compression ratio

[sv650s]

i just got into forced induction and i found out that compression ratio has a lot to do with it, i've done my research but there one question that i can't get answered. when is compression ratio considered high?

ps if you're going to do any math go easy cos i'm not really good at that stuff.

[beef_bourito]

10:1 is generally concidered to be high for forced induction. but it all depends on how much boost you want to run and what you're going to do to cool it down. you can run alot of boost on 10:1 compression if you use water or alcohol injection and an intercooler but without them you can't run nearly as much.

here's a few sites for info:
http://kb-silvolite.com/article.php?...3b0040bfe053cc for compression ratios and boost
http://www.goodvibesracing.com/Compression%20Ratio.htm for calculating final compression ratio with cr and boost
http://sdsefi.com/meltdown.htm for info on knock and detonation and tuning for them

the highlighted numbers are the limit for 93 octane fuel.

[sv650s]

ok i think i got it right, since i'm not that good at math it took me a long time to figure out the basics of compression but i think i finally figured out how to calculate compression ratio.
bdc= tdc+(bdc*stroke)= CR
tdc tdc

can somebody correct me if i'm wrong?

[beef_bourito]

CR=volume @ tdc/Volume @ bdc

[sv650s]

what does the @ mean?

[beef_bourito]

it means at

[sv650s]

oh my bad didn't look at it right, i was expecting it to be all addition and multiplication not just division. so the whole thing i wrote wasn't necessary it's just v@tdc and the answer is always :1 ?
v@bdc

[Alastor187]

Originally Posted by sv650s

oh my bad didn't look at it right, i was expecting it to be all addition and multiplication not just division. so the whole thing i wrote wasn't necessary it's just v@tdc and the answer is always :1 ?
v@bdc

Where:

V_BDC = Volume inside cylinder when the piston is at bottom dead center.
V_TDC = Volume inside cylinder when the piston is at top dead center.


The right side of the colon is always one, so just remember that and you are set. The reason for this is the compression ratio is a “relative” measure, and always measured with respect to the smaller volume.

[534BC]

All compression ratio specs are not a true ratio unless they have 1 added to them. A compression ratio should really read in text form "8 into 1" instead of "8 : 1" It has alway been that way, it is that way in books and specs yet when figuring out a true ratio the ccv is alway added to the formula. Go figure???

I can't really say I agree with the posted chart as the ratios and/or boost seem quite high to me. For a naturally aspirated engines I generally just convert the first number in the ratio to the octane of fuel used.

8:1 can run 80 octane
13:1 can run 130 octane

many variables, but it is a general guideline.

[534BC]

Nowadays it seems that the limits can be pushed a bit higher, but I seemed to add 1/2 of the boost to the first number in the ratio also to get the limit for the specific octane of fuel. Extra cooling let you push the limits quite a bit also.

[Alastor187]

Originally Posted by 534BC

All compression ratio specs are not a true ratio unless they have 1 added to them.

Can you explain that in more detail?

[534BC]

The ccv is added to the total so that a 8:1 ratio is really a 9:1 ratio, or it is 8 units squeezed into 1 unit.

The ccv is added to the cid before dividing. It is really nothing to worry about at all as the whole industry uses the same standard, maybe it is just the way I look at a true ratio,,,,

[534BC]

The ccv is added to the total so that a 8:1 ratio is really a 9:1 ratio, or it is 8 units squeezed into 1 unit.

The ccv is added to the cid before dividing. It is really nothing to worry about at all as the whole industry uses the same standard, maybe it is just the way I look at a true ratio,,,,

[jveik]

yeah i just to clarify for that one dude, that compression ration includes the volume of the combusion chamber... common sense should tell ya that so sorry if i seem like an idiot, i just wanted to clarify lol

[nissanfanatic]

http://e30m3performance.com/myths/mo...comp_ratio.htm

Static vs Dynamic compression ratio:


Basically, static compression ratio is only important if you have specific goals on a pump gas/air-to-air intercooled setup. If you do not have any specific requirements to meet on a certian fuel or any restrictions on advanced charge cooling, then higher compression ratios can be used with boost in conjunction with higher octane fuel or water injection/water-to-air intercoolers.

Most people tend to go to lower compression ratios in order to make a lot of power on street applications where it is most convienient to buy 93 octane fuel avaliable at most fuel stations.

Although, I have been pushing 16psi+ on a 9.5:1 CR on 93 octane...