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OK guys I’m trying to sort out my catch can situation on my 383 build
Here is my situation and my question..
I’m running an elite engineering catch can.. I am feeding the dirty side from the driver side valve cover.. around the back of the intake and along the passenger side valve cover to the catch can. My intention is to cap off the back port of the passenger valve cover and provide a fresh air feed to the front port of the passenger valve cover. And of course, naturally run the catch can outlet to the intake manifold port behind the throttle body.
My main question is this. I’m running a fast 102 that has an LS2 throttle body on it.. so there is no positive pressure fresh air port on that throttlebody. From everything I’ve been told and read I should be able to grab fresh air anywhere after the MAF.. and prior to the throttlebody. My my question is, wouldn’t that make it a negative pressure source under throttle and not "feed" the valve cover fresh air? And if so, does that matter for the purpose I’m trying to achieve.
Last edited by LS1Silverado05; Apr 23, 2026 at 11:01 PM.
Not the prettiest, but I installed a fitting in the oil fill cap and then added a small 3" ?filter ...........Filter on catch can has since been replaced.with a cap.
I am working to resolve an oiling issue with my set up. Started out with a Saikou Michi dual can, dirty coming from drivers rear valve cover (updated cover with orifice) and fresh air via a bulkhead fitting installed in my intake tube before throttle body.
With two different engines it has sucked oil and now even more so with my current set up. Thinking there was a problem with the Saikou cans I switched to an Elite single can/clean air system with no change, it actually might be catching even more than the Saikou. Then I started doing even more research.
I found that GM instructions for the LS3 crate direct you to use the LS3 valley cover and cap the driver valve cover. With a 454 LSX both the valley and valve cover are used. I installed an LS3 valley cover and capped the valve cover however my current combo is a 427. I will be testing this out over the weekend, hoping for a good result.
Dont seem to have any issue with oil spray or film from the element?
Rings are gapped for up to a 200 shot, and before adding breather/catch can, first pass down the track, it blew out the dipstick which sprayed oil everywhere... not fun on the top end shutdown.
After adding breather/catch can, that hasn't happened. I do see some oil vapor (smoke) from the breather once in a while, but nothing sprayed.
My cam only makes 10-ish on the vacuum gauge at idle.
I guess you could add the filter remotely to alleviate some of that ?
wouldn’t that make it a negative pressure source under throttle
Only if your filter and MAF are restrictive. Butt in no case are they as restrictive as the throttle, under any conditions besides WOT.
You are correct: if you have a MAF, you MUST tap the PCV fresh air feed off from between the MAF and the throttle. Otherwise, tuning will be near impossible, due to unmetered air getting into the intake. Most stock throttle bodies provide a port like this, butt since yours doesn't, just install a bung in the air duct between the MAF and the TB for the fresh air, and you'll be golden. Not sure about 2 ports in the pass side VC though; I'm not familiar with that. Butt plugging one and feeding fresh air to the other seems like the right thing to do, as long as the port you use, has good baffling.
Doug's setup works for him because he's not running a MAF. Doesn't apply to your situation.
Not sure about gjohnson's situation. Sounds like it's routed correctly butt something else is clearly going on. Like maybe the baffles in his VCs, whatever they are, aren't effective enough.
On the Gen III LS1 valve covers the passenger side has two ports and the driver side had one.
The driver side also connected to the rear port of the passenger valve cover and then from there it went up to the intake manifold port with an inline PCV valve in the line just before connecting to the port on the intake manifold. The other port on the passenger VC went to the port on the throttle body. Since your TB does not have a port the best place to install one would be in the air intake tube. Dosen't matter if it's before or after the MAF but I would prefer to install it in between the MAF and the TB as to not have the contents of what's coming through that line gumming up the MAF sensor.
Doug is venting to atmosphere hence not using his intake manifold to pull a vacuum on the crankcase and suck that crap back into his engine and like Doug I too am venting to atmosphere. I have a 427 which has 10 AN lines coming off each valve cover to a breather can.
Last edited by 01CamaroSSTx; Apr 25, 2026 at 07:21 PM.
Pretty simple setup. Just cap off the TB and intake manifold and run large enough lines in order to relieve the crankcase pressure. These kinds of setups allows fresh air and only fresh air to enter the intake manifold!
It ABSOLUTLEY DOES MATTER whether the fresh air is taken off before or after the MAF.
If it's NOT after the MAF, then whatever air gets drawn into the intake through the PCV valve, will NOT have passed through the MAF, and the ECM will therefore not know that it's there, and will therefore be unable to properly deliver fuel for it. Basically it will act like it has a giant intake leeeeeeeeek. At idle especially, butt also at cruise, it will run WWWWWAAAAAAAAAAYYYYYYYY lean. At WOT, when vacuum is low, it won't make much difference, butt at all other times and most especially idle, you'll end up with a weird runs-like-crap problem. It can be somewhat tuned out, by just fattening up the idle a whole bunch, butt setting it up RIGHT - which is, a closed system with NO "breathers" or any other vents directly to atmosphere, and the fresh air picked up from between the MAF and TB - will make tuning MUCH easier.
Doug has a carb, so, no MAF. His setup can therefore vent to air if he wants, without causing him a problem. You have a MAF, therefore CANNOT do this. (or at least, shouldn't) His setup is not applicable to your car.
It ABSOLUTLEY DOES MATTER whether the fresh air is taken off before or after the MAF.
If it's NOT after the MAF, then whatever air gets drawn into the intake through the PCV valve, will NOT have passed through the MAF, and the ECM will therefore not know that it's there, and will therefore be unable to properly deliver fuel for it. Basically it will act like it has a giant intake leeeeeeeeek. At idle especially, butt also at cruise, it will run WWWWWAAAAAAAAAAYYYYYYYY lean. At WOT, when vacuum is low, it won't make much difference, butt at all other times and most especially idle, you'll end up with a weird runs-like-crap problem. It can be somewhat tuned out, by just fattening up the idle a whole bunch, butt setting it up RIGHT - which is, a closed system with NO "breathers" or any other vents directly to atmosphere, and the fresh air picked up from between the MAF and TB - will make tuning MUCH easier.
Doug has a carb, so, no MAF. His setup can therefore vent to air if he wants, without causing him a problem. You have a MAF, therefore CANNOT do this. (or at least, shouldn't) His setup is not applicable to your car.
Wrong, I have a MAF on my car and I'm venting to atmosphere..So with that being said I'll just say that any air coming through my MAF sensor has been accounted for in the tune.
On a stock system where you're using the intake manifold to pull air through the crankcase. That air that's being pulled through the lines of the valve covers from the intake port and throttle body is already metered air. A MAF or even an SD tuned engine that uses the intake manifold to pull a vacuum on the crankcase has that air already accounted for. SO IT DOES NOT MATTER if you elect to run the line off the valve cover to the the throttle body, air induction tube or the air box. Now if you were to put a breather on a valve cover and you're still using the intake manifold to pull air through the crankcase then you're now introducing unmetered air into the system. Another issue I have about this whole fresh air feed BS is flawed to me because the line that goes from the valve cover to the throttle body or air induction tube has the same blowby gases as the line from the other valve cover or valley cover that's connected to the intake manifold. Same oil/gases/condensation coming from both lines and I can only assume that it's done this way to help circulate the air better.
Like I said "The only air that's going into my engine is the air that's entering the intake manifold by way of the throttle body which is being drawn through an air filter in the air box" If you look carefully you'll notice that the TB and intake manifold are capped off. There is a 3/8" line coming off the valley cover to a small breather that's close to the battery next to the passenger fender.
Last edited by 01CamaroSSTx; Apr 26, 2026 at 06:41 PM.
Only thing a PCV does is reduce emissions by burning off any vapor produced by the combustion process (contaminates) that gets past the rings along with any water vapor produced from the humidity in the air along with the heat cycling of a motor from a cold start to full temp, and back again.
Running low tension rings, guys will use a vacuum pump to create a negative pressure in the crank case which helps the rings to seal better and making more horsepower.
I greatly appreciate the input on this guys. Seems like there's some conflicting opinion, which is what led me back to my initial question of the intake between the MAF and TB being under negative pressure. In my mind, under any significant throttle that intake tract goes under negative pressure and would be drawing in unmetered air. If in fact is an ambient pressure or positive pressure then obviously its "feeding" fresh air to the VC and there is no real concern of unmetered air being drawn in. I just don't want to fight erratic levels of metered and unmetered air.
what led me back to my initial question of the intake between the MAF and TB being under negative pressure. In my mind
How restrictive are your air filter and MAF? Those are THE ONLY things that will cause "negative pressure", aka vacuum, there.
However all that may be, it doesn't matter even if so. The intake manifold itself ALWAYS has more vacuum (lower pressure) than that section of tube NO MATTER WHAT, so air will ALWAYS flow from the tube, into the one valve cover, through the engine to the other VC, and back out through the PCV valve into the intake plenum; and NEVER any other way. Since ALL of these connections are behind the MAF, ALL the air passing through them will also go through the MAF to get there, and therefore will be metered properly.
PCV was originally designed and intended as an emissions control system. It was one of the first that was created. However it was soon realized that it also flushes all manner of nasty vapors such as water and combustion blowby out of the crankcase and draws them into the intake where they are completely harmless (at least if all the liquid oil has been baffled out of them), and can be harmlessly burned, thus keeping the oil clean. Back when oil only lasted 3000 miles it wasn't such a big deal, butt nowadays with oil that can go 2 - 3 times longer than that as long as it stays uncontaminated, keeping it free of contaminants is ALOT more important. Don't believe anybody that says you don't need it. On a pure race-only engine, they may be right; on anything street-driven, it's needed.
I should mention also, that the factory gets the fresh air from that exact spot. In most TBs there's a port on the front of the TB, facing into the intake tube, in front of the blades and behind the MAF, that goes through the TB and into the intake without any other openings or branching off, there's a hose nipple attached to that port, and the pass side VC gets its makeup air from that. Your 05 is typical of ones that were built like that.
I'm turbo, so I need the can to vent, I'm needing to bleed off crankcase pressure.
You'll find that people like to move the valve cover outlet to the middle or front of the valve cover, when you accelerate where does the oil want to go... to the back of the valve cover.
I'm running flipped stock valve covers with -10 AN fittings added.
How restrictive are your air filter and MAF? Those are THE ONLY things that will cause "negative pressure", aka vacuum, there.
However all that may be, it doesn't matter even if so. The intake manifold itself ALWAYS has more vacuum (lower pressure) than that section of tube NO MATTER WHAT, so air will ALWAYS flow from the tube, into the one valve cover, through the engine to the other VC, and back out through the PCV valve into the intake plenum; and NEVER any other way. Since ALL of these connections are behind the MAF, ALL the air passing through them will also go through the MAF to get there, and therefore will be metered properly.
PCV was originally designed and intended as an emissions control system. It was one of the first that was created. However it was soon realized that it also flushes all manner of nasty vapors such as water and combustion blowby out of the crankcase and draws them into the intake where they are completely harmless (at least if all the liquid oil has been baffled out of them), and can be harmlessly burned, thus keeping the oil clean. Back when oil only lasted 3000 miles it wasn't such a big deal, butt nowadays with oil that can go 2 - 3 times longer than that as long as it stays uncontaminated, keeping it free of contaminants is ALOT more important. Don't believe anybody that says you don't need it. On a pure race-only engine, they may be right; on anything street-driven, it's needed.
I'm clearly overthinking it. Logic just tells me that if an engine goes under throttle its going to PULL air into it from any open source, to include a line that is connected inline with the intake tract. However, I guess I wasn't accounting for the fact that basically the fresh air being drawn in my valve cover effectively will potentially be drawn OUT the dirty side and thus should be metered for once it ends up in the engine? Anyway I'll just hook it up and see. Thanks!
Last edited by LS1Silverado05; Apr 30, 2026 at 10:52 PM.
Pretty simple setup. Just cap off the TB and intake manifold and run large enough lines in order to relieve the crankcase pressure. These kinds of setups allows fresh air and only fresh air to enter the intake manifold!
yours is exactly like mine, except since mine is a truck and i reached the point to where i didnt want to put anything else under the hood so I put the catch can in the bed of my truck and ran 10an lines under the truck through a grommet in the bed.
@LS1Silverado05 the more power the engine makes, the more crankcase pressure will increase. A 700HP NA engine will have more crankcase pressure than a stock one. And a power adder combo will have even more. My dual -10 lines might in fact be even undersized for future levels of boost. We want to avoid having the pressure pushing out in a spot where we don't want it to... rear main seal for example.
yours is exactly like mine, except since mine is a truck and i reached the point to where i didnt want to put anything else under the hood so I put the catch can in the bed of my truck and ran 10an lines under the truck through a grommet in the bed.
I think that's a great way to set it up!
More lines just means more room to relieve the crankcase pressure and having it behind the cab IMO is better than inside an engine bay. I can tell you that I when I drain my can it's not oil pouring out, only condensation with some traces of oil but it's only when I drive the car in the colder months otherwise I never have to drain it.
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Fresh Air feed to Valvecover? 
