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Static compression 

 

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 Post subject: Static compression
Posted: Sun Dec 21, 2008 4:29 pm 
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I know bugger all about static compression. I think the 302s are somewhere around 9 - 9.5 stock. Is that right? What happens as the static compression increases on a NA motor? I've heard that performance increases. Is this the case and how so? How high can you go on a street build? 10:1? 11:1? 12:1?

 

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Posted: Sun Dec 21, 2008 7:25 pm 
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10.5-11 depending on state of tune, alloy heads and 98-100 fuel all the time
Will yeild good results in both bottom end torque as well as top end stonk
Brett

 

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Posted: Sun Dec 21, 2008 8:17 pm 
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8.9-9.0:1 stock for E7s.

 

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Posted: Sun Dec 21, 2008 11:53 pm 
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The max we've got readily available over here is 98ron. So I should be looking to make comp of around 10.5 then?

 

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Posted: Mon Dec 22, 2008 12:06 am 
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Here's a static compression calculator I found earlier.

http://www.rbracing-rsr.com/compstaticcalc.html

 

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Posted: Mon Dec 22, 2008 12:35 am 
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Scaffy wrote:
The max we've got readily available over here is 98ron. So I should be looking to make comp of around 10.5 then?


Not a 302, but the current FPV BOSSes run 10.8:1 and recommend 98RON, require 95RON.


There's not only static compression to worry about, dynamic compression (and that depends on other things too. cams, cam timing etc.) is what you really have to pay attention to to ensure a reliable ping-free run.

 

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Posted: Mon Dec 22, 2008 12:48 pm 
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data_mine wrote:
There's not only static compression to worry about, dynamic compression (and that depends on other things too. cams, cam timing etc.) is what you really have to pay attention to to ensure a reliable ping-free run.


+1 for the importance of dynamic compression over static... work out the rev range or power you want and the fuel you want to run and go from there.
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Posted: Mon Dec 22, 2008 2:38 pm 
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One step at a time. Can you explain dynamic compression to me then please and relate its importance?

 

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Posted: Mon Dec 22, 2008 2:51 pm 
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Static comp is when the motor is not running and dynamic is calculated when it is running, you could have a motor with 10-1 static comp as it sits not running but that figure could rise when the motor is run because of different cam profiles, overlap etc and that would be the dynamic comp ratio.
That is my rough understanding but im sure my reply will be well and truly tuned up by those who know better :lol:

 

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Last edited by green car on Mon Dec 22, 2008 2:52 pm, edited 1 time in total.

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Posted: Mon Dec 22, 2008 2:52 pm 
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Ah ok... sounds a lot more in depth then!

 

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Posted: Mon Dec 22, 2008 2:55 pm 
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Dude its a real f**k but if you are keen, i think kosteckis used to do engine building, modifying courses-seminars that told you all the in depth info you seek.

 

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Posted: Mon Dec 22, 2008 3:01 pm 
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Very Basically:

Static compression is the measure of how compressed the volume of mixture could be based on the swept volume of the cylinder versus whats left after (the combustion chamber). e.g the volume decreases from 10 to 1.

Dynamic compression includes valve opening and closing times and engine efficiencies.

eg. Your engine at high rpm may only comsume 80% of its full cylinder volume so your 10:1 ratio is now only 8:1. Your cam may be designed for 12,000rpm so only compresses 50% of the full cylinder volume below 2000rpm... now your dynamic compression ratio is only 5:1...

edit: (dynamic compression ratio is only at that particular engine rpm)
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Posted: Mon Dec 22, 2008 3:35 pm 
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So you use the volumetric efficiency to work out the dynamic compression? Or am I off target?

 

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Posted: Mon Dec 22, 2008 3:50 pm 
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Scaffy wrote:
So you use the volumetric efficiency to work out the dynamic compression? Or am I off target?


Thats always been my definition, (volumetric efficiency includes cam profiles) however I believe others use only the cam profile to determine dynamic compression..making it less dynamic and more static in situ..:)
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Posted: Mon Dec 22, 2008 4:01 pm 
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Is there an ideal dynamic compression range that I'd want to be aiming for on an NA application?

 

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Trickflow Street Heat top end kit, 70mm BBK TB & EGR, 24lb injectors, Underdrives & 3.73s: 275rwhp - 13.15@105mph NA / 12.37@116mph N20

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