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    Maximum Boost vs Compression Ratio


    Posts : 196
    Join date : 2011-04-13
    Age : 28
    Location : Seattle, WA

    Maximum Boost vs Compression Ratio Empty Maximum Boost vs Compression Ratio

    Post  g3t_t0rk on Wed Apr 13, 2011 7:35 pm

    Again, I've found an awesome read that I just had to share with everyone. It's also from NSXPrime (damned good forum!).

    I hear this from a lot of people, "Damn he's running 30lbs of boost! That is so sick!" What I never hear people say is, "Damn he's running 30lbs of boost but his compression ratio is only 6.3:1." Compression ratio is just as important if not more important then boost. Before you get into the NSXPrime article, make sure you know what compression ratio is or else this whole read goes to waste.

    "What is compression ratio?
    Compression ratio refers to the volume, or amount, of an air and fuel mixture that the combustion chamber in a combustion engine can hold when it is empty and at its largest size compared to the volume it holds when the mixture is compressed down to the smallest size possible. This ratio applies to both internal combustion engines, such as those found in modern-day vehicles, and seldom-used external combustion engines. Diesel- and gas-powered engines alike each have a compression ratio, though the design of the diesel engine encourages a higher compression ratio. Engines with higher compression ratios are generally considered better because they create more power while still maintaining efficiency.

    To calculate the compression ratio of an engine, an engineer would first calculate the volume a cylinder in the engine can hold when the piston is at the bottom of the cylinder. During one stroke of the engine, the piston moves from the bottom up toward the top and compresses the air-fuel mixture. After finding the volume of the cylinder when the piston is down, and therefore not compressed yet, the engineer would then need to calculate the volume when the piston is up and the air-fuel mixture has been compressed. A ratio such as 13:1, for example, means that the engine holds 13 times more volume when the piston is down than when it is compressed. The amount of air-fuel mixture does not change, but rather is simply pressed into a significantly smaller space to create a large explosion.

    Diesel engines use compression to create power while gas engines use a spark to ignite the air-fuel mixture that creates the necessary power to drive the vehicle forward. High compression ratios in gas engines often cause a problem known as engine knocking. Diesel engines, on the other hand, are designed for high compression in order to function. A ratio of 13:1 is considered high in a gas engine while a diesel engine can range from 14:1 up to 23:1 depending on the type.
    The disadvantage of a higher compression ratio in a gas engine is the possibility of engine knocking or pinging. This occurs when a larger explosion than desired occurs and causes the piston to move upward or downward too quickly. A loud knocking noise results and, if not fixed, continuous engine knocking can permanently damage the engine. Cars using gas with a higher octane rating or a knocking sensor can use higher compression ratios, but still cannot match the high ratio of a diesel engine."

    High compression ratios cause more power by compressing the air and fuel even tighter than average and thus creating a more forceful explosion. The tight packing of the air-fuel mixture helps both air and fuel to blend better and when the explosion occurs more of the mixture evaporates. More evaporation is a sign of higher thermal efficiency, meaning the engine performs better without using too much extra energy to gain this power."

    "We’ve all wondered it before - how much boost can the NSX engine withstand, while at the same time, not imbedding the crankshaft into the pavement? Perhaps this may shed some light onto the subject, based on thermodynamic gas laws. In simplified form, an equation can be presented as P1 x V1 = P2 x V2, where P1 is the intake manifold pressure, P2 is static combustion chamber pressure (more on this in a sec) and the ratio of V1 to V2 is the piston’s compression ratio.

    Heat of compression is excluded for two reasons - inlet temperature does not have as significant affect on stresses imposed onto the rotating elements of the engine as boost does and an aftercooler will negate the heat of compression. My own opinion is that any forced induction engine should have an aftercooler. BTW, the proper term from an engineering point of view is aftercooler, not intercooler. The latter implies that there is a second stage of compression on the engine. So unless the engine is running a supercharger and a turbocharger, it has no intercooler. And if it did have both modes of forced induction, the cooler immediately upstream of the intake manifold is still called the aftercooler.

    The resulting pressure P2 is called the dynamic compression is affected by engine speed, camshaft lobe profile, piston rings sealing, valve seats sealing, air density, etc. Since this varies from engine to engine, what follows is based on the static compression ratio, just to keep things simple.

    Boost pressure is measured in "gauge" as x PSIG; however, in common speak, the G is omitted. A normally aspirated (0 PSI boost) engine ingests air at atmospheric pressure, which is 14.7 PSI at sea level. To use the above equation, the gauge pressure of your boost reading must be added to the atmospheric pressure. A mountainous region has lower atmospheric pressure than a coastal region because of the altitude, but what follows is based on 14.7 PSI, because that is a standard default value to correct against. If your forced induction creates 6 PSI of boost, this equals 20.7 PSI absolute to use in the above equation.

    The factory compression ratio of the pistons is 10.2:1 and a common aftermarket compression ratio for a boosted NSX engine is 9.5:1. The downward thrust of the piston that causes the crankshaft to rotate is affected by net combustion pressure, which is influenced by boost and compression ratio. For a stock NSX, the "static combustion pressure" is 10.2 x 14.7 = 150 PSI. Install a 6 PSI forced induction atop a stock engine, and this number becomes 211 PSI which is a 41% increase in downward forces acting upon the connecting rods, wrist pins, crankshaft, bearings and main caps. Now recognizing that a popular supercharger kit from California and Japan have combined hundreds of installations that generate 6 PSI of boost, and with no know recorded failure of the engine bottom, one could argue that the 41% increase in stress is significant but does not exceed the design safety factor of the engine components. I don’t know what safety factor Tochigi used, but 2:1 is plausible so the 41% increased load is still within the design window. (Failure of other internal engine components due to increased temperature because of no aftercooler is a different story. BTDT, which is why mine is now aftercooled.)

    Ok, if you’re convinced that 6 PSI of boost with a stock engine is no big deal, how about different boost levels with various compression ratios? What if 8 PSI boost with 9.5:1 pistons? 22.7 x 9.5 = 216 PSI = 44% increase over stock but only a 2% increase over what a CT or GM supercharger results in. Are you comfy with stretching things a bit more? Let’s try 10 PSI with low comp pistons.....24.7 x 9.5 = 235 PSI = 57% increase over stock and 11% increase over CT / GM. Finally, a look at 12 PSI with the low comp pistons.....26.7 x 9.5 = 254 PSI = 69% increase over stock and 20% increase over CT / GM.

    While still within the postulated safety factor of 2:1, in theory, a forced induction engine that results in the last set of numbers should still hold up without significant bottom-end mods. Yes, components such as head gaskets and the quality of the valve job come under closer scrutiny, but that’s a different issue. Cylinder wall distortion geometry becomes magnified with elevated combustion pressure, but this post is focused on the rotating elements of the engine that are subject to the dynamic loads. However, I am interested to know when is a deck plate considered a “must have” in a boosted NSX.

    Closing thought.....using the static compression ratio as a guideline and assuming the design safety factor is 2:1, this suggests that with 9.5:1 pistons as much as 16 PSI of boost can be run on an NSX engine before things start to break. I think the “upper teens” is where the high output turbo guys draw the line. Caveat emptor - boost wisely."

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