Soaring

Any math gurus on here? How do your figure compression ratio? In other words, let's say I have a 10:1 compression ratio. How do you come up with that figure?

aamilo

I think ????? it is a calculation of the based on the total volume of air/fuel inside the cylinder chamber and how much the piston compresses it before it reaches TDC. So a 10:1 compression is saying that the air fuel mixture is compressed 10 times the value achieved by normal atmospheric pressure. SO it should be a measurment in volume based on bore/deck height/gasket thickness etc when compaired to the stroke. OWWWW too much physics

http://www.bgsoflex.com/cr.html
http://www.wallaceracing.com/cr_test2.php

Soaring

From what I've read, for best performance and efficiency, try to get the best CR for the fuel you plan to use, with a squish height of less than .040", and use only flat-tops or dishes. Domes get in the way of the quench effect created by the squish area. Is this true?

aamilo

unfortunatly you have gone beyond my level of knowledge but it sounds right. I know it is very important not to "over" compress because the higher the compression the higher the octane rating your fuel needs. I believe anything over 10:1 will usually require you to use premiem fuel. I believe the big advantage to dished pistons is that you can push the compression a "little" higher while still maintaining a good valve clearance. But again it is best to check with the professionsals you buy your parts from and maybe some other posters will have some more indepth knowledge.

Soaring

Yeah,I realize that anything over 10:1 will require a higher octane fuel, but my question basically is how do you figure compression ratio mathmatically. Let's say I have 600cc at when the piston is TDC, and 60cc at Bottom. Will that equal to 10:1 or will it equal 11:1? Is there still 60 left at bottom, so that equals 660? So, therefore it would be 11:1?

aamilo

Hey Soaring, My wife likes your car. (and all this time she thought I was looking at **** ) I think you have your TDC and Bottom reversed for volume However to answer your question I really don't know. I would imagine it would be total volume including cylinder chamber volume (cyl bottomed out ie 600cc) divided by volume remaining at TDC (ie 60cc) leaving you with 10:1 but I am unsure on how you would calculate volume in an assembled engine unless you have the specs from your engine assembly (ie bore/stroke/chamber volume). Again we are above my level of knowledge so I would need better advice before I could be sure. I am going to wait to see if another post can answer your question before I jump in again.

white68fb

If you'd like, I can give you a web site with a CR calculator. It's takes a fair amount of data to do it right like volume of your piston reliefs, how close your piston gets to the top of the deck etc. Let me know but I can't send it until Monday or Tuesday.

67coupe

This website has some great engine building info. They also have an article on quench height.

Speed-o-motive

JD1969

Well the octain rating that you need is more related to your engine as a whole. For example alumiminum heads will tolorate a higher compression than iron heads. Your cam profile will affect it as well, a cam that bleeds off cyl. pressure becacuse of a huge amount of overlap will allow for higher static compression. The last two engines I put together were both well over 11:1 (one was 11:33 and the other was 11:48)and both ran great on 93 octain, but it was because of the alum. heads and the cam that I chose. BTW here is a link to a compression ratio calculator
http://www.smokemup.com/auto_math/compression_ratio.php you have to register but the site has a ton of usefull info.

Soaring

Thanks for all the info and sites guys. Now, I guess I need to do some studying.