Head CC change versus comp ratio

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    I know this is fairly easy to find out if you have the liquid and fixture to do so and actually CC the head, but I don't have that here. It's just a curiosity thing for now.

  Does anybody know a way of finding out what the change in chamber CC would be if a spark plug that is 1/4 inch longer than stock were inserted into a known 258 CID head chamber of 65 CC? Once I know this new number, I know it will be easy to re-compute the compression ratio from established formulas. 

  A typical plug for my 258 would be an Autolite 765. Thread size of this plug is 14mm. Stock reach is .750 inch, the replacement plug would be 1.0 inch reach.

 Get out your slide rule folks...and thanks for any help.

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Pinballer Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Apprentice Joined: Feb/10/2016 Location: Central IL Status: Offline Points: 63	Post Options Post Reply Quote Pinballer Report Post Thanks(0) Quote Reply Topic: Head CC change versus comp ratio Posted: Nov/04/2016 at 5:23pm
	I know this is fairly easy to find out if you have the liquid and fixture to do so and actually CC the head, but I don't have that here. It's just a curiosity thing for now. Does anybody know a way of finding out what the change in chamber CC would be if a spark plug that is 1/4 inch longer than stock were inserted into a known 258 CID head chamber of 65 CC? Once I know this new number, I know it will be easy to re-compute the compression ratio from established formulas. A typical plug for my 258 would be an Autolite 765. Thread size of this plug is 14mm. Stock reach is .750 inch, the replacement plug would be 1.0 inch reach. Get out your slide rule folks...and thanks for any help.

6PakBee Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Charter Member Joined: Jul/01/2007 Location: North Dakota Status: Offline Points: 5457	Post Options Post Reply Quote 6PakBee Report Post Thanks(0) Quote Reply Posted: Nov/04/2016 at 6:32pm
	I get about 1 CC. !4mm is about 0.55". With the additional protrusion, you have a cylinder 0.55" in diameter by 0.25" long. I get 0.0597 in3 for the volume of the cylinder and at 16.387 CC/in3 the result is 0.9776 CC.
	Roger Gazur 1969 'B' Scheme SC/Rambler 1970 RWB 4-spd Machine 1970 Sonic Silver auto AMX All project cars. Forum Cockroach

Pinballer Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Apprentice Joined: Feb/10/2016 Location: Central IL Status: Offline Points: 63	Post Options Post Reply Quote Pinballer Report Post Thanks(0) Quote Reply Posted: Nov/04/2016 at 8:02pm
	That's a lot less volume than I came up with today at work trying to do it on paper in long hand! But I was having trouble convincing myself it was going to go down to 51 cc which is what I came up with, so that's why I came on here for help. Apparently my math was really flawed! At 60 years old now, trying to extract geometry class memories from back in 1972 to search for area of a circle X length and convert it into cc's might have been a lesson in futility. Your answer appears quite reasonable, and thanks! Now: some of you would say why would I care about all this? In a Stocker, you're looking for any chance to increase compression in a motor that basically has none and the rules don't allow you to change a whole lot of things very much like deck height and gasket thicknesses. -IF- the plug doesn't hit the piston and this easy modification can gain me even a couple horsepower, then all the better.

PROSTOCKTOM Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Jun/20/2010 Location: Indiana Status: Offline Points: 2450	Post Options Post Reply Quote PROSTOCKTOM Report Post Thanks(0) Quote Reply Posted: Nov/05/2016 at 11:00pm
	If your making reference to a stocker, as in a NHRA Stock then there are a lot of machining that can and must be done to make more compression while still being legal. Having raced NHRA V/S, V/SA, W/S, W/SA I have never seen any engine that didn't require a lot of machining within the rules to get the most out of it while still being legal. We deck the block to the specs, deck the head(s), offset the crank stroke .015"(.017" allowed) using the smallest rod and main bearings allowed and run the maximum piston overbore. 1mm (.03937") on the W/SA all of which will cause an increase of 1.25 to 1.5 points of compression. The word, STOCK, is true in some respects in theory, but not STOCK in real life. Tom Edited by PROSTOCKTOM - Nov/07/2016 at 6:21am

Pinballer Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Apprentice Joined: Feb/10/2016 Location: Central IL Status: Offline Points: 63	Post Options Post Reply Quote Pinballer Report Post Thanks(0) Quote Reply Posted: Nov/06/2016 at 4:53pm
	Definitely in agreement with you Tom, I know there's a lot of machine work I can do and yet stay legal. First I have to have the correct head though! For now, I'm just exploring "creative" ways to make things a tiny bit better if I can.

amcenthusiast Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/02/2012 Location: SW Atlanta GA Status: Offline Points: 1778	Post Options Post Reply Quote amcenthusiast Report Post Thanks(0) Quote Reply Posted: Nov/07/2016 at 6:23am
	One of those 'tiny things' a person can do is index their spark plugs ...and a longer plug might help to use a double gasket on those that don't line up as good...