A couple questions on compression and rod length
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A couple questions on compression and rod length

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    Default A couple questions on compression and rod length

    Looking at the 509 Twin turbo build and trying to settle on a piston...dish or flat top. I imagine whatever set of heads I go to will be a 118-124 cc chamber. So, I am trying to see what thickness I would do for a cometic gasket to get me right around 8.5 on the compression. I want to stick with pump gas and down the road will probably do meth. I am ok with giving up a little power on the bottom as I don't want to start creating problems with blowing out the pump. The 468 with the little turbos and what I think was about an 8 CR actually launched the boat pretty well.

    Anyway, I am looking at JE's site and I can't figure out how they come up with their #'s. For instance, the flat top piston with 124cc chamber shows 8.6, but when I do a CR calculator, I can't come up with the number. Do they figure on a certain thickness head gasket?

    Speaking of head gaskets, how thick would you go with a cometic?

    last question, I notice for a 509 combo they list both 6.385 rod and a 6.135 rod. What is the reason for going with one length over another?

    Thanks guys!

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    Quote Originally Posted by terrible_buddhist View Post
    Looking at the 509 Twin turbo build and trying to settle on a piston...dish or flat top. I imagine whatever set of heads I go to will be a 118-124 cc chamber. So, I am trying to see what thickness I would do for a cometic gasket to get me right around 8.5 on the compression. I want to stick with pump gas and down the road will probably do meth. I am ok with giving up a little power on the bottom as I don't want to start creating problems with blowing out the pump. The 468 with the little turbos and what I think was about an 8 CR actually launched the boat pretty well.

    Anyway, I am looking at JE's site and I can't figure out how they come up with their #'s. For instance, the flat top piston with 124cc chamber shows 8.6, but when I do a CR calculator, I can't come up with the number. Do they figure on a certain thickness head gasket?

    Speaking of head gaskets, how thick would you go with a cometic?

    last question, I notice for a 509 combo they list both 6.385 rod and a 6.135 rod. What is the reason for going with one length over another?

    Thanks guys!
    How far off are you,, it works for me,, 4.5" bore, 4" stroke -3cc volume for intake valve relief, .040" thick head gasket? Gasket bore makes a difference too, I actually didn't show 8.6,, just under, used a 4.580" bore on gasket (which is what the Fel Pro 1075 is).

    6.135 is stock rod length, the 6.385 is commonly used, even with a 4.25" stroke crank for better rod angle geometry.

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    Quote Originally Posted by terrible_buddhist View Post
    Looking at the 509 Twin turbo build and trying to settle on a piston...dish or flat top. I imagine whatever set of heads I go to will be a 118-124 cc chamber. So, I am trying to see what thickness I would do for a cometic gasket to get me right around 8.5 on the compression. I want to stick with pump gas and down the road will probably do meth. I am ok with giving up a little power on the bottom as I don't want to start creating problems with blowing out the pump. The 468 with the little turbos and what I think was about an 8 CR actually launched the boat pretty well.

    Anyway, I am looking at JE's site and I can't figure out how they come up with their #'s. For instance, the flat top piston with 124cc chamber shows 8.6, but when I do a CR calculator, I can't come up with the number. Do they figure on a certain thickness head gasket?

    Speaking of head gaskets, how thick would you go with a cometic?

    last question, I notice for a 509 combo they list both 6.385 rod and a 6.135 rod. What is the reason for going with one length over another?

    Thanks guys!
    They are figured like Barry said, zero deck, usually a SAE thickness gasket at .039-.040, and on your combo, a 4.580 gasket bore.

    With a quick and dirty figure I came up with 8.9 to their 9.0 on a flat top and a 118 chamber.

    You didn't look at all the available flat tops, there is also a 1.24 CH piston for a 6.535 rod. (+.400)



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    Quote Originally Posted by gn7 View Post

    With a quick and dirty figure I came up with 8.9 to their 9.0 on a flat top and a 118 chamber.
    Yep, I actually forgot to state, but I used 124cc chamber since that was his example with the 8.6

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    Quote Originally Posted by obnoxious001 View Post
    Yep, I actually forgot to state, but I used 124cc chamber since that was his example with the 8.6
    Yep, like you said, just a shade under 8.6 with a 124 chamber.
    I think the oppsie in his calculations is the gasket bore size, or the deck clearance, or possibly the entire formula.



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    Quote Originally Posted by gn7 View Post
    Yep, like you said, just a shade under 8.6 with a 124 chamber.
    I think the oppsie in his calculations is the gasket bore size, or the deck clearance, or possibly the entire formula.
    Or maybe used a positive or negative on the -3cc dome volume, depending on whose calculator you are using, I have seen it stated both ways. It would change the number about .39 with the 124cc combustion chamber,, a fuzz more with the 118cc.

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    Quote Originally Posted by obnoxious001 View Post
    Or maybe used a positive or negative on the -3cc dome volume, depending on whose calculator you are using, I have seen it stated both ways. It would change the number about .39 with the 124cc combustion chamber,, a fuzz more with the 118cc.
    The +/- of the valve notch/dome and the +/- of the deck height on some "calculators" is different and you have to be careful. That's why I tend to use the same one once you have it down. On the deck height I usually just enter .0001 and then it don't make much doesn't make much difference if its +/-. Can't explain why, but some calculators won't let you enter .000



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    Quote Originally Posted by gn7 View Post
    The +/- of the valve notch/dome and the +/- of the deck height on some "calculators" is different and you have to be careful. That's why I tend to use the same one once you have it down. On the deck height I usually just enter .0001 and then it don't make much doesn't make much difference if its +/-. Can't explain why, but some calculators won't let you enter .000
    Right, I was saying that may be his issue.

    I have one saved onto my desktop that's pretty good. The one I have combines gasket thickness and deck height as one entry.

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    Quote Originally Posted by obnoxious001 View Post
    Right, I was saying that may be his issue.

    I have one saved onto my desktop that's pretty good. The one I have combines gasket thickness and deck height as one entry.
    It makes it a little easier, but the problem with that Barry, is that any difference in the bore diameter and the gasket diameter will skew the number a little. But for mere commoner use, it doesn't make much difference. I just wouldn't use that formula for NHRA Stock classes, because they won't be using a formula like that.

    Can you even tell from the entries if they are using the bore X deck height, or the gasket diameter X deck height? One would make the number higher, the other would make it lower than actual.



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    Quote Originally Posted by obnoxious001 View Post
    How far off are you,, it works for me,, 4.5" bore, 4" stroke -3cc volume for intake valve relief, .040" thick head gasket? Gasket bore makes a difference too, I actually didn't show 8.6,, just under, used a 4.580" bore on gasket (which is what the Fel Pro 1075 is).

    6.135 is stock rod length, the 6.385 is commonly used, even with a 4.25" stroke crank for better rod angle geometry.
    I think the gasket bore is where I made my mistake...I will have to double check that. So with a 4" crank, the 6.385 gives a better rod angle?

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    You guys were right...I assumed the gasket bore was the same as the engine bore.
    Compression Ratio Calculator | Tools | Diamond Racing Pistons


    Bore: 4.5
    Stroke: 4
    Rod Length: 6.385
    Gasket Bore: 4.580
    Compressed Head Gasket thickness : .04
    Block Deck Height: 9.78
    Top of 1st ring to top of pistons (FLAT): 1.395
    Chamber Volume: 118
    DOME (-) or DISH (+) vol. (theoretical) in CC's: -3 (oddly, if you put a positive # in lowers the CR)
    Piston to Deck: 0


    Puts it at 8.9CR, about a half a point higher than I want to be. Is it better to cut the piston down a bit, or go with a thicker gasket?

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    Quote Originally Posted by terrible_buddhist View Post
    You guys were right...I assumed the gasket bore was the same as the engine bore.
    Compression Ratio Calculator | Tools | Diamond Racing Pistons


    Bore: 4.5
    Stroke: 4
    Rod Length: 6.385
    Gasket Bore: 4.580
    Compressed Head Gasket thickness : .04
    Block Deck Height: 9.78
    Top of 1st ring to top of pistons (FLAT): 1.395
    Chamber Volume: 118
    DOME (-) or DISH (+) vol. (theoretical) in CC's: -3 (oddly, if you put a positive # in lowers the CR)
    Piston to Deck: 0


    Puts it at 8.9CR, about a half a point higher than I want to be. Is it better to cut the piston down a bit, or go with a thicker gasket?
    You have the top of 1st ring dimension wrong, that 1.395" is "CH" or pin height. Also, how did you arrive at 9.78" deck height?

    They do properly explain that a positive number is used for a "dish",, and your 3cc valve relief does remove volume from the piston.
    Last edited by obnoxious001; 12-29-2013 at 09:32 AM.

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    Quote Originally Posted by terrible_buddhist View Post
    You guys were right...I assumed the gasket bore was the same as the engine bore.
    Compression Ratio Calculator | Tools | Diamond Racing Pistons


    Bore: 4.5
    Stroke: 4
    Rod Length: 6.385
    Gasket Bore: 4.580
    Compressed Head Gasket thickness : .04
    Block Deck Height: 9.78
    Top of 1st ring to top of pistons (FLAT): 1.395
    Chamber Volume: 118
    DOME (-) or DISH (+) vol. (theoretical) in CC's: -3 (oddly, if you put a positive # in lowers the CR)
    Piston to Deck: 0


    Puts it at 8.9CR, about a half a point higher than I want to be. Is it better to cut the piston down a bit, or go with a thicker gasket?
    Using the 10751 gasket which is .053 thick will lower your compression roughly .5.
    Like Barry said, the 1.395 is the CH, the distance from the top of the piston to the center of the pin. You don't include the top ring measurement in to calculation unless you are picking fly shit out of pepper. It changes the compression, but not enough to discuss here.

    Yes, I would use the longer rod. Better rod ratio, and the piston is lighter.
    Quote Originally Posted by obnoxious001 View Post
    You have the top of 1st ring dimension wrong, that 1.395" is "CH" or pin height. Also, how did you arrive at 9.78" deck height?
    couple guesses here. The JE tables explain the compression is figured with 9.78 deck height, and he is misinterpreting the CH listed as the distance to the top ring.



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    Last edited by gn7; 12-29-2013 at 09:56 AM.

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    both correct guesses. Thanks for being patient...trying to learn

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