Nismo280zEd Posted July 7, 2004 Share Posted July 7, 2004 OK... while thumbing through the forums the other day i came accross an arguement about the Arizona Z car 4 bbrl intake setup vs. conventional... Anyway... i was readn' about how the crankcase ventilation tube should be capped off, not vented. So i'm thinking about this today.. and it dawns on me... I completly forgot the L series engine is pressurized compared to other engines. So it makes since that it should not be left open.. thereby causing a vacum leak liek when you take the oil fill cap off or the dipstick. So i blocked it off... Then i'm thinking... well it that shouldn't be vented, maybe the opening on the valve cover shouldn't be either... then i start thinking about my old EFI setup.... So i'm wondering... do i blocke them both off, or? Do i connect the two up with a PCV valve? or should it be tied into my carb intake manifold? Basically i'm asking the proper way to do this. Cause i know everybody with triple webers had to have conquered this issue. Thanx, Ed Quote Link to comment Share on other sites More sharing options...
datsunlover Posted July 7, 2004 Share Posted July 7, 2004 I to am curious about this... PCV stands for Positive Crankcase Ventilation.. right? so.. hmm.. now Im confused.. Where does it go from the factory? The name suggests it should be vented.. so what benifit is there to 'caping' it off? Im thinking mine is wrong right now.. I'v got it going into the air breather I think... Ahhh!!!!!! It's been a long day..... Quote Link to comment Share on other sites More sharing options...
Nismo280zEd Posted July 7, 2004 Author Share Posted July 7, 2004 Well, the factory had it plumbed up into the intake manifold with a PCV valve screwed into the manifold. So basically it vented into the intake manifold (This is how the EFI setup was on my 280z). Also the breather hose from the valve cover went into the intake manifold, along with a bunch of other crap. I capped off my crank case ventilation tube today... made a big difference, gained alot of low end torque on my car, and ran alot smoother. But i would like to know the proper way, or atleast what other people have done with their converted triple webers, or mikuni, or other. -Ed Quote Link to comment Share on other sites More sharing options...
280Zone Posted July 7, 2004 Share Posted July 7, 2004 The valve cover vent tube on the fuel injected cars runs back to the intake. The reason is that the fuel injected cars are verysensitive to vacuum leaks. They are designed to inject fuel based on the amount of air that is entering the engine. Disconnecting it on a fuel injected car will cause the car to not run properly (some people will dispute this as they have retuned their car to run with it disconnected). Carbed cars are not as sensitive so it will not matter as much which ever way you decide to go. Quote Link to comment Share on other sites More sharing options...
stagefumer11 Posted July 7, 2004 Share Posted July 7, 2004 so what is the benifit of capping it off, or chucking a small filter on it? Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted July 7, 2004 Share Posted July 7, 2004 I've asked about this soooo many times... and I think have gotten enough answers to formulate a clear understanding of how it works. Originally, the crank case vent was routed to the intake manifold (thru the PCV valve) and the valve cover vent vented into the intake before the throttle body. Of course, this was designed this way to recirculate engine blowby. Lots of guys do just run the open filters on them. And lots of OTHER guys will always claim how this creates vacuum leaks. But I think this is misunderstood. The PCV system is really a loop... air can pass either way in this loop. Air that enters in thru the valve cover vent can pass thru the block and vice versa. The air that circulates thru there is air that has already been measured by the AFM. Any openings in the loop (like removing the dip stick) will cause a vacuum leak, of course, because now you are allowing UNMETERED air to be drawn in and circulated into that loop. Same would occur if you were to open one of those vents to atmosphere while leaving the other routed to the intake (a mistake I made, not knowing that air does freely circulate throughout the entire engine, the air in the block and valve cover are not seperate). HOWEVER, if you disconnect BOTH the valve cover and the crank case vents from the intake completely, and put filters on both of them, and of course block off the hole in the intake pipe and remove and block off the PCV valve on the manifold, there is no more loop. Air can enter and exit the valve cover and block freely, blowby will still be allowed to freely exit the engine. There is no more connection at all to the intake, and hence air that enters the engine thru those vents will NOT effect your air/fuel mixture... no vacuum leaks. You can remove the dipstick while the engine is running and the idle will not fluctuate. As far as 'tuning around it' I run the stock EFI, well known for it's relative inability to really tune, and I really haven't ever tried to tune it other than just tune UPS, and it suffered no problems when I opened both vents to atmosphere... and ANY vacuum leaks on stock EFI will definately make a noticeable difference. If there is any vacuum leak, I assume it is possible that air could be getting sucked up into the cylinders thru the rings on the intake stroke, like an opposite effect of blowby, but I dont think it does or it's very minimal, not enough to negatively impact air/fuel ratios or cause the car to run badly. On the other hand, the car probably runs a bit better without the PCV hooked up, as you're not recirculating oil steam into the engine. Your intake stays cleaner, and there's no oil to possibly contribute to detonation. However, I would NOT block off EITHER vent. If you block them off you are a) allowing blowby to remain in your engine, contaminating and reducing the effectiveness of your oil, necessitating more frequent oil changes at the least, and if you're running a turbo, and possibly even in an N/A engine, you will be pressurizing the block big time, which can cause main oil seals and oil gaskets to start leaking, usually pretty badly and usually fairly quickly. Even if you were to only block one off you are greatly reducing the ability of pressurized blowby to exit your engine... not a desirable situation. Basically, you either want to leave them both open, or you want to leave them both routed in their stock positions. And of course if you're leaving them routed be sure you dipstick seals well and you dont have leaky gaskets on the motor (valve cover, oil pan) as these will be possible vacuum leaks. I decided it was easier to leave them both open.... keeps the intake cleaner, and gives me less to worry about as far as leaks or potential leaks, and of course it looks cool as well EDIT: I find it odd that your car ran better with the crankcase vent capped off... that suggests to me that there is some problem there, maybe there already is/was a vacuum leak, or maybe your PCV valve was bad? That doesn't seem normal to me. My Z runs exactly the same either vented to the intake or vented to the atmosphere. Quote Link to comment Share on other sites More sharing options...
Nismo280zEd Posted July 7, 2004 Author Share Posted July 7, 2004 I hinted at this but i guess i didn't clearly state it... I'm running tripple webers with a TWM intake manifold. So i can't not simply "hook them up" to their stock locations as there are none. You said capping if off would pressurize the block, do you mean create too much pressure? cause it's pressurized anyways. I also dont' really see how blocking it off would contaiminate the oil anymore than having it open, like you said it's only dirty air going in and out there. Not like carbon deposites are being re-circulated. Or atleast that's my understanding from your post. So are you saying if i ran a hose from the crank, and a hose from the valve cover, and hooked the crank hose up to a PCV valve and into my vacumn, then installed the valve cover hose into the vacumn system, (like the stock manifold) that the car would have OEM properties? Nissan designed the engine to be pressurized for a reason, I would think that is kinda essential to keep. Is the right way of thinking here? -Ed Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 7, 2004 Share Posted July 7, 2004 If you plug the vents you're guaranteed an oil leak. Either take the hoses and run them to a catch can which is vented to atmosphere or do as you said and plug them into the intake. Problem with triples is that if you plug the PCV into one runner, then that runner will burn air/fuel, and whatever oily crap comes out of the PCV. You can split it into 6 lines with its own little manifold, and run one to each cylinder (I think Dan Baldwin did this). Catch can is the easiest. Quote Link to comment Share on other sites More sharing options...
Nismo280zEd Posted July 7, 2004 Author Share Posted July 7, 2004 catch can, like an overflow canister? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 7, 2004 Share Posted July 7, 2004 Yeah, summit, jegs, etc have them Just a can with a couple hose fittings on them and a little filter on top. Quote Link to comment Share on other sites More sharing options...
TimZ Posted July 8, 2004 Share Posted July 8, 2004 Nissan designed the engine to be pressurized for a reason, I would think that is kinda essential to keep. I'm confused here - when did this happen? If the engine is pressurized, then why does pulling the dipstick cause a vacuum leak? Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted July 8, 2004 Share Posted July 8, 2004 yeah I was trying to say, engine is NOT supposed to be pressurized. As a matter of fact, it's a proven fact that an engine would run better if there was strong vacuum present in the block at all times. This is the principle behind 'evacuation' kits, I may be wrong but I think that's what their called. I've seen setups that were just a regular belt driven vacuum pump, with hoses plumbed to block and valve covers, to constantly provide vacuum. This has been proven to provide an increase in HP. Something to do with lessening the resistance to the moving crankshaft, I believe. Either way you definately do NOT want pressure in your block. This WILL cause oil leaks in your engine, and will presumably have the opposite effect of evacuation, which is to say, it will lessen HP, though probably not noticeably. Stock carbed L28's had the vent from the valve cover dumping into the air filter housing. Stock EFI L28's had it dumping into the air stream after the AFM but before the TB. I run mine open... this is the easiest way to go but not the cleanest, hence the suggestions to run a catch can, which is the better way to go if you dont mind the small cost and bit of work to get it set up. If you wanted to do a cheaper, easier evacuation setup, you can route the two vent hoses into a one way valve plumbed into your exhuast. They sell these setups from jegs or summit or somewhere, way cheaper than the vacuum pump setup. This will actively suck the dirty air out of your block and may give a small increase in power. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted July 8, 2004 Share Posted July 8, 2004 I found another theory as to why you'd get more power with neg crankcase pressure. It has less to do with resistance to the internals spinning inside a pressurized crankcase and more to do with the rings. Apparently according to the article I was reading the more pressure differential there is on either side of the piston rings, the better they seal. So basically the rings seal better with vacuum in the crankcase. Quote Link to comment Share on other sites More sharing options...
Guest 2slo4u Posted July 9, 2004 Share Posted July 9, 2004 I thought the reason for Positive Crankase Ventilation was to remove the corrosive fumes from the crankase (the ones that leak past the piston rings) and to keep pressure from building and keep oil from leaking out because of slight vacumm. It seems that vacumm or pressure couldn't help make more hp though because either you'll put resistance on the piston when it is rising -vacumm, or you'll put resistance on the piston when it is being pushed down-pressure. Either way it probably doesn't matter because the pistons rise and drop in a symmetrical fashion which can't produce positive or negative pressure in the crankcase, right? Kinda like a two cylinder when one is rising and the other is falling at the same time, no pressure build in the crankcase, only pressure flowing back and forth between the bottoms of the cylinders? Correct me if I'm wrong, I don't want to misinform. Maybe when they taught us this in school it was outdated? Quote Link to comment Share on other sites More sharing options...
Cody 82 ZXT Posted July 9, 2004 Share Posted July 9, 2004 I read an article once in Hot Rod that tested a vacuum pump on a 500 hp engine. In the test they picked up 16 HP with no other changes. I may have some of the facts off by a few HP but, you get the point. I believe that the system should be evacuated by whatever means but, if you have oil blow by it should be routed into a catch can. I just ordered a catch can off E-bay and I'll be rerouting my system soon to see if there is any difference in the amount of oil layer on the intake. Quote Link to comment Share on other sites More sharing options...
80LS1T Posted July 9, 2004 Share Posted July 9, 2004 Here my take on this..... The catch can idea is a good idea. Diesels often use them and they work great just as long as you empty the catch can once and a while. Otherwise it will just overflow and defeat the purpose all together. As for hooking up your exhaust to your valve cover breather ports? Um NO. The exhaust will not suck the fumes/blow-by into the exhaust. If you've ever had an exhaust leak you will know that there is a "positive" pressure in the exhaust. If you hooked it up to your valve cover breather port you would pressurise your engine and you would have a lot of oil leaks fast. Even if you did have a check valve in the tube, the pressure from the engine would not over come the pressure from the exhaust. Thats just IMOP! Since when does pulling your dipstick cause a vacuum leak? The air that is in the block doesnt not need to be metered since it does not enter the cylinders with the A/F ratio. Maybe I am just misunderstand what is being said here? Guy Quote Link to comment Share on other sites More sharing options...
stagefumer11 Posted July 9, 2004 Share Posted July 9, 2004 so in term i would have a better a/f ratio if i run a catch can off the rocker cover, and block off the pcv vavle. and it looks smarter Quote Link to comment Share on other sites More sharing options...
Xander Posted July 9, 2004 Share Posted July 9, 2004 80lt1, I don't think its a good idea to hookup the port to the exhaust either but don't think that the exhaust can't suck . If you have a leaking exhaust you can feel it blowing when you are idling. But when the exhaust gas speed goes up you can create a venturi efect and this wil creat a low pressure. This is why a leaking exhaust header gasket can throw of your O2 sensor readings. At higher velocities it will start pulling in air. Quote Link to comment Share on other sites More sharing options...
Cody 82 ZXT Posted July 9, 2004 Share Posted July 9, 2004 There are kits out by Moroso that you can hook up you valve cover breathers to a check valve and then to your exhaust. The high flow of exhaust when in full throttle creates a vacuum to pull from the valve cover breathers. Any oil or vapor get thrown into the exhaust. I'm sure some or almost all is burned up. Heres a look at the one I just bought. I'm sure Nismo has no affiliation with this seller but, I wanted one and this one looked good for the price. Most of the ones that I've seen go for about 100.00 plus. I also saw some on the web for 1.00. Very cheesy looking. Yes, I think a more consistant A/F ratio could be acheived by riding any oil or oil vapors from your intake. http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7909601824&category=6763 Quote Link to comment Share on other sites More sharing options...
TimZ Posted July 9, 2004 Share Posted July 9, 2004 Since when does pulling your dipstick cause a vacuum leak? The air that is in the block doesnt not need to be metered since it does not enter the cylinders with the A/F ratio. Maybe I am just misunderstand what is being said here? The stock EFI has always been this way - in fact, most mass aiflow type systems are this way. Pulling the dipstick or opening the oil filler cap lets unmetered air into the intake via the PCV system. The reason air gets in is that the PCV system uses manifold vacuum to try to maintain negative pressure in the crankcase. If you create a leak via the dipstick for example, this translates through to the intake manifold, and the additional air does not go through the AFM. Quote Link to comment Share on other sites More sharing options...
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