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

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

    So I've been reading about this pressure ratio hype that people have been jawwing on about, something about making more power with a 1:1 pressure ratio in both the intake and exhaust manifolds or somethin'. I think this one feller with a slick fast Charger is the one raisin' heck about it. Don't you want a higher pressure and heat in the exhaust so you can spool that there turbo up right quick in a hurry? What's the big deal?

    In all seriousness, what is the science behind trying to achieve an intake/exhaust pressure ratio of 1 to 1 or lower?

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

    Quote Originally Posted by BoostedDrummer View Post
    So I've been reading about this pressure ratio hype that people have been jawwing on about, something about making more power with a 1:1 pressure ratio in both the intake and exhaust manifolds or somethin'. I think this one feller with a slick fast Charger is the one raisin' heck about it. Don't you want a higher pressure and heat in the exhaust so you can spool that there turbo up right quick in a hurry? What's the big deal?

    In all seriousness, what is the science behind trying to achieve an intake/exhaust pressure ratio of 1 to 1 or lower?
    Because, simply put, exhaust backpressure robs a lot of HP!

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

    Since the intake gasses are being pushed in with a compressor and the exhaust gasses are only being moved by the force of the piston, you don't want too much pressure in your exhaust manifold. Otherwise you can't get the fresh air/fuel mixture in because the exhaust gasses haven't had a chance to get out. So if you're boosting 20 psi you don't want 40 psi (1:2 ratio) in the exhaust. A 1:1 ratio would be 20 psi intake pressure and 20 psi exhaust manifold pressure.

    On my SRT-4 van, I built a log header with a Chrylser flanged S60 turbo. I welded in an O2 bung in the end of it to measure back pressure in manifold, between the head and turbo. When I was boosting at 20 psi I had 25 psi measured in the exhaust manifold. From what I've read, that is really good for a street car. Street cars tend to be higher because they usually have smaller turbos to spool up quicker.

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

    Doesn't a higher pressure in the exhaust manifold spool the turbo up faster due to the high pressure trying to shoot out the turbine? Basic law of fluid dynamics is that high pressure will always try to flow to an area of lower pressure, so, that being said, wouldn't all of that pressure trying to escape into the exhaust downpipe have a faster velocity?

    I made this thread for information purposes, in case anyone in the future is reading this and would ask the same questions. So, these would basically be laymans term questions lol.

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

    Not only does the backpressure itself cost horsepower, but it also can cause reversion when you want run a cam with a bit of overlap.

    When the intake and exhaust valves are both open, high exhaust backpressure can be a hinderence to fresh air and fuel entering the chamber.

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

    So for our turbocharged engines, it would be ideal to have a low exhaust pressure/high intake pressure in order to clear the cylinder for the next cycle? This is also where valve overlap comes into play? If there is a 1:1 ratio in the system, wouldn't the boost pressure still be fighting the exhaust backpressure as it tries to fill the cylinder? It would seem that having the lowest pressure ratio that is possible to achieve would only benefit the engine. By this, I mean that if there is 20 pounds pushing against 20 pounds, then it becomes neutral; wherein the only relief comes from the exhaust gasses exiting the turbine. Wouldn't the greater pressure from the turbo benefit by pushing out a smaller, lesser amount of pressure?

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

    Pressure measured somewhere before the turbine wheel isn't necessarily generated by the turbine wheel. It can be head port, exhaust manifold, turbine wheel, swingvalve, exhaust system. Imagine a little straw as the exhaust manifold. Now you have a very high pressure differential between the exhaust valve and the turbine wheel but the flow is extremely low and very little power reaches the turbine wheel.

    Since the exhaust system is connected to the atmosphere and the turbine wheel requires a source of power to spin the compressor wheel, there must be a pressure differential across it and the absolute pressure on the engine side of the turbo must be above atmospheric. So a ratio of 1:1 would mean for 25psi boost theres 25psi of pressure before the turbine (or somewhere before the turbine..not sure this is precisely defined as to measurement location).

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

    Quote Originally Posted by BoostedDrummer View Post
    So for our turbocharged engines, it would be ideal to have a low exhaust pressure/high intake pressure in order to clear the cylinder for the next cycle? This is also where valve overlap comes into play? If there is a 1:1 ratio in the system, wouldn't the boost pressure still be fighting the exhaust backpressure as it tries to fill the cylinder? It would seem that having the lowest pressure ratio that is possible to achieve would only benefit the engine. By this, I mean that if there is 20 pounds pushing against 20 pounds, then it becomes neutral; wherein the only relief comes from the exhaust gasses exiting the turbine. Wouldn't the greater pressure from the turbo benefit by pushing out a smaller, lesser amount of pressure?
    I think if the pressure ratio goes below 1:1 then you have the intake charge being sucked out the exhaust during overlap = no bueno.

    But this brings up a thought experiment:

    Take a given compressor wheel.
    Pick a certain amount of air to move through it for our thought experiment (will correspond to engine output power).
    Pick a certain boost pressure for it to generate.
    Assume atmospheric for the inlet pressure.
    Now using the compressor map you can calculate how much power that work requires.

    Now pick out your turbine wheel
    Figure out how much air needs to flow through it and what pressure differential it needs to have in order to generate the power required by the compressor wheel above.
    Assume the exit side of it is atmospheric.
    Now you know what pressure the input side of it will need to be. Lets call it "turbine input pressure".

    And it should be noted that the compressor and turbine in this thought experiment are linked only by a shaft. There is NO connection of the airstream between the two.

    So where things get funny is when you power the turbine using an internal combustion engine. Now that "turbine input pressure" starts effecting how the cylinder fills because its attached to the cylinder, so you have to play a game to get it to be the right pressure (around the same as the pressure you are trying to fill the cylinder with). But also simultaneously you have to consider if your ICE will make the power you want at that turbine input pressure as boost pressure as a 1:1 ratio.

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

    It takes a certain amount of work on the compressor wheel to deliver a certain flow rate (determined by physical properties of the compressor wheel, rpm, pressure). The work is imparted by the turbine as the turbine and the compressor are linked.

    So, the exhaust energy is what has the potential energy to do the work. It is up to the turbine AND the turbine housing to convert that potential energy into mechanical energy.

    If your intake/exhaust pressure ratio is 1:1, then the system is working very efficiently. If the ratio is higher than 1:1, that means there is more potential energy built up than the turbine can convert or needs to convert into mechanical energy to drive the compressor. At some point the turbine will not be able to convert all of the potential energy into the needed work to spin the compressor hard enough (make enough torque) and the potential energy overruns the efficiency of the system and power suffers due to the fact that all of the potential energy isn't being converted to mechanical energy, so it has to go somewhere. It basically backfeeds the system and starts to prevent more potential energy from entering the equation...so you see power fall off in the upper rpms of the engine, the engine isn't as responsive as it could/should be, boost can fall off.

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

    I think everyone is hitting upon it, you want to drive the turbine with the Least amount of restriction possible, while getting the job done. Anything more and you're disrupting the potential flow of the motor. Remember, Air Pump! So your #1 objective is still to move as much air through the motor as possible. You can't do that with too much restriction on the turbine side, there has to be a happy medium.

    Think about the turbine like a paddle wheel in a river. That paddle wheel can only work properly when it is Balance to let enough water through to Not disrupt the flow of the river, but still take enough energy out of the flow to do the work. Try to make the wheel do more work by Restricting the river's flow, and the river ends up overflowing Before the paddle wheel because it Can't pass enough through.

    Now, we have a closed system, it Can't overflow, so it builds up pressure. That pressure is Not helping the tubine wheel spin faster, because it's there from RESTRICTION. (it's trying to Overrun the system) What it ends up doing is slow the Entire system down, because now everything from the intake on can't flow as freely anymore.

    So you're thinking about it Correctly when you say that the Important piece of the puzzle is the Greatest difference that you can create between the before and aft (pre turbine/ Aft turbine) pressures, but you don't want to do that by placing a restriction in the path of the motor as a whole.

    You Want to do it by Removing as much restriction as you can AFTER the turbine! This is how you create the difference in pressures that = Great flow and Power!

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

    Exactly what I was about to say.

    Excessive back pressure before the turbine not only robs power from the engine and creates a cork, it also makes the turbo work harder to produce boost on the compressor side. I have read but have no real life experience and would take r&d ( rob can you possibly fill this in) that if you were creating let's say 15psi on the compressor sode , but 30psi in back pressure on the turbine side, you could be realistically creating 22.5 psi if you were to bring the ratio down to 1:1. (I hope this makes sense...)

    I have seen cars drop lower than a 1:1 ratio and still create MORE power. How low can you go before you start flat lining, I have no idea. But as Rob stated, engines are air pumps. They need, want to FLOW!!!!!
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    I don't think supercharged engines have any trouble making power with an excellent exhaust-to-intake pressure ratio.
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by 85boostbox View Post
    Exactly what I was about to say.

    Excessive back pressure before the turbine not only robs power from the engine and creates a cork, it also makes the turbo work harder to produce boost on the compressor side. I have read but have no real life experience and would take r&d ( rob can you possibly fill this in) that if you were creating let's say 15psi on the compressor sode , but 30psi in back pressure on the turbine side, you could be realistically creating 22.5 psi if you were to bring the ratio down to 1:1. (I hope this makes sense...)

    I have seen cars drop lower than a 1:1 ratio and still create MORE power. How low can you go before you start flat lining, I have no idea. But as Rob stated, engines are air pumps. They need, want to FLOW!!!!!
    I prefer to think of it as how much more Power per lb of boost you can make vs how much more boost you will make.

    So lets say @ 15psi intake boost and 30 psi drive pressure you are making an avaerage of 8hp/ lb of boost. Drop the drive pressure to 15 and now you're making 11-12hp/ lb of boost.

    Make sense?

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

    Yes. Had it in my brain, just suck at trying to figure out how to types stuff out.
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Efficiency; the number 1 goal to work toward.

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

    Quote Originally Posted by 85boostbox View Post
    Yes. Had it in my brain, just suck at trying to figure out how to types stuff out.
    You are correct that it also allows for more boost. By it's very principle, running close to 1:1 will allow you to run out the compressor at whatever boost it is capable of sustaining against the rest of your build environment.

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

    I'm actually surprised that no one has pointed out EGTs yet...
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    Quote Originally Posted by 85boostbox View Post
    I'm actually surprised that no one has pointed out EGTs yet...

    EGTs are a very old school way of monitoring things and really don't come into play IF you're even in the ballpark of tuning and running any kind of decently thought out set-up.

    I Almost ran an EGT gauge when we first started to get things going, but after reading all of the people melting down and gernading mtrs And EGT sensors failing, I decided to not run/ trust one. So, I proved to myself, if no one else, that IF your AFR's are in the "safe" range And you run the appropriate octane for your boost/ timing And your over-all setup is put together decently and well thought out, you really don't need to monitor EGT because it will simply never be an issue.

    Having said that; Yes, EGT's will be lower when you run a better (closer to 1:1) pressure differential. (all other perameters being in check)

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

    Sure you don't want to change slugs in the pits each run? ^^^
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    Re: Intake/Exhaust Pressure Ratio's - what's the big deal?

    That is what I was meaning. With a 1:1 ratio your egt's will be lower and yet again, create more power.
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