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Thread: Boost effects on low compression engines vs high compression engines

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    Quote Originally Posted by IPD View Post
    i swear this is the same guy who loudly protested when he was told that superchargers aren't that great of an option for these cars. i don't know if he'd take the advice of people who run turbochargers.
    I think the only person that was loud was you. I will listen to anyone, especially if they are knowledgable and have first hand experience.

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    Quote Originally Posted by futurevr4man View Post
    blown3000, you need to look at the effects of gasoline at a high pressure. as you increase the pressure, the gasoline will ignite sooner. when you increase the pressure high enough, gasoline will ignite with no spark, thats called pre-detonation (or knock). so when you have a high compression engine, then throw boost on top of that, you can cause the gasoline to ignite before the piston is at the top of the stroke. as you can imagine, that is extremely hard on your engine internals, and more specifically, your bearings. this is how you flatten out your main bearings (rod bearings are affected more by oil starvation).

    and just so you are aware, what i just explained is how a diesel works. they dont have spark plugs. they also run like 16:1 compression ratio so they can get a whole lot of cylinder pressure built up. also, diesel burns at a lower temp than gasoline FWIW which is why gasoline wont run in a diesel... ignition wouldnt occur as it should. diesel: ~210C, gasoline: ~230C
    I understand the principles of pre-detonation and have addressed that issue. Wondering whether there are limits among other issues. Also like to hear the upside to boosting higher compression engines.

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    Now with more poop-smear Not Verified Feedback Score 8 (100%) IPD's Avatar
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    Quote Originally Posted by Blown3000 View Post
    I understand the principles of pre-detonation and have addressed that issue. Wondering whether there are limits among other issues. Also like to hear the upside to boosting higher compression engines.
    the only upside to boosting a higher-compression engine is that you "peak out" at a lower PSI...which means that you don't have to have your boost system held together with massive amounts of force to prevent boost leak.

    you will make less power on the same octane.

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    VR-3747 :D Not Verified Feedback Score 1 (100%) futurevr4man's Avatar
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    the limits are your cylinder compression. you can lift heads on these cars readily without taking the proper precautionary measures. fuel is your biggest limit in general though, as it is what is creating the power. obviously if you knock early enough and frequently enough, your limits would be whatever your pistons/rods/crank/bearings can take.

    upsides are that you can make the same power at a lower boost level. to be honest, the honda platform doesnt even tune for knock detection, they tune for most power made. i have a lot of honda buddies that are actually knowledgeable and not just honda fanbois, and they have made some impressive numbers out of their little engines. they last as well, but that has a lot more to do with honda materials engineering than anything. they are durable little engines.

    there is also a benefit of more torque with higher compression, whereas you can have a higher horsepower with lower compression. higher compression changes quite a few things, and depending on your ecu, it may not like it as well.

    im being nice and playing this question game here, but what is your goal? high compression boosted engines are nothing new, not even for this platform. there is no secret to it, its physics and intuition that will tell you what you can and cannot, should and should not do. if you are sticking to super charging, you very well may want higher compression because you probably wont push that much boost. you will need to make sure your intake temps dont get too high because that will set off detonation earlier as well.

    are you the older (40's maybe) guy that came to NG with the red supercharged 3k?
    rise and rise again until lambs become lions

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    Quote Originally Posted by Blown3000 View Post
    Can you be more specific.
    I'm not sure what you're looking for...? A lesson in simple physics? how a motor works? Maybe I'm not grasping the question, but it seems pretty simple to me... Do you understand why you can't run 20:1 compression in a gasoline motor? If you increase the volume of air in the cylinder by one atmosphere, you've already compressed it 2:1. Do the math, and you'll find that the difference in compression between our N/A motors and turbo motors is no accident...


    Real Performance Automotive (541)816-4500 www.FB.com/RealPerformanceAuto

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    Forum User Feedback Score 0 ibsorgn's Avatar
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    Quote Originally Posted by Blown3000 View Post
    What are the benefits of boost on a low compression engine VS high compression engine? Is there really a need to lower your compression ratio if you are going to use sometype of forced induction? If so why?
    You raise a good question, maybe I can help shed some light without talking about specifics.

    The higher the compression ratio (CR) the more power an engine will deliver. With that said The rule of thumb is that a higher compression engine (HCE) will perform much better 'off boost' than a lower compression engine because it has its own natural compression to generate power. Generally speaking a HCE will not have as big a jump in power as will a lower compression engine (LCE) will have as boost is applied. Normally the boost threshold will be much lower in a HCE than in that of a LCE.
    Normally speaking the higher the compression ratio, the more basic or natural torque the engine will produce. By adding boost, regardless of the means, increases the effective compression of an engine. This increase in cylinder pressure will translate into more power and torque due to greater expanding exhaust pressures.
    By dropping the compression ratio allows a higher amount of boost to be used, which translates into a greater volume of fuel and air to be introduced into the cylinder. Naturally this generates more hp and torque - so long as the greater volumes of air and fuel are being delivered. Off boost the LCE will perform more poorly than that of the HCE.

    As said by others, forced induction devices which increase effective compression ratios can and will cause denotation but that problem and well as other issues, due to increasing cylinder pressures, will have to be dealt with as boost pressures increase.

    IMHO, a higher compression engine with moderate levels of boost make a good combination for a street machine.

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    Hitokiri Battousai verified Feedback Score 0 Jeremy C's Avatar
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    Quote Originally Posted by IPD View Post
    the only upside to boosting a higher-compression engine is that you "peak out" at a lower PSI...which means that you don't have to have your boost system held together with massive amounts of force to prevent boost leak.

    you will make less power on the same octane.
    There's also the benefit of better off-boost performance. Turbo lag is not as noticeable.

    As for why you can't run higher compression and higher boost? There's no reason why you can't really. You'll be spending more money to use less boost, and I don't believe the higher comp is going to offset the hp loss from less boost. This chart isn't for our cars specifically (don't think anyone has done one specific to our vehicles) but it's not going to change much:



    Not taking credit for this chart at all, but I can't find the one I had made for the Starion/Conquest group a decade ago so I have to make do.

    Using this chart and knowing how much work 3S guys put into their cars to run specific psi on their cars can give you an idea of how much you will have to do as well. Want to run 15psi on 10:1? Prepare to build the car as if you were going 22psi on an 8:1 motor and you should be safe.

    There are other variables that will change this depending on your setup (direct injection motors run higher compression and higher boost due to how the fuel charge is injected), but general rule when not talking about new, high tech motors is that higher boost + higher compression = lot more build money and more things to break.

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    Quote Originally Posted by futurevr4man View Post
    are you the older (40's maybe) guy that came to NG with the red supercharged 3k?
    Nope - not me

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    Quote Originally Posted by RealMcCoy View Post
    I'm not sure what you're looking for...? A lesson in simple physics? how a motor works? Maybe I'm not grasping the question, but it seems pretty simple to me... Do you understand why you can't run 20:1 compression in a gasoline motor? If you increase the volume of air in the cylinder by one atmosphere, you've already compressed it 2:1. Do the math, and you'll find that the difference in compression between our N/A motors and turbo motors is no accident...
    Sorry, You may think your answer is pretty simple but I'm not understanding what you are trying to say. Why don't you put it in simplier terms and help me with the math!

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    Quote Originally Posted by ibsorgn View Post
    You raise a good question, maybe I can help shed some light without talking about specifics.

    The higher the compression ratio (CR) the more power an engine will deliver. With that said The rule of thumb is that a higher compression engine (HCE) will perform much better 'off boost' than a lower compression engine because it has its own natural compression to generate power. Generally speaking a HCE will not have as big a jump in power as will a lower compression engine (LCE) will have as boost is applied. Normally the boost threshold will be much lower in a HCE than in that of a LCE.
    Normally speaking the higher the compression ratio, the more basic or natural torque the engine will produce. By adding boost, regardless of the means, increases the effective compression of an engine. This increase in cylinder pressure will translate into more power and torque due to greater expanding exhaust pressures.
    By dropping the compression ratio allows a higher amount of boost to be used, which translates into a greater volume of fuel and air to be introduced into the cylinder. Naturally this generates more hp and torque - so long as the greater volumes of air and fuel are being delivered. Off boost the LCE will perform more poorly than that of the HCE.

    As said by others, forced induction devices which increase effective compression ratios can and will cause denotation but that problem and well as other issues, due to increasing cylinder pressures, will have to be dealt with as boost pressures increase.

    IMHO, a higher compression engine with moderate levels of boost make a good combination for a street machine.
    Let me see if I understand, by lowering compression you are able to run more boost effectively by adding more air to the cylinder thereby generating more power. In higher compression engines power is derived not so much by adding air but more along the lines of compressing the air tighter.

    Is this why big supercharged dragsters run compression ratios as low as 5 to1?

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