coopersmotors wrote:
The popping/crackle on over run is essentially artificial, created by the mapping of the PCM. In the case of a normally aspirated carburated engine, with significant tuning, valve overlap & (preferably stainless) 4:1 or 4:2:1 branch exhaust header; Fuel supply is more akin to a controlled leak. On throttle shut off, fuel/air mixture is still admitted to the engine via the idle/slow running circuit. If exhaust temperatures at above the flash point of the fuel &/or the engine has significant valve overlap, popping will occur.
In injected turbocharged engines for example,this tendency can be exploited to reduce turbo lag, leading to a lick of flame & a fair bit of crackle, whilst the expanding gas produced kept the turbo spinning at a faster rate than it otherwise would have done.
In a modern day normally aspirated injected engine. on throttle shut off & whilst over running, ordinarily no fuel is used at all. In fact the same vehicle running down hill in neutral uses more fuel than one in gear.
Sadly this is very boring. The solution:- is to admit 3 or 4 spits of fuel as the engine over runs. Result:- snap, crackle & pop syndrome. Notice it only happens for a split second or so. To do so for longer would adversely affect fuel consumption.
Older carburated engines, like for like, will always use more petrol because the idle/slow running circuit is operational all the time the engine is turning. Consequently popping occurs all the way down the rev range on over run, provided there is sufficient heat &/or overlap to ignite the fuel. To my experience this is not the case with the FRP, which tends to do so as it passes through 4/5kRPM. I.e On changing up the box in the 6/7kRPM zone popping does not occur. It's on slowing/braking/passing lower RPM that popping occurs. This behavior, reminiscent of days & cars past, is part of the PCM map & sounds delicious, regardless of whether it serves any practical purpose or not.
Cracking info....Cheers Dude...