Calculating 2-stroke compression ratio (NOT PSI)

[Zero Client]

I'm trying to get my head-dome size correctly spec'd for C12 race fuel and need to know the best most accurate way of determining over-all CR for the fuel. I need to know if most calculate from the bottom of the stroke (IE full stroke aka 68mm for stock stroke), or from the top of the exhaust port.

When I put on my mathematical hat; I'm coming up with a CR of about 15.5:1 measuring from full stroke, but I also understand that's more the dynamic CR, while the static CR based on my calculations are more like 7.5:1 from the top of the exhaust port.

So - which is correct? I know basic cranking would probably be closer to the 7.5:1; but with exhaust tuning/backpressure and the natural 2-stroke 'super-charging' effects, the higher CR comes into play.

c12 fuel is rated for CR below 15:1.
Pump gas is debatably rated for CR around 12:1

 

[ridetrp]

I'd just run diesel or contact your engine builder

Sent from my SM-N950U using Tapatalk

 

[Zero Client]

LOL! I am my engine builder - in training

 

[Big Kahuna]

http://www.csgnetwork.com/compcalc.html

 

[Zero Client]

http://www.csgnetwork.com/compcalc.html

Yup - I'm right at 15.2:1 on that calculator. That's a dynamic calculator.

 

[Harbor]

For 4 stroke drag Racing and street cars compression ratio calculations were always used.

For these jetskis 2 stokers, cranking compression pressure, porting, pipe type, Timing, ride style, as well as the hull type, used to determine octan was news to me.

looking for cheaper race gas I asked a current sportsman drag racer, Who owns a automotive machine shop and is a machinist what fuel
He uses, that led to asking what his cranking compression pressure is, he had no idea. It was all CR calculations to him.. he knew and used that number for fuel calculations.

When my skis been at the watercraft factory or the ski clinic, before they ran the engine they took a cranking compression test together with the other for mentioned build info to determine what fuel my ski needed before they ran it, since they didn't know what fuel
I used and the tank was low....

Nothing wrong with calculating compression ratios, but it seems like
The jet ski guys have it more dynamicly figured out for what we're doing by using build specs, ride style and cranking compression pressure to determine fuel
Requirements.

CR numbers and calculations don't seem like a universal language that we're all using in our sport.

 

[Big Kahuna]

It all depends on setup...........

RIVA use to have port specs that would show pump gas or race gas. But here is the kicker, some of the race gas would only have about 165 lbs compression. and yet your saying, That is pump gas range. SPLASH did a build on a 96 or 97 Superjet, Protec did the build for them. This was set up as a bouy buster........ Guess what, Pump Gas and blowing just over 200 lbs compression......... It is more than just about numbers........

 

[Harbor]

Exactly!!'

 

[CARBONX2]

Full stroke CR is the correct way

 

[kevbo]

Watercraft engines can stick pistons faster than anything. Do application specific research, there is plenty of knowledge of every type. Retard the timing if you are tuning for long full throttle runs. You usually end up finding the meltdown point.

 

[Nuklear6]

Warning hopefully this doesn't get to long.

The link to the compression calculator above is static compression ratio not dynamic. It doesn't does not factor the exhaust port height.

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

This online calculator is what you need to live by. Keep in mind you need to measure the ACTUAL head volume at top dead center on the engine you are seting up. There are many variables that can affect this, you cannot just use the cut CC of the Dome.

It is a must to use the dynamic compression ratio of a two-stroke. The exhaust Port height will change the trapped compression which greatly effects the pressure in the combustion chamber and therefore the cause of heat and potential detonation.

That being said the fuel manufactures generally rate the fuel compression limits more towards 4 strokes that have a lot less difference between static and dynamic compression ratios. This due to having a full compression stroke and normally only minimal intake valve timing that overlaps the compression stroke.

Keep in mind that there are a lot of tuning/ environmental factors that also will limit or allow different fuel grades. I can make my 205 psi 800cc stroker run on good 93 pump fuel and run wide open with no problems. But things like carb tuning, cylinder temp, and pump load must be dialed into exactly where they belong. A slight decrease in cylinder temp and bam your squeaking a piston. That being said I never send a 205 psi engine to a customer and suggest pump fuel. No offense but i just don't have faith they'll get it dialed well enough to keep the engine alive.

I've been very busy but when Im near my notes and get a chance I'll post some static compression ratio to cranking compression numbers I've found first hand.
To keep it simple use a good compression gauge and crank it with a good battery.

True 93 octane up to 185psi
103-105 octane up to 207psi
112 octane up to 225psi
Those are fairly safe numbers

Chuck Palmucci
CPT Enterprise LLC
609 510 3062

 

[Zero Client]

Thanks Chuck, I may be confused, as that calculator is calling above the exhaust port TRAPPED compression ratio. I've definitely seen that calculator a bunch of times during my travels. That and similar calculators are calculating CR from above the exhaust port. I'm coming up with 8.5:1 Trapped ratio.

Bore - 90mm
Stroke - 68mm
Exh port height - 37mm
Head volume - 31cc (46cc dome - 10cc piston top - 5cc piston above deck)

Interesting though - that calculator mentions taking Dynamic ratio readings from the top of the INTAKE ports. That - I have not done. Not sure how accurate that is for this - and actually mainly just throws another CR into the mix.

The compression ratio can also be calculated based on the swept volume after the closure of the intake port. This is often called the "dynamic compression ratio".

Head CC volume - I calculated based on piston dome cc (10cc measured), plus piston height above the cylinder - 0.78mm. Together this comes out to 15cc for the piston dome plus piston extension. I then took 44cc domes and relieved squish by 20-thousands for needed squish relief (Target 40-45thou total squish). Doing this turned 44cc domes into 46cc domes again by way of calculations. These measurements subtracted from each other make the total calculation for the combustion chamber at about 31cc (46cc dome minus 15cc piston head/extension).

C12 fuel is leaded 112RON/108MON.

 

[Nuklear6]

@Zero Client

If you read the text at the bottom of that calculator again it talks about using the intake valve closing to get dynamic compression of a 4 stroke. Not a 2 stroke. You use the closing of the exhaust port for dynamic or trapped compression ratio on a 2 stroke. I can't stress how important it is to do a dry fit, find top dead center with a dial indicator, and measure the actual top dead center combustion chamber volume. Calculating is good to get you some what somewhat close, but an actual measurement is best.

I should have also been more clear when talking about the compression ratio listed as the limit by the fuel manufacturers. That is mostly a static compression ratio. They have Engineers that can accurately measure the dynamic compression ratio however they do not have enough faith in the average buying consumer to know their Dynamic compression ratio. So it's best for them to give a conservative static compression ratio as a limit for their fuel.

Your set up will be fine on c12 as long as your actual measurements show similar to your calculations. You should however try and get your exhaust timing a degrees lower on the atdc opening. That is if free ride/freestyle is your desired application like most on this forum.

Chuck Palmucci
CPT Enterprise LLC
609 510 3062

 

[Zero Client]

You should however try and get your exhaust timing a degrees lower on the atdc opening. That is if free ride/freestyle is your desired application like most on this forum.

Hard to-do without some physical changes like sleeves, bottom decking, stroking etc.

For the piston dome cc, I measured the dome height of the piston to be 4.78mm. This measurement was attained by flat-edge leveling the CENTER button of the piston with the cylinder while the head was removed using a hard straight edge. I then used a micrometer to measure the piston depth average on the edge of the cylinder in multiple places for an average reading of 4.78mm. This turned out to be the same for both front and rear.

From then calculating the volume of the piston dome I used the following equation; which is standard geometry for finding the volume of a 'cone', as the dome of the piston actually is a cone with regards to flat angles except for the 1/2" button on the very top of the piston.

`````(3.14159) * r * r (h / 3)
V = ----------------------------------
```````````1000

r is Radius. In my case 90mm bore would be 45mm radius.
h is height - found above at 4.78mm.

V, or better, piston dome volume ends up being just about 10.1cc

Add to V, the cc volume of how much edge of the piston (Either positive or negative) to the top edge of the cylinder is. In my case this is found by standard geometry known as volume of a cylinder:

V = (3.14159) * r * r * h

r is 45mm
h was mic'ed at +0.77mm
V ends up at about 4.9cc.

10.1cc piston dome + 4.9cc piston extension = 15cc total piston displacement within cylinder head.

Then take the head dome which is 44cc, and relieve squish by 20thou. Through cylinder dome calculation this turns the 44cc dome into a 46cc dome through volume of a cylinder calculations.

46cc modified dome - 15cc piston = 31cc total head volume for the above compression ratio calculator.

 

[Nuklear6]

Got a min to reread all your numbers. 2 questions.

Does your dome cc measurement include the spark plug hole volume? Spark plug hole volume is 1.5cc but i only subract 1cc for calculation because 1/3 of the volume stays open even after the plug is installed.

You give 37mm as your measurement of port height Does this already factor in the .78mm of your positive piston/deck height? You have to add or subtract your piston to deck height to your overall port height for calculation. If your 37mm is not factoring that, your exhaust port timing is actually pretty good at 37.78mm, 88.43 deg atdc, and 183.1 deg of duration. The corrected port height must also be used in the trapped compression calculator.

Heres what i got for your engine once i correct for spark plug volume and exhaust port height
bore 90mm
stroke 68mm
ex port height 37.78mm
trapped head volume 31-1cc for spark plug =30cc

Trapped compression ratio 9.01:1
Static compression ratio 15.42:1
ROUGHLY on my compression gauge is would be somewhere around 210psi cranking

Your engine if spark plug volume is already factored in
bore 90mm
stroke 68mm
ex port height 37.78mm
trapped head volume 31cc

Trapped compression ratio 8.75:1
Static compression ratio 14.95:1
ROUGHLY on my compression gauge is would be somewhere around 206psi cranking

Wanted to change head colors on my engine to better match my current hull so i took actual measurements yesterday while i changed the head (getting ready for DAYTONA Wootwoot)

My engine real world
bore 82.5mm
stroke 72mm
ex port height 40mm
trapped head volume 30cc - 1cc for spark plug volume =29cc (measured with low visc oil with piston atdc)

Trapped compression ratio 8.36:1
Static compression ratio 14.27:1
Actual cranking psi on my compression gauge 194 (checked both cylinder before i measured head volume so residual oil didnt boost cranking psi)

final point: calculation is good but make sure you take actual measurements as you assemble. If your real world findings are close to your calculations youll be just fine on c12.

Chuck Palmucci
CPT Enterprise LLC
609 510 3062

 

[Storbeck]

@Nuklear6

Any chance you'd be willing to give a ballpark figure for a trapped and static compression ratio that is safe (conservatively) for pump gas?

If it makes a difference:
My plan is to use a 64x cylinder bored 85.5mm, planning on shaving the base of the cylinder to give me exhaust duration of around 182 degrees or so, basically planning on matching stock 62t port timing (so a little more than a 61x, but not as much as the 64x starts with)

I'm planning on running about .060" squish clearance and using the thinner "64x" type metal gasket, modifying my head to suit.

I'm thinking I want to be in the nieghborhood of 170psi cranking pressure for pump gas safe with that setup.

I have access to the machine tools to modify the cylinder and head, and I have a burrette to accurately measure the total installed volume of the head, but I don't have much of a feel for how that volume translates to cranking pressure.

Pump gas around here is 91 octane.

 

[Nuklear6]

@Storbeck
You would want to be around 7.5:1 dynamic compression. To achieve that with your bore size and port timing you'd want your trapped head volume to be 33.5cc. That's assuming your engine is stock stroke. One helpful hint is if your modifying stock head start with a 701 head not 64x. The hemisphere of the front and back combustion chambers are different on the 64x. This way you only have to cut the swish area and don't have to mess with the radius of the hemisphere.

Chuck Palmucci
CPT Enterprise LLC
609 510 3062

 

[Storbeck]

Thanks Chuck!

I really appreciate you and a few of the other folks on this site taking the time to answer these technical questions.