Compression Ratio Estimate

There are quite a few different estimations of CR floating around and I understand there are many variables in this estimation. I am slightly concerned about getting proper fuel for my bike to prevent engine damage since I reduced the squish band significantly. I started mixing 2 gallons of 116 race fuel to  4 gallons of 92 pump gas just to be safe since I have no good estimate of current CR.  I have race gas readily available so this is no problem, and I like the way it smells.

Would someone take an educated guess at my CR just for my peace of mind, please? 

Here are my engine specs:  67 mm BBR piston; 4mm stroked crank; some head porting by Terry Miller; .010 base gasket and .016 head gasket for a squish band of .038.

 Also, approximately how much did I raise CR by reducing squish from 0.085 to 0.038? This adjustment really made a noticeable difference in performance according to my butt dyno.

The tricky part to me is measuring any pop up dome volume on the various pistons available.  Wossner has a very short or thin pop up area, the BBR lots more.  Might you know what the dome volume is?  Do you know what the CR was supposed to really be with just bore and OE gaskets or the gaskets BBR includes?  We could determine what the pop up dome volume is with some reverse mathing.

Making the squish a more proper 0.038" would reduce cylinder volume by 4.2cc.  With a flat top piston your CR should be about 11:1.  

If the dome volume is 2cc that would bump CR up to 11.9:1

If dome volume is 3cc that would bump CR up to 12.4:1

4cc would be 12.9:1

5cc would be 13.5:1

This is of course if I am doing the math correctly. :)

 

I think you are going to be higher than expected because the 4mm stroke with no other changes will raise CR because it increases swept volume. I just did a 67mm BBR piston with stock stroke and the same squish you have. I even machined .025 off the dome and still came out to about 12 to 1. Your 4mm stroke will make it much higher. 

Did another with 6mm stroker and Wiseco piston with half the dome the BBR piston has. I set the squish to .040 with gaskets and the CR was over 12.  I didn't have time to machine the dome so I had to widen the squish to get down where I needed it. Sorry, Im going by memory because the bikes are not with me. Your setup with stroke and tight squish sounds like 13.5ish to me?

The bottom line for anyone doing a stroker is that even the low dome pistons will probably be too much for pump gas. You will either have to machine some of the dome off (leave .110 min thickness), run a thicker squish band or use flat top pistons to get under 11:1

If your bike is together, finding the actual CR is easier. The reality check is to fill the chamber with light oil. Position the engine or bike so the plug hole is level. Fill with a large syringe marked in cc's about 3mm up into the plug threads to account for plug volume. Then use swept volume (bore x stroke) plus chamber volume divided by chamber volume.

I'll bet most people come out higher than they expected. My calculations were always low.

Terry Miller uses 22-23 cc chamber volume and 3cc dome for wiseco. The BBR dome is twice the height but tapered so I estimated it at 5cc.

There are many ways for error to creep in,.. Its hard to know what actual gasket crush will be and aftermarket gaskets can be way off. Aftermarket valves often have thicker heads or non dished valve heads, Piston domes with valve pockets are a guess,..

Filling the chamber with oil is the most accurate way to confirm. 

Tip,. its hard to hold the engine at TDC for this check because the rotor magnets pull it away. I found a metric drain plug at the auto parts store that fit the top inspection plug threads. Drill the biggest hole you can through the plug so you can see the line on the flywheel. Position the flywheel and snug it down to hold it in place. 

 

Edited by woodsryder

Well I’m running Terry’s own 13-1 69mm Pistons (with std stroke) in an engine with a 4mm stroker (262cc) and another with a 6mm stroke. (269cc) on “100%” Sunaco 100 Octane with out any detonation or starter issues at all. Both with .038 squish.

 

I have another 273cc (69.5 Bore with 6mm stroker) engine on the bench ready to install with the same compression and squish. I do not expect any issues there either.

 

So why run a flat top piston at all unless your running Colman Lantern Fuel?

 

IMG_2139.jpg

IMG_2137.jpg

I went riding today where my only fuel choices at the pump were 87 octane 10% ethanol or 90 octane non ethanol.

I wish I had some lantern fuel.

On ‎1‎/‎29‎/‎2018 at 7:56 PM, jeffrow68 said:

I reduced the squish band significantly

 

On ‎1‎/‎29‎/‎2018 at 7:56 PM, jeffrow68 said:

for a squish band of .038

Technically speaking you have reduced the "squish gap" which is a good thing.

From what I've found in my recent research, a small squish gap, less than 0.040", improves the engines ability to resist detonation by eliminating small pockets of air/fuel mixture.  The worst size gaps are in the 0.060 to 0.120, IIRC.  These promote detonation.  Trying to lower the compression ratio by using a thicker head gasket can actually increase detonation if the squish gap gets too wide.

Another thing that needs to be done to improve detonation resistance is to use less timing advance.  Squish and a small compact combustion chamber allow for the mixture to burn completely with less timing advance.  Less timing advance makes for less residual heat in the CC as well as moving peak cylinder pressure closer to TDC.  All win-win!  The OE ignition box might just work the best for this.  Instead of compensating with more timing advance, compensate with more compression.

17 minutes ago, MetricMuscle said:

 

Technically speaking you have reduced the "squish gap" which is a good thing.

From what I've found in my recent research, a small squish gap, less than 0.040", improves the engines ability to resist detonation by eliminating small pockets of air/fuel mixture.  The worst size gaps are in the 0.060 to 0.120, IIRC.  These promote detonation.  Trying to lower the compression ratio by using a thicker head gasket can actually increase detonation if the squish gap gets too wide.

Another thing that needs to be done to improve detonation resistance is to use less timing advance.  Squish and a small compact combustion chamber allow for the mixture to burn completely with less timing advance.  Less timing advance makes for less residual heat in the CC as well as moving peak cylinder pressure closer to TDC.  All win-win!  The OE ignition box might just work the best for this.  Instead of compensating with more timing advance, compensate with more compression.

+1 BUT my experience has been that the stock 230 cdi has a timing curve that comes in to late even with the flywheel advanced 4° the used early 150f box I bought was no good so I could not test it. Went back to stock initial timing and put the pro com back in. Much better response down low. This is on my 8mm stroke 65.5mm wiseco piston build. 

I have a Pro Com Box on my bikes for above 4K elevation and A 2005-Older 150f Box for less than 4K elevation (if required)
Example:
I went to Carnegie Cycle Park this last weekend at 1K elevation, 70d temps, perfect weather, no detonation at all. I was expecting it and was prepared for it but none that day.

But then I run Sunoco California Pump legal race fuel like I always have...maybe this batch had some 110 leaded in the can that I was not aware of?

My riding partner complained that my new rear tire was throwing way to much debris so I think that I was on the throttle as much as I normally am?

What is the advertised compression ratio of the BBR 230 piston?

10 hours ago, ricky racer said:

+1 BUT my experience has been that the stock 230 cdi has a timing curve that comes in to late even with the flywheel advanced 4° the used early 150f box I bought was no good so I could not test it. Went back to stock initial timing and put the pro com back in. Much better response down low. This is on my 8mm stroke 65.5mm wiseco piston build. 

So OE bore Wiseco piston which has a 3cc dome I believe.  

What head gasket, base gasket, deck height?

What cam shaft?

With OE gaskets and 0.030 deck height you should have just under 10.5:1 CR.

Things which allow you to run more compression -

-  Less timing advance.

-  Small combustion chamber with proper squish / quench area.

-  Hot camshaft that bleeds a bit of compression at lower rpm.

-  High octane fuel.

It is my understanding that the ProCom advances ignition timing in the first half of the rpm range but then it is the same as OE from half way to redline.

5 hours ago, MetricMuscle said:

So OE bore Wiseco piston which has a 3cc dome I believe.  

What head gasket, base gasket, deck height?

What cam shaft?

With OE gaskets and 0.030 deck height you should have just under 10.5:1 CR.

Things which allow you to run more compression -

-  Less timing advance.

-  Small combustion chamber with proper squish / quench area.

-  Hot camshaft that bleeds a bit of compression at lower rpm.

-  High octane fuel.

It is my understanding that the ProCom advances ignition timing in the first half of the rpm range but then it is the same as OE from half way to redline.

.007 single layer stock head gasket. .010 base gasket. .038 squish after touching up cyl. and head surface. should be 11.77-1 cr ST2.5 cam The procom not only brings in more but brings it in much sooner. More of a performance curve. Stock box's are way to lazy even with 4* advance. Mine feels better from idle to the top with the procom tested back to back with the stock box. Yes I mean to say topend feels stronger also. 10.5-1 lol you forget to change the stroke to 74.2mm in your calculation?

13 minutes ago, ricky racer said:

.007 single layer stock head gasket. .010 base gasket. .038 squish after touching up cyl. and head surface. should be 11.77-1 cr ST2.5 cam The procom not only brings in more but brings it in much sooner. More of a performance curve. Stock box's are way to lazy even with 4* advance. Mine feels better from idle to the top with the procom tested back to back with the stock box. Yes I mean to say topend feels stronger also. 10.5-1 lol you forget to change the stroke to 74.2mm in your calculation?

LOL, no, I was using OE gaskets in my calculations so more like a 0.073" squish.

So more advance means longer burn so maybe making the CC smaller would help the OE CDI work better.

24 minutes ago, MetricMuscle said:

LOL, no, I was using OE gaskets in my calculations so more like a 0.073" squish.

So more advance means longer burn so maybe making the CC smaller would help the OE CDI work better.

I really doesn't matter what compression you build into anything. The stock cdi box is to lazy for big smiles. Most engine builders shoot for full advance no later than 2500 rpm. I bet the stock box is not full in until 5000rpm. TO lazy. I'm not saying more total advance just bring it in sooner=bigger smiles.

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