Most liked posts in thread: Can someone explain compression ratio?

I should not, nor should anyone else feel that they need to post a résumé to answer a question on this site.

@Nitrominis, you may be surprised to know that I am capable of getting out bed and finding the floor everyday without having to search Google. I, and many others that visit this site, do know stuff even if we do not have the experience you claim. Believe it or not, many of us have also changed our brake pads too without having a wall full of ASE Certifications. We haven't killed ourselves or others either.

 

Boy......you really know how to take a thread OT now don't ya.




:biggrin5:

 

I choose the one that shows attention to detail and would spend the extra 2 seconds it takes to proofread before showing the world they don't care enough to fix a simple item like using the term "ration" when "ratio" is the proper term.

 

Dude, are you seriously comparing turning a wrench to a medical degree?

Fine. I hold an ICAR gold class certificate in collision repair, pretty much an industry standard comparable to ASE, but for body men and painters. All it took was about 40 hours of class time, a few open book multiple choice tests and $800. When do you want me to come over to work on you car?

 

:deadhorse:

really...........?

 

I didn't read all the knowledgeable answers above.

My though is it is the amount of squish before you get a bang. Suck, squish, bang & blow 4 strokes of fun!

 

Back in my day we'd let the pistons get all carbon'd up to raise the compression ratio. Oh the sound of that powerful pinging.

Back on topic: direct injection donuts.... :donut1::donut1::donut1:

 

Some people want products that have 'better numbers', indeed they may base their buying decision only on those numbers - like a 200hp car must be better than a 150hp one, a 12Mp camera better than a 8Mp, or a 6-airbag car safer than a 4-airbag one.

Sadly there are many characteristics that are not amenable to simple measurement - just look at the (particularly American) obsession with using g on a skid pan as a measure of cornering, which tells us nothing at all about handling. Similarly a 8Mp camera with a good lens will generally produce better photos than a 12Mp one with a mediocre lens.

So there isn't a compression ratio that's 'better' than another - get a diesel and that will have a compression ratio over 15:1 and maybe over 20:1. Them's real man's numbers....

 

The bore and stroke can stay the same but if the dimension from the piston pin to the top of the piston changes the CR changes. Or if all stay the same but the chamber volume in the heads are different the CR changes. So different heads or different pistons can change the CR.

 

I see what you are getting at here...

The stroke is the distance the center point of the piston ring travels.



Altering the "height" of the piston above that point changes the size of the space between the top of the piston and the bottom of the head. Given no other changes a "taller" piston would increase the compression ratio while a "shorter" one would decrease the ratio.

Altering the head gasket to be thicker or thinner than originally spec'ed is another method of changing the ratio.

 

Yup....

 

The 'how' is well described above. The 'why' is because the turbo on the S model forces more air into the cylinder and reducing the compression ratio reduces the stress on the engine - though it also reduces the power. But the combination of the turbo on full song with the original compression ratio would end up with cylinder heads making extra unwanted power bulges in the bonnet/hood.

I never realised till now how 'undersquare' the 1.6 engine is (ie, stroke much greater than bore). This is very unusual (and very old-fashioned?) in a modern high-performance engine - even the 1960s Cooper S pushrod engine was nearer 'square'! The Toyota is 'square (bore equals stroke). Modern sports motorcycle engines go very 'oversquare' (up to bore equals 1.5 times stroke) - but then they are revving to 16,000rpm.

I'm surprised the Toyota is only 'square' as the engine power and torque numbers make it sound like a very 'top-end' motor - full power at 7000rpm and peak torque at 6600rpm sounds a nearly impossible combination - and nearly impossible to drive at less than max revs all the time.

 

The bore x stroke is partial formula for the engine size which along with other measurements is needed to calculate the CR.



And Nathan you are almost complete with your response found I presume on the internet? Your information (source?) forgot to mention the pistons volume. You can have a higher piston as your description and actually lower your CR with some piston specifications.


Basic engine rebuilding information:
The calculation for determining compression

CR=(D + PV + DC + G + CC) / (PV + DC + G + CC)
CR = Compression Ratio
D = Displacement
PV = Piston Volume
DC = Deck Clearance Volume
G = Gasket Volume
CC = Combustion Chamber Volume

 

The MINI engine is more of a "stroker"..which I believe helps to bring in more usable torque at lower RPM .....

 

That's not really explaining it, is it? That's like me quoting a text book on IP protocols to a layman just to show off my amazing memory.

 

Jerry there is not a simple answer and it does take a bit more then a simple response on achieving the CR and the calculations and measurements to achieve the CR of an engine.
As for memory it is more experience that some may not be aware I have given many responses on other threads as well forums. To qualify having own and operated several years a Performance Engine Machine Shop this was as you said a text book summery what it takes to get the CR of an engine Jerry.

I got a pm from a MU member not altogether pleased about my response and want to clear something. I did mean to discredit anyone's response here it is my bad post technique. Some responses may be helpful but just needing a bit added. It is so easy to run to the internet for the instant answer however may not always supply the complete answer.

Apologies.

 

Yes but how did you turn/machine your rotors when you changed the brake pads? My point is I do have both the experience and the credentials and offer them on a forum without asking for anything in return.
Oh and you failed to mention on my wall is also M Series,P Series and L1 certifications. :wink:
The M certification helps me to be able to post factual information here as well with questions I asked on other threads here on MU. The M certification signifies Engine Machinist.

 

That is very interesting. In my field we specifically look for people with a lot of certificates. We especially like ones that come with "Microsoft" printed at the top of the page. It saves us time interviewing them. We can move on to the next candidate right then and there.

Lynn, perhaps if you feel the need to tout your credentials at every turn, they are not as impressive as your perception of them?

 

The Compression Ration has a direct effect on the fuel used and vs verse. So as long as the engine is an internal combustion Compression Ratios will always be a factor in its design.


Most every "forced induction" engine is NA until you "boost" psi with Turbo,NOS,Supercharger,variable lift camshafts, etc.
So to kinda answer your question you would have to compare them without the boost for a fair conclusion about "efficiencies"?



@Jerry we have been down this road many times, the question is do you go to your dentist without credentials hanging on the wall? Do you want a mechanic or a trained technician working on your MINI?