How do you calculate Compression ratio?

cbtoyz

cbtoyz

Vaginatarian
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Apr 4, 2008
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I just put some different pistons in my 12 valve and I am curious to see what the compression ratio will be. The pistons I installed are Marine 370, part number 3935591, standard size, and the numbering on the piston says 101.88 SPO.12, 09E131. I measured the piston protrusion at .018, and the pin height is 2.026 which is .010 shorter than the OEM pistons. I was planing on using a .010 over Head Gasket. Does any body have an idea what the compression will be at, or is there a formula I can use to calculate it? Thanks, Colin
 
It may not help you a whole lot, but I've always used .010" as one point of compression. For example, I had a set of 6.2 diesel pistons I had .030" taken off the tops. I went from a stock of 22:1 to 19:1 on compression. This may or may not be totally accurate, but close. The only other way that I know of would be to CC the head an combustion chamber with water and subtract that from the total volume of the whole completed cylinder, including head and gasket. I don't know the difference in bowl sizes between your two pistons, but just as an example your lowering 2 points of compression from the head gasket and shorter piston. But you may be gaining some back from the protrusion and possibly smaller bowl on your new pistons.
I've probably not helped at all, but hey I tried!!
thanks,
Mike
 
cbtoyz said:
I just put some different pistons in my 12 valve and I am curious to see what the compression ratio will be. The pistons I installed are Marine 370, part number 3935591, standard size, and the numbering on the piston says 101.88 SPO.12, 09E131. I measured the piston protrusion at .018, and the pin height is 2.026 which is .010 shorter than the OEM pistons. I was planing on using a .010 over Head Gasket. Does any body have an idea what the compression will be at, or is there a formula I can use to calculate it? Thanks, Colin
Click to expand...

Total cylinder volume at BDC divided by total cylinder volume at TDC. You will need to know bore and stroke,protrusion, compressed gasket thickness,Gasket I.D., and bowl volume. Don't forget to add the volume of the chamfer at the top outer edge of the piston, and the corner radius at the edges of the gasket.
 
Just giving you a bad time about the piston chamfer and gasket radius. Those values are pretty small and could be ignored. If you want me to double check your findings, pm me the diminsions and I'll let you know what i get. Funny you should ask this.I have to check this on the 6.7. I had the pistons filled in and re-machined. No clue where it's going to end up(i think it's going to be a bit on the high side.
 
RonA said:
Just giving you a bad time about the piston chamfer and gasket radius. Those values are pretty small and could be ignored. If you want me to double check your findings, pm me the diminsions and I'll let you know what i get. Funny you should ask this.I have to check this on the 6.7. I had the pistons filled in and re-machined. No clue where it's going to end up(i think it's going to be a bit on the high side.
Click to expand...

You had the pistons filled in? Why? More compression for more low-end power?
 
Muddin_dude06 said:
You had the pistons filled in? Why? More compression for more low-end power?
Click to expand...

Bowl was designed for common rail injection. I was told the design would create alot of smoke with the poor atomization of the p-pump.
 
'96 piston:

Pin to crown - 71.6mm
Dome to crown - 7mm
Piston bowl - 52/61mm
42cc's

Marine piston:

Pin to crown - 71.5mm
Dome to crown - 8mm
Piston bowl - 64/67mm
53cc's
 
RonA said:
Total cylinder volume at BDC divided by total cylinder volume at TDC. You will need to know bore and stroke,protrusion, compressed gasket thickness,Gasket I.D., and bowl volume. Don't forget to add the volume of the chamfer at the top outer edge of the piston, and the corner radius at the edges of the gasket.
Click to expand...

don't forget the volume around the valve faces... if I remember right I came up with about 5cc's up there. Depends where the face of your valves are in relation to the surface of the head.

So what bowl design are you going to use Ron?
 
Ron, your special rods will be on their way to you this week...let me know what compression ratio you have after install.
 
Thank you Piers. i just found out a few minutes ago that i won't be able to use the crank that's in this 6.7. Something about the way they are set up for a rear engine motor home. I didn't know they were were different. Oh well, it wasn't the first mistake on this project. I'm sure it won't be the last. I knew I was in over my head before i started. Maybe I can trade electronics, pump, or injecters towards a crank?
 
RonA said:
Thank you Piers. i just found out a few minutes ago that i won't be able to use the crank that's in this 6.7. Something about the way they are set up for a rear engine motor home. I didn't know they were were different. Oh well, it wasn't the first mistake on this project. I'm sure it won't be the last. I knew I was in over my head before i started. Maybe I can trade electronics, pump, or injecters towards a crank?
Click to expand...

Well that's a bummer. I'm not sure if Jeff sold the one he had or not but he had one at one time: New 6.7 Crankshaft - Competition Diesel.Com - Bringing The BEST Together

Edit: oops, looks like I'm about 10 seconds too late :doh::hehe:
 
I have several 5.9 cranks kicking around, I'd be happy to donate a crank towards your special little project. Let me know if you want one & I'll build a shipping container this weekend.
 
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