Other Compression VS Octane

[PancakePete]

Just wondering what or how to figure this out, I am just wondering.
Alot of Gas sites go by ratio like 15:1 or what not.
How do you figure that out in each motor ?
Why would people say leaded is good for motors, but then I look
on VPfuels site and the c-12 is 108 octane and says its good for skis
then you go to there table and only lists Unleaded Fuels for skis..

What octane for what compression has always stumped me.
Cant there be a table or formula like for bore and stroke to figure
out compression and octane.
Like 190-200 should be 100 oct.
200-210 should be like 105 oct.

Ok, I am retarded ! :dunno:

 

[Crab]

check ada, they have some reccomendation. http://adaracing.com/personal-watercraft/head-kit-yamaha-701-super-jet-girdled/ya701mhdkit

 

[2lick]

Good question, I was Googleing this the other day, no result...

Please help us o great ones!!!!!!!!!!!!!!

 

[Kaveman]

Pete, I'm pretty sure that there are other things that figure into the necessary octane...like ignition timing, ambient temperature and how much WOT time is spent while riding.

As far as VP listing Unleaded only for skis, some of the Yamaha Waverunners have a catalytic converter that is "allergic" to lead poisoning. Other manufacturers may have as well IDK. Also the IJSBA dis-allows the use of leaded fuels in competition.

I have heard that a trace of lead will adhere to any plastic or rubber in the fuel system giving a fail on the fuel test. Therefore if you use leaded fuel to practice then you need to change the fuel tank and all hoses, fuel selector valve, fuel filters (if plastic) along with the gaskets and needle valve in the carbureter. (I am neither agreeing nor disagreeing to the validity of the claims, but won't be taking the chance of being disqualified if it is so)

 

[crammit442]

I have heard that a trace of lead will adhere to any plastic or rubber in the fuel system giving a fail on the fuel test. Therefore if you use leaded fuel to practice then you need to change the fuel tank and all hoses, fuel selector valve, fuel filters (if plastic) along with the gaskets and needle valve in the carbureter. (I am neither agreeing nor disagreeing to the validity of the claims, but won't be taking the chance of being disqualified if it is so)

If the fuel test is a simple litmus type test it's easy to fail if you practice on lead. You're correct that the only safe way to be sure is to change all fuel system parts and clean the carbs before testing.

 

[Shonuff]

Compression divided by two equals recommended octane.

 

[vitaly]

Compression divided by two equals recommended octane.

Yes, pretty close for the 15:1 ratio, but only for the conservative ignition timing. Should be fine for the freeride with some more aggressive timing where you never go WOT for more than a few seconds, which will kill a race ski when prolonged WOT runs are needed.

 

[2XLR8]

Normally even at Havasu if the strip shows a slight color change from previously running leaded fuel they will let you slide. It only takes a small amount of leaded fuel to turn it very dark.

 

[Moving-On]

Time at WOT is a big factor, so is timing(scavenging, compression, load and octane actually determine timing needed)
Straight guage reading is not only indication of octane needed, rate of compression rise is a big factor, that is why it is easier to refer to compression ratios in 4-stroke terms

 

[PancakePete]

Thank you Kave !

I can pick up the phone and bug chuckie, pat and Radney.

I do try and learn stuff and search for things.

Still wondering on the question above.
How to figure out compression ratio?


Other things factor in, but what will add, each degree timing=1/2 oct.

I have no clue ?
Reason I am asking is the gas here is so Crappy its not even funny anymore. And I would rather mix half and half like 92 and 108(c12)=100 oct.

 

[PancakePete]

I got it ! I am retarded !
http://www.csgnetwork.com/compcalc.html

I was searching wrong.. But this is what I was looking for
simplified... THank you for your help ! :fing02:

 

[Crab]

I got it ! I am retarded !
http://www.csgnetwork.com/compcalc.html

I was searching wrong.. But this is what I was looking for
simplified... THank you for your help ! :fing02:

That calculator probably wont help much since it is for valve in head calculations unless you change the stroke to represent the stroke from the top of the exhaust port to TDC. It also won't factor in the cylinder stuffing effect or supercharging that happens from a good pipe and porting.

 

[tor*p*do]

1 atmosphere = 14.7 psi @ sea level
so a compression ration of 15:1 should give
15 x 14.7 = 220 psi

 

[PancakePete]

That calculator probably wont help much since it is for valve in head calculations unless you change the stroke to represent the stroke from the top of the exhaust port to TDC.

Ya in post.. I was saying something like that but smimplier... Just busy here today. Going to look at lunch again. Thanks Crab.

Just intresting to me ! :thinking:

 

[D Slicker]

Pete, Im running 185psi on 92 octane with no issues.

 

[PancakePete]

Pete, Im running 185psi on 92 octane with no issues.

Werd, old domes 210.. new ones should be 200 ish Probly.
Still alot for pump gas.. and Crappy pump gas at that.

Its all good. :biggrin:

 

[crammit442]

The safest thing to do is to get your builder to give you what he thinks is a safe Motor Octane recomendation for your setup. Most(not all) race gas motors will live on a Motor octane number of around 100 give or take if used in a freestyle boat. Compression ratio/psi# is only a part of the equation. When you mix race/pump fuels make sure to calculate the number using motor numbers, not the research number. Also, remember that all race fuel is not the same. Modern fuels can make substantial power gains even on a stock boat. This assumes you get the best fuel for your application. Oxygenates and other chemicals can make big power.

 

[Big Kahuna]

Normally even at Havasu if the strip shows a slight color change from previously running leaded fuel they will let you slide. It only takes a small amount of leaded fuel to turn it very dark.

People have been DQ'd at havasu just by buying unleaded 92 octane gas at the shell station. That unleaded gas failed the test. Go to PWCToday, search the racing section from 2004/2005/2006. There are plenty of threads about this, the test they use is bogus and not accurate.

 

[crammit442]

People have been DQ'd at havasu just by buying unleaded 92 octane gas at the shell station. That unleaded gas failed the test. Go to PWCToday, search the racing section from 2004/2005/2006. There are plenty of threads about this, the test they use is bogus and not accurate.

That has happened in the past and may continue to, but because of past issues with this, more discretion been shown in the last couple of years. The problem with the test isn't that it's bogus, it's just pass/fail. A legal amount(trace) of lead allowed. The test used doesn't differentiate between trace and actual leaded fuel. There have been a couple of motorcross DQ's reversed by further testing. Testing for exact amount of lead is considerably more difficult and expensive. The easiest thing to do is just run the same thing in practice that you run in races. I think more often than not, people get caught out because they run AVgas or the cheapest leaded fuel they can find in practice to save money.

 

[Big Kahuna]

That has happened in the past and may continue to, but because of past issues with this, more discretion been shown in the last couple of years. The problem with the test isn't that it's bogus, it's just pass/fail. A legal amount(trace) of lead allowed. The test used doesn't differentiate between trace and actual leaded fuel. There have been a couple of motorcross DQ's reversed by further testing. Testing for exact amount of lead is considerably more difficult and expensive. The easiest thing to do is just run the same thing in practice that you run in races. I think more often than not, people get caught out because they run AVgas or the cheapest leaded fuel they can find in practice to save money.

At WF's, they are not going to take the steps that Supercross/AMA does after the initial test. If there was a trace, then you got DQ'd, because they could not determine if it was trace element or not. Granted, I have not heard much about it in the past 1-2 WF's as to being as much of an issue like it was in 05.

Pete, listen to Charles, or call your builder, they should have told you what your fuel requirements should have been from the get go.