Having run a Four-Valve mod motor in the past with a Roots-style, twin-screw, and centrifugal supercharger, along with a twin-turbo system from HP Performance (both for MM&FF and my book on modular performance), I was excited about the recent combination of the turbos on an '03-'04 Cobra motor. On the cool scale of 1-10, the introduction by Hellion of the twin-turbo system feeding the factory supercharger rates a solid 200!
Loyal readers of this column will remember my frequent calls to action for this combination, and it's time some company finally stepped forward with the real deal. Truth be told, the combination of blowers and turbos on a single mod motor is not new; it was accomplished in the past by more than a few Lightning and Cobra enthusiasts alike. That it appeared on a Lancia Delta Integrale (Group B) rally car shows the idea is indeed not new, but a cool one nonetheless. It's nice to know, however, that Hellion finally made it available to modern Blue Oval enthusiasts.
The idea behind compound forced induction originated in the aviation industry (as did just about every other neat idea on internal combustion). Turbos and blowers, or multiple turbos, were used successfully in aviation applications long before automotive enthusiasts got their hands on it.
Currently, compound forced induction is most prevalent on diesel motors, especially those employed in tractor pulls, where maximum, short-term power is the name of the game. Naturally, the diesel-truck world has taken a liking to the massive pressure ratios made available through such a system and has put it to use for quarter-mile and dyno-drag assaults. It's not uncommon to see 1,000-plus horsepower and over 2,000 lb-ft of torque from these low-rpm torque monsters.
While certainly impressive in the aviation and diesel applications, the use of compound forced induction has its limitations as well, especially when applied to gas motors. This is especially true when choosing a Roots-style blower as the secondary (compound) compressor.
Though it's difficult to argue with 1,188 hp (thanks partially to a correction factor of 23 percent due to elevation-an inaccurate reading which will be covered in next month's column), it can be argued that the twin-turbo motor would be even more efficient without the Roots-style blower to hinder its performance.
In concept, having the Roots-style blower pres- ent to improve the low-speed response is a good idea. However, it eventually becomes not only a flow restriction to the more efficient 67mm turbos, but it also creates further chaos by having to com-pound the boost pressure from roughly 27 psi to 44 psi. I shudder to think about the inlet charge temp exiting a Roots-style blower at 44 psi when I've seen temps near 400 degrees at just 20 psi.
Despite the use of a pair of efficient intercoolers (air-to-air after the turbos and factory air-to-water after the Roots-style blower), I have to wonder if the reading of 120 degrees indicated in the story was 100 percent accurate. The number likely came from the factory air-temp sensor, which is nowhere near quick enough to react to a megaboosted run that lasted but a few seconds. The fact that Lancia didn't employ the inefficient Roots-style blower as the secondary compound compressor should further illustrate this point. In both cases, the blower was employed upstream of the turbo to enhance low-speed power but was eventually bypassed altogether.
It's important to first understand the science behind compound systems in general. While space constraints in this column limit the discussion to a cursory overview, it should help illustrate why it might be better to just run turbos on your Cobra motor, unless the cool factor outweighs maximum efficiency. In a compound forced-induction system, one form of forced induction is fed by another.
This differs from multiple turbos or blowers running in parallel (as in a conventional twin-turbo setup), as this only increases the volume of air supplied to the motor and not the pressure. Compound systems, such as those employed on diesel tractors, require elevated pressure ratios (basically boost levels) that cannot be supplied by a single turbo. Having one turbo that is efficiently capable of only 29 psi of boost (two-bar) fed by another with the same capability means a greatly increased boost level (though not likely the desired four-bar) and therefore power output.
In the case of the Four-Valve mod motor, these elevated (compound) boost levels are not necessary to achieve the desired power output, as they can be produced running just the 27 psi supplied by the 67mm turbos, not the 44 psi supplied by the compound system.
In fact, the power output would likely be increased without the blower (at max power level) in the way, but this is something we'll find out as I plan to run a series of tests that will illustrate the power offered by a conventional turbo system compared to the compound setup. This will include the usual array of datalogging, so stay tuned for yet another force-fed Ford shootout.