13:1 Compression Pistons in 340 Small Block

DodgeMoparV2 said
View attachment 1715380317 View attachment 1715380315 Hello, let me just start off by saying I’m new here and had some questions regarding engines. I know basics and some advanced terms regarding engine building. My questions today regard more advanced knowledge then what I know and that’s why I am here.

Recently I decided to pull out an old 340 block my old man had sitting in the back and throughout the years he’s acquired tons of parts. One of the pieces he had are .030 13:1 compression pistons (that’s what he was told they are) he pulled out of a 340 motor that he swapped all back to original. This other motor I want to rebuild my goal is to have something street but also build a big block buster with (or at least give them a run for their money).

One essential key that I know makes an enormous difference and I’ve been trying to research on is Camshaft specs and what types you run with performance. I know quite little compared to everything there is to know about camshafts and I rebuilt a couple engines with just mild build but I know with getting into these higher performance pistons you need to run special cams.

This is where the big question come into play, what type of camshaft should I run with this build? I’ve seen lots a people saying 10.5:1 pistons with certain cams run the best but I’d really like to use this 13:1 pistons. I’ve also seen people saying to get them machined down to 12:1 or 11.5:1 but I would also like to not go that route.

If it helps I was thinking about running a Mopar Performance Purple Cam with a Mopar Performance intake manifold and I’m running J heads. I’ve yet to get into Rocker Arm ratios but I’m probably just going to run the stock 1.5 ratio with double spring valves. I’ll probably end up getting some special hardens pushrods and whatever else the cam specs require.

Thank you all!

First ---> Welcome to FABO!!!

Just because the manufacturer says that they are 13:1 or whatever compression that they are rated at, doesn't mean that they will be that in your engine... Chrysler runs the head decks on the blocks and heads fat (tall to the high end of the spec), so if they get any scratches or gouges in them during manufacturing, they can mill them and make them good again and they will still be within the recommended factory specifications...

Use this thread as a guide to check the actual compression that they will be in your engine... The domes will make things a little complicated, you will have to find a good reference point for the piston preferably a flat area that you can use to measure the depth in the cylinder at different points... You must measure how much volume the domes are to be able to calculate the cc's of them for the compression calculation... That may take measuring them at two or three different levels... One level with the dome below the head deck of the block, one with the dome bottomed out to the plastic plexi glass cover plate that you make (with the highest point even with the head surface of the block), then measure how high that reference point is when the piston is at TDC... You will have to be able to measure that reference point at all three levels so we can calculate the volume of the dome... The dome will stick above the block head surface when at TDC... Then you/we may be able to calculate the true compression that those pistons will make in YOUR engine...

How to Check Compression

I would install one piston in the block (I would recommend in the #1 cylinder) and measure the volume of the piston with the link that I provided above...