Detonation, Now only at high RPMs

[NICAT]

Hi.
I have a few previous threads about detonation problem. By your helps i have almost fixed it. Thanks

So there is no more detonation during daily driving, even at higher loads.
But at high RPMs, it still knocks. At third gear. If it goes beyond 4500rpm-5000 rpm the knock sound is auidable. (But if i drive at low speed, at 5th gear, where engine under very high load, still it does not knock )
I unfortunately can't retard the timing any more, because engine likes to hesitate when pressing the throttle at the traffic lights.
I have already made some researches before posting here.
There could be many reasons, such as carbon buildup, vaccum leak, wrong ail-fuel ratio and so on.
But i wonder if its existence only at higher RPMs would give me any clue.
Engine has electronically distributed ignition.

 

[A_Harman]

How is the PCV foul air return hose plumbed?

 

[NICAT]

Originally Posted by A_Harman
How is the PCV foul air return hose plumbed?


Great idea

Let me check it tomorrow

 

[oilpsi2high]

Check the plug gap.

 

[2oldtommy]

I did not read your first post. However, the problem may be that you could use a cooler range spark plug. Check the plug insulator to see if it shows signs of blistering. Champion spark plugs at one time had a good lesson on 'reading' spark plug problems on their website.

At 4500-5000 RPM the plug insulator may be overheating and causing the detonation.

FWIW---- Oldtommy


.

 

[NICAT]

Originally Posted by 2oldtommy
I did not read your first post. However, the problem may be that you could use a cooler range spark plug. Check the plug insulator to see if it shows signs of blistering. Champion spark plugs at one time had a good lesson on 'reading' spark plug problems on their website.

At 4500-5000 RPM the plug insulator may be overheating and causing the detonation.

FWIW---- Oldtommy


.

I used to belive that spark plug is not big enough to cause significant increase in temperature inside the combustion chamber or be able to help cool the combustion chamber faster. So it shouldn't lead to detonation.
But when the spark plug is too hot, it can cause preignition if the temperature in combustion chamber is already high. Please correct me if i am wrong

 

[Kawiguy454]

under normal circumstances you should not need to adjust spark plug heat range. Look at the spark plugs if they are tan they are ok. If they are very clen/white they are running lean. Make sure your fuel pump has correct pressure and filter is clean. High rpm needs more fuel and a marginal fuel pump will run lean at higher rpm's generating heat and knock. A rich mixture does not knock so easily.

 

[NICAT]

Originally Posted by Kawiguy454
under normal circumstances you should not need to adjust spark plug heat range. Look at the spark plugs if they are tan they are ok. If they are very clen/white they are running lean. Make sure your fuel pump has correct pressure and filter is clean. High rpm needs more fuel and a marginal fuel pump will run lean at higher rpm's generating heat and knock. A rich mixture does not knock so easily.


Thanks bro.
Another important thing i learnt today.
If they are not running lean, should they be a little dirty ?
I think i will also need to test (without having to buy) new map sensor.

 

[NICAT]

By the way,
Engine compression ratio is 9.5, Fuel octane is AI-92

 

[A_Harman]

Originally Posted by NICAT
Originally Posted by A_Harman
How is the PCV foul air return hose plumbed?


Great idea

Let me check it tomorrow


It would be a cheap and quick test to disconnect the foul air return line going back to the carburetor (or throttle body), cap off the vacuum port, and just let the foul air vent to a catch can. If you are getting oil carryover into the intake manifold at high rpm, the oil can cause detonation.

 

[2oldtommy]

Nicat,--- To further explain; the design on the insulator is to remove heat in order to keep the CORE at the proper temperature to prevent preignition. The longer the insulator, the hotter the plug. when you have a shorter insulator. you allow it to dissipate the heat to the outside more quickly, therefore, a cooler plug. If you have some old plugs around, Take a look to see the length of the insulator from the firing tip up into the body of the plug. A trained eye can tell easily if this is a 'hot' or 'cold' plug.

The purpose of the insulator is to keep the wire core at an optimum temperature .My guess is that when you exceed 5000 RPM, the current plug you are using is preventing the dissipation of heat quickly enough.---hence-- detonation.

For source material on plugs, check out Champion, NGK, Denso and others to get a better understanding of spark plug design. Check for lesson on reading racing plugs. This will give you that will help you understand the above info.


FWIW---Oldtommy

 

[mk378]

The car in question is your Daewoo that is cobbled together from parts of several similar but not quite the same Daewoos, is it not?

You really shouldn't tempt fate here. Drive it like a grandma would. A Daewoo that can still move by itself is rare.

 

[dogememe]

Originally Posted by mk378
The car in question is your Daewoo that is cobbled together from parts of several similar but not quite the same Daewoos, is it not?

You really shouldn't tempt fate here. Drive it like a grandma would. A Daewoo that can still move by itself is rare.

lol

 

[JeffKeryk]

Better gas or less total timing at rpm. PLEASE remember, if detonation is audible you are well into detonation.
Plot your total curve.
Do you have a distributor? What year is vehicle? Is it stock?
You can shorten the advance range which allows for more lead in the usable range while limiting total timing.
Good luck.

 

[NICAT]

Originally Posted by mk378
The car in question is your Daewoo that is cobbled together from parts of several similar but not quite the same Daewoos, is it not?

You really shouldn't tempt fate here. Drive it like a grandma would. A Daewoo that can still move by itself is rare.


Not true at this time.

It was a different cylinder head once. But we have installed stock head recently.
But that is not the only point.

I have seen many threads about detonation. And all i have learnt was "engine tends to detonates more at lower rpms than at higher rpms"
And the opposite is true in my case. So i was eager to know the possible reasons. That is the reason why i started a new thread here. (also made some google search before jumping to here.) So if there is an easy fix, i will do it. if not, then i won't. But in any case, i learnt some new things.

The problem is that, in my country, there is almost no professional mechanics. I have another car bmw e60. i have always suffered from unprofessional mechanics.
For simple problems, they removed gearbox, disassembled the engine, asked me to buy expesive spare parts (that i did not actually need).
Just like, once they replaced the gearbox fluid. and didn't follow the instructions. And the level was high and rpm was fluctuating. To solve this, they disassembled the engine, then removed the gearbox. asked me to buy spare parts and so on. Tons of money spent, and issue was not solved. So i decided to learn those things in very details.
That is the reason why i am trying to fix every single issues in this car. Because I am not just fixing, but also learning by practicing. Thanks everyone for helps.

 

[Ducked]

If you replaced the head fairly recently that should eliminate carbon build up from the list of suspects.

 

[Ducked]

A while ago I improvised something to draw water into the filter housing at high revs, though I had to add restriction to the air filter to get it to work. If you don't do high revs often this might work around your issue. If you do, you might need something mores sophisticated, akin to a carburetor, to give a constant level, and you might need quite a lot of water.

 

[Ducked]

I suppose it's possible simply adding restriction would work, but maybe not on an FI engine, which might compensate by trimming the fuel so things stay the same