Dale Amy
December 1, 2002

Horse Sense:
Boost must be accompanied by timing retard since it raises both cylinder pressure and temperature, the prime causes of detonation.

When we left off, our '95 GT ragtop had just been given the gift of boost in the form of Paxton's potent Novi 2000 centrifugal supercharger ("Winds of Change," Mar. '02, p. 71). Through our J&P-ported Edelbrock Performer heads and intake and MAC long-tubes, the out-of-the-box Novi huffed and puffed its way to more than 435 rwhp at a mere 5,700 rpm-the point at which we had no choice but to get out of it. Trouble was, during dyno testing, we quickly found and exceeded the limits of our convertible's fuel and spark systems. We had no means of boost retard onboard and, under double-digit boost, our ACCEL 40-lb/hr injectors were able to dispense fuel faster than the Novi kit's 255-lph in-tank fuel pump could supply it. And, in a final fit of unpreparedness, we had no chip. Just another day in the life of a magazine project, but all things considered, it's a wonder the small-block survived the dyno session intact.

Though we were able drive it on the street, fear for the health of our stock hypereutectic pistons limited us to part throttle and about 4,000 rpm, because introducing 10 psi of boost into cylinders starving for fuel under a factory timing curve would be a recipe for certain meltdown. On the bright side, driven in this pansy, balloon-foot fashion, the GT had no really bad manners other than a determined proclivity for stalling at hot idle and a tiny bit of light throttle surge. But good manners are of little comfort when you really just want to hammer the thing. After an intervening six months of winter storage, it was finally time to address our support systems' shortcomings, so that-hopefully-the only items at risk under full throttle would be the rear tires, the factory clutch, and our drivers' licenses, not necessarily in that order.

For our '95 5.0, we are using MSD's combination of 6-BTM (Boost Timing Module) capacitive-discharge ignition box (PN 6462), Blaster TFI coil (PN 8227), and TFI coil harness (PN 8874). The 6-BTM's timing control knob allows dialing-in between 0 and 3 degrees of timing retard for every pound of boost the unit detects. Through experimentation, Lidio Iacobelli ended up pulling about 1.3 degrees per pound, for a total retard of around 13 degrees at our Novi's observed max of 10 psi.

We could have gone overboard with some sort of aftermarket EFI and a whole new stern-to-bow fuel system, but unlike most of this magazine's other projects, this one is strictly a street vehicle and is therefore unlikely to gain much more power, so we decided to take a minimalist approach. We polled the supercharger-savvy crew at Lidio Iacobelli's Alternative Auto, who suggested all we really needed was MSD's well-known 6-BTM boost-retard ignition box, one of Vortech's equally ubiquitous T-Rex inline supplementary fuel pumps, and one of Lidio's blower-friendly electronic tune-ups. Lidio favors Mike Wesley's Autologic chips and tuning software, and since we'd had little prior experience with the Autologic system, we were anxious to try it firsthand. Judging by our results, we think it was a good choice.

Check out the accompanying photos for a look at the hardware installation, and our tuning sidebar for the story on Lidio's approach to a powerful, but safe, forced-induction tune-up. We now have a Viper killer with all the manners of a stock GT.

Electrons In Order
In the past decade, Alternative Auto has installed tons of blowers on both pushrod and modular Mustangs, so when it comes to knowing what it takes to tune for boost, Lidio Iacobelli is a good man to talk to. Lidio is of the street-tuning philosophy, meaning he plugs an air/fuel ratio analyzer and a laptop into the car he's tuning and drives it around extensively, under cold, hot, idle, part-throttle, and full-throttle conditions, instead of using full-throttle pulls on a chassis dyno or just generalized guesswork. For the A/F analyzer, he temporarily plumbs a pricey, wide-band oxygen sensor into the subject vehicle's exhaust.

For this vacuum/boost source, Matt T'd into a vacuum line off the throttle body. To do this, he simply replaced an existing single T fitting with this double T.

But before he even drove our GT after its ignition and fuel upgrades, he went underhood, unplugged the SPOUT connector, and set initial timing on the distributor to about 29 degrees BTDC. Unplugging the SPOUT interrupts the timing control signal from the EEC IV, so spark timing remains at initial no matter the engine speed, load, or temperature. This means we now have 29 degrees of spark advance continuously until the 6-BTM starts seeing boost and retards it at (in our case) the rate of 1.3 degrees per pound of boost.

Why does Lidio use this method on blown pushrods, rather than specifying a desired spark curve through the chip as many tuners do? Mainly because he feels some of the EEC IV's basic, unalterable spark strategies can be destructive under boost conditions. "We found a few years ago," Lidio explains, "that we couldn't trust [the EEC]. What would happen is when we left the timing to the EEC, while we were ripping through the gears on a car, as we let off the throttle to push the clutch in, and then refloored it again in the next gear, it would spark knock for an instant." Lidio surmises the EEC adds a bunch of timing as soon as the throttle is lifted and engine load drops. It then takes some time to retard all that advance again once back under full throttle, causing the momentary knock. Spark knock and boost don't coexist peaceably. "We found that by locking the timing," Lidio says, "the problems went away."

So he fixes total timing and leaves it, but any number of other functions, such as the fuel curve, he can adjust, real-time, using Autologic's ChipMaster software on his laptop as he drives. With Chip-Master, the laptop plugs into the EEC's service port-like a chip-and in fact acts like a chip, replacing the EEC's data lookup tables with its own. Changes made on the laptop-such as altering target air/fuel ratios at a given rpm, for instance-take effect immediately. If they don't have the desired result, another adjustment can be made on the fly, and its results observed immediately.

The MSD adapter harness plugs in line between the factory coil harness and the plug on the TFI coil, which bolts in place of the factory coil. The four color-coded wires from the 6-BTM then plug into the corresponding connectors provided on the adapter harness. It's a no-brainer. The 6-BTM's final connection is to full-time battery power, for which Matt uses the SN-95's power distribution post near the battery. Needless to say, the battery ground cable should always be disconnected before making any electrical connections.

This real-time tuning ability is one of the things that attracted Lidio to the Autologic system. Instead of waiting to download a file, he can just tune as he drives. Once he has everything the way he wants it, he can burn a chip right off the laptop, and he's done. Lidio also believes he can adjust more parameters using the Autologic stuff.

Specifically, some of the things he did in our case were to raise warm idle speed and mess with injector slopes to eliminate our hot stalling problem; raise the rev limit by 200 rpm; lower the low- and high-speed electric fan engagement points; and reduce the throttle angle at which the EEC switches from closed-loop (correcting air/fuel ratios to Stoichiometric, based on O2 sensor feedback) to open-loop (setting A/F based on lookup tables) operation. The '94-'95 EEC IVs also have a timed delay for going into open-loop, which Lidio virtually eliminated.

Air/fuel ratios are absolutely critical with a supercharger. Whereas a naturally aspirated engine might normally have a wide-open-throttle ratio of 12.5:1 at higher rpm, Lidio likes to run as rich as 11.8:1 to 12:1 with a blower. Prior to installing our T-Rex pump, we could sustain that mixture only up to about 5,500 rpm, after which fuel pressure suddenly plummeted, meaning we were running out of volume. With the supplementary pump, pressure rose steadily all the way up to 6,400 rpm, where the chip started shutting down cylinders.

After a day with Lidio, our spark, fuel, and tuning issues were history. Though we haven't since visited a dyno (yet!), our little 5.0's power is now utterly tire shredding, yet driveability is good enough to make a factory calibrations engineer happy. And the porky old ragtop still delivers better than 20 mpg cruising down the highway. Who says you can't have your cake and eat it too?