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Old 01-11-2013, 04:01 PM
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Finishing up BBC build quench and dynamic compression questions

So I have all the parts to the puzzle

.30 454 2 bolt block
Arp main and rod bolts on truck rods
L2465f pistons 25.7 cc dome
802 heads with big valve treatment an cleaned up

Quench is a concern , was told deck was machined 5 or 10 thou so my piston should be 13-18 thou down the hole , if I run a felpro mls head gasket my quench would be .044-.048 thou , is .048 too much ?

My static should be right at 10.50:1 and the dynamic 7.9:1
Cam is comp xr288hr

Going from carb to performance efi now if it matters

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Old 01-11-2013, 04:40 PM
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Quote:
Originally Posted by jumbo2 View Post
So I have all the parts to the puzzle
.30 454 2 bolt block
Arp main and rod bolts on truck rods
L2465f pistons 25.7 cc dome
802 heads with big valve treatment an cleaned up
Quench is a concern , was told deck was machined 5 or 10 thou so my piston should be 13-18 thou down the hole , if I run a felpro mls head gasket my quench would be .044-.048 thou , is .048 too much ?
My static should be right at 10.50:1 and the dynamic 7.9:1
Cam is comp xr288hr
Going from carb to performance efi now if it matters
OK, correct me if I'm wrong.....
L2465F pistons are for open chamber heads only.
3909802 oval port heads are closed chamber heads for '67-'68 396/427, 101 cc chambers.
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Old 01-11-2013, 04:47 PM
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820s are open chamber heads oval port almost identical to 781s but Stan Weiss seems to think they flow better on the exhaust side , 113cc and some measured up to 118 over the years and depending on year

Last edited by jumbo2; 01-11-2013 at 04:52 PM.
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Old 01-11-2013, 04:49 PM
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Stupid me its a typo 2x 820 heads is what I have not 802 , thanks techinspector1
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Old 01-11-2013, 04:59 PM
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Have you cc'd the chambers? If not, why not?
Have you measured the exact piston deck height? If not, why not?
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Old 01-11-2013, 05:06 PM
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No and no , I lost 95% of everything I own about a year ago in a fire , I will try and cc the heads and I'm looking at a dial indicator right now as I need to set cam end play and degree the cam ect

Just going off theory for right now and will make changes if I have too , worst case it would only change such a small amount anyways

Physically looking at both heads 781 vs 820 I don't see much different at all , I'm not a engine builder by any means , I like to play with my junk and try to ask questions and spend every night on the computer reading forum after forum and build after build doing research
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Old 01-12-2013, 06:10 AM
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Finishing up BBC build quench and dynamic compression questions

First unless you have the correct figures everything is a guess. Assume that the 820 heads have 113cc chambers and the pistons are .015 down in the block. Using a .027 compressed thickness head gasket, a 4.375 gasket bore, and 44+15=59 for intake closing point. Your static compression would be 10.561 and the DCR would be 8.826. I am sure you would hear detonation with these figures. On a BBC you don't want to go over appoximately 9.6 static compression with iron heads.The DCR should be in the range of 7.5-8.5 for 91 octane gas. One option to lower the compression is to machine the dome to .016 which will lower the compression to 9.705 and DCR to 8.125. Like I said this is guessing until you cc the heads and measure how much the pistons are down in the hole. BTW I am using the KB compression calculator. http://www.kb-silvolite.com/calc.php?action=comp
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Old 01-12-2013, 10:16 PM
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Originally Posted by cdminter59 View Post
First unless you have the correct figures everything is a guess. Assume that the 820 heads have 113cc chambers and the pistons are .015 down in the block. Using a .027 compressed thickness head gasket, a 4.375 gasket bore, and 44+15=59 for intake closing point. Your static compression would be 10.561 and the DCR would be 8.826. I am sure you would hear detonation with these figures. On a BBC you don't want to go over appoximately 9.6 static compression with iron heads.The DCR should be in the range of 7.5-8.5 for 91 octane gas. One option to lower the compression is to machine the dome to .016 which will lower the compression to 9.705 and DCR to 8.125. Like I said this is guessing until you cc the heads and measure how much the pistons are down in the hole. BTW I am using the KB compression calculator. United Engine & Machine Co. Incorporated
how did you come up with the number 44 ? for the intake closing at .050 ? I know why you added the 15
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Old 01-13-2013, 12:17 AM
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I found techinspetor1's post and how he came to 44 deg , here is my other question , the program doesnt take other things into effect , like having a sparkplug will change the cc , the head gasket is not round its more oval with the valve reliefs so that must add a cc or 2 and when the to of the bore is notched to unshroud the valve that must be a cc or 2 ?

When I use the KB calculator and go from 113cc head to 118 cc the comp drops down to 9.98:1 , so if the plug and valve reliefs and head gaskets make up a couple cc's wouldnt it be the same ? wouldn't you have to literally fill the head on the block torqued down with gasket and piston at tdc to get the real a real number ? Or cc the head gasket and chamber seperate?
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Old 01-13-2013, 11:21 AM
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Finishing up BBC build quench and dynamic compression questions

This is why you cc the heads chambers. Piston manufactures give you the piston head volume on their website. Example: Flat top pistons 5cc, D-dished pistons 18cc, domed pistons -20cc. The valve reliefs are included in these measurements. Fel-Pro gives you the compressed thickness, gasket bore volume. Your machinist can give you the finished bore of the cylinders. You must mock up the engine to get the deck clearance. There are formulas for calculating cylinder volume but if you grind on the top of the block for unshrouding a valve you would have to actually measure the cylinder for the volume. These calculators are more or less a guide.
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Old 01-13-2013, 05:33 PM
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In any other compression calculator I get numbers that work , Im not sure why KB gives me such weird numbers but im going to stick to some numbers from better calculators and finish measuring everything when my tools come and hope im semi close , im more trying to get a true understanding about a couple things but feel like right now so just going to wait . Will leave results either way if anyone cares
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Old 01-14-2013, 08:11 PM
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Well I got my dial indicator and stand today!!! a trip to the local vet and got some large and small syringes and a chunk of plexiglass from a friend to cc heads .

So after measuring how far down the pistons are in the block , ontop of the wrist pin on one side because of my piston head shape , I got .014 as the highest and .016 as the lowest

Next im going to cc the heads , should I grease the valves ?
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Old 01-14-2013, 08:42 PM
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Quote:
Originally Posted by jumbo2 View Post
Well I got my dial indicator and stand today!!! a trip to the local vet and got some large and small syringes and a chunk of plexiglass from a friend to cc heads .

So after measuring how far down the pistons are in the block , ontop of the wrist pin on one side because of my piston head shape , I got .014 as the highest and .016 as the lowest

Next im going to cc the heads , should I grease the valves ?
Put the springs on and use trans fluid it is thick and wont leak any leak and you got to fix it and start over. 1 cc is very small and you are certainly looking to get that close a measure.
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Old 01-14-2013, 09:14 PM
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so after checking 2 chambers 2x with a spark plug im very confident to say I got 114.5cc chambers on these 820 casting heads

So if I average the piston down the hole I would call it .015 , so im looking for a head gasket from .025 to .030 to keep my quench in check considering im right at 10.5:1 comp ratio I proabably want to keep the quench as tight as possible to avoid detonation

Looks like the only 2 options are cometic or copper , gmpp used to make a .025 but it looks like its discontinued , can anyone add anything ? feels good to get these measurements finally done !!!
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Old 01-14-2013, 09:38 PM
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using the Keith Black comp calculator i got 10.47:1 static , and using 59 deg is my intake closing point +15 deg I get 8.73 dynamic

using Static/Dynamic Compression Ratio Calculator

I get static 10.48:1 dynamic 8.06:1 using what they asked for

using Jeep Strokers • Dynamic\Static Compression Ratio Calculator

I get 10.36:1 static 7.95:1 dynamic and off my duration and LSA the program came up with a 70 deg closing point wich would explain why the diffrence

so what gives , what would you listen too ? most calculators I can find give me the same 10.5:1 and around 8:1 dynamic except the keith black but thats the only one that doesnt ask about the +4 ground into the cam from comp hmmmmmm maybe
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