Compression Ratio

"Technical Notes 12/15/94"

One of the purchasers of "The Engine Analysis Program" also bought my "Prop Duplicator" and really liked the Prop Tech Notes section. He suggested that we do the same for T.E.A.P.

Great suggestion!!

I will offer my experiences every month - hope you find something which will help you.

Lets look at two items which I feel will gain you the most in terms of performance and reliability. They are: COMPRESSION RATIO first and EXHAUST TIMING second.

COMPRESSION RATIO: When considering compression ratio you should recognize some fundamental items. The compression ratio will change with the following:

1. Changing of the volume of the head.
2. Changing the Exhaust Timing.
3. Changing the Squish Band/Piston Clearance.

Changing the Head Volume: If you change the volume of the combustion chamber (Head) you greatly affect the compression ratio. You should use the Head Design Module of T.E.A.P. only after you have set the Exhaust Timing.

Changing the Exhaust Timing: Remember that any change in the height of the exhaust port will change the amount of volume of fuel/air which will be compressed after the exhaust port closes, as the piston moves up toward TDC. I like to have enough exhaust timing to allow the engine to gain the highest practical RPM and allow my pipe length to be long enough not to lose power coming off the turns. (If you are looking for record trial straightaway speeds, you can use a shorter pipe length to great advantage but not for heat racing and acceleration) I currently use timing of 183-185 degrees for my 21's, 185-187 for 40's, and 188-190 for 67's. I don't run 80's and 90's but I work closely with several who do and we use 190-194. You can see that there is a pattern that the larger the engine the higher the exhaust timing. There are several well respected people who believe that a raise of the exhaust timing should be accompanied by a raise in intake timing also. I really don't use this theory. I have always felt that the intakes should be left within a range of from 123 to 127 degrees. I feel that in the hottest time of the year you lose a GREAT deal of performance if your intakes are any higher than 126 or 127 degrees.

Changing the Squish Band/Piston Clearance:

The clearance of the piston to the squish band at Top Dead Center is VERY important. This determines several things including detonation (sand blasting of the piston top and head combustion chamber), and power output as a result of the close squish band piston clearance forcing the fuel mixture toward the center rapidly with a huge rise in pressure before detonation. If you get a lot of detonation, most people raise the clearance of the piston and squish band. Don't do that since the detonation will be made worse. Change the volume of the chamber and keep the clearance tight. This tight clearance allows the rich charge to be in a very thin layer between the piston and squish band, which allows a rapid cooling between these surfaces. So, decide what clearance you want for the squish band/piston at TDC and then work from this point. I suggest what has worked well for me is .008 to .011 for 21's, .010-013 for 45's, .011 to .014 for 67's and .014 - .017 for 80's and 90's. I usually run on low end of these measurements for maximum acceleration. If you will watch race boats with a different vantage point - ACCELERATION - I think you will see some very important heat racing considerations demonstrated. You have seen some really fast guys who get beat badly in heat racing because they don't accelerate off the buoys. Watch closely and I think you will see a different vantage point!

What then is the best compression ratio?

This is a very difficult question to answer since there are several very important variables to consider:

  • 1. What does the boat weigh?
  • 2. How easy is the boat to push? ( is it setup loose or tight)
  • 3. How is it propped? (large/small blade area, medium/high pitch)
  • 4. What Percentage Nitro do you run?
  • 5. Are you an "On the Buoys" racer or "Fast Outside"?

As you can see this is a VERY difficult thing to indicate since there are many variables to consider, and each affects the other.

So, what is the order to correctly set the compression ratio?

  • 1. Decide what Exhaust Timing you want.
  • 2. Decide what squish band/piston clearance you want.
  • 3. Decide where you fit on the course.
  • 4. Decide what squish band width you prefer. (The wider the more violent)

Remember that the fuel/air mixture is the main coolant for the engine. Keep the mixture on the rich side of the needle and you will see consistent performance and longer engine life. You don't need to burn a plug every run to know that your engine is performing correctly. I normally don't use many plugs, I try to stay on that rich side of the needle and with the correct compression ratio, to keep the plug, and get the performance.

I hope this will stimulate your thought processes. I can't guarantee that these figures will be perfect for you, since the variables are huge (I guess that is what makes this so much fun!)

ENGINE Analysis Software for the Serious RC Competitor



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