Blaster best way to set squish
- Thread starter catpower
- Start date
DAG
Yes, my balls tickled from that landing
- Location
- Charlotte, NC
What's your current squish and what's your target squish?
- Location
- Victoria BC
If you need more squish @JetManiac sells ADA domes with extra squish relief and base gaskets with extra thickness. He helped me out quickly.
62t 61x ported decked. protech head. without cutting domes I had nearly .015" squish. was told for this head I needed .050' min. So last time I cut domes myself. Problem is cylinder is ported and I'm thinking I've already lost some low end so now I've bought 35cc domes but wondering this time should I raise cylinder via gasket to get porting back or cut domes for proper squish.
- Location
- NYC/Long Island
Well a thicker base gasket will raise exhaust port height. This equals more power, but takes away from response. The porting shouldnt have messed with any port heights or durations so as long as you didnt deck off the bottom of the cylinder for any reason.. I would just cut the domes for proper squish. ADA Racing does this. You have to cut the squish band with a angle if you're doing yourself.
I have used a thicker base gasket in a pinch to set up squish. I couldn't tell the difference, but it was 62t cylinder with riva racing sleeves so it already made crazy power and lacked on response.
I have used a thicker base gasket in a pinch to set up squish. I couldn't tell the difference, but it was 62t cylinder with riva racing sleeves so it already made crazy power and lacked on response.
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Finally got around to check squish with new domes. .040" I'm thinking a thicker gasket would be easier besides I can't imagine .010"- .015" raising cylinder porting with a thicker base gasket would be noticeable. I'm just trying to understand port height in relation to a fast ski vs. a ski with good low end response. If the porting (removing material from cylinder port) raised the exhaust port would this be good for power (speed) but would take away from bottom response due to loss of compression. Smaller domes should help get my compression back up but I'm still compressing a smaller area before the port work was done. Correct me if I have this wrong.
- Location
- NYC/Long Island
At the core of cylinder porting is heights of ports, width of ports and duration of ports. Ideally it's best to measure these in degrees of rotation in a full power cycle. Just throwing numbers, but say on the downstroke the exhaust port opens at 40 degrees and then closes at 80 degrees. The reason we say "height of ports" is because it's much more user friendly to identify a cylinder's porting characterists by measuring from the top of the sleeve.
It's not a change in compression although it may have a side effect of affecting compression on a tester. Changing domes based on a tester is risky and may cause detonation. When you have a smaller dome size, you are now changing the compression ratio in the engine and that's the important spec. You are now taking 700cc of air/fuel mixture and compressing it to 32cc of air/fuel mixture as opposed to compressing it to 35cc of mixture. Just changing the port timing alone won't affect your compression ratio, but it will affect "entrapment". This may cause a change in compression (PSI) reading on a tester. A compression tester should be left alone for testing the health of a cylinder and thats it. I actually never done a compression test on any of my engines. IMO a compression test is only useful to keep track of how healthy the piston rings are in the motor. Especially on motors that are tough on rings (wide exhaust ports)
During the power stroke of the engine both ports are open, there is bound to be some unburned air/fuel mixture to escape outside the exhaust port (short circuit). It's in this stroke, and the height of the exhaust port is how much will escape. If the exhaust port is low, there is less chance of the fuel mixture escaping and it's much more efficient at lower RPM's. However, this lower exhaust port at higher RPM suffers because there is a noticeable delay from ignition and the opening of the exhaust port to clear out the burnt gasses in the cylinder. Power valve cylinders give best of both worlds since at lower RPM the exhaust valve lowers and at higher RPM the exhaust valve goes higher.
Will you feel it? I don't know.
It's not a change in compression although it may have a side effect of affecting compression on a tester. Changing domes based on a tester is risky and may cause detonation. When you have a smaller dome size, you are now changing the compression ratio in the engine and that's the important spec. You are now taking 700cc of air/fuel mixture and compressing it to 32cc of air/fuel mixture as opposed to compressing it to 35cc of mixture. Just changing the port timing alone won't affect your compression ratio, but it will affect "entrapment". This may cause a change in compression (PSI) reading on a tester. A compression tester should be left alone for testing the health of a cylinder and thats it. I actually never done a compression test on any of my engines. IMO a compression test is only useful to keep track of how healthy the piston rings are in the motor. Especially on motors that are tough on rings (wide exhaust ports)
During the power stroke of the engine both ports are open, there is bound to be some unburned air/fuel mixture to escape outside the exhaust port (short circuit). It's in this stroke, and the height of the exhaust port is how much will escape. If the exhaust port is low, there is less chance of the fuel mixture escaping and it's much more efficient at lower RPM's. However, this lower exhaust port at higher RPM suffers because there is a noticeable delay from ignition and the opening of the exhaust port to clear out the burnt gasses in the cylinder. Power valve cylinders give best of both worlds since at lower RPM the exhaust valve lowers and at higher RPM the exhaust valve goes higher.
Will you feel it? I don't know.
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Good info but I don't want to turn this discussion into something very technical. To understand this it may be unavoidable . Compression ratio= ( related to two stroke due to ports found in cylinder ) maximum volume of fuel and air to be compressed. Just using arbitrary numbers, 300cc in cylinder 30 cc in head dome. Ratio 10-1. If exhaust port was raised ( measured from top of sleeve to top of port) foolishly to the point that available volume of fuel and air was reduced in half from original design thus compression ratio would be now 5-1. To get back to a 10-1 ratio my dome size would need to be reduced to 15cc. I know my exhaust port was raised thus reducing available volume to be compressed. So unless my dome size is not respectively reduced more than my cylinder volume now reduced and stay with in same compression ratio all should be fine. These are of course exaggerated numbers. My point would be raising exhaust ports DOES have an effect on compression ratio. My apology for questioning this, I'm no engine builder just trying to learn more.
- Location
- NYC/Long Island
Sorry I have to turn it into something very technical because it's best to understand exactly whats going on by raising the cylinder. I am no engine builder, however I have little experience setting up my own cylinders and this is what I have learned taken from multiple sources like graham bell and another book (forgot the name?).
Compression ratio is static. It is measured by comparing the volume in the cylinder at BDC (Bottom Dead Center) to TDC (Top Dead Center). The ports in the cylinder have nothing to do with it. It is called static because this ratio never changes. OEM is 7.2:1 (Wikipedia) it is set rather low to be safe on pump gas.
Compression reading is dynamic. This is taking a tester and figuring out how much PSI. IMO this is accurate for a 4-stroke engine because pumping losses is minimal. However on a 2 stroke those ports are open for majority of the power cycle and pumping losses happen. More so at lower RPM like cranking the engine over. Testing PSI at 200 cranking RPM is no way going to be the same PSI if the motor is operating at 7000 RPM. It will change because pumping losses are minimal. Thats why I consider cranking PSI to be no way relative to what fuel/domes you should run, its pretty close to guessing.
In your case since your exhaust port was raised then you will most likely notice a loss of "dynamic compression" on the tester because of pumping losses, but you are still compressing your air/fuel to the same ratio as it was before. At higher RPM the motor isn't going to notice that pumping loss.
If you would like to put smaller domes, you may be risking detonation unless you keep it inline with the correct fuel.
Compression ratio is static. It is measured by comparing the volume in the cylinder at BDC (Bottom Dead Center) to TDC (Top Dead Center). The ports in the cylinder have nothing to do with it. It is called static because this ratio never changes. OEM is 7.2:1 (Wikipedia) it is set rather low to be safe on pump gas.
Compression reading is dynamic. This is taking a tester and figuring out how much PSI. IMO this is accurate for a 4-stroke engine because pumping losses is minimal. However on a 2 stroke those ports are open for majority of the power cycle and pumping losses happen. More so at lower RPM like cranking the engine over. Testing PSI at 200 cranking RPM is no way going to be the same PSI if the motor is operating at 7000 RPM. It will change because pumping losses are minimal. Thats why I consider cranking PSI to be no way relative to what fuel/domes you should run, its pretty close to guessing.
In your case since your exhaust port was raised then you will most likely notice a loss of "dynamic compression" on the tester because of pumping losses, but you are still compressing your air/fuel to the same ratio as it was before. At higher RPM the motor isn't going to notice that pumping loss.
If you would like to put smaller domes, you may be risking detonation unless you keep it inline with the correct fuel.
That's the question. I'm always hearing talk about compression as a guide for what octane to run. Most on this site I believe speak of using 35cc domes and run premium 91 if not less. 35cc domes on fresh motor and stock ports is advertised to be approx 170-180 psi. Depending on 61x or 62t. On my motor due to cylinder decked and or porting I'm getting much lower like 160-165. This has been discussed on an older post. So is compression ratio or compression the better guide for what fuel to run. I ran 110 mixed with 91 in past to give me about a 94. I did so just to be on the safe side and at the time I was unaware of squish or what is even was. Like a fool I bought parts and you know bolt and go. Squish was.020-.025" after more reading I corrected that problem for following year of riding. Just trying to learn more, if I were to do all of this over again I would have ponied up the big bucks and had one engine builder put together an engine rather than piece work. Live and learn.
- Location
- NYC/Long Island
There's too many variables with a compression test to pick a octane rating. The motor could be cold, hot, somewhere in between, been sitting for a few days and are relatively dry, maybe the rings are a little worn, maybe port height yielding pumping loss at cranking RPM, relation to sea level, density of the air, ambient temperature etc. etc. You're guage could be inaccurate as well.
How did you notice the tight squish? Did you detonate? You could put higher compression domes in it, but you may be risking detonation. I would just monitor the cylinders for detonation and dont do any extended WOT runs until you're confident. It's risky, but it will definately get back some response.
Hey nothing wrong with learning, you would've never knew what squish ratio was if you had a engine builder take care of it for you. Also engine builders make mistakes too, and some people who you thought were reputable are not. Only thing guaranteed is the wrench you turn.
How did you notice the tight squish? Did you detonate? You could put higher compression domes in it, but you may be risking detonation. I would just monitor the cylinders for detonation and dont do any extended WOT runs until you're confident. It's risky, but it will definately get back some response.
Hey nothing wrong with learning, you would've never knew what squish ratio was if you had a engine builder take care of it for you. Also engine builders make mistakes too, and some people who you thought were reputable are not. Only thing guaranteed is the wrench you turn.
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I would assume if one were really lucky and checked piston just prior to Boom he might find top of piston eaten away? I rode all summer 26 hours (tiny tach) with no problems with gas mix as stated. I start reading more on XH2O and thought I should check mine. Elevation 940 temps 70-95 deg. Multiple testers, they all agree. Checked motor hot and cold, throttle wide open and throttle closed. I've heard what your saying but I personally have never seen any significant difference checking compression. What gets me Is tuning high speed circuit based on rpms. Again I've read rpms will gain or drop if lean or rich. This motor will not get over 6800 regardless of carb setting and this is with an MSD limiter set about 8000. But that's a whole conversation alone. Humidity now that's a carb tuners nightmare. Ski has always run like a dog when hot and humid.
88kawi5fiddy
pew pew lazers!
- Location
- So Utah
humidity = more water in the air = less oxygen per unit volume of said air. now, Hot AND humid is a bigger problem. Hot air = less dense = less oxygen per unit volume. Add the density and humidity and its = less and less oxygen.
if it is 50F outside and 100% RELATIVE humidity, there isn't that much water in the air, as cold air cannot hold as much water as hot air. clouds raining on ocean side mountain ranges make sense now? colder air is at the top of the mountains.
these are just numbers im throwing out, but 50F and 100%H equals 100F and 65%H . Per cubic foot, there is the same amount of water in the 100% and 65%, but the hotter air CAN hold more. 100F and 100%H could be 1.5x times as much water per cubic volume as the 50F and 100%H.
Turn your screws IN == less fuel since when it's hot and humid there is less oxygen. Less oxygen = less fuel since we are chasing 14.7:1 Oxygen to fuel ratio.
if it is 50F outside and 100% RELATIVE humidity, there isn't that much water in the air, as cold air cannot hold as much water as hot air. clouds raining on ocean side mountain ranges make sense now? colder air is at the top of the mountains.
these are just numbers im throwing out, but 50F and 100%H equals 100F and 65%H . Per cubic foot, there is the same amount of water in the 100% and 65%, but the hotter air CAN hold more. 100F and 100%H could be 1.5x times as much water per cubic volume as the 50F and 100%H.
Turn your screws IN == less fuel since when it's hot and humid there is less oxygen. Less oxygen = less fuel since we are chasing 14.7:1 Oxygen to fuel ratio.