Michael Galimi
July 23, 2010

"Making horsepower is all about building adequate cylinder pressure," proclaimed Mike Dezotell of Dez Racing, as we embarked on our latest adventure into the world of compression ratios for boosted applications. The question for this technical adventure being how much performance can be gained (or lost) by altering compression ratio on a blown mod motor. This tale begins with a typical Three-Valve modular engine, which we all know loves boost in its stock form.

Most supercharger systems set at 10-12 psi, and with custom tuning will produce anywhere from 450 to 500 rwhp. We give credit to Ford for providing a suitable platform that incorporates three valves per cylinder, variable camshaft timing, and a relatively tough short-block. The stock combo yields 300 hp (315 hp for '10 models) in naturally aspirated trim-that normally equates to 245-250 rwhp with an auto trans and 265-270 rwhp with a stick.

One reason for its success, other than the cylinder heads, VCT, and intake, is the bump in compression ratio from 9.4:1 on the '99-'04 GT to 9.8:1. Our plan was to take one engine and test it with two different compression ratios. Yes, this meant setting a baseline, yanking the engine, swapping pistons, and retesting. This amounted to a big effort, but it was worth the work.

Dez Racing performed the surgery on Steve Gelles' '06 Mustang GT, already equipped with a 300ci stroker mill, steel crank and H-beam rods, Fox Lake-ported Three-Valve heads, stock cams, a JPC Racing intake, and a Vortech Si-trim H.O. supercharger system. The car produced 538 rwhp and 482 lb-ft of torque on the DynoJet dyno at Dez Racing, and the blower cranked out 15 psi of boost 6,500 rpm. The crutch, according to Dezotell, was the low 8.5:1 compression (the engine was built by another shop).

"Gelles' car was down a bit on power when compared to other S197s with similar modifications, and I was adding a lot of timing in order for it to respond," states Dezotell. For our test, timing was set to 19 degrees for all baseline runs. His solution was to bump the compression a full point to 9.5:1 to get more power and torque.

Traditionally, engine builders lower the compression ratio into the 8.5:1 to 8.8:1 range for boosted applications to reduce the chance of detonation. But today, with tuners gaining better control of ignition timing and fuel mapping, compression can safely be raised to make more power. (Editor's note: We'll see the effects of this when shops begin supercharging the new 11.0:1 compression 5.0L engine.)

"These days I run most centrifugal supercharged street cars around 9.5:1 compression; Roots-blown, twin-screw blower, or turbos at 8.8:1. Those ratios are normally safe and produce good results." To be "normally safe," he takes into account not just the type of forced induction, but also the cylinder heads, camshafts, bore size, and the desired use. He explains: "The Three-Valve combustion camber is open, unlike the Two-Valve, which has a swirl-dam. I find this to be the biggest difference between the two styles of engines. The Two-Valve head changes dramatically when the swirl-dam is removed. I've run 10:1 compression on a Two-Valve bullet with 14 psi and no detonation. Four-Valve engines also like compression due to the Hemi-style combustion chamber."

A set of Diamond forged pistons was swapped into the 300ci powerplant, which effectively brought the compression from 8.5:1 to 9.5:1 (see photos). The full point of compression, according to Dezotell, would allow the Three-Valve to produce a lot more power and torque, but still be safe and reliable on 93-octane pump gas.

After baseline testing, the engine was torn down and the block shipped off to the machine shop for honing. The bore size remained 3.572 inches, but the cylinder walls were cleaned up. The heads and valvetrain were also inspected, as Gelles had quite a bit of mileage on the engine. All the pieces came back from the machine shop with a clean bill of health, and Dezotell had his guys reassemble the engine.

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