The final test involved running...
The final test involved running the Vortech supercharger at a higher boost level. This was accomplished by machining a 7.5-inch crank pulley to replace the smaller 6.5-inch (stock) crank pulley.
The Kenne Bell 1.7L blower assembly was then applied to the low-compression 4.6 with equally impressive results. The blower upped the power output of the low-compression short-block from 365 hp (in normally aspirated/low-compression form) to 533 hp at 6,300 rpm. The peak torque jumped from 368 lb-ft to 500 at 4,400. Compared to the high-compression supercharged motor, the peak power was off by 67 hp while peak torque suffered just 39 lb-ft. Note that the change in compression (see Kenne Bell Effect of Compression graph) reduced the power output across the board from 3,000 rpm to 6,300 rpm. It is interesting to note that the boost pressure was slightly lower on the low-compression motor than the high-compression version. The peak boost registered on the low-compression motor was 9.3 psi at 3,800 rpm, while the boost finalized at 8.7 psi at 6,300 rpm. Remember, we ran the same pulley ratios on the two motors, so the compression was the only variable responsible for the loss in power and boost pressure. Once again, the lower-compression motor would allow higher boost levels given an octane-induced detonation threshold.
The same scenario was repeated on the high- and low-compression versions with a Vortech centrifugal supercharger. Equipped with a 6.5-inch crank pulley and 3.33-inch blower pulley and the air-to-water Aftercooler, the Vortech supercharger pumped out 655 hp at 6,400 rpm and 556 lb-ft at 5,600 rpm on the high-compression motor. The peak boost registered 12.6 psi at 6,400 rpm. Installing the same set up on the low-compression motor, the 4.6 pumped out 607 hp at 6,400 rpm and 518 lb-ft at 5,600 rpm. The peak boost registered 11.5 psi, again down compared to the high-compression motor.
It is obvious that the change in compression ratio had a major effect on the power production--in normally aspirated and supercharged form. If we were building an all-out drag race motor, we might opt for the elevated compression ratio, but not so for a street vehicle. After reviewing the power loss, we would likely opt for a slightly higher static compression than 8.1:1, running it closer to 9.0:1, especially with a centrifugal supercharger. We feel that 600 (flywheel) hp is not a problem with a compression ratio near 9.0:1 and on 91-octane pump gas given absolute control of the timing curve. Naturally the air-to-water intercoolers used on both blower systems helped that detonation situation.
After running the compression ratio comparison, we decided to test the effectiveness of the new GT blower upgrade from Kenne Bell. The highlight of the blower upgrade for the two-valve motors was naturally the 2.2L twin-screw Autorotor supercharger. The 2.2L replaced the smaller 1.7L used on the standard 4.6 GT kit. In addition to the larger blower, the kit included a unique inlet system incorporating the large oval throttle body and aluminum inlet casting similar to that used on the four-valve Cobra upgrade.
The 3.33-inch blower pulley...
The 3.33-inch blower pulley necessitated the use of a six-rib drive belt. The custom short drive system allowed us to successfully use a six-rib belt all the way to 16.8 psi.
The benefit of the larger blower was that it is possible to move much more air at a reduced blower speed. Less blower speed means a reduction in the parasitic losses associated with spinning the supercharger, not to mention a slight drop in charge temperature thanks to increased efficiency. When it comes to positive displacement superchargers, it is always more efficient to spin a larger blower slower than a smaller blower faster. The smaller (faster spinning) blower will usually produce better boost response (at a given maximum boost level), but spun the same speed (with equal drive ratios), the larger blower will produce more boost throughout the rev range. The increase in airflow from the larger blower equates to more power. When you combine more boost with less charge temperature and a reduction in parasitic loss, you're looking at some pretty impressive power gains.
To accurately test the blower upgrade from Kenne Bell, we installed the smaller 1.7L twin-screw supercharger on the low-compression 4.6. The blower was configured with a 75mm throttle body and a 2.75-inch blower pulley. So equipped, the 4.6 produced 533 hp and 500 lb-ft of torque. The boost pressure peaked at 10.2 psi and finished up at 9.3 psi at a maximum engine speed of 6,300 rpm.
After running a back-up run, we removed the 1.7L and installed the larger 2.2L blower assembly. Both blowers were run using the air-to-water intercooler incorporated into the Kenne Bell lower intake manifold. The 2.2L blower was equipped with the same drive ratio (2.75 blower pulley), which helped produce a peak boost pressure of 14.5 psi. Naturally the power was up substantially with the larger blower, from 533 hp to 664 hp. Obviously the larger blower had a great deal more power to offer than the 1.7L, though the 1.7L was no slouch.
We never got the opportunity to run the 2.2L blower on the high-compression motor, but figure on the same relative difference in power as experienced with the smaller blower. Know that we eventually produced a hair over 700 hp (and 750 lb-ft) with the Kenne Bell at 22 psi of boost on the low-compression motor, but belt slippage kept us from getting solid, accurate high-boost numbers.
The final test run (that we will tell you about in this issue) was to crank up the boost pressure on the Vortech supercharger. After running the 6.5-inch crank pulley and 3.33-inch blower pulley, we stepped up to a custom 7.5-inch crank pulley. The 7.5-inch crank pulley was machined for use with the Kenne Bell supercharger and featured an eight-rib setup. Unfortunately, we could not take advantage of the extra two ribs, as the standard (4.6 GT kit) Vortech blower pulley was a six-rib. Though we couldn't utilize the extra grip offered by the eight-rib crank pulley, we did take advantage of the larger size, thus increasing our effective drive ratio from 1.95:1 (6.5/3.33) to 2.25:1 (7.5/3.33).
Increasing the impeller speed of the blower by increasing the size of the crank pulley was desirable since it did not reduce the belt wrap or leverage on the blower pulley. As it turned out, the custom (short belt) six-rib setup performed flawlessly, producing a repeatable boost curve that peaked at 16.8 psi, where the Vortech supercharged 2V produced 710 hp at 6,700 rpm. The peak torque checked in at 590 lb-ft at 6,100 rpm, and the supercharged and Aftercooler motor produced more than 700 hp from 6,250 rpm to 6,800 rpm. The power was all the more impressive considering yours truly forgot a 1/8-pipe plug in the discharge tube used to measure pressure and temperature. The boost leak obviously diminished the power potential somewhat, but 710 hp with a six-rib belt is pretty impressive.
8.1:1 versus 10.1: Compression-Normally...
8.1:1 versus 10.1: Compression-Normally Aspirated 4.6 GT
It is obvious from this graph that dropping from 10.1:1 compression down to 8.1:1 had a dramatic effect on the horsepower and torque curves. The loss in compression was evident from 3,000 rpm to 6,200 rpm. Dropping two full points in compression reduced the peak power numbers from 401 hp to 365 hp. The peak torque numbers dropped from 389 lb-ft to 368 lb-ft. Note also that the rpm where peak power and torque occurred dropped by 200 rpm.
8.1:1 versus 10.1: Compression-Kenne...
8.1:1 versus 10.1: Compression-Kenne Bell Supercharged 4.6 GT
Since the idea behind reducing the compression ratio is to allow use of a supercharger, we installed a Kenne Bell supercharger on both the high- and low-compression motors. The drop in power mirrored those experienced normally aspirated. The peak numbers dropped from 600 hp to 533 hp on the Kenne Bell supercharged 4.6L. The peak torque figure was off by 39 lb-ft (539 lb-ft vs. 500 lb-ft).
8.1:1 versus 10.1: Compression-Vortech...
8.1:1 versus 10.1: Compression-Vortech Supercharged 4.6 GT
Would the results be any different with a centrifugal supercharger? The answer is yes and no, as the reduction in compression dropped power by roughly 50 hp. This drop was slightly less than that experienced by the Kenne Bell combination, but slightly more than the normally aspirated motor. We did not run the low-compression motor from 3,000 rpm to 6,700 rpm, but it appears that the losses experienced were much less than with the positive displacement supercharger.
Kenne Bell 4.6 GT Blower Upgrade-1.7L...
Kenne Bell 4.6 GT Blower Upgrade-1.7L vs 2.2L
While we had the low-compression supercharged motor at our disposal, we decided to test the new Kenne Bell blower upgrade for the 4.6 GT motor. The standard blower supplied with the GT kit measured 1.7 L, while the upgrade stepped things up to a whopping 2.2 L. Running the standard 1.7L blower with a 2.75-inch blower pulley resulted in 533 hp and 500 lb-ft of torque. Installing the 2.2L blower upgrade increased these numbers to 664 hp and 578 lb-ft of torque. Spinning the larger blower at the same speed produced more boost pressure, from 10.2 psi to 14.5 psi, and power right along with it. Size really matters on positive displacement blowers and if big power is what you are looking for from your GT motor, go for the blower upgrade.
Vortech Supercharged 4.6-6.5-inch...
Vortech Supercharged 4.6-6.5-inch vs. 7.5-inch Crank Pulley
The final test was to increase the size of the crank pulley from 6.5 inches to 7.5 inches on the Vortech set up. The 65-pound injectors and Aeromotive fuel pump (with KB BAP) gave us plenty of fuel, so decided to up the boost on the Vortech. Despite being saddled with a six-rib belt (required by the Vortech blower pulley), the 7.5-inch crank pulley and 3.33-inch blower pulley produced a peak boost pressure of 16.8 psi. With ambient water running through the Aftercooler, the 4.6L produced 710 hp and 590 lb-ft of torque. Had the idiot author remembered to install a missing 1/8 pipe plug, the numbers would likely have been even better, but 700+ hp from 6,250 rpm to 6,800 rpm is nothing to sneeze at.
The air-to-water Aftercooler...
The air-to-water Aftercooler helped reduce the charge temperature, though we were pushing the flow limit of the street-oriented intercooler design.
Running 24 degrees of total...
Running 24 degrees of total timing and an 11.8:1 air/fuel ratio, the (low-compression) Vortech supercharged 4.6 pumped out 710 hp and 590 lb-ft of torque at 16.8 psi.