i know very little about this and it seems to be very often overlooked when many inexperianced and some experianced people are designing engines. it seems this could be very important when you want to get the most out of your engine build. i am planning on running pump gas (91 to 93 octane) in my mostly street and sometimes track driven 75. can someone help educate me on this subject? what is the best dynamic compression to have for this type of engine and can some one help me figure it out with my planned combo? i have most of the components picked out but this calculation will help me decide what cam to use and if if my target static compression will be correct for this application also. i am sure quite a few of you can help me learn more about this.

 

Have a look here:

http://www.chevytalk.org/threads/sho...rt=1&vc=1&nt=2

 

The DCR is a compression ratio that begins as the intake valve closes on the compression stroke. The amount of volume that is trapped just as the intake valve closes is divided by the fully compressed volume when the piston is at top dead center. The ratio calculated is the "dynamic compression ratio" or DCR. Simmilar to static compression ratio but we only count the compression that occurs after the piston is on the way up AND the intake valve is fully closed.

In an effort to save you and I from the tedious geometry and trigonometry calculations that are required, some people have been nice enough to create spreadsheets that will do it for you. They are free to you to download on the web.

Trial and error shows that using a typical aftermarket camshaft profile and pump gas you can run in the area of 8.3 to 8.4 DCR and still be without detonation. Higher than that requires careful tuning. Less than around 8.0 and the engine will be lazy and not perform to it's full potential. Factory grinds have slow lift ramps and the valve event specs are not easy to nail down so it is not easy to apply DCR to them and even if you do, the recomended DCR is not as applicable because of the ramp profile.

The DCR spreadsheet I like to use is Pat Kelly's. Keep in mind you must use actual intake valve closing points. Some manufacturers make this tricky to locate but you need this to make the program work.

http://cochise.uia.net/pkelley2/DynamicCR.html

-Mark.

 

thanks for the info. now all i have to do is get the proper cam specs for the cams i am looking at and plug everything in.

 

Figuring DCR is the best way to get optimum power for your investment and make sure you are pump gas compatable.

 

thanks for the heads up on this. it was your reply in my other post that woke me up to this. i have gotten some very good links off of this post and this should keep me going in the right direction. thanks for all the help guys.

 

http://www.kb-silvolite.com/calc.php?action=comp Here is a DCR calculator.

 

 

i think i have found the combo that should work now. i am going to use the new 75cc afr 195's with a forged eagle crank and rod package (3.875 stroke and 6 inch stroker rods) with a set of srp 5cc forged pistons with a comp hight of 1.062 and .028 head gasket. my block has been decked to 9.011. the cam i am going with will most likely be the crower hyd roller # 00471

PERFORMANCE LEVEL 4
Street/strip applications, rough idle,
2500 stall required.

283 - 327 cid / 2900 - 6000 rpm
350 - 372 cid / 2800 - 5950 rpm
383 - 400 cid / 2400 - 5900 rpm

110° 286° 292° 236° 240° .555" .559"

this will give me a static comp of 9.6 to 9.8 depending how the head cc works out i may shave the heads to get to 9.8. that would give me a dynamic comp of 8.2. with this cam. i still want to run on 92 or 93 pump gas with no ping. i am looking at an rpm airgap intake or the weiand stealth air strike dual plane. i will be using stans tri-y headers to finish things off. anything else i should look at?

 

That sounds like a real good setup based on the numbers. You could go a little tighter on the lobe centers if you want a little bit more peak HP. The engine "as spec'd" will have no shortage of torque and the long stroke can take the tighter lobe centers. Either way it looks good to me.

-Mark.