318 pistons to bring compression up
Wanted to see if someone can point me to the correct stock type pistons to bring a 318 to the earlier 9.2 compression. I currently have a 73 core with flat tops. I see some for sale that have 4 valve reliefs are those higher compression than flat tops?
Thanks
Well, let me bust one myth right off the bat, and that is, no 318 ever had compression that high from the factory. It didn't happen. Lemmie splain.
Chrysler had terrible machining tolerances. They left deck heights tall, combustion chambers big. That resulted in compression ratios a lot lower than where they were advertised. Some were down in the 7s. Probably not the ones rated at 9.2. They were likely closer to 8.5......or at least 8:1. Sorry to break it to you, but that's just how it was.
Here's what I would do if I was looking to do it on a budget and was building a mild street 318. Go on the Summit site and search 318 pistons. Choose the one with the tallest compression height. There are a few different ones. The stock cast pistons will be fine. BUT, if the pistons you have are already the tallest compression height (and they might be), then no need to buy more.
You need to "blueprint" your compression ratio. In other words, look at all the factory specs for your year engine. Deck height, combustion chamber size everything. Have the machine shop deck the block to the factory spec. Have them CC the chambers to the factory size by milling the heads. You'll also want to use the factory style think steel head gaskets, IF you can find any. They're hard to find now. If you cannot, you can mill an additional .020 off the heads to compensate.
Essentially, that's what you're gonna have to do. Of course, you could get some different pistons like some from KB, but then you still won't be at "whatever" advertised compression ratio they are rated for, without all the blueprinting work.
