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

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    Forum User Not Verified Feedback Score 2 (100%) RealMcCoy's Avatar
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    Quote Originally Posted by Blown3000 View Post
    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?
    You're starting to catch on....

    There are two completely different factors at play. A motor can be envisioned as simply an air pump... The more air you can pump, the more power you can make. The secondary factor is how much energy(power) you can extract from the mixture you have. The more you compress it, the more energy will be released during combustion. This is not only mechanical compression ratio in play, but also camshaft profile and cylinder head design. There is always a limit to the amount of cylinder pressure any given motor can withstand on the fuel used.... The higher you go with the octane rating of the fuel, (burns slower with higher flash point) the more pressure you can get away with...

    Boyle's law states that gas volume and pressure are inverse at the same temperature. As one doubles the ether halves for a fixed amount of gas... That can also be interpreted by if you double the amount of gas in a fixed volume, you double the pressure. Let's imagine you have a perfect intercooler, and you get no temperature or pressure drop as you compress the air: If you increase the the amount of air by one atmosphere(14.7 lbs/sq.in) you have doubled the air charge in the cylinder, and doubled the pressure.

    At 10:1 ratio, you decrease the volume by 90%, so your pressure rise on a 100% efficient N/A, with no thermal expansion calculated, would net 147 psi.

    At 8:1 compression you decrease the volume by 78%, so your two atmospheres of boost would net you a pressure rise to 134 psi.

    At the same boost level (two atmospheres) a 10:1 motor would net 291 psi... Seeing a problem with this picture?

    Of course those are all theoretical numbers that would be complete bullshit in the real world when you factor in thermal expansion, pressure drop, and fuel mass... But I think it makes the point.


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    Forum User Feedback Score 0 ibsorgn's Avatar
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    Quote Originally Posted by Blown3000 View Post
    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?
    Yes. Of course you can boost higher compression engines for performance improvements but to build for significant boost the CR should come down to be effective.

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    Forum User Feedback Score 0 ibsorgn's Avatar
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    Quote Originally Posted by IPD View Post
    you will make less power on the same octane.
    I would like to hear more details concerning your statement. Please be specific. Thanking you in advance.

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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 ibsorgn View Post
    I would like to hear more details concerning your statement. Please be specific. Thanking you in advance.
    based "unscientifically" on me never having seen any 10:1 3/s run above 350awhp on pure pump. 8:1's can break 500 without breaking much of a sweat on pure pump.

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    Quote Originally Posted by ibsorgn View Post
    I would like to hear more details concerning your statement. Please be specific. Thanking you in advance.
    Pretty simple statement... You will reach the knock limit of your fuel with less boost. Therefore you will move less air, and make less power.

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    Quote Originally Posted by IPD View Post
    based "unscientifically" on me never having seen any 10:1 3/s run above 350awhp on pure pump. 8:1's can break 500 without breaking much of a sweat on pure pump.
    Was looking for a technical explanation. Might want to quantify your statements so that folks know when you're writing from a point of knowledge or just making an off-handed remark.

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    Forum User Feedback Score 0 ibsorgn's Avatar
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    Quote Originally Posted by RealMcCoy View Post
    Pretty simple statement... You will reach the knock limit of your fuel with less boost. Therefore you will move less air, and make less power.
    Yes that's a pretty simple statement, however I thought IDP was equating octane to energy not resistance to ignition. Actually I still don't know, maybe he was. Many times these off-handed remarks are very nebulous while others are very direct and informative such as your answer in post #21.

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    Quote Originally Posted by ibsorgn View Post
    Yes that's a pretty simple statement, however I thought IDP was equating octane to energy not resistance to ignition. Actually I still don't know, maybe he was. Many times these off-handed remarks are very nebulous while others are very direct and informative such as your answer in post #21.
    i put in my $.02 from what i've observed. i'm not the resident expert in fluid-dynamics, physics or engineering. i'll let everyone else fill in the blanks as to why something is the way it is--which mccoy did nicely.

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    Here are my opinions from driving both a stock TT and now having a boosted 10:1.

    At 6-8 psi the 10:1 TT felt stronger in the bottom end and had more punch to it that a stock TT. This is due to the higher compression giving it a better bottom end and helping the turbos spool faster. About 6-8psi on a 10:1 TT would probably be about what a stock TT runs HP wise.

    I was able to get to 12 psi before having to add a FMIC to keep the intake charge temps down as the stock SMIC were not up to the task.

    At 14psi I had to add 50/50 meth to keep knock under control. I was able to get it to 16psi and stable before started having knock issues again. I relate this to running out of injector where even bumping the base fuel pressure to over run the stock 360's would not help.

    I now have a set of 450's I'll be adding come spring and re-tuning. Hoping to get to 18psi but I'm pretty doubtful. At the 16psi limit I hit I'm guesstimating 360-380 hp. Never was able to get it on the dyno as I don't have one close by me.

    I'm basically testing the limits of boosted 10:1 to see what my limit will be. This is on a complete stock cast crank bottom end and stock heads.

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    Quote Originally Posted by DrGonzo View Post
    Here are my opinions from driving both a stock TT and now having a boosted 10:1.

    At 6-8 psi the 10:1 TT felt stronger in the bottom end and had more punch to it that a stock TT. This is due to the higher compression giving it a better bottom end and helping the turbos spool faster. About 6-8psi on a 10:1 TT would probably be about what a stock TT runs HP wise.

    I was able to get to 12 psi before having to add a FMIC to keep the intake charge temps down as the stock SMIC were not up to the task.

    At 14psi I had to add 50/50 meth to keep knock under control. I was able to get it to 16psi and stable before started having knock issues again. I relate this to running out of injector where even bumping the base fuel pressure to over run the stock 360's would not help.

    I now have a set of 450's I'll be adding come spring and re-tuning. Hoping to get to 18psi but I'm pretty doubtful. At the 16psi limit I hit I'm guesstimating 360-380 hp. Never was able to get it on the dyno as I don't have one close by me.

    I'm basically testing the limits of boosted 10:1 to see what my limit will be. This is on a complete stock cast crank bottom end and stock heads.
    Don't want to hijack Blown's post but out of curiosity did you try any high octane fuels (racing fuel)? What type of Air fuel ratios (if you monitored) did you hit at 16psi? Very nice to have a fellow that has actual experience in this matter.

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