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  #1  
Old 03-20-2003, 10:57 PM
djswwg djswwg is offline
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Default Stroker Comp. Ratio Question

I've tried to figure this out using several different formulas from the internet and I never get the same result twice. If someone can figure this out for me, I'd be very appreciative. Here's the specs, what's the comp. ratio???
bore-4.375
stroke-3.75 (1/2 of which is 1.875 I think)
84cc. combution chamber vol.
4cc. valve relief vol., flat top piston
head gask.compressed height-.0394" (9.86cc vol?)
block height-9.96"
rod length-6.965" (yes, that's the right spec.)
piston comp. height-1.12"

Thanks to everyone who attempts this seemingly collosal undertaking!!!!!!!!!!!djs
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  #2  
Old 03-20-2003, 11:00 PM
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fastmopars .inc fastmopars .inc is offline
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i think you need to assemble the engine at least and provide some deck measurements. all of the calculators i have found seem to only need the deckheight instead of rod length and compression height, etc
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Old 03-20-2003, 11:23 PM
djswwg djswwg is offline
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Fastmopars, That's the whole point of my question. I want to know what the end result will be before I buy all the parts and find out I screwed up. So far I only have the rods. What formula do you use?.... thanks djs
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Old 03-21-2003, 12:14 AM
345Dart 345Dart is offline
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Default djswwg

Assuming a 4.380 gasket bore you should end up with a CR of 10.456 and zero deck clearance with the figures you have provided.
Go to www.kb-silvolite.com and plug in the numbers for yourself and any changes that you may wish to make before buying any more parts.
Sounds like a good combination to me.
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Old 03-21-2003, 03:14 AM
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You are talking about a 4.41 bore head gasket. Thats robbing you of compression. Don (cuda66273) has a company on his webpage (www.4secondsflat.com) that will make you head gaskets to your particular bore. As 345dart said, you should go with a nice tight bore gasket, 4.380 or 4.390. Not 4.41 like you are saying you are looking at. But, for your sanctity. 10.44 w/ the 4.41 bore/.039 thickness gasket. 10.81 w/ the 4.30 bore/.025 thickness Cometic gasket offered through Don @ FBO. These are all at zero deck height. I'd have to sober up a bit and get out of bed with my girl (both of which aren't happening tonight) to handle that math. Good luck and hope that helped....

-Fox Deacon
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Old 03-24-2003, 10:29 PM
djswwg djswwg is offline
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92 whitedak and 345Dart, thanks very much to both you guys for the help! I'm off to those websites to check this further.......djs
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Old 03-24-2003, 11:21 PM
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The math is great reading but it's only a ballpark.

After seeing specs, and comparing them to real measurements in the car, the only way you will get it right is to assemble it and measure. The "specs" only get you a ball park.

Assemble it prior to balancing the assembly. Do your measurements. Then, mill the piston down to get the compression you want. Get a piston a little taller than you want.

That's what I'd do. I'm making plans for a 451 myself. Budget permiting.
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Old 03-25-2003, 09:46 PM
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thanks dave571, all information (and it seems like I can never get enough!) is greatly appreciated ..........djs
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Old 03-26-2003, 04:33 AM
DartGT66 DartGT66 is offline
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The bore size of the gasket does not make a big difference. In a .040" thick gasket, the cc difference of a 4.38 and 4.41 gasket is a little over 0.1cc = meaningless. If the heads used are closed chamber and the pistons are zero deck, you need to use thick head gaskets. I like to do it the other way round; use pistons that will sit slightly in the hole and mill the decks to achieve zero deck height.
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Old 03-26-2003, 04:48 AM
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I'm not saying that you are wrong, but the bore size of the gasket, in relation to the thickness of said gasket, do make a "significant" difference. If this is to be a performance engine or he is trying to specifically define his CR to certain standards, particular to the last .01:1, they both mean a great deal. Perhaps with an equal thickness of .040" between the 4.38 and the 4.41 only yields a .13 cc difference, the two different gaskets I proposed (4.41/.039 and 4.30/.025) differ in the realm of 3.81 ccs. This is enough to raise compression ALMOST half a point (.37:1). I suppose if I was trying to narrow down my compression or even trying to squeeze the last bit of compression out of the engine I could, I'd want to know these things. But you were correct on two things. With the two gasket sizes you gave, the difference in compression is a meager .01:1. Not much but still something. And also what you said about lowering the deck to meet the pistons rather than going with higher deck pistons to begin with. Makes sense.

-Fox Deacon
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Old 03-26-2003, 05:10 AM
DartGT66 DartGT66 is offline
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The thing affecting the CC's more is the thickness. He didn't mention which heads he was going to use, but if it's a closed chamber, he can't go with much thinner than what he already has because the piston to head clearance becomes too small. BTW, I'm using Fel Pro 1039's, .051" thick 4.56" bore gaskets that are 13.9 cc's!
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Old 03-26-2003, 06:00 AM
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I don't understand how the thing affecting the cc's the most is the thickness. Using your fel pro 1039's and the formula for the displacement of the bore in a head gasket (essentially a cylinder)

(bore/2)^2*H*PI:

(Your fel pro)
(4.56/2)^2*.051*3.14 = .8324 inches^3
.8324 * 16.38706 = 13.64 cc (not 13.9)

Consequently, if you change the thickness to, say, .071 (for demonstration purposes), we arrive at:

(Your fel pro with a 1/.7813 gasket height increase)
(4.56/2)^2*.071*3.14 = 1.1589 inches^3
1.1589 * 16.38706 = 18.99 cc (not very realistic, but again, for demonstration purposes)

But, on the other hand, if we were to leave the height at .051 and increase (increasing the thickness would be like increasing the bore so we increase) the bore to a directly proportional number to what we changed the thickness ( .051/.071 = .7183, therefor 4.56/.7813 = 5.834(again, unrealistic):

(Your fel pro with a 1/.7813 gasket bore increase)
(5.834/2)^2*.051*3.14 = 1.3579 inches^3
1.3579 * 16.38706 = 22.25 cc (again, completely non-realistic)

That puts it at 13.64 the way it is now. If you really wanted to increase your CCs, your best bet is to increase your bore. As shown, if you increase on the same scale that you increase your height, you will obtain more cubic centimeters with the bore increase. We increased the height and bore BOTH proportional to their original numbers. I understand that increasing your thickness is usually a means to correct/help other things (close chambered heads and deck height) but if you were looking for CCs (lower or higher) your bet would be increasing/decreasing your bore size. Unfortunately, as I understand it, running higher compression engines, it is often a problem running outragiously bored head gaskets. I guess the ultimate answer is what DartGT66 and the others said. You've got to take A LOT more things into concideration.


Now here is the disclaimer. This is the first serious head gasket math I've ever done in my life (all 18 years of it). If I'm wrong, I NEED to know about it. But I think I could be right on this one. Also, I'm not trying to overthrow anyone here. Lord knows I've got absolutely 0 experience with this stuff. I'm just turning it into a thing of numbers. So please don't take any offense to this. It wasn't meant as a "I'm better than you" attempt. Again, if I'm wrong or did something wrong, point me in the right direction.


-Fox Deacon
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Old 03-26-2003, 06:08 AM
DartGT66 DartGT66 is offline
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Your math is right, but there is not that much variation in the bore diameters. I think it should rather be comapared with the same amount of thickness and diameter change, and in that case the thickness is the one which has way more effect. I rembered the cc's wrong, they advertise it at 13.7cc, so yourmath works there. However, in most BB head gaskets, the gasket opening is not round. The chamber is actually bigger than the bore on the side where the valves are, and that makes things a bit more complicated. In hte case of my 4.56" gaskets they are round, and because of the hickness I have actually slightly positive deck height to get a reasonable quench distance.
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Old 03-26-2003, 11:13 PM
djswwg djswwg is offline
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WOW! thanks for all the input. Here's what things look like at this point. The constants are as follows.
Flat top pistons with 4.0cc valve reliefs
Gasket thick. .020", bore 4.80"
Cyl bore 4.375" Stroke 3.75".
The variables are; cyl. head cc's. Don't know yet but I'll be using the p/p 906's from my 440. Once they're off I'll get the volume checked and adjust the deck height from there.
Using the KB pistons calculator, here's whats up. If the head cc's are 84 and the block ht. is left stock, (9.98") the compression should be 10.443:1. If the deck is cut .010" (9.97'), the comp. goes up to 10.687:1. If milled .015" (9.965"), comp. is 10.814:1 and so on. It's easy to plug in different values for comb. chamber cc's and deck hts. to come up with various comp. ratios. Thanks to all for your help...............djs
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Old 03-26-2003, 11:18 PM
djswwg djswwg is offline
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The other thing I forgot to mention was the pistons will be Ross with 1.12" comp. distance, the rods are Manley 6.965" centre to centre. If anyone wants to check my results using the info I've posted I'd be more than happy to compare just to see if I've done this correctly..........djs
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