*waits for answer like Ben Stein*

I'm guessing static?

 

Your dynamic compression depends on variables including static compression.

Short answer, both.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 92TypeR &raquo;</TD></TR><TR><TD CLASS="quote">Your dynamic compression depends on variables including static compression.

Short answer, both.</TD></TR></TABLE>

Doh! You're right, somehow that slipped my mind...and intuition tells me a low compression engine w/super "tight" low lift/duration cams will have nowhere near the detonation/pre-ignition issues of a high compression motor with basically race cams... stupid question, my apologies.

 

Does'nt static cr dictate what dr cr is, after the cam specs come into play? Well here is an article that wont answer your question but gives some insight...

http://www.motorcycle.com/mo/mcrob/rt-fuel2.html

 

Considering that a combustion engine is a VERY DYNAMIC machine, its pretty safe to say that static/calculated CR mean nothing.

Ultimately the factors contributing to detonation of numerous, with dynamic CR being only a small piece of the puzzle.

 

There is really no such thing as static compression in a running motor; only dynamic or 'net' compression.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Top Ramen &raquo;</TD></TR><TR><TD CLASS="quote">There is really no such thing as static compression in a running motor; only dynamic or 'net' compression.</TD></TR></TABLE> Ditto

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BryanPendleton &raquo;</TD></TR><TR><TD CLASS="quote">Considering that a combustion engine is a VERY DYNAMIC machine, its pretty safe to say that static/calculated CR mean nothing.
</TD></TR></TABLE>

Of course it means something. If you dont know what your starting with, how do you know what to do to get where you want? As your dynamic compression increases, does your VE? If static compression does not matter, i cant wait to build my next 16:1 motor to run on 85 octane. I figure i can stick some .700 lift cams in there to reduce dynamic compression right?

 

Only if you fill the cyl about 60% or have a high of 60%VE. LOL

 

I wouldn't same they matter as much as fuel type and timing.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Combustion Contraption &raquo;</TD></TR><TR><TD CLASS="quote">

Of course it means something. If you dont know what your starting with, how do you know what to do to get where you want? As your dynamic compression increases, does your VE? If static compression does not matter, i cant wait to build my next 16:1 motor to run on 85 octane. I figure i can stick some .700 lift cams in there to reduce dynamic compression right?</TD></TR></TABLE>

The topic is what is detonation dependant on? Static or dynamic CR. The motor does not operate at Static CR, so it is pointless and useless with regard to causing any detonation. Now the problem is how do you measure dynamic CR. We typically use Static CR, because is it easily calculated.

 

Bryan, you are right in the fact that the engine never really operates at its advertised static compression. My comment was more to allude to the fact that the static CR is still an important figure.
Its a good 'baseline' number, for lack of a better term.

 

it is my personal belief that detonation is a factor of many small things that add up to basically 3 main ingredients. cylinder pressure, flashpoint of fuel and burn rate.

static and dynamic will both influence cylinder pressure, piston and chamber shape will influence burn rate, octane is obviously the dominant concern for flash point.

its basically the idea of -can my fuel sit on the cylinder walls collecting heat without igniting long enough for my flame to burn it, before it pops?

a lot more than compression ratios will come into play IMO and it shouldnt be looked at as "okay my compression ratio is this and im running 93, im good." theres more to consider

 

But like someone said, would a 16:1 motor with like 359 degree 20mm cams be very prone to detonation? Or would the dynamic compression be so low it wouldn't make more power than a motor with normalish compression and cam specs?

 

its a lot of money to spend on custom pistons and a welded head only to bleed off the cylinder pressure they were intended to generate in the first place.

a well designed engine has ENOUGH dome and ENOUGH cam to make good power, but not so much of one that it requires a compromise in the other at the expense of power. if the cylinder pressure is very low from too much overlap then the cylinder temperature will be low as well. the tendency for detonation will be reduced, but so will the power production. and back to the expense of those parts, if it doesnt make power, there isnt much point in spending the money just to say "im running 16:1"

yeah, so'd my shitbox f350 diesel, and it ran better too. all 187 HP from a 7.3L

 

cams will make a difference on the dynamic ratio,the more overlap you have the lower the ratio will be becuase some of the intake charge will go out the exhaust

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by the DOT &raquo;</TD></TR><TR><TD CLASS="quote">cams will make a difference on the dynamic ratio,the more overlap you have the lower the ratio will be becuase some of the intake charge will go out the exhaust</TD></TR></TABLE>

Not necessarily. The event that most determines dynamic CR is intake valve closing.

 

cold intake charge cooling a hot exhaust valve was the subject of a 1940's SAE paper that documented an experiment which showed a supercharged engine would continue to make more power as overlap was increased, up to an obscene amount because of this cooling effect on the hot exhaust valve. i understand supercharging is different than NA, but as long as the intake pressure is higher than the exhaust pressure the effects should be similar, and this is possible with very well designed NA race engines.

its intake charge going back out the intake that will screw you.

 

Problem with that is in 1940 10 degrees was alot of overlap. Also head design sucked and engines were not efficent.

 

if it makes any difference i think it was more like 50 degrees overlap if i recall Don... ill try to find the paper again sometime. working out of a diesel shop, i certainly agree on the last two. atrocious.

 

Suscribed, because I'm still learning

 

Anyone want to post up a link that explains static/dynamic without boggling my mind?

 

static CR is simply the ratio of combustion chamber volume with the piston at BDC and TDC. If the valves remained closed your would be compressing the air volume at BDC to the volume at TDC. Your valves bleed off the air though, so you don't a 'true' CR equal to the simple static CR.

 

So does a static cr generally read lower or is it subjective to the way the cams are degreed?

Am I making sense?

 

[QUOTE=Vtaaak y0]So does a static cr generally read lower or is it subjective to the way the cams are degreed?QUOTE]

No. Yes. Static will always be higher. Dynamic is dependant on cams and how they are timed relative to the crank.