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Old 05-12-2007, 04:57 PM
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Milling Heads for more compression

383 SBC w/Dart iron Eagle heads 200/64cc. Replacing cam with a Herbert solid roller and since heads are coming off for new springs I thought I need to increase compression as well. Presently 9.67:1 Speed Pro dished. Want to mill heads to achieve around 10.3. and smaller chambers will result as well 59cc - 61cc ??? Would appreciate hearing from those that have gone this route to achieve slight compression increases. Cam is .608 lift with 1.6 rockers 244 duration. I know I need more compression with this cam but dont want to mill over .030. Thoughts appreciated.

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Old 05-12-2007, 07:24 PM
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Mill .030 off the heads. Use thin .015" steel shim head gaskets.
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Old 05-12-2007, 07:37 PM
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Depending on valve size, you may end up milliing into the intake valve seat, necessitating sinking the valve, which would kinda make the chamber bigger, necesstating milling the heads more, in which you may end up milliing into the intake valve seat............


tom
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Old 05-13-2007, 07:01 AM
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Milling Heads - Compression ratio formula

DART sez heads can be milled .060" and that .0065" = 1cc.
I am trying to calculate what the compression ratio would be milling at .030"
I know there is a guide to milling that states chamber size and computes compression. Anybody aware of this???????
THNAKS!!!
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Old 05-13-2007, 08:45 AM
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Quote:
Originally Posted by NovaHO
DART sez heads can be milled .060" and that .0065" = 1cc.
I am trying to calculate what the compression ratio would be milling at .030"
I know there is a guide to milling that states chamber size and computes compression. Anybody aware of this???????
THNAKS!!!
the 1cc per .00065" cut is an approximate guide. The only way to detremine the amount removed acurratly is to cc before and after.
To calculate your compression ratio before and after head milling you need to know some accurate dimentions of your engine. Other wise you are guestimating at best.
There are many online java script compression calcs. All you have ton do is enter to required info.
bore size, stroke, cylinder head volume, piston valve relief volume/dish volume/dome volume, piston to block deck clearance @ TDC. Head gasket volume. (head gasket diameter is not bore size.)
"64cc heads" are not often exactly 64cc. Usually larger unless have been milled before. Blocks are not all the same deck height.
If you want to "blueprint" the cr you need to measure and calculate these variables accuratly.
Will you be running enough fuel octane to support your new cr?
A little too much compression is a lot too much.
Java CR calc

Like MST advised, you need to check and establish if the heads you have will allow the amount of deck shave you want without cutting into the intake valve seat. A head with large 2.05"or 2.08" valve will not be the same as a Dart head with 2.02's. Measure twice, cut once.

If your present cr is 9.67:1 and;
If your 383 dished pistons have a 12cc dish and are .025" from the top of the block @ TDC and your heads are actually 64cc and you were using a standard felpro head gasket before (.041), then you can raise the cr of your motor just by replacing the head gaskets with felpro .015" shim gaskets. Your new cr will be 10.27:1
I'll leave it up to you to check all the if's.....

Last edited by F-BIRD'88; 05-13-2007 at 09:05 AM.
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Old 05-13-2007, 09:23 AM
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Milling Heads - Compression ratio formula

Good advice F-BIRD, I thought about just changing gaskets but didn't know if I would achieve exact effects of milling. Seems that .023"milling would put me around 10.3. (if all numbers are close as you stated) I normally run a blend of 97 + 104 octane, no pinging @35total timing, however did ping @37.
Motor loves the higher octane now, so I don't have a problem running the pricey stuff, car doesn't see many miles. My thoughts : increased bottom end torque with smaller chambers (around 59s) part of what got me headed in the milling direction plus need for higher compression with this duration cam. Now I might consider just cleaning them up .010 and installing the new springs. Thanks for the input, very helpful.
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Old 05-13-2007, 10:05 AM
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My thoughts: If you're that close to the edge now at 9.67:1 I would not nesessarily go to 10.3. If you just replace the head gaskets with thin .015" ones the quench clearance effect will be improved (detonation resistance) as well as raising the cr. This is better than milling the heads alone. and is reversable if it turns out to be too high. I would try cooler heat range non- projected tip spark plugs for additional ping tolerance. AFR (jetting) may be too lean also.
If you want more bottom end torque rework the igntion advance for more advance at idle possibley locking out the distributor. Further consider advanceing the cam in the motor a bit more. Closes the intake valve sooner creating more cylinder pressure. If the new cam is ground on say 108 LSA degree it and advance it 4 to 6deg from straight up. intake will be on a 102 to 104 lobe center position.
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Old 05-13-2007, 11:06 AM
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Milling Heads - Compression ratio formula

Looks like your advice may be the way to proceed. Cam is a 110 LSA and states best to install @ 106 my application. Guess I'm wrong about plugs, I thought colder would ping more. Jetting needs to go up 1 or 2 sizes I think (new carb with base Holley 750 jetting). With large tube headers and now the bigger carb I can use more bottom. Thanks again!
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Old 10-12-2007, 06:11 PM
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"1cc per .00065" " seems like an extra zero?

I shaved my heads .030" & I don't think my combustion chambers decreased by 46cc , It looks like about 3.2cc to me.

BBP

Last edited by BUFFALOBILLPATRICK; 10-12-2007 at 07:31 PM.
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Old 10-12-2007, 06:50 PM
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With a .600 lift cam milling your heads will probably nessesate fly cutting the pistons which for compressions sake will be be defeating the purpose.
I'd mock it up and clay your pistons first befor you go milling anything.
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Old 10-14-2007, 02:24 PM
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.0065" not .00065" sorry a typo. The peak valve lift does not indicate how close the pistons get to the valves. When the valves are at full valve lift the piston is way way down in the bore. No where near the valves.
The cam duration and overlap is a much better indication of how close the valves will be to the pistons at or near TDC.

Set up the motor with heads installed with the new gasket. set the valve lash at 0 lash. Turn the motor over slowly twice BY HAND Now tighten the rocker arms (both intake and exhaust on 1 cylinder on each cylinder bank.) 2 full turns.
again turn the crank over slowly twice BY HAND if valves do not contact the pistons you are good to go. if the valves do touch the pistons, stop and loosen the rockers off by 1/2 turn. repeat. if the valves now clear the pistons with the rocker arms set 1.5 turns tighter than 0 lash. You now have more than enough clearance once the valves are lashed properly.

if you advance the cam from this point then intake clearance will be reduced a small amount. if you retard the cam the exhaust gets closer to the piston near TDC. the critical zone is 0 to 30 degrees either side of TDC.
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Old 10-14-2007, 02:31 PM
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Generally valve to piston clearance is not a problem on SBC's using conventional design 23deg heads till the block and heads have been shaved a good bit and the cam is in the 250@.050+ range on tight 106 or less LSA.
Its up to you to check this clearance for yourself at any rate.
If your cylinder head uses a non stock valve angle or position in the combustion chamber or non stock valve guide spacing always check V to P clearance reguardless of the cam. TFS twisted wedge is a example of a head using a modified valve position and angle.
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