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Thread: 455 Pistons?

  1. #1

    455 Pistons?

    Ok, heres the deal. I have a 1970 455 with the 25cc dish pistons and a set of 68cc heads with the standard valves. I was looking into the Speed Pro pistons to increase the compression. After a little research I notice that Speed Pro pistons are 23cc dish. Not much of an improvement for 250 bucks. Also Ive heard that the Speed Pro pistons are sensetive to detonation. I was wondering if keeping my stock pistons and milling the heads down to 66cc is an option. I was wanting to achive about 11-1 comp ratio. Stock its suppose to be about 10.25-1. Does anyone have any tricks to achive this keeping the stock pistons? Ive heard that the stage one valves help, but is it worth the cash to put the stage one valves in? I want to put my money where I will see the most improvement. Do you have to change out all exhaust and intake valves for the Stage one upgrade? Can I just enlarge the intake valves? Any opinoins would be useful...

  2. #2
    If Ive calulated it correctly milling the heads down to 66cc with the stock pistons would give me a CR of 10.868856643235844. Going with the Speed Pro 23cc dish pistons would put me at 11.082029039776451. Thats right where I want to be. How much volume do the stage 1 valves fill in the cylinder heads?

  3. #3
    Stage I valves are both intake and exhaust, although there is no rule that you have to change them both.


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  4. #4
    Thanks for the reply. I was thinking about just doing the intake valves. Im still curious about how much volume they fill in the cylinder head though. the stage II heads with the same valves were 58cc but that was a totally different head as far as I know.

  5. #5
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    What is the part number of the Speed Pro piston that you want to use?
    Installing larger valves for that small of a gain in compression is ridiculous. It would be way cheaper and make much more sense to deck the block to gain compression. If you want to improve high RPM flow through the heads then by all means put in big valves, but do both the intake and exhaust. And if you were only going to do one or the other, I'd for sure start with doing the exhaust valves. Larger intake valves will affect your low RPM port velocity in a negative way. Which could make it less responsive at lower speed. When you do get the RPM up enough to make use of the big intake valve you will be trying to push more combustion gases past the stock exhaust valve. Installing just the exhaust valves will give you the potential gains of a more efficient exhaust system without any impact to your low speed driveability.
    All pistons are sensitive to detonation.
    The performance difference between 10.86 to 1 compression and 11 to 1 compression are practically nil. And neither will run on pump gas.

  6. #6
    I agree. Changing the valves for compression reasons alone is not a good idea, but it is important to know so I can calculate compression. If i decrease the volume to much it can cause issues. I dont know the part number of the pistons but its the only one they make for the 455. Do you know if there is any clearance issues if the cylinder heads are milled down?

  7. #7
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    Troy, I realize that this post is a couple of days old but I think you still cruise around on here fairly often so I am gonna put another reply on here.
    I ran across my Federal Mogul (Speed Pro)piston catalog yesterday and looked up the pistons for the 455. They do not and have not made a piston that you can achieve 11 to 1 compression with in the last 20 years or maybe ever. The catalog lists a cast stock replacement, a forged stock replacement, both low compression, and a hypereutectic H392NP. I've used the hypereutectics and they work good for a street/mild performance engine. The Speed Pro performance catalog lists some fairly accurate compression figures for these pistons with the different head CC's. They take into account the deck clearance, bore oversize etc. And none of the pistons will make 11 to 1, even if you zero deck the block and mill the heads to 66 cc's. Where are you getting the dish CC figures for the pistons in question? The catalog lists the dimensions of the dish but not the actual CC's. Are you measuring them or are you getting them from a published source? Are you taking into consideration the CC's of the head gasket thickness? I'm just curious as to how you got the 11 to 1 figure.
    Your original post was about the installation of bigger valves and their effect on head CC's. I've put bigger valves in tons of small block Chevy heads and for the most part the effect on the CC's has been negligible. On stock factory castings with years of use and having a non machined combustion chamber to begin with leaves too many variables to control. There can be variations in size from chamber to chamber in the factory castings that can be larger than the change from installing larger valves. You are on the right track though. I give you kudos for trying to figure this out on your own. Most people never have a clue or take the time to educate themselves in this area. You can't trust the advertised compression ratio's in most of the catalogs. These people want to sell parts and don't like things like the truth getting in the way. The piston manufacturers themselves will almost always have the correct data you need as it is in their best interest to give accurate information. The specialty manufacturers are the best at this as they deal a lot with professional engine builders who need accuracy when planning a build. The big guys who sell at the general parts stores can be kinda vague. They mostly sell the stock replacements and have a "Trust us, we know what you need more than you do" kind of attitude. Which in a lot of cases may be true. The exaggeration of compression ratios probably keeps a lot of shade tree experts from getting themselves into trouble while allowing them to brag to their buddies about how much power they must be making with those "higher compression" pistons they bought.

  8. #8
    Sorry it took so long for me to get back. To continune on the subject, I first started looking into this issue when I noticed that Speed Pro advertizes a 9.5-1 "high compression" piston H392NCP for the 455. I was curious as to why the specifications for the 1970 SF 455 Buick block stated that it had a 10-1 CR, but yet the Performance pistons were only 9.5-1. I figured it may have been based off the later 72-76 lower compression engines.

    I decided I was going calculate the based on the actual build specification to get a more acurate idea of what was going on. Understaning that this is mearly a calculation that be affected my multipule variables, but the basic calulation goes like this.... well lets skip the mathmatics and take the easy road... you can use this link to calculate it, there are many out there:

    http://www.race-cars.net/calculators...alculator.html

    Cylinder Diameter 4.3125
    Stroke Length 3.90
    Number of cylinders 8
    Chamber in cc's 68 (stock)
    Piston dome in cc's -25 (stock 1970)
    Gasket Thickness .020

    You can fiddle with you measurments a little to get what you want.

    The displacement is cubic inches 456 or 7.47 liters.You also have a Compression ratio of 10.5:1
    in
    Buickperformance.com has a good article on the measurments of the different stock pistons.
    http://www.buickperformance.com/pistons.htm

    Thanks.

  9. #9
    Troy,

    Your calculator is missing the piston to deck clearence. The pistons will generally be .050" down the hole.

    Here is a better one that includes deck clearence: http://cochise.uia.net/pkelley2/crc.htm. Using that one, it comes outt to 9.5:1 at .050" down the hole with your other specifications. You do not want to have the pistons closer then .035" from the head including the gasket, but you do not want that distance to be greater then .060" for a good quench area. I think that is part of the reason that earlier posters suggested decking the block instead of milling the heads. That way you can increase compression and have a better quench area.
    Steve B.



    67 GS 525 Buick Stage IV
    66 GS Convertible
    65 GS HT
    63 Riv
    02 Subaru WRX Turbo
    03 Ford Cobra Convertible (Factory Supercharged)

  10. #10
    Thanks for the link. How did Buick come up with 10-1 CR?


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