I have just finshed figuring out what my DCR is. I used 3 different calculators and got the same thing. With my 370 build, pistons, custom cam and all the numbers plugged in STATIC im at 11.1:1. Well to my suprise the DCR is 9.8-9.9:1 which is awefully high with only 11.1 static. Is this thing going to be pump gas friendly? I was under the impression that anything over 8:1 is pushing it. Cam has over 15* of overlap in it so im sure this has a lot to do with it. Opinions?

 

What's your intake lobe duration and installed intake centerline?

 

Intake lobe duration is 235*, center line 109*

 

That's your duration at .050, DCR is calculated using advertised duration (@.006 for hydraulic to account for small deflection and @.020 for solid lifter to account for lash). The idea is to calculate swept area at the point when the intake valve actually closes. A 235@050 LS intake lobe is probably 290-300 @ 006 depending on manufacturer.

 

It's a lunati custom grind by Ed Curtis. I'll try to re calculate to see what I get. Unfortunately the cam cars did not come with anything advertised at .006. All the specs were at .050

 

My old TSP cam was ground by Lunati, its 231 @ 050 intake lobe and is 289 @ 006. So yours should be just a few degrees more if it helps you ballpark your stuff.

 

Thanks alot! I really do appreciate it.

 

No problem. Also, you can push toward 8.5:1 DCR on 91 octane. I tested this limit with my old 383 and various small changes near 8.5.

 

That really did help. Im right around 8.5 +/- .1. That makes me feel better. E85 is scarce and race gas is expensive!

 

my 10.9:1 conventional SBC 383 was just shy of 8.4 DCR with 68cc Holley heads and a tiny Lunati solid roller installed fairly early. I could run 37 degrees of timing n/a with a hot plug on 91 octane. I ran across a pair of AFR 195 Eliminators that were 65cc chamber, made my DCR 8.55 and I ate two plugs out of it the first night out. In my defense, was racing against some loud azz stuff in the other lane and wasn't hearing the detonation.. Anyway, it was happy at 34-35 degrees with fresh set of plugs. But basically reinforced the 8.5:1 DCR threshold for "full timing" on pump gas.. for my application anyway.

I usually aim for 8.2-ish now-a-days, and can make minor adjustments with cam installation angle. If you have an adjustable timing set, roll it back (higher number) a degree to close that intake valve just a skoshe later.

 

Ran approx. 8.6 DCR on my 11.3 to 1 385, 93 octane was the answer. Thank God they still have it everwhere in Texas!

 

Not to threadjack, just food for thought... Is dynamic compression related to power out put like static compression is? Ive been told about a half point bump in compression is worth about a 4% bump in horsepower. Does the same go for DCR? Asking because everyone here seems to be about 8.5:1, and according to a calculator and a few people im at 7.9:1 or 8:1.

 

In fact DCR is related to power output, and SCR has absolutely no relationship to it at all. I can build a 15:1 motor and cam it with a 8:1 DCR and it will make less power to a 10:1 motor with a 9:1 DCR over the same RPM range. theoretically speaking of course
If you can run strictly E85 like I do my motor has a DCR of 10.6, the TQ numbers are unreal for this size motor

 

Ok, anyone have a link to a good read on understanding dynamic compression and how it works?

 

My 408 is set up at 12.5:1 scr, but with my cam I am at a good safe 8.22:1 dcr. On pump gas I wouldn't try and hit over 8.5:1 in a street driven car personally

 

Static compression is how much the volume in the cylinder is squeezed from bottom dead center up to top dead center.
Dynamic compression includes your cam specs (intake valve closing point). You cant build pressure until the valves are closed, which might be after the piston has come up in the cylinder as much as an inch or more, which is also known as "effective" compression.

Hope that helps!

 

Yeah it does thanks!

 

DCR is still a static compression. Just wanted to point that out.

With that said, I usually aim for 8.3:1 on 91 as the safe limit and 8.7:1 on 93. Can you go higher? Sure, but you need to make sure you account for head design, quench, and total timing.

I run 8.5:1 on 93 because of the horrifying humidity in Florida.

 

4430-4343-0171-9n™;17266999]DCR is still a static compression. Just wanted to point that out.

With that said, I usually aim for 8.3:1 on 91 as the safe limit and 8.7:1 on 93. Can you go higher? Sure, but you need to make sure you account for head design, quench, and total timing.

I run 8.5:1 on 93 because of the horrifying humidity in Florida.[/QUOTE]

I've seena very few people run as high as 9:1, but that is still pushing your luck even with everything being perfect.

 

I have as well. Like I said, if you tune it with that in mind, you can get by and make HUGE numbers. But if you get some bad gas, get into some extremely muggy conditions, etc, you are taking a risk with the car. Ideally 8.7:1 on 93 gives you the most power at a reasonable risk as long as you have a tight quench and well designed heads.