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Thread: 95 kPa at WOT, exhaust too small?

  1. #1
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    95 kPa at WOT, exhaust too small?

    I recently swapped from a mild 5.3 to a LFA, basically converted to LS2 specs like a 2005 GTO and running off a 411 PCM.

    It has a Summit 8706 cam with recommended springs, flat top pistons, 243 heads, NBSS/TBSS intake, 92mm TB.

    The truck has always had a Summit exhaust kit, it's 2 1/2" from shorty headers all the way out to the bumper. It has a dual in dual out muffler.

    When I start the scanner the manifold pressure reads 101 kPa as usual, but at WOT it'll raise(?) to 95 kPa.

    Is there anything else that would cause this other than the exhaust being too small?

    Can't attach scan for some reason.

  2. #2
    Advanced Tuner Montecarlodrag's Avatar
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    Exhaust has nothing to do. You have a restriction in the air filter or somewhere in the path to the intake.
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    Senior Tuner Ben Charles's Avatar
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    Agree with above! It’s all about inlet restriction with low KPA at WOT (which shows restriction)

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    Just got back from some testing. I took the MAF and filter off and ran SD. 4" intake pipe, one 90. Maybe a little better at times. On average about the same. Still sits at 95 kPa at WOT. It might go to 99 at first but maybe 5 seconds at WOT it's at 95. Both with no maf and filter, and with filter and maf installed.

    Sort of like an engine with clogged cats.

  5. #5
    Senior Tuner kingtal0n's Avatar
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    well hang on. dont be so quick to judge these things.

    If you start out at 100kpa and gradually climb to 95kpa then fine- its a pressure drop due to something, probably the air filter.

    But if you go WOT at say 2,500RPM and see 95KPA then it can't be a restriction in the air filter or anything in the air path. Because at 2,500rpm the airflow demand of the engine is likely near half of what it is at say 5,000rpm. In other words, the engine only makes 200hp or so around 2,500 rpm (if that) so there is no way the air filter or anything else can restrict the engine to 200hp and then magically flow more as rpms rise later. See what I am saying?

    If you see the same 95kpa at low rpm as high rpm then its likely a calibration issue, or the map itself is 'bad' (it might not be bad, it might just be reading slightly low voltage "bad" but still working).

    It could also be a voltage issue, a ground issue, if the return voltage path is difficult due to poor grounding for example it can have that affect.

    So next lets say it really is taping out at 95kpa. Well if the engine has a proper paper air filter, and an intact pcv system, then 95KPA is ideal. You need about 5 to 10kpa of pressure drop post air filter to drive WOT pcv action- to pull a suction on the crankcase at WOT. This does MANY things,
    1. helps keep oil inside the engine and out of the oil baffles (out of the intake manifold)
    2. protects oil seals (front/rear main & cover seals)
    3. provides pressure drop for piston rings which improves piston ring seal and reduces blow-by
    4. scavenge crankcase for combustion gasses which would otherwise degrade engine oil

    So you really NEED and WANT a pressure drop post air filter, and a fully intact factory PCV system to pull on the crankcase at WOT, otherwise the engine will begin to lose oil seal integrity and start to leak, and it will continuously pollute the engine oil with combustion gasses, and the piston rings can't effectively do their jobs the way they were designed. This is perhaps the most overlooked aspect of high performance engines, neglected and dismissed at trivial when in fact it is one of the most important and highest topics of performance there is.

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    Quote Originally Posted by kingtal0n View Post
    well hang on. dont be so quick to judge these things.

    If you start out at 100kpa and gradually climb to 95kpa then fine- its a pressure drop due to something, probably the air filter.

    But if you go WOT at say 2,500RPM and see 95KPA then it can't be a restriction in the air filter or anything in the air path. Because at 2,500rpm the airflow demand of the engine is likely near half of what it is at say 5,000rpm. In other words, the engine only makes 200hp or so around 2,500 rpm (if that) so there is no way the air filter or anything else can restrict the engine to 200hp and then magically flow more as rpms rise later. See what I am saying?

    If you see the same 95kpa at low rpm as high rpm then its likely a calibration issue, or the map itself is 'bad' (it might not be bad, it might just be reading slightly low voltage "bad" but still working).

    It could also be a voltage issue, a ground issue, if the return voltage path is difficult due to poor grounding for example it can have that affect.

    So next lets say it really is taping out at 95kpa. Well if the engine has a proper paper air filter, and an intact pcv system, then 95KPA is ideal. You need about 5 to 10kpa of pressure drop post air filter to drive WOT pcv action- to pull a suction on the crankcase at WOT. This does MANY things,
    1. helps keep oil inside the engine and out of the oil baffles (out of the intake manifold)
    2. protects oil seals (front/rear main & cover seals)
    3. provides pressure drop for piston rings which improves piston ring seal and reduces blow-by
    4. scavenge crankcase for combustion gasses which would otherwise degrade engine oil

    So you really NEED and WANT a pressure drop post air filter, and a fully intact factory PCV system to pull on the crankcase at WOT, otherwise the engine will begin to lose oil seal integrity and start to leak, and it will continuously pollute the engine oil with combustion gasses, and the piston rings can't effectively do their jobs the way they were designed. This is perhaps the most overlooked aspect of high performance engines, neglected and dismissed at trivial when in fact it is one of the most important and highest topics of performance there is.

    I don't really agree.
    I said I ran it with no filter, and no MAF.

    The 5.3 I just took out would hit 101 kPa at WOT with the same intake and exhaust. And map sensor even.

    My two LS1 camaro's hit 101 at wot.

    I've even done this test with an old 468 big block with a vacuum gauge taped to the windshield, was running a 2 1/4" exhaust with a small block and swapped in a big block and got more than zero on the gauge at wot. I changed the 780 carb to a 900 and the exhaust to 3", then it got zero gauge vacuum (inches of mercury) at WOT and felt like it picked up 1/3 more power.

    So lets rule it out. Shorty headers with 1 5/8" primaries to 2 1/2 collectors to 2 1/2 exhaust to a single dual in dual out muffler to tail pipes turned down at the bumper is definitely not the problem at all no matter what? Because my next step is to cut it and hang it up somehow and test run it. If that doesn't work just band clamp it back together.

    Just wondering why everything I own with fuel injection shows atmospheric pressure KOEO and atmospheric pressure or 1 kPa less at WOT. I have 9 running vehicles.

  7. #7
    Tuning Addict blindsquirrel's Avatar
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    Using the key-on MAP value vs MAP at some unknown, unspecified engine speed isn't really useful.

    Comparing MAP KPA between low RPM, initial WOT hit vs MAP at peak HP is a better indicator.

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    Quote Originally Posted by blindsquirrel View Post
    Using the key-on MAP value vs MAP at some unknown, unspecified engine speed isn't really useful.

    Comparing MAP KPA between low RPM, initial WOT hit vs MAP at peak HP is a better indicator.
    Using key on MAP values tells me that the map sensor will read atmospheric pressure. That it's working.

    I don't understand your point, at max hp the map (the actual manifold pressure) does the same thing it would do if it had clogged cats, except less extreme.

    Comparing MAP KPA between low RPM, initial WOT hit vs MAP at peak HP is a better indicator.

    Which is exactly what I'm doing.

    At peak hp the MAP value doesn't read atmospheric pressure.

  9. #9
    Tuning Addict blindsquirrel's Avatar
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    My point is you don't understand how this works, or how the MAP reading changes from KOEO to low speed WOT to max RPM WOT and what those changes mean.

    No naturally aspirated engine in existence will read atmospheric with no losses at WOT. Air has mass and inertia. There will always be some losses unless you remove enough parts that the engine will no longer function. You can make changes to minimize the losses, but there will always be some.

    You can chart all kinds of interesting info in the scanner, torque and HP and RPM and MAP. Make a pull and watch how the MAP changes at initial hit, peak TQ, and peak RPM/HP. It's neat.

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    Quote Originally Posted by blindsquirrel View Post
    My point is you don't understand how this works, or how the MAP reading changes from KOEO to low speed WOT to max RPM WOT and what those changes mean.

    No naturally aspirated engine in existence will read atmospheric with no losses at WOT. Air has mass and inertia. There will always be some losses unless you remove enough parts that the engine will no longer function. You can make changes to minimize the losses, but there will always be some.

    You can chart all kinds of interesting info in the scanner, torque and HP and RPM and MAP. Make a pull and watch how the MAP changes at initial hit, peak TQ, and peak RPM/HP. It's neat.

    You say No naturally aspirated engine in existence will read atmospheric with no losses at WOT.

    Whats this? A dodge Hemi, my old 5.3 with a summit 8719 cam, a summit 8708 cam. And whatever else I dug up. I don't know what I'm doing? This kind of disproves what you just said here. I have different vehicle scans with the same results, some OBD1 vehicles. Scanner reads Barometric pressure at WOT or very, very close to it. I'd appreciate it if people would not look for what I'm doing wrong and MAYBE advise on what might be wrong regarding the question I asked.

    Why do these vehicles read atmospheric at wot? Click on the image to see full size.



    Last edited by Jason B; 09-27-2020 at 09:51 PM.

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    Quote Originally Posted by blindsquirrel View Post
    My point is you don't understand how this works, or how the MAP reading changes from KOEO to low speed WOT to max RPM WOT and what those changes mean.
    Thats a pretty low blow right there. Maybe I don't explain things well and you don't understand what I'm talking about. Try to not make assumptions and ask if you don't understand. Thanks.

  12. #12
    Senior Tuner kingtal0n's Avatar
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    Oh boy

    No map sensor can read perfectly. What you see is really a voltage input to the computer.

    When it says 101KPA or whatever, its really just a 4.95 or 5.01v or whatever input. Once a threshold is surpassed, say 4.92vish, than anything over that will be max KPA. i.e. 4.93 to 5.12v = 101KPA max

    So even the slightest disruption to the voltage will affect the map reading if it cannot achieve a full voltage signal.

    Next, Map KPA is taken at manifold (pressure) and has nothing to do with exhaust or combustion pressure unless the intake valve(s) are/is leaking.
    In other words the exhaust pressure can be 5psi but if the path between the air filter (or lack of air filter) and the intake manifold is completely open it will still be 100KPA because atmospheric pressure goes right in through the huge opening. In fact the more restrictive the engine exhaust is, the more restrictive the valves, etc... the more atmospheric pressure will be able to stay inside the intake manifold. Pressure only drops when the engine's cylinders are able to take it away from the intake faster than the atmosphere can push it back in.

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    Quote Originally Posted by Montecarlodrag View Post
    Exhaust has nothing to do. You have a restriction in the air filter or somewhere in the path to the intake.
    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

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    Quote Originally Posted by kingtal0n View Post
    Oh boy

    No map sensor can read perfectly. What you see is really a voltage input to the computer.

    When it says 101KPA or whatever, its really just a 4.95 or 5.01v or whatever input. Once a threshold is surpassed, say 4.92vish, than anything over that will be max KPA. i.e. 4.93 to 5.12v = 101KPA max

    So even the slightest disruption to the voltage will affect the map reading if it cannot achieve a full voltage signal.

    Next, Map KPA is taken at manifold (pressure) and has nothing to do with exhaust or combustion pressure unless the intake valve(s) are/is leaking.
    In other words the exhaust pressure can be 5psi but if the path between the air filter (or lack of air filter) and the intake manifold is completely open it will still be 100KPA because atmospheric pressure goes right in through the huge opening. In fact the more restrictive the engine exhaust is, the more restrictive the valves, etc... the more atmospheric pressure will be able to stay inside the intake manifold. Pressure only drops when the engine's cylinders are able to take it away from the intake faster than the atmosphere can push it back in.
    Look dude I'm not asking how to build a rocket. In other words you don't need to over explain everything. I know how it works. Do you agree that a partially clogged cat will effect the manifold vacuum? I posted a video above to show you that it does. So seeing how it does, then an exhaust thats too small will have the same effect. Yes, or no?

    Again I was seeing barometric pressure at WOT with the same air intake, same intake manifold, same map sensor, same exhaust, but in a mild 5.3. Now all thats same stuff is on a cammed 6.0, bigger displacement bigger air pump. You think if I took the 325 out and put a 840 inch mountain motor in would the same exhaust system work? NO. Do you see the point?

    My ONE QUESTION was, could it be the exhaust. I'm not asking everyone to go out of their way to explain the basics. Just asking for an opinion.

    So if you say no then thanks I'll move on.
    Last edited by Jason B; 09-27-2020 at 10:17 PM.

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    Asking a question you are 100% certain you already know the answer to is just... well, something. Have fun conquering the world.

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    Quote Originally Posted by blindsquirrel View Post
    Asking a question you are 100% certain you already know the answer to is just... well, something. Have fun conquering the world.
    Mad because you said something was impossible and I proved it wrong? With facts.

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    Intake manifold pressure is always lower than barometric pressure.
    Exhaust manifold pressure is always higher than barometric pressure.

    The more restriction in the intake the farther(lower) from barometric pressure it will be.
    The more restriction in the exhaust the farther(higher) from barometric pressure it will be.

    Free flowing is when both intake and exhaust are as near barometric pressure as possible.

    They do have a direct inverse effect on one another.
    When one has a restriction it lowers overall airflow and the other gets closer to barometric pressure, some people could look at that and falsely think its a benefit. IE: "some exhaust back pressure is needed or you lose torque."

    What you are seeing with your MAP to Barometric sensor relationship is not cause by an exhaust restriction, and could only be caused by a restriction from TB to intake opening. Seeing what you already did to rule most of this out and that you have a 92mm TB, I would say its not big enough or it isn't opening all the way.

  18. #18
    Senior Tuner kingtal0n's Avatar
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    Atmospheric pressure for an Naturally aspirated engine is the supercharger in this situation, it can provide 101KPA of 'boost'.

    If you shut the engine off i.e. completely plug the exhaust pipe and stop the pistons from moving, the pressure in the intake manifold will be atmospheric = 101KPA

    Thus, anytime you see anything less than 101KPA its because the atmosphere is having trouble to reach the map sensor for some reason.

    If you plug the exhaust or stop the engine (air pump) whether its 1% or 100% (engine stopped) the pressure will increase, not decrease.

    The less the engine (air pump) can move OUT of the intake manifold, the easier time the atmosphere can maintain air INTO the intake manifold.

  19. #19
    Tuning Addict blindsquirrel'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
    How does a restriction result in a lower pressure upstream of the restriction? Please explain.

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    Quote Originally Posted by blindsquirrel View Post
    How does a restriction result in a lower pressure upstream of the restriction? Please explain.
    He said manifold vacuum drops, implying a higher absolute pressure.