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Tuning a Carbureted Street Rod Engine - Part 1 of 2

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spacer Planning Engine Selection

By: Henry P. Olsen

Planning your engine selection and set-up before you buy your parts or build a car can not only provide you with a hot-rod that runs like a dream, but can also save you a fortune in time, money, and frustration. Build or buy an engine that has been built for the way the engine will be used; a race motor will not make a good everyday cruiser. An engine designed to produce max power above 3000 rpm may not have much usable power from idle to 3000 rpm, while an engine designed to produce power from idle to 3500 rpm may not have much power above 3500 rpm. If the proper engine is matched to your designed use you can have all the power you want, have a reliable engine with the look you want, and also have an engine that runs great.


Here are some questions you should answer before modifying, buying or building an engine:

  • How will the car be driven?
  • Is the car driven daily around town at low speeds or as a highway cruiser?
  • Is the car a weekend hotrod or a full-on racecar?
  • Is the car for show only or does it not only have to look & sound good but also drive good as well?
  • Will the engine be fuel injected or carbureted? Will the engine be supercharged?
  • Does the car have an automatic or manual transmission? Does it have overdrive? If it has an automatic transmission, does the torque converter stall speed match the cam/engine?
  • What is the rpm range of the engine at the normal driving speed?
  • What is the rear end gear ratio? What is the tire diameter?
  • Is the car going to be used at sea level or mainly above sea level?
  • Does the car have a big enough cooling system to cool off all the horsepower you are adding and is there enough airflow thru the radiator to cool the engine?

The answers to these questions will determine how the engine should be built and the power range the engine should be designed for. This will allow you to get the best engine for the way you will use it.

The heart of the engine is the camshaft; select the cam and matching valve train components that are designed for the way you will use the engine and make sure the valve springs are designed for both the cam profile and cylinder heads used. Pick the camshaft designed for your application; the camshaft for a fuel injected, carbureted, or supercharged engine may use a different camshaft lobe separation. There are no free lunches- if a �hot cam� is used to gain power at high engine RPMs you will lose power at lower engine RPMs. Conversely, if you select a cam to give the engine a lumpy idle, the low rpm engine performance will suffer. A cam will have both a usable rpm range in which it will produce its best horsepower and an ideal cruise rpm; camshaft manufacturers such as Crane Cams supply this data in their product catalog. Obtain and use this information to select a cam that matches your use. �


An engine package cannot idle for around town driving and also be a high rpm race engine; it must be built for the purpose it will be used for. A cam designed to provide the most power from 3000�6000 rpm has very low power from idle thru 2500 rpm and the ideal cruise rpm may be 3800�4200 rpm. A cam designed to provide the most power at 1600 rpm thru 4600 rpm may have an ideal cruise rpm of 2200�2600 rpm; another cam designed to provide the most power at 1200�4200 rpm may have an ideal cruise rpm of 1800�2400 rpm. If the vehicle will normally be cruised at 65 to 70 mph, use an engine package designed for a cruise rpm of 1800 rpm at 65 mph.


If you are going to use your hot-rod mainly as a cruiser, build or buy an engine for that type of use; if your main use will be as a racer, build or buy an engine for your type of racing. The main point is to know what you expect the engine to do and buy or build the engine that will fit the way the engine will be used. In order to gain power at higher RPMs, you must give up low end torque unless you add cubic inches or a supercharger. Therefore if you build a high rpm race engine, don�t expect the engine to run well at low RPMs. �


One of the most common mistakes made is putting a hot cam in an engine without matching the rest of the engine to the cam. If the engine is going to be used in the 4500 rpm & up range, the whole engine must be built to match; from the cylinder heads, valve springs, connecting rods and forged pistons to the intake and exhaust system. Furthermore, the engine must be used in the rpm range it was built for; you can go fast with a mild engine, but a race engine can�t run well when used as a driver. When an engine is built for high rpm power using a hot cam or an intake system designed for high rpm, the low rpm performance suffers due to a lack of air/fuel mixture velocity created by those components. �


If you are building an engine and selecting cylinder heads, there are many aftermarket cylinder heads available; always use a head designed for the way the engine will be used. Heads, like cams, have a flow or rpm band that they are designed to operated within; if you use a head with runners that are too large for the engines use, you will only hurt the engines power output. There is no use in putting a big cam in an engine if the cylinder head, valve springs, and intake and exhaust systems are not matched to the cams flow demands.


Is the engine carbureted or fuel injected? If fuel injection is used, the cam must be designed for the fuel injection system you are using; a fuel injection must have a cam that is matched to its program or the program in the computer must be able to be corrected for the engine�s needs. If not, you may get a headache that may never be cured. A carb is much more forgiving if the cam used is not designed correctly for the engine use. A carb will supply a fuel mixture that is easy to �jet� to match the engine requirements. A computer-controlled fuel injection injects the fuel that its program tells it to; any changes in the engine airflow may create fuel mixture problems. If the program is not corrected for any engine airflow changes made, it will not supply the correct air/fuel mixture or ignition timing! As with every computer, the computer needs all the correct information the program expects to see or it�s �garbage in garbage out� and the engine does not run correctly.


You must know what rpm band you are going to use the engine within and what the normal cruise rpm will be. What is the final gear ratio/tire size? If the cruise rpm used is the 1500�2000 in high gear at 70 mph, don�t use a cam designed to cruise at 2500 rpm and up.


On a carbureted engine, what type of intake manifold and carburetor(s) will be used? The carburetor-intake manifold combination should be selected based on the way the engine will be used. If a tunnel ram or race style intake is used just for the appearance, the engine may lose power at low RPMs. The engine must be designed for each component or it may take a lot of work to overcome the mismatch in parts; a race intake system is designed for high RPMs at the cost of low rpm performance. The best-carbureted set-up for a mild performance engine is a single 4 bbl on a dual plane intake, but there are many options that, with a little tuning, can provide the look you want along with the drivability and power you need. There are a lot of options on the carbs being used from Stromberg 97�s to an original carb such as a Rochester Quadrajet or even a pair of 1295 cfm King Demons and everything in-between. Use the carburetor style and airflow designed for your engine�s needs or it may take some serious tuning to attempt to tailor the fuel curve to be correct for your use.


A dual plane intake will help an engine make more low-end power, while a single plane intake can make more power at higher RPMs. Air-gap style intakes may create a little more power, but at the expense of cold engine operation and low rpm power due to the lack of heat in the intake manifold that helps keep the air/fuel mixtures atomized. If you are going for a 3x2 set up, a hot cam may be a waste since the manifolds available will not flow enough air for the engine at high loads. The cam selected can determine the type of intake manifold and carb you use; a mild cam can use a variety of carburetor set-ups. If a tunnel ram or other race style intake manifold is used on a mild engine for �normal driving�, it may take a lot of tuning to help get the air/fuel mixtures correct in the lower rpm range. The tuning required may be in the idle / off-idle / accelerator pump systems of the carb and the initial timing and ignition advance curves may need to be modified in order to help overcome the lack of velocity in the air/fuel charge. When building a high rpm engine with a hot cam, you may need a fully tunable high performance carb with 4-corner idle circuits to get some idle quality such as the Race Demon series of carbs from Barry Grant, Inc.


Always use an intake manifold designed for the power curve of the cam and cylinder heads; a big carb on an intake manifold with runners that are too large may cost you horsepower. If you use a cam, carb, or intake manifold that is too big, this will create lean and rich pockets in the air-charge at lower engine revs. Thus the spark plugs may not be able to ignite the fuel/air mixture in the cylinder creating a loss of engine power. Most intake manifold catalogs will show the rpm range that the manifold is designed for; this can be a good way to match an intake manifold that can work well with the rest of your engine�s components.


If a factory fuel injection is used, the cam must be designed for that fuel injection; the cam overlap and duration are crucial on a fuel injected engine; if it is incorrect, you may have a nightmare. A hot cam on a closed loop fuel injection may cause the o2 sensor to see the extra o2 created by the overlap of the cam as a lean fuel mixture. The computer will then try to enrich the fuel mixture, creating a misfire which results in even more o2! The computer then gets confused and the engine does not run as it should. If you use a cam that is not matched to the computer�s program along with the proper flow size fuel injectors, when the computer is in open loop - cold operation / wide open throttle / etc., it will supply a pre- programmed air/fuel mixture and ignition advance curve. This may cause the engine to run poorly until the o2 sensor can adjust the mixtures. A fully programmable fuel injection can be programmed to overcome many of the problems created by a hot cam but it will take time and money. A fuel injection system using a mass flow air sensor will be much more forgiving of engine changes than a speed density system; this is because the computer can �see� the extra airflow and therefore adjust for the engine changes. Both fuel and ignition advance curves must be customized on a hot cam engine.


If engine size is increased, cam changed, or other engine changes made, you may need to increase fuel injector flow. A programmable fuel injection system such as Edelbrock�s Pro-Flo requires the engine to have 10 inches of vacuum at idle and they recommend a camshaft with a max duration of 234-244 degrees @ .050� with 112 degrees of lobe separation or higher for the fuel injection program to properly function. The general rule of thumb is it takes one pound of fuel per hour to make two horsepower; therefore a 300 horse power V-8 engine will need fuel injectors that can provide at least 150 pounds of fuel per hour. The fuel injector flow must be large enough to supply the fuel needed for the engine, yet if the injectors selected are too large, part throttle performance may suffer because a large injector may not atomize the fuel properly at low flow demands (part throttle). This also assumes the computer is programmed for the larger fuel injectors.


The fuel supply system must also have a fuel pump that can maintain the proper fuel pressure along with a computer programmed to provide the correct air/fuel ratio. Not all aftermarket fuel injection systems have altitude compensation; this is a necessary feature that, if not used, may cause the vehicle to run incorrectly at altitudes above sea level. Some injection systems that do have an altitude compensation option will only update the computer�s altitude data when the engine is shut off and then restarted; this can be a pain if the car is driven from sea level to the mountains without stopping and shutting off the engine. Do your research before buying an aftermarket fuel injection system or modifying an engine on an OEM fuel injection in order to be sure you get a system that will work for you.


If the car is driven continuously at high altitude, the proper engine component selection is even more crucial. If the cam is too big, the low rpm performance will suffer the effects of altitude even more than a stock engine. A hot cam will create lower manifold vacuum, especially at higher altitudes; the engine must create enough vacuum at idle to supply vacuum for power brakes. In general, use a stock or a torque camshaft to help create low-end power at higher altitudes. The main method of creating power at high altitude is to increase the compression ratio to help overcome some of the lack of oxygen in the air or add on a supercharger.


The ignition system, along with the mechanical and vacuum advance curves, must be tailored to the engine�s demands and the fuel being used; if not, even when you use all the correct parts in the engine it can�t deliver all the engine�s potential. Just because the distributor came supplied with the engine package does not mean the mechanical advance curve, and especially the vacuum advance curve, is correct for the way you will use the engine. A high performance engine will need a high performance ignition system properly tuned for the engine in order to deliver all of the engines power.


The compression ratio should only be as high as the gasoline used will allow; the pump fuel sold at most gas stations will be good for use with 9-9� to 1 compression.


Always design a cooling system that can cool the engine since making horsepower creates heat! The bigger the engine, the hotter the cam, the higher the compression and the more heat the engine will create; thus the more heat for the cooling system to dissipate! If the car has a/c, this will add more heat for the cooling fans to get rid of. This heat will take a good cooling system along with proper airflow through the radiator. Cars such as a '40 Ford ran warm with a stock flat head engine and in a hot-rod engine with a/c and you now have a cooling problem. The problem seems to be that airflow can get into the radiator but not out; one solution is to lower the wheel wells to let the airflow through the radiator. �

Proper engine planning will give you an engine that is built for your needs and that can supply all the power you want designed for the way you drive. Find a good engine builder/supplier that knows how all the engine parts will work together and use their advice. If the engine builder doesn't ask the proper questions on how to build or supply the engine that you need, you may not be happy with your engine performance.