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455 Compression Ratio Changes


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I'm also interested in this topic, as in where did all the power go from 70 to 76. Was it just decreased C/R or was it that and cam, intake, carb, etc. changes? It would be nice if I could make 70 power in my 76 with just a head change (if it is bigger chambers vs. dished pistons that dropped the C/R) but I am probibly dreaming right?

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The difference is in the piston dish.The combustion chambers are essentially the same. The difference in power was partially because they were rated differently. The 70 was rated bare and they started rating the later engines with all accessories and exhaust manifolds . they also retarded the timing

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Hey Old Guy,<P>You sure it's all in the pistons? The NHRA General Specifications give the minimum head cc as 55.5 cc for the '70 Stage I and 67 cc for the '71 Stage I. There's also a part number change for the heads between the years: '70 head part number is1234602, '71 head is 1237660. This info is from Dove's Guide to Buick Performance Engines.<P>The difference of 11.5 cc between '70 and '71 combustion chambers takes the Stage I compression ratio from 10.5 down to 9.5.<P>If this same increase in combustion chamber size were applied to the non-stage 10:1 compression '70 455, the compression would drop to 9.1:1. The '71 non-Stage however, was 8.5:1. I just noticed in Dove's book that head gasket thickness also seems to have changed from '70 to '71 from .020 to .028 accounting for another 2.7 cc increase. This would drop the compression further to 8.9:1, still not down to 8.5:1. <P>So, this doesn't leave much for a big difference in the dish of the piston. The question remains - is there a difference in the amount of dish in the pistons between '70 and later engines?<BR>Can anybody verify that the above is correct or just a bunch of crap?

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Please, someone correct me if I am mistaken, but I agree with 'The Old Guy' - compression was changed on 1971 and later engines with a piston change only. Heads from 1970 are no different than heads from later engines. As far as rated horsepower and torque are concerned, 1971 ratings are 'net' ratings as opposed to 'SAE Gross' ratings used in 1970 and back.<P>Further, I have read that a 455 block from 1973 and up is a better block block to hot rod, due to extra material in the main bearing webs. That info was posted here on this board in another thread. Also, rods for the 350 engine were improved for the 1973 model year. I don't know if this holds true as well for the 455 engines. Anybody know about that?<P>Matthew<P>p.s. - to '70GS: I would check TA Performance or Poston Enterprises for replacement pistons. My catalog for TA shows 8.5:1 and 10.1:1 pistons available as either cast or forged. If you opt for the 10:1:1 pistons, using a Fel-Pro (or similar) head gasket supplied in a rebuild kit will probably take you down to about 9.6:1 - 9.8:1 compression which should be okay running with premium pump gas.<p>[This message has been edited by 67GSCalif (edited 11-21-2000).]

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The "dimple" in the '70 pistons may account for some of the CC differential. I just went to the basement to look at my '70 pistons and they are "dimpled" in the center. I don't believe they have ever been changed. The dimple rises up from the center of the piston and does consume some of the CC's.<P>------------------<BR>George C. Thomas<BR>70 GS 455 12.teens<BR>86 GN Low 13's/ high 12's

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I dont know for sure either but I think it was mostly in the pistons and possibly there were some different heads. Maybe the difference in the heads depended if it was stage 1 or not that is a definite possibility NO? I also read ALL big block Buicks used Forged Steel rods and All 350's used cast iron. This all comin from all that book lernin and no real sperience an all!<P>------------------<BR>Lee C.<BR>81 Electra Park Ave (350 original)<BR>83 Electra Park Ave (307 that wont die)

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While not specific to the V8 rebuild, I want to relate my experience rebuilding a 3800, with the express purpose of increasing the compression ratio. I found that most of the aftermarket piston manufacturers destroke the piston .010-.020", assuming the block will be decked to straighten it, plus, the book values listed in the catalogs, relating to piston dish volume, are generally inaccurate. Silv-o-lite actually sent me a free piston after talking to them on the phone. I was told the cup volume was 26 cc and they sent me one to prove it. I cc'd it myself and it was just under 31 cc's. Federal Mogul was even worse; it was 36 cc's. There is also a variation in the cup volume between pistons. My set of six varied by 1.4 cc's. By selecting a small cup and mating it to a cylinder with a larger chamber, I was able to get the total volume within .8 cc's across all cylinders. If I hadn't done this, the total varience could have been two cc's. One other item is the head gasket. I purchased a Fel-Pro set and the cylinder opening was 4.2". Since the maximum overbore on this engine is 3.84"(similar to the 350 Buick), the extra area between the head and the block seems like sloppy tolerances, and it increases the area for unburned fuel/air mixture (plus it drops the compression ratio). I bought a set of stock head gaskets and the cylinder opening is 3.85". The point of this long post is this: don't believe what a vendor tells you, or the catalogs say, measure it for yourself. <P>------------------<BR>Hal, btk@vbe.com

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67GSCalif,<P>I have the TA catalog. It appears to be the same piston for 10.0 and 8.5 C.R. since it's the same part number. Maybe the easiest way to be sure is to call TA.

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Its in your heads, guys. TA was very adamant about changing the HEADS on my 76 block. The (67-69) 430 heads and 455 heads from 1970 were the "high compression" heads, later years increased combustion chamber size for deacreased compression. I have the old ones from the 76 block free if you want to pay shipping! They would make good boat anchors!<P>------------------<BR>Ted Nagel<BR>65 WildcatConvSuperWcat<BR>65 WildcatAutoSuperWcat<BR>65 Wildcat4spdSuperWcat<BR>65 Riviera Wildcat 445<BR>65 Special Wagon 455<BR>70 GS Stage1 4speed

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Wildcat65:<BR><B>Its in your heads, guys...</B><HR></BLOCKQUOTE><P>The head chamber sizes did increase as the years went on. But, I think there was only about a 12 cc difference between the 1970 and 1975/76 heads. I don't know if this alone would account for a drop in compression ratio from 10.5:1 to a 7.9:1 compression ratio. It may be.<P>I really don't know if the 10.5:1 compression ratio is really correct for the 1970 455 Stage-1 either. More than one person has told me that it should be 10.25:1<P>Sorry if I only added confusion to the subject.<P><p>[This message has been edited by Straycat (edited 11-22-2000).]

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by 67GSCalif:<BR><B>...Also, rods for the 350 engine were improved for the 1973 model year. I don't know if this holds true as well for the 455 engines. Anybody know about that?...</B><HR></BLOCKQUOTE><P>1973 may be the year the 350 rod switched to capscrews. Not sure though. As far as I know, the 455 rod never changed in design.<P>

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How did Buick drop the compression on the 455 after 1970, deeper dish on pistons, increased cc combustion chamber, or a combination of both? The reason I'm asking is I'm rebuilding a '70 455 and the replacement pistons available say they're for'70-'76, 8.5 compression ratio. Anybody know how many cc in the dish of Federal Mogul replacement pistons?<BR>Thanks for your help.

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Ahh. Trying to be funny at the expense of truth. I knew I'd get caught...<BR>Call TA for the best info. They have a list of casting numbers for higher comp heads. I am not sure of all the other reasons for power decreases as the years went by, but the ratings ("brake" vs "SAE net) changed, the insurance companies went crazy and high HP went out of style. So the ratings went from optimistic in 1970 to realistic and reported differently in 1976. A lot more was going on than just changes to the engine.

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Justin,<P>Increasing the dish in the piston would only increase the combustion chamber volume and thus decrease compression ratio. The displacement (cubes) is still the area of the bore times the stroke.

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yes there was different cc heads i know that the stage 1 heads have a higher compression because they have a larger valve and in 1970 at least used a steel shim head gasket. there are 3 head cc that i know of smallest being 66 then 71 and i think a 76 cc not sure on the last on i also think there was 3 different pistons at least that what the parts guy said. IN THE BUICK WEBRING IS A SITE BIG BLOCK BUICK. great site dont know the web address but worth the search lots of info mike

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Let me try to clarify a little. <BR>1. The horsepower ratings changed in '72 not '71.<BR>2. As far as I know the only GM cars with Air injection from 68-72 were manual transmission cars. <BR>3. Starting in '72 EGR was installed and as far as I know the '73 engine is rated the same as the '72.<BR>4. If you want high performance pistions you can bore your block to .038 and use Mopar pistions.<P>Tomsriv<BR>'71 Riv

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