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Hi, I'm just turning my attention to boost, doing some reading etc while thinking about boosting my car.
One thing I am noticing is a contradiction, web sites show charts saying an engine with 10.5 to 1 compression can only handle 2 or 3 psi before detonation will destroy them.
Yet other sites and forums say they're getting 8, 10 or more psi, eg Nissan V350z with VQ35de stock internal ad 10.3 to 1 compression ratio.
What I can't find as an explanation of how they do this, any info or links to articles would be welcome.
Detonation occurrence highly depends on intake air temperature and ignition timing lead. Of course compression ratio and boost pressure also play big role in that picture but detonation can be avoided by lowering intake air temperature or retarding ignition timing or using a fuel with higher octane rating or switching to more aggressive camshafts to reduce dynamic compression ratio. Moreover, aftermarket spares can withstand some detonation much better than the stock ones. So it is not fair to say that the engine with 10:1 compression ratio can withstand only 5 ( or 8 or 10 psi) of boost as it depends on many factors, not just boost level.
ok, I see I think.
How do the camshafts reduce dynamic compression. These were definitely stock pistons and conrods they were talking about and not aftermarket parts. Is it through over lap and wasting some air/fuel down the exhaust? And wouldn't retarding ignition timing increase the likelihood of detonation through letting the mixture compress more before spark?
Compressing the mixture longer does not help detonation at all as it only happens after spark introduction. Self ignition after long compression is the function of diesel fuel engine not gasoline.
Some engine combustion chamber designs are also better than others, and using a better fuel will also give more 'headroom' for increasing compressive temperatures*.
*why diesels are called compression ignition engines, they compress the air in the cylinder to a temperature where the fuel that is injected into the cylinder will spontaneously ignite on contact.
What about fueling, does running super rich help avoid detonation and allow higher boost with more safety?
Rich mixture helps to reduce thermal load in the combustion chamber thus increasing detonation threshold but super rich mixture sometimes has an effect of helping detonation . For instance, if I run my engine 10.8:1 AFR at high load and high revs it runs clean but if I make it 10.5-10.4:1 AFR ECU can detect some knock -of course it varies from engine to engine.