Project C7 Corvette Stingray, Gaining octane with Moroso’s Air Oil Separator

Getting oil in your intake tract is never a good thing.  The worse thing is that an oil mist that gets sucked into your cylinders can cause the engine to detonate.  You all know that detonation is the #1 killer of high performance engines. Oil has a significantly lower auto ignition point than gasoline so when it is introduced into the combustion chamber during the intake stroke it acts just like low octane fuel in your tank. Oil also causes sludge that gums up your intake manifold and fouls your ports, contributing to performance hampering valve deposits.  Since our LT1 engine has a high 11.5:1 compression, it needs all the help it can get to avoid detonation on our crappy 91 octane California pump gas. To make sure as little oil mist gets into our intake manifold as possible, we installed Moroso’s air oil separator into our C7’s PCV crank vent to clean the engines blowby gasses before they are introduced into the intake manifold.

The Moroso air oil separator is a polished billet aluminum can that basically acts like a filter for blowby gasses.  It is machined in two parts so it can be disassembled for cleaning.  It is full of stainless steel mesh which is rolled like a jelly roll and acts like the filtering medium.  When blowby is introduced into the can, the oil sticks to the mesh and any water vapor in the blowby gasses condenses on the mesh and flows to the bottom of the can.  It’s very simple but effective.

The air oil separator kit comes with all the fittings, and hoses to do the installation.

A bracket specific to the C7 assures that this will be a clean and simple installation.

First we applied liquid teflon thread sealer to all of the fittings.


  1. The Moroso “catch can” is probably one of the WORST designs out there. It really saddens me that you didn’t use one that actually works. If you look at the SAE paper on crankcase ventilation filters, you can see how awful that design is. Something like the Mann+Hummel ProVent 100 is not significantly larger than the Moroso catch can, and at least an order of magnitude better at getting oil vapor out of the blowby gasses. Not to mention that, when properly installed, it automatically drains back into the sump, which means you don’t have to drain it. You just need to replace the filter element when it gets clogged, which isn’t very often. (Get the pressure relief version, and it will start to dribble out the side, letting you know when to change the filter)

    1. In track testing the Moroso works flawlessly so far, not one bit of oil mist past it. Do you have any empirical evidence that it doesnt work well? I feel there is nothing at all wrong with the design and would not hesitate putting it on any car. I would run something else with a higher volume, more baffles and an oil return on flat engines that have venting problems like Subaru’s and Porsche but no problem for most things. Provents are designed for diesels that do not have a vacuum in the intake manifold so there is a check valve that must be removed. Then you have to make brackets and buy hoses, etc. to adapt it to the car. Times money and this is a simple kit that installs in about 30 minutes.

      1. Tested a design that is nearly identical in execution to the Moroso design. Just because you can’t SEE the oil mist doesn’t mean it isn’t condensing on your intake valves, or altering your combustion. Unless you’re actually MEASURING the oil mist before and after the catch can, you can’t definitively say that nothing is getting by it.

        The Provent 100 that I have on my car does not, in fact, have a check valve that needs to be removed. People say that, not actually understanding what the valves are for, nor how they work. The valve in the top of the Provent 100 is the “control valve”, which, if you install the filter correctly, will remain completely open, until crankase pressure drops into vacuum (only takes ~2in-hg for mine to close. My magnehelic gauges don’t go that high, and my vacuum gauges aren’t super precise that low), at which point flow is restricted, until crankcase pressure rises back above the cutoff. It is self-regulating, since the vacuum source in a diesel is the intake hose between the filter and turbo. If your PCV valve is too aggressive, and your crankcase inlet is completely blocked, then it can get pegged shut at idle, but that’s not terribly likely, in my experience, and it opens back up, as soon as you roll onto the throttle. The only other valve is the relief valve, which bleeds off excess crankcase pressure, when and if the filter is clogged.

        Your comment about brackets and hoses is valid, but when I’m looking at ~20-50% efficiency with the MOROSO style design, and 75-98% with the MANN+HUMMEL design, There’s no way I’m going to use the MOROSO.

        1. Do you have quantitative evidence that the Moroso separator performs that badly in this application or is this your opinion? Do you any evidence that invisible oil light end fumes which are very similar to gasoline and in fact are parts of the hydrocarbon mix that can be called gasoline, contribute heavily to valve deposits and or detonation or is this again your opinion?

          1. I notice that you have not even acknowledged any of my points, other than to contradict them with no evidence of your own. I’m not the author of the article, and thus I have no vested interest in either product. You’ve not presented any evidence that your choice works, other than manufacturer’s propaganda “marketing”, while effectively ignoring the SAE documentation that I presented. I wish you all the best, but I miss the old NPM quality of writing.

          2. No I am wondering why you spend so much time and mental power on a catch can which is a super simple thing to add on an engine. I am prodding you to see why you think there is such a need for something like this or is this like when people get all anal and argue for days about what is the best oil to run in their totally stock car. I would prefer to devote more thought to other things on my motor and just buy something quick and easy that works well like this part. The only time I would bother is if there was some sort of problem that would require it and if there was, there was probably something seriously wrong with the engine or it is a Subaru. In my experience I haven’t had to “engineer” a catch can system even on high dollar, high boost, dry sumped race engines. Of course we evaluated this system, we ran it hard all day on the track, probably a few hours of wide open throttle and close to it and nothing got past it. NPM was a joke compared to what we do now. At NPM no engines were built, nothing was track tested, part were rudimentary, nothing was raced, etc. At NPM no one was a subject expert. At MotoIQ we do extremely high power builds that compete in all different types of motorsports and do well. We do track testing, most of our staff are professionals, etc.

        2. In no way does this SAE article support any of the claims you’re making Josh. Just because you believe the Moroso is “nearly identical” doesn’t actually make it so. Moroso is a highly respected company and their catch cans are actually recommended by a boutique adjustable PCV valve manufacturer because of their quality, efficiency, and low pressure drop. Bad mouthing one of the original aftermarket manufacturers based on one largely irrelevant technical paper is not just poor form, it makes you look like a fool too.

  2. I read comments here actively because the dialogue is interesting and debate is a productive process. However, the time Mike takes to curate this site for us should be respected. You may have valid points and we are certainly all fallible, but show a little tact and respect.

Leave a Reply

Your email address will not be published. Required fields are marked *