How we as enthusiasts can address the problem – Part of what we can do is based on examination of parts that have been run. For example, it’s possible to “read” combustion chambers and piston crowns to determine where mixture ratio problems may be occurring. Even in an engine we might otherwise believe to be combustion efficient, it’s possible to make improvements.
On combustion chamber surfaces and piston crowns, look for light and dark spots and note where there appears to be little or no combustion residue. The dark spots signal overly rich conditions, lighter ones are on the lean side and clean surfaces are typically from fuel wash and with no burn at all. You can do a couple of things for these conditions and if you’re successful in providing at least a partial solution, the benefit will be obvious. Then you may be able to take some fuel away from the engine and make more power. Less fuel is wasted. Burn rate will increase, thereby enabling you to remove a measure of sparking timing. This is important because there’ll be less “negative work” done on the piston as it ascends just ahead of BTDC during combustion. Throttle response also tends to improve.
Measures of your own can include roughening up areas where little or no combustion appears to be occurring. As simple as it may seem, dimple patterns in critical areas that I’ve advocated for years work well, to the point some that some of these techniques are now under patent. If you remove the valves and look down the intake ports of a Chevy cylinder head (early or late design small- or big-block), you’ll be able to see a portions of the combustion chamber’s back wall. Heads that have been in operation will exhibit “clean” areas that you’ll be able to see on this line-of-sight path. Dimpling these areas has proven to help direct some of the separated fuel back into the inlet air charge for combustion. Dyno tests run at WOT while a reduction in unburned hydrocarbon levels (raw fuel) are measured at the exhaust ports have clearly demonstrated increased combustion efficiency, higher cylinder pressure, and gains in power…as also reflected by brake specific fuel consumption (BSFC) data of lower numerical value. Some experienced dynamometer operators and engine builders pay as much or more attention to BSFC data as they do to power numbers. At least the smart ones do.
Roughened intake port passages may flow slightly less air than smooth ones but you can rely on the boundary layer disturbances of the former to aid fuel suspension and power. Exhaust ports can be slick as possible. Dimples in the floor of a manifold plenum and along runner floors can also be of benefit, and you’d be mistaken if you thought strategically-placed dimples on piston crowns didn’t work. This gets back to determining where the fuel is washing across the surface. Check out the “Swirl-Quench” pistons offered by Hi-Tech Engine Components (www.hi-techengines.com) in their High Performance Engine Parts section.
Even if you’re dealing with later-model Chevys and MPFI systems, you can enhance the combustion process by the methods I’ve described. I recall an LS1 package that was treated only to the piston crown dimpling approach and it was worth 10-15 horsepower throughout the range. The method also required about 2 degrees less spark timing for optimum power. By itself, that’s a benefit.
Whatever approach you my take, stay focused on the need to provide a well-blended air/fuel charge just prior to combustion. Once the burn process commences, dramatic changes in mixture density and propagation are enhanced by pre-combustion conditioning of the air/fuel charge. The more homogeneous the mixture, the more uniform the flame rate, the faster the flame travel, the less spark timing is required and the greater the gain in combustion efficiency in power. As I said, the secret’s in the burn…
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