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?

FieroZ34

iTrader: (1)

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.

jdime20

so it's not the pressure that raises CR, but the actual amount of air that is put in the cylinder?

FieroZ34

iTrader: (1)

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.

FieroZ34

iTrader: (1)

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).

jdime20

so this is why some n/a guys run low cr- big cube motors?

jdime20

and if so, there would be an advantage to having more CID on a FI motor as well, right?

Runn

What would be best a highcomp engine with low boost or a lowcomp engine with lots of boost?

FieroZ34

iTrader: (1)

I can't explain that, except that big bore engines do have poor ignition characteristics. But I see little reason to run low compression. That may just be poor design/execution of a build.

Yes, totally. As the old rule says, there is no replacement for displacement. However they limit displacement, and put intake restrictions, in an effort to control horsepower output, as the 700-800 they have makes for an insane go kart.

Best is relative. Best in terms of maximum power? Low compression, high boost. As I said before, it's better in terms of power to put more air in, than to squeeze the existing air more. However I'd rather a medium compression engine (9.5-10.5), with mild boost, just because in my experience it makes for a nicer driving car (As far as turbochargers go). For blowers, low end and response doesn't really matter, as they have near instant boost anyways, thus you can have great drivability and low end response even with the low compression.

6techniques

iTrader: (7)

Well I think that depends on your idea of a low C/R...IMO, anything at 10.5:1 or higher is pretty good, but some guys run a little lower in an effort to curb the price of using a higher octane fuel. So I wouldnt say it was poor execution, but probably a budget effort and using the extra CID to make the power...Granted not as much as it can with the combined CID and higher C/R, but enough to make a bit more power on a budget.



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)?

jdime20

forgot to mention that being a pump gas motor

jdime20

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

jdime20

as for gaining more power by moving more air rather than more pressure, it would make better sense to run a turbo that would move more cfm, and to run less boost than it would be to have a smaller turbo with more boost, right?