Let’s assume that the break-in process and tuning are being done on an engine dynamometer. A dyno test on a new engine build is highly recommended, although not absolutely necessary. Beyond the obvious benefit of identifying the actual powerband of the engine, the dyno allows full access to the engine. This makes spotting any problems, required repairs, and adjustments far easier than if the engine were in the car. Most at-home mechanics or enthusiasts do not have their own dyno or hands-on access to one. However, the tuning and set-up information in this chapter gives you the knowledge to properly guide your engine through the dyno tuning process, so you wind up with the strongest-running engine.
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Connecting the Engine to the Dyno
Mounting the engine to the dyno is similar to, but simpler than, installing it into a car. With the DTS dyno, the dyno I’m most familiar with, the engine is mounted to a portable cart and then the engine is connected to the dyno. The engine is attached to the cart with a large plate bolted to the rear bellhousing surface of the block, along with a pair of adjustable legs serving as motor mounts up front. A manualtransmission SFI-legal flywheel is attached to a coupler for connecting to the dyno itself. With the engine physically mounted to the cart, it is wheeled into the dyno room and attached to the machinery.
When connecting the engine to the dyno, you need to hook up water for cooling and install a water-temperature sensor. A modified drainplug sensor needs to be installed to measure oil temperature. The throttle linkage is attached and checked for WOT actuation. Headers are routed to exhaust plumbing in the dyno room. My dyno headers also use a pair of wide-band oxygen sensors to monitor fuel mixture—one in each collector. Most dyno rooms have an integrated fuel-supply pump and an ignition box, so all you need to connect are the trigger leads for the distributor and the fuel lines onto the carb.
Each time the engine is run through the RPM range on the dyno, it is referred to as a “pull.” The dyno controls the amount of load on the engine and the rate of acceleration through a combination of electronics and hydraulic pressure.
Before making any engine pulls on the dyno, you need to perform some final procedures and checks. Hook up a timing light, check for fuel pressure and leaks, ensure that the engine is full of the proper break-in oil, and check for distributor position and spark. In addition, you need to check the oil to be certain that there are no internal water leaks. I also assume that if you are running a flattappet cam, the inner valvesprings have been removed for proper breakin— it’s an absolute must.
Breaking It In
If everything is set correctly, the engine should fire up instantly after a couple pumps of the throttle. It is particularly important on flat-tappetcam engines that you do not spend a lot of timing cranking; you want all that cam lube to stay in place for the first few revolutions.
Once it lights up, try to quickly get the engine up to around 2,000 rpm and start checking on the operation of the engine. The base timing needs to be set. Small drips are not an issue, but big leaks or unusual noises are good reason to shut the engine off and fix them. Shutting the engine down does not hurt the break-in process, and is much smarter than continuing to run with a major problem.
Once satisfied that everything looks and sounds healthy, put a light load on the engine and let it run for about 20 minutes. You need to place a load on the engine to help the rings seat in the cylinder. Vary the load and the RPM a bit during this time, and keep an eye on temperatures and oil pressure. Walk into the dyno room while the engine is running and look for leaks, listen for noises, and feel for vibrations.
When the break-in period is over, I usually do a very short dyno test pull from 2,000 to perhaps 4,000 rpm just to get a starting reference for fuel mixture. Remember that you only have the outer valvesprings if you are doing a flat-tappet-cam break-in, you cannot rev it up very high without risking damage.
With the initial run completed, it’s time to get the engine ready for the real work. Remove the oil filter and cut it open to check for debris. A few speckles is normal; a damaged cam fills the pleats of the filter with shavings. A magnet can help identify anything you find; bearing and piston material is nonmagnetic. If you find specs of material and minimal debris, you should install a new filter filled with oil and proceed with the dyno run.
The next step is to remove the valve covers, check your lash settings, and then reinstall the inner valvesprings on flat-tappet-cam engines. You need to reset lash again after the engine has been run. At this stage, verify that the engine has been correctly assembled and there are no problems. This is your chance to fix any small leaks you have spotted. Most are quickly cured with a dab of teflon paste or the turn of a wrench.
Tuning for Power and More
An engine dynamometer is great at testing for WOT. It’s reasonably good for testing steady-state/steadyload RPM characteristics. And it’s darn near useless at testing for transient throttle response and behavior. Although an OEM has expensive and sophisticated software-driven dyno cells to do such testing, you’re not going to find one in any average race shop this side of a NASCAR team. You are testing for a baseline, with the expectation of final tuning in the car itself.
Tuning, whether on a dyno or in a car, is a three-step process. First, you need to establish control. In the case of our subject engine this means a safe and repeatable fuel curve as defined by the dyno’s oxygen and BSFC data. The second step—with control established—is to look for trends. Trends are defined by making a single change and gauging the results of that change. If the engine responded favorably (increased horsepower or torque), you should go further in that direction and test again. If the engine didn’t respond well to the change, go in the opposite direction and test. It takes at least three dyno pulls to define a trend.
Keep in mind: you cannot extrapolate information beyond the range of your tests. If 32 degrees of timing does better than 30 did, it doesn’tmean that 34 is better yet. The last step is to let the engine decide what it “wants.” If best power calls for goofy timing or fuel numbers, the odds are that something is wrong with a sensor or marking. Do not tune to a test value; tune for performance.
On a street/strip-style engine, you test for the best fuel mixture, best timing, and to define the RPM ranges for torque and power.
A dyno provides a lot of data, assuming that all the sensors are operational and connected. Obviously, it provides horsepower and torque numbers, but it also provides fuel efficiency data as measured by the BSFC curve, and fuel mixture as defined by oxygen-sensor readings. These are both measuring fuel usage in different ways. You can tune with only one, but having both is a very good idea. There are no “most correct” numbers for either measurement, but there are commonly accepted ranges that you should expect to be within.
The BSFC measures the amount of fuel used to get 1 hp out of the engine. Typically it’s between .350 and .600 at WOT. It used to be said that a BSFC number of .500 was a good one, but you commonly see far lower values in very good engines today. A lower value is considered leaner or more efficient as long as no detonation is present.
Wide-band oxygen sensor essentially measure the amount of oxygen in the exhaust stream. This correlates pretty well to the actual fuel mixture, and the data normally tracks along with the BSFC as the mixture is made leaner or richer. Output from the oxygen sensors is expressed as an “air/fuel ratio,” with the ratio value going up as mixtures get leaner. Expectations of between 12.5:1 and 13.5:1 are reasonable at WOT for naturally aspirated engines.
Assuming that everything is operating properly, you are ready to make some real power—well, almost ready. Get the timing set to your baseline position, get the engine fully warmed up, and reset valve lash with the proper springs in place.
I’ll start the engine and run it up to around 3,000 rpm to check total timing. I like to start out with a conservative value, somewhere around 32 degrees. I quickly scan the readouts for oil pressure, fuel pressure,and temperatures. If all is okay, I close the door to the dyno room and make the first real test pull. The first one is usually only to around 5,000 rpm but under full load. I amlooking for a safe fuel curve and smooth torque delivery. If the data looks good, I make a few subsequent pulls in higher RPM increments until I establish the engine’s RPM peak forhorsepower. I like to see a clear roll-off for a couple hundred RPM at the top of the power curve; otherwise a small dip migh fool you into thinking the engine is done when it has more yet to give.
Unless the fuel curve appears dangerous, I do a few timing loops first, finding the best power in 2-degree increments. Because most FE packages are comfortable somewhere between 28 and 38 degrees, a good timing baseline should be established within a half-dozen pulls.
Getting control of the fuel curve can be more challenging. Most often, things come in pretty close to usable for a baseline on a new carburetor. I am not going to get into fullscale carburetor tuning because that’s a book in itself, but I can provide a few working guidelines that will be helpful.
The total fuel-delivery curve is determined as a combination of flow through several circuits. The main circuit is responsible for the majority of fuel delivery at higher speeds, with an additional 20 to 40 percent of the total flow coming through the power-valve-channel restrictions. The idle circuit is still active even at WOT and delivers a fair percentage of the fuel package at lower RPM. However, its significance diminishes as the main circuit becomes fully active.
Changing the main jets alters the entire WOT fuel curve, as well as the steady state unloaded fuel delivery. Changing the size of the power valve-channel restriction alters the high-load and WOT parts of the fuel curve without adversely affecting fuel mixture at constant or steady state throttle application. Often it’s better to keep the part-throttle lean a crisp by leaving the jetting slightly lean and opening up the power valve channel restrictions for peak power fuel. Changes to t main air bleed affect both the timing of the main circuit (where it starts flowing) as well as the fuel-mixture quality and ratio, which bleeds “add air” into the fuel mix coming through the boosters.
Reading spark plugs is the “old school” way of monitoring fuel mixture, and still has some merit. The plug color can be impossible to read when running unleaded gas, though, because the fuel tends to color the shells black and leave the insulators bone white. I normally check the plugs after I think I’ve got the tune optimized as a cross-check for the data from the sensors. Remember: Tune for the best power, and use the sensors as a guide. You don’t want to optimize the sensor readings; you want to optimize the engine!
With everything close to optimal, you can safely test the performance of a variety of parts, such as carb spacers, different headers, mufflers, air cleaners, plug gaps, and other timing settings, etc. But to eliminate variables, you should add one part or make one change at a time, so you are certain of the source of the change. The changes you make will be clearly reflected in the data, without the concern of damage or unknown variables.
Once satisfied, you can remove the engine from the dyno and get it ready for the car. Drain and inspect the break-in oil. It usually looks a bit dark and “sparkly” from break-in material, but it should otherwise be free of water or debris. Cut open the filter for final inspection. Check carefully for any leaks and fix them immediately. Mark your distributor for position in case it gets bumped during engine installation. And remember, you still need to tweak a bit once it’s in the car to optimize the transient behavior.
Get out there and enjoy your new engine!
Written by Barry Robotnik and Republished with Permission of CarTech Inc