Ford 347 Build And Max Compression Ratio

I'm in the process of gathering parts for my small-block Ford rebuild. I'm thinking about a 34?ci stroker kit with S.4-inch rods. Here is what I have already:

• '90 roller lifter 302 block

• AFR 185 heads, S8cc chambers, 2.02-inch intake valves (PN 1388)

• Comp Cams 280HR hydraulic roller cam (PN 35-314-8)

• Comp Cams steel full-roller rockers, 1.6:1 ratio

• Comp Cams rocker stud girdle

• Comp Cams double rollertimingchain (I'll use the position that gives the most valve clearance)

• Fel-Pro head gasket (PNZ10112)

• Edelbrock RPM Air-Gap dual-plane intake fitted for port fuel injectors

• A9L Ford computer recalibrated with TwEECer

• 80mm MAF meter and throttle-body

• Headers, 1.5-inch-od x 32-inch-ol primaries

The motor will go in a '66 Mustang with a built T5,1350 U-joints, and a 9-inch, small-bearing Ford rearend.

What's the highest compression ratio I should use with 100-octane gasoline? (There is a ?6 station close to where I live that sells it.) Also, what temperature thermostat should I use? What heat range aftermarket multielec-trode spark plugs should I use?

Jeremy Elsberry Via email

Jeremy, this combo looks pretty good overall. A 302 stroked to 34? runs strong and fits in a smaller envelope than a 351W. Consider full-length headers with at least lVs-inch primary tubes. Standard single-electrode plugs like the Autolite 3924 have proved reliable for consistently firing the high cylinder pressures developed by a high-compression engine.

The A9L factory Ford computer, typically used circa 1989 to 1990 on manual trans cars with California emissions, has an unusually wide window of compensation, making it very tolerant of hot rod mods without the need for lots of tweaking. That said, if need be, the TwEECer connects to the Ford computer's J3 service port allowing you to reprogram using a Windows-based PC with an available USB port. It can store four different calibrations, permitting easy reversion to a milder tune if out of range of your 100-octane service station.

The 5.0 L Mustang engines have no factory knock sensor. Again, since you want to push things to the limit, consider adding a J8cS Electronics Safeguard individual cylinder knock control system. This device is compatible with the factory computer. The 5.0L block has a blind hole on the block's bellhousing flange behind the intake manifold that accepts a knock sensor (used stock on some Ford 5.0L V-8 trucks).

When it comes to aftermarket MAF meters, Ford tuner Mark Sanchez (contact him at AEW or through Westech Performance) recommends the Pro-M MAF as the most reliable unit: "Others may go overvoltage." Use at least 36-lb/hrfuel injectors, which should handle up to 550 hp. The 192-degree stock thermostat is too hot, a 160 is too cold, and a 180 is just right.

As to the static compression ratio achievable without getting into spark knock, it's influenced by the engine's combustion chamber design, spark plug location, and quench configuration. AFR heads have a modem chamber with good quench characteristics, but you also want to maximize quench potential by shooting for about 0.040 inch of piston-to-head clearance at TDC (includes head gasket compressed thickness).

All the preceding factors being equal, the smaller an engine's bore size, the higher the allowable compression ratio because the spark plug's flame front has less distance to travel. Tiny-bore-size motorcycles can run extremely high compression ratios even on low-octane fuel; small-block engines can tolerate more compression than big-blocks.

At high altitudes, because air density decreases, you can run more compression than at lower altitudes (or run the same compression ratio on lower-octane fuel).

The cooler the engine's inlet air temperature, the higher compression you can run: An efficient cold-air induction system is absolutely vital for successfully running high compression. In a perfect world, the temperature of the air flowing through the MAF sensor and throttle-body should be no higher than the outside ambient air temperature.

Electronically controlled engines with port fuel injection and spark control can run more compression than a carbureted engine. Some people claim that aluminum heads permit more compression because they reject heat quicker than cast-iron heads.

Also critical in determining octane sensitivity is the intake valve closing point. The later the valve's closing point, the higherthe piston will be in the bore during the compression stroke (on its way toward TDC). This means the actual running compression pressure of the engine will be significantly lower than the actual calculated static compression seemingly indicates, and the later the intake closes, the greaterthe difference. At low- and midrange through the engine's torque peak where detonation is most likely to occur, the reduction of cylinder pressure induced by a later-closing intake valve makes the engine less detonation sensitive [lowers its octane sensitivity). On similar-duration cams, the cam with the wider lobe-separation angle will have a later-closing intake valve. Retarding the cam also closes the intake valve later. And bigger cams in general have later-closing intakes than smaller cams.

This concept of looking at the effective running cylinder pressures of an engine instead nTRnn r.n

> The 302 Ford-based 34? stroker kit is a powerful, hard-running combination. Cast and forged billet cranks, as well as complete billet assemblies, are available from Scat, Eagle, and other sources.

of merely the static compression ratio is sometimes referred to as dynamic compression ratio. The dynamic compression ratio number is a calculation based on the engine's bore, stroke, connecting rod length, static blueprint compression ratio, and camshaft intake valve closing point. At sea level, a well-tuned, normally aspirated, pump-premium, 4-inch-bore engine can run about an 8 to

8.5:1 dynamic compression ratio; all-out nonrestrictor plate racing engines on the best high-octane race gas run around 9 to 9.5:1. Your 100-octane fuel fells somewhere in between, about the 8.5 to 9.0:1 range. EFI and aluminum heads would fell to the high side of these ranges. You can jack up these ratios another point or so if you routinely drive more than 4,500 feet, assuming the fuel octane

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