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.