Pontiac V8 engine

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m (→Pistons)
 
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In 1967 the 400 4bbl engines went to new valve angle and placement and all Pontiac V-8's 1968 and newer. Newer valves don't line up correctly with the 67 2bbl, any 326 or all 1966-back piston valve reliefs (and obviously vice-versa), the correct piston has to be used if any sort of aftermarket high lift cam is used, or else there may be destructive valve to piston interference. There are 8-valve relief pistons available to fit all the valve angle differences used through the years on Pontiac 400 engines. While these pistons will get the job done, from a high performance standpoint they are not desirable partially due to the lower compression.
 
In 1967 the 400 4bbl engines went to new valve angle and placement and all Pontiac V-8's 1968 and newer. Newer valves don't line up correctly with the 67 2bbl, any 326 or all 1966-back piston valve reliefs (and obviously vice-versa), the correct piston has to be used if any sort of aftermarket high lift cam is used, or else there may be destructive valve to piston interference. There are 8-valve relief pistons available to fit all the valve angle differences used through the years on Pontiac 400 engines. While these pistons will get the job done, from a high performance standpoint they are not desirable partially due to the lower compression.
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You must use the 8 valve relief pistons if you bore out your 67 400 2bbl engine if you use the 67 400 2bbl heads, but you will gain around around 15 cc of piston dish displacement due to the extra valve reliefs, lower compression height and the crown cut on the edges of the piston.  However, if the standard 400 bore is not more than .003" worn, you can hone them and install .060" over-sized 65-66 389 pistons and .060" 389 rings.  They will have the correct valve reliefs for the 2bbl engine, a true flat-top and you will retain close to stock compression.  67 400 2bbl engines were offered in 8.6 or 10.5:1 compression ratios (different cylinder heads) so the 8 valve relief pistons may work better if need to lower compression on a 10.5:1 engine down to around 8.5:1 give or take depending on deck, head gasket, etc.
 
  
 
Pontiac went to an advertised 7.6:1 compression in about 1975, due in part to the design of the top of the pistons. Pontiac pistons from the low compression years have a 45 degree machined outer edge of the piston top.  
 
Pontiac went to an advertised 7.6:1 compression in about 1975, due in part to the design of the top of the pistons. Pontiac pistons from the low compression years have a 45 degree machined outer edge of the piston top.  

Latest revision as of 17:30, 27 September 2022

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