Dynamic compression ratio questions
Thread Starter
Race Director
Joined: Jul 2004
Posts: 12,703
Likes: 329
From: Scottsdale Arizona
Dynamic compression ratio questions
Good morning all,
Long story short, I'm finally building the SBC block that I've had sitting around for a couple of years. Because of the o-rings that were in the block, I had to deck it below 9 inches which made finding pistons hard. The guys at United Engine in Carson NV called last week and they had a set that worked moderately well. The only issue is that the compression ratio, even with a 0.100 Head gasket, is fairly high 10.1:1. Part of the package is thatI have a very nice set of Dart Iron Eagle heads left over from the motor that I blew up.
In any case, I was planning on reusing the very nice, custom cam that Comp made for the original 383, a solid roller with a nice combination of smooth (ish) idle and tolerable vacuum. Using the dynamic compression ratio calculator at that site, I was able to get numbers from 8.1 to 8.3. Most of the reading that I've done suggests that with premium fuel I should be ok in the "8 range." I know that I can play with the timing some but am I pushing my luck (again)?
http://www.kb-silvolite.com/calc.php?action=comp2
Long story short, I'm finally building the SBC block that I've had sitting around for a couple of years. Because of the o-rings that were in the block, I had to deck it below 9 inches which made finding pistons hard. The guys at United Engine in Carson NV called last week and they had a set that worked moderately well. The only issue is that the compression ratio, even with a 0.100 Head gasket, is fairly high 10.1:1. Part of the package is thatI have a very nice set of Dart Iron Eagle heads left over from the motor that I blew up.
In any case, I was planning on reusing the very nice, custom cam that Comp made for the original 383, a solid roller with a nice combination of smooth (ish) idle and tolerable vacuum. Using the dynamic compression ratio calculator at that site, I was able to get numbers from 8.1 to 8.3. Most of the reading that I've done suggests that with premium fuel I should be ok in the "8 range." I know that I can play with the timing some but am I pushing my luck (again)?
http://www.kb-silvolite.com/calc.php?action=comp2
Last edited by CA-Legal-Vette; Dec 8, 2012 at 02:02 PM.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Good morning all,
Long story short, I'm finally building the SBC block that I've had sitting around for a couple of years. Because of the o-rings that were in the block, I had to deck it below 9 inches which made finding pistons hard. The guys at United Engine in Carson NV called last week and they had a set that worked moderately well. The only issue is that the compression ratio, even with a 0.100 Head gasket, is fairly high 10.1:1. Part of the package is thatI have a very nice set of Dart Iron Eagle heads left over from the motor that I blew up.
In any case, I was planning on reusing the very nice, custom cam that Comp made for the original 383, a solid roller with a nice combination of smooth (ish) idle and tolerable vacuum. Using the dynamic compression ratio calculator at that site, I was able to get numbers from 8.1 to 8.3. Most of the reading that I've done suggests that with premium fuel I should be ok in the "8 range." I know that I can play with the timing some but am I pushing my luck (again)?
http://www.kb-silvolite.com/calc.php?action=comp2
Long story short, I'm finally building the SBC block that I've had sitting around for a couple of years. Because of the o-rings that were in the block, I had to deck it below 9 inches which made finding pistons hard. The guys at United Engine in Carson NV called last week and they had a set that worked moderately well. The only issue is that the compression ratio, even with a 0.100 Head gasket, is fairly high 10.1:1. Part of the package is thatI have a very nice set of Dart Iron Eagle heads left over from the motor that I blew up.
In any case, I was planning on reusing the very nice, custom cam that Comp made for the original 383, a solid roller with a nice combination of smooth (ish) idle and tolerable vacuum. Using the dynamic compression ratio calculator at that site, I was able to get numbers from 8.1 to 8.3. Most of the reading that I've done suggests that with premium fuel I should be ok in the "8 range." I know that I can play with the timing some but am I pushing my luck (again)?
http://www.kb-silvolite.com/calc.php?action=comp2
Thread Starter
Race Director
Joined: Jul 2004
Posts: 12,703
Likes: 329
From: Scottsdale Arizona
Great quench is the key to running higher than optimal DCR. Comp will give you an exact intake closing point. Use that on the United Machine calculator instead of doing the add 15 degrees thing. Use the biggest bore head gasket that will get you .040 quench with your - deck and post up your actual DCR number. You will not want to be over 8 to 1 DCR with iron heads and california fuel even with perfect quench. Increasing quench to reduce dcr thinking this will help is a mistake. Tight quench and higher dcr is more resistant to preignition than big quench and less drc if your borderline. If you take your heads in and unshroud the intake valves by removing some on the outer edge of the chamber you can gain flow and reduce DCR in 1 shot. Some race shops will have a CNC procedure to do this that will gain you 2-3 CC on the chamber and is quick and easy.
The quench as of right now will well and truly suck. I went with thick head gaskets to reduce the attic CR.
