Compression ratio math

[CEO]

Can someone please help me figure out what the middle number in the compression ratio is. For example what is the diff between 10:3:1 vs 10:5:1 or even 10:53:1? And how the hell is Ferrari running 14:1 compression on the new 458 speciale without knock or detonation?? 458 Speciale - Ferrari 458 Speciale - Official Website

Kenneth

[tharbert]

You have the notation incorrect. It's a simple ratio. In your Ferrari example, if you bumped the compression down half a point, it would be 14.5:1. I think you are mistaking a decimal for an extra colon.


I know you can run higher compressions ratios with higher octane gas. Alcohol engines also run those type of numbers.

[CEO]

14:1 is higher than 14:5:1? And the Ferrari say that it runs on 93 octane.

[Dodge Man]

CR is all about the motor design. Static CR is a can of worms these days with variable vale timing, electronic engine controls, etc.. BACK in the day Static CR had a lot to do with cam profile and when the intake valve & exhaust valves mechanically closed. FAST FORWARD to present day engine management computers and variable valve timing to get the high static CR to work properly in the modern real world. Porsche has a lot of great VVT systems in late model engines. The old school motors were way different from the modern engines with a boat load of control systems. That is why a lot of folks love vintage racing because it requires a lot of mental skills to build and tune a motor without the modern control systems. I have an OEM 9.8 CR motor with CIS and that was a lot of CR back in that day. The 930/10 was designed to go against motors with twice the volumetric displacement(911 vs. Corvette). I embrace that period motor design for how special that was in its day. IMHO. However, Ferrari is cutting edge high dollar motor design so do not expect an article in Popular Mechanics magazine next month to explain Ferrari's millions of F1 research & development. IMHO You are also probably reading marketing material spec sheets to sell cars to Nuevo rich so take it with a grain of salt. Happy CR research!!!!

[tharbert]

Originally Posted by CEO

14:1 is higher than 14:5:1? And the Ferrari say that it runs on 93 octane.

Oops, yup, up.

Not an expert but knock sensors can be used to adjust the timing on the fly. About the highest octane I see anymore is 91 with absolutely no stations offering non-ethanol options.

[Bob Kontak]

Originally Posted by CEO

14:1 is higher than 14:5:1? And the Ferrari say that it runs on 93 octane.

14:1 is lower. Tharbert was probably thinking air fuel ratio. :-)

To add to Dodge Man's comment, I am pretty sure the modern engines can change the lift of the valves as well as the timing. Not all cars but some.

[Bob Kontak]

Originally Posted by tharbert

About the highest octane I see anymore is 91 with absolutely no stations offering non-ethanol options.

I have never confirmed any stations on this list except one in Texas (called them) but here is a site that indicates stations in Illinois (and others) that have non-ethanol gas. I bet you can find it at one of these.

Ethanol-free gas stations in the U.S. and Canada

[nickyclyde]

I suspect he meant to write that 13.5:1 is lower than 14:1. I would suspect that 14:1 would cause detonation of our current premium fuel but with modern fuel injection systems fuel is delivered to the cylinder before and/or after the 14:1 condition or that the variable valve timing can , when called upon , reduce the compression based on the analysis that the electronics do on the current available fuel in the fuel line. I have no idea if I am right. Just the way I suspect it works?

[wrxnofx]

At its core, there is a correlation between compression ratio and fuel octane. Back in the '60's and early '70's before all the emissions requirements, muscle cars had high compression ratio engines, 10.5:1 and 11:1. Higher compression = higher pressure = higher temperature in the combustion chamber. Higher temperature in the combustion chamber means a higher propensity of pre-ignition, or detonation, of the fuel in the combustion chamber. To counteract that, lead was added to the fuel to keep the temperature down in the combustion chamber. In the U.S., when we switched to unleaded fuel in the mid '70's, horsepower took a dump because compression ratios dropped significantly due to there no longer being lead in the fuel.

The RON, or research octane number, of fuel is all about the amount of energy bonds in the fuel. The higher the octane number, the more bonds of energy there are to be broken in the combustion process. The more bonds there are, the slower the fuel burns, and therefore the less volatile it is. Because the higher octane fuel burns slower in the combustion chamber, the pressure at the flame front doesn't increase as fast, and therefore there is less of a chance of detonation, which is nothing more than the fuel igniting outside of the flame front.

The original, theoretical calculation for compression ratio is nothing more than (volume in combustion chamber @ BDC) : (volume in combustion chamber @TDC). That calculation ceases to be accurate when "artificial" methods are used to modify the compression ratio. Turbocharging, for example, artificially increases the compression ratio due to the compressor side of the turbo pumping additional air into the combustion chamber.

[NICE 69 S]

CEO,
You have too many colons in your example. 14:1 compression means you are taking 14 units and reducing it to 1 unit (cubic inches or cc, doesn't make a difference). If it is 14.5:1 , then you are taking 14.5 units and reducing it to 1 unit. The higher the first number, the more compression you have.
Please note that these are static numbers, which means you just take cylinder volume, add in combustion chamber volume and divide it by combustion chamber volume to come up with a "static" number. When you add in camshaft overlap, and the opening and closing degrees of crankshaft rotation, the compression number will usually be different.
Bob B

[Josh D]

One of the latest technologies that have allowed even higher CR is direct fuel injection. Rather than fuel entering the combustion chamber with air via the intake valve and getting heated as it is compressed, it's injected directly into the cylinder at high pressure, much like a diesel compression engine. As it is injected at the precise moment, I'd assume it would have a cooling effect on the compressed air.

I read somewhere they are working on gasoline compression ignition engines that won't use a spark ignition at all, and have very high compression ratios. Maximum work from the least amount of fuel.

[CEO]

NICE 69 S, Yes I did get a bit happy with with my colons, sorry.
All this info has been very helpful. Thanks guys.

So is there an optimal way to figure out what CR I should use or shoot for? Its for a 3.45 NA motor. I started with a 3.0 and bored it out to 102mm Nickies from LN Engineering with dual plugs and ceramic coatings, ported heads and an 45mm ITB Tec3r injection setup with custom cams that I have yet to decide on. 10.5:1? 10.3:1? We have 93 octane here in Dallas and I have easy access to race gas but don't want to spend the money every time I fill up.

Kenneth

[Dodge Man]

Originally Posted by CEO

NICE 69 S, Yes I did get a bit happy with with my colons, sorry.
All this info has been very helpful. Thanks guys.

So is there an optimal way to figure out what CR I should use or shoot for? Its for a 3.45 NA motor. I started with a 3.0 and bored it out to 102mm Nickies from LN Engineering with dual plugs and ceramic coatings, ported heads and an 45mm ITB Tec3r injection setup with custom cams that I have yet to decide on. 10.5:1? 10.3:1? We have 93 octane here in Dallas and I have easy access to race gas but don't want to spend the money every time I fill up.

Kenneth

Your CR & Octane requirement is based on Dynamic CR. The 10.5:1 or 10.3:1 is just part of the equation. The higher static CR is usually dictated by cam timing in motors without VVT. The intake valve probably closes later than stock SC and you need a higher static CR to get the "dynamic" compression back into range. a.k.a. When the intake closes later you lose some piston stroke so the TDC volume has to be smaller(higher Static CR) to get the proper "squeeze" for good combustion. The cam timing probably will drive your exact static CR requirement. Cam grinders usually know what will work and not collapse pistons or tag valves. A cam grinders rep is based on how the motor runs so cam grinders will give lots of info on other motor requirements to keep their rep intact. Just read an old CRANE or ISKY cam catalogue for fun. IMHO Do not forget to putty test valve to piston clearance with the motor on the stand. Dual plugs solve the flame travel issue in a tight combustion chamber. However, I am a little jealous about the dual plugs. Dual plugs can save your bacon(pistons) if timing or octane gets a bit off. I worked with some 13.5:1 Chevy 302 roller cam street guys back in the day. Sounds like a great 911 build so best of luck.