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Old 04-15-2013, 03:08 PM
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Best Compression Calculator

Using various compression calculators, along with the dynamic ratio calculators, which one is the most accurate and used by most of you for doing this math?
I used the United Engine, KB/Silvolite, and the RSR ones, and keep getting inconsistent DCR numbers

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Old 04-15-2013, 03:15 PM
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Quote:
Originally Posted by BigMo View Post
Using various compression calculators, along with the dynamic ratio calculators, which one is the most accurate and used by most of you for doing this math?
I used the United Engine, KB/Silvolite, and the RSR ones, and keep getting inconsistent DCR numbers
In my opinion, all the calculators are accurate if you use the same one all the time on every build. I have used the KB calculator for DCR for decades now and so I know what to expect. If you use different calcs all the time, you will get different results all the time.

I know for instance, that a motor that sits at somewhere between 8.0:1 and 8.5:1 on the KB calc, will be a good build that will operate well on pump gas. Using an extremely tight squish (0.035" for instance), a fellow may be able to run as high as 9.0:1 on the KB calculator on pump gas if the camshaft intake closing point was chosen exactly. I've never built a motor past 8.7:1 DCR on the KB calc, so I'm not sure you could get away with 9.0:1, I'm just supposing. Old Bogie, whose opinions I value highly, has suggested that 9.0:1 would be doable if you paid attention.
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Old 04-15-2013, 05:16 PM
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Quote:
Originally Posted by BigMo View Post
Using various compression calculators, along with the dynamic ratio calculators, which one is the most accurate and used by most of you for doing this math?
I used the United Engine, KB/Silvolite, and the RSR ones, and keep getting inconsistent DCR numbers
I'll X2 techinspector1. The KB tracks really tight to my home brew SCR and DCR calculator running in Excel.

That begs the question of how far apart are the answers you're getting?

One thing that swings these calculators really hard is the assumption that they request you make if all you have is the closing degrees of the intake measured at .050 inch lift. They add a specific number of degrees to get at where the valve actually seats. But this can be a very large number in degrees or a not so large number depending upon the ramp design. New fast acting lobes tend not to have a lot of duration between .050 inch lift and actual seat. (Most measures place the actual seat while the valve is still .006 open. Some cam companies use .008.) Older lobe designs can have upwards of twice the duration in the gap between .050 to whatever the manufacturer deams as valve closure compared to newer lobe designs. I point this out as it can introduce quite a bit of Kentucky Windage into these numbers when you calculate different brands of cams but use the consistent "adjustment" the calculator asks for.

Bogie
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Old 04-15-2013, 05:40 PM
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I can't say its the most accurate, or used by "most of you", but I've had consistant results with Pat Kelly's calculator. I installed the calculator with the link"DRC Calculator with VB6 Runtime files" near the end of what I believe is an excellent article on compression. FWIW
ssmonty
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Old 04-16-2013, 05:25 AM
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You mean the KB ah la United Engine one? Thats what i have been using, but on a search here, someone said that Compcams shows timing at .006, and i have assumed .050....so now im having some jitters.....my latest entry i have 11.3 static with 7.56 dynamic, using a Comp roller (XR286R) and adding 15 deg to the listed closing point.....i checked on several others using their timing inputs and things jump to a 8.7 dcr.....thats definately pushing the limit....seems like the only way i can get some good mid range dcr numbers is to use a smaller cam, and i aint like'n it dammit!! I dont see any issues with the 11.3 static (i know someone is gonna give me hell) but with some bleeding off by the cam timing it should be ok....but i think the 7.5 dcr will be weak.....opinions welcomed here.....especially whether Comp really uses .006 or .050 timing on the sheet .
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Old 04-16-2013, 06:29 AM
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Why are there articles after articles on street engines with pretty much the same setup, making approx. 525 horse, and running on pump gas (ie. 93 octane)? I can change the chambers to 60cc and end up with 10.9cr and 7.3 dcr, which seems pretty weak to me, especially when 7.8-8 is the target. With a very light car (2200lbs) low gear, plenty of stall, i dont see why this engine wouldnt be right on.....do i have to go back to the "sissy cam" to get the engine to work.....
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Old 04-16-2013, 06:38 AM
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why not post the recipe you used for the 525 HP engine with both cam specs?
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Old 04-16-2013, 07:46 AM
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Ok, thats what i want and will do.......PM me and give me suggestions. I doing a .040 over 350 block, AFR 195's (comp ported) 65cc stock will cut as needed, flat tops w 4cc, zero deck, cam was a XR286R roller from Comp, balanced rotating (all good stuff, 4340 crank, good rods, etc.) standard stroke and rod length......will use head gasket in the .039 range......I have put together lots of high end drag engines, but havent fooled with "street car" stuff in years.....
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Old 04-16-2013, 07:53 AM
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Here is abunch of formulas you might find useful.

AutomotiveCalculators and Converters
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Old 04-16-2013, 08:02 AM
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Using stock cc on head (65 with .035 gasket (4.166 bore), block, pistons, rods, stroke as described, gets rite at 10.5, but dcr is very weak with that cam.....
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Old 04-16-2013, 08:25 AM
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I understand what youre saying.....no cam in hand yet, just what i had decided on. Using a single plane already in hand (Vic Jr), and will be using an 850DP worked by the Carb Shop. I just thought that with good cooling, which the car has, plus an oil cooler to help more, and the aluminum heads, that pushing to 11:1 would be possible. No big deal, I will shoot for the lower comp, in the 10.5-10.8 range, and go for it. Oh yeah, 1 3/4 headers is what fits the car without major custom fab........
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Old 04-16-2013, 08:55 AM
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I appreciate the insight.........what are you thinkin in the way of the custom cam....im not against getting one ground to fit the engine instead of an off the shelf stick.....i plan on a 6800 or so RPM limit, with occasional 7000 hit depending on who and what i am after.....but this will definately be a street/strip motor
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Old 04-16-2013, 10:15 AM
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Quote:
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Ok, thats what i want and will do.......PM me and give me suggestions. I doing a .040 over 350 block, AFR 195's (comp ported) 65cc stock will cut as needed, flat tops w 4cc, zero deck, cam was a XR286R roller from Comp, balanced rotating (all good stuff, 4340 crank, good rods, etc.) standard stroke and rod length......will use head gasket in the .039 range......I have put together lots of high end drag engines, but havent fooled with "street car" stuff in years.....
My Excel model drives SCR at 10.57 with a DCR for the XR286R cam with an unadjusted closing point of 69 degrees at .015 inch (Comp's fact sheet) of 8.038.

I would consider this on the low side for an aluminum headed engine.

Given the XR286R has a power range of 3000 to 7000 RPM, you're going to be forced to run a high stall converter if an automatic and stiff gears out back regardless of transmission type so running the SCR up to around 11 on pump gas shouldn't be a problem for detonation. Certainly if this was a configuration trying to cruise at 75 while only turning 2000 RPMs that amount of compression wouldn't work.

However, the problem you've got considering the components available, how are you going to get to 11.X SCR? A domed piston will slow the burn forcing more spark lead which may get you into a detonation situation. the other is a smaller combustion chamber, to really be effective that looks a lot like angle milling but I think that's the best solution if you want more compression short of replacing the heads with something having smaller chambers.

At this point, if it was mine, I'd accept the lower DCR and call it good, it will still be an engine worthy of "braggin' rights".

Bogie
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Old 04-16-2013, 10:41 AM
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To make the comp. i would cut the heads for the correct chamber size for the build.....run the calc with 62 cc heads, and a .036 gasket w 4.166 bore....then the .040 block and same cam...etc etc
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Old 04-16-2013, 10:47 AM
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Oh, Im running 3200-3500 stall, 3.90 gear, car weight 2200.....
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