Squish Velocity as a Part of Head Design


Some time ago I realized that I MUST solve the detonation on my .21 engines if I wanted them to last more than a few races.  I had to replace the piston and head button several times during the season.  Engine was running terrific, but there must be a better way…….


I talked to some people who I value their opinion and found that I was forcing the engine to detonate with a combination of WRONG design criteria.


OK, what to do to solve this problem and not lose performance.  I found that there was a combination that would solve this problem EASILLY and not lose any performance.  In fact, I found that I gained a ton of performance.


There are several things that contribute to the efficient head button design which does not detonate.


1.                  The material of the button.

2.                  NO FLAT AREA on the squish band at the outer edge.

2.                  The Squish Band width as a percentage of the total bore area.

3.                  The Compression Ratio.

4.                  The Head Button Clearance (Head button to Top of Piston at TDC).

5.                  The Squish Band Angle.

6.                  The Maximum RPM desired.  (If torque engine is desired a different setup is required)

7.                  The fuel which is run.

8.                  The Squish Velocity (MSRV)



To make all these things work together is the hard part.  UNTIL NOW…


Let’s talk about each of these items and how they contribute to the efficient design of a head button.


The Material:

I read a post on IW that was from Dave Marles.  He is someone that I consider to be at the top of competitors.  He indicated that he was using Brass as his head button material.  I decided to try it because it seemed logical that this material would expand at a similar rate as the liner.  I selected 360 brass available from http://mcmaster.com.  It was indicated by Dave that the brass button would not detonate as much as aluminum.  I found this to be true. 7075 aluminum also works well for material and also available at McMaster.com


Squish Band as a Percentage of Bore Area:

The can only be found by trial and error and is probably dependent on several factors.  Boat weight and ease of launch, fuel nitro content, etc, etc.  I have found that between 70 and 75% is optimum for our use. 


Compression Ratio:

This is one that I have struggled with over the years.  I have run engines that have HUGE compression ratios and have recently softened to a lesser ratio.  The lower the compression ratio (within reason) the more RPM that you can use without burning your glow plug. However, the less
HP you will make and the engine will work harder.  This is dependent on many things though including boat weight, nitro percentage, pipe, etc etc.  I am currently running a compression ratio on my .21 engines of 10.25:1 where I was running 11+ last year.  My RPM is much higher and I still have about the same amount of launch ability.  I suggest that you determine this for yourself.


Head Button Clearance:

The clearance of the head button squish band to the piston at TDC is super important and I suggest that you use .006” for a .21 engine, between .008” and .010” for a .45, .010” to .012” for a .67, .013” to .015” for an .80-.101 engine.  These clearances will allow you to have good low end torque.


Squish Band Angle:

The squish band angle can be used to “TUNE” squish velocity.  You can run a flat squish band, but I would suggest that you use no less than 1 degree.  I would use between 1-3 degrees as your tuning factor for the final squish velocity. I personally use 2.5 degrees.  Run the angle ALL the way to the edge of the squish band.  NEVER leave the slightest amount of flat area at the outside edge as it makes the squish velocity off the chart in that area and will detonate like crazy in that flat area and on the top edge of the piston.


Maximum RPM Desired:

This is something that is required to input to calculate squish velocity.  It is important for you to have a close approximation of the maximum rpm that your engine is turning.  We sure have a need for a good, inexpensive Doppler rpm measurement device.  You can use the currently available add-ons that will measure the rpm during runs.  Not super expensive, and very accurate. 


Fuel Percentage Run:

The fuel percentage is important in the generation of heat and POWER.  The squish velocity calculation does not consider fuel nitro percentage, but it is important in the amount of heat that you will generate and finally probably a contributor to detonation.

I would suggest that I have found the optimum nitro percentages for various engines as follows:  .21 and smaller engines = 60-65%, .45 engines = 65%, .67 and up engines = 50%.  These nitro percentages will produce about maximum power for heat racing without excess heat.


Squish Velocity:

The final part of the puzzle and the calculation of all the input items is Squish Velocity.  Until now, we have NOT had an easy way to calculate SV.  I have developed an applet that will do this in an efficient manner and output the MSRV (Squish Velocity).  After extensive experimentation, I have found that we can get maximum output of our engines with a head button that develops a SV of between 68m/s and 74m/s.  At 75m/s you get some detonation and below 68m/s you do not get optimum performance.


How do you get the Squish Velocity Applet?  It is available at http://1nitrorc.com.  There is no information there except for a place to order it currently.  I have not had time to update the website to discuss the newest software.  I have plans to do that.  I wanted to offer this important tool to the racing public even before the website was updated.  The link to the order form is:  http://1nitrorc.com/eaporder.htm  I will send via email rather than send a disk for this applet.


The Engine Analysis Software is not required to run this stand alone tool. Microsoft Excel is required to run the applet.


Note:  Getting close to releasing the newest Engine Analysis Software with MANY new features and enhancements and the squish plugin as part of the new version.



Marty Davis





ENGINE Analysis Software for the Serious RC Competitor



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