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PostPosted: Sat Feb 15, 2014 5:42 pm 
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Joined: Wed Sep 29, 2004 4:48 pm
Posts: 1037
Location: CT
lol, no, that was supposed to have a quote from dave's post in which he was discussing vw engines, but I messed up and did not carry the quote over. BUT all engines are air pumps and operate on the same principles.


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PostPosted: Wed Feb 19, 2014 7:44 am 
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Location: New Jersey
CCR for clarification is cold compression ratio correct?
What is difference between static and ccr?


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PostPosted: Tue Feb 25, 2014 3:11 pm 
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Posts: 3620
Location: Wichita ks
CCR
"Compression Ratio (CR) - Volume of cylinder and combustion chamber with piston at exhaust port opening, divided by, volume of combustion chamber with piston at TDC (Top Dead Center). This is the "corrected" compression ratio."

Mud, for the most part we just call it CR.

Static
I call it static reading or cranking compression number in psi. I base it off of the static CR or calculated CCR.
Then when I crank the Engine with no combustion the gauge reads a psi number. This number is based upon ring seal and the atmospheric pressure with in the cylinder at exhaust port closure.

In an operating Engine the atmospheric pressure within the cylinder at exhaust port closure is subject to the pipes tuned length at rpm. If correct there will be a positive pressure if not there can be a negative pressure. Negative pressure along with poor crankcase pressure (PCR: Primary compression ratio)will make an Engine run hot no matter what jet you run.

A pressure loss can translate into a weak mix or lean it out, the revers is also true if there is a vacuum leak the fuel density will be week and the air to fuel ratio gets high. Whenever we alter the transfer/intake ports or add a reed spacer the static PCR changes. When we have an air leak we have a dynamic PCR change. A lower static PCR will require a larger reed tip ratio which leads to a bigger main jet and smaller needle number to maintain a good fuel density. Basically less vacuum to pull fuel. A change in CCR and tuned length needs a change in PCR. The PCR is a highly over looked item.

That leads us to port velocity/timing and the ability to clear the cylinder and charge it with fresh fuel mix. The time/area the reed is open will have an effect on the dynamic PCR. The denser the fuel to air at a given velocity the better the Engine will operate.

May be getting of topic so I will stop here. I hope I answered your question. Maybe "H" and others can do a better job explaining.


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PostPosted: Wed Feb 26, 2014 7:59 am 
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Location: New Jersey
Thank you for the explaination it helped me greatly better understand the true psi.
basic atmospheric pressures can change a ccr based upon the explanation as do several factors.
How can 1 give an accurate compression ratio? Is there a base setting to do so?
Let us say i am selling a pilot and wish to provide the purchaser with accurate readings. Is the basic crank the Engine cold throttle wide open till pressure maxs out proper?
Is static compression a better reading?
With all the factors listed density of fuel, atmosphere pressures can we establish a base line?


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PostPosted: Wed Feb 26, 2014 8:44 am 
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Joined: Mon Dec 15, 2003 2:40 pm
Posts: 22512
Location: Chicago
Mudbogger wrote:
Thank you for the explaination it helped me greatly better understand the true psi.
basic atmospheric pressures can change a ccr based upon the explanation as do several factors.
How can 1 give an accurate compression ratio? Is there a base setting to do so?
Let us say i am selling a pilot and wish to provide the purchaser with accurate readings. Is the basic crank the Engine cold throttle wide open till pressure maxs out proper?
Is static compression a better reading?
With all the factors listed density of fuel, atmosphere pressures can we establish a base line?



Try not to over think it too much, to me a baseline would be to check the fully assembled machine with the throttle wide open and crank the Engine over until the pressure on the gauge stops rising.

I say fully assembled because if you test the compression on a Pilot with no intake or exhaust system in place the compression reading will be higher, test this yourself the next Engine install, install the Engine hook up the starter then check compression before installing the intake or exhaust then as you install each part of the system check the compression again, install the exhaust pipe, test, install the reed cage and intake, test, install the carb, test install the intake boot and air box then test, you will want the battery charger on between test with all this cranking lol

Once a 2 stroke is running compression will go way up because of the way the exhaust system works on a 2 stroke, you have felt a 2 stroke come up on the pipe and felt a slight surge in power when it hits the pipe? On my Pilot with the rev pipe at about 6k if you roll on the throttle real slow and it came on the pipe it felt like a 5hp gain all at once, wonder what the compression was reading then when the pipe was sucking the fresh intake charge of air and fuel out of the crank cases out into the pipe then stuffing all that mixture back into the cylinder right before the piston closed the exhaust port?

Watch the GREEN fresh air/fuel being sucked out into the pipe (this is what creates a 'wet line' in the pipe ) then stuffed back into the combustion chamber like a super charger packing air/fuel into a Engine, you wont see that happen in a 4 stroke without :-)
Image


The wet line happens where that fresh air/fuel enters the pipe, the line usually forms where the mixture meets the part of the pipe where its so hot it burning on contact, all before that the pipe wall has been cooled enough by the previous charge of fresh air/fuel it cant burn and stick to the pipe, this is why you want to locate your EGT probe past that wet line so the probe is not cooled by cooler fresh air/fuel mix.


See in the attached pic I made a capture of one of the frames of the animation the green is the air fuel at its extent into the pipe the 3 lines represent the EGT probe, if you had it located in the wet line where the blue line is you get a false reading if you located it where the yellow line is you might be too far from the saturated heat as the heat of the products of combustion
start to fade the further you get away from the source, the red line would be a better location for the probe.

This is all a mock up the distances and locations of the probe in the pipe in the pic don't mean anything real life will vary, you need to find your wet line before you install your probe on a Pilot that is about 6" away from the face of the piston stock or modified...

Image


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PostPosted: Wed Feb 26, 2014 12:56 pm 
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Joined: Fri Jan 12, 2007 2:17 pm
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Location: Wichita ks
Mud: basic atmospheric pressures can change a ccr based upon the explanation as do several factors.

Adnoh: atmospheric pressures; Effects the static cranking compression number in psi
CCR: is math or calculated, Corrected Compression Ratio

Mud: How can 1 give an accurate compression ratio? Is there a base setting to do so?

Adnoh: You have to measure several item and then do the math. In order to get the number you will need.
(1) The exhaust port closure in degrees or distance from deck to top of exhaust port to calculate volume
(2) The head gasket thickness and ID for volume
(3) The heads volume
(4) The piston dome volume
(5) The volume within the cylinder at TDC (Top Dead Center)
(6) Do the math
As far as a base setting, the pilot manual list the stoke number. Once the cylinder is bored, different piston is installed, different gasket are used, the head is altered or cylinder ported. The calculated CCR changes.


Mud:Let us say i am selling a pilot and wish to provide the purchaser with accurate readings.
(1) Is the basic crank the Engine cold throttle wide open till pressure maxs out proper?

(2)Is static compression a better reading?

Adnoh: (1) yes, (2) yes. There is some related item one can do as well
When doing your psi reading take one cold. Then take one cold with a cap full of two stroke oil poured into the cylinder through the spark plug hole. Then compare the reading over 10% it may need rings. Then take a warm reading and do the same with the oil and compare all the numbers. Even if the numbers are all in line there’s no guarantee that the Engine will last just the condition of the ring seal.
One needs to ask a few questions or supply the new owner with the info when the cranking psi number is taken.
(1) What bore is it at
(2) What piston is in it
(3) Any head work
(4) Any port work
(5) What gasket kit was used
(6) Approximant hours( the funny number is 40)
(7) What fuel, what oil at what ratio



Mud: With all the factors listed density of fuel, atmosphere pressures can we establish a base line?

Adnoh: Yes we can establish a calculated base line. The Honda manual gives us this. What we read on the board when there’s a failure help establish the limits. Every Engine is different and ever part we put on it has an effect. “H” and others has done a good job providing us with information on what works well without going deep into details.

I really like the animation of the Engine it explains a lot.


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PostPosted: Wed Feb 26, 2014 3:01 pm 
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Joined: Tue Dec 23, 2003 10:31 pm
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Location: New Jersey
as always thank you my friend!


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PostPosted: Tue Nov 21, 2017 9:19 pm 
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Just bought a Honda pilot compression is 185 cold? Running vp112 but still worried that it's too high makes a loud squeak during shutdown almost sounds like an air noise? Possible blow by I don't know. Only comment would be greatly appreciated. Thanks


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PostPosted: Tue Nov 21, 2017 10:09 pm 
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Joined: Tue Jan 17, 2012 3:19 am
Posts: 7698
Location: Hope, B.C Canada
I did comment in your other post in the projects section (hoser's world).
If you are even remotely unsure about an air leak you should pressure/vacuum check that Engine.


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PostPosted: Wed Nov 22, 2017 12:51 am 
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Joined: Wed Jan 14, 2009 2:10 am
Posts: 4678
Location: Carson City NV
Henry wrote:
Just bought a Honda pilot compression is 185 cold? Running vp112 but still worried that it's too high makes a loud squeak during shutdown almost sounds like an air noise? Possible blow by I don't know. Only comment would be greatly appreciated. Thanks


If you are hearing a loud squeaking noise during shut down and your compression is 185, its not blow by.

Its either an air leak as Can Odd mentioned or a clutch squeak or chirp. My money is on clutch chirp but better safe than sorry, check your Engine for air leaks with a leak down test. If you don't have an air leak then I would start looking hard at the clutch. When was the last time it was lubed?

Rand


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PostPosted: Thu Feb 15, 2018 10:47 am 
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Posts: 1070
Coming from a Jetski background, I may have more to add to this conversation.

1st off, I hear folks are having issues with getting replacement heads from Speedster. You can send your head off to Jim Holiday in Cartersville GA to be re-lathed and re-sized. Jim knows all about how to re-shape for different sized dome requirements, squish band, and the like. All you'll need to do is send him a head and give him squish clearance, angle, and required dome CC volume and he should be able to help you out for a very reasonable price. Call Jim at 770-386-6510.

2nd, CR or Compression Ratio is the actual figure you are trying to meet, not PSI. While pros very well may have figured out max PSI (Which is easily measured on a gauge) of 160 PSI for pump gas, or ~190 for ~110 octane, your actual calculated CR measurements are what it takes to fully calculate your CR, and ultimately your fuel requirements. Pump gas shouldn't run over about 11.5:1 CR. Race gas can go much higher (like 14:1 or even 16:1). CR is also affected by over-boring your cylinder of course.

3rd - cooling. The issue here with land-vehicles is over-all cooling capacity. See, with my jetski, I have an infinite amount of ~ambient temperature water around my craft, and so I can run 15:1 CR (220PSI) on 108/112 octane and still have the cyl/head stay at 150F. You can help by increasing cooling any way you can - bigger radiator/fan, water lines, pumps, cooling passages. One thing found with jet skis is if the water is flowing too fast through the head then the head actually gets hotter, and so restricting some flows give the head a better chance at heat transfer and allow the head to run cooler.

4th - Tuning. Engine compression does not have very much impact on tuning, meaning an Engine at 140psi would very likely have the same jetting specs at as it would at 160psi, if only the head was changed out for one with smaller CC/dome. This makes tuning your Engine easier, as you can tune at a safer compression first before running the higher more detrimental compressions.

5th - at the end of the day, many race jetski-ers had found that lower Engine compression actually yields more power on the top-end - which is important for us land vehicles where often we are rung-out much the same. This has a bit to do with stresses added and heat-soak from the higher compression, EGT, pipe temps, and more. In the jetski world, we generally run much higher compression engines for requirements of bottom end hit (Like for stunts) which don't have much time to heat-soak, and lower compressions for free ride and race crafts (Top end pro-longed speeds).

6th - Run VP fuel like C12 or VP110 in your machine if you can. There are benefits from the lead additive which arguably can add lubricity to a 2-stroke - which is a nice if you believe it. Also since we're not running the machines every day the cost of the race fuel ($8-$14/gallon) doesn't really hurt that much in the grand terms of things. Especially if you have a local pump somewhere - just do it, but also plan on never going back to pump fuel again (Tune and what-not).

7th - Timing. With race fuel, bump the timing (Not sure if this can be done statically at the stator?) The calculation is about 1/8" per 4 degrees of timing - outter rim of the stator. Bumping the timing is something which usually goes hand-in-hand with introducing race fuel to things.

8th - Any Engine porting, specifically around lowering or raising your exhaust ports, affects cylinder port timing, relating to CR, and ultimately what fuel should be run. Expect a modified cylinder specifically around the tops of exhaust and transfer ports to be totally different than for a stock cylinder.

9th - I mentioned SQUISH band, or SQUISH Clearance as it is so called. On jetskis, we run minimum of 35thou clearance on race fuel for freestyle (Bottom-end) but upwards of 50-60 thousandths for to Mid/Top and race boats. Squish needs to be measured and is EASY!!! Two sticks of solder down the plug hole parallel to the crank shaft with the solder up against the cylinder wall side, then carefully turn the Engine past TDC (Top Dead Center) by hand. Mic the 'squished' solder and there you have your squish clearance in-hand. If your squish is too tight, it will severely throw off your compression readings on the gauge! Also could be cause of piston-edge over-heat, preignition, and detonation. I'd think our machines would need to run quite loose in this regard (Maybe 50 or greater squish?) Does anybody know what is the ideal squish?

10th - Gaskets. A thin or thick or lack of a Cylinder base gasket changes port timing and squish band. Port timing changes CR and octane requirements, and squish is well a static thing which you need to aim for. If the base gasket is thinner/thicker than stock, MEASURE YOUR SQUISH (#9). Cylinder head gaskets change your squish clearance only. The same thing goes if you've ever had your cylinder or head decked. MEASURE YOUR SQUISH.


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PostPosted: Thu Feb 15, 2018 10:58 am 
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My thoughts -

1. Measure your squish band clearance. Aim for what is recommended for an FL400r, or perhaps look at what the CR500r dirt bikers use as a baseline (Probably 55thou??)
2. Measure from the top of your exhaust port to the top of the cyl.
3. Measure your piston dome volume (Should be around 7~10CC) Its volume of a cone at cyl bore and piston dome height
4. Measure in CC what your head volume is.
5. Measure at TDC (Top Dead Center), where the edge of your piston is in relation to the top of the cylinder (Could be below or above deck)
6. Calculate your static CR in this way

7. Measure your dynamic CR but use full-stroke instead of #2 above.
8. Find your absolute top end figure your CR could potentially be (Based on dynamic 'things') from this

9. Measure your 'potential' dynamic/static CR
10. Measure from the tops of your TRANSFER/Intake ports
11. Find CR using this measurement.

Because we don't generally know what the intake and pipe will do for adding CR, I like to take an 'average' of the 3x above CR measurements and aim your octane at the average, add 1CR to that figure for safety. Then purchase your fuel based on that number.


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PostPosted: Thu Feb 15, 2018 2:52 pm 
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Location: Hope, B.C Canada
ZeroClient wrote:
My thoughts -

1. Measure your squish band clearance. Aim for what is recommended for an FL400r, or perhaps look at what the CR500r dirt bikers use as a baseline (Probably 55thou??)
2. Measure from the top of your exhaust port to the top of the cyl.
3. Measure your piston dome volume (Should be around 7~10CC) Its volume of a cone at cyl bore and piston dome height
4. Measure in CC what your head volume is.
5. Measure at TDC (Top Dead Center) (Top Dead Center), where the edge of your piston is in relation to the top of the cylinder (Could be below or above deck)
6. Calculate your static CR in this way

7. Measure your dynamic CR but use full-stroke instead of #2 above.
8. Find your absolute top end figure your CR could potentially be (Based on dynamic 'things') from this

9. Measure your 'potential' dynamic/static CR
10. Measure from the tops of your TRANSFER/Intake ports
11. Find CR using this measurement.

Because we don't generally know what the intake and pipe will do for adding CR, I like to take an 'average' of the 3x above CR measurements and aim your octane at the average, add 1CR to that figure for safety. Then purchase your fuel based on that number.


When I did mine this way the computer printed out the letters: "AVGAS"


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PostPosted: Thu Feb 15, 2018 6:17 pm 
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AVGAS is only 100LL octane. Also the molecular structure is of smaller molecules and is a little different. Flash points are higher as I understand - more for high-altitude. Flash point being higher may cause pre-ignition (Probably not), but also has reduced capability toward downward piston push compared to non AVGAS fuel. Also I'm of the understanding that some pre-mix oils do not suspend well in the AVGAS due to the molecular structure.

You cant beat the price though if you can get it. Get what you can get I guess?


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PostPosted: Fri Feb 23, 2018 2:34 pm 
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Location: Pennsylvania
ZeroClient wrote:
AVGAS is only 100LL octane. Also the molecular structure is of smaller molecules and is a little different. Flash points are higher as I understand - more for high-altitude. Flash point being higher may cause pre-ignition (Probably not), but also has reduced capability toward downward piston push compared to non AVGAS fuel. Also I'm of the understanding that some pre-mix oils do not suspend well in the AVGAS due to the molecular structure.

You cant beat the price though if you can get it. Get what you can get I guess?


Ok, I mean no disrespect, however each part of this contradicts another part.

- "Also the molecular structure is of smaller molecules and is a little different." ok, smaller molecules versus? and a little different versus? Each hydrocarbon fuel is based on a distillation fraction of the base material it's made from. In general, higher octane fuels, ie: VP. CAM2, AVGAS, (or even 89 versus 87) are fractions that have more branching of the hydrocarbon chains versus lower octane fuels. Keeping it simple a certain amount of energy must be put into the system (combustion process) to result in the breaking of the bonds holding the branch chains together and thus less propensity for pre-ignition to occur. The backbone chains are not always longer or shorter then a comparable fuel, but the over all structure may be vastly different.

-"Flashpoints are higher, more for high altitude." Ok, the higher the altitude the lower the adiabatic O2 concentration. Aviation fuels are higher in octane by design, one cannot put a low quality fuel in an airplane and hope for positive outcomes. Every decision with avionics is made to stack the deck on a plane taking off and landing under it's own power, not because of a damaged Engine. Fuel flash points must be low enough to allow the vaporized fuel to ignite, but sufficiently lower then the auto ignition point to prevent pre-ignition.

-"Flash point being higher may cause pre-ignition (Probably not)" This is a relative statement, higher versus what? A lower octane fuel? In reality a higher octane fuel will have a higher flash point, yet it should still not readily auto ignite close to it's flash point. The result, fuel that resists auto ignition as it's introduced to the combustion process and a level of complete flame propagation emanating from the ignition source (spark plug)

Ok, I'm not sure even were to go with the piston push statement. As fuel octane increases, there is found a general resistance to pre-ignition and a move to a more CONTROLLED burn rate. Again, as octane goes up, one should expect the fuel to burn in a more control and complete manner (not violently and uncontrollably). There are all kinds of caveats to this, however the trend is in that direction. If a fuel burns in a more complete manner with out demonstrating multiple flame fronts, then for a given Engine the force introduced in the combustion chamber will be greater then the force introduced by a fuel that does not express the same attributes. But think about this, a shitty fuel that detonates, will in fact introduce much higher forces in the combustion chamber, but they will be pressure peaks and may and normally do cause damage (think pinging). Again everything is relative.

I have not heard of any issue with 2 stroke oil mixing or not mixing with AVGAS, in fact the absence of alcohol allows the oil to mix with AVGAS very well. All in all AVGAS is an absolute upgrade from most gasoline found at the pump (87 to 94 octane). AVGAS also has the advantage of being alcohol free. Since methanol and ethanol are both hydroscopic and thus corrosive, they are by mandate not mixed into AVGAS. As to weather one uses AVGAS or a higher octane race fuel should be tied to the over all design of their Engine: comp chamber, compression ratio, MSV..etc. oh, and also there wallet :)

I'm not being a smart ass, but information should be accurate and consistent, for every ones benefit. I typed this off the top of my head, if I screwed the pooch with anything, please, post what is accurate, but please explain details.


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