Dynamic Compression Ratio Calculator

I just spent the last three hours of my life using every DCR calculator I could find on the web to check how a 401 would react with one of the milder Crane roller hydraulics that a member was kind enough to recommend.  Was shooting for around 8.1 or so DCR for 87 octane gas.  Of the six or so that I tried, none of them came up with the same answer when given the same inputs.  Varied from high to low by nearly a full point.  I thought (assumed?) that this was a mature, verifiable approach.  So......what gives?  Anyone have any thoughts?  Recommendations?

I played around with Summit's calculator. Amazing how much the gasket thickness can effect c.r. What was the AMC head gasket thickness on a 1968 390? Also, what was the effective dome volume? I used a hit-and-miss method and came up with 15.8 cc for effective dome volume. And, .045" for deck clearance to get a c.r. of 10.2:1

Boris Badanov wrote: http://www.summitracing.com/expertadviceandnews/calcsandtools/Compression-Calculator/ This one is correct.

That's correct for static compression ratio, couldn't find anything for dynamic compression ratio on Summit's site.

tsanchez wrote: When it asks for timing figures it's actual closing not at .050

Tony, some actually ask for the timing figure @ 0.050.  Some ask for advertised duration and lobe separation.  Then you get the ones using the @ 0.050 approach that recommend adding 15 degrees or 25 degrees to the @ 0.050 value!  One would think that if you provided consistent inputs, you would get comparable outputs.  Haven't found that yet.

I am just trying to develop an engine combination that will run on the horse urine that is called 87 octane and get the most dependable power possible.  I could easily just build an 8:1 ratio motor that would probably run on kerosene but I think I'd be leaving a lot of potential unused capability on the table.  I could also build a 10:1 motor that would sound like a marimba band at WOT and last as long as a good drunk.

Just trying to find the best combo. 

Ken_Parkman wrote: One thing to remember is there is nothing "dynamic" about those calculators. They just factor static for the valve closing point. That really has very little to do with octane requirement; it's way more complicated than that. IMO it is only valuable if you have a database of similar engines to be able to see a trend. And using "accepted" values for a sbc may be misleading. I do not use dynamic compression ratio at all, as a matter of fact for street performance I've found compression to be almost the least important factor. To artificially try to run a higher ratio by screwing up the combination tend to be much less effective than the correct combination and then the compression to match. Remember how little is gained by compression, about 2% per full point depending. You will lose way more than that with an incorrect cam.

At last some one. Who understands that the cam choice is critical desicion in the process.


Steve