Project Light Speed Part 3 - 49hp LS1 Engine Cam Swap

Knowledge Is Power
We continue to find out more about the complex factory computer that runs our LS1 engine. In talking further with Steve Cole and also with TPIS' Myron Cottrell, we learned a funny thing happens on the way to wide-open throttle (WOT). The factory computer is designed with what are called short-term fuel trim and long-term fuel trim. Let's say our LS1 engine is running at part-throttle with a short-term fuel trim condition that requires fuel to be added to the base fuel map, and let's say that number represents 5 percent additional fuel.

Lube the new cam with a viscous lube like Engine Assembly lube from Federal-Mogul, but don't use that black moly lube. Also remember to install the cam thrust plate to prevent the cam from moving. We didn't degree the cam, since we figured that most in-car swaps will not include this.

If the engine goes to WOT at the same instant it adds 5 percent part-throttle fuel, the computer is programmed to add this same 5 percent of additional fuel to the WOT fuel trim. This is done to compensate for a possible weak fuel-delivery situation, a slightly clogged fuel filter, or any other condition that could possibly reduce fuel flow both at part- and full-throttle.

Given this situation, let's say that in an attempt to tune our engine, we reduce the fuel pressure, which is a common approach. Since the lower fuel pressure reduces the amount of fuel delivered, the oxygen sensors at part-throttle will see a leaner air/fuel ratio as well. The computer will compensate and add fuel with the short-term fuel-trim setting, which appears as a negative number if you use a diagnostic-link scanner like the EFI Live system we use. Increasing numbers like -5 or -8 will dictate this same amount of correction when the engine experiences WOT. This is the main reason why our baseline tends to shift over time. In an attempt to have as consistent a baseline as possible, we have resorted to warming up the engine, then shutting it down and disconnecting the battery to clear the computer memory of any "learned" fuel trims. However, if the computer is adding fuel at part-throttle when we hit WOT, then the WOT tuning will go richer by that amount.

One trick you can try is to increase fuel pressure to ensure a positive part-throttle trim number. When the computer generates a positive short-term trim number, it does not reduce the base fuel map for WOT operation. This way, the computer never delivers a leaner base fuel trim than what the engineers programmed in, and there is less of a chance the engine will run lean at WOT and burn a piston.

This explains why our engine will baseline with different power levels: When it rolls into WOT, the short-term fuel trim could be in many different trim modes adding varying amounts of fuel. The more we learn, the better our engine will run, because we can make this engine work for us rather than against us.

GM uses a special Kent-Moore tool to align the front timing-cover seal to the crank. You could make an aluminum tool to duplicate the hub to come close. The tool would be easy to build and much cheaper.

It's also a good idea to pull the cam sensor slightly up out of the back of the block. This will prevent it from getting damaged during the swap.

The crank bolt is a torque-to-yield fastener. Press on the damper and torque the old bolt up to 250 lb-ft. Then remove that bolt and throw it away-a new bolt is $3. Now torque the new bolt to 37 lb-ft and lock the engine to prevent it from turning. Then turn the crank bolt 140 degrees. You'll need a 11/42-inch drive breaker bar and a "cheater" to do this effectively.

With the cam in and the front dress all buttoned back up, the Hot cam and spring change was worth as much as 49 hp at 6,200 rpm, but average power and torque improved as well. This is a good swap.