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Thread: Intake/Exhaust Pressure Ratio's - what's the big deal?

  1. #41
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    This is a GREAT thread as many of the items discussed are the issue that we realize when selecting a turbo, its boost level, the induction volume, induction pressure losses, turbine size, etc...

    A few items worth mentioning:
    Cam duration and/or over-lap 'falsely' manipulate the pressure ratio with respect to intake VS exhaust gas pressures.
    'Falsely' because its manipulating trapping efficiency which does not address the true source of the imbalance.
    For example, an excessive amount of over-lap can reduce the pressure ratio between intake and exhaust but is reducing back pressure due to reversion back into the combustion chamber.
    Therefore the balance becomes, as mentioned, the port work and upstream/downstream set-up configuration.

    The talent in playing this balancing act is the willingness to support trade-offs, for the most part.
    If a 1:1 ratio is desired on a single turbo application, expect that it will come at the expense of lag, loss of low end torque, etc..
    As the pressure ratio climbs, the inverse will occur.

    With this, the larger the turbine side, turbine wheel, exhaust ports and supporting exhaust system - the closer you'll be to 1:1.
    In contrast, it can also be accomplished by restricting the induction system, reducing cylinder pressures (via spark, boost, cam timing, etc..).
    Therefore, it narrows down to how far anyone is willing to go and what the over-all performance expectation is while realizing its manipulation of one piece of a very large puzzle.

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by shadow88 View Post
    Ok, how about I re do the question.

    Let's say I have the typical set up, that's a 2.2l turbo 2 with a 2 piece, hybrid 50 trim stage 2 .63 housing and a 3 inch exhaust. I run 25 psi boost. Let's also say the pressure ratio is not 1:1, let's go with 1.5:1 as the revs climb to the upper end.

    What can be done to reduce the pressure ratio closer to 1:1 regarding cam duration, lift,advance/ retard, port size, intake design, charge pipes, tb size or anything else you want to mention?

    I'm trying to get an idea of things to help reach the "ideal" pressure ratio that isn't a different turbo.
    The best thing you can do short of changing the turbo is make the exhaust plumbing post turbine as large as possible with as few bends as possible, and those bends should be as large and as wide as possible. It's also important to have an intake plumbing (pre and post compressor) that is as free flowing, yet thermally efficient. Any extra work that your compressor has to do results in more turbine back pressure.

    The real gains come from picking a newer tech turbo that is much more mechanically and thermally efficient than the old T series.
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by 5DIGITS View Post

    The talent in playing this balancing act is the willingness to support trade-offs, for the most part.
    If a 1:1 ratio is desired on a single turbo application, expect that it will come at the expense of lag, loss of low end torque, etc..
    As the pressure ratio climbs, the inverse will occur.

    With this, the larger the turbine side, turbine wheel, exhaust ports and supporting exhaust system - the closer you'll be to 1:1.
    In contrast, it can also be accomplished by restricting the induction system, reducing cylinder pressures (via spark, boost, cam timing, etc..).
    Therefore, it narrows down to how far anyone is willing to go and what the over-all performance expectation is while realizing its manipulation of one piece of a very large puzzle.
    Agreed. A 1:1 one ratio is a much more attainable goal for a race application, where you really don't care about low-rpm response too much. But it still remains that reducing turbine backpressure even from 3:1 to 2:1 would be a worthwhile effort.
    Mike Marra
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  4. #44
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Thanks for the answer, I'm sure we're getting somewhere for the average guy.

    As a disclaimer, I'm not interested in changing anything on my car, I don't have an exhaust pressure reading, nor do I really care. I have a satisfactory amount of lag as well as max power so I'm not changing much.

    Thanks to 5 digits for mentioning that the closer the ratio gets to 1:1 the more turbo lag you will have. I know shadow liked to refer to that as "drive pressure" and it makes perfect sense it'll increase lag.

    Now, the reason I chose the "typical" 2.2l set up is many people have this and may be looking to increase power and already spent $800 or more for the turbo and exhaust, so they're probably looking for anything else they could change, like cam advance/ retard, new cams on the market, tb size, charge pipes ect ect.

    5 digits mentioned increased overlap leading to reversion, and if you didn't read the link contraption posted a couple days ago regarding cams, you really should. Maybe things like head flow comparison numbers or something like that the average person can use.
    Last edited by shadow88; 12-19-2014 at 05:00 PM. Reason: spellink

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by shadow88 View Post

    Thanks to 5 digits for mentioning that the closer the ratio gets to 1:1 the more turbo lag you will have. I know shadow liked to refer to that as "drive pressure" and it makes perfect sense it'll increase lag.
    I don't think he said or implied that. Certainly many of the steps one could take to reduce turbine backpressure would also improve turbo response. It really depends how bad your backpressure is to start with. Certainly you can free up your post-turbine exhaust and increase your charge cooling efficiency without increasing lag.
    Mike Marra
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by contraption22 View Post
    I don't think he said or implied that. Certainly many of the steps one could take to reduce turbine backpressure would also improve turbo response. It really depends how bad your backpressure is to start with. Certainly you can free up your post-turbine exhaust and increase your charge cooling efficiency without increasing lag.
    When I read 5 digits post, paragraph 3, that's what I get out of it.
    The talent in playing this balancing act is the willingness to support trade-offs, for the most part.
    If a 1:1 ratio is desired on a single turbo application, expect that it will come at the expense of lag, loss of low end torque, etc..
    As the pressure ratio climbs, the inverse will occur.

    Did I interpret this incorrectly?

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    I think from what I understood reversion to be, is that during the overlap time when the exhaust and intake valves are both open, that when the exhaust pressure at the turbine is 30psi (this is all for example) and the intake pressure is only 15psi from the turbo (compressor), then reversion means that the 30psi will overcome the 15psi and force some of that hot exhausted air back into the intake. Sort of like an exhaust gas recirculation(EGR) situation, which we all know robs power and not to mention heats up the intake charge thats being delivered into the next combustion cycle. Now if the opposite were true, being 30psi intake/15psi exhaust, then the overlap would help blow out the gas that was just used during the combustion stroke out through the exhaust valve faster, being that 30psi overtakes 15psi. This would also cool the valves a bit I'd imagine.

    Someone please correct me if I'm wrong, I was only a C+ student in high school lol

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    You deserved higher marks in high school. That's how it works.

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by shadow88 View Post
    When I read 5 digits post, paragraph 3, that's what I get out of it.
    The talent in playing this balancing act is the willingness to support trade-offs, for the most part.
    If a 1:1 ratio is desired on a single turbo application, expect that it will come at the expense of lag, loss of low end torque, etc..
    As the pressure ratio climbs, the inverse will occur.

    Did I interpret this incorrectly?
    I think you did a little bit if you interpreted it to mean that a a better ratio definitely means more lag. Certainly the magical 1:1 ratio will be in more race oriented setups, but again you can move closer to that ideal by doing things that lower back pressure AND improve response.
    Mike Marra
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Then there is the twin scroll setup which helps with overall response by concentrating pulses from same firing cylinders to spool the turbo and keep pressure down by not having other cylinders fighting over who gets out first. Reason why most manufacturers like Ford have strictly gone over to twin scroll technology.

    And then there is the VNT/VGT. We all know what those do. Or you should lol.
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by 5DIGITS View Post

    The talent in playing this balancing act is the willingness to support trade-offs, for the most part.
    If a 1:1 ratio is desired on a single turbo application, expect that it will come at the expense of lag, loss of low end torque, etc..
    As the pressure ratio climbs, the inverse will occur.
    I just wrote a novel on why this is an old school Myth. Then the site Froze......AGAIN! Don't know how to check if it auto saved.

    Regardless, Lag does NOT have to be a big concern IF you do it Right

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    How do we know that the various engines running at low pressure ratios (i.e. 1:1) are seeing benefits besides those which would just be associated with drastically lowering exhaust restrictions?

    In other words, couldnt this scenario be observed:

    1- a given engine running at say 35psi plenum pressure, with an exhaust pressure at 50psi measured at the head port. (1.42:1)
    2- that engine makes say 400hp
    3- changes to the exhaust manifold and swingvalve are made to drastically reduce exhaust restrictions
    4- at same 35psi plenum pressure, engine now makes 450hp, exhaust pressure is now 35psi at head port (1:1)

    How do we know that extra 50hp gained isn't simply because now the engine isn't pushing ~800cfm at 15psi through the exhaust restrictions?

    In other words, how do we know its simply reduction of the power required to force air through restrictions, versus special efficiencies gained by manipulating how the exhaust and intake charges directly interact during overlap?

    For instance: it takes about 30hp to compress 400cfm of air from atmospheric to ~30psi. 400cfm is roughly 200hp at typical AFRs. So an engine consuming twice that much would take about twice as much power. So lets say it takes 60hp at 400hp to push the exhaust gases from valve seat to turbine input with around a 25psi drop, at 800cfm engine air filter air flow. So if we remove all those restrictions, and now that drop is 1psi, cant we claim thats where the 60hp comes from? Why do we conclude its coming from special efficiencies with the exhaust and intake interacting? We know that the restriction alone requires 50hp+ at 400hp+ power levels so how much benefit is left to assign to "neutral" effects of 1:1?

    What exactly is the model of how the exhaust and intake charges supposedly interact during overlap at 1:1? If the pressures are the same then wouldn't the exhaust and intake have equal tendency to fill the cylinder as its volume increases, besides their different valve openings?

    How do we know a 1:1 is actually 1:1 unless its measured directly behind the valve seats? If thats where the action of these special benefits are, then how can a 1:1 measurement made further downstream align with a model that explains things by describing pressures as 1:1 further upstream, where they certainly would be different than the pressure measured downstream?
    Last edited by acannell; 12-20-2014 at 01:13 PM.

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    I've kind of thought about the pressure acting against eachother, but from my little knowledge, seems like the only time it is to be concerned is during the valve overlap period. Add to that you also have a changing "environment" going on as well(couldn't think of a better word to describe). By that I mean that the piston is also moving a volume of air just by its movement inside the cylinder. So what I would guess is that when the piston is driving upwords, it is creating an environment of increasing pressure even though it is expelling the exhaust gasses. When the intake valve opens during overlap, won't the piston in effect be trying to push some of the intake charge out?

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by Shadow View Post
    I just wrote a novel on why this is an old school Myth. Then the site Froze......AGAIN! Don't know how to check if it auto saved.

    Regardless, Lag does NOT have to be a big concern IF you do it Right
    I'll bite off of this "old school" myth. The old school myth was once true in my eyes. But look at technology compared to today to just 10 years ago. With advancements like we have now in technology and tuning lag can almost be eliminated with proper tuning. I helped tune my buddys Honda that had a WAY oversized turbo. Lag was horrendous and wasn't seeing boost til almost 4k. I tuned it he sees boost at 2500 now.
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by acannell View Post
    How do we know that the various engines running at low pressure ratios (i.e. 1:1) are seeing benefits besides those which would just be associated with drastically lowering exhaust restrictions?

    In other words, couldnt this scenario be observed:

    1- a given engine running at say 35psi plenum pressure, with an exhaust pressure at 50psi measured at the head port. (1.42:1)
    2- that engine makes say 400hp
    3- changes to the exhaust manifold and swingvalve are made to drastically reduce exhaust restrictions
    4- at same 35psi plenum pressure, engine now makes 450hp, exhaust pressure is now 35psi at head port (1:1)

    How do we know that extra 50hp gained isn't simply because now the engine isn't pushing ~800cfm at 15psi through the exhaust restrictions?

    In other words, how do we know its simply reduction of the power required to force air through restrictions, versus special efficiencies gained by manipulating how the exhaust and intake charges directly interact during overlap?

    For instance: it takes about 30hp to compress 400cfm of air from atmospheric to ~30psi. 400cfm is roughly 200hp at typical AFRs. So an engine consuming twice that much would take about twice as much power. So lets say it takes 60hp at 400hp to push the exhaust gases from valve seat to turbine input with around a 25psi drop, at 800cfm engine air filter air flow. So if we remove all those restrictions, and now that drop is 1psi, cant we claim thats where the 60hp comes from? Why do we conclude its coming from special efficiencies with the exhaust and intake interacting? We know that the restriction alone requires 50hp+ at 400hp+ power levels so how much benefit is left to assign to "neutral" effects of 1:1?

    What exactly is the model of how the exhaust and intake charges supposedly interact during overlap at 1:1? If the pressures are the same then wouldn't the exhaust and intake have equal tendency to fill the cylinder as its volume increases, besides their different valve openings?

    How do we know a 1:1 is actually 1:1 unless its measured directly behind the valve seats? If thats where the action of these special benefits are, then how can a 1:1 measurement made further downstream align with a model that explains things by describing pressures as 1:1 further upstream, where they certainly would be different than the pressure measured downstream?

    I kinida see your reasoning of the question for the sake of discussion, but when it comes down to it, we know exhaust backpressure a detriment to power. Regardless of theory behind the motivation to remove or reduce it, the end result of reducing backpressure is more power.
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by contraption22 View Post
    I kinida see your reasoning of the question for the sake of discussion, but when it comes down to it, we know exhaust backpressure a detriment to power. Regardless of theory behind the motivation to remove or reduce it, the end result of reducing backpressure is more power.
    Concluding that reducing exhaust restrictions = good and leaving it at that doesn't really address the question in the thread title.

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by acannell View Post
    Concluding that reducing exhaust restrictions = good and leaving it at that doesn't really address the question in the thread title.
    True enough. Did you read the Hot Rod article about camshafts for turbo cars?
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Heres the R5 cam as measured by turbo2point2 and 4Lbodies. Also on the plot is the piston location.

    Maybe the R5 is a bad example for this discussion for some reason (low overlap?). But anyways:

    Look at how long the exhaust pressure has to influence the cylinder volume.

    When the piston stops moving upwards on the exhaust stroke and is at TDC at the beginning of the intake stroke, the exhaust and intake valves are both open about 0.025".

    At that time the cylinder volume is at its minimum, basically combustion chamber + piston dish sized.

    This is the time where exhaust gases will possibly "influence" the intake charge and/or its influence on the cylinder volume.

    At say 3500 rpm, the crank is turning once every 17 ms.

    In about 25 crankshaft degrees from that TDC, the exhaust valve is fully closed.

    That takes about 1 millisecond.So we are talking about how much the exhaust system pressure can influence the cylinder volume (basically ~50cc's at that point in time), with a valve open 0.025", in 1 thousandth of a second.

    How much does an exhaust port flow, backwards, at 0.025" lift? Maybe 150cfm at 1bar at 0.5" lift. At 0.025" lift..maybe 15cfm? Thats about 7000cc's per second. So in our 1 millisecond, thats about 7 cc's. Maybe double that if our absolute pressure is 30psi in the exhaust manifold versus 0 in the cylinder.

    But remember, the exhaust valve is CLOSING. So its only 0.025" for that one instant in time. So without doing the integration I'd say that we are looking at the exhaust pressure possibly filling up the combustion chamber with single digit cc's of exhaust. Compared to the maximum cylinder volume at BDC something less than 1%. How much can that really matter?


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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by contraption22 View Post
    True enough. Did you read the Hot Rod article about camshafts for turbo cars?
    im not sure if it was posted in one of these TM threads probably

    to be clear I think reducing exhaust pressure (and therefore pressure ratio) is obviously a good thing

    but what I'm trying to focus on is whether the benefits come simply from those restrictions not consuming power directly, versus the benefits coming from some kind of special efficiency gained in the combustion chamber from intake and exhaust pressures interacting

    If the benefits are just from the reductions in backpressure no longer consuming power then I dont think there is any need to focus on a specific "ratio", since really it would just be a matter of reducing exhaust restrictions period, without reference to matching boost pressure.

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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    wait a second...

    The cylinder is already filled with exhaust at TDC of the exhaust stroke. Its just exhaust that hasn't been pushed out. When the piston is at TDC there is still a combustion chamber volume left and that volume is filled with exhaust.

    So the idea that exhaust gases are flowing back into the cylinder during overlap doesn't really make sense. Its already filled with exhaust gas. And exhaust gas that is above exhaust manifold pressure at least until exhaust stroke TDC and then probably at least equal to exhaust manifold pressure for several crank degrees until the volume of the cylinder increases again on the intake stroke, enough to reduce that pressure.

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