Mods For Mods Part IV - Comparison Ratio On Ford Mustang GT 4.6L Engine

With all of our compression-ratio calculations behind us, we ran the two on the dyno. First up was the high-compression combination. The 4.6 GT motor was equipped with the TEA-ported PI heads, a set of Comp Xtreme Energy XE274H cams, and the PI manifold. Additional goodies included 19lb injectors (and Keith Wilson fuel rail), a set of 1 3/4-inch Hooker headers, and an Accufab 70mm throttle body and matching elbow. Ran with a MSD coil pack and Meziere electric water pump, the 10.1:1 4.6 produced 401 hp at 6,100 rpm and 389 lb-ft at 4,900 rpm. This combo was impressive, never dropping below 320 lb-ft from 2,900 rpm to 6,200 rpm. Obviously the Xtreme Energy cams and CNC-ported PI heads were well-matched.

Next came the mad thrash to swap everything to the low-compression block. It should be noted that all of the tuning was performed using a F.A.S.T. fuel-injection system, allowing us to optimize the fuel and timing tables for each combination. Both setups were run with a 3-inch inlet tube and free-flowing cone filter attached to the Accufab throttle body. After swapping on all the components from the high-compression motor, we were ready to run once again. As expected, dropping the compression ratio by two full points (10.1:1 to 8.1:1) resulted in a dramatic drop in power. The peak numbers on the low-compression motor checked in at 365 hp at 5,900 and 368 lb-ft at 4,700 rpm.

Note that not only were both numbers significantly lower than the high-compression motor, but they also occurred 200 rpm lower in the rev range. In looking at the power curves, it is obvious that the combination of ported heads and aggressive cam timing worked best with the higher compression. The drop in compression reduced the power curve from 3,000 rpm all the way to redline.

For those of you who have replaced your stock-compression motor with a low-compression version for use with a blower (or turbo) and want to know why it feels a bit more sluggish off boost, here is the answer. It looks like the old adage that each point of compression is worth roughly four percent in power is pretty accurate. If we multiply 401 hp x .92 (this takes four percent for each compression point for a total drop of eight percent), we get 369 hp. I would have to say that 369 hp is close enough to our peak reading of 365 hp for the percentage to be considered accurate.

While lesser magazines may have stopped here, we here at MM&FF were just getting started. To further illustrate the effect of the change in compression, we ran both the high-compression and low-compression combinations with both a Vortech and a Kenne Bell supercharger. In fact, we also used this opportunity to test the new Kenne Bell 4.6 GT blower upgrade. Fresh off its new 4.6 GT kit featuring the 1.7L twin-screw Autorotor supercharger, Kenne Bell decided to offer the GT owners a larger blower option. Naturally, we wanted to illustrate the extra power available from the blower upgrade and our new Sean Hyland low-compression short-block was the ideal candidate for testing the two blowers back to back.

Before we get to the results of that blower comparison, we need to take a look at the effect of compression using both the Kenne Bell and the Vortech superchargers. After all, boosted applications are why we lowered the static compression in the first place.

It should be noted here that in all instances, an engine with higher compression will make more power. It is equally important to point out that the combination of high compression and elevated boost pressure will severely limit performance in a street car due to the resulting detonation. The ideal choice for pump gas (on a daily driver) is moderate compression (9.0:1) and moderate boost, but it is possible to improve on boost performance by further reducing compression and increasing boost.

The trade-off in detonation threshold is skewed in favor of less static compression and more boost pressure, but off-boost response (and cruise fuel mileage) will definitely suffer with a drop in compression. This is especially true with centrifugal superchargers, as the centrifugal blowers tend to run best near the top of the rev range where they make maximum boost pressure. Due to their climbing boost curve and inherent increased efficiency (defined as hp per pound of boost), centrifugal superchargers can get away with and will respond better to higher static-compression ratios. The instantaneous boost response of a twin-screw and roots blower will not tolerate as much static compression (or timing), but the efficiency of the positive displacement blowers diminish with elevated boost levels (more so on the roots design than the twin screw).

To see how the high-compression and low-compression motors responded to boost, we ran both combinations with the 1.7L twin-screw Autorotor. The pulley ratios (6.5 crank and 2.75 blower) were not changed between the two. Installation of the supercharger required upping the injector size from 19 lb/hr to 65 lb/hr (we wanted plenty of injector for higher boost levels run later). The inlet into the blower was equipped with a 75mm Accufab throttle body. The total timing was reduced with the supercharger (from 28 degrees total to 24 degrees) and the air/fuel ratio was reduced from 13.2:1 on the normally aspirated version to 11.5:1.

On the high-compression short-block, the Kenne Bell supercharger pumped out 600 hp at 6,300 rpm and 539 lb-ft of torque at 4,400 rpm. The pulley ratio produced a peak boost pressure of 10.2 psi at 3,800 rpm and a final boost pressure reading of 9.3 psi at 6,300 rpm. We mentioned the boost readings since the pressure actually changed from one motor to the next, despite identical pulley ratios. We attribute this to the fact that more flow was needed to fill the volume in the low-compression motor. To keep detonation in check, we ran the supercharged combination on 100-octane race fuel (necessary because of the relatively high compression).