Edit, thanks guys, my math was off, I'd been working in the shop for quite a while after a long shift at work.... Checking my math vs a few online calculators, i'm not getting the same numbers, It appears that the deck clearance in these calculators is the actually the quench thickness???(gasket thickness included), instead of what i've always heard deck clearace called out as, the top of the piston to the top of the block. Anyone know if that's right, or have i been in the shop too long?? https://www.uempistons.com/index.php?main_page=calculators&type=comp http://www.csgnetwork.com/compcalc.html And for my own math 41.25013ci displacement = 675.96852cc 62cc head+9.15 cc gasket - 2.40cc dome = 68.75cc All this comes to 9.83 to 1 not the 10.8 that i get from the calcs For real specs, it's a .020 327, with a 0 deck height, 2.40cc dome(i cc'd it out in the block), 62cc camel humps and .041 head gasket at 4.166 diameter
And according to the guy at uem pistons this setup should get me to 10.5ish region.... According to some places like cnc motorsports these should put me at 10.8 to 1..... Looks like i might have to do some chopping on my heads or get new slugs, kinda really pissed!!
I'd written a response and then reviewed your original post-- I then deleted my response because I didn't fully understand your question first time around. Can't figure what's accounting for the discrepancy, but regarding your own math remember the loss for the distance from the rings to the piston crown and the spark plug hole. Take a look here: http://www.lunatipower.com/Tech/Pistons/HelpfulCalculations.aspx
Mike, the piston has a .150 dome above, but the flat of the piston is exactly flat to the deck, i already measured that all out. I had the block 0 decked when i had it machined BTW incase anyone is wondering, when i measured the cc's of the piston i dropped it .200, cc'd it and figured out the math to get the dome, which is less than 1 cc less than called out but i guess that factors for rings and such I should add, the more i think about it, the more i think, yea 10 to 1 is a reasonable compression for a daily, instead of the 10.5 i was wanting, mabey i should run with it....
Huh...... so the L79 vette motor which is rated at 11 to 1 had a 5.3 cc dome according to stuff i'm reading online.... So with stock 64 cc camel humps it's pretty much the same cc as i have minis the 0 deck??? Meaning they are less than 10 to 1 also?? This blows my mind if it is true
Static compression ratio is sort a meaningless (although it is a good reference point) since nothing happens till the intake valve closes a the beginning of the compression stroke so the effective stroke (used for compression) is not really what the physical stroke is. I use cranking compression divided by 14.7 X 1.31 to get the effective ratio. 1.31 being an accepted factor of expansion for air fuel under compression.
Dolmetsch is right, plus you can get a lot of performance from head flow and cam choice while still running pump gas. And remember advertised compression ratios from the factory often differed from reality.
...cranking compression is great...after the engine is assembled. Then what do you do with the engine when the pistons were 'wrong' and you're off by a full point or more? .
Shaggy, you said 62cc chambers, as in 461 "X" castings, but then 64cc as in the more commonly seen 461 castings, after a few responses were posted. The factory referred to these .125 dome pistons as 10.35:1, but the same pistons are used when referring to 11:1 in various books (????). Are you wanting/expecting lower than roughly 10.5:1, or higher? If this is a daily driver, what gas do you plan on. The L-79 I was "forced" to run in my truck for a while eventually burned an exhaust valve; even with "high test" gas and gas additive. I have some good 70cc heads you could HAVE to drop the CR if that's your intent, as opposed to 76cc heads. I am Butch/56sedandelivery.
And for my own math 41.25013ci displacement = 675.96852cc 62cc head+9.15 cc gasket - 2.40cc dome = 68.75cc All this comes to 9.83 to 1 not the 10.8 that i get from the calcs For real specs, it's a .020 327, with a 0 deck height, 2.40cc dome(i cc'd it out in the block), 62cc camel humps and .041 head gasket at 4.166 diameter[/QUOTE] I got 10.83: 1 with these figures. You add chamber volume to the swept volume [this is the total volume at BDC] then divide by the chamber volume [this is the total volume at TDC]
I take it the 2.4 cc is the valve reliefs. Given this, Dyno2000 shows: head chamber volume: 62cc head gasket bore: 4.166 head gasket thickness: .041 head gasket volume: 9.16 cc deck height: 0.000 volume above piston: 2.40 swept cylinder volume: 675.97 cc total combustion volume: 73.56 cc compression ratio: 10.19 : 1 I think you need to look at your math again.... 62+9+2=73, not 68 like you have added up. (using rounded numbers)
I understand the definition of static compression ratio to be the total volume of the cylinder, including the chamber/gasket volume, and cyl swept volume, at BDC, divided by the chamber volume at TDC The chamber volume is REDUCED by 2.40 due to the piston dome.....not added. so...62.00 + 9.16 gasket volume = 71.16 - 2.4 dome = 68.76 Chamber volume 68.76 + swept cyl vol. 675.97 = 744.73 /68.76 = 10.84 So, I still agree with Kerrynzl's method/math. If wrong....where and why? Ray
Okay, thanks that's where I screwed up, thanks guys!!!! So this makes me feel better, I'm trying to get it into the 10.3 or 10.4 region and it has a solid dome, so the higher compression right now the better!!! Now I'll make some jaws up for my mill and shave a bit off each piston, which should make them lighter too!
The most accurate formula's I've seen are at the link below. It's not simple at first but very simple in reality. I turned this page into an Excel spreadsheet once but have lost track of it. http://fordracingparts.com/download/catalogs/virtual/index.html#78-79/z
The following is going to be kind of esoteric.....but it has been bothering me about how the piston dome affects the cylinder volume. Before I go into that, I ran the figures with some variables, based on that notion, and the lowest CR I came up with was 10.49.........so it's kind of moot, except as an mental exercise. I feel certain the dome volume has to be deducted from the chamber volume when the piston is at TDC. What I am struggling a bit with, is using the 'swept volume' of the cylinder...which I think presumes a flat top piston, when in fact the piston is domed, and that reduces the actual volume of the cylinder accordingly. So, regardless of the bore, and stroke length, the cylinder volume is reduced by the dome. So, I deducted the dome volume from the swept area, as well as from the chamber volume at TDC, and ran the numbers again.........result was 10.49. Close enough to be negligible, in my opinion. However, I still am not sure of my reasoning about the effect on "swept volume"......any higher math whizzes out there that care to comment? Ray edit: I was composing this while the above post (#16) was being made. I did look at the linked article after seeing the post......and it is sooooo detailed it made my head hurt )
Dome volume does not alter swept volume. With a pop-up piston the dome removes cylinder volume at BDC but adds cylinder volume at TDC [ so the shape cancels itself out ] Dome shape only alters chamber volume. Here is quite a comprehensive compression ratio calculator http://www.crt-performance.com/compression.htm It helps if you know the maths , if you're trying to "reverse engineer" everything for a targeted compression ratio. Hey shaggy , before you chop into those slugs [which will require rebalancing] have a look at "Cometic" head gaskets. I know that decking a block gives a bigger compression bump than milling the head the same amount [because the area of the bore is greater than the area of the combustion chamber. The reverse applies also when dropping the compression ratio. I reduced the compression of my 331 SBF from 11:1 down to 10.17:1 by bolting on a set of .066" Cometic head gaskets [ .025 thicker ] Cheers Kerry
I was already planning on balancing everything anyway, everything is ready to go, and I bought these pistons figuring I was going to need to trim a little off of them anyway since nobody makes quite what I want. I think it'll be around .06, but i contacted uem pistons for some conformation
With zero deck clearance you will need to cut the pistons to flat top with 2.30 cc valve reliefs [2 valve reliefs] [instead of 2.40 cc dome intrusion] This will net you a 10.14 compression ratio with 62 cc heads. If the heads measure at 63 cc you will get 10.01:1 compression
I spent 7 years as an instructor in the provincial institute of trades program . My subjects I was asked to teach most were engines electrical and fuels because of my work background so I have taught compression ratio calcs many many times. The dome thing is measured a little different then described in practice but the results you are getting are so close it is probably of no value to go there. And what actual figure do you want and why? I run an 11to1 426 MW on the street only. (Momma don't allow me to race our cruise nite car) I use Sunoco 94 octane which is available here if you know where to look and it is fine with that. I run a big cam (310 adv duration and 525 lift)because that is the reason , the only valid reason for extra compression but that is a long subject and probably one of the most significant speed secrets there is. (relationship of cam duration or intake closing to engine compression ratio.)If the 94 wasn.t available I could run 92 octane which is available here everywhere but would need to drop it to around 10.to 1 and would have to step down one step in cam as well. It is a complicated subject but practical solutions are around. For instance if you ran on the street use flattops and a moderate cam. If you are racing put in a huge cam to suit max RPM you intend to see and use a domed piston to restore the compression (actual cyl pressure to a level similar (a bit higher is fine) to what it would have been otherwise. A long time ago I spent many hours on this subject here and it used to be archived at one time. don
Dolmetsh, oh, i know, Ive been working on this for months. Mostly i wanted a full nut and bolt traditional late 60's hot setup, but it was impossable to pass up switching to a 6" rod to take advantage of 80 gram lighter pistons(then again 6" rods were availible then). According to what i can figure i should try to keep the l79 under 10.5 for what i'm doing, but i will be relying on the slow open and close of that cam a bit. Worst case i can recurve the distributer(i have a machine), and run a little rich, as often as i wind stuff out i should be able to pretty well avoid a little loading up. I'm not figuring this on being a drop in crate motor, and that's what i love about it. I'm not ashamed to pull over, pull out my tuning kit and dial stuff in on a daily basis until it is right, ive done it on plenty of motors before Anyway i dont trust anything i cant measure (i'm a machine shop lead), so that's why i started rechecking everything by hand and apparently botched my math, and that is why i started this
Ah you'll be fine. I am waiting for the performance report down the road. I don't know about you but I am always nervous stepping out on a new combo even if I am sure I have all my ducks in a row. Usually things wind up better then I thought. I think you'll be fine. Now git to it. :>) don
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