Why? Why would you want to install a carburetor on an LS engine rather than use EFI? That is a question that is frequently asked on many of the message boards and forums that support the GM LS based engines. Contrary to what some might lead you to believe, let us assure you, there’s no increased risk in killing your motor if you run a carb over a fuel injected system. The reasons why a carb on an LS makes sense are pretty simple. In LS swaps or street/strip/race cars a carbureted LS is simple, cheaper, and often can make more top-end power than EFI.
In this tech article, we are going to show you how to install an LSX ignition specifically for carbureted engines with help from our friends at MSD Ignition.
MSD ignition has earned it’s reputation as a leader in ignition technology while at the same time keeping their product packaging looking very similar throughout the years. The ‘if it’s not broke, don’t fix it’ attitude is a big plus to performance car people, and the MSD ignition boxes are easily identifiable by sight and let people know that you are serious about speed.
Carburetor technical support. Email a Tech. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual. Some of the aftermarket carburetors we see need a lot of tuning work to get the A/F mixture correct for the engine’s demands with the reformulated unleaded gasoline of today. Air/Fuel Mixture Tuning.
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MSD 6LS Ignition Controller
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MSD's 6LS ignition control only requires connections for the coils, crank sensor, MAP sensor and the camshaft sensor.
The MSD 6LS allows the user to map a timing advance curve, program the two step rev limiter or vacuum advance curve for your needs. There’s even a step retard in case you want to run a little nitrous in the mix. -Todd Ryden
MSD Ignition has come up with the perfect solution for guys like us that want to push the LS engine’s power curve to the max with the added benefit of no wiring headaches. We talked to MSD’s Todd Ryden about their cool ignition controller for LS engines, the 6LS, “For guys that want the advantages of GM’s aluminum small blocks without worrying about miles of wiring with typical EFI systems, our 6LS ignition controller allows them to mount a carburetor on the LS1/LS6 engines or the 6LS-2 on the LS2/LS7 engines. These controllers allow the user to map a timing advance curve, program the two step rev limiter or vacuum advance curve for your needs. There’s even a step retard in case you want to run a little nitrous in the mix.”
The MSD 6LS is a dream for guys that don’t care for wiring or want a clean retro looking engine bay with modern LS muscle. Requiring only connections for the coils, crank sensor, MAP sensor and the camshaft sensor, this ignition system is a simple way to go old school with natural aspiration.
Features
- Runs a carbureted LS engine without complicated EFI hardware
- Map a timing curve using Pro-Data+ software
- Programmable two-step rev limiter, vacuum advance curve and step retard
- Direct plug-in to factory components
- Programmable via a PC through MSD’s Pro-Data+ software
With the exception of a couple of wires, all the wiring for the controller wiring harness is routed into factory style connectors for simple and easy installation. A tach signal, ground and hot wire are the only wires not in factory style connectors. The 6LS ignition controller box can be mounted like any other MSD controller, which is practically anywhere under the hood or on the firewall.
Setting Timing and RPM Limits with the 6LS
The included plug in modules give you six different pre-programed timing curves to choose from based on how your LS engine is set up. Just plug them in, cycle the ignition on and off, and the curve is saved. Easy as that.
MSD’s 6LS controller is programmable by means of simple plug-in modules, an optional hand held programmer, a laptop computer; giving the user a variety of ways to adjust RPM and timing. Using a laptop computer for adjustments may be intimidating to some racers, where as simply plugging in an MSD module or using the handheld controller may be a bit easier. The versatility of this ignition controller makes it a another big plus for car builders wanting to convert their LS EFI to distributorless carb-based system.
MSD Pro-Data+ Software
For those looking for maximum control over the timing and rev limits of the 6LS, MSD has the Windows based Pro-Data+ software. With Pro-Data+ you can plot timing curves based off of both RPM and Manifold Absolute Pressure (MAP) for vacuum or boost. You simply right click, add a plot, and drag it to the point on the table you want it to be, and you’re well on your way to creating your own completely custom timing curve. You can also set your desired maximum and minimum timing limits through out the curve by using the Data Editor Menu as safety precautions.
Step Retard For Nitrous Applications
The 6LS also has a built in step retard (Sw Retard) that can help you dial back your timing at a specific RPM point, making it ideal for cars running nitrous. The step retard is activated by introducing the pink wire in the 6LS’s wiring harness to 12v, and the amount of retard can be adjusted from zero to 15 degrees of timing in 1 degree increments through the Pro-Data+ software.
Laptop tuning - with a carb? MSD's Pro-Data+ software is nothing to be afraid of. It makes customizing the timing curve, rev limits, and step retard of the 6LS easy.
Built-In 2-Step Rev Limiter
Another great feature of the 6LS is that it has a built in 2-step rev limiter. The “RevLo” setting is for limiting your launch RPM on the starting line at the drag strip, while the “RevHi” setting is your maximum RPM safety net. Setting up the 6LS’s rev limiters is a snap; all you have to do is hook up the blue wire from the wiring harness to a 12v power source and wire in a button or switch. When ever the blue wire is receiving 12v, the RevLo setting will be activated. Let your finger off the button when you launch, cut power to the wire, and the RevHi setting will take it from there. Of course, the exact values for both rev limits can be set in the Dat-Pro+ software or the hand held programer.
Delivering The Spark: MSD Coil Packs and Plug Wires
MSD also makes two versions of stock-replacement coil packs for performance LS builds that compliment the 6LS controller. MSD’s Multiple Spark Coils (MSC) are direct replacements for the stock LS1/LS6 coils (Part #: 8245) or the newer style LS2/LS7 coils (Part #:8247). Both versions of the MSC coils are capable of producing 44,000 volts of spark energy in addition to their muliti-spark capabilities. Ryden explained to us that the, “MSC coils have a higher spark energy and voltage plus they are designed to work with MSD ignition controller’s multi spark capabilities. All this translates into an improved combustion process in the engine’s combustion chamber that you will feel in throttle response and a smoother idle along with better high RPM performance.”
The MSD MSC coils are a direct replacement for your stock coils, and feature multi-spark capability when used with the 6LS.
MSD recommends using their Super Conductor 8.5 spark plug wire set (Part # 32813, 32819, 32823 or 32829) to take full advantage of the spark energy from the coils to the spark plugs. The spark plugs can only use the energy transferred by the plug wires, so if the plug wires don’t transfer all the energy created by the coils, you’re losing potential energy and more complete fuel combustion.
The truth is that all the spark voltage in the world won't do you any good if you can't get it all transferred to the plugs. The MSD Super Conductor 8.5 plug wires make sure you're getting full use of all the power in your ignition system
Moroso Remote Coil Mount Brackets
While there's nothing wrong with mounting the coils on your valve covers, for easier maintenance or tight engine bays, Moroso offers a trick remote coil mount setup that we can see ourselves using in a few upcoming projects.
So you’ve got the old-school look of the carb and intake manifold going in your hot-rod, but coil packs on the valve covers keep giving you away that your engine is something more modern. Relocating the coils isn’t just about looks either – moving them away from the heat radiating off the engine could greatly increase the coils working life span.Well, Moroso has a creative answer to relocating the coils off the valve covers with their Remote Mount Coil Brackets (Part # 72395). These stainless steel brackets are meant to prevent cross-firing from one coil to another, and can accommodate both OEM and aftermarket coils.
With the Moroso coil brackets you just simply find a spot anywhere in your engine bay that it makes sense to mount the coils, and mount up the bracket.
Intake and Carburetor
A look at Edelbrock’s LS Carb Manifolds…
Edelbrock Performance RPM for LS: (Part #71187)
- Dual Plane for Broad RPM Curve
- Cathedral Port LS1/LS2/LS6
- Great for Street Use
- Single Plane for Up to 600 HP
- Available for Carb or EFI
- Up to 7,500 RPM Capable
- Single Plane Competition
- Serious Racing Potential – 8,000 RPM
- LS1/LS2 Cathedral Port
- Perfect for LS3 and L92 conversions
- Works great up 7,500 RPM
- Single Plane Street/Strip – Excellent for large CI engines
RPM Air-Gap Dual-Quad LS1: (Part #75187)
- Dual Carb LS Intake for Twin Carbs!
- Cathedral Port LS1/LS2/LS6
- Show Car Look!
For our engine, we actually chose the Edelbrock Super Victor LS1 #28095 EFI intake. Even though we were using a carb, we liked the idea of being able to switch to EFI down the road. With the Edelbrock intake, we simply made plugs that blocked off the injector bungs. That’s really the only difference. The Super Victor LS1 is a serious high-RPM intake perfect for 600 HP and up engines that are either large displacement or rev up to 8,000 RPM. It’s a perfect fit for a 427 ci engine like that on our project engine. It’s about 1-inch taller than the GM intake or the standard Victor LS intake, so you’ll need to make sure you have adequate room under hood. We utilized a K&N drop-filter that lowered our filter 1.5-inches for hood clearance on our Chevelle.
The Other Option – A Distributor?
Another option for adding a carb to an LS engine involves converting to a distributor spark control system. Why consider a distributor? There are arguments that a distributor system can handle much higher horsepower and RPM levels than the stock or M50 coil-on-plug system can handle.
Chevrolet Performance offers an LS front distributor drive cover (Part #88958679) that allows for the use of a small block Ford-style distributor and mechanical fuel pump. Most aftermarket kits for this conversion include your choice of Carter fuel pumps (part # 6904 for street application, #60454 for street/strip, #60968 for race applications) with the recommendation to use ProForm distributor (Part # 66969R), or MSD’s small diameter Pro Billet Distributors (#8382 or 8579) with an MSD ignition controller.
For maximum weirdness, team up your carbed LS with Chevrolet Performance's front-drive setup and a Ford-style distributor...
Full Race Distributor Setup
Though definitely not street-oriented, a Jesel Valvetrain belt-drive distributor setup like the one we’re employing on Project BlownZ is another option for the truly hard-core. Driven off the cam via a toothed belt, the Jesel distributor incorporates a cam position sensor for timing data sent to the standalone ECU, and the large-diameter cap sends the spark to the appropriate lead from the high-output coil.
Mounting the Chevrolet Performance front cover and retro looking Chevrolet Performance Bowtie valve covers (Part #25534398 and 25534399) change the entire look of the LS engine to a more traditional looking GM powerplant. The Chevrolet Performance front drive kit comes with a machined front cover, fuel pump and distributor drive gear, color coded spacers, assembly bolts, and the front cover seal.
Converting to a distributor controlled ignition timing setup using Chevrolet Performance front cover requires a few additional components for the front drive system to work properly. A Stewart long style GM water pump is required for correct drive spacing. An accessory drive kit is also required so that the power steering pump, air conditioning and the alternator drives can be driven by the engine. Wegner Automotive makes a complete kit (part # WAK 010) for the conversion that includes all necessary brackets, pulleys, stainless hardware, belts, OE tensioner, Long Stewart water pump, ATI Dampner, 140 amp alternator, GM type 2 power steering pump, and Saden AC compressor. Wegner’s WAK011 contains the same components except the air conditioning unit.
Our research showed that Mr. Gasket’s distributor hold down clamp (Part #9860) was an often overlooked item would be a necessity to complete the distributor conversion. Along with the hold down clamp, a nice set of plug wires and spark plugs should be added to the list.
Taking a Step Back
While a lot of people will consider taking an LS engine and retrofitting a carburetor and stand-alone ignition system as a step in the wrong direction, for many hot rodders and racers it makes perfect sense. The truth is that carbureted LS engines can make just as much horsepower and torque as ones running EFI (if not more), and when you couple that with the simplicity that a carb offers it’s easy to see why many gearheads go this route. Hopefully we’ve shown you that even setting up an ignition system for a carb’ed LS can be an easy job, that can even give you more control and tuning ability.
Over the past 50 years, the venerable Holley carburetor hasn’t changed much. It’s simple design and near infinite adjust-ability have long made it a favorite among enthusiasts. What has changed are the tuning tools available to us. While EFI tuners routinely use this new technology with great success, surprisingly few carb owners have warmed up to the idea yet.
There isn't a whole lot to a carburetor from a parts perspective, but knowing how each of the components effect each other is crucial when it comes to tuning a carburetor.
Modern fuel injected engines deliver precise air/fuel ratios which allow for good power and drivability, while at the same time netting impressive gas mileage. A large part of that success is due to modern-day engine management systems, where a variety of sensors inform the computer of current engine operating conditions, so it can make accurate fuel calculations. The same technique can be used to tune a carbureted engine. All that is needed are a few key sensors, and a method of recording their readings over time. Let’s take a look at some basic sensors for this process.
RPM Signal
An engine speed signal can come from the negative side of the coil, a tach output from an aftermarket ignition box or factory ignition module, crank trigger, or an inductive pickup of some sort. Viewing the RPM graph in a datalog gives us a quick visual reference of where we stand.
Wideband O2 Sensor
By detecting the amount of unburned oxygen content in the exhaust after combustion, a wideband sensor is able to indicate the engine’s actual air/fuel ratio. This tells us how rich or lean the engine is from stoichiometric. The stoich air/fuel ratio for standard gasoline is around 14.7:1 (lambda 1.00). Consider this to be the chemically correct air/fuel ratio to completely burn all available oxygen during combustion, leaving no air or fuel left over afterwards. Though stoich is ideal for many circuits of operation, it is typically too lean at wide open throttle for the vast majority of engines and will be most happy between 12:1-13.5:1.
Don't be persuaded by bad tuning; carburetor-powered vehicles can benefit from EFI-like drivability as well.
Once considered too expensive for the average enthusiast, wideband kits have become surprisingly affordable in recent years. Look for a kit that has good datalogging software, several analog inputs, RPM input, free air calibration, and analog output. An in-car display is also nice, and mandatory if the software does not display the air/fuel ratio in real time. For this example, we used Innovate Motorsports LM-1. This is the older version to their newly updated LM-2 that is two channel capable and records directly onto the included SD card.
Even Factory-Supplied Sensors Help
Many factory sensors, including those listed below, output a signal ranging from 0 – 5 volts depending on the sensor readings. In order to use these sensors, we have to supply them with a five-volt power source, an engine ground, and a signal wire to one of our datalogger‘s analog inputs. To reduce the vehicle’s 12 volt power down to the required five volts, use an LM7805 voltage regulator. They’re available from electronics stores for a very modest price.
The TPS is a potentiometer that measures rotation of the throttle shaft. The sensor puts out between 0 and 5 volts linearly, depending on throttle position. In reality, these sensors are at their idle position between .5 and 1 volt, and register full throttle around 4.5 volts. We’ll plug the signal wire into one of the wideband’s analog inputs. A clever mind can easily adapt a TPS to the end of a Holley primary throttle shaft.
Measuring manifold pressure will help tuning different parts of the load cycle
GM makes good, inexpensive MAP sensors. They measure the absolute pressure inside the intake manifold. While we normally think of vacuum and boost as being on two different sides of the atmospheric pressure line, the MAP sensor simply sees vacuum as a positive absolute pressure and boost as just a higher positive pressure. MAP sensors are available in one, two or three bar versions. The one bar has the best resolution for a naturally aspirated motor, while a two bar is good for 14 pounds of boost, and a three bar goes up to 29 PSI.
● GM 1 bar MAP Sensor part# 12569240
● GM 2 bar MAP Sensor part# 12569241
● GM 3 bar MAP Sensor part# 12223861
Identifying the Carburetor’s Circuits of Operation
Now that we have a good understanding of the tools available, let’s take a look at what we’re trying to achieve. Holley carburetors have four distinct circuits of operation: idle, off-idle, cruise, and wide open throttle. They also have an accelerator pump to compensate for quick throttle actuation.
Idle – At idle speed, the carb runs solely on the curb idle discharge port. Air/fuel ratio is adjusted by turning the idle mixture screws. Target an air/fuel ratio of stoich (14.7:1 – lambda 1.00) for standard gasoline. Engines with longer duration camshafts may not be able to maintain good combustion this lean. If necessary, you can richen it up to as much as 13.5:1 (lambda .92). Try to achieve the highest manifold vacuum you can, at the leanest ratio that provides stable combustion. Set idle speed as low as reasonably possible to avoid entering the off-idle circuit.
Off Idle – Just above idle at very light throttle positions, the transfer slot now provides the fuel. Idle feed restrictions and idle air bleeds are used to tune this circuit. The elevated RPMs should allow us to maintain a 14.7:1 (lambda 1.00) air/fuel ratio, even with a lopey cam. Engines with very large cams may require a slightly richer mixture. If the cruise circuit comes in too soon, it can overlap the off-idle circuit too much and cause a very rich condition. You can delay the cruise circuit by enlarging the main air bleeds.
The metering block houses the power valve and jets. Primary (front of carburetor) jets are your primary tuning jets for cruising and low throttle use. Moderate and wide open throttle is where the secondaries come into play from a tuning perspective.
Cruise – Around 2000 – 2500 RPM, the transfer slot circuit begins to fade out and the main jets take over all fuel supply duties. Here’s where gas mileage can really be improved! Jet the carb for an air/fuel ratio between 14.7:1 and 15.3:1 (lambda 1.00 – 1.04) in all cases. But don’t lean it out to the point where the engine begins to stumble.
WOT – Anything over about half throttle reduces manifold vacuum to a point where the power valve should open. This uncovers the PVCR’s (Power Valve Channel Restrictions), allowing additional fuel enrichment under heavy load. To make maximum torque safely, we need to richen the mixture up some. A good target would be an air/fuel ratio of 12.5:1 (lambda .85). Modify the PVCR size until your target lambda is achieved. This typically means enlargement. Boosted cars will want to run considerably richer, such as 11.5:1 (lambda .78) or more.
Power valves are rated by the amount of manifold vacuum required to close them. Any vacuum amount below this level opens them. The trick here is to size the power valve roughly 1.5-inches below the minimum amount of vacuum your engine produces at idle.
Final Word on Power Valve and Accelerator Pumps
Adjustable metering blocks often have changeable PVCR jets. If using regular metering blocks, adjustment is done by modifying the PVCR diameter. Proper WOT air/fuel ratios could be achieved by enlarging the main jets instead, but that would richen up the cruise circuit beyond ideal. Poor gas mileage and plug fowling will result.
Carburetor Jetting Calculator
Power valves are rated by the amount of manifold vacuum (in inches of mercury) required to close them. Any vacuum amount below this level opens them. The trick here is to size the power valve roughly 1.5-inches below the minimum amount of vacuum your engine produces at idle. This ensures the power valve will not open prematurely. At the same time, you want the valve to open any time you’re much over half throttle. Comparing MAP and TPS readings will help you find that balance.
Accelerator pumps come with a cam adjustment slot for controlling the timing of the accelerator pump and sit right behind the '1, 2' adjustment screw. Pump cam kits can be purchased to change the rate in which the accelerator pump will ramp in the additional fuel.
Accelerator Pump – Anytime the throttle is opened rapidly, manifold vacuum drops and the car momentarily runs lean. Pumping an extra shot of fuel in prevents this lean condition, and instead creates a torque-producing slight rich condition. The air/fuel ratio should richen up for a second or two, and then quickly fade back to your desired ratio without ever going lean. Shoot for a light acceleration air/fuel ratio of 13.8:1 (lambda .94). Full throttle acceleration should produce and maintain your total WOT enrichment ratio (12.5:1 or lambda .85 on a naturally aspirated engine). Change accelerator pump squirter size, pump cam, and pump cam position to tune this circuit.
Demon Carburetor Jetting
Once you install the sensors and build a simple wiring harness for them, set up your laptop for datalogging. For best results, tune the circuits one at a time in the order listed. After using a wideband sensor to tune your carb, you’ll wonder how you ever got along without one. Before you know it, you’ll be tuning like a pro!
Holley Carburetor Jetting Guide
A cabin mounted air/fuel ratio gauges allows you to read what the car is doing in real time through a variety of driving styles