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Thread: 95 kPa at WOT, exhaust too small?

  1. #21
    Tuning Addict blindsquirrel's Avatar
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    The initial proposition is that a restricted exhaust is causing MAP to be 95KPA when baro is 101. This is false. Restrictive exhaust will not cause that, the pressure measuring device is in the wrong location to measure a lower absolute pressure (numerically higher vacuum, numerically lower KPA) upstream of the restriction - if anything, it would read above atmospheric (if engines worked that way, which they don't).

  2. #22
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    Right, if he had an exhaust restriction instead of the low 95kpa map, he would see a higher pressure closer to the 101 Baro, like on all his other cars. Theres 6 kpa of pressure that could be filled. It would be better opening up the intake restriction tho.

  3. #23
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    Guess I'm on my own again. I'll remove the exhaust and test when I don't have to work.

    I just don't understand how people say that exhaust back pressure will not effect manifold pressure when it's basic mechanics. I've known this from reading my dads Motors manuals a long time ago. I know it will cause this but I'll post this so those who don't believe don't have to say it's my word.

    Heres one: Low vacuum also can be an immediate clue to a plugged exhaust. To check further, run the engine at about 2500 rpm for about 15 seconds. If vacuum drops during this period and does not increase when you close the throttle, you're almost certainly looking at a restricted exhaust. from here. https://www.motor.com/magazine-summa...a-vacuum-gauge

    Heres another: One method is to use a vacuum gauge connected to the intake manifold vacuum and raise the engine rpm to 2000 rpm for 2 minutes. If the vacuum gauge falls as the engine is held at 2000 rpm, then suspect a restricted exhaust system.
    From here http://prod.lv2014.gener8cms.net/ind...-pressure-test

    Of course these are extreme examples, but if the exhaust system is too small it may only do it at WOT. And we're all on the quest for more power, so even if this is small (reaching 95 instead of barometric) it counts as an improvement can be made.

    If you have a hard time understanding it, think valve overlap.

  4. #24
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    Don't confuse less vacuum with less pressure. Less vacuum is more pressure. Barometric - MAP= Vacuum. So yes the exhaust restriction can effect manifold pressure, but it will make MAP closer to barometric, not the other way like you are seeing. I'm not implying the solution is to plug your exhaust up and get MAP closer to Barometric. Its to unrestrict intake manifold and get more airflow. Did you check that your TB is opening all the way?

    Unrestricting the exhaust more, if possible, may make the difference you already see increase, as more air can escape faster out the exhaust, the engine will pull more on the intake.

  5. #25
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    I thought the late model tbss intake was good for the power I'm making. I don't think the TB is too small IMO.

  6. #26
    Senior Tuner kingtal0n's Avatar
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    because you seem to ignore electrical ground issues and map sensor calibration with engine on vs off (which relfects an alternative ground resistance pathway to the voltage sensors) The only way to resolve this issue is to post a LOG of the engine in question at WOT from say 2,500 RPM to around the 5,000 rpm range so we can compare the map sensor voltage or KPA values first to see if there is truly a restriction or issue.

    Next, the intake manifold (any) at WOT with no air filter and no intake pipe should be fairly atmospheric for all but the most high performance engine vs displacement, therefore if there is some pressure drop present under those conditions I Would suggest that the intake valves are not sealing or there is some leak somewhere

  7. #27
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    Quote Originally Posted by kingtal0n View Post
    because you seem to ignore electrical ground issues and map sensor calibration with engine on vs off (which relfects an alternative ground resistance pathway to the voltage sensors) The only way to resolve this issue is to post a LOG of the engine in question at WOT from say 2,500 RPM to around the 5,000 rpm range so we can compare the map sensor voltage or KPA values first to see if there is truly a restriction or issue.

    Next, the intake manifold (any) at WOT with no air filter and no intake pipe should be fairly atmospheric for all but the most high performance engine vs displacement, therefore if there is some pressure drop present under those conditions I Would suggest that the intake valves are not sealing or there is some leak somewhere

    I don't see it as an electrical problem. I made the harness, I know it like the back of my hand.
    You seem to be thrown way off by me saying that I checked my KOEO map reading. Well I mentioned that because it shows that the sensor will read barometric pressure and it shows what the pressure is.
    I've read about people looking at their weather service as reference to baro pressure. I wish you'd get off that.
    Last edited by Jason B; 09-29-2020 at 08:06 PM.

  8. #28
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    Here are a few from this past weekend.

    Quick initial tests. Tune isn't fully worked out yet.

    Attachment 103668
    Attachment 103669
    Last edited by Jason B; 10-02-2020 at 08:07 PM.

  9. #29
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    Have you checked the KOEO MAP when its heat soaked or only when the engine bay is cold? IAT's get pretty high, its probably pretty warm where you are. Might be exacerbated by sucking right off a hot radiator.

    Looks like you are getting great cylinder filling, and need a better flowing intake manifold, or to dial in your fuel a bit more/ make sure your injector flow rates are accurate for the fuel rail pressure and don't have any exhaust leaks causing a false lean making you raise VE/MAF too much.
    Another thought something could also be slipping in the transmission and you are not actually getting full load on the engine, that would take a dyno to see if power at the wheels is where you would expect it to be. 10 seconds 50-110 isn't lightning fast but this may be a big heavy vehicle.
    Last edited by murfie; 09-30-2020 at 11:24 AM.

  10. #30
    Advanced Tuner Montecarlodrag's Avatar
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    Quote Originally Posted by Jason B View Post
    Stuff your tail pipes with rags, use the scanner. Run it, give it some throttle. You'll see the manifold vacuum drops.

    I don't need a youtube video to tell me this but I'll link to this just to prove a point. https://youtu.be/d-jp1IIJVVk?t=212
    You are mistaken. Your problem is not losing vacuum but the opposite, the engine is making 6 KPa of vacuum at WOT so that means something is not letting the engine breath. There is a restriction somewhere on the intake side.
    You ask for help then act like you know anything (which clearly you don't). Good luck with your car then !
    9 sec Montecarlo SS

  11. #31
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    Quote Originally Posted by Montecarlodrag View Post
    You are mistaken. Your problem is not losing vacuum but the opposite, the engine is making 6 KPa of vacuum at WOT so that means something is not letting the engine breath. There is a restriction somewhere on the intake side.
    You ask for help then act like you know anything (which clearly you don't). Good luck with your car then !
    Sorry your feelings got hurt. I need to remind myself that there are emotionally delicate people out there.

  12. #32
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    The fuel trims were plus/minus 4 until I took the maf off and put it in SD. Then with the maf and no filter that didn't change. Not sure what's up with that. Not sure if I checked the koeo map hot but I was logging the baro pid , assuming it reads that off the map. I also got rid of the trans output pid . It has a 3600 converter and with the input and output pids I can do the math to show there's no trans slippage.

    This is in an s10 that weighed 3240 with the iron 5.3. The air intake filter touches the pass side fender.

    By intake do you mean the manifold?

  13. #33
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    I think Bernoulli's principle is at play here.

  14. #34
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    Quote Originally Posted by KDS View Post
    I think Bernoulli's principle is at play here.
    I've ruled out any restrictions in the intake side except for the manifold.

    From what I understand a 92mm tb will flow more than 1300 cfm and a 2 1/2 inch exhaust will flow 510 per pipe approx. That's 1020 cfm for dual exhaust.

  15. #35
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    To compare the static MAP condition to the Dynamic MAP condition is difficult. The air column in the manifold with no movement at key on engine off is different than when the engine is running at WOT. At WOT the plenum area has air movement so its not possible to duplicate the pressure readings in a static state without something else impacting the air column. Sounds to me like you have a great flowing low restriction intake system. I run an LQ4 with an ls1 top end and have more restriction than you do. The MAP values at WOT are in the 90 - 95 range with some incursion into the 100's in lower gears for short periods. I use cylinder fill as my barometer for intake improvements. I've removed the MAF as I'm running SD enhanced and I installed the SS camaro air intake that draws air from below the bumper. I'm sure that adds some positive pressure to the system. There are to many variables in play to try and make an apples to apples comparison and you probably have less of an issue that you think you do.
    That said, your exhaust is not a contributor.

  16. #36
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    Quote Originally Posted by KDS View Post
    To compare the static MAP condition to the Dynamic MAP condition is difficult. The air column in the manifold with no movement at key on engine off is different than when the engine is running at WOT. At WOT the plenum area has air movement so its not possible to duplicate the pressure readings in a static state without something else impacting the air column. Sounds to me like you have a great flowing low restriction intake system. I run an LQ4 with an ls1 top end and have more restriction than you do. The MAP values at WOT are in the 90 - 95 range with some incursion into the 100's in lower gears for short periods. I use cylinder fill as my barometer for intake improvements. I've removed the MAF as I'm running SD enhanced and I installed the SS camaro air intake that draws air from below the bumper. I'm sure that adds some positive pressure to the system. There are to many variables in play to try and make an apples to apples comparison and you probably have less of an issue that you think you do.
    That said, your exhaust is not a contributor.
    Thanks for that info. Seems I'm making a bigger deal than what it is. I'll get a new filter on there and continue fine tuning it.

  17. #37
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    Thanks to everyone who replied, you've all helped in one way or another.