Compression Ratio

[SonexEZ]

Hi All I didnt build this engine but have it all a part due to oil leaks and also reground one exhaust valve , My question is the copper head gasket is .040 what dose that tell you for compression and what fuel do i use do i need to get more information to find this out still have two cylinders to do . I have flown this Sonex aircraft several times but have it home in my garage at this time . I have put two months into it just trying to get it to be safe aircraft

[Bryan Cotton]

There is a calculation of how many shims you need to achieve the right compression ratio based on measured deck height. Worth reading page 21 of the Aerovee manual, which you can get for free here:
http://www.aeroconversions.com/support/manuals.html

Actually the whole manual is good, not just page 21.

[mike.smith]

Long answers to short questions:

To really know if you are getting your desired compression ratio you need to measure the displacement (cc's) of each of the four cylinder head chambers. Then you can calculate the thickness of the barrel shims and the head gasket to get the right figures. I found that the copper gaskets would compress by about 33% after torquing, so I calculated based on that. All of the chambers "should" be within a cc or so of one another, but one of my chambers was way off, so I had to account for that when figuring the shims and gasket. Another reason to cc the heads yourself. I would recommend doing everything you can in metric, and only convert when you have to.

Here's where you can start on my Kitlog page the start of this process: http://www.mykitlog.com/users/display_l ... 205&row=22

My calculations (for 8:1 compression):
http://www.mykitlog.com/users/display_l ... 690&row=12

And here are some references:

Engine Compression Ratio Calculator:
http://www.csgnetwork.com/compcalc.html

Convert inches and mm:
http://www.metric-conversions.org/lengt ... meters.htm

Misc:
http://www.aircooledtech.com/deck_height/
http://www.aircooledtech.com/calculating_cr/

[SonexEZ]

thank you for all the information my head is spinning , i will never sleep tonight lol

[Rynoth]

thank you for all the information my head is spinning , i will never sleep tonight lol

Provided this is an Aerovee engine with factory parts, following the steps in the Aerovee manual that Bryan linked should remove all guesswork. It's a fairly simple process of measuring the piston crown at TDC, and selecting shims per the chart in the manual.

[mike.smith]

thank you for all the information my head is spinning , i will never sleep tonight lol

Provided this is an Aerovee engine with factory parts, following the steps in the Aerovee manual that Bryan linked should remove all guesswork. It's a fairly simple process of measuring the piston crown at TDC, and selecting shims per the chart in the manual.

The problem is that the Sonex published chart assumes a certain value for the cc's of the chambers. If I had gone by the Sonex chart my second time around I would have ended up with a good bit less than 8:1 compression in 3 cylinders, and even less in the 4th. CC'ing was not difficult. It took me one evening to cc them, and one evening to figure out my numbers. After my re-build my engine is absolutely performing better than the first time, where I just went with the numbers in the AV manual. For me, CC'in was worth the minor added effort, and I can confidently know my true compression ratio.

[rizzz]

Perhaps Sonex has not updated their manual since they made the switch from CB Performance to Mofoco heads.
Volume in the chambers would almost certainly be different between the two brands.
I measured all 4 chambers on my Mofoco heads and found even slight differences between those (minor though).
From memory I think I measured between 63 and 65cc (mine are a 94mm bore though)

If you want to be certain do a CC check, a kit to do this costs next to nothing or can be made from some plexiglass and a syringe if you want:
http://vwparts.aircooled.net/Cylinder-H ... ng-kit.htm

Once you have the measured CC's it's pretty easy to calculate the deck height/compression ratio either using the links others have provided above or this one below (which is what I used):
https://www.cbperformance.com/v/enginecalc.html

[Rynoth]

Not to poo poo on progressive and innovative ideas, but if it's an Aerovee engine with Aeroconversions support, I'd follow the Aerovee manual first and ask Aeroconversions second. I've run into my own issues, yet following these steps have solved most of my issues thus far.

[mike20sm]

https://www.youtube.com/watch?v=K6c2cg0TFkU
[youtube]https://www.youtube.com/watch?v=K6c2cg0TFkU[/youtube]
a video on CC'ing

[niamod58]

Long answers to short questions:

To really know if you are getting your desired compression ratio you need to measure the displacement (cc's) of each of the four cylinder head chambers. Then you can calculate the thickness of the barrel shims and the head gasket to get the right figures. I found that the copper gaskets would compress by about 33% after torquing, so I calculated based on that. All of the chambers "should" be within a cc or so of one another, but one of my chambers was way off, so I had to account for that when figuring the shims and gasket. Another reason to cc the heads yourself. I would recommend doing everything you can in metric, and only convert when you have to.

Here's where you can start on my Kitlog page the start of this process: http://www.mykitlog.com/users/display_l ... 205&row=22

My calculations (for 8:1 compression):
http://www.mykitlog.com/users/display_l ... 690&row=12

And here are some references:

Engine Compression Ratio Calculator:
http://www.csgnetwork.com/compcalc.html

Convert inches and mm:
http://www.metric-conversions.org/lengt ... meters.htm

Misc:
http://www.aircooledtech.com/deck_height/
http://www.aircooledtech.com/calculating_cr/

I recently joined this forum and I love it! I had the same question of asked by SonexEZ and the answer provided by mike.smith was really informative, especially with its referred links.

Thank you for all your shared info. :)