September 1, 2004
Lowering the static compression ratio of your forced-induction motor is a good idea to help minimize detonation, but how much does the drop in compression cost you in terms of power?

In our previous adventures with Mods for Mods, we subjected our '98 4.6 two-valve GT motor (supplied by Mustang Parts Specialties) to all manners of abuse. The early short-block went through cams, throttle bodies, and even nitrous in Part 1, where we managed to up the power from 260 hp to 307 hp in normally aspirate trim and to an honest 400 hp with a shot of Zex.

Part 2 brought a set of CNC-ported PI heads from Total Engine Airflow and a matching PI intake. Adding an Accufab throttle body allowed our engine to top the 400-hp mark--without the spray. Installation of the Kenne Bell 1.7L supercharger pushed the power levels over 550 hp and finally to an even 600 hp. We continued the abuse to our motor in Part 3, with a Reichart Racing intake (good for 10 hp) and then a Vortech supercharger. The blower/Aftercooler combination eventually took our test mule to 655 hp. Along the way, we retested throttle bodies and intake elbows, as well as the effect of cam changes on the supercharged combination.

The installation of the PI heads on our early (non-PI) GT motor naturally upped the compression ratio. The chambers on the TEA-modified PI heads checked in at 45 cc, slightly higher than the stock PI measurement of 42 cc. The chambers on the '98 non-PI heads checked in at 52 cc. The difference in chamber volume meant that the installation of the TEA PI heads raised the compression ratio of our early short-block by nearly 1 full point (from 9.2:1 to 10.1:1). While the 10.1:1 hybrid motor ran well in all of our testing, we knew that the high compression was not the ideal choice for forced induction, especially for street use. Knowing this, we decided to take a closer look at lowering the compression ratio to facilitate running elevated boost pressures. While considering the new short-block configuration, we began to wonder about the effect of the change in compression ratio. Obviously the power would drop as we decreased the compression ratio, but to what extent? This is an important issue, as many engine builders currently offer low-compression 4.6s designed for blower and turbo applications.

Test motor number 1 was this 1998 short-block equipped with factory 11cc dished pistons.....

Since our 4.6 was still in excellent condition (despite all the abuse), we decided not to subject it to a rebuild. Instead, we decided to secure a new low-compression short-block dedicated to forced-induction testing. Working with the gang at Kenne Bell, which was naturally interested in the effect of the reduced compression to work with its supercharger kits, we contacted Sean Hyland Motorsport about a suitable low-compression test mule. At the request of the folks at Kenne Bell, SHM built a 4.6 short-block featuring a steel Cobra crank, forged connecting rods and a set of forged pistons. The low-compression, forged pistons featured reverse domes (dish) to reduce the compression ratio.

Common 4.6 piston designs include flat-tops and dish volumes of 11 cc (stock early 4.6), 17 cc. and 23 cc. Combining the 23cc dish pistons with a stock PI chamber of 42 cc results in a compression ratio of 8.95:1. The guys at Kenne Bell wanted to further lower the compression ratio, so Sean Hyland obliged them by building a custom piston with a massive 28 dish. Combined with a stock PI head, the large dish dropped the compression ratio to 8.44:1. The installation of our TEA-ported heads (with 45 cc chambers) dropped the final compression ratio of the Sean Hyland short-block to just 8.1:1 (after measuring the deck clearance of .012).

Rather then take the easy route by building the low-compression short-block and tossing on a blower, we decided to do a direct back-to-back comparison, MM&FF style. Direct back-to-back tests require a great deal of work, which is why so few people go to the trouble of doing them. Unfortunately, without direct testing, results are, shall-we-say, somewhat less than accurate. Some back-to-back tests are pretty simple. Things like throttle bodies and intake-manifold swaps take very little time, whereas things like cylinder heads and camshafts are a bit more involved.

.....Combined with the TEA-ported PI heads, the static compression ratio worked out to 10.1:1.

When you start talking about testing changes in compression ratio, you're looking at a ton of work regardless of the test procedure. For our comparison, we decided to use two different short-blocks but to duplicate every other component on the two test motors. To that end, we ran both our high-compression early-GT motor and the low-compression Sean Hyland short-block with the same heads, intake, and camshafts. Additional common components included headers, throttle body, and air intakes, as well as ignition, injectors, and even the oil pump. That's right folks, we even went to the trouble of swapping the oil pumps since we knew that oil pressure can ultimately affect the power output.

Know too that when we say we installed the same components on the two motors, we mean the very same components, not just duplicates. The cams, intake, and heads were removed from the high-compression '98 GT engine and installed on the Sean Hyland short-block. The same holds true for every other component as well. Sure, this was very time-consuming (and ultimately expensive in terms of dyno time), but we wanted to ensure that every aspect of the comparison was absolutely identical. Only then would we know the results of the change in compression.

Both the high- and low-compression motors were first run in normally aspirated trim. The 10.1:1 version pumped out 401 hp and 389 lb-ft of torque, while the 8.1:1 version produced 365 hp and 368 lb-ft of torque.

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.

Before testing each combination in supercharged configuration, we swapped out the stock 19lb injectors for a set of RC injectors that flow 65 lb/hr The injectors gave us plenty of fuel flow for our testing.

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.

After running the normally aspirated test, we installed the Kenne Bell supercharger on both combinations for testing. The low-compression motor produced 533 hp and 500 lb-ft while the high-compression version made 600 hp and 539 lb-ft of torque.

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).

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 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.