Today's
Fuels by Larry Peto
Today's
Fuels
by Larry Peto
I would like to try to enlighten you on some of the problems associated with today's gasoline in carbureted engines. These problems exist whether an engine is rebuilt to stock specifications or modified for increased power and include engines for tow vehicles, 4x4, and marine use. Some street performance and race engines share some of these problems but generally operate with a higher-grade fuel.
I am by no means a Petrol-chemist but I do know engines and how different types of fuel will react in different engine combinations.
One of the biggest problems of today's fuel in carbureted engines is drivability i.e. vapor lock-hesitation, loss of power, inconsistent throttle response, and black smoke all relate to vapor lock.
Gasoline today has a low vapor point, which means it turns gas into a vaporous state as a lower temperature. A carburetor is designed to meter fuel in a liquid state not a vapor. The vaporization should take place in the manifold for the engine to run correctly. This problem is aggravated by the fact that fuel blends vary depending on the time of year. We see those problems less in marine applications than towing because of engine temperatures. Marine engines can run consistently at 140 -150 degrees where a tow vehicle can run constantly at 210 - 220 degrees.
We have observed this problem for many years but has become worse. You might ask why we are forced to use this fuel? The answer is emissions. Every new vehicle built today is fuel injected. Fuel injection requires the fuel to be under pressure right up to the injector. When you pressurize a liquid it raises its boiling point. Most throttle body systems require 14psi and most multi-port injection systems require 40-45 psi. End of problem.
This doesn't spell death to the carburetor but is helping in its demise. There are a few things that can be done to make the problem livable, but not totally eliminate it short of retrofitting to a modern EFI system. An electronic fuel pump installed at or in the tank will help by providing pressurized fuel all the way to the carburetor. Remember that delivering fuel under pressure will help keep it from vaporizing so easily. Unfortunately, the fuel can still vaporize inside the carburetor because it is no longer under pressure. The only thing that can be done here is to keep the carburetor and fuel cooler. Heat shields under the carburetor, fresh air intakes to the carburetor from outside the vehicle (not hot air coming thru the radiator), and lower cooling system temperatures are about it.
The second problem we are faced with is detonation or pinging under load. Uncontrolled detonation can cause catastrophic engine damage in a big hurry. If you hear pinging or engine knock, get out of throttle, switch of higher-grade fuel, and reduce engine temperature. Myself or no other custom engine builder will warranty an engine that has detonation-induced damage. What we are doing to help prevent this problem when we build an engine, knowing that it is going to run pump gas, is to reduce cylinder pressure be reducing compression and on supercharged engines reduce boost.
This is a tough situation when we want to build more power. Obviously temperature, ignition timing, air fuel ratios, piston and chamber design, engine construction materials i.e. aluminum or cast iron, and of course the load that the engine will see all affect this problem. The thing to remember is that we can't just jack up the compression like we did 20 years ago with today's fuel.
When it comes to newer style engines with modern electronic controls this is again less of a problem. With sophisticated knock sensors and electronics the ECU's can modify ignition timing and fuel delivery to help reduce the problem. I presently own a '94 and '96 vehicle with these features but at times we still hear pinging under certain circumstances even with 92 octane unleaded.
Today's gasoline also doesn't store well. This is a problem that relates more to marine use but can be a problem with vehicles that sit unused for any period of time. Supposedly when fuel sits its oxygen compounds dissipate and what was once 92 octane is now 86. Heat and time seem to be the contributing factors here.
Today's fuel is also hard on carburetors. We rebuild more carburetors today then ever before. Not from dirt and varnish but from the fuel attacking various components requiring replacement. In light of all this t6he owner of the vehicle needs to assume a certain amount of responsibility and realize in an extreme duty application such as those discussed and deal with them as best you and your engine builder/tuner can.
In closing another matter that needs to be mentioned is that the stated octane value on the pump is not always what it should be. One way to deal with this is to buy fuel from the same station as much as possible and hopefully it is one that sells a lot of fuel keeping its stock fresh.
LARRY PETO at larry@larrysengine.com
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