Pretty boring Sunday at work. Forums are also slow this time of year. Thought I would put a question out there that the engine gurus could answer and explain that would give understanding and insight to those that might be curious ( like myself ). I have abasic understanding what it is, but what does to much or not enough do? What are some of the ways you check it? How does it affect BB or higher compresion? Thanks FRed
What is "squish"
- Thread starter fc8464
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Squish is the distance from in the top of your piston to the bottom of your head. Changing your squish is changing your compression ratio. I check mine by using solder bent in an "L" shape red through the spark plug hole. Pull the rope and let the piston smash the solder. Use two different pieces one at a time in two different spots in the hole. The best reading will be directly above the pin. Measure your smashed solder with a caliper.
Your assessment is accurate other than the bolded sentence. Compression ratio has nothing to do with the "squish". Compression ratio is the volumetric difference between when the piston closes all the ports off and compression starts and the volume left at TDC. Squish is the narrow outer diameter of the combustion chamber that most builders like to see in the 0.050" to 0.0625" range (from the engines I've had). It should also be smallest at the outer circumference and taper slightly larger towards the center dome and spark plug. It has been too many years since I was on a first name basis with the high end engine builders to remember the theory and thoughts regarding the rest of your questions. Too narrow, bad stuff happens (det and preignition?)-too loose, no power ( flame front is not concentrated in the dome, resulting in a lot of wasted unburnt fuel escaping).
It has been fifteen to twenty years since I was building high end Titanium rodded engines so if I'm off base feel free to correct me.
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I used the PRO TI rods in a '95 storm based triple (took better than 3#'s off the crank total after balancing) for the Alaska hillclimb circuit (three races a year back then). I was young, single, with a great job; better outlet for me than the other alternatives; alcohol, drugs, etc. I did really well with it when I farmed the riding out to friends with natural ability. As I'm known for "outriding my ability" and "my thumb writing checks my body can't cash".
I wish the TI rods were still available, but to my knowledge the one owner is now deceased and the other did not choose to continue production. They would be killer on the new lightened Polaris crank.
In a nutshell Squish controls gas velocity, To tight will create det to the piston, Too loose gives up performance, both squish clearance and angle control the velocity of the gases.the clearances need to grow with the bore size to maintain a ratio for peak performance. It also controls how a motor will obtain RPM,s. Ive had tight motors that would not hit 8K even with weight removed.Its why two identicle sleds will not perform the same and is the one factor thats slightly different in every build.tight squish motors typically are explosive off the flipper but fall off and fall behind on a long pulls.
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Identical to how I check mine.
Good info from loudhandle and byeatts too. They pretty much summed it up.
:thumb:
ALL good info except one point...
squish clearance can change compression ratio...granted changing a gasket .004" to achieve the proper squish clearance is more important than the minor change in compression...
same motor 2 base gaskets...
5 hole (.020") yields a .035" squish and a head volume of 32 cc
8 hole (.032") yields a .047" squish and a head volume of 33 cc
pretty sure 33 cc is lower compression than 32 cc
granted this is a little extreme but does show its applicable to "fine tune" compression ratios, be it safer or hotter for your application/fuel...BJ
squish clearance can change compression ratio...granted changing a gasket .004" to achieve the proper squish clearance is more important than the minor change in compression...
same motor 2 base gaskets...
5 hole (.020") yields a .035" squish and a head volume of 32 cc
8 hole (.032") yields a .047" squish and a head volume of 33 cc
pretty sure 33 cc is lower compression than 32 cc
granted this is a little extreme but does show its applicable to "fine tune" compression ratios, be it safer or hotter for your application/fuel...BJ
A little off the squish topic - crank phasing is another reason two identical motors make different HP; e.g., doo S3 cranks have had a rep for sometimes being out of phase OEM a couple of degrees (in a two cylinder, pistons TDC/BDC are not 180 degrees opposite each other). Port and ignition timing challenged. Other posts are great in this thread.
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