid) was 70 degrees F, the absolute temperature is 830 degrees:

460 + 70 + 300 = 830

A 70-percent-efficient intercooler would lower the compressor discharge temperature by 210 degrees:

0.7 x 300 = 210

That reduces the system's total temperature gain to only 90 degrees, for a final absolute temperature of:

460 + 70 + (300 210) = 620

The density change (in percent) is then:

Density change = [Original absolute temp. / Final absolute temp.] — 1

= [830 / 620] — 1 = 0.339 34%

With the charge now about 34 percent denser when entering the combustion chamber, in theory this should yield a commensurate power increase. But not so fast. These numbers don't take into account drag-induced boost-pressure losses as the charge flows through the ducting and intercooler. If the system lost 1 psi of boost from duct/intercooler drag for every 10 psi of boost generated on the compressor's outlet side, the power lost due to aerodynamic drag through the system can be calculated by dividing the absolute pressure with the intercooler by the absolute pressure without the intercooler, and subtracting from 100 percent (1):

HP loss = 1 [Std. pressure + pressure w/ intercooler] / [Std. pressure + pressure w/o intercooler]

= 1 —[14.7 + (10— 1)]/ [14.7 + 10] = 0.040 4%

Even with this 4 percent parasitic loss, our hypothetical intercooler would still be worth about a 30 percent power gain:

34% 4% = 30%

That means, for instance, that if your turbo setup without an intercooler could make 800 hp (assuming it didn't get into detonation with all that hot air), it has the potential to pound out 1,040 hp with our hypothetical 70-percent-efficient intercooler.

800 + (800 x 0.30) = 1,040

The next step would be to actually size an intercooler so it has the capacity to achieve the required efficiency level with your turbocharger, engine, and vehicle. In some cases, intercooler-efficiency graphs for various intercooler models under differing conditions can be beneficial in aiding selection and sizing, but unfortunately they're generally not widely available to the average consumer. It is possible to model the design mathematically, but the math involved is fairly complex and literally would take about two more magazine pages to fully flesh out. If you want to check out the math involved and how it's derived, see Jeff Hartman's Supercharging Performance Handbook (Motorbooks 2011, ISBN 0760339384, $22.26 at Amazon.com) or Corky Bell's Supercharged! (Bentley Publishers 2001, ISBN 0837601681, $25.51 at Amazon.com).

And the math is no panacea: Even after doing all the calcs, the end product can only approximate the real-world performance of a specific manufacturer's intercooler. This is because much of the math's input assumptions are highly variable, depending on the actual design characteristics of different manufacturers' real-world intercooler models. For example, one important factor in the math—intercooler internal fin density—can vary greatly between different intercooler designs. The density of the internal fins affects how much real-world intercooler internal flow area is needed to reach your temperature-reduction goals. The usual rule of thumb is to allow 6 to 7 square inches of internal intercooler flow area for every 100 hp of engine output, but this can increase by up to 40 percent with really dense internal fins. Consequently, the amount of internal flow area actually needed affects the overall intercooler's size envelope, including the intercooler's frontal-area requirement. Yet another input needed to accurately assess how much frontal area you need—the heat-transfer surface area per volume of intercooler core—is likewise usually obtainable only from the specific intercooler manufacturer.

Because of these complexities in turning theory into reality, the preferred sizing method really boils down to a personal consultation with your intercooler company of choice to discuss particular requirements. Johnny Wang, a spokesman for Turbonetics' Spearco intercooler division, says ideally the following info is needed to most accurately determine the proper intercooler for the given application. The more of this data you have (assuming it's accurate), the closer the ideal intercooler match will be.

* Flow rate out of the compressor in cfm

* Boost pressure

* Compressor discharge temperature (Turbonetics can supply this value if it's also the turbo supplier)

* For an air/air intercooler, the cold-side ambient air temperature (if not known, assume 65 to 70 degrees F for a typical street-driven car)

* For an air/air intercooler, the airflow efficiency in percent through the intercooler's frontal area (for example, if there were no obstructions such as grille area or sheetmetal, airflow efficiency would be 100 percent)

* For an air/coolant intercooler, the auxiliary pump's flow rate in gallons/hour (gph)

* For an air/coolant intercooler, the temperature of the coolant medium and/or the efficiency of any auxiliary radiator

From the forgoing, you can see that the addition of an intercooler has the potential to significantly increase power on high-output boosted engines. The limiting factor on a production-based street car will almost always be packaging constraints that limit the overall intercooler size (and, hence, its efficiency potential). When all is said and done, I think you'll find for nearly any street car operating in the real world, there's literally no such thing as too big an intercooler.

Sources

Bentley Publishers; Cambridge, MA; 800/423-4595 or 617/547-4170; BentleyPublishers.com