Full Version Block Nut Destroyed
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Is there possibly some confusion here? Perhaps the head was shaved by 0.2 mm, not 2mm. There is not enough clearance between the valve and the piston head to reduce it by 2.0 mm surely?
What hasn't been mentioned yet is the effect of throttle opening and engine speed.
In the first case you have to have a good weight of charge in the cylinder for detonation to occur, it cannot for example happen going down a slope on the overrun.
Similarly, the time taken for the flame front to part-traverse the combustion chamber and detonation to occur is fixed, whereas when the engine is rotating at high speed the angle between ignition point and TDC takes a shorter time to cover, so the effect of detonation is reduced or nullified. Correction. As soon as I went to bed I realised that is not true. The advanced ignition at high engine speed means that the angle for detonation to occur is increased so detonation at high speed is a possibility, you're less likely to hear it because of the racket the engine is making but it will do real damage.
The worst case scenario for detonation is when you are pootling along at low engine speed and slam the throttle open. The natural reaction is to ease the throttle, thus reducing the charge weight. So the ECU automatically retarding the ignition (thereby reducing the angle to TDC) does a similar job at low speeds without affecting max power output.
High charge temperature makes the problem worse because the mixture is that much closer to detonation. So supercharged and turbocharged engines are more liable because of greater charge weight and higher charge temperature. Fortunately they tend to give less boost at low speed, they are usually fitted with intercoolers, and they tend to run lower compression ratios.
I could go on but I expect you're all asleep by now
!
Derek
In the first case you have to have a good weight of charge in the cylinder for detonation to occur, it cannot for example happen going down a slope on the overrun.
Similarly, the time taken for the flame front to part-traverse the combustion chamber and detonation to occur is fixed, whereas when the engine is rotating at high speed the angle between ignition point and TDC takes a shorter time to cover, so the effect of detonation is reduced or nullified. Correction. As soon as I went to bed I realised that is not true. The advanced ignition at high engine speed means that the angle for detonation to occur is increased so detonation at high speed is a possibility, you're less likely to hear it because of the racket the engine is making but it will do real damage.
The worst case scenario for detonation is when you are pootling along at low engine speed and slam the throttle open. The natural reaction is to ease the throttle, thus reducing the charge weight. So the ECU automatically retarding the ignition (thereby reducing the angle to TDC) does a similar job at low speeds without affecting max power output.
High charge temperature makes the problem worse because the mixture is that much closer to detonation. So supercharged and turbocharged engines are more liable because of greater charge weight and higher charge temperature. Fortunately they tend to give less boost at low speed, they are usually fitted with intercoolers, and they tend to run lower compression ratios.
I could go on but I expect you're all asleep by now
!Derek
As mentioned before, 2 or 3 mm milled off the head is probably me mishearing something.
Just to avoid any longer discussions over things that don't exist.
Just to avoid any longer discussions over things that don't exist.
Sorry for hijacking your thread, John, but glad my advice concerning wire thread inserts bore fruit.
Derek
Derek
Be my guest, I just didn't want members to lose sleep over what is an obvious misinfo. And thank you again for advice.
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