Any performance difference in Sunoco Purple? - Page 2

Adger Smith · 02-25-2015, 07:50 PM

Quote"Mike in your description, would you go up or down a jet size with the SG lowering? From Say a .760 to a .740?"end Quote
There are 2 variables with fuel weight (SG)
The closer you get to 1 ( water) the more dense a fuel becomes.
The density causes 2 or more changes in the carb. The 2 most dramatic are the flow through a jet and the way the float "floats" in the fuel.
A lower (SG) or viscosity fuel will flow more through an orifice (jet) and a higher (SG) or viscosity will flow less.
A lower (SG) allows the float to sink into the fuel and raises the float level.
When dealing with air mixes(air bleeds) in the fuel the less dense will pick up/mix air more readily than the dense.
Below is a copy and paste of a general rule of thumb....
Density of the fuel. (a) This is the weight of fuel compared to water, water being at 1.00. If race fuel is .750 (specific gravity) it would weigh ¾ the weight of water. If water at 60 degrees Fahrenheit weighs 8.125 pounds per gallon (ppg), then race fuel at .750 will weigh 6.09 (ppg). (b) The less dense the fuel (lower specific gravity) the higher the BTU content and the better the stability of the fuel at higher RPM's. (c) The lower the specific gravity of the fuel, the more crucial the jetting becomes, and the easier it is to run the engine lean, possible causing internal damage. (d) The combustion speed is determined primarily by the pressure (density) of the fuel mixture. A lower (SG) or viscosity fuel will flow more through an orifice (jet) and a higher (SG) or viscosity will flow less.

Mike Croley · 02-26-2015, 05:09 PM

One other thing to keep in mind, when you change specific gravity, always check and re-adjust float levels. That goes for even small changes in SG that wouldn't require a jet change.

02-25-2015, 07:50 PM	#11
Adger Smith VIP Member Join Date: Dec 2001 Location: Texarkana Ark/TX Posts: 2,348 Likes: 433 Liked 676 Times in 260 Posts	Re: Any performance difference in Sunoco Purple? Quote"Mike in your description, would you go up or down a jet size with the SG lowering? From Say a .760 to a .740?"end Quote There are 2 variables with fuel weight (SG) The closer you get to 1 ( water) the more dense a fuel becomes. The density causes 2 or more changes in the carb. The 2 most dramatic are the flow through a jet and the way the float "floats" in the fuel. A lower (SG) or viscosity fuel will flow more through an orifice (jet) and a higher (SG) or viscosity will flow less. A lower (SG) allows the float to sink into the fuel and raises the float level. When dealing with air mixes(air bleeds) in the fuel the less dense will pick up/mix air more readily than the dense. Below is a copy and paste of a general rule of thumb.... Density of the fuel. (a) This is the weight of fuel compared to water, water being at 1.00. If race fuel is .750 (specific gravity) it would weigh ¾ the weight of water. If water at 60 degrees Fahrenheit weighs 8.125 pounds per gallon (ppg), then race fuel at .750 will weigh 6.09 (ppg). (b) The less dense the fuel (lower specific gravity) the higher the BTU content and the better the stability of the fuel at higher RPM's. (c) The lower the specific gravity of the fuel, the more crucial the jetting becomes, and the easier it is to run the engine lean, possible causing internal damage. (d) The combustion speed is determined primarily by the pressure (density) of the fuel mixture. A lower (SG) or viscosity fuel will flow more through an orifice (jet) and a higher (SG) or viscosity will flow less. __________________ Adger Smith (Former SS)

02-26-2015, 05:09 PM	#12
Mike Croley Member Join Date: Apr 2010 Posts: 182 Likes: 0 Liked 0 Times in 0 Posts	Re: Any performance difference in Sunoco Purple? One other thing to keep in mind, when you change specific gravity, always check and re-adjust float levels. That goes for even small changes in SG that wouldn't require a jet change.