Gasoline, Detonation, Timing, and Twin-Ignition

This column is about Fuel, Detonation, Timing and Twin-Ignition for air-cooled cars. These subjects are all inter-related in the optimization of performance.


Fuel quality problems in the USA have been present since the EPA mandated the removal of lead in automobile gasoline and the major oil companies have struggled to maintain minimum octane requirements as well as comply with oxygenated and reformulated fuel regulations. The challenge to maintain seasonal and regional drivability and maintain resistance to detonation has resulted in motor fuels that have some unpleasant characteristics. In addition, the compression ratios in Porsche cars have gradually crept upward since 1982 in the effort to improve performance, fuel economy and overall efficiency. The air-cooled Porsche racing engines and 3.6 litre versions of street engines equipped with twin-ignition haven’t suffered quite as much from the latest gasoline formulations and the 4-valve, water-cooled heads used in the 996/997/991, Boxster/Cayman cars are much more tolerant of differing fuels.

Performance issues that affect all cars including Porsche naturally aspirated as well as turbocharged models are the constant variations in the chemical makeup of motor fuels and the seasonal changes made by the oil companies to comply with Federal Clean Air Act regulations. Not all areas of the USA mandate the use of oxygenated or reformulated gasolines, however there are changes in the fuel composition due to seasonal temperature changes. The net effect is the change in power and driveability that most Porsche owners regard as normal or perhaps the need for a tune-up. Throttle response, acceleration, knock resistance, and overall power are affected by these changes in gasoline. Racing gasolines have been widely accepted now as the main solution for maintaining peak performance in Porsches used for DE events and competition. Besides the obvious octane issues, the main reason why most racers use racing gasoline is to eliminate the variables in specific gravity, vapor pressure and octane present in today’s pump fuel. Racing fuel formulations and offerings do vary among the different manufacturers as well for different applications so one should try several brands & variants to optimize power.

Each gasoline formula performs differently in Porsche cars and one should try several brands within a similar octane group to determine which one offers the best throttle response and power. There is a big difference! Pump gasoline octane rating numbers posted at the gas pumps do not tell you very much about what to expect using that fuel. That yellow sticker is the average of the Research Octane Number and the Motor Octane Number of that particular fuel. The RON of a fuel affects low to medium speed knock and engine run-on. This number is typically 8-10 numbers higher than the MON. The Motor Octane Number is determined under higher RPM and high loads. This affects the high speed and part throttle knock characteristics and this is of greater importance to owners of high-performance cars. If the MON is too low, you could experience detonation during passing maneuvers and when climbing hills under load. If the RON value is too low, then you might experience part-throttle detonation.It is VERY important to realize that some engines respond better to a higher RON and others will perform better with a higher MON depending upon combustion chamber shape,compression ratio and spark plug position. Since RON and MON values ratios are proprietary to each manufacturer of gasoline, you must try different fuel to see which one performs best in your car. Load dyno testing will also help determine what fuel to use. These variations in RON and MON will not change the posted average octane rating at the pump.

Although some unleaded racing gasolines are illegal to use in cars that are driven on public highways, blending unleaded racing gasoline with pump fuel can help eliminate these differences in the fuel used in your Porsche and raise the overall octane rating of the blended fuel.  Now you can find EPA-approved racing gasolines which are oxygenated for compliance. This is always a good idea when participating in Driver Education or any other Track events. Be absolutely certain that the racing gasoline that you may use is UNLEADED. Do not use leaded racing gasoline in any car with a catalytic converter, even for track use.

Other factors that affect driveability in Fuel injected and Weber-equipped 911’s are the volatility and inclusion of ingredients that attack the rubber components in the fuel system. Gaskets, O-rings, fuel lines, and float needles are all affected by the compositions of gasoline. Some brands of fuel have been known to dissolve the interior of rubber fuel hoses so these items will need constant observation and regular replacement to prevent engine problems.  This is why its foolish to simply add some Toluene, Aniline, and Zylene to the fuel tank, even though those compounds are proven ingredients in racing fuels. For example, a leaking rubber-tipped float needle in a Weber-Carbureted 911 can fill a cylinder with fuel causing hydro-lock and result in engine damage as well as a major fire hazard. Unleaded racing gasolines generally do not cause these problems since they are formulated not to attack rubber and plastic such as the foam inside racing fuel cells. This can very among fuel manufacturers. We strongly recommend using ethanol-resistant fuel hose and Viton seals/O-rings in the fuel system for durability.


Now is the time to mention one of the most common octane-enhancing agents present in in pump gasoline, Ethanol. With an octane rating of 112, Ethanol does a commendable job of increasing octane when used in correct quantities, however it has some drawbacks that you need to know. In most cases, the Ethanol content is posted on the pump so that you know how much has been blended in the product. These blends can change during the fall and winter seasons. Ethanol is an alcohol that has very clean-burning characteristics, but its also hygroscopic, miscible, and will scour out any dirt or impurities present in the tank and deposit them downstream into the fuel filter plugging it up very quickly. The use of Ethanol will require the frequent replacement of fuel filters. In addition you will  experience a drop in fuel mileage due to Ethanol’s lower energy content. The mixture changes caused by blending Ethanol is somewhat self-compensated by most fuel injection systems, however carbureted cars may require some adjustments or jetting changes to maintain performance. Today fuel formulations requires the constant use of fuel system cleaners to maintain throttle response and power due to carbon buildups. This is a slow process and sometimes the performance loss isn’t apparent until hesitations, misfires, hard starting and rough idling manifest themselves. Regular use of Chevron’ Techron or LubroMoly’s Jectron together with Ventil Sauber products will prevent these problems. In addition, some Porsche dealers and independent repair facilities that employ the MotorVac fuel system cleaning process, can restore full power and driveability.

Here is some quick data about 3 major brands of Unleaded Racing Gasoline that illustrates some differences that you should consider when choosing which fuel to try. Leaded racing fuels are illegal for road use and will contaminate a catalytic converter. Unless you have a static compression ratio over 11.5:1, these unleaded racing fuels should work just fine, especially with twin-ignition.

 UNOCAL 76Trick UnleadedSunoco GT+

As you can see, changing the RON and MON combinations may not make a change in overall octane, but their burning characteristics and anti-knock qualities can vary enough to make a significant difference in how your Porsche runs! The Reid Vapor Pressure, or RVP, is an indication of volatility which affects the fuel’s resistance to vapor lock, its ability to start easily when cold, and its throttle response. The fuel’s specific gravity specifications, also affect the float levels in carbureted cars and mixture settings. If the fuels’ RVP is too low, 5.5 or less, this can cause poor throttle response and stumbling when the throttle is suddenly opened. RVP’s that are too high, 8.0 or more, can create vapor lock and fuel percolation in float bowls.

Here is a mixing chart for blending 91 Octane Unleaded Premium with 100 Octane Unleaded Racing Gasoline.

Gallons100 oct race gas1234567891011
91 oct pump gas

Porsche has used twin-ignition in their air-cooled racing engines since the 4-cam Carrera engines. The high-domed pistons necessary for high compression ratios requires another spark plug to start another flame front on the other side of the piston dome. The best place for a spark plug is the middle of the combustion chamber to ensure even flame propagation.  A centrally positioned plug allows the flame front to travel the least distance for complete ignition. This reduces the need for ignition advance to start and finish the combustion process when the piston reaches Top Dead Center. Since the spark event is starting closer to TDC in the compression cycle, there is less pressure from the beginning of ignition that is pushing ‘back’ down on the piston crown as the combustion event progresses. This lessens the ‘negative’ work done by the expanding gasses and allows all of the pressure building in the cylinder to push the piston in the correct direction, making the engine more efficient.

Single Plug combustion pattern

The offset-plug position in the air-cooled 911 delays the combustion process since it takes longer for the flame front to progress across the piston to the opposite side. By installing two spark plugs per cylinder, the combustion process is accelerated and can reduce the required advance by as much as 10 degrees, thus lowering cylinder head temperatures. In terms of power, twin-ignition will add some 3-5% or more depending upon compression ratio over a single ignition system. RPM can increase as much as 700 RPM at top speeds. If high compression ratios are to be used, twin-ignition allows all of the power benefit to be gained from the increase. Another benefit is twin-plug equipped 911 (and 930’s) run much crisper and cleaner with lower cylinder head temperatures and improved throttle response. Plus, a twin-plug 911 is much less prone to plug fouling with today’s fuel.


Twin plug combustion pattern

Besides the damage to one’s bank account, there isn’t one drawback to installing this system and enjoying the benefits in throttle response, power and great drivability that twin-ignition adds to any 911 or 930. Quite a difference! In some cases, it is a necessity to realize the maximum gains from a complete set of engine modifications. The water-cooled engines (Mezger/M96/9A-series) have a much more optimal spark plug location at the top and center of the combustion chamber that negates the need for two spark plugs per cylinder.

Twin-ignition Hardware

Now comes the spendy part,…how to implement this in 911 and Turbo engines.

There are 4 basic methods to installing and using dual plugs in these engines:

1) Stock Distributor converted to RSR configuration.

This is done by taking the Bosch OEM unit and mounting a billet adapter ring to accept the Bosch 12-nipple RSR cap and machining the distributor shaft to accept the RSR rotor. The cap and ring must be phased to the rotor alignment using a distributor machine and the advance curve should be modified to suit. can be converted This setup can trigger either the OEM Bosch CD boxes or a pair of MSD 6AL’s with MSD matching coils for best performance. This setup is ideal for all 2.0-3.5 litre engines with carburetors, EFI or MFI systems.  For 930’s, this is the hardware of choice since this retains the all-important boost retard feature that is critical to engine life and proper throttle response.

2) 964 Dual-distributor converted for 2.4-2.7-3.0-3.2 litre Engines.

This one is done by installing a trigger from a donor SC distributor and using the appropriate crank gear, depending upon the engine being used. This unit will not fit the Turbos’ due to interference with the boost plumbing. Again, these can be triggered by OEM Bosch CD boxes, MSD’s or a proprietary splitter unit for Motronic-equipped engines. This setup is ideal for any Motronic motor and works very well in carbureted or MFI-equipped engines. The 3.6 crank gear must be used for these conversions.

3) Crank-triggered Distributorless Ignitions.

The ubiquitous Electromotive coil-pack and TEC-III Engine Management systems fall into this category as does the superb but expensive MoTec ones. These can be challenging to install and require additional RFI and EMI shielding to prevent stray ignition signals that create random misfires. We use additional grounding straps and high-quality plug wires to eliminate this possibility and maintain the integrity of the ignition signals. Some of these coil-pack ignition systems operate at system bus voltage and do not produce the amount of ignition current needed to keep the plugs clean in a rich mixture environment. Carbureted and MFI-equipped engines do operate with rich idle mixtures needed for best throttle response and will foul plugs MUCH easier with these ignitions.

The other big drawback with some of these DIS is that you cannot use plug gaps larger than .032 without misfires. Its well proven and documented that plug gaps in the .040-.045 range make more overall power and result in better idle qualities due to the larger flame kernel and the resulting more thorough combustion process.

4) Custom Twin-plug distributors.

There are several options for brand new twin-plug distributors that employ magnetic triggering, adjustable advance mechanisms, and ball-bearings. While not cheap, these are one of the best solutions for installing twin-ignition on any engine.


There is a wealth of information available on these subjects and we have only scratched the surface here. If you would like more information on these issues or other performance-related Porsche questions, please contact: