By: Henry P. Olsen

Setting up the fuel and ignition systems to get a blown engine to run properly may take some time, but the results are worth it. The engine must be built with the supercharger in mind, the compression ratio, as well camshaft should be matched to the application and designed use. Always select the camshaft to match the rpm band the engine will normally be operated in, a race cam may make power at high rpm but it will not run well at lower engine speeds. There are a few “set-ups” not used on a normally aspirated engine that can make all the difference in how the engine runs. The camshaft lobe separation that seems to work best for a supercharged engine is 112 to 114° along with a split duration camshaft where the intake lift and duration may be mild yet a more aggressive exhaust cam profile. This type of camshaft can be used on a supercharger since you are blowing the air in to the cylinder; this type of camshaft creates more horsepower without hurting low rpm torque. Using tools such the OTC/SPX PerformanceGas or MicroGas exhaust gas analyzers and advance timing lights allows us to tailor the fuel and ignition advance curves to unleash all the horsepower the supercharger is blowing into the engine.

Blow through carburetor supercharged set-ups

The first concern for an engine using a blow-through supercharger where the carburetor is under boost pressure is fuel pressure. A special fuel pressure regulator that is boost pressure referenced along with a high pressure/volume fuel pump to keep the fuel pressure correct for this type of supercharger system must be used. The boost referenced pressure regulator senses the boost pressure and then regulates the fuel pressure to keep the fuel pressure at a set amount above the boost pressure. The pressure that we use most is 5 ½ to 6 lbs. over the boost pressure, so when you have 6 lbs. of boost the fuel pressure would be 11 ½ to 12 lbs. The floats used in the carburetor on a blow thru supercharger must be made of nitrophyl or of some other solid material, brass or hollow plastic floats will collapse from the pressure the supercharger puts into the fuel bowls. The 2 main ways of pressurizing or blowing through the carburetor are #1 where the carburetor is in a pressurized box and #2, is the use of a hat in place of the air cleaner on top of the carburetor. If the carburetor is not inside of a “box” but using a hat to pressurize the carburetor bowls & venturii, the throttle shafts will have to have seals installed to keep the fuel from blowing out thru the clearance in the throttle shaft. We always recommend a marine type of flame arrestor be used inside the air box or hat to defuse the air-charge, if this is not used the air charge blowing from just one side of the carburetor will cause the air/fuel mixture to be incorrect as the boost changes.

The idle and off-idle systems are the hardest part of the fuel system to get the fuel curve correct with a blow-through supercharger system, extreme care must be taken to properly size the idle fuel and air restrictions in order to obtain the correct idle and part throttle fuel mixtures. The reason this care must be taken is because this type of supercharger is pressurizing the idle air bleeds thus blowing the fuel into the engine thru the idle system. The rest of the carburetors fuel circuits operate in a normal fashion. The fuel mixture at idle speeds and cruise speeds seen most are idle CO of 1 to 3%. The cruise mixture on a low boost, mild cam engine would be 1 to 1.5% CO while the high boost pressure hot cam cruise mixture is 3% CO. A power mixture of 6.6% CO (12 to 1 air/fuel mixture) is used on most engines, yet you can go richer if needed to help control detonation.

Pull-through Carburetor Supercharger Set-ups

An engine using a carburetor on top of a supercharger uses a standard set up for fuel pressure; the fuel pressure we use most is 5 ½ to 6 psi. The most important point is to use a high volume fuel pump along with fuel lines large enough to keep fuel pressure constant at all engine loads.

Selecting the correct carburetor

Selecting the correct carburetor for a supercharged engine is very important; the fuel curves for a low boost engine versus a high boost engine make proper carb selection very important. The Demon carburetor line from Barry Grant Inc. offers carburetors designed for use on supercharged applications; these carbs are in their Race Demon series of carburetors. These carburetors come with removable sleeves; this feature allows you to change the carburetor airflow (cfm), so if you change boost pressure, engine size, or cam, you can resize the carb for the new engine without buying a new carb. The Race Demon series carb also has changeable air bleeds and idle feed restrictions; this feature makes it easy for your tuner to dial in the fuel mixture curve for the engine’s needs.  

The blower Mighty and Race Demon carburetors have power valves with vacuum tubes that allow the power valves to be boost referenced, just connect this port to boost pressure and the power valve can see engine load or vacuum instead of the vacuum created by the supercharger. A boost referenced power valve helps make it possible to get the fuel mixture curve correct for all engine loads and should be used on any high boost street driven engine. One of the more common problems I have seen is with the engine at idle speed; the engine will go into a supercharger roll (the engine speeds up the slows down then speeds up….). This supercharger roll is caused by the superchargers vacuum signal to the power valve not being the actual engine vacuum, but the vacuum created by the supercharger. The power valve reads the higher vacuum created by the supercharger, this vacuum closes the power valve, causing the fuel mixture to go lean, the engine then slows down due to the lean air/fuel mixture so the vacuum signal from the supercharger causes the power valve to open as the superchargers vacuum drops, so again the engine again speeds up due to the richer air/fuel mixture, rolling from rich to lean causing the engine to speed up then slow down.  

The fuel pressure we use most is 5 ½ to 6 lbs. and we always suggest a high volume fuel pump that can supply enough fuel to keep the pressure constant at all engine loads. We have seen Holley style carbs where we had a fuel leak from the plugs on the metering block; this was caused by a carb air scoop creating a high pressure in the fuel bowls pushing fuel past poorly fitting metering block plugs. The cure for this was to epoxy the plugs in the metering block and putting a few vent holes in the carb scoop to help lower the air pressure (this problem has not been seen by us on Demon carbs or on AFB style carbs).

AFB style carbs on a low boost pressure - mild blown engine can work well, but since they do not offer a boost referenced power system, care must be taken to avoid overly lean fuel mixtures, especially at off-idle to part throttle. A street driven dual 500-cfm afb set-up can be a very good running set-up on a mild blown engine that gives a great appearance and also can run well under normal street driving conditions. The 500 & 600-cfm carbs can be modified to avoid an off-idle lean condition, so they can work well on a low boost pressure single or dual carburetor set-up, but we avoid the 750 and 800 cfm AFB’s on street driven blown engines because of a lean at part throttle problem that is inherent with this type of carburetor.

Jetting a supercharged engine with the MicroGas

The method we use to determine and then obtain the correct air/fuel mixture (jetting) is on an engine is the use of a tool from OTC/SPX tool company, the PerformanceGas and MicroGas portable exhaust gas analyzers, both units have provided us with accurate data that allows us to obtain great results. The CO reading that an exhaust gas analyzer provides is an accurate indicator of the air/fuel mixture (jetting). Jetting or obtaining the correct fuel mixture on a supercharged engine is different on a low boost engine when compared to a high boost application.   On a low boost application the air/fuel mixture is almost the same as a normal hot rod fuel curve, but when a set-up is used with a lot of boost, the fuel mixtures used can be much richer. On a low boost engine the power mixture we use is a CO reading of 6.6% or a 12 to 1 air/fuel ratio, but when using a lot of boost a power mixture 0f 11.5 to 1 air/fuel or CO reading of 8.0% is not uncommon. This richer mixture can help in controlling detonation created by the high boost pressure, but if you go too rich you will lose power. The cruise mixture on a mild cam low boost pressure engine would be 1% TO 2% CO (14.1-13.8 TO 1 air/fuel ratio) but on a hot cam high boost engine the target mixture would be 3% CO (13.4 to 1 air/fuel ratio).

A blown engine will require the accelerator pump to be tailored to its needs; some engines will need a quicker accelerator pump squirt, others will need more accelerator pump volume. Always avoid drowning the engine with too much accelerator pump. A street driven supercharged engine with 2 Holley style carbs will very seldom need 50cc pumps on the primary side of the carbs.

Intercoolers / cooling the charge

The act of compressing the air charge into the engine creates heat; it is not uncommon to see air charge temperatures in the 200 to 250 degree plus range. This heating of the air charge hurts engine performance by reducing the air density. By cooling the air charge you can gain back about 1% in horsepower for every 11 degrees you can drop the air temp, by using an intercooler, it is easy to get a 10% increase in power or more.

Fuel injection with an add on blow through supercharger

If factory fuel injection is used and you do not use an intercooler the vehicles computer may “see” air charge temperatures that are not in the computers program. If a computer sees data that is not in it’s program, it may cause the engine to run poor or not at all. In most cases when adding on a supercharger to a fuel injected car, higher flow fuel injectors be needed and the computer may need reprogramming for the engine to perform properly. A mass air flow equipped fuel injection system will be the easiest type of fuel injection to add a supercharger to since this type of injection can “see” the extra air being feed to the engine and therefore can calculate the correct amount of fuel needed. Speed density fuel injection systems will require reprogramming for a supercharger.

Ignition system set-ups with a supercharger

The ignition system used on a supercharged engine must be up to the job; the higher the boost pressure, the more ignition output is going to be required to fire the spark across the spark plug gap. This higher ignition output will also require the wires and the rest of the ignition system to be up to the job of delivering the extra ignition output to the spark plugs. Always consult the ignition supplier of the product you are using for the recommended spark plug gap when using a supercharger; use these specs as a guideline for where to start. Some experimentation may be needed to find the spark plug gap that works best for your engines demands and the air-fuel mixture inside the cylinder.   The use of a engine ignition analyzer to check the condition and output of the ignition system can confirm if the ignition system has enough spark output to handle the extra ignition output needed to fire the sparkplugs because of the higher combustion chamber pressures created by the boost from the supercharger. The ignition system analyzer we use to check a ignition system is the Vision Premier or the Genisys Scope Module from OTC/SPX, either unit allows us to easily check the output and condition of the ignition system.

An MSD ignition system such as a #6btm (boost timing master) may use a spark plug gap of up to .042” with 6 psi of boost. The higher the boost (combustion chamber pressure) the smaller the plug gap, we have seen spark plug gaps of as small as .018 - .022” on a high boost engine with a Msd ignition system. (Note a 9 to 1 non-supercharged engine may use a plug gap of up to .055”.)

The ignition advance curve, in general, is quicker yet shorter than an engine without a supercharger, this helps give better throttle response until the boost pressure kicks in. The advance curve most seen is an initial timing of 18 degrees with a total of 32 degrees; this is just a starting point and must be tailored to each engine. MSD can supply a boost retard ignition system, which allows an advance curve that can supply enough advance to give good throttle response yet, as the boost comes in, it can retard the timing to avoid detonation. The boost retard system can retard the timing from 1 to 3 degrees per pound of boost and total retard can be as much as 20 degrees. A vacuum advance should only be used in a low boost application and should be limited to 10 degrees of advance with engine vacuum above 10 to 12 inches of vacuum. The best way to check and set the advance curve is on a distributor test stand, then confirm the results with an advance timing light such as the type we use from the Black Light series of timing lights supplied by OTC/SPX Tool Company.

So there’s the long and short of it. If you are going to run a blown engine, the right carburetors and ignition system that are tuned for your engine with the proper tools will make your hot-rod experience enjoyable and trouble free.

Ole’s Carburetor & Electric  

120 Camino Real  

San Bruno, CA 94066

650.589.7377  

olescarb@aol.com  

Reggie Jackson’s High Performance Engines  

1137 San Mateo Ave 

San Bruno, CA 94066  

650.873.7492  

reggiejacksonhp.com

John Bishop – Hot Rod Tuning

808 Burlway#2

Burlingame, CA 94010

650.343.4860  

jfb396@aol.com   

SPX/OTC Corp.

655 Eisenhower Dr.

Owatonna, MN 55060-0995

507.455.7000      

www.otctools.com   

Barry Grant Inc.

1450 McDonald Road

Dahlonega, GA 30533

706.864.8544

www.barrygrant.com