Quote:
A14force wrote:
HOWEVER, the bloke who does my boring/honing etc has advised me that too low a deck height, can actually promote detonation!
I read a thread ages ago at a mopar site, where they were discussing minimum piston to head clearance. The general consensus was that the tighter the squish, (And therefore the higher the CR) the less likely a motor was to ping.
Of coarse a forced motor is a bit different.
The advantages of 'squish' seem to have been discovered by an engineer named 'Ricardo' back in the 1920's What he found, in side valve engines, is that if the flat top pistons had zero deck height, & the portion of the head that covered HALF of the bore before opening up to the valve/combustion chamber was dead flat with the gasket face, then the piston & head were separated with only the gasket thickness.
When the piston rose to TDC, the gas that was trapped between the piston & this flat portion of the head was 'squished' & it jetted out into the combustion chamber at high speed. It 'swirled' around the combustion chamber & promoted rapid combustion of the burning fuel charge, rather than simply having a relatively static compressed charge that has a slower moving flame front burning across the fuel charge, creating a rising pressure wave in front of it untill the remaining [unburned] charge reached a point of self ignition & it spontaneously combusts [explodes] resulting in what we call detonation [ping]
These Ricardo cylinder heads allowed compression ratios to increase from about 3.5 to 1 up to as much as 6.5 to 1 in the space of only a few years & this was staggering when you remember that the fuel of the day was little better than Kerosene & Tetra Ethyl Lead in fuel was still over twenty years off.
It didn't take engineers long to realise that 'high swirl' was critical to detonation control in all combustion chamber shapes, so a good squish area became a vital part of combustion chamber design.
Now lets look at our Datsuns
If you lower the deck height of the piston in the bore, and/or raise the height of the head above the block with a shim, you reduce the efficiency of the squish area & you reduce the intensity of the resulting swirl. This then increases the possibility of detonation.
If you increase the diameter of the bowl, & thereby reduce the width of the band at the top of the piston, you decrease its ability to create a jet of gas from the squish area & thereby reduce the swirl, which results in a higher probability of detonation at any given degree of ignition advance.
So even in a blown engine, the higher the level of 'swirl' the better, even at lower compression ratios as this then allows more ignition advance, & up to a point, that's where the power is.
The amount of 'squish' need not be directly related to compression ratio.
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