Fairly technical question regarding detonation

[Martin]

Hello there!

I dont know if this is the right forum to post this question, because I think is more theorical than practical, if not please move it where it belongs.

I want to know IF and WHY delaying intake valve closing will not generate more detonation at higher rpms than a stock camshaft timing in a highly compressed NA engine.

I tried very hard looking for answers to this on the internet but I couldnt find an explanation.

A bit of background on why Im asking this? (you can ignore what is down below if you want, the important question is above)

[spoiler]Well I rebuilt my engine using some high CR pistons, also I bored the sleeves to 0.40. Unfortunately I did this 2 years ago before knowing a lot of “stuff” and I did it based on friends advice who run the same configuration on other country without issues.

Now the problem is that they have fairly better fuel at their disposal. also they are just 0.20 overbored, so looks like that makes my engine have some knock counts (1-4) on high load but not under WOT (lets say between 20-60 TPS). But Im keen on instead of reverting to older spec pistons, embrace the high CR and make something out of it.

Im attacking the problem from all the sides, Im trying to slightly richen the mixture, i already added colder sparkplugs AND im planning to work on the timing to use the high compression on my advantage. One of the keys to reduce detonation is to mess with the camshafts, blowing off some pressure either through intake in the intake stroke. Less air, less compression no knocking right? But I also understand that at higher engine speeds this tends to make more power because of the airflow that fills the cylinder better at higher speeds with a delayed intake valve closing.
[/spoiler]

NOW back to the first question, IF delaying intake closure builds more pressure on the cylinder at higher RPMs, that should cause more detonation and probably fatal engine failure, BUT MY GUESS is that at higher RPMs flame travel dynamics change and that should make detonation less of a factor.

That is my guess but i would like to have a confirmation or explanation, even some material to read because until now I could find anything proving or rejecting my theory.

Thanks a lot in advancement.

EDIT: If you are curious about the engine im working on is a Mitsubishi 4G93 DOCH 1.8L 10.5:1 stock CR raised to about 12:1

EDIT2: Current Mods are: 4-2-1 Exhaust manifold / removed cat / Autotechnic Fuel Pressure Regulator / 4G92 (1.6) pistons / Innovate LC2 Wideband ||| Ready to install mods: K&N Cold Air Intake / HKS Regulable Camshaft Pulleys

Topic

[Mike]

I don’t think there’s a “carved in stone” answer to your question. Something that has real-world benefits for a particular engine may not do anything for another engine. Another person’s experiences won’t reliably predict what you’ll experience. You’ll simply have to experiment and see what the results are.

Your engine’s volumetric efficiency and swirl within the cylinder will have an effect on detonation. Likewise, the efficiency of the entire cooling system will have an effect on detonation, since, at the heart of the matter, the root cause of detonation is simply too much heat within the cylinder and combustion chamber for the fuel to handle.

One concern for me in your question is that you say your friend has access to better fuel than you have. If your fuel quality is sketchy, detonation will likely be an issue, especially at 12:1 compression. Rather than messing around with valve timing, perhaps running a lower compression ratio will keep detonation away. On the other side of the coin, you could always use an octane booster, but that is a never-ending expense, and a bit of a hassle, since you’ll have to measure and add the appropriate amount every time you visit the pumps, and you won’t always be adding the same volume of fuel to the tank.

[Martin]

[quote=”Evil-i” post=189911]I don’t think there’s a “carved in stone” answer to your question. Something that has real-world benefits for a particular engine may not do anything for another engine. Another person’s experiences won’t reliably predict what you’ll experience. You’ll simply have to experiment and see what the results are.

Your engine’s volumetric efficiency and swirl within the cylinder will have an effect on detonation. Likewise, the efficiency of the entire cooling system will have an effect on detonation, since, at the heart of the matter, the root cause of detonation is simply too much heat within the cylinder and combustion chamber for the fuel to handle.

One concern for me in your question is that you say your friend has access to better fuel than you have. If your fuel quality is sketchy, detonation will likely be an issue, especially at 12:1 compression. Rather than messing around with valve timing, perhaps running a lower compression ratio will keep detonation away. On the other side of the coin, you could always use an octane booster, but that is a never-ending expense, and a bit of a hassle, since you’ll have to measure and add the appropriate amount every time you visit the pumps, and you won’t always be adding the same volume of fuel to the tank.[/quote]

Thanks for your answer. I already considered both lowering the compression ratio and using octane boosters. But lowering the compression ratio would mean a complete rebuild just to revert to a less powerfull spec. So is a last resort move. Then you clearly explained why octane boosts is not a nice solution.

So I know running higher compressions like 13:1 is achivable, maybe you lose drivability but thats not my concern right now. Usually people use high CRs with performance camshafts. But what im trying to understand here is the physics behind that at high rpms.

[Jason White]

Every process in the engine takes time. As engine speed changes, the timing of these processes needs to change so they all come together at the right place for the most efficiency. Early engines everything was mechanically set, and they had a very narrow range of efficiency and even then they weren’t very efficient. Of course the first to be made to adjust with engine speed as ignition timing. Fuel take about 0.003 seconds to burn in the combustion chamber, so you need to set the spark so that it has completed the burn at top dead center. So, you have to advance that timing as the engine goes faster. Then of course we changed valve event timing and with Direct Injection even in gas engines we have been able to alter fuel timing. There are many elements of physics involve in the operation of the internal combustion engine. Thermal, hydraulic, pneumatic, electronic as well as electronic. All these come together to determine how your engine operates.

Now I don’t know exactly what leaving the intake valve open longer does to reduce detonation but it is changing the attributes that caused detonation which is most likely thermal or the pneumatic factor to how the gasses are behaving as the mixture is being compressed.

[Author]

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