Hello all, I have a 1965 YF block Pontiac 389 thats been bored out by .060 and I'm looking for a set of pistons to install in it. I seem to have two stock style options, the standard "flat top" ones that 389s use that have 4 valve reliefs (Silvolite 1516.060), and then there are later 400 style pistons with 8 valve reliefs and have a 10cc dish (Silvolite 1526.std). What I am curious about is that when I look at UEM Pistons' catalog, they list the 1526 as having 8.2:1 compression, which I am having a difficult time reasoning to why it lists that. Both pistons list a 1.7 inch compression height and the only substantial difference is that the 400 pistons have that 10cc dish. Is there something special about the 400 style piston that results in it having substantially lower compression. I would benefit from having a dropped compression ratio, as I have #76 heads that are either 69cc or 72cc chamber size, which with a 400ci engine results in nearly 11:1 compression, so I would like to get that into the high 9s or very low 10s. If anyone has any good ideas to achieve this other than these pistons, I'm also open to hear it.
I’d imagine one reason it’s lower, is the , stroke comes into play also. I was fiddling around with an online calculator recently, same bore at 4.030 but went from 3.48 to 3.75 stroke, both zero decked blocks. The 355 was 9.5 : 1 and the 383 was 10.7? Maybe. Don’t recall.
Pontiac compressions were over rated from the factory so don't go by what you read. Wallace Racing has really good, accurate information. Your #76 1965 heads should have 70 to 72 ccs, to be sure, they need to be checked. Most Pontiac heads were fairly close to the CC's stated since they were all machined. 71cc would be close enough to buy piston with. Use this chart, http://www.wallaceracing.com/cratio0001.htm your .060" over 389 will be 400 cubic inches. The 400 dish piston is not a standard piston, Pontiac used flat tops with valve reliefs. Some had 8 some had 4 relief, but never a dish. The 400 pistons with 10cc dish will put you right around 9.0:1 compression.
Call Butler Performance, they will know what piston you need. https://butlerperformance.com/c-1234874-engine-components-internal-pistons-389-pistons.html Piston choice will be determined by what heads you use.
Thanks for your wealth of knowledge. With that I think what Silvolite is listing as a dish is simply compensating for the 8 valve reliefs since they seem like they'd roughly make up a 10cc dish to the piston. I just had dropped my cylinder heads off at the machine shop and the machinist said that he would take the volume on the heads for me, so that'll be good to know what its sitting at, and I'm waiting for the final report on what the cylinder bores will be at to choose a piston.
I believe the stock compression height for the 389 is 1.720, so they probably sit 0.020 down in the hole, which is going to reduce compression ratio (maybe by 0.3), as well as screwing with the quench area. Adding a 10cc dish is going to subtract about another 1.0 from the CR. I am in process of rebuilding a 1960 389, will be using 0.030 Sealed Power 288P flat top 389 pistons (1.700 CH, 4 valve cuts), likely with some home machined D-dish recess work and block decking to help out the quench area.
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