West TX RX-7

What do the Initial and final boiling point of a race fuel determine in reality. I'm having some knock issues. The car was tuned on C16

Specification Sheet: VP C-16

Specific Gravity: .735 @ 60°F

Lead: Leaded

Color: Blue

Motor Octane: 117

RVP: 1.85

Oxidation Stability (min.) 1440+

Distillation:
10% evap @ 211.5°F
50% evap @ 213.1°F
90% evap @ 215.8°F
E.P. @ 233.3°F


but I'm running This:

Specific Gravity 0.715
Antiknock Index (R+M)/2 112
RON, Research Octane 114
MON, Motor Octane 110
Reid Vapor Pressure 8
Distillation, ºF
Initial Boiling Point 90
10% Evaporation 155
50% Evaporation 215
90% Evaporation 235
Final Boiling Point 260
Oxidation Stability, Min 1440+
Copper Strip Corrosion No. 1
Existent Gum, mg/100ml 1


Whats the difference?

Also what would this fuel below do differently?


Specification Sheet: VP Blue

Specific Gravity: .717 @ 60°F

Lead: Leaded

Color: Blue

Oxidation Stability (min.) 1440+

Motor Octane: 108

R+M/2: 109

Research Octane: 111

RVP: 7.20

Distillation:
10% evap @ 140.1°F
50% evap @ 214.0°F
90% evap @ 248.4°F
E.P. @ 389.3°F

West TX RX-7

BTW: I'm NOT leaning out

Evil Aviator

It is part of the distillation curve (volatility curve), which shows the temperatures at which the fuel will vaporize. This pretty much just affects starting, vapor lock, and acceleration & throttle response. It may affect other things, but this is a bit beyond my realm of knowledge, and is probably best explained by the fuel vendor.

Knocking problems are usually reduced by moving to a higher Motor Octane Number (MON). If your car was originally tuned with 117 MON fuel, it is possible that your current 110 MON fuel is the problem. Moving to the 108 MON fuel will only make things worse.

IMO you should always have your engine tuned to the exact fuel you plan to use.

The car was originally tuned with fuel that has a .735 specific gravity. You are currently using a fuel with a specific gravity of 0.715. Therefore, your current fuel weighs about 2.8% less per volume than the original fuel. Does your induction system meter fuel by volume? If so, then you are running lean.

RETed

How do you know that?


-Ted

Evil Aviator

Correction: This assumes that both fuels have the same stoich mixture.

West TX RX-7

I use a J&S knocksensor with A/F and knock guage. I can watch the timing being retarded when I'm on the gas and also see the A/F.

Evil Aviator

Is your A/F gauge set to AFR or Lambda?

West TX RX-7

Hmmm That I am unsure of but I'll find out.

Evil Aviator

I guess the main question is whether your AFR gauge uses 14.7 to convert the lambda from the O2 sensor into an "AFR" number. The problem with this is that race fuel usually doesn't have a stoich mixture of 14.7, so the AFR gauge reads the improper AFR due to the incorrect conversion factor. If your AFR gauge reads the lambda (or voltage) directly, then there is no conversion factor, and the reading is therefore always correct (in this respect, anyway). If your AFR gauge has the capability to use a different conversion than 14.7, then you should enter the stoich value of the fuel as determined by the fuel manufacturer.