AKSSS Posted March 14, 2011 Report Share Posted March 14, 2011 I just want to start by saying that I am not worried about fuel milage. I just am curious as to what the heck is going on to have such a drastic change. Ok, So with my old setup I was running a sts turbo with their pcv setup. I was getting around 16-19 mpg on the highway taking it pretty easy. I was quite impressed. I was doing better than my wifes 08' tbss. lol. Well I swapped over to a kbracing front mount and I changed my pcv setup to basically the same setup as what Mr. P had talked about in an old post a few years back (http://www.silverado...__1#entry675006). I now have ran my dr. side vent to a catch can, then to the manifold. Inbetween the manifold and the catch can, I put in a one way check valve that will stop any boost from back feeding to the catch can or pressurizing the crankcase. Then I put a filter on the oil filler cap that I got from Summit (thanks retired99, I was to lazy to build it). This should have vented everything correctly. Well now, I am averaging about 11-14mpg. Yes, I have been working on the ve tables (with alot of help from wheatley), and either way about it, my truck is running in closed loop 2bar. It always should, and is right around 14.7 at cruise. So what is causing the reduction in fuel milage? I would have thought with the gains in efficency that the milage would have gone up. Could changing the way that the crankcase is vented change the milage? Ideas? Quote Link to comment Share on other sites More sharing options...
chpspecial Posted March 14, 2011 Report Share Posted March 14, 2011 I doubt that that is your problem, but why don't you set up the PCV system like it was withthe STS kit and see for sure if that is what is causing your drop in mileage... Just keep the driverside check valve in place. That will tell you for sure if it is due to the PCV routing. Quote Link to comment Share on other sites More sharing options...
AKSSS Posted March 14, 2011 Author Report Share Posted March 14, 2011 I doubt that that is your problem, but why don't you set up the PCV system like it was withthe STS kit and see for sure if that is what is causing your drop in mileage... Just keep the driverside check valve in place. That will tell you for sure if it is due to the PCV routing. This might be a good idea. I might try this. Are you still running the sts pcv system? And if you are what boost levels have you ran it up to? I still have my entire sts setup sitting here in a box. Been thinking about sell it, just not sure. Now it will at least be put to use again. Quote Link to comment Share on other sites More sharing options...
stg313 Posted March 14, 2011 Report Share Posted March 14, 2011 Sorry but what the heck is a PCV system? Quote Link to comment Share on other sites More sharing options...
chpspecial Posted March 14, 2011 Report Share Posted March 14, 2011 This might be a good idea. I might try this. Are you still running the sts pcv system? And if you are what boost levels have you ran it up to? I still have my entire sts setup sitting here in a box. Been thinking about sell it, just not sure. Now it will at least be put to use again. Yes, and I have taken it to 15# with no issues... other than I need a bigger exhaust housing. Which I already have but need to install. Sorry but what the heck is a PCV system? Positive Crankcase Ventilation. It vents crankcase gases and and pressure into the intake manifold, but with a boosted vehicle, boost changes the direction of the venting and causes a few problems. I.E. HP, emmisions, and dipsticks flying out of the dipstick tubes pushing oil out of the tube on to the headers and starting an oil fire Quote Link to comment Share on other sites More sharing options...
misterp Posted March 14, 2011 Report Share Posted March 14, 2011 The PCV system when setup correctly should not change the fueling/behavior of the engine at all. There are two reasons for sucky fuel economy - either your AFR is off, or your ignition timing is off. 30-years ago on our roundy-round cars we would remove it completely so as to be able to dial in optimal jetting on our carbs. PCV is a way for air to get into the intake manifold without regulation at the throttle body. The PCV is nothing more than a controlled (or calibrated) vacuum leak. If you suspect that the re-worked PCV system is a contributing factor, it's easy to take 5-mins and change it back to stock. But I doubt that will immediately restore economy. Not trying to sound defensive or invalidating here, but I have modified several PCV systems including my own and it had no effect on economy. I would have to see logs to be able to suggest a economy culprit, do you have any you can share? Mr. P. Quote Link to comment Share on other sites More sharing options...
AKSSS Posted March 15, 2011 Author Report Share Posted March 15, 2011 The PCV system when setup correctly should not change the fueling/behavior of the engine at all. There are two reasons for sucky fuel economy - either your AFR is off, or your ignition timing is off. Not trying to sound defensive or invalidating here, but I have modified several PCV systems including my own and it had no effect on economy. I would have to see logs to be able to suggest a economy culprit, do you have any you can share? Mr. P. Well this is what I was looking for. I was just wondering what other people would think of this theory of mine. I didn't think it would have an effect. Just wanted to ask other opinions. By the way, Mr. P. You have never lead me astray. And no I haven't taken any real good long logs. I am kinda curious how you would log for fuel economy. Unless what you are getting at is how close my afr is to my commanded? Quote Link to comment Share on other sites More sharing options...
misterp Posted March 15, 2011 Report Share Posted March 15, 2011 The logging I am talking about is tuning-101 stuff, PIDs I would include in monitoring would be: Throttle Position Spark Advance STRIMs, and/or LTRIMs RPM MAP (Kpa) KR IAT Front O2s DYN Cylinder Air Commanded AFR Wideband O2 (AFR) If you are running a MAF - MAF Freqency, & MAF Airflow Data from these PIDS should be able to suggest or pinpoint lazy O2s, incorrect fueling, or retarded ignition spark. Tuning issues can be caused by how you implement your PCV circuit because it is a direct air-leak into the intake manifold behind the throttle body - it is a purpose-built calibrated vacuum leak; however that air leak is compensated for during the normal tuning process and the adjustments required are only significant during engine idle. The percentage of air coming in through the PCV circuit under power is a statistically insignificant amount of air (compared to the opened TB). AFR tuning is something you CAN do yourself, if you have the time and software. First/easiest thing to look for is the commanded AFR versus actual measured/observed AFR at the wideband sensor, the wideband should always report within 2% of commanded AFR or else your VE table is off enough to affect your economy. Front O2s - when they get old or fouled, your economy will go to crap because they are not accurate! Even though your PCM is in "closed loop" in theory IF the front O2 sensors are in correct working order then the exhaust will be stoich, but you need to verify this with the wideband (check commanded AFR versus observed AFR during cloosed loop operation). Another economy killer, is aggressive PE settings combined with a driver that does not have a 'smooth right foot'. Also, your commanded AFR vs. RPM table, the EQR might have been richened in the 70-85 Kpa area and in a heavy truck we operate in those higher-demand areas frequently. Sorry to digress, there's a lot to look for in optimizing AFR! But there is no reason that you shouldn't be getting at least stock economy, unless you've changed heads and/or camming or driving habits. Mr. P. Quote Link to comment Share on other sites More sharing options...
misterp Posted March 15, 2011 Report Share Posted March 15, 2011 When you changed to your front mount, did you change: Cam? Heads? Injectors? PCM or PCM tune? Different O2s? Mr. P. Quote Link to comment Share on other sites More sharing options...
AKSSS Posted March 17, 2011 Author Report Share Posted March 17, 2011 When you changed to your front mount, did you change: Cam? Heads? Injectors? PCM or PCM tune? Different O2s? Mr. P. Well I changed the front o2's with new bosch ones, and I worked on the tune a little bit. I am just getting ready to start doing some real tuning up here in Ak. It was 24 above today!! I even found some dry pavement!! yoohoo!! I will start logging this weekend and so how close I am on everything. I also just ordered a fuel pressure sensor that I can log using hpt. I was kinda waiting for that to get here before I start logging and tuning. It is going to be interesting to see what I can get out of this thing up around 14-15 psi as compared to my old setup. Maybe my afr in my tune is off that much. I did a "quick" tune in the middle of winter after I got it running again after doing the setup swap. I will try to get some more info this weekend and see. I hope that is all it is. That would be awsome if all it took was a little more tuning. Quote Link to comment Share on other sites More sharing options...
Krambo Posted March 17, 2011 Report Share Posted March 17, 2011 Tuning issues can be caused by how you implement your PCV circuit because it is a direct air-leak into the intake manifold behind the throttle body - it is a purpose-built calibrated vacuum leak; however that air leak is compensated for during the normal tuning process and the adjustments required are only significant during engine idle. The percentage of air coming in through the PCV circuit under power is a statistically insignificant amount of air (compared to the opened TB).Mr. P. I would challenge this and say that there should be NO leak of unmetered air through the PCV system. The fresh air source for the PC system should come from the intake tube AFTER the MAF and BEFORE the throttle body. Being that the vacuum is far greater in the intake manifold (AFTER the throttle body) than the intake tube (BEFORE the throttle body) in part throttle/idle condition, METERED air will be pulled from the intake tube to flush the crank case and have no influence on fueling. In a WOT condition, vacuum in the intake tube AND the intake manifold will be essentially equal causing no influence to the air fuel ratio as well. Quote Link to comment Share on other sites More sharing options...
misterp Posted March 17, 2011 Report Share Posted March 17, 2011 (edited) I would challenge this and say that there should be NO leak of unmetered air through the PCV system. The fresh air source for the PC system should come from the intake tube AFTER the MAF and BEFORE the throttle body. Being that the vacuum is far greater in the intake manifold (AFTER the throttle body) than the intake tube (BEFORE the throttle body) in part throttle/idle condition, METERED air will be pulled from the intake tube to flush the crank case and have no influence on fueling... I agree you are correct in the factory/stock implementation; in a modified setup using a vented oil fill cap (myself and AKSSS) this is not so, but (I maintain) it is only significant at very low RPM (idle & off-idle). In a WOT condition, vacuum in the intake tube AND the intake manifold will be essentially equal causing no influence to the air fuel ratio as well. I agree you are correct in an engine with perfect ring seal and valve guides/seals; however even in the most well-engineered engines there is blowby at high RPM, meaning that there will ALWAYS be more pressure in the crankcase than the atmosphere (WOT N.A. motor situation) - the result is an oily intake manifold as crankcase vapor is pumped into the intake manifold at WOT by blowby gasses; you can 'filter' this with a catch can, or avoid it completely by using a one-way PCV valve and using a vented oil fill cap, or doing like old carburated SBC's did and using an additional 3-sq in. filter inside the air cleaner to stop oil vapor from entering the carb inlet via the valve cover fresh air tube. You're thinking is solid, you just forgot about the extra blowby. Mr. P. Another thought on this - if you are going to use a catch can to implement a CARB-legal solution to filtering blowby gasses, you have to (1) use a catch can on the SUPPLY line (hose between pass valve cover & TB), and (2) use the one-way FoMoCo PCV valve. 99.99% of all people plumbing a catch-can to their LS-motor (truck, vette, f-bod, GTO) are all doing it wrong and still getting oil into their intake at WOT! - Mr. P. Edited March 17, 2011 by Mr. P. (see edit history) Quote Link to comment Share on other sites More sharing options...
AKSSS Posted March 20, 2011 Author Report Share Posted March 20, 2011 All good information guys. It all adds to my learning curve. Lol. It gets a little small all the time. Quote Link to comment Share on other sites More sharing options...
zippy Posted March 22, 2011 Report Share Posted March 22, 2011 A correct pcv system is by far not this complicated. I have found incorrect pcv systems to cause a poor idle issue, but I would say that if it were to cause a fuel economy issue, it would have to be really bad. How much timnig are you seeing at a cruise? Are you tuning in open loop? Quote Link to comment Share on other sites More sharing options...
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