Compression Ratio Measurements

[zgerbic]

Just did a complete set of measurements for a new 390 I am building. I have pistons, Silv-o-lite, that are dished across the center for valve clearance, a freshly rebuilt set of heads with new valves/seats and head machined flat. Bores are 4.090, stroke is 3.781", Rods 6.54" and compression height is 1.76".

It is a bit of work to put one of these engines together so thought I would measure everything while it was apart. I wanted to know what the compression ratio would be based on actual measurements, not just a guess. I measured the original 360 in the truck and it measured 6.45:1. A bit lower than I would have expected but eye-balling the clearance between the piston at TDC and the block looked like nearly 1/4" when I pulled the heads off. Turned out to be .218". Compression height was 1.66" on 6.54" rods.

I measured the new engine. The piston dish for the valves was 7.5CC and the head CC was almost exactly 70CC. I have some chemistry glassware so used a 50cc graduated cylinder and 25cc burette to measure these correctly. The calculations come out to 9.43:1, which is much closer to what I was shooting for. Without taking actual measurements, it originally looked like about 10.15:1.

Has anyone taken the time to accurately measure their 360 or 390 to determine the actual compression ratio? I have heard estimates of head volumes from 65CC to 72CC. I generally don't hear anything about the volume of the valve dishes in pistons which makes a big difference in CR. In my case, a flat top piston would give 10.14:1 compression instead of 9.43:1 with the valve clearance. I think all the pistons I have seen for 360s/390s have had valve dishes machined in them but no measurements taken.

Please let me know what you have measured and what the CR turned out to be. Hopefully the measurements I took will help out someone with a similar 390.

[zgerbic]

It has been a while. Just wondering if anyone has actually measured the CR for their FE engine, either dynamic or static.

At the time I originally wrote this I created a spreadsheet that calculated both static and dynamic measurements. There are probably calculators out on the web somewhere but I thought I would create my own since I work with both imperial and metric engines. The static compression on my engine was 9.43:1 and the dynamic is 7.79:1. Was wondering what other people building FEs, especially 390 to 400 were ending up with.

Regards

[DuckRyder]

I have done a couple (the same engine really, once as a 390 and once as a 445. They both cam out in the 10.6:1 neighborhood static and in the high 7-low 80 dymamic. I played with head gaskets and cam timing a little to get the 445 down a bit.

Pistons measured pretty much spot on the specs for dish or VR. Heads came out between 65 and 66. They have been heavily worked....

P Kelley is one good calculator, though it needs a windows box, silvolite has an online one and there are a few others. They don't always agree...

[zgerbic]

My calculator and the P Kelley calculators are within 0.3% of each other. Our calculators require many more values to make the calculations and probably use the same math. The Silvolite calculator provides a much different set of compressions since it asks for only a small number of values. The Silvolite calculator is probably geared more to someone that has limited info or is comparing published specs. I think it is accurate for what is put in.

I think it is good to know the actual ratios, if you are actually going to tune them to get what you want. Less accurate calculators probably are ok if you are just wondering what you might get with the components you are buying. Different people want different things.

I installed and ran the Pat Kelley calculator on my Fedora Linux PC under the WINE environment, so you don't actually need a windows box to run it. It installs and looks the same as in Windows.

I found a set of rebuilt heads for my 390 and expected to find something like 60cc for head volume, expecting them to be decked a bit since they were used. They were exactly 70ccs which surprised me. Actually I was very surprised I could actually find a near perfect set of rebuilt factory heads at all a couple of years back. I was looking for about 9.5:1 compression and about 7.5:1 dynamic as I was expecting to run regular gas in the engine for some light hauling around town.

[sparky72]

I, too, cobbled together an Excel spreadsheet to calculate SCR and DCR. With the pistons I used (Sealed Power? It's been a while...) and the heads I used (C6AE-R), along with my block (which had been decked at least once before I had it surfaced), I came up with an SCR of 9.77. The Howards hydraulic roller cam I used, my DCR is right on 8.0. I run 91 and have never had any detonation issues (as far I know; knock on wood). I've never tried running regular and probably never will.