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HighTQ 04-28-2010 06:59 AM

Cam selection math
 
Since so much information is available today vs 25yrs ago I am finding various reads to update myself on this subject.One of you in helping another member posted a link to Dimitri Elgin where he discussed compression ratio factors vs octane ratings.In the article he states that there is three parts of the compression equation.There is static,effective and dynamic cr factors to calculate when selecting a cam.If I understood his article correctly dynamic compression is computed when an engine is operating at 100% efficiency.Now correct me if I am not understanding this but doesn't an engine have to be operating in the lowmid rpms,say 2000/2500 before this occurs?Most street driven vehicles like my 2000 Silverado w/5.3 liter for example,run at an idle to 1800 under normal around town driving.

This is where my question comes in.I have played with the online calculators,and read quite a few post on this topic and it seems like static and dynamic compression are the most used guidelines.Why is that?

ap72 04-28-2010 07:31 AM

Because in most cases the induction and exhaust are close enough so the dynamic compression at 100% VE can be disregarded. These results are not calculated but empirical. They come from running conditions, and in most cases the running conditions these engines are put under are similar enough where you only need static and dynamic compression.

VERY well tuned engines or poorly tuned engines fall outside of these rules. There are some engines out there with 9:1 compression that require race fuel because their induction system is so well tuned they can see VE over 125%.

HighTQ 04-28-2010 09:40 AM

Quote:

Originally Posted by ap72
Because in most cases the induction and exhaust are close enough so the dynamic compression at 100% VE can be disregarded. These results are not calculated but empirical. They come from running conditions, and in most cases the running conditions these engines are put under are similar enough where you only need static and dynamic compression.

VERY well tuned engines or poorly tuned engines fall outside of these rules. There are some engines out there with 9:1 compression that require race fuel because their induction system is so well tuned they can see VE over 125%.

Ok I follow you on the VE over 125%,but got lost at the part where you said "dynamic compression at 100% can be disregarded" and then completed your explanation saying static and dynamic were all needing to be calculated.Is this meaning dynamic only needs to be a factor when the engine operates below 100% VE,which would be part throttle operation?

ap72 04-28-2010 10:39 AM

dynamic compression at a particular VE would be the effective compression, through VE out of the equation for most NA street engines; just set it to 100%, or 1.

HighTQ 04-28-2010 03:12 PM

Ok I get now.Thanks


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