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Old 08-04-2005, 10:47 AM
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Another DCR question.

OK I did a search and waded through some very interesting threads on the subject but came to no conclusion only more confusion. I am gathering parts to build an 8.9 to 1 355 SBC. I will be using a set of 416 casting 58cc 305 HO heads with 1.94/1.50 valves, a set of -22cc D-cup pistons and probably a Summit K-1102 cam. I ran this through a DCR calculator and came up with a DCR of 8.16. This will be going in a 4200 Lb. vehicle with a 200-4R OD and 3.73 gears. Question..... can anyone tell me if this DCR is too high to run with 87 or 89 Octane? My static CR tells me I'd be OK but the DCR seems higher than what I've seen some others post for Satic CR's in this range. I used 2 different calculators and got nearly identical results. Do I need a cam with wider LSA to bring the DCR down? HELP !!!!
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Old 08-04-2005, 10:56 AM
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My very minor understanding of DCR tells me that you need to find a cam with the intake valve closing (IVC) a little later.
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Old 08-04-2005, 11:09 AM
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dcr

Don't get all to excited over your dcr number. There ia alot about this subject that many people including most of the web sites and my self that is not understood.....

One of the problems is that those calculator give you a one cycle number,,,, this all changes when the same engine is at 5000 rpms......

The deal with a dynamic compression calculator is to help you avoid detonation.... Now we all know there are at the very least 6 or 8 major factors that can change the engines ability to resist detonation. So if you build a engine with a perfect dcr for the fuel you are going to run but have all the rest of the combo wrong it's not going to work anyway....

You don't have enough info for me to figure it out but, what do the calculators tell you the cranking compression is going to be..... That is a much better indicator,,, to me anyway....

I have run engines with the dynamic over what is recommended for pump gas with no issues, but the rest of the contributing factors were correct....

keith
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Old 08-04-2005, 11:24 AM
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Thanks, the ones I used didn't give me cranking compression. I know my current 350 has an honest 160 PSI and runs fine on 87 Octane. I'll check out some other calculators and find one that gives cranking compression estimates. I'm thinking I should be OK but I'd hate to get this all done and find out I have to run 93 all the time.
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Old 08-04-2005, 11:43 AM
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Dcr

try this one

http://www.rbracing-rsr.com/comprAdvHD.htm

Keith
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Old 08-04-2005, 12:47 PM
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Thanks,
I ran my current 350 through and the one I plan to build, my current motor came out at 8.1 DCR and 162 PSI, actual cranking compression is an honest 160 so that's pretty darn close. I have run it with 87 Octane with no trouble. The one I am planning came out at 8.4 DCR and 170 PSI. Think the extra .3 DCR and 10 PSI will make that much difference? I'm thinking the "real world" numbers will probably be a bit lower. The new motor will have headers, better ignition, intake manifold and will be backed with a 200-4R and 3.73 instead of the tired TH350 and 2.73.
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Old 08-04-2005, 12:50 PM
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dcr

I have found that i need to play with the altitude on that one to get the cylinder psi to come out but once i found the correct number it's always with-in 5 psi for me....

If you keep the engine cool and have the correct timing curve it will be fine...

keith
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Old 08-04-2005, 01:00 PM
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Thanks Keith, I'm "pretty sure" it will be OK but I only want to build this one once, there's no room in the budget to do it over.
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