Combustion Chamber Design

9/19/2012

As you know, I spend a lot of time refining my engines with new ideas. I also spend a great deal of time cutting new combustion chamber designs.

Recently, I have found that making my head buttons from 360 brass seems to reduce the amount of detonation on my engines. I will stay with this material until I find something that is better in regard to the detonation problem.

As for the recent findings of combustion chamber design and specifically head button design, I will give you some information that will be helpful to you.

First, I have found that Squish Velocity should be in the area of 55-60 m/s in order to not get a lot of detonation and also not burn a lot of plugs.

I found that you can gain a ton of low and mid range power by making the volume of the chamber smaller and using a squish band with 3 degrees of angle on 95% of the band. The outer 5% remains flat.

It is also good to keep the plug as close to the center of the chamber as possible. This can be done in 2 ways. Either you can make the bowl shallow or you can pull the plug element a little to place the element in the middle (centroid) of the chamber. Both will work well. I have started using the first one with the chamber shallow.

The amount (in percentage) that the squish band represents as the total area of the bore is also very important to the power that you can develop in your engine. I personally have found that a squish percentage of 65-70% works best for me. Not to say that my opinion might change as I try and test more buttons.

One additional VERY IMPORTANT characteristic of the great head button is the fit between the button and the liner. I have found that a total clearance of between .001” and .0015” does not make the button expand the liner. I learned this the hard way. I lost seal in recent engines because I had the fit to close. Norris Sparks figured out why I was having this problem and since have been using the wider clearance with perfect results.

In Review (things that contribute to a great head button/combustion chamber:

1. Squish Velocity of between 55-60 m/s
2. 360 Brass as the material for the button
3. Smaller Volume combustion chamber
4. Squish angle of 3 degrees for 95% of the band width
5. Squish angle of 0 degrees for 5% of the band width
6. Shallower combustion chamber or pull the plug element
7. .001”-.0015” button to liner clearance

I hope this technical information is useful to you and if you have comments, please don’t hesitate to start a thread on International Waters.

One additional thing that I might not have indicated in the past…. The Squish Velocity Plugin, for calculating squish velocity, is a free standing application. You do not have to have the Engine Analysis Software to use this valuable tool.

A side note: WoodRCBoatProducts (Steve and Chris Wood) have available brass head buttons for many of the popular engines that have many of the characteristics noted above. Visit them for your requirements. I know that the .101 racers are super happy with their new brass buttons.

MartyDavis
Developer of the Engine Analysis Software