Static Compression vs. Dynamic Compression Ratio
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Joined: Sep 2009
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From: PA/MD
Forged hydraulic roller, melling 10296 pump, but I'm running, and atribute lower and top end to my ported Edelbrock proflo xt
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Joined: Mar 2012
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I too think that the safe recommended dcrs are just that, safe. I have a first gen sbc that I run a dcr over 9 ( not sure exactly what it is but it is below 9.6). My static compression is 12.1 with iron heads and I run 93 in it. The cam is a solid roller and it has 49 cc chambers with flat tops and steel shim head gaskets. I have not dialed the timing yet and it is currently conservative at 28. I need to do some test and tune and find out how much it can take. I think that dcr and the octane you can run have a lot to do with combustion chamber shape, quench and pistons. I ran the quench at .035 in this engine. I am currently building a 408 lq4 with 11.5:1 compression. I was afraid to push the dcr as high in this engine as it is a daily driver. If I get lucky I should be able to run 91 octane in this one due to the fact I have aluminum heads and FI.
TECH Addict
Joined: Sep 2009
Posts: 2,481
Likes: 5
From: PA/MD
That was my problem when I built my 408. I read and searched, but always came up with the max "safe" dcr of 8.5:1.
Here is another thought I had on high compression FI. When you are in the lower rpms (under 4000) you have to worry about knock burning or blowing your engine to peices, but after you are past a certain rpm (somewhere just over 4,000rpm) the motor is spinning too fast for knock to occur and do any damage....so what if you either build lag into your setup do you aren't building boost till this point, or have an rpm activated switch that holds the wastegates opened to bleed off boost, then close after this set rpm and start boosting with a ramp up to where max boost is hit around 7,500-8,000 rpms? Right now I spin mine to 8,000, but could spin it faster.
Any thoughts?
Here is another thought I had on high compression FI. When you are in the lower rpms (under 4000) you have to worry about knock burning or blowing your engine to peices, but after you are past a certain rpm (somewhere just over 4,000rpm) the motor is spinning too fast for knock to occur and do any damage....so what if you either build lag into your setup do you aren't building boost till this point, or have an rpm activated switch that holds the wastegates opened to bleed off boost, then close after this set rpm and start boosting with a ramp up to where max boost is hit around 7,500-8,000 rpms? Right now I spin mine to 8,000, but could spin it faster.
Any thoughts?
