Help figuring my CR

[Mrturner1]

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.

[dazza]

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.

[dazza]

I guess Summit has one as well.

[Mrturner1]

Originally Posted by dazza

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.

That's the one I found but I don't know piston stroke on a 355 or what piston CC is on stock 4 VR pistons

[haysonj]

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

[Mrturner1]

This says my CR would be 10.9, is that even possible?

[davepl]

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.

[Mrturner1]

Originally Posted by davepl

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.

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

[Mrturner1]

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.

[davepl]

Originally Posted by Mrturner1

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.

Can your quench be improved? Perhaps. Is it the cause of the problems you noted? I actually doubt it. Rattling on 93 octane would make me suspicious, but that's a fair bit of timing, probably close to the right amount.

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.

[Mrturner1]

Originally Posted by davepl

Can your quench be improved? Perhaps. Is it the cause of the problems you noted? I actually doubt it. Rattling on 93 octane would make me suspicious, but that's a fair bit of timing, probably close to the right amount.

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.

I don't, and what's funny is a few weeks ago I was all worked up about a possible blown head gasket and having to pull the heads off. Now I kinda wish there was a good excuse lol

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)

[haysonj]

Originally Posted by Mrturner1

I don't, and what's funny is a few weeks ago I was all worked up about a possible blown head gasket and having to pull the heads off. Now I kinda wish there was a good excuse lol

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)

Best thing to do is call Comp cams and tell them the spec on the block and heads and the use of the truck they will give you a cam recomendation. Your CR is around 10.08 if you have flat top pistons which would need primium gas. The target for a good quench is .050 Back when your motor was built they ran .015 steel gaskets. More than likely the reason your piston is so far in the hole is it has rebuilder pistons in it. There is a .028 gasket that would get you a lot closer to .050.

[Mrturner1]

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

[Mrturner1]

Originally Posted by haysonj

Best thing to do is call Comp cams and tell them the spec on the block and heads and the use of the truck they will give you a cam recomendation. Your CR is around 10.08 if you have flat top pistons which would need primium gas. The target for a good quench is .050 Back when your motor was built they ran .015 steel gaskets. More than likely the reason your piston is so far in the hole is it has rebuilder pistons in it. There is a .028 gasket that would get you a lot closer to .050.

I actually gave them a call while it was still on the engine stand and the guy recommended a XE268, which was too small for what I wanted.

.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.

[haysonj]

Originally Posted by Mrturner1

I actually gave them a call while it was still on the engine stand and the guy recommended a XE268, which was too small for what I wanted.

.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.

I have seen a few guys on here saying they run a .040 quench but you better check valve clearance if you are running it that tight. I think the factory is around .060 but most of the posts I've seen on here say .050. My motor I ended up at .049 quench and 9.6 CR but I have fuel injection and a roller cam so I can run regular gas

[Mrturner1]

This looks a lot better than my current 280H. Which numbers on this graph affect CR?

[haysonj]

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