compression ?

[powermizer]

Glen I would like to ask your opinion on whether one would be better to figure static compressio or dynamic compression to figure what octane to use. Building blowen big block chevs for boats I have found it best to just use static compression, but when doing natural asperation I like to figure dynamic compression. I could only guess that static compression would be best for two strokes, but not sure. Could you please enlighten me. Thanks

[100hp honda]

dont meen to cut in front of glen but this will shed some light on your question. it doesnt talk about dome design, squish angle, width, or clearance...... which is another factor into the eqaution.

http://www.2strokeheads.com/tech2.htm

[powermizer]

Yea RPM, Elevation, Timing, cooling system, and so on.

[glen howell]

In a 4-stroke engine With a hi-lift long duration camshaft you have very little vacumme at low speed as the revs. come up you have more.I relize vac. and comp. are to different things but they are both relivent to each other as a 4-stroke engines stroke is mesured from the bottom of the cylinders stroke . a 2-stroke engine is mesured the same way but the true mesure would be to the closing of the exhaust port because it only makes comp. and vac. at closing and opening of the port , So as you can see the raising or lowing the exhaust opening of a 2-stroke engine is like changing a camshaft in a 4-stroke engine,it has a great to do on both engines with the static and running compression. The head settings has the most effect on the octane of gas to run ,The squish and the quinch and the piston to head clearence ,If you get a comp.reading with the throttle wide open in 5 kicks of over 180 LBS. you most likley will need race gas,any thing below that will dictate the condition of the engine and the using of pump gas. Please ask me on the question answer stickey cause I might not always have a good answer and what I answer which might sound like dribble to some.GLEN

[100hp honda]

If you get a comp.reading with the throttle wide open in 5 kicks of over 180 LBS. you most likley will need race gas,any thing below that will dictate the condition of the engine and the using of pump gas.

so as long as the gauge reads 180+ then you need race gas ? according to RK tek theres alot more to it than that. everything you say seems very vague . not trying to start a fight but just saying how i see it

[2strokeforever]

of over 180 LBS. you most likley will need race gas

seems like a great rule of thumb to me

you and glen need to go see who can get higher up a hill and the winner will never be questioned by the other. ever

[100hp honda]

CRANKING COMPRESSION VS OCTANE REQUIREMENT



Generally, when you increase compression past a certain level, you need to increase the fuel octane requirement in order to combat detonation.

Detonation occurs when the cylinder pressure is high enough to ignite the end gases without the aid of the spark plug. This usually occurs in the combustion chamber end zones (near the bore edge of the dome). This ignition of the end zone gases will create a shock wave that will travel and collide with the on-coming flame front. It is this collision that will do the damage to your engine.

So, think about it..if you increase the fuel's resistance to auto ignition by raising its octane level then it MAY not self ignite before the flame front reaches it.


So, what does this have to do with compression? It stands to reason that higher compression RATIOS will yield higher cylinder pressures (given that nothing else has changed before the compression ratio was raised) Well.. we just determined that the higher pressures are a major player in causing the fuel to auto ignite (not good) hence, the need for higher octane with higher compression RATIOS.

OK, we have been talking about compression ratios NOT cranking compression. What about the cylinder's cranking compression?

Continuing along the above lines:

What about cranking PSI vs Octane requirements?? My opinion is that they are not very closely related. In other words.. knowing what PSI your engine "cranks" out will NOT tell you whether or not you need a higher octane fuel, unless, of course, the PSI number is extremely large (like 200). Then, it may carry more weight and tell you , "yes", you do need some high octane fuel BUT it will NOT tell you which octane you need.

OK, "WHY?"

Let's do some deductive reasoning here:

Take an engine with an exhaust duration of 182 degrees ATDC and install some domes (any domes) that yield.. say 170 PSI cranking compression.


OK, take another engine, exactly the same, except the exhaust duration is at 198 degrees ATDC (ie higher exhaust port) Now, install the SAME domes that the other engines has. Now, the cranking compression reads only 150 PSI.

Hmmm.... so does this engine require a different octane of fuel than the other one? Probably, but not what you think.. It , most likely, requires, a HIGHER octane fuel NOT a lower octane.

I know that the general conception among riders is that the higher the PSI .. the higher the octane needed.. Well, I just told you completely the opposite.
With the general consensus.. it would stand to reason that when ever you raise the exhaust port... you need to run a lower octane fuel because the cranking compression will, indeed, get lower with higher exhaust heights. Does this seem right?? What if you lowered the exhaust height? Would you then need a higher octane fuel because the cranking PSI would surely go UP?


Let's look a few of the determining factors behind the above statements: Please realize that I will be over simplifying the processes but it should get the point across.

1: With an increase in exhaust height or area.. the probability of having more escaped fuel/air charge "stuffed" back into the cylinder just prior to exhaust port closing increases. This will raise your dynamic cylinder pressures and combustion temps.

2: Along those same lines... during the peak scavenging phase the larger exhaust CAN effectively pull more F/A charge in the cylinder and out the exhaust which then leads us back to #1.

3: Crown temps are higher with a raised exhaust. Anytime you have elevated crown temps, you increase your chances for detonation.

IMHO, there is no direct relation between cranking PSI and octane required. I wish there were.

For example.. one of my race engines (snowmobile) cranks only 140 PSI but I know that if I don't run at least 110 octane , the engine will not live for very long.

As we are beginning to realize... these PSI charts really give no relative information unless your engine is stock ported (and even then, I find that they are not useful)

This all relates to information needed when choosing a dome for your engine. I know that when you call up the head shop or go to the counter and ask the parts guy the conversation goes something like this:


YOU:
"So, if I want to stay with premium pump gas, which size domes should I purchase?"

THEM:
"Well, if you want to stay with premium fuel your cranking compression needs to stay below 160 PSI. These XXcc domes will put you right at that correct PSI for premium fuel"


I DO NOT AGREE WITH THIS LINE OF REASONING!


We see that the porting arrangement plays a MAJOR role in the resultant cranking compression. It is NOT only about the head!!!

Then.. You factor in the bore size in the equation and you REALLY can get into a pickle!!

So, you have to be very careful in choosing domes for your engine.

The head (dome) design itself, plays a large role in determining octane requirements of an engine. If the dome is not correct for the engine, you could be forced to run race fuel in order to avoid engine failure. OR on the other extreme, you may THINK your engine requires race fuel, when, in fact, it would live just fine on pump fuel. So, you could be throwing money and convenience right out the window.

[glen howell]

!00 If RK TECH believes like you both of you don't know shit about a 2-stroke engine!!!! Or a 4-stroke either!!! If you lower the exhaust port you would have to take some meterial off the bottom of the cyl. then you would raise the compression,and throw the port timings off to the point the engine would not have a lot of big end revs. but the bottom and mid. range would be better because of the raise in compression and it still might need race gas even after getting the other ports matched. Now if you were to raise the exhaust ports you would get more over revs. but you might lose some of your bottom and mid range .Providing the ports are reworked to except the higher top end and the head was modded also in either case.,what your saying is only what you've read or someone has told you ,you have no personal experence in any or either case ,because you haven't the ability or knowledge or tools to do any of the things you referenced to, I have personally been there and done that good and bad ,I know what works and don't work. Most riders don't like or have the money to buy a cool head and some just plain don't like them. And as far as a paitent on a head design any good machine shop can duplicate a head design and paint it red,blue,or green..Glen

[glen howell]

2-stroker he doesn't have the ability or any of his motorcycles (HONDA'S OR K.T.M.s) that could follow one of my bikes up this.Glen


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[powermizer]

Thanks glen, I understand when you build a 4 stroke engine with a high lift long duration cam that at low rpm the engine is less efficiant, but as higher rpm's come the engine becomes more efficiant than the smaller cam engine, thats why it builds more power. I just thought a two stroke engine worked the same way, by raising ports longer duration and widening would be higher lift. On a 4 sroke engine with a big cam you have less compression at low rpm, but more at higher rpm. Just wonders if the same would be true for a 2 stroke. Thanks again Glen

[rsss396]

Using Static compression numbers is how almost all top builders build their domes and figuire octane requirements.

with a properly set up dome
premium fuel is normally good for around 12 to 1
100 octane for around 14 to 1
108 for around 16 to 1

[100hp honda]

this was the original question

Glen I would like to ask your opinion on whether one would be better to figure static compressio or dynamic compression to figure what octane to use. Building blowen big block chevs for boats I have found it best to just use static compression, but when doing natural asperation I like to figure dynamic compression. I could only guess that static compression would be best for two strokes, but not sure. Could you please enlighten me. Thanks

from what i can see in glens dribble posts, this seems to be his answer i guess

The head settings has the most effect on the octane of gas to run ,The squish and the quinch and the piston to head clearence ,If you get a comp.reading with the throttle wide open in 5 kicks of over 180 LBS. you most likley will need race gas,any thing below that will dictate the condition of the engine and the using of pump gas.

i know when me and danny raised the exhaust we experienced a loss of cranking compression just as RK tek explained. not sure where danny was but i was well below 180psi. glen by your analogy i would need pump gas ? if i was potential customer that called and inquired if i need race gas for my engine, wouldnt you ask a few more questions than just whether or not my gauge says 180 ?

[rsss396]

here is a good write up


http://www.sacoriver.net/~red/uccr.html

[100hp honda]

someone posted that a while back on a qaud site i think. damn good info

[rsss396]

here is a free calculator for corrected and uncorrected compression ratios

along with other free calculators such as squish velocities .....



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

[glen howell]

POWERMIZER to answer your orginal question? I use the static compression because it's easer to get a reading than setting up a complete engine and running it on my tester for dynamic compression Most of the time I only get a cylinder,and head from the customer then I have to mount them on a short block to test them .Gosh that was simple.Glen

[bigred1977]

this is funny,and the main reason why i hate been a mechanic,know all cunts.