# 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?

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 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 -

-  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 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 -

-  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.

## Create an account

Register a new account

• ### Similar Content

• By 230F
Jetting the 230F
By: Phil Vieira
This project takes no less than 2 hours if you have never done jetting to a bike before. It took me 1.5 hours, to take my bike apart, take out the needle, change my pilot jet and the main, and take pictures along the way, but I have seen the inside of my carb 3 times, so I know my way around it pretty well…
You should be jetting this bike right when you get it home. This bike comes lean from the factory. If you don’t know what that means, it means that the bike is getting too much air, in terms, a hotter engine, and your plugs will get hotter, and a decrease in HP. To make your engine last longer, do this.
These jetting combos are for a 2000 feet and below scenario. Any altitudes higher, you should do a search on the forum. If it cannot be found, post on the forum. Please don’t post on the forum “How do I do this…” You have all the answers here.
This project comes to a grand total of less than 30 dollars. The needle is 20, the main jet is about 3 dollars, and the pilot is 5 dollars. You may not need to do the pilot jet depending on your situation, but again, if you’re riding 2000 feet and below, it’s a good idea to get a pilot jet.
The jets I used consist of a 132 main, 45 pilot and the power up needle with the clip on the 4th position.
Part numbers:
16012-KPS-921 – Needle (Includes Power up needle, Clip, and needle jet)
99113-GHB-XXX0 – Main jet (Where XXX is the size)
99103-MT2-0XX0 – Pilot jet (Where XX is the size)
For the Jets, just tell them you need jets for a regular Keihn carb, (also known as a Keihn Long Hex) main jet size XXX, pilot jet size XX. They should know the part numbers. For the needle, bring the number along. If you are lazy, they should have a fiche and they can look up the numbers. Then again you can take in the old jets, and make sure they match up to the new ones.

Now, the tools you will need are as follows:

~A collecting cup of some sort. I used a peanut butter jar.
~Ratchets for the following sizes:
- 6mm, 8mm, 10mm, 12mm
- Extension for the sockets needed
~Phillips and Flathead screwdriver (Be sure these are in perfect condition. A badly worn screwdriver will strip the screws)
~Needle nose pliers
~”Vise grips” or known as locking pliers (Two)
~Open end wrench 7mm and 12mm
~ It’s a good idea to have a extra hand around
(Not needed, but I highly recommend tiny Phillips and flathead screwdrivers (Pictured next to the jar and the ¼” extension) I recommend these for removing a couple things since you can put pressure with your thumb on the end and unscrew it with the other hand. This insures that you will not over tighten any parts, and ensure that you will not strip the heads of the bolts.
Ok, now that you have the tools, let’s start by putting the bike on a bike stand. I put it on the stand rather than the kickstand because it’s more stable and sits higher. I hate working on my knees. Start by taking the number plates off. Yes, both of them. The right side, you take off one bolt and the top comes off of its rubber grommets, pull the top off, and the plate comes right off. The left hand side, use the 10mm socket to take the battery bolts off, and then take the Phillips bolt near the back. Again, rubber grommets are used to hold the top in place. Take the seat off. There are two mounting bolts on the back:

Those two bolts are both a 12mm socket. Use the open end wrench on the inside, and use the socket on the outside. You may need to use an extension if you don’t have a deep socket. Once you have the two bolts off, slide the seat back, and lift it up. This is what you have. Notice there is a hook in the middle and a knob on the tank. That is what you are sliding the seat off of.

Now that the seat is off, you must take the gas tank off. Don’t worry, you won’t spill any gas any where, I promise. On the left hand side of the bike where the valve is, slide down the metal clip holding the tube in place. Turn off the gas supply, and slip the tube off slowly. Now take off the two bolts in the front of the take. This is on the lowest part of the gas tank in the front, behind the tank shrouds. The socket you will use is an 8mm socket. Take the bolts all the way off and set them aside. Now look back at the last picture posted. On the back of the tank, there is a rubber piece connected to the knob and the frame. Slip that rubber piece off of the frame. Pull the vent tube out of the steering stem and lift the tank up. Don’t tip it, and lay the tank aside where you won’t trip on it. This is what you’ll end up with:

It may be a good idea to take a rag, and wipe all the dirt off the top of the bike if any. You don’t want anything dropping down into the carb. If you do, engine damage is the result. A clean bike is always a good thing! Now we must drain the gas out into that container. This is very easy. Make sure you open the garage door, windows, whatever, to let the fumes out. Breathing this crap is bad. Here is where the drain screw is:

(Don’t worry about removing the carb, that comes later) This is on the right side of the carb, on the float bowl. The vent tube that goes down to the bottom of the bike is where the gas drains to. Put the jar under that tube and start to unscrew that screw, enough so that the gas leaks into that jar. Once the gas doesn’t drip anymore, close the screw all the way. Now on to the top of the carb. We are going to take this cover off:

This cover comes off by removing the two screws. Once removed, the lid comes off as well as the gasket. Flip it over and set it aside. Do not set the gasket side down on the ground, as it will get contaminants! Here is what you are facing:

The angle of the camera cannot show the two screws. But one is visible. It has a red dot, and opposite of that side is a darker red dot. I made it darker because it’s not visible, but that is where it is. This is where I use the miniature screw drivers to get the screws. I magnetize the screwdrivers, and use care to make sure I don’t strip the heads. Metal pieces in a piston are not good! Remove the two screws. Put these screws on a clean surface so they do not get contaminants. Now get your vise grips and set it so that it will lock onto the throttle, not too tight, not too loose. Set the vise grips on the seat. Start to open the throttle slowly as you guide that “plunger holder” (as I call it) up to the top. Once you have the throttle all the way open, take the vise grips, and lock it so that the throttle does not go back any more. What I do is I hold it pinned and lock it up against the brake so it doesn’t rewind on me. If you don’t have locking grips, a friend will do, just have them hold the throttle open all the way until you are finished. How fold the plunger holder to the back of the carb and pull the piece up to the top. Take care not to remove it, as it is a pain to get back together! If it came apart on you, this is what it should be assembled to:

Once you get the holder out of the slider, set it back like this:

As you can see, the bar is back 45 degrees, while the holder is forward 45 degrees to make a S. Here is what you are faced with when you look down on the carb:

Where the red dot is where the needle lies. Grab needle nose pliers and carefully pull up the needle out of its slot. This is what the needle looks like once it is out.

Now we must move the carb to take the bowl off. Untie the two straps on the front and back of the carb. Don’t take them off; just loosen them until the threads are at the end. Take the front of the carb off the boot and twist the bowl as much as you can towards you. Tie the back tie down to that it does not rewind back on you. This is what you have:

Now we must take off the bowl. Some people take that hex nut off to change the main jet, which you can, but you cannot access the pilot jet, and you can’t take out the needle jet (a piece the needle slides into), so we need to take it off. It’s just three bolts. As we look at the underside of the carb, this is what you will see:

The bolts with the red square dots are the bolts you will be removing. These are Phillips head bolts, and the bolt with the blue dot is your fuel screw. This is what you will adjust when the time comes, but keep in mind where that bolt is. You need a small flat blade to adjust it.
Well, take those screws off, and you are faced with this:

The blue dot is for cross reference, which is the fuel screw once again. The green dot is the pilot jet. You can remove this using a flat blade screwdriver. Just unscrew it and pull it out. Once you pull it out, set it aside and put in the 45 pilot jet you got. The red dot is the main. You remove this by using a 6mm socket. Just unscrew it. If the whole thing turns, not just the jet, but the 7mm sized socket under it, don’t worry, that piece has to come out as well. If it doesn’t, use a 7mm to unscrew it off. Here is what the jets look like:

Pilot Jet

Main jet attached to the tube. Take the main jet off by using an open end wrench and a socket on the jet. Again, it screws right off.
Here is what you are faced with if you look form the bottom up.

From left to right: Main jet, Pilot Jet, Fuel screw. Now in the main jet’s hole, if you look closely, you see a bronze piece in the middle of that hole. We are going to take this off. Since I did not do this part (I only changed my pilot jet when I took these pictures) there are no pictures taken for this section but this is really simple to do if you’ve been a good student and know where things go. You should know anyways, you have to put the bike back together!
(Notice: There have been discussions about these needle jets being the same. Only change this needle jet if the one you have is worn out. If you do not have the old needle, a older drill bit bigger than 3/20ths (.150), and smaller than 11/100 (.11") Use the tapered side of the bit, set it down in the hole and tap it out carefully.)
Now take your OLD needle, I repeat, the OLD needle because what you are going to do next will ruin it. Pull the clip off with your needle nose pliers, or a tiny screwdriver to pry it off. Then put the needle back in the hole where it goes. That’s right, just to clarify, you took off the needle, and you put the needle back in the hole with no clip. Slide the point side first, just as it would go normally. Now if you look at the bottom of the carb, the needle is protruding past the main jets hole. Grab another pair of locking pliers (vise grips as I call them) and lock it as tight as you can on the needle. Pull with all your might on the needle. Use two hands. Have a friend hold the carb so you don’t pull it off the boot. Tell them to stick their fingers in the hole that goes to the engine, and pull up. After pulling hard, the needle jet should slip right off. Then notice which side goes towards the top of the carb. There is one side that is a smaller diameter than the other. Take the new needle jet, and push it up into the hole the way the old one was set. Just get it straight. Take the tube the main jet goes into, and start threading it in. Once you can’t tie it down anymore with a ratchet, unscrew it and look at the needle jet to make sure it’s set. That’s it for the needle jet. Now let’s start putting the carb back together.
(Notice: Many people have destroyed jets and such by overtighting them! Use the thumb on the head of the wrench and two fingers on the wrench to tighten it down.)
Thread the main jet into the tube it goes into, and then start putting it back on the carb. Thread the pilot jet in as well if you haven’t done so already. Remember these carburetor metals are soft as cheese, so don’t over tighten the jets very much. What I do is I put my thumb on the top of my ratchet, and use two fingers closest to the head of the ratchet to tighten the jet. That’s how tight I go when I tie them back in.
Now before we put the carb back together, let’s adjust the fuel screw. Take a small screwdriver, and start screwing in the fuel screw until it sets. Again, do not over tighten, just let it set. Then count back your turns. Count back 1.75 turns.
Now we must put the bowl back on. The white piece that came off with the bowl goes back as followed:

If you look directly under the carb, the round hole is aligned with the pilot jet. Take the float bowl, and put it back on.
Untie the rear clamp and the front clamp as well. Slip the carb back the way it used to. Make sure that it is straight up and down with the rest of the bike. The notch on the front boot should be aligned with the notch on the carburetor, and the notch on the carburetor should be in that slot. Tie the clamps down securely.
Let’s put the needle in. These are how the needle numbers go:

The top clip position is #1, the lowest one, closest to the bottom, is #5. (The picture says six but it is five in this case) For reference #1 is the leanest position, while 5 is the richest. I put the clip in the 4th position. Read at the bottom of the page and you can know what conditions I ride in, and you can adjust them to your preference.
Put the clip in the new needle, slip it in. Take the vise grips off your grips and start guiding the plunger holder down to the bottom. Remember not to let that assembly come apart because it is a pain in the ass to get it back together! Once you get it to the bottom, put the two screws on, and then put the cover on.
Now that you have done the carburetor mods, there is still one thing you want to do to complete the process. Don’t worry, this takes less than a minute! On the top of the air box there is a snorkel:

As you can see, you can slip your fingers in and pull it out. Do that. This lets more air in to the air box. Don’t worry about water getting in. There is a lip that is about 1/8” high that doesn’t let water in. When you wash, don’t spray a lot under the seat, but don’t worry about it too much.
The next thing you must do is remove the exhaust baffle. The screw is a torx type, or you can carefully use an allen wrench and take care not to strip it:

The screw is at the 5 o’clock position and all you do is unscrew it, reach in, and yank it out. This setup still passes the dB test. The bike runs 92 dB per AMA standards, which is acceptable. Just carry this baffle in your gear bag if the ranger is a jerk off. I’ve never had a problem, but don’t take chances.
That’s it! Start putting your tank on, seat, and covers. After you put the seat on, pull up on the front, and the middle of the seat to make sure the hooks set in place.
Turn on the bike, and take a can of WD-40. Spray the WD-40 around the boot where it meets the carburetor. If the RPM rises, you know you have a leak, and the leak must be stopped. You must do this to make sure there are no leaks!
Here is my configuration:
04’ 230F
Uni Air filter
132 Main Jet
45 Pilot Jet
Power up needle, 4th clip position
Fuel screw 1.75 turns out
Riding elevation: 2000ft - Sea level
Temperature – Around 60-90 degrees
Spark Plug Tips
When you jet your carb, a spark plug is a best friend. Make sure your spark plug is gapped correctly, (.035) but that’s not all that matters. You want to make sure the electrode is over the center, and you want the electrode to be parallel, not like a wave of a sea. Put in the plug, and run the bike for 15 mins, ride it around too then turn it off. Then take off the spark plug after letting the bike cool. The ceramic insulator should be tan, like a paper bag. If it is black, it is running rich, if it is white, it is running lean. The fuel screw should be turned out if it is running lean, and turned in if it is running rich. Go ¼ turns at a time until your plug is a nice tan color.
Making sure your bike is jetted correctly
While you are running the bike for those 15 mins to check the plug color, you want to make sure it’s jetted correctly now. Here is what the jets/needle/screw control:
0- 3/8 throttle – Pilot jet
¼ to ¾ throttle – Needle
5/8 – full throttle – Main jet
0-Full – Fuel screw
Pin the gas, does it bog much? Just put around, is it responsive? When you’re coming down a hill, the rpm’s are high and you have no hand on the throttle, does it pop? If it pops, it is lean and the pilot jet should be bigger. If it’s responsive your needle is set perfectly. You shouldn’t have to go any leaner than the 3rd position, but I put mine in the 4th position to get the most response. Your bike shouldn’t bog much when you have it pinned. If it does it is too rich of a main jet.
Determining the plug color, you will have to mess with the fuel screw.
That’s it, have fun jetting, and any questions, post on the forum, but remember to do a search first.
Also, if your bike requires different jets due to alititude, humidity, or temperature, please post the following so we can better assist you:
Average temperature
Altitude (If you do not know this, there is a link in the Jetting forum that you can look up your alititude)
Average Humidity
What jets you are currently running
What the problem is (If there is one)
Just do that and we'll help you out the best we can.
EDIT: The girl using this login name is my girlfriend. You can reach me on my new login name at 250Thumpher
Then again, you're more than welcome to say hi to her!
-Phill Vieira
• By jason230
Well I figured this would be a cool thread to have seeing as how quite a few of us have gotten pretty far into upgrading our 150/230s. This will be helpfull for those looking into getting things for their bike so they can see what everything looks like.
'05 CRF230:
-Acerbis Rally brush guards
-Factory FX #'s
-Renthal 7/8ths MC bend w/ soft half waffle grips
-BBR +1/2" shift lever
-BBR revbox
-White Bros. R-4 Full Exhaust
-Twin-Air and White Bros. airfilters
-White Bros. 112main and 48pilot jets
-Dunlop D756's w/ MSR Ultra Heavy Duty tubes

• By werra
In order to help 230F riders set up and fine tune their Emulator equipped forks, it was suggested that we start a new thread to complie set up data.
Okay people, start posting your set up details with the template below:
Bike: CRF230F/Year
Forks: Stock
Fork Springs: Stock/Spring Rate
Damper rod holes drilled: diameter, number
Emulators: spring color, spring weight, turns of preload
Fork Oil: Brand, weight/viscocity
Oil Level: inch/mm from top (measured with springs removed, emulators installed)
Rider weight: with/without riding gear
Riding Conditions: Woods/trail/track