boost/compression
Originally Posted by jdime20
is there a way to know how much compression ratio is raised for every psi you add? is there a formula for this or just a general rule of thumb?
Flow > boost.
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Staging Lane
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Originally Posted by FieroZ34
Well that'd be essentially your Dynamic Compression Ratio, and it has nothing to do with PSI, but everything to do with CFM.
Flow > boost.
Flow > boost.
Originally Posted by jdime20
so it's not the pressure that raises CR, but the actual amount of air that is put in the cylinder?
That being said, I do not want to spend the time to figure out how much CFM it takes to get a desired dynamic compression ratio.
Static compression ratios don't mean much, they are just a good starting point for average engines. But fact is, you'll gain a lot more power by getting more air IN (And out), than you will by squishing the existing air more.
If I were building a boosted engine (Which I'm not because the Government isn't paying me enough), it'd have 10:1 compression, semi-low boost (6-10psi), and really good heads, cam(s), and other "bolt-on" components. This, IMO, makes for the best driving car, and will still make a lot of power. The compression is high for a boosted engine, but boost is kept low, thus the air's temperature stays lower, and with good ignition characteristics I wouldn't have to pull too much timing, and wouldn't detonate. However it would have its limitations, and on an average LS1 700hp would be about the max. At this point, the 6-10psi just isn't enough air to go higher, and I'd need more exotic bolt on parts (But you can't go much more exotic), thus you boost it to high heaven (20+ psi).
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Originally Posted by jdime20
so this is why some n/a guys run low cr- big cube motors?
Originally Posted by jdime20
and if so, there would be an advantage to having more CID on a FI motor as well, right?
Originally Posted by Runn
What would be best a highcomp engine with low boost or a lowcomp engine with lots of boost?
Originally Posted by jdime20
so this is why some n/a guys run low cr- big cube motors?
And on the note of running a C/R of ~10:1 on a turbo car, I wouldnt do it for a long period of time, unless youre happy with having a "justOK" amount of power. Now, Ive seen a few centrif. guys run at that level of C/R and make big numbers (again, that depends on your idea of big power, which IMO, is ~700+fwhp) but you have to look at the fact that youre STILL limiting yourself to the amount of PSI that you can push into that engine...
Fiero let me ask you this...Which do you think has the most POTENTIAL power for a street car (that sees a little side action every now and then) that has a stock C/R of say, 9.5:1.... +1 C/R w/ 7psi or -1C/R w/ ~14.7-15psi (1 atmosphere)?
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Staging Lane
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as for my personal project, i'm looking to make around 650-700 hp on pump gas, 91 octane is easy to find where i live, SOME places keep 93. with that in mind, i'm thinking 9.5, 10:1 is still to much compression. motor will be a 383, i'm thinking 8:1 CR with 7-8 psi, and for the OCCASIONAL track day have the ability to run more boost with better fuel or methanol injection. what are your opinions on this, and thanks for all the above explanations
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Staging Lane
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Originally Posted by FieroZ34
Exactly. Boost pressure is relative. I can build an LS1 that has 5psi and makes more horsepower and torque than an LS1 at 15psi. It's all about using good components, like heads, cam, intake, etc. And this is the better way to go because the more pressure you run, the more the air gets heated, the harder the compressor has to work, etc. However there aren't many N/A engines that have the capability to flow the amount of air that boosted engines do. And the ones that can make 700-800hp, out of 3.0l, with airflow restrictions. Really impressive, but these also turn 22,000rpm, aren't designed to live for more than a few races, and are not in my definition of cheap (Not even in my definition of expensive, they are above any measure of cost...lol). Thus boosted engines become more practical, as they are able to get the same amount of air in, without the exotic parts, RPMs, and price.
That being said, I do not want to spend the time to figure out how much CFM it takes to get a desired dynamic compression ratio.
Static compression ratios don't mean much, they are just a good starting point for average engines. But fact is, you'll gain a lot more power by getting more air IN (And out), than you will by squishing the existing air more.
That being said, I do not want to spend the time to figure out how much CFM it takes to get a desired dynamic compression ratio.
Static compression ratios don't mean much, they are just a good starting point for average engines. But fact is, you'll gain a lot more power by getting more air IN (And out), than you will by squishing the existing air more.
