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Help figuring my CR
I need a little help figuring out my compression ratio. I know most of the requirements for the calculator, but I'm missing a few still. I don't know piston stroke, or the number for factoring in piston valve relief CC into the equation. I have a stock 350 short block, 4 valve relief stock pistons .025 in the hole. It's a .040 over bore, and 67cc combustion chambers on the heads. And the head gasket thickness is .039 I think that's everything but let me know if I need more info.
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Re: Help figuring my CR
Since I don't know, I was going to jokingly say, search for an old fashion CR calculator on google...
then I did! This may help. |
Re: Help figuring my CR
I guess Summit has one as well.
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Re: Help figuring my CR
Stroke on a stock 350/355 is 3.48 the dome volume could be anywhere from 5 to 7 cc depending on who made the piston. You need to either deck the block or find a thinner gasket to get the quench closer to .050
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This says my CR would be 10.9, is that even possible?
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Re: Help figuring my CR
If you can find a metal 0.035 gasket you'd wind up with perfect 0.060 quench, I think. That's likely more important than the compression ratio difference the thinner gasket would make. But really now that I look at your gasket, at 39 + 25 in the hole you're at 64, so I wouldn't mess with it. I've heard 0.060 is the target, though the fellow above is looking for 0.050. It'll still be plenty turbulent and great for your compression ratio.
I think you want to enter your Piston Dish as a negative number. If you use 7, you get 10.9:1, but if you use -7, you get 9.33:1 which is pretty decent. Changing it decreases the result, so it must be negative for a dish (otherwise the number would have gone up when put in backwards). Get a second opinion, but if your numbers are right it sounds like you're golden to me. For iron heads that's about the limit, 9.5:1 I think. |
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Well after all the reading I've done, it seems as though my quench is much too big, and I can see all the symptoms that are in the articles I'm reading. I'm getting half the MPG I used to, and the parts I rebuilt the top end with are only marginally bigger than what I had. I also noticed that it's rattling at 34* on the timing when running 93 octane. FelPro makes a 1094 head gasket that is .015 compressed thickness, I think that's the missing puzzle peice for my build. From what I understand, the OPTIMAL quench is between .038 and .044 to get the most efficiency from each combustion cycle (I sound like I know what I'm talking about now lol thanks for the reading material haha) then you have less pinging and less of a chance of detonation, and more power and better MPG's. But I'm still learning and that was only one article I read, so I'm not 100% positive. I'm using a Comp 280H cam which has a max lift of .480 intake and exhaust, so for clearance I wouldn't have any issues either. Which brings me to a whole different issue, after reading all those articles and some of the books you recommended, the Comp 280H isn't what I'd go with if I could do it again, but I'll save a cam swap for a different build as this one makes good power :) |
Re: Help figuring my CR
I think that'll be the next thing I really dive into is cams and how they work, and what all the numbers mean besides just the lift and duration.
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Tightening up the quench can't hurt, but I wouldn't pull my heads for it, unless you have some other reason to do it. |
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Which part of the cam grind or characteristics affect compression btw? Which numbers should I be paying attention to there and how do they change CR (probably not as simple a question as I make it sound I'm sure) |
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I also found out yesterday after re-checking all the numbers on the heads and calling Comp, that my heads are do not have 180cc intake runners as they were advertised. I thought for sure I had checked the numbers and found that the guy selling them was right and they were 180cc, but this time I called Comp to be certain and the tech said he knew for sure they are 167cc heads...so that was a bummer lol that makes two fairly expensive parts I've bought used on this build and got a little less than was advertised both times. Guess you gotta learn somehow :)
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.050 really? I guess there are a lot of different opinions as with anything else. I probably should read more than one engine builders tech articles, that way I get a nice broad span of expertise on engine building. From my reading, .040 is the goal for quench, but that's just one guys opinion. If my compression is around 10-1, closing in on that .040 quench with a .015 shim gasket would likely put me at much too high a CR. Although I am not sure on how much compression my cam will eat up as of yet. |
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This looks a lot better than my current 280H. Which numbers on this graph affect CR?
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Re: Help figuring my CR
https://www.uempistons.com Go to this website and they have a calculator to figure your dynamic and static compression. It is the KB pistons website
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