Calculating compression ratio - which method?

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Tim van Schyndel

Well-Known Member
Joined
Jan 22, 2013
Messages
99
I have read two different ways to calculate the compression ratio which involves either the full cylinder swept volume or volume from top of exhaust port to TDC.

My questions are:

  1. does either method matter as long as you always use the same method?
  2. what ratio to aim for with 50% nitro (non toroidal head buttons) for either method?
  3. When measuring squish, does the "slack" (which is taken up by the measuring) in the main and big end rod bearings have an impact on the measurement. e.g. I use a small piece of solder through the glow plug, turn the motor over and measure the squashed lead solder. As this would take any force all bearings down, does it represent the true squish. I thought it would considering it is the compression stroke although it would not take into consideration oil and the crank counter balance applying forces in the opposite direction.
Any advice much appreciated.

Tim
 
I use top of exhaust to top of liner in EAP program for compression. Head clearance I also use solder, but cut a piece width of bore then install head button. Turn engine over and measure both sides or some use plasti gauge. OMHO
 
I feel that the "compression ratio" in a two stroke is an arbitrary measurement that you can argue about for ever. For simplicity I always use the geometric ratio because it doesn't depend on exhaust port height so it's a simple calculation. Except for the Japanese, that's usually the standard method for comparison. In any case the compression ratio is only a starting point. Our one piece cylinder engines can't have a high enough compression ratio to matter. See the chart for reference.

Compression ratios.JPG

The squish is another tricky subject. I always use a dial indicator in the head, see below. Push the piston up until it hits and set the indicator to zero. Assemble the engine and measure again for the clearance. This works well down to around .010" (.25 mm) squish clearance. If you want to run less than that you need to match machine the head and piston. For reference, we ran .002" (.05 mm) in a Quickdraw. There were very light piston strikes on the head button. That gives an idea of the "stretch" in the assembly at around 20,000 rpm.

Timing measurement.JPG

Lohring Miller
 
I feel that the "compression ratio" in a two stroke is an arbitrary measurement that you can argue about for ever. For simplicity I always use the geometric ratio because it doesn't depend on exhaust port height so it's a simple calculation. Except for the Japanese, that's usually the standard method for comparison. In any case the compression ratio is only a starting point. Our one piece cylinder engines can't have a high enough compression ratio to matter. See the chart for reference.

attachicon.gif
Compression ratios.JPG

The squish is another tricky subject. I always use a dial indicator in the head, see below. Push the piston up until it hits and set the indicator to zero. Assemble the engine and measure again for the clearance. This works well down to around .010" (.25 mm) squish clearance. If you want to run less than that you need to match machine the head and piston. For reference, we ran .002" (.05 mm) in a Quickdraw. There were very light piston strikes on the head button. That gives an idea of the "stretch" in the assembly at around 20,000 rpm.

attachicon.gif
Timing measurement.JPG

Lohring Miller
Thanks Lohring

Can you please expand on what you mean by "Our one piece cylinder engines can't have a high enough compression ratio to matter. See the chart for reference."

I was of the impression that too high CR was a major contributor to detonation?
 
Oops, I'm so accustomed to writing about gasoline fueled engines that I forgot about nitro. Adjusting the head button on a nitro engine is critical for performance. That's also true for the very few gasoline engines that don't have one piece cylinders. The head volume on a 26 cc Zenoah is around 2.4 cc while we tested 26 cc Quickdraws with 1.8 cc on the same fuel. Even lower head volume is possible with a toroidal combustion chamber.

My last experience with nitro engine testing was on some 11 cc engines. We ran 3 different engines on our inertial dyno and melted the pistons on every one. The load was too high and the detonation in the lower rpm area caused the problem. These were highly tuned engines with the heads optimized for their operating rpm. That means they were on the edge of detonation and the higher load pushed them over that edge. The chart I posted is for larger engines, but the methanol values might be a place to start. I usually try to get the squish clearance below .010" then adjust the head volume while checking for the sand blast effect of detonation. Smaller engines can run tighter squish and higher compression than larger engines. The head volume for a particular engine is the key value. The compression ratio is a general reference.

Lohring Miller
 
I tend to go with the volume of the head button for a particular motor size more than anything, which would equate to full volume C/R I guess.

"Effective" trapped C/R can be influenced a bunch by porting and the pipe and how they let the cylinder charge with the return wave IMHO.

Most motors had somewhere between 12 & 15:1 outta the box until the CMB 45 VAC came along at 24:1! I've since played with much smaller head buttons and they can work well, but if you cross the line with the needle the plug will black hole immediately. I've also found that the higher your nitro content the smaller the button you need which is kinda opposite to most thinking.

Run a little rich with a small button, 60-70%, hot plug and water cut back = hang on!
default_smile.png


Here's some specs for CMB's:

https://www.intlwaters.com/gallery/displayimage.php?album=70&pid=17627#top_display_media

https://www.intlwaters.com/gallery/displayimage.php?album=70&pid=17634#top_display_media

https://www.intlwaters.com/gallery/displayimage.php?album=70&pid=14048#top_display_media

https://www.intlwaters.com/gallery/albums/userpics/10068/2005_CMB_CAM_45.xls

https://www.intlwaters.com/gallery/displayimage.php?album=70&pid=13980#top_display_media

ps: a little frosting on the head and piston isn't a bad thing, just means you're "there".

pss: I too use solder to check the squish & run everything between 0.010" - 0.012", pretty sure the rod is under compression ATDC so this works to take out any play.
 
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Thanks Terry and Lohring for all he information.

It is a 45 VAC that I am experimenting (and learning with) in a JAE rigger running a cupped 1860 (which it loves) prop on 50%.

The head clearance without any shims is measuring at 0.010" (0.254mm) at the very outer edge of the squish. I was running a 1.2mm hole diameter for the head cooling and was finding it impossible to get heat into the engine without leaning to the point of blowing plugs and some detonation. I have to admit though that while running cold the engine was a bullet. The engine also loved pulling the plug element out.

I have dropped back to a 1mm (0.040") cooling hole which seems to be more on target. I will also try a 0.8mm cooling hole to keep the heat while running a richer needle.

Terry - is this on target with your experience with the VAC?
 
Few things I was told to do with 45 vac, from a good source.

1 Lap the carb barrel

2 Open button 24 thou with 1/2" ball mill. Don't change the depth. Gave me .29cc head vol.

3 On open side of drum cut .010, not the housing.

4 Polish bottom rod journal with 1000 grit.

5 Check oil slit bottom of rod.

6 Check run out on flywheel.
 
Thanks Terry and Lohring for all he information.

It is a 45 VAC that I am experimenting (and learning with) in a JAE rigger running a cupped 1860 (which it loves) prop on 50%.

The head clearance without any shims is measuring at 0.010" (0.254mm) at the very outer edge of the squish. I was running a 1.2mm hole diameter for the head cooling and was finding it impossible to get heat into the engine without leaning to the point of blowing plugs and some detonation. I have to admit though that while running cold the engine was a bullet. The engine also loved pulling the plug element out.

I have dropped back to a 1mm (0.040") cooling hole which seems to be more on target. I will also try a 0.8mm cooling hole to keep the heat while running a richer needle.

Terry - is this on target with your experience with the VAC?

This sounds like Deja Vu, I just had this very conversation with a buddy yesterday. He too was trying to run cold plugs (OD99 IIRC) and overcooling. Couldn't get it to "go" w/o leaning down the needle and consequently taking out the plug. The OD287 is my go to plug after burning the seals on K&B HP's. The 277 is good too but doesn't hold up as well in the bigger motors.

Got a flow meter? Stick it on the outlet of the motor and flow the entire cooling system out the rudder. Just checked my 45 VAC and I'm down at 2.5! The mid point on my fuel needle is at 7.25 (old school propane meter). If I run a bunch of hot laps and bring it in quick I can hold the cylinder for only a sec or two, not real scientific but works for me.

I've found it best to restrict the water right at the rudder rather than at the head, I've split cooling lines restricting at the head. I use drilled out 4-40 brass set screws (Mcmaster Carr has them) and tap the water fitting in the rudder (Carl Brey started this). I have a range from 0.010 to 0.040" by 0.005", trying to burn 70%+ in the SAW boat at 35F air and 35F water needs the 10 thou! One of my FAST buddies Mark Stein soldered up a few fittings and drilled them out, the Murany's came up with a cool t-fitting with a rounded off screw on one leg, etc.

10thou_004.jpg


A full blade 1860? That's a big wheel unless you have a lot of it blanked out with your center ski. I'm running them cut down to 56mm with a little tongue/back cut and some cup, low 80's if I want but like to pull the pipe out a bit, run high 70's but still be able to idle around and GO if needled.

Disclaimer (lol): If you try this sneak up on it slowly. The Nova Rossi's for example come with smaller cooling jackets and other factors (pipe stinger size, cowling config etc.) can affect cooling. Richen the needle WAY up to start! Run lean and hot with a heavy fuse (cold plug) and you'll hurt stuff!

ps: wasn't my Crew Chief that came up with this, Andy Brown "enlightened" me years ago...
 
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Few things I was told to do with 45 vac, from a good source.

1 Lap the carb barrel

2 Open button 24 thou with 1/2" ball mill. Don't change the depth. Gave me .29cc head vol.

3 On open side of drum cut .010, not the housing.

4 Polish bottom rod journal with 1000 grit.

5 Check oil slit bottom of rod.

6 Check run out on flywheel.
My early (2012's) came 0.29cc outta da box, piston has a 0.2cc dish as well.

I run mine bone stock (honest!).
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Using the same method that is in Terry's gallery for measuring the cc on the head, all 4 of them that I have had come out to .35cc. These motors are 2014 and newer.

Terry,

Thanks for the info on the phone and sharing this. We'll give it a shot.
 
Using the same method that is in Terry's gallery for measuring the cc on the head, all 4 of them that I have had come out to .35cc. These motors are 2014 and newer.

Terry,

Thanks for the info on the phone and sharing this. We'll give it a shot.
No problem!

Did your motors come with a dished piston as well?
 
The last few years I've measured the heads at .36cc , close to Mike, they work awesome. Never had the earlier .29cc head. The latest V-5 head has a wider squish and deeper bowl, it also works very well.
 
Do the later .36cc heads come with a completely flat squish band like the earlier .29cc? How 'bout the squish area? These are about 70%.

Mike, here's the head and piston after about 25 runs as discussed:

DSCN3104.JPG


DSCN3094.JPG
 
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Few things I was told to do with 45 vac, from a good source.

1 Lap the carb barrel

2 Open button 24 thou with 1/2" ball mill. Don't change the depth. Gave me .29cc head vol.

3 On open side of drum cut .010, not the housing.

4 Polish bottom rod journal with 1000 grit.

5 Check oil slit bottom of rod.

6 Check run out on flywheel.

Thanks Ray

Can you explain 4 & 5?

What does the polishing on the rod journal do?

What do you mean by "check" the old slit?
 
Hi Tim oil slots on the rods should always be checked on a new engine for burrs which can cause blockages ..Get a sharp razor blade and scrape the slit clean for good oil flow,,and could balance flywheel if keen..
 
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Do the later .36cc heads come with a completely flat squish band like the earlier .29cc? How 'bout the squish area? These are about 70%.

Mike, here's the head and piston after about 25 runs as discussed:

DSCN3104.JPG


DSCN3094.JPG
Terry, I have one of the last VAC's sold here in Aus. The head has an angled squish and a very small flat section on the outer edge. Flat piston.

It's interesting that the detonation on your motor occurs across the entire piston. Mine only seems to occur mostly on the exhaust port side. I have also noticed that a majority of the pitting occurs on the head, not the piston.
 
Hi Tim oil slots on the rods should always be checked on a new engine for burrs which can cause blockages ..Get a sharp razor blade and scrape the slit clean for good oil flow,,and could balance flywheel if keen..
Thanks David

What do the polishing of the rod journal do? Are we attempting to literally "polish" or add a type of honing grove for oil or as you have described with the rod, remove any burrs?

Cheers
 
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