Questions about race gas

[dburnexx]

I have been running a 50/50 av gas mix in my trail bike. lookin to switch to a race gas mix what kind do I need I didnt realize that there was so many kinds.

 

Sent from my GT-S5360L using Tapatalk 2

[dozer]

just run straight av gas

[SAVAGE420]

Yes.

[trickedcarbine]

Av is nice, if VP is available and you got coin it's nice. To really take full advantage of the expensive fuel you need to have a builder set up domes for it.

[trickedcarbine]

Av is nice, if VP is available and you got coin it's nice. To really take full advantage of the expensive fuel you need to have a builder set up domes for it.

[WINDYCITYJOHN400]

If you have access to AV fuel....Run that.

It's the same as 108 octane race gas.

[Derrickc14]

Where do you get av fuel at? I'm in the philly area.

[special06shee]

Small air ports.

[dburnexx]

Yea I cant run pump gas on a compression gauge im at 157. I was told goin to race gas i would see a major increase in power but im goin to try full av gas first its alot cheaper and easy to get

 

Sent from my GT-S5360L using Tapatalk 2

[BUILDER]

So you need race gas at 157?????

[dburnexx]

I was having detonation problems after goin to a 50/50 av gas mixture the problem went away I am running 18cc domes in a cool head, alot of port work on cylinder and case, hot rods crank stock lenth, toomy t5 pipes, k&n air filter open box, v force reeds, stock carbs jetted, timing advanced + 2.

 

Sent from my GT-S5360L using Tapatalk 2

[uman1030]

You can run pump at 157?

depends!

 

Cranking compression is not the same as compression ratio and heavy porting can give you what seems to be a pump gas cranking compression when in actuality you have a race gas compression ratio. here is a copy paste explaining what i mean.

 

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.

[uman1030]

Copy and paste.......

 

 

 

Anyways, my response was a statement, not a question.

Was directed at the thread in general. And statements usually dont end with a question-mark.  Regardless I said here is a copy paste explaining what i mean. so your statement is rather redundant. way to go captain obvious.

[swhitey917]

I have ran both VP C-12 and Turbo Blue racing fuel. Had great results with both. VP is pricey but it is what I can get the easiest since I have a distributor about 20 min from me so that's what I run in my R. I have never ran AV fuel so I cant comment on that but I would like to look into it more. 

[Texas2stroke]

So. While we are on this topic, I am running a 4mill crank. 110 rods. 68mm pistons with 19cc domes in a cool head and a cub cylinder. What octane rating should I be running? And what fuel/oil ratio would be recommended?