Figuring out my Static Compression
There are quite a few good references to get formulas for calculating compression ratios, so I will not discuss the formulas for that here. If you are interested in reading about that, I liked this site:
https://www.omnicalculator.com/everyday-life/compression-ratio
To get my compression ratio for my engine, I needed the following information. Listed next to that are my values, with actual values in
green and advertised values in
red:
- Bore: 4.0702"
- Stroke: 4.00"
- Volume of the combustion chamber (head): 65 cc
- Volume of the piston dome (- value) or dish (+ value): 25 cc
- Compressed head gasket thickness: .035"
- Clearance at TDC (how far "in the hole" is the top of the piston? .0015"
By entering this into a calculator, my static compression ratio ends up being
9.7:1
I was gunning for upper 9's for compression ratio, ideal probably would have been 10.0:1, but I was happy with that.
But is that accurate? So I verified.
First I measured the cc's in the heads. Here is how I did it.
You want one side of the head to be higher than the other. This allows the placement of a plate (see other pictures below) so the hole where fluid enters is higher than the low side. This allows the full cavity to be filled.
The you need to be sure the fluid does not leak out. Putting some silicone grease around the valves helps keep the fluid in (and insert a spark plug too!). Wipe away excess grease.
Then you put the grease around the combustion chamber - a THIN layer!!!. This allows the plexiglass plate to seal to it without adding any appreciable volume.
Here is a picture with the plate stuck on:
Then you need a graduated cylinder. You drip fluid into the hole until the cavity is completely filled and compare the starting level in the graduated cylinder with the final level. I used isopropyl alcohol with some red dye to make it easier to see. Here is the rig:
Here is a picture of the filled combustion chamber. Try to get all air bubbles out by tapping on the Plexiglas, but a couple of tiny bubbles won't make a difference for a street engine. Note I have one bubble shown here, but its volume is likely less than .1 cc.
So the result for the volume of the combustion chamber???
My actual volume recorded was 66 cc (vs. 65 cc advertised).
Then it was time to do the volume of the piston dish. Same process as above, except one addition. The piston is .0015" in the hole at TDC. So the final measured volume needed to subtract that volume of fluid to get the actual dish volume.
Here is my rig set up for that:
So the result for the volume of the piston dish???
My actual volume recorded was 27.5 cc (vs. 25 cc advertised).
OK, so who gives a crap? What did I do here? Well, here are my new values:
- Bore: 4.0702"
- Stroke: 4.00"
- Volume of the combustion chamber (head): 66 cc
- Volume of the piston dome (- value) or dish (+ value): 27.5 cc
- Compressed head gasket thickness: .035"
- Clearance at TDC (how far "in the hole" is the top of the piston? .0015"
This nets me a final compression ratio of
9.4:1
While this is not all that far from 9.7:1, it doesn't make me happy. I wanted compression to be in the upper 9's. At least I now know. Instead of
thinking I had 9.7:1 compression, I now
know that I only have 9.4:1.
Can I increase the compression? Well, my options are limited. I can:
- Buy new pistons (not going to happen. Remember, this is a budget build!)
- Reduce the head gasket thickness. If I use a .020" head gasket, then I will need to mill .020" off the pistons for clearance. My estimated new compression would then be about 9.6:1. This would require me to do the machine work on the pistons. That can be done, but a thin head gasket is also prone to failure. If I want reliability (and I do), then this has some risks.
- Cut the deck of the block AND mill my pistons too. Lots of work. Likely not going to happen.
- Live with it. This is likely what I will do, and adjust the camshaft profile to make the most out of my more limited compression.
I'll discuss with my father-in-law and see where we end up.
Sorry for the long post, but hopefully folks doing a detailed build can get some tips from this.
Hawk