DusterBoy15
THE WISE(guy) MEMBER
i dident see if its a dished piston part
octane levels
- Thread starter sweatybetty
- Start date
-
Thrashard340
Well-Known Member
It's not labeled as a dished part. The -5 cc's is for the valve reliefs - kb107's.
AND apparently wasting money.
AdamR
Big Member
The higher octane burns slower and to slow of a burn will hurt power. That is why you dont run race fuel with low compression.
340mopar
Sorgermotorsports
The easiest way to find out what octane to run (because there are so many variables) with say 10:1 is build your engine the way you want. Put in the highest octane pump gas you can get (if this will be a street car) and get the motor dialed in as far as fuel curve and timing curve to achieve maximum power. Now lower the octane of the fuel you put in until you get to the point of detonation then go back up one step in octane. This is the minimum level of octane that your motor needs to not detonate and will put out the most amount of power. Going higher than this minimum will not do any thing more for power output. All it will be is a waste of money and octane out the pipe.
Chuck
Chuck
Chuck has this exactly correct. I tune my 11.5 to 1 383 for max power including the maximum initial and total advance that the combination will handle and through just this kind of experimentation I was able to determine that 5 gallons of purple per tank of 93 is exactly what I need, no more, no less.
2 Darts
A-body Addicted
Octane is not "just a number". It relates to the flash point (temperature at which the fuel will spontaneously ignite) of gasoline. The higher the flash point, the more heat required to ignite. Low octane gas in a high compression engine knocks because it is igniting before the engine gets sufficiently into the "power" stroke.
http://www.howstuffworks.com/gasoline3.htm
Since heat is the determing factor, it makes sense that an aluminum head can utilize a higher compression ratio on a given octane fuel. Conversely, engines with aluminum heads will be less prone to knock at a given compression ratio and fuel octane.
http://www.howstuffworks.com/gasoline3.htm
Since heat is the determing factor, it makes sense that an aluminum head can utilize a higher compression ratio on a given octane fuel. Conversely, engines with aluminum heads will be less prone to knock at a given compression ratio and fuel octane.
jysnflem
Well-Known Member
- Joined
- Feb 29, 2008
- Messages
- 499
- Reaction score
- 37
I was filling up the Dart at CostCo with 93 octane and had a conversation with the attendant about how much cheaper their 93 was compared to 94 at Sunoco. Everywhere in Michigan the gas has 10%ethanol. He mentioned that the ethanol increases the octane but they don't advertise it. So I looked this up on line and found out this to be true. So how much increase in octane is there I wonder. Jysnflem
moper
Well-Known Member
I can't think of a more shortsighted way to build an engine than to build it, and then find by trial and error after the fact which fuel is the least destructive. A true race engine had best be built with the exact fuel type in mind to take advantage of all the traits of that fuel. A street engine for the environment in which it will be living. You want the fuel that puts the most pressure on the piston for the right number of crank degrees(burn time) at the right time in crank degrees and is stable in vapor form during the cylinder fill and squeezing. Period. It's not just heat. It's stability during compresson. If the fuel can stay in vapor form until the spark lights it, you're golden. Whether that's 7.5:1, 10:1, or 15.5:1. Adding ethanol does add octane, however, the amount of ethanol must be much higher than 10% to reach a point of additional power produced. Alcohol has a bit less energy for a given volume than gasoline. So you need a lot more of it to do the same work. But, the octane rating on it is much higher, and the heat absorption of it is amazing. So you can run much higher compression ratios with it, and reap the benefits of more pressure at the right time. 10% ethanol actually replaces the gasoline molecules with less potent ones, so you are losing efficiency in an engine deisgned for gasoline. Your milage is worse on it because you are leaning out the A/F ratio and either the pc in your regular car, or your dyno guy has to compensate for it. But it does burn cleaner by the sniffer. By building an engine to make use of modern design and technology, you are not just band-aiding a built in problem of pre-ignition or detonation. You're designing those issues out so you can run common available fuel. BTW, the Ethanol levels are gogint o go up between now an 2012, IIRC up to 15%. So the builders that ignore fuel type/octane will have more issues coming as the levels rise. The best way to determine what fuel you will need is cylinder pressures. And static ratios mean next to nothing. One can build an 11.8:1 505 that runs 93 pump fuel and makes about 590hp; or one can build a 422 LA with 9.5:1 that runs on 87 pump fuel and makes around 430hp; or one can build a 12.5:1 440 that must run VP C12 that makes 480hp. The first two have dynamic compression ratios of 8.5:1 and 8.2:1 respectively. The last one has a dynamic of 10.1:1 and needs the race fuel to stay healthy. It's the interaction of piston, chamber, stroke, rod length, and cam intake lobe that gets added to the formula of static compression to figure dynamic. Play with the calc on kb-silvolite.com for some interesting relationships between parts choices. I'd rather fill a car with 87 at $4.25/gallon than C12 at $8.75/gallon. Not to mention being easier to find it at 10pm on a cruise night.
-
