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Choose the correct Compression Ration

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Hello everyone,

is there any way to calculate the correct Compression ratio for an engine?

I explain my issue:

Its the third time now i rebuild my entirely sleeved and forged engine (1,6 liter 4 cylinder direct injection with OEM 10.5 CR)

The first time one piston melted and piston rings on all pistons were broken,the second time one spark plug electrode melted away under load.

The problem is that i did not hear any knock with my "phormula" in ear knock detector,also on the logs from my SCS standalone ECU i dont see any knock,EGTs are also ok,so i suppose that my engine suffers from self detonation before the spark even occours.The result is a broken engine in a few seconds.

Some additional info:

Stock,the engine runs a CR 10,5 and max boost is 1bar absolute.

I am still running 10,5 CR with the build engine but boosted it up to 1,8b absolute.

Running 98 octane fuel.

Spark plug indicator 8.

Intake temps not higher than 34 degrees.

Am i missing something?

If i am right,how to find the limits to avoid self detonation (No normal knock!)

I just dont want to build the engine a 4th time...

Thank you everyone

Best Regards


From factory your engine is ballanced in terms of combination CR-boost-fuel octane rating in order to produce specific amount of power. By almost doubling the boost you change the balance by increasing the combustion chamber presdure (and therefore the temperature). That change should have been compensated by some means: decreasing CR, increasing fuel octane rating or retarging ignition timing. If none of that was done you probably getting combustion chamber temperature too hot and as a result parts are melting because of pre-ignition... For DI engine and 1.8 bar of boost (2.8 bar absolute) i would suggest CR not higher than 8.8-9 :1 or if you want to squeeze a little bit more power from your set up maintaining the same 10.5:1 CR switching to E85 fuel.

Thank you for your reply

I will reassemble the engine with a CR of 9.5:1 then and give it a try on 102 octane with the same boost

But how do you recommand setting spark advance here to not pass the cylinder temperatures?

Because pre-ignition is not detectable by the knock sensor?

Pre-ignition, which is probably what was occuring, is different from detonation, and may not be detected as it isn't the result of colliding flame pressure fronts.

I say probably, because the spark plug may have been acting like a glow-plug, igniting the fuel too early as the compressive temperatures rose - basically, acting like a compression ignition diesel.

Oh, you said "spark plug indicator 8", if that's the heat range, what was the OEM, and you do need to be careful as some brands go colder as the number increases, others get hotter as the number increases - you really need to pay attention! I'd look at at least two heat ranges colder/harder and work back from there towards the OEM if fouling is an issue. There are plenty of guides on-line on how to 'read' spark plugs, and what to look for.

Spark plug timing is still done the same way - mean best torque short of detonation - a couple of degrees back should give head-room for fuel variations and heat soak.

If you can post pictures of the spark plugs, and the pistons, it may be possible to give better advice?

When dealing with high octane fuels where it's very difficult to find MBT ignition timing without a dyno i use one method which at least gives me a clue on ignition timing trends. I reduce the boost very low (0.6-0.7 bar max) and use very retarded ignition timing along with very low octane fuel. By slightly advancing ignition timing (1-2 degrees at the time) i find the detonation threshold where it just starts to take place so chances to damage the engine are close to 0 - it gives me RPM-load-ignition timing baseline. When i change the fuel for higher octane and repeat the exercise, after which i have two charts that alow me to see the trends in ignition timing affected by different octane rating fuels. Usually it's 2-3 degrees difference between 92 and 95 pump gas in my area. Also 2-3 degrees between 95 and 98, 3-4 degrees difference between 98 and 100 but this is all just very general numbers as it all depends on particular place and particular engine.

So when i switch to vey high octane fuel i pretty much already have an idea what ignition timing the engine is going to like but i always start very conservative. And needless to say that knock detection system must be very sensitive and accurate for that type of activity..

And as far as pre-ignition goes usually the knock sensor wouldn't detect it as detonation because technicaly it's not, since there is no cylinder pressure multiplication but just way too early ignition timing caused by some overheated spot (usually it's the tip of the spark plug). When it happens also EGT goes down significantly as most of the energy goes into heating metal parts but it happens so quickly the chances are you wouldn't even notice...

And yes, another very important thing i forgot to talk about is spark plug heat range ( thanks for picking it up, Gord). It would be very wise to go at least one step colder on those...

Thank you guys for the detailed answers,i appreciate this!

The engine runs OEM the NGK plugs with heat range 7, i was running heat range 8 now (Sorry i missed this detail)

I will probably try heat range 9 now

At the moment i have only one photo of the plug melted (attached) and i will disassemble the engine soon to see the rest of the damage an post photos here

Thank you georg for the hint,i will try it this way the next time by starting low boost,low octane fuel and very retarded spark and see what it gives

I was tuning on a dyno,not in steady state but with power runs to not overheat the engine,i was going with 2 degree steps to advance the timing,every run resultet with higher torque and power,also no knock was heard with the in ear knock detector,as well no knock shown on the datalog,so for me everything was fine,i was happy with the torque and power for the moment so i decided to give it a try on the road,and thats the moment where it happened,i did not even recognised it when driving,only when stopping by a red light,i felt and heard that the engine was running only on 3 cylinders anymore

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