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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
I got a 383 stroker in 85 z28. It seems I'm always chasing new oil leaks. Currently I got the rear of intake/oil pressure sensor standoff leaking, Oil pan gasket leaking, Fuel pump gasket leaking, valve cover gasket leaking (passenger side). I will note the rear of intake happens at high rpms.
It's strange because everything is fairly new (~10-15k kms on rebuilt motor)
I just got a new pcv valve, hose and breather as they're all pretty inexpensive and wanted to see if it'd help, it didn't. Got new valve cover gaskets (tried the felpro blue rubber ones and they weren't good, went cork w/ a bit of silicon and it's working better so far).
I'd say measure the crankcase pressure. You can use a 1-bar map sensor or gauge which reads inches of water (We desire 20" Water in the crankcase) to do this.
And then modify the PCV system to bring the pressure down below atmospheric so the oil leaking stops. Target 10-20" Water (1" to 1.5" Hg) at idle and WOT, both.
I see a breather which is not going to do it. It will require an active suction on the crankcase using the air filter tract IMO.
Some people are satisfied with breathers-only. However when using a breather the pressure will always rise over atmospheric at WOT which will always facilitate oil leaking.
For SBC the oil seals are not nearly as great as a modern engine, so IMO its even more important to target low crankcase pressure at WOT to prevent leaks.
Remember blow-by causes crankcase pressure, so if you are measuring and noticing very high crankcase pressure it must be due to something in the bottom end causing blow-by for some reason. Perhaps a compression test would be a good place to start. If it's got forged pistons those generally use larger piston-wall clearances and require much warming up, hot engine oil, hot engine to fully expand in the bore before the blow-by diminishes to acceptable levels. But I Don't see the need for forged pistons given there is no forced induction or nitrous so I assume they are no forged.
I'd say measure the crankcase pressure. You can use a 1-bar map sensor or gauge which reads inches of water (We desire 20" Water in the crankcase) to do this.
And then modify the PCV system to bring the pressure down below atmospheric so the oil leaking stops. Target 10-20" Water (1" to 1.5" Hg) at idle and WOT, both.
I see a breather which is not going to do it. It will require an active suction on the crankcase using the air filter tract IMO.
Some people are satisfied with breathers-only. However when using a breather the pressure will always rise over atmospheric at WOT which will always facilitate oil leaking.
For SBC the oil seals are not nearly as great as a modern engine, so IMO its even more important to target low crankcase pressure at WOT to prevent leaks.
Remember blow-by causes crankcase pressure, so if you are measuring and noticing very high crankcase pressure it must be due to something in the bottom end causing blow-by for some reason. Perhaps a compression test would be a good place to start. If it's got forged pistons those generally use larger piston-wall clearances and require much warming up, hot engine oil, hot engine to fully expand in the bore before the blow-by diminishes to acceptable levels. But I Don't see the need for forged pistons given there is no forced induction or nitrous so I assume they are no forged.
I have done a compression test and all cylinders were between 170-180.
Do you have pictures or any information on how to connect a line to the air filter? Every video I see of people setting these up use a breather but I have heard lots of people say what you are saying
What is the best way to measure crankcase pressure? I don't own an air compressor or leak down kit so can't do a leak down test. But if that's the only way then I guess I don't have much of an option.
Easiest way is take a 1-bar map sensor provide it with 5v and ground and attach to the crankcase
Heres mine on my car, just a factory 1-bar sensor from any engine
Use 2 different map sensors to repeat the test and compare results. 1 of the sensors could be bad, I had a bad one the first time i tried this so now Im careful to test with different 1bar map sensors each time.
Heres how alot of people do it instead like for dyno, just get a spare oil cap and drill/tap the cap for the test. then put back the old oil cap when finished.
You can make 12v into 5v using a small voltage converter device, something like these.
eBay item number: 384892160106
eBay item number: 114720734903
Most USB devices are 5v so you may have one already.
I recommend using a OEM 1-bar map sensor from a Chevrolet vehicle with the 5v input from one such device, keep the power separate from the car power, provide with a different battery.
For example I use 6x AA batteries to make 12v input to my step down to 5v. You don't want the car battery input power because it could influence the results during WOT when alternator voltage snakes around.
Do you have pictures or any information on how to connect a line to the air filter?
You just need something between the filter and carb with an attachment for the hose
Quick paint draw
At WOT the pressure between the filter and carb butterflies will drop based on the air filter flow rate. This suction pulls on the crankcase, pulling blow-by out of the crankcase, producing a pressure drop inside crankcase which prevents oil leaking, and protect oil quality.
Air filter flow rate is setup to provide the desired pressure drop at WOT
For example if the air filter flows 500CFM @ 0.5" Hg pressure drop then when the engine is flowing 500CFM the crankcase will be supplied 0.5" Hg which is just barely enough
IF you install a 800CFM air filter and the engine only uses 500CFM then it will not develop any pressure drop at all, crankcase pressure will rise and the engine will begin to blow oil
Thus the air filter flow rate is a SET value based on crankcase pressure requirement, desired crankcase pressure is set by air filter flow rate pressure drop
Another example, air filter flowing 450CFM total and the engine uses 500CFM is perfect, it will develop a 1.0" to 1.5" Hg pressure drop and protect the engine.
Yes this will cost power but the choice is either power supplys PCV action and protects the engine and prevents oil leaking, OR You sacrifice the engine health and allow oil to blow out from the engine (Catch oil in a catch can and blow oil from every seal) and get the full power of the engine.
Alternatively you can use a vacuum pump to get both the power AND crankcase pressure drop, but vacuum pumps are unwanted extra maintenance and potentially unreliable for long term daily drivers.
However there are even more solutions available, such as exhaust driven PCV and electric vacuum pump driven PCV.
Bottom line is no matter what you choose, the crankcase pressure needs to be measured, and set purposefully. Just like a fuel pressure setting. Never assume
05-23-2022, 09:03 PM
