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Discussion Starter #1
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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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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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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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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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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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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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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Discussion Starter #10
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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Discussion Starter #11
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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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.
 

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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?
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.
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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Discussion Starter #15
What SHOULD the values be for that? 9.8 on the deck? No idea on the ring question...

It looks like the ultra rod only comes in the 6.385, so that is good!
 

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What SHOULD the values be for that? 9.8 on the deck? No idea on the ring question...

It looks like the ultra rod only comes in the 6.385, so that is good!
The deck height needs to be the measured deck height to give you proper compression ratio. As Bob said, the ring height is a small difference (as is the difference in cylinder bore and gasket bore for the few thousandths of the deck height), but it would be the dimension from the top of the piston down to the top of the first ring. Thing is, then they haven't asked the actual piston size versus the bore size.

The "deck height" is commonly checked with a piston and rod in the block, which tells you how far "in the hole" the piston is with your actual combination. If the rods and pistons (and crank) are not all the same, you will see some variation. Same goes if the block is not "square", deck height will vary from end to end or side to side. A good machine shop should be able to square the block up and also give you a measurement for finished deck height.

Depending on how serious or casual you plan your build, you might try using .010" and zero deck heights and see the difference in calculated compression. Maybe try looking for a different compression calculator that may be a little simpler for you to use.
 

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What SHOULD the values be for that? 9.8 on the deck? No idea on the ring question...

It looks like the ultra rod only comes in the 6.385, so that is good!
Forget about the ring distance. Its meaningless in calculating compression unless you are picking fly shit out pepper.

Formula for figuring deck height"

1/2 stroke + rod length + piston CH=assembled length.

2.00 + 6.385 + 1.395= 9.78 Any block deck height more than 9,78 is deck clearance. I.E. 9.79= .010 deck clearance.

Did you mean to say the 6.385 is the SHORTEST Ultra rod, not the ONLY??
Yes, the 6.385 Ultra is an excellent rod for the application.



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Is there any downside to leaving the piston further in the hole?
Only that it lowers the compression. But if the block is used and needs to be decked , and it probably does, then the deck will not be the stock 9.8, It will be what ever it takes to clean it up and get it straight and flat. This is something you need to discuss with your machinist, and have him tell you how much he took off and what your finished deck ended up being.
If it ends up being 9.79, then you figure you compression ratio based on that, and adjust your compression with the gasket if you think you need to make an adjustment.



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