By: Henry P. Olsen

Authors note: This is an article about how to tune a carbureted engine. It will apply to any engine from stock to high performance and will help a tuner with tuning an engine to run with today's reformulated gasoline.

In this technical tuning article will explain how to diagnose and tune an engine’s ignition and fuel systems so you can get all the power built into an engine while getting as many miles of driving fun per gallon of gasoline. These tuning methods can be used to tune any engine from a stock flat head Ford V-8 to a high performance engine such as the 427R Roush performance engine that is in the 36 Ford we will be using as basis to illustrate this story. We will first explain the tuning process and then show how we apply this process to the engine in the 36 Ford street rod we are working on.

The 427 cubic inch displacement - 550 horsepower Roush built small block Ford engine that is in a 1936 Ford street rod definitely had that killer look to match the power the engine was advertised to have, but the engine did not run as well as it was expected it to. The performance related complaints were: a hesitation when you hit the throttle under hard acceleration, the engine had a audible ping under hard acceleration, the engine run good to about 4,000 rpm but above 4,000 rpm the engine lost power & you could hear the engine backfire thru the carburetor, the last complaint was the exhaust fumes made your eyes water & also made it hard to breathe, Since the Roush Performance engine had been dyno tested at the factory before it was shipped, the cars builder decided to have John Bishop’s Hot Rod Tuning Service check the 36 Ford out and find out what was causing the engine performance problem.

The Tuning Challenge

Tuning a high performance to run its best with race (leaded) gasoline is easier than tuning the same engine to perform its best with the unleaded reformulated gasoline that most of us run in high performance engines. This unleaded reformulated gasoline is designed for use in a modern computer controlled electronic fuel injected engine with the goal of lowest possible exhaust emissions. The computer that operates with a modern fuel injected engine automatically adjusts the air/fuel mixture and the ignition spark timing in order to obtain the lowest possible exhaust emissions along with the best possible power and drivability. A street rod with a carburetor must have both the ignition spark timing and air/fuel mixture tuned for the blend gasoline that is sold in your part of the country.

Race gas and the leaded gasoline that was sold at your corner gas station until the mid 1970’s was easier to tune for because a tuner could “read” the spark plugs and look at the tailpipe “color” to determine the air/fuel mixture. Today’s reformulated gasoline does not leave any “color” on the spark plug unless the air/mixture is very rich and the tail pipe color is almost meaningless. Most people think their engine is running too rich because the exhaust fumes burn their eyes; this is not true. An engine with a rich air/fuel mixture will have excessive carbon monoxide (CO) in the exhaust; carbon monoxide is an odorless gas that is quite deadly if you breathe too much of it. A engine that has a misfire from things such as: a rich air/fuel mixture, a lean air/fuel mixture or incorrect ignition spark timing will have a lot of unburned hydrocarbons (HC) in the exhaust. Exhaust gas with a high unburned hydrocarbon (HC) content will burn your eyes and make it difficult to breathe.

Checking Basic Engine Condition

Before any tuning is done you should confirm the engine is in good shape by checking the cylinder compression, cylinder leak down, oil pressure, fuel pressure, and all the tune-up related parts such as spark plugs, wires, points are all in good shape. The next step in the tuning process is to check ignition timing and ignition spark advance systems are functioning and set to factory specifications. The ignition spark output and the condition of the spark plug wires should also be checked out with an ignition scope.

Ignition Spark Timing

Reformulated unleaded gasoline burns different at a different rate than leaded gasoline so ignition spark advance curves will need to be tuned for the gasoline you will be using. The first step in the process of tuning an engine designed for leaded gasoline to use today’s reformulated unleaded gasoline is to modify the ignition spark advance. The proper tuning of the initial and the ignition spark timing advance curve(s) may be the easiest, cheapest method to unlock the power that was built into your engine.

The engine takes a mixture of air and fuel mixed together, the piston then compresses the air/fuel mixture into the combustion chamber, then at the ideal time a spark is then supplied by the ignition coil thru the distributor and spark plug wires to the spark plug. This spark creates a controlled burn in the combustion chamber that pushes the piston down and thus creates the power to send your street rod accelerating down the road. If you want your engine to perform it’s best, the proper air/fuel mixture must be ignited by the spark plug at the correct time so the engine can turn the energy in the gasoline into power for it’s driver.

The distributor ignition advance curve that is comes in most distributors must be designed to allow the engine to perform well with many different drivers with widely varying driving styles and operating conditions, this means the ignition advance curve in most cases is very conservative to avoid the possibility of engine damage from detonation. Detonation or pinging can result if the ignition spark advance is too advanced for the octane of the fuel and the needs of the engine. The variables that need to be taken into account when you pick a ignition advance curve are the weight of the vehicle, how the driver will operate the engine (the rpm range of engine operation or if the driver tends to lug the engine), octane of the fuel, compression of the engine, altitude of the vehicles main usage and the heat of the air. The distributor’s mechanical and vacuum advance curves must be correct for the engine and the fuel being used or the engine’s performance will suffer as well as the possibility of engine damage from detonation

Ignition Spark Advance Curves

The ideal ignition timing for power and fuel economy is just short of the point where detonation or pinging occurs, the correct ignition timing (the initial timing plus the advance from both mechanical and vacuum advance mechanisms) will cause the pressure created by the fuel being burned in the combustion chamber, that is pushing the piston down to be at its peak / maximum when the piston is at about 12 and 15 degrees ATDC (after top dead center). If the peak cylinder pressure is reached before the 12 to 15 degrees ATDC you will lose power as the piston fights to compress the burning air/fuel mixture and detonation may also be experienced which can lead to engine failure. If peak cylinder pressure is reached much after the 12 to 15 degrees ATDC, the engine will just be wasting the energy the combustion process creates as wasted heat out thru the exhaust system.

The amount of ignition advance needed changes with engine speed, engine load, engine temperature, air temperature, compression ratio, the octane of fuel being used and the air/fuel mixture. A fast burning - rich air/fuel mixture (12.5 to 1 air/fuel mixture) that will produce the maximum power and will require less advance than the slower burning - leaner air/fuel mixture (14 to 1 air/fuel mixture) that is used for cruise speeds to properly burn all the fuel. A vacuum advance adds a little more ignition advance to allow the engine to fully burn this slower burning - leaner air/fuel mixture that is seen at part throttle conditions such as a 65 mph cruise. A 12.5 to 1 air/fuel mixture burns the fastest and will supply the best power, but a leaner air/fuel mixture of 14 to 1 is much better for fuel economy and also will not tend to foul spark plugs like the richer 12.5 to 1 air/fuel mixture will tend to do.

If you do not have enough ignition spark advance the engine may lack power and the engine may tend to run hot or overheat. If you have too much ignition spark advance the engine will also want to run hot and lack power, if excessive pinging or detonation is present you are also risking engine damage. Any distributor, whether it is original or performance replacement should have the mechanical and vacuum advance curves checked to confirm they are correct for use with your driving style, engine package and today’s reformulated unleaded gasoline.

The best way to check both the vacuum and mechanical advance curves of a distributor is in a distributor test stand; this is because you can check the spark advance curve at any rpm without fear of over-revving the engine. Back in the 60’s and 70’s almost every quality engine tuner had a distributor test stand in their shop so they could check the advance systems in the distributor, but now days it is getting hard to find a shop that has one.

If you do not have access to a distributor test stand, an optional method that you can use to check the vacuum and mechanical advance curves is the use of a dial-back timing light. A dial-back timing can allow you to read the advance curve of an engine at different engine speeds even if it does not have a degreed vibration dampner, just take care not to over-rev the unloaded engine. The vacuum advance curve can also be checked with the use of a hand vacuum pump to vary the vacuum supplied to the vacuum advance, just use the timing light to read the amount of advance given by the vacuum advance at different amounts of vacuum from 1 to 23 inches of vacuum.

Ignition Spark Advance Guidelines

The “hot-rod” advance curve used most on a 9.5 to 1 compression engine with a mild camshaft (duration less than 220 degrees at 0.050”) is 10-12 degrees initial timing plus 22-24 engine degrees of additional advance from the mechanical advance mechanism. In most cases, full advance (32-36 degrees) is in by 3500 rpm. An engine with camshaft duration above 220 degrees at 0.050” will like more initial timing; however, the total timing will stay the same (32-36 degrees). In most cases when you are using a “hot rod” mechanical advance curve, the amount of additional advance from the vacuum advance should not exceed 10 degrees and not be in before 10 inches of vacuum, or performance may suffer from too much advance.

The mechanical advance should not start advancing until just above the base idle speed, too much advance with the engine speed too low may cause a ping or detonation problem, which can lead to engine damage. The best guideline we have found for determining what initial timing is best for a gasoline engine is in the Barry Grant Inc. catalog and/or web site in the Demon carburetor selection guide; they recommend 10 to 12 degrees of initial timing when the camshaft duration is less than 220 deg. @ .050 valve lift, 14 to 16 degrees of initial timing with less than 240 deg @ .050 and 18 to 20 degrees of initial timing with a cam with less than 260 @ .050 valve lift. The total ignition mechanical advance from the distributor must be reduced when you increase the initial timing because engine damage will result if the total advance is excessive for the engine compression and fuel being used. A modern 9.5 to 1 compression Chevrolet, Ford or Chrysler engine will in most cases will use 36 degrees of total ignition advance including the initial timing set plus a maximum of 10 additional degrees of advance from the vacuum advance (if used).

A engine with a high performance camshaft designed to create power above 3000 rpm will respond well to 18 degrees of initial timing because the air/fuel mixture is not uniformly mixed at lower engine rpm, so the additional initial timing allows more time for this air/fuel mixture to burn in the cylinder; the same theory also applies to the use of a supercharger or a race designed intake manifold such as a Edelbrock Victor or any of the new air-gap style intake manifold. A high performance cam and/or intake manifold will decrease the velocity of the air/fuel mixture at low rpm’s, this low velocity causes the air/fuel mixture to not be as well mixed as a engine package that has been designed for low rpm operation.

Tuning the Ignition for Fuel Economy

The ignition spark advance curve an engine needs for maximum power with the rich air/fuel mixture the engine has during wide open throttle acceleration is different than the ignition spark advance it needs for the leaner air/fuel mixture the engine has when you are cruising at 65-75 mph on the highway. In almost every case we would prefer to use a distributor with a vacuum advance tuned to provide an additional 10 degrees of advance when the engine vacuum is above 10 inches. The extra 10 degrees of ignition spark advance that a properly tuned vacuum advance can provide will give the extra spark advance a engine will need to burn the leaner air/fuel mixtures an engine will use a light throttle while driving your street rod down the road at normal highway speeds. The extra timing from a vacuum advance will also help the engine run a little cooler and give you better fuel mileage when you are driving at normal highway speeds. Many of the original Ford and Chrysler distributors have adjustable vacuum advances, for General Motors and MSD replacement distributors we use a vacuum advance we buy from Ole’s Auto parts that have been limited to 10 degrees of advance.

Ignition System Upgrades

In most cases your engine will perform its best with electronic ignition, but an engine can perform quite well with point-triggered ignition. The original equipment electronic ignition systems that Chrysler, Ford and General Motors have for most engines are a very good and reliable ignition system that can work very well in a street rod. These original equipment electronic systems have the advantage if you happen to have an ignition failure while on a road trip you can find parts to repair it at any auto parts store; this is not always the case with many of the aftermarket electronic systems you may buy.

Any original or high performance aftermarket replacement distributor must have the ignition spark advance curve(s) tuned for your engine and the gasoline you will be using. Most of the high performance replacement ignition systems such as a MSD distributor come with a very conservative ignition advance curve installed in the distributor. A new MSD distributor comes with a selection of advance springs & bushings so you can set the advance curve you want, however most vacuum advance equipped distributors have too much advance from the vacuum advance for the reformulated gasoline of today.

Spark plug wire condition & maintenance is very important consideration in the proper tuning process on any engine, as the spark plug wires age the insulation material will break down allowing the spark that is being sent to the spark plug to take a shortcut to ground. Many of the aftermarket spark plug wire suppliers offer heat resistant spark plug wires with improved insulation materials such as silicone, these improved spark plug wires are available in 7mm or you can upgrade to 8mm spark plug wires.

Tuning the Roush Performance 427R Ignition System

The ignition timing that we read when we checked it with a timing light was 30 degrees BTDC (before top dead center) total all in by 2000 rpm, the initial timing that varied from 15 to 22 degrees BTDC. The next step was to remove the distributor from the engine and check the advance curve on a distributor test stand. The actual advance readings when the distributor was checked on the distributor test stand show us the distributor had 14.5 degrees of mechanical advance was all in by 1800 engine rpm. The fact that entire spark advance curve was in by 1800 rpm was most likely the reason for the ping problem the engine and the reason the timing was varying at idle was the advance springs were so light that the advance was starting at idle speeds.

Using the Barry Grant initial timing guide we decided that 18 degrees of initial timing would work quite well with the camshaft that is in this engine. We then recurved the distributor so it would give us 16 degrees of mechanical advance that starts advancing at 800 rpm and is all in by 3000 rpm to give us 34 degrees of total mechanical advance. The MSD distributor that is in this engine did not come equipped with a vacuum advance and since the reason the customer selected this engine package is maximum power the vacuum advance was determined to not be a priority. We then reinstalled the recurved distributor into the engine, set the initial timing at 18 degrees BTDC and confirmed the total advance to be 34 degrees BTDC. The tech literature that came with the engine specified the total timing was not to exceed 35 BTDC.

The changes with the initial timing and spark advance curves made an improvement in how the engine in this 36 Ford performed, the idle was smoother and the engine had more power at low and mid range rpm driving conditions. The engine ping problem the engine had when we first started the tuning process was gone but the throttle response is still not up to what an engine like this should have. The idle quality still left a lot to be desired and the problem with backfiring at engine speeds above 4000-rpm still needs to be addressed. Now that the ignition is tuned for the unleaded reformulated gasoline it is using, we can go to the next in the tuning process, which is tuning the air/fuel mixture for power and fuel economy.

To be continued in Part 2: Fine Tuning the Fuel System.

Sources:

Ole’s Carburetor & Electric, Inc.

Ole’s Carburetor & Electric Inc.

120 El Camino Real

San Bruno , CA 94066      

650.589.7377

olescarb@sanbrunocable.com  

John Bishop/Hot Rod Tuning

Burlingame, CA

Ph#650 343 4860

jfb396@sbcglobal.net

Demon Carburetion

Barry Grant, Inc.

706-864-8544

www.barrygrant.com

OTC/SPX Corporation

1-800-533-6127

www.otctools.com