[hotrod]

On turbocharged cars you want heat in the exhaust to get good spool up. In fact, some turners intentionally run lean AFR's at low boost to enhance the spool up of laggy large turbos. I would suggest the turn on point be as late as practical for that reason on turbocharged cars.

I would suggest you set the turn on point just a bit below the boost that under high load (with a given fuel) just begins to show mid-range rpm knock. Engines will typically knock first at rpm's near the torque peak rpm.

What I would, do is find a good steep hill, fill the gas tank and throw some crap in the back seat and make a series of pulls up the hill after you get your basic tune dialed in. Gradually increase boost until you just see signs of knock on a quality knock monitor like a Knock Link or on reading your plugs for signs of detonation. This becomes you ceiling for operation without WI. Then set your turn on point a bit below that boost setting, so you have a little cushion for extreme condions and any delays in the spray actually getting into the air stream.

(another way to determine your knock limited boost for the WI turn on point, based on high load, would be to intentionally run one grade poorer octane fuel than you typically run. Find the first knock limited boost point with that fuel, and set your WI turn on point at that boost level so you would be safe on a poor fuel grade one step lower in octane than you normally use)

Keep in mind your knock point will be lower in hot temps and on long duration high gear pulls, where you spend more time at high load. This is one reason dyno tunes frequently kill engines that are never double checked with a road tune. Unless the dyno can hold a load at a set rpm you can never see peak cylinder temps during a quick pull through the rpm range on most chassis dynos.


As far as timing, the folks at NACA always did their tests at MBT (Minimum Best Torque) timing. They did that by tuning for maximum torque than backing off the timing until torque dropped 1%. That guarantees your on the safe end of the torque/timing plateau. For example on a 400 ft/lb torque engine, you give up 4 ft/lbs of torque for a very large measure of safety.

As mentioned above the tuning for WI is not a single pass process but rather an iterative multi step process.

With that said, I would tune for best power without WI, and find your MBT timing under that condition. This should change very little as you add WI.

Find your best/latest safe turn on boost point.

The old school approach used by the Buick GN folks then involved an iterative process of adding water until the engine lost power, then add boost, or leaned fuel until it "wakes back up", then keep repeating that cycle until you learn what your engines tolerance level is. Usually by watching EGT's and knock indications.

All the NACA studies indicate that max power always occurs with WI at AFR mixtures leaner than max power without WI. WI at AFR richer than about 11:1 are a waste of time, as your drowning the engine in fuel and water. The ideal lean AFR will change with WI rate and mixture.

I would be slow to move away from the MBT timing, as over advanced timing can create huge cylinder pressures, with small net gains in power.

I would deal with timing as the last careful tweak to the step wise tuning process. As mentioned above your WI mixture will modify your burn speed and that in turn will slightly modify your MBT timing. So every so often you would double check to see your still near MBT, then go back to tuning via AFR, boost and WI mix.

It is interesting to note that engines of similar design, frequently cluster around a very small range of ideal ignition timings. To the point that many tuners believe you can get a given high performance engine family (say a small block chevy) very close to max performance with a widely recognized timing recipe.

Larry