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Old 12-03-2010, 08:57 PM
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Compression Ratio Calculators

My head is spinning.
I am trying to cook up a good recipie for my 389.After much research I am going to opt to ditch the old 1965 close chambered heads for some later ones.I will be running 400 piston with the valve relief for 14 degree valve angle heads.
I have used a few different compression calculators to determine dynamic and static ratios as I want to drive it daily without feeding it race fuel.
Here is my prospect
4.12 bore
3.75 stroke
87 CC chamber heads (hope i can find)
0.05 deck ( I hope i can)
.039 head gasket (or as close as possible for good quench)
6 cc valva relief pistons with 1.720 compression height
.3 ring height
256 intake duration
110 lobe seperation angle

I used one calculator and came up with 9.01 static and 7.54 dynamic.
Should be good to go on pump swill.
Trouble is A different calculator came up with a 10.1 static and did not offer the dynamic figure.
I could do the math myself(well maybe not) or I would at least try but MY HEAD IS SPINNING NOW
Anyone have a calculator that they can swear by? I dont need a bunch of cast aluminum grenading my pocketbook. Feel free to correct my recipie also if you think I am missing something

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Old 12-03-2010, 09:06 PM
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Quote:
Originally Posted by latech
My head is spinning.
I am trying to cook up a good recipie for my 389.After much research I am going to opt to ditch the old 1965 close chambered heads for some later ones.I will be running 400 piston with the valve relief for 14 degree valve angle heads.
I have used a few different compression calculators to determine dynamic and static ratios as I want to drive it daily without feeding it race fuel.
Here is my prospect
4.12 bore
3.75 stroke
87 CC chamber heads (hope i can find)
0.05 deck ( I hope i can)
.039 head gasket (or as close as possible for good quench)
6 cc valva relief pistons with 1.720 compression height
.3 ring height
256 intake duration
110 lobe seperation angle

I used one calculator and came up with 9.01 static and 7.54 dynamic.
Should be good to go on pump swill.
Trouble is A different calculator came up with a 10.1 static and did not offer the dynamic figure.
I could do the math myself(well maybe not) or I would at least try but MY HEAD IS SPINNING NOW
Anyone have a calculator that they can swear by? I dont need a bunch of cast aluminum grenading my pocketbook. Feel free to correct my recipie also if you think I am missing something
I'll be glad to help you figure it correctly, but first, you need to get your decimal points in the right place. On the deck, did you mean 0.05" as in fifty thousandths or did you mean 0.005" as in five thousandths? Also, do you have the intake closing point @0.050" tappet lift on the cam?
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Old 12-03-2010, 09:19 PM
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Mechanical CR is 8.27:1 @ 0.050" deck, 8.96:1 if the deck is 0.005" and allowing 4.180" for the head gasket bore size, and not computing any volume above the top ring. Used THIS calculator.
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Old 12-03-2010, 09:39 PM
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Need rod length as well, I believe (I don't use dynamic CR, so I may be mistaken). FWIW, the stock rod length is 6.625".
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Old 12-04-2010, 07:33 AM
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I was going to deck theblock to 5 thousanths from the stock 23 thousanths to keep a good quench.
The cam I chose that seemed to be a streetable choice was a comp cams
51-221-4 grind number XE-256-H
http://www.summitracing.com/parts/CCA-51-221-4
The 51-222-4 looked real good but I was thinking the smaller cam was a safe choice.
Rods are a stock6.625 and compression height stock is 1.717,Funny thing is I saw a set of pistons that had the spec at 1.720
Dont think I will run any extra cam timing on this one.
I am changing to 400 pistons and newer heads too.Check out this attachment from KB pistons.Some of the specs are wrong????what gives???rod legnth is not 6.3000 from what I can gather and compression height is listed as 1.720 in not the stock 1.717. Also I believe these have no offset in the pins
Attached Files
File Type: pdf DOC120110-002.pdf (470.3 KB, 66 views)

Last edited by latech; 12-04-2010 at 07:42 AM.
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Old 12-04-2010, 08:27 AM
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This page has a good calculator that does dynamic compression.
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Old 12-04-2010, 10:31 AM
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Or here's some calculators.. http://www.kb-silvolite.com/calc.php
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Old 12-04-2010, 01:40 PM
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OK, starting from scratch, we know that we need 5 values to figure static compression ratio.
1. Cylinder volume in cc's
2. Chamber volume in cc's
3. Piston crown volume in cc's (If using a domed piston, dome volume will be deducted from chamber volume. Flat-tops will usually have a few cc's of valve reliefs. Dished pistons will list the cc volume in the dish).
4. Piston deck height volume in cc's
5. Head gasket volume in cc's

We'll use the formula (.7854 times bore times bore times stroke times 16.387) to find the cc's in the cylinder.......

1. (.7854 times 4.12 times 4.12 times 3.75 times 16.387) = 819.24 cc's in the cylinder.

2. Chamber volume is 87 cc's.

3. Pistons have 6 cc's of valve reliefs.

4. Piston deck height volume is figured just like cylinder volume.
(.7854 times 4.12 times 4.12 times .005 times 16.387) = 1.09 cc's in the piston deck height.

5. Gasket volume is figured just like cylinder volume and piston deck height volume. We'll use this gasket with a 4.200" bore and a thickness of 0.040".
http://www.summitracing.com/parts/FOG-945044200/
(.7854 times 4.2 times 4.2 times .04 times 16.387) = 9.08 cc's.

Now, add all the volumes together, 819.24 + 87 + 6 + 1.09 + 9.08 and find a total of 922.41 cc's. This is the total theoretical volume that will be filled with air/fuel mixture on the intake stroke.

As the piston comes up on the compression stroke, we will compress these 5 values into the chamber volume, piston crown volume, piston deck height volume and gasket volume. So, if we deduct the cylinder volume from the total volume (922.41 less 819.24), we find that we are squeezing the fuel/air mixture into a space of 103.17 cc's with the piston at the top of its stroke.

Now, we can divide the total volume of 922.41 by the squeezed volume of 103.17 and find a 8.94:1 static compression ratio.

Using the Keith Black DCR calculator (if you use the same one time after time like I do, you begin to get a feel for it) and entering a compression ratio of 8.940, a stroke of 3.750, a rod length of 6.625 and an intake closing point of 47 (intake closes at 32 ABDC @0.050 on the cam you listed, so you add 15 to that number and enter 47 into the calculator) and the calculator shows a dynamic compression ratio of 7.981:1. In my experience, anything between 7.8 and 8.3 on the KB calculator should make a real nice street motor. This combination of yours should be a real nice daily driver motor that should run on cat pee pump gas without any fuss.
http://www.kb-silvolite.com/calc.php?action=comp2

Oh, and you made the remark about the KB pistons having a compression height taller than stock, yeah they do that at KB. Matter of fact, I wish some of these piston manufacturers would pull their heads out of their derrieres and make pistons 0.020" taller than stock so we could get the squish right without cutting the block decks to get there.

Forgot to mention that the constant I use, 16.387, is the amount of cc's in a cubic inch. So, that's your conversion from cubic inches to cubic centimeters (cc's).

Last edited by techinspector1; 12-04-2010 at 01:52 PM.
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Old 12-04-2010, 03:14 PM
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Excellent. Thank you Techinspector.I will have to read your post a few more times to fully digest it, very good info.
The Compression height was a big question mark for me, I am glad you cleared that up. I am rather skeptical as I see quite a few misprints and bad info on a daily basis.
I have been really going crazy trying to get the right recipie for my engine.
I wish I could run the older closed chambered heads but I havent the funds for that. It would be cheaper and the engine would perform better with the 14 degree angle valve heads.
I have my Beret setup for my headwork so I can get them even.Now I can go back to shopping for pistons. I like the ones I posted here earlier.
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Old 12-04-2010, 08:29 PM
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here is the easy way.. http://www.kb-silvolite.com/kb_car/p...tails&P_id=195
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