understanding compression ???

[TecateDan]

Clymer say my compression should be 8.4:1 or 123.48 psi

My compression is measured at 180PSI or 12.25:1

People have mentioned a one kick ??? Just want ot know what the deal is and what my real ratio is?

[El Camexican]

Do you trust that gauge? I’d check that first. Is it rattling when you lay on it? 180 sounds pretty high, but keep in mind that once all them fancy power-valves start moving a lot of squeeze will be bled off.

[MTS]

Where are you getting your 12:25:1 number at 180 psi? Corrected or uncorrected ratio? Need to know your head volume installed at tdc, your exaust port height from the top of the cyl, bore and stroke to do the math, the aux exaust ports and powervalve will drop the compression as they open as mentiond

[El Camexican]

http://www.torqsoft.net/compression-ratio.html

As MTS mentioned you can calculate it if you have all the numbers (see above). Any chance you CCed your head before you buttoned it up?

Are you sure that the piston, rod and head are stock? Did you measure the base and gaskets and check them against stock thickness?

As far as I know the CR is just a number (how did you conclude 12.25:1?) and the PSI mainly for noting to see when it’s time to do the rings. What really matters is how it runs and if it is overheating and pinging which has more to do with the shape of the combustion chamber and piston crown than the PSI. Unless you are using a high end pressure gauge it would not be surprising if it was off by 5 to 15 PSI. So how does it run? If it's not knocking or boiling over drive it!

[barnett468]

Hello Tecatedan


Unfortunately since I don’t quite understand exactly what you want to know, I have provided some general info below.



Clymer say my compression should be 8.4:1 or 123.48 psi ?

The 8.4 number is the factory correct number for the compression. I don’t know if it is a static or dynamic [trapped] compression number. If accurate and the engine is stock than that number more accurately reflects that of a dynamic compression number than a static compression one based upon the cranking compression number you generated.



My compression is measured at 180PSI or 12.25:1 ?

How did you generate the 12.25:1 number?



C People have mentioned a one kick ??? Just want ot know what the deal is ?

See the article below from Harry Klemm.



and what my real ratio is?

See info below.


The only 2 stroke compression ratio calculators I have seen determine dynamic [trapped] compression ratios which are a useless number to you unless you can find the equation to convert it to cranking compression. Many also do not use the cylinder head gasket thickness or piston dome volume etc. in their calculation so they do not generate a very accurate number. Cranking compression is the 180 number you have now and is the primary one that most engine builders are concerned with. You can actually calculate cranking compression numbers by first calculating your dynamic [trapped] compression then using the dynamic compression calculator below simply enter your known values EXCEPT for “compression”. Simply enter 10.0 in the compression window, hit calculate button and a “dynamic” and “cranking” compression number will appear below the box. Simply hit the return button and re entering a higher or lower “compression” number as needed and recalculate it until the calculated “dynamic” compression number matches the one you previously calculated. The cranking compression number is automatically below the dynamic one.

The compression calculators and mathematical equations used for determining dynamic and cranking compressions are theoretical and not a concrete equation to determining the EXACT dynamic and cranking compression of a particular engine. A few reasons for this are because they do not take into account how much air the engine will be compressing. This value can also be estimated but it requires much more data such as opening and closing times of both intake and exhaust ports or valves, swept area under the curve [partially determined by valve lift and valve duration in a 4 stroke] port size, engine scavenging and air density etc. If you have a 500 cc engine that is compressing 1/4 cubic foot of air it will have the exact same static compression ratio as the same motor compressing 1/2 cubic feet of air however the additional 1/4 cubic foot of compressed air will increase the dynamic and cranking compression providing the density of the air is the same. Some equations also take into account the rod length however I have found that varying the rod length only in a particular equation by 50% only changes the dynamic and cranking compression values by approximately 4%.

To determine your static and estimated dynamic and cranking compression ratios you will need your cylinder head volume, the volume of the piston dome [available from the mfg and if not just use negative 5 cc], exhaust port closing time in crankshaft degrees, connecting rod length [just use 5”] and deck height which is the distance the outer edge of the piston sits below the cylinder head surface. Just use 0 if you do not have it.

1. Take the info above and enter the required info into the STATIC COMPRESSION RATIO CALCULATOR below.

2. Take the static compression number it generates and enter it along with the other requested info into the DYNAMIC COMPRESSION RATIO CALCULATOR below. This will give you both your dynamic and cranking compression numbers.


If I am simply trying to eliminate detonation/ping in a particular engine the method I use if I am stuck with using only one octane of gas is and the cylinder head squish band width and angle is correct is to simply keep installing thicker head gaskets when available until the pinging is minimal then lower compression further by approximately 5% to account for varying conditions like air temp, density, and minor inconsistencies in octane levels. If thicker head gaskets are not available, then it’s head machining time.

I noticed the bottom of your export is 3.205” [81.28 mm] from the top of your cylinder. The stroke is 86.0 mm. Since the outer edge of the piston is commonly even with the bottom edge of the ex port this would mean that either your piston deck height is -3.72 mm or your piston sits 3.72 mm above the bottom of the ex port or somewhere inbetween. Any distance your piston is below the top of the cylinder at tdc will have to be used to calculate more accurate compression ratios.


STATIC COMPRESSION RATIO CALCULATOR

http://www.wallaceracing.com/cr_test2.php


DYNAMIC COMPRESSION RATIO CALCULATOR

http://www.wallaceracing.com/dynamic-cr.php


The following are excerpts from an article written by my friend Harry Klemm who was the engine tuner for the Kawasaki team green 80cc bike program and engine builder of Rex Staten's famous Carlsbad CZ as well as Jimmy White’s Championship Tecate etc.


A Brief Note About Compression Gauges - It bears noting that most compression gauges are made for automotive applications where V8 engines often have a swept compression volume of 700+ccs. Unfortunately, the average 125cc 2-stroke has a swept volume of about 70cc. This big difference in swept volume means that a valid compression test has to be done in a certain way with a particular type of gauge.

The Best Way to get a Valid Compression Reading

A) Within 5 days of a normal “operating cycle - Engines that have sat for a long time may have excess oil that has leaked in from somewhere. The added ring sealing offered by this excessive oil presence can greatly increase the normal valid reading.

B) Cold Engine only - Hot engines will yield lower numbers, and varying levels of temperature will yield varying levels of lowered readings.

C) Fuel Petcock “off” - To avert fire risks.

D) Throttle “must” be full open - To consistently allow the same air access to the lower end.

E) Spark plug mounted in cap and grounded to motor (to avoid harming ignition)

F) Put your machine in 1st or 2nd gear (use the same gear each time), and push the bike at a walking speed for about 10 feet (road-racers with very tall gearing should go 20ft). Doing the test in this way assures that the engine will always spin at the same speed, and that gauge gets enough "pressure hits" from the cylinder to correct for a gauge that may have an unusually large volume hose.

G) Repeat - It’s not a bad idea to do two or three tests (back to back) to assure consistency


Why not just use the kick starter ? - Because engine rotation speed, and the number of pressure "hits" will vary alot during "kick tests".... and so cause variations in the reading. Sadly, these variations are greatest on large displacement singles .... and large displacement singles are the hardest to push-test. Despite that, the effort is worth the accuracy. If you have doubts, do both a push-test and a kick-test on your machine is see what difference you get.... you'll be surprised.


http://www.klemmvintage.com/squish&comp.htm


Hope this info helps.

[TecateDan]

Sadly the motor has been sent to machine shop to have deck and head squared... I used the kick starter but did it twice....... 192 psi cold 180 hot. I divided psi by 14.7 to get compression. I've been talking to a pipe builder for am inframe pipe build after tf and CR I one of the numbers he needs. Clymer was wrong on port specs so wanted to confirm CR is 8.4:1... Ill try all the calculators u sent me and see what I come up with..

[barnett468]

Hello TecateDan.



http://www.torqsoft.net/compression-ratio.html

As MTS mentioned you can calculate it if you have all the numbers (see above).

Unfortunately the calculator above is one that generates only the dynamic “trapped” compression ratio [which I mentioned how to use to generate the cranking pressure number and it does not take into account the head gasket thickness or inside diameter, the volume of the piston top [yours would be a NEGATIVE number because it has a “dome”], or the “deck height” so the number it generates is inaccurate off enough to be meaningful under some circumstances.

Xxxxx


What really matters is how it runs and if it is overheating and pinging which has more to do with the shape of the combustion chamber and piston crown than the PSI.

The following does not address the effects cylinder head squish bands and “dome” shapes have on detonation and/or pre ignition.

Xxxxx


the PSI mainly for noting to see when it’s time to do the rings.

The purposes of this number can be explained a few different ways. I’ll try to explain at least one of them and butcher my explanation as little as possible. If someone thinks they can explain it better please do so.

The cranking compression number is used as a guide when engine building. There are cranking comp numbers that have been established throughout years of testing etc that tell an engine builder the maximum amount of cranking compression he can use in a particular build that will give an “ideal” balance between maximum cranking pressure vs minimum detonation risk for the particular fuel he is using.


Here’s a few examples.

a. Let’s say for example, the maximum recommended cranking pressure number when using 93 octane gas is 170 psi and the maximum for 87 octane gas is 155 psi and so on. The lower a cranking pressure is below those that are recommended for a particular octane level the lower the potential for detonation to occur.

b. Let’s say “Grinder” is building a high performance 1200 cc single cylinder 2 stroke motor for multiple Championship Winner Jimmy White to use in his pit bike but it MUST be able to run on 93 octane gas or less because Jimmy isn’t getting free gas from Kawasaki anymore and he currently can’t afford the exorbitant price of race gas [but who can]. He grinds away at the exhaust port thereby raising it let’s say 1 mm. Ok, he has now obviously lowered the dynamic compression ratio and cranking pressure below what was stock, however the static compression is not affected by this so it’s value has not changed.

“Grinder” now wants to increase the cranking pressure to the maximum pre established recommended level for 93 octane gas. To do this he takes the predetermined number and uses all the necessary values to calculate exactly how many cc’s the combustion chamber needs to be to achieve this pressure.

c. “Grinder” now has a customer that thought removing .100” off of the cylinder head of his 250r by tying it to the back of his pickup truck and dragging it around on the asphalt for a while so he could get more “compression” out of his motor so he could kick ___ at the Trikefest drags was a good idea except he found out afterwards, that not even 190 octane race gas could stop the incessant pinging his engine was now making and came crawling into the back door of “grinders” shop begging for help. “Grinder” does a cranking pressure test [compression test] and see’s that “boneheads” engine has 350 psi of cranking pressure and that he needs to lower it to 170 [for example] because “bonehead” can’t afford the exorbitant price of race gas either.

“Grinder” takes the head off, cc’s it, measures exhaust port height, bore and stroke etc., and enters these numbers into an equation or the appropriate online calculators etc., to determine exactly what cc’s “bonehead’s” cylinder head needs to be to run happily on 93 gas octane gas without melting his piston while “bonehead” is riding it at full throttle in first gear through the pits all day.