Oil Cooling vs CR

4Cyl Inline 2L on 98 Oct petrol/gas

Who has experience with Piston oil cooling jets and how much compression ratio can be increased keeping the oil temperature controlled to 90-95*C /195-205*F, keeping the cooling water temp as well between 92-98*C / 198-210*F

I understand that there are a lot of things to take in account and consideration, this question is based on who has experience or played with this.

lets say OEM engine was 9.5CR but oil temp was above 100*C/212*F as well as Coolant.

Good questions.

The oil cooling to the underside is to reduce crown temperatures and increase the durability of the piston - the alloy being weakened by heat, but there must be at least some, if small, reduction in charge heating.

It's going to depend on where you want to go, but a lower temperature coolant thermost with a better radiator if required, and an oil cooler, should be of much greater benefit, and be both easier and cheaper than fitting the 'jets - you may also need to fit a higher capacity oil pump to counter the additional "internal leakage" and resultant oil pressure drop.

Modern vehicles may be able to run on a much lower octane than 98, assuming it's N/A - my 'shopping basket' has an 11:1 CR, and does need 98.

There is a very thin layer of gas right above the piston top surface that prevents piston from overheating from very high combustion temperature. When knock occures it removes that layer and allows combustion temperature to overheat the piston often causing it to be seazed or even melted down the bore. So all being normal there is huge difference in temperatures between combustion chamber and the piston. Improving piston cooling by oil or water temperature wouldn't make noticeable difference especially in short pulls like in drag racing. Making those things cooler benefits to lowering overall knock threshold (by cooling down the entire engine and cylinder head) which is important during long runs ( rally, time attack etc)

Will the idea is to avoid the engine build to be knock limited, if the piston and cylinder head can be kept cooler.

I don't know at which point increasing the CR will be pointless if timing need to be retarded.

I'm trying to achieve 12.5CR but it will be pointless if with a 11.5CR I can run more timing.

It mostly depends on octane rating of fuel in use- that is the most limiting factor as far as knock threshold goes. The other issue with very high compression ratio is that sometimes the coolant temp goes too high not helping to reduce knock threshold but that can be usually solved by installing a big thick aluminium radiator.

I wouldn't give any concern to additional oil cooling, I would be more interested in the specific engine* characteristics, quench clearances, controlling coolant temperatures, ensuring as cool a charge as possible - cold air induction, heat shielding, phenoic/heat barrier gasket between manifold and head, possibly painting intake and exhaust systems with heat barrier paints and maybe lagging the latter, etc.

12.5:1 should be possible with most engines, especially with longer duration camshafts that reduce dynamic compression.

CR and timing is sometimes a trade-off - the former may limit MBT timing, but if the net torque is higher, that's going to be acceptable. Similarly, while the best AFR may be around 13-13.5 for torque/power, you may find a little richer (12.5, or even more so), allows more timing and more net power.

*Exactly what engine are you using, some are much better than others?

Project car, Mitsubishi Colt with a 4g63 NA, on 98 Oct pump gas., class regulations is NA and 2000cc, single butterfly, manual gear box, car need to wait 1100kg, 17" wheels max ( I will run 16") an a bunch of FIA rules and regulations for safety..

Almost everything internal is custom made to my specs, crank and block is the only OEM parts with little mods.

Happens that I got the car for little to nothing, and I have spare engine, so why not.

Ultimately testing is likely required to find what's optimal for your particular package of parts, fuel, conditions, use case.

Personally I agree with temperature control being part of making the most of an engine. Like you said, if you have to reduce timing to avoid knock, the extra compression won't help as much.

You don't necessarily want to regulate the engine at a cooler base temperature though. The engine may make best power while relatively hot, right up to the point where you'd have to retard ignition unfavorably.

Since squirters are easy on that engine may avoid the oil pump sitting on the relief much of the time, it may be a benefit. If it's going to prevent you from having sufficient oil flow to other areas at high RPM though, then I'd skip the squirters.