We had more problems once we got the engine fired again. The stock Caprice ECM has a rev limiter set at about 5,500 rpm and would cut fuel before the engine was making peak power. We had a Hypertech programmer on hand but had to jumper the pins because the ALDL on our Painless harness was the newer-style OBDII connector and the programmer had an OBDI connector. These computers have a very slow baud rate (information transfer speed) and it takes almost 10 minutes to reprogram the ECM. Hope you're not paying for dyno time by the hour.
With the rev limiter raised to 6,000 rpm, the Edelbrock combination made 343 hp at 5,800 rpm and 378 lb-ft at 4,300 rpm.
Our final combination to test was Edelbrock's new RPM Xtreme heads and LT4 intake manifold. These heads were brand-new to the LT1 scene last year, and we had hopes for big power numbers. We combined them with Edelbrock's Performer RPM LT4 Air-Gap intake manifold but kept the Performer Plus cam and exhaust.
On the dyno, however, we struggled to make much more power than we did with the previous combination. Looking at the datastream on the scan tool, we noticed the ECM was taking out 11 percent of ignition timing in knock retard and suspected that the overly sensitive LT1 knock sensors were picking up the extra clatter of the roller rockers and pulling timing out. From the factory, LT4 engines came with roller rocker arms but with less-sensitive knock sensors. With a pull of 366 hp at 5,200 rpm and 404 lb-ft at 4,200--well down from Edelbrock's advertised numbers for this combination, we realized we had the limit of how much power we could make with the factory ECM programming.
We also scheduled a day at Westech's engine dyno. Fuel-injection guru Ernie Mena ran the engine, making fine-tuning adjustments to the timing and fuel maps in between each pull. Thanks to his tuning, we finally got the engine to churn out 412 hp at 5,600 rpm and 402 lb-ft at 5,900 rpm. To take advantage of the Edelbrock heads' more efficient combustion chambers and airflow characteristics, we needed to reprogram the ECM, so we downloaded Tuner C.A.T.S., ECM reprogramming software for GM computers of that era. We also needed a set of cables to connect a laptop computer to the ALDL. We got ours from AKM Cables.
For grins, we swapped the Edelbrock cam with a custom-ground one from Comp. Though it only improved on the Edelbrock cam by a few horsepower, the wider lobe-separation angle smoothed out to the point where the engine sounds almost stock.
At the same time, we swapped the stock throttle body for a trick 52mm unit from Holley and added MSD's new Pro-Billet distributor. This is MSD's version of the OptiSpark, which allows a total of 14 degrees of adjustment--7 degrees advanced and 7 retarded. This also allows fine-tuning of the ignition curve without having to reprogram the ECM for each timing adjustment. Stock OptiSparks are not adjustable.
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