Compression Ratio

[nivvy]

4.375 bore
4.380 gasket bore
4.150 stroke
78 cc indy aluminum heads
.013 deck height
-6 dished piston

so for me to run no higher than 11 - 1 compression I would need a .060 head gasket minimum ...do they even make these

http://www.csgnetwork.com/compcalc.html

[Chryco Psycho]

Cometic does , up to .080 thick  & I would recommend using them with alum heads

[nivvy]

I went to there website and the only show .040 but can custom make a set for whatever you want..so my next question is since they can make what I want how close should I come to the 11-1 ... or dead on at 11-1

.060 will take me to 11.01 / 1
.065 to 10.90 / 1
.070 to 10.78 / 1


 

[Chryco Psycho]

assuming you want to use pump fuel I would be looking at 10.8 range 

[HP2]

Don't forget your cam timing will have an impact on your cylinder pressure too. Cylinder pressure is a bigger determing factor to pump gas compatibility than your static compression ratio.

[PlumCraZRT]

Don't forget your cam timing will have an impact on your cylinder pressure too. Cylinder pressure is a bigger determing factor to pump gas compatibility than your static compression ratio.

It all really comes down to how much quench your combo can provide.  You can get away with high cylinder pressures if you provide adequate quench to support it.  Difficulty is in making sure your quench area is big enough and effective enough.  For our intensive purposes, we need to rely on intuition, guess, or go off experience.  Any way about it, we are relying on less than scientific methods.

Try to look around for combos that have worked and make a good guess at it.  Again, experience helps with this, so get it while it's hot...

[HP2]

Quench is definetly a part of it. But rather than just guessing, I try to use some math to tilt the odds in my favor. Here are some links to help with that.

The first one deals primarily with motorcycle engines, but they are still 4 cycle and the impact is the same regardless of actual displacement. http://www.rbracing-rsr.com/comprAdvHD.htm

This is one i found recently with becoup lotsa formulas.  http://www.wallaceracing.com/Calculators.htm

[PlumCraZRT]

Quench is definetly a part of it. But rather than just guessing, I try to use some math to tilt the odds in my favor. Here are some links to help with that.

The first one deals primarily with motorcycle engines, but they are still 4 cycle and the impact is the same regardless of actual displacement. http://www.rbracing-rsr.com/comprAdvHD.htm

This is one i found recently with becoup lotsa formulas.  http://www.wallaceracing.com/Calculators.htm

I've made my own dynamic compression ratio calculator using an excel spreadsheet.  It's useful at knowing what dynamic compression ratios you will see, but again it's all somewhat useless unless you know what the max dyn comp ratio can be before you start to detonate... and that's largely determined by quench.

To give a brief explanation, quench and swirl is basically created by making an area on the head where the piston will come relatively close to contacting... most often this area is just a flat spot with a flat top piston that will get to the head gasket thickness away from contacting the head.  This low near-contact area forces air and mixture from that side of the cylinder into the middle of the chamber and results in a complete burn by swirling the air and creating lots of motion in the chamber.  Detonation is caused when the mixture ignites due to high compression or hot spots, but not from the provided spark.  By creating the motion and reducing the stagnation of the mixture in the cylinder/chamber detonation is prevented and the mixture is forced to burn at the time of spark.

Getting back to dynamic compressions... Your dynamic compression is determined by you intake closing angle and your rod/stroke combination.  If you want I can send you my excel chart.  Just PM me.  I've never really gotten a good answer on what dynamic compression ratios are acceptable with what sorta quench or with aluminum heads or ANYTHING for that matter.  I've heard like one generic number which i want to say is 8.5 and that you have problems above 8.5... sounds a little hokey.

[HP2]

I've heard around 180 psi is about the limit for pump gas. Certainly quench can factor into that, as can combustion chamber design, intake design, ignition lead, spark intensity, and other factors.

CP, what's your take on cylinder pressures?

[Chryco Psycho]

about the same , real cylinder pressure is what really determines fuel octane need simply put to much compression will build too much heat & ignite the fuel before spark , keep increasing pressure & you have a diesel !! There are other factors too , lighter cars with more gear will get away with more cylinder pressure due to reduced load against the engine , powrband also makes a difference for example my Duster used Speed Pro 6 pack pistons with milled closed chamber iron heads & steel shim gaskets so the mathmatical compression was approx 11:1 but it had no sign of detonation on pump gas  but it was never loaded below 3000 rpm & used the Mopar .590 solid cam , but quench was perfect , it could be easily made to detonate at low rpm but was never used this way , put that engine in a 1 ton truck & pull a trailer with it , it won`t live long !!
 there is no calculation for real world use with all the variables possible

[PlumCraZRT]