SO YOUíRE dead-set on screwing huge horsepower out of your favourite HolFalVal. But before you rush out to buy carbs, heads and a cam, you must that you have a known good engine. Itís no damn use if you bolt on all this stuff and blow the bum out of the thing, or a piston lets go and you run into an electrical problem ícos an escaping conrod has punched a hole in the block and taken out the alternator!
Enough said. Lots blokes get into strife when of their modified mill doesnít run right, because they donít understand the basic tuning laws. Bolt on more or bigger carbs, and you have to jet far richer to get back to the original fuel/air mix of around 13:1, as the bigger carb throats donít have the same amount of air rushing past the jets, so less fuel is being picked up and carried through into the cylinders.
So you reckon you need more compression, through milling the head face, fitting lumpy pistons, or buying performance heads. So you go from 8:1 to 12:1, and your engine detonates like crazy. Thatís because the higher you go with a compression ratio, the less ignition advance an engine will tolerate, and you have to retard the timing.
Same goes after you drop in a modified cam. This lifts the valves higher for a longer period, increasing the valve opening area. That change allows more fuel and air into the cylinders, increasing the effective compression ratio, even if a compression gauge says you got less. The longer the valves are off their seats, the more incoming gas escapes through the exhaust valves at starter cranking speeds. But as the revs rise, you get an increase in the effective comp ratio, and the ignition timing has to be retarded some more.
The other thing that happens after you upgrade to a performance cam is that the engine will run really rich at low revs, but you have to jet for a 13:1 fuel/air mix in the upper rpm range.
So the bigger the cam, the more cubic feet of air the intake system will deliver, and the higher your effective compression ratio will be. This all demands a major change in the ignition advance curve, which is divided into two parts.
Thereís the static timing, where the ignition is already set to fire the plugs at around 12 or so crank degrees before a piston is top dead centre. The other half is known as the centrifugal advance, where we need to fire the plugs earlier and earlier as the engine revs rise, as there is less and less time for the fuel/air mix to ignite and spread across a combustion chamber.
So we have 12 initial or static degrees built into a distributor computer, or and a total advance of 44 crankshaft degrees. But as we have messed with the valve trimming and lost a lot of cranking compression, we need to trigger the plugs to fire much earlier, to give the engine a chance to fire and run without stalling. The easiest way to fix this is to retard the centrifugal advance back to maybe 26 degrees, and up the static advance to 18. In practice, most engines will only tolerate a maximum static and centrifugal total advance of 50 degrees before a side blows out of a piston through detonation. Unless youíre running on alcohol fuel, that is, where slowerburning ethanol or methanol allows you to get away with much more advance.
Supercharged or turbocharged engines are a different ignition timing story, but the same basic rule applies. As the revs rise and the blower pressure increases, the ignition timing has to be retarded, to avoid detonation destroying pistons. But we still need a lot of static advance to start the engine anyway, so ideally we should retard the total ignition timing as the boost pressure rises. As itís usually not practical to modify an ignition system to do this, most home engineers opt for a lock distributor as a compromise.
Blown engines, apart from possible piston problems, are easier on their internal bits than a higherstressed, normally aspirated engine. They donít need big revs to produce the same amount of horses, have a massive mid-range torque advantage, and cost less for the bits you will need.
Unblown engines require tougher crank stuff, conrods that will cope with higher rev-created vibrations, and forged high-compression pistons. You just cannot slam a full-race cam in there without fitting different and stronger valve springs to cope with the new high-lift loads and avoid early valve bounce. Usually, these springs will have fewer coils so they donít bind up at full valve lift, or you may have to go to beehive springs.
With all this, the valve operating gear gets reinforced, from heavy-duty pushrods and cam followers, stronger roller rocker gear, right down to an uprated cam drive and even titanium valve caps. Thatís if you want your engine to live!
So there you are. I hope all this helps in your pursuit of power, apart from time, effort, and spending a truckload of munny on the murder. partner Ė Yours! vital in keeping s them from committing murder yours!