Squish band

[Deleted member 12676]

Fabian, just call him at (603) 899-9871 or email him at fred@crmachine.com

getting back to the topic at hand...
more replies from mopedarmy:
>Squish bands are important, but they also rob some of the compressed mix from contributing to Power. (about 10% of volume is squish area)
>After I get done modding a head to get a reasonable compression ratio there is very little squish left and yet they run fast and cool.
>Squish increases the gas velocity which wrinkles the mix giving the flame front more surface area for convection of the flame to burn faster also it makes a more homogeneous mixture. both effectively act as if your timing has advanced, more so at higher rpms as faster piston speeds means more gas velocity from the squish. If you're losing power at high rpm it could be that the engine is now fighting itself due to the pressure rise being to far advanced. try retarding the timing a bit.

I just calculated my 55cc engine and from a 7:1 CR with 200psi and 5.5:1 CR with 150psi I figure 6.5:1 is almost 185psi. (My piston is from a motorcycle and both ring ends have a normal gap to seal better than the stock Grubee rings). So when Jennings indicated that a squish band is appropriate over a 6.5:1 CR he meant 185psi (more or less depending on how accurate my gauge is).

I am going to try a head comparison again. I have two heads for my 55cc, one of which I can modify to make a squish band. I will make the clearance .8mm and match the compression. And I will retard the timing for the head w/squish.

 

[Deleted member 12676]

I, admittedly being human, forgot to take into account my piston is domed (1.8mm peak) which according to the calculator at this site occupies 1.3cc of space. Taking that into consideration the normal head with 150psi gives a compression ratio of 6.56, and the head with squish that has 185psi gives 10.3 CR. So Jennings was saying that over 150 psi needs a squish band. And I can tell you from my making a squish band head today, that in order for the band to be wide enough for a minimum squish velocity of 15m/s (Torqsoft calculator) and to have a squish clearance between .6mm and .8mm that the measured compression is always going to be over 150psi. It is like a self fulfilling prophecy. A squish band, set close enough to the piston to work right (give enough velocity), makes it very hard to design a head that gives less than 150psi. If you buy a head with a squish band you'd better be prepared to mill the cylinder top or add extra gaskets in order for you to have the right clearance from head to piston. And make sure that the head will match the piston. All my 48cc pistons are flat but rise up .8mm from the piston edge. So with a common .8mm deck (distance from piston edge to cylinder top at TDC) the piston top is even with the cylinder top. So if you have a flat top piston the squish has to be flat or almost flat. Any angle (between piston and squish) more than 1 degree lowers the squish velocity below the minimum desired 15m/s.
Calculating for a 69cc Grubee engine I figure, with a squish head of 6cc volume, that the standard compression ratio will be increased to 9.5 and the measured psi to 178. If the exhaust port is raised 2.4mm then the CR is 8.8 and the psi is 171. That is with a flat top piston.

 

[Deleted member 12676]

I can't tell any power difference between the two heads, both at 170psi. But the one with the squish band lets it rev out further. That is when I am using an expansion chamber. It either is increasing the top end power or it is causing the exhaust gas to be hotter which affects sound wave speed which extends the pipes powerband. A longer burn (and therefore hotter exhaust) may be due to 10% of the mixture being protected by the squish band from the initial stages of the flame. Next I need to test them using only the stock muffler because it won't be affected by exhaust temperature. That will tell which of the two is happening. (So please no "I told you so"'s yet.)
The squish band head runs hotter at 390 degrees vrs 340 for the regular head but that may be due to the extra rpm and the fact that less heat is absorbed by the piston, and more heat absorbed by the head. The piston absorbs less due to the protection from the hottest happening of the flame, partially protected by the squish band. The head absorbs more due to a more rapid and therefore hotter flame when it is burning inside the "dome". I took off the piston and looked at its underside and there was no darkening which would of indicated excess piston heat.
I used a modified Jaguar CDI to test it. I reduced the normal input resistance to delay the ignition more. And it liked no secondary jumper so I just redid the CDI for even more top rpm ignition advance. I will probably start making them with the extra adjustability for people using these heads with squish band.
The 50 extra degrees doesn't worry me since it is borrowed from the piston, which is a good thing. What worries me is the 170psi. That is higher than most motocross bikes. It will shorten the life of the conrod bearings, no doubt. I may reduce the pressure in the normal head to 150psi for continued testing and I'll leave the other head at 170psi since that parallels heads being sold. Here is a picture of the two heads:

 

[Fabian]

?

Check out this experimental head from CR Machine.

Copper inlayed squish band and a compression release.


View attachment 48721


View attachment 48722

Oh my gosh - that's one heck of a good looking cylinder head.

Can i bolt three of them on my engine

 

[Fabian]

Previous post in this space deleted due to Jaguar taking issue with it's content as well as respecting the OP's wishes.

 

[Deleted member 12676]

Fabian, why oh why are you talking business on this thread? There's no reason you can't do that by email or clicking on his username and sending him a text message via this forum.
Do I ever do that on any of your threads?
a little common courtesy, please

 

[Fabian]

I didn't mean to offend Jaguar.
Just that i am somewhat interested to get my hands on the CR Machine Manufacturing cylinder heads to start messing around with the different compression ratios of the respective cylinder heads that use a single centrally mounted spark plug location.

I have made mention of using your Jaguar CDI's; in fact i have two of them mounted on my bike (and i have purchased a total of 5 Jaguar CDI's) so it is not in any way a different situation.

however

I will respect your feelings and remove my offending post in this thread which seems to have caused an issue.
Scroll up three posts and you will see that the content has been removed.

 

[Deleted member 12676]

final test results with muffler exhaust (not expansion chamber):
160psi standard head
55.6kph 320 degrees

160psi squish head
56.1kph 370 degrees

The squish head was run with the most retarded total ignition curve setting.
Both heads got the best top speed with the most advanced final curve setting.

I just took off the squish head and measured the squish clearance and it was .82mm.
The difference between the two speeds is .9%. So most of the speed increase previously recorded was due to a longer burn (burning the squish-hidden 10% of fuel/air) increasing the exhaust temperature which caused a higher peak rpm of the expansion chambers powerband.

My final conclusion: Squish bands are detonation preventors for high compression. For engines that rev higher than the 8300 rpm of mine they may also significantly boost top rpm power. Until I test at those rpm I can't do anything but speculate with what I know about how they act on the fuel/air mixture. On these small engines it's really hard to design a head with a squish band that allows less than 160psi. I had to increase the dome size and add an extra gasket (.2mm thick) in order to get the psi down to 160. I designed a head for a 125 with squish band and there was 25% more inner dome diameter which means more volume so that the compression doesn't have to be so high.

 

[HeadSmess]

i got bored.... so i made a jig. it turns hemispheres it needs more work still, but initial trials are a success! mounting the tool is tricky still....

this is for the making of heads, obviously.



not sure what order these pics are in so lets say, this is the jig!



im guessing this might be the first hemi i tried? hard to focus. got a good pic of the motor rating plate?



and all the jig is, is a rack on my cross slide, engaging with a gear driving a shaft connected to a small turntable.

i left all the shavings lying around cus it gives me this subconscious urge to blow it off when i look at a pic... sometimes i actually try!

maybe i should start my own thread now?

 

[Deleted member 12676]

Show us the final product when it's done.
I forgot to mention that a squish band head is also good for extending the rpm range when used with an expansion chamber. That is in lieu of raising the exhaust port.