Compression ratio???

[Dodgezilla]

I have a 73 440 engine with 8.5 to 1 pistons. I also have a set of 516 Closed Chamber heads with the larger exhaust valves to put on it. I plan on assembling it with steel shim head gaskets. I was gonna get some 10-1 pistons for it, and may still, but if I assemble it with what I already have can someone tell me the compression ratio I will have? I have had a couple people tell me I will be around 10 to 1 but I want to get more opinions. I have also been told that with the 8.5 to 1 pistons I will be very low on HP. I have a 509 cam for it too. I do not plan on racing the car but may put my foot into it on occasion. Am I gonna be real disappointed if I run the 8.5 to 1s?

Thanks

 

[eric383]

Check out this calculator.....
http://www.summitracing.com/expertadviceandnews/calcsandtools/compression-calculator

 

[Cranky]

First off you need to know the compression height of a piston before buying it. Pistons that are advertised at say 10-1 will only give that ratio if EVERYTHING possible is done to achieve it and sometimes even then you won't achieve the advertised ratio. Also, with a low compression engine, you can use a cam that will build cylinder pressure in the lower rpm ranges instead of using a cam that reduces it. The 509 is designed to build pressure in the upper rpm ranges and thus operate better in the mid to high rpm range and does not do as well as some others would in the lower ranges. IE, you can make a low compression engine think it has more just by using the right cam. Cornfused yet? There is also some calculators on the net that will tell you what your compression will be by filling in all the blanks but you need to know how far in the hole your pistons are, CC's of the heads, bore diameter and stroke etc....

 

[mentor70]

ive run the .509 in a low compression 440 and it sucks on low speed, and esp. idling....it needs 10 to 1 at least...maybe a little more.

personally, ive never liked the 516 heads, but you could make them work.

BUT if you gonna buy new pistons anyway, you could get the Keith Black 235 i believe, and re use the heads you already have, but you will need to have them CCed by a machine shop, and milled down to the the proper compression, which you would have to do with the 516s anyway to find out exactly where your at.

 

[Matrix2]

You need to consider the entire build. Everything needs to match to get the best results. Exactly what do you want at the end? A mild street build that idles well, provides vacuum for power brakes, or something more radical? What carb, what intake, what exhaust, what budget? That will help determine which cam. There are calculators on the web that can help with all this as Eric says. Pump gas?

 

[Dodgezilla]

Thanks for the replies. Either way I am using stock factory pistons. Also have a smaller Crane cam similat to the Hemi grind that I can use. Have the stock 346 heads I could use too. Not really looking for engine building advise though. Just wondering if, off the top of their head, someone could tell me the compression ratio.

 

[Cranky]

Well, the most accurate way to find your exact compression ratio is to pull a head and measure your piston height at TDC then measure the gasket and head volumes and plug in the numbers into a calculator but from charts I'm finding on the net, CR for a 73 440 is 8.2-1

 

[lewtot184]

if it's factory stock you may not have 8.5:1, maybe closer to 8:1. closed chamber heads may raise the compression a 1/2 point. i think the .509 cam would be a terrible choice.

 

[747mopar]

Like these guys are saying nobody on here will be able to tell you what the compression will be, you have to have the heads cc'd and know how far the pistons are in the whole. For instance I had to mill my block to zero deck height to get my motor up to 10:1 and that was figuring the bore, stroke, 88cc combustion chamber, cc of the piston valve reliefs etc. You have to do the math and not just guess and without all of those variables we'd all just be guessing.

 

[Dodgezilla]

Thanks for all the replies. Not really looking for a real accurate answer. Just a roundabout guess. I think I got my answer and do not think I will use the 8.5 to 1 pistons. The 509 cam is great for my combo when I am running 10 to 1s as that's what I ran many years ago. I will just make ashtrays out of the 1973 pistons... LOL

Thanks again..

 

[383man]

As everyone has said you need to measure everything to now what the comp really is. Of course if you are at say about 8.2 and you replace the open chamber heads with the closed chamber heads you will have more comp but my bet is would only be around 9.0 or in that area if I had to guess and I would not run the .509 cam with that comp. If you have the money and can afford to do the build the way you really want to then I would replace the pistons with a higher comp set. But make sure you know the comp height on the pistons you buy so you get the ones to give the comp you are looking for. Ron

 

[Meep-Meep]

Closed vs. open chamber is about .4 point of compression. Steel shim vs. composition gasket is also around .4 points of compression difference. The late model 440's can have a very large bore chamfer at the top of the block and that will further reduce the compression. Can't remember how much but it's significant.

Any cam with a late intake closing and high overlap will be a disaster in that engine so choose wisely if you intend to stay with 8-8.5:1 CR.

 

[Dodgezilla]

Definitely going to use the 10-1 pistons now... Thanks again everyone for your input...