Sharp edges on combustion chamber cause of light load knock

[scoobydo123]

Hi all

Head had previously been skimmed at least once FSM states 8 thou max at least 23 thou has been skimmed as timing cover was not skimmed with the head so that was a witness to of at least one head skim. Engine seemed to run OK but could hear light load knock through knock sensor connected to headphones. Corrected ign timing with adjustable cam sprockets but still getting light load knock even if I retard ign 12 degrees ;-( Had to take head off due to different issue and have found that the combustion chamber has what I would deem to be sharp edges. After cleaning old gasket off it is sharp enough to rip kitchen towel when wiping the head face off. Could this be a cause of the pinging? Engine is ka24de and running Tesco or Shell premium fuel. Picture of the head included

[hidetheelephants]

It won't help, but I'd be looking at the compression ratio more TBH. Bit sloppy to leave a sharp edge like that regardless of whether it's in the combustion chamber or not.

[996Keef]

I'm not familiar with KA24DE specifically but when a lot of material is removed from a cylinder head face, the timing is affected and will need to be corrected with an adjustable cam sprocket.

Provided your compression ratio isn't like 12:1 it should be OK on super.

Maybe it's over-advanced now timing wise making it ping

[ridds]

That has been skimmed a lot, still not fully flat and looks more heavily skimmed on the Intake side.

More likely you Static CR is pretty high. Have you measured it?

[scoobydo123]

996Keef said:

I'm not familiar with KA24DE specifically but when a lot of material is removed from a cylinder head face, the timing is affected and will need to be corrected with an adjustable cam sprocket.

Provided your compression ratio isn't like 12:1 it should be OK on super.

Maybe it's over-advanced now timing wise making it ping

Have already used adjustable cam sprockets to take up the retard. Prior to adjustable cam sprockets IGN timing was way retarded and had to advance the dizzy to the end stop to get near correct ign timing ( dizzy runs from back of exhaust cam) now it sits mid way for base timing of 15 degree.

Have pulled 12 degree of timing and still it pings.

[scoobydo123]

ridds said:

That has been skimmed a lot, still not fully flat and looks more heavily skimmed on the Intake side.

More likely you Static CR is pretty high. Have you measured it?

No I haven't measured static CR. Didn't have any perceived head sealing issues, but yes attention to detail on the previous head skim maybe not the best. Car gives decent performance and very good economy but would like to cure the light load knock. Hence investigation whilst head is off. Do you think the sharp edges could be the problem?

When I come to reassemble was going to get a head saving shim which is 20thou to de compress it.

[stevieturbo]

scoobydo123 said:

Have pulled 12 degree of timing and still it pings.

Is this the same engine you've been complaining about "pinging" for a long time now ?

If you've pulled 12deg and it still genuinely does it, there is a major problem, far beyond a few sharp bits in any chamber.

[scoobydo123]

Yes same engine that was pinging and hydro-locked at idle when an injector pintle decided to part way :-( Got any suggestions for me. Obviously whilst head is off assume I can debur the edge with a dremmel so that is as least as good as it can be prior to putting back together with a 20thou decompression shim.

Plan is to re-assemble without shim and see if cranking compression is back to balanced and then order shim from USA and new headbolts. If compression still down then have to see if I can get some rings / pistons from USA

Thanks

[Boosted LS1]

Have you got safe piston to head clearance?

[GreenV8S]

Have you double checked the mixture?

[scoobydo123]

Boosted LS1 said:

Have you got safe piston to head clearance?

There is no contact but haven't measured.

[scoobydo123]

GreenV8S said:

Have you double checked the mixture?

Have checked the output of the narrowband o2 sensor and it seems to respond as it should. Injectors have been flow checked and cleaned, changed out fuel pump to a bosch and checked flow delivery to fuel rail and got a decent flow rate at the fuel rail.

[richhead]

While the head is off, i would def be checking the compression ratio, to high may cause your problem

[stevieturbo]

scoobydo123 said:

Have checked the output of the narrowband o2 sensor and it seems to respond as it should. Injectors have been flow checked and cleaned, changed out fuel pump to a bosch and checked flow delivery to fuel rail and got a decent flow rate at the fuel rail.

narrowband a complete waste of time for mixtures other than stioch

And really, even some wet'n dry would be fine for smoothing things off. Wouldn't even need a dremel

[GreenV8S]

scoobydo123 said:

Have checked the output of the narrowband o2 sensor and it seems to respond as it should.

While it's knocking?

I guess that by 'as it should' you mean the output is cycling rich/lean reasonably quickly. That shows ths feedback loop is working, but under the circumstances it would be worth checking the actual AFR when the problem happens. The symptoms suggest you've got too much compression and/or too much advance and/or too little fuel - addressing any of those will help.

[Pete54]

The standard compression ration is 9.2:1, it would be easy to calculate the new compression ratio. If you have already retarded the timing by 12 degrees then the suspicion has to be the current compression ratio is causing the problem. The sharp edges will accentuate it, but they are easily dressed out. How much? Well again it depends on the compression ratio.......

[scoobydo123]

GreenV8S said:

While it's knocking?

I guess that by 'as it should' you mean the output is cycling rich/lean reasonably quickly. That shows ths feedback loop is working, but under the circumstances it would be worth checking the actual AFR when the problem happens. The symptoms suggest you've got too much compression and/or too much advance and/or too little fuel - addressing any of those will help.

Hi one thing i did do is fudge the coolant temperature resistor to report 50c in an attempt to provide some enrichment and this had no effect in knock reduction but maybe enrichment is turned off at 50c certainly idle speed was increased so ecu was detecting a warm up condition required. The ping is really on the lightest of throttle increase.

[stevieturbo]

scoobydo123 said:

Hi one thing i did do is fudge the coolant temperature resistor to report 50c in an attempt to provide some enrichment and this had no effect in knock reduction but maybe enrichment is turned off at 50c certainly idle speed was increased so ecu was detecting a warm up condition required. The ping is really on the lightest of throttle increase.

If things are tuned correctly, hotter air is irrelevant for mixtures....as the AFR should remain constant regardless.

Timing may be reduced though.

But as it seems there is no mixture information....all guesswork anyway

[Pete54]

There is no point is asking for opinions if you are not prepared to listen.

What is the current compression ratio? - without that you have no idea where you are starting from. Massively retarding the ignition timing and fiddling with other parameters is not addressing the basic problem, but suggests it is pretty severe.

If the compression ratio is very high none of this will work. If it is marginally higher than standard, then is it genuinely 'pinking'. You seem to think so, so maybe there is pre-ignition due to the sharp edges - but maybe.

Fault finding requires some knowledge and a methodical approach - only then will you get to whatever the 'answer' actually is.

[InitialDave]

OP, I would start with the very basics - what is the volume of the bowl in the head (and are they all the same?).

Thickness of the compressed head gasket, piston crown depth below block surface, bore, stroke etc - what is the compression ratio coming out as when calculated, is it what you think it should be?

Is actual TDC where the timing marks claim it is?

Establish what your starting point is based on measurements of what you physically have, not what it "should" be, and work from there.