Jim Smart
May 23, 2006

There's a lot of myth and folklore out there for those seeking additional power from classic Mustangs. Stroker kits, superchargers, bigger carburetors, hot ignitions, high-flow exhaust systems, less restrictive air cleaners, fuel additives, and more will theoretically achieve gains in power. However, did you know your engine can make more power with simple analysis and performance tuning?

We visited Jon Enyeart of Pony Carburetors in Las Cruces, New Mexico, to discuss engine tuning and carburetors and how both directly affect performance. We also consulted with Marvin McAfee at MCE Engines in Los Angeles to get his take on how to unleash hidden power.

Pony Carburetors has been building concours-restored Mustang carburetors for 17 years. The company's nice-looking, factory-original Autolite, Holley, Carter, and Rochester carburetors look good on any concours restoration, not to mention a daily or weekend driver.

More than good-looking, Pony Carburetor's atomizers are high on function and improved performance because Jon wouldn't have it any other way. His approach to engine tuning is methodical, taking place one system at a time. He begins with the carburetor, then moves to the ignition system to ascertain a proper baseline tune. Most of the time, rough idle and lackluster performance are rooted in fuel- and ignition-system tuning. However, fuel and ignition systems cannot do their jobs effectively unless all other areas of the engine are healthy.

Any tune-up should include a compression check and cylinder leakdown test. Compression should be uniform within 5-10 psi across all cylinders. Leakdown differs from compression in that it shows whether or not the cylinder can maintain a proper seal. Compression is mechanical proof that each cylinder is making compression as the piston rises to the top of the bore with both valves closed. Leakdown is a test of cylinder pressure once compression happens and the fuel/air mixture ignites.

When compression and/or leakdown are poor, there's no point in tuning further until the fault is determined and repaired. Low compression and cylinder leakdown issues can be caused by burned exhaust valves, bad rings, piston damage, or a cracked casting.

Step By Step

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Mump_0605_08_z Vintage_ford_mustang_engine_tuning Accelerator_pump
Accelerator pumps look different on the various types of carburetors, but their mission is the same: to enrich the mixture as the throttle opens to prevent stalling.
Mump_0605_09_z Vintage_ford_mustang_engine_tuning Accelerator_pump_nozzle
Stumble, hesitation, and flat spots are caused when the accelerator doesn't provide enough of a fuel shot as the throttle is opened. This accelerator pump and nozzle is what you can expect to see on Holley and Barry Grant carburetors.
Mump_0605_10_z Vintage_ford_mustang_engine_tuning Autolite_4100_idle_mixture_adjustment
This is the idle-mixture adjustment on an Autolite 4100 four-barrel. With the throttle at idle, fuel and air mix here to keep the engine idling. When adjusting idle mixture, you are adjusting the air/fuel mixture. If the engine surges at idle, one side is too lean. A rough idle indicates a mixture that's too rich. At idle, the smaller transition hole should be covered by the throttle plate.
Mump_0605_11_z Vintage_ford_mustang_engine_tuning Transition_slot
This is the transition slot that delivers fuel as the throttle is opened to provide a smooth transition from the idle circuit to the power circuit.
Mump_0605_12_z Vintage_ford_mustang_engine_tuning Secondary_throttle_plate
Secondary throttle plates remain closed until we mash the gas. At that time, a strong vacuum signal in the primary bores opens the secondary throttle plates. Of course, on carburetors with mechanical secondaries, the secondaries open via linkage. In either case, as the secondaries open, fuel is drawn from the secondary fuel bowl via the secondary boosters.

Doctor of Tune
Jon tells us the best tuning comes from putting an engine through its paces on a chassis or engine dynamometer. On a chassis dyno, the engine is run through the driveline under load. Fuel mixture, rpm, exhaust-gas temperature, and more, are an indication of what the engine is doing as it operates. The proper air/fuel mixture can be determined with a tailpipe sniffer that measures oxygen content, NOX, and hydrocarbon emissions. All are important to proper tuning. We can also determine volumetric efficiency (VE), which helps determine carburetor and manifold sizing for the application.

It's easy to assume that dyno testing only determines power at wide-open throttle, but it can also indicate how the engine behaves at cruise power. It's good to know what the engine does at 70 mph as well as during deceleration and idle.

Air/fuel mixture is primarily determined by jet and throttle-bore size. Main metering jets in the carburetor control fuel flow to the boosters in the carburetor throttle bores. Boosters mix the fuel and air. The larger the jet size, the more fuel reaches the boosters. If the mixture is too rich, reduce the jet size to reduce fuel flow to the boosters. By the same token, if the mixture is too lean, a larger jet size is needed for greater fuel flow to the boosters.

Boosters and main metering jets do their work when the throttles are open, and there's airflow through the boosters. When the throttle plates are closed and the engine is at idle, fuel flows through the idle circuit and atomizes below the throttle plates. As the throttle opens, fuel moves from the idle circuit to the transition circuit, which coupled with the accelerator pump shot, helps the carburetor move from idle circuit to power circuit. One objective for tuning is to help the engine make a smooth transition from idle to power making. Clean fuel and air circuits are required, coupled with a good accelerator-pump shot as the throttles open.

Proper air and fuel distribution is needed under all conditions. If you step on the throttle and the engine stumbles, this indicates a weakness at the accelerator pump or transition circuit. This is when you want to examine accelerator-pump shot. Does raw fuel spray from the accelerator-pump nozzles? If it doesn't, the accelerator pump is at fault. If there is fuel spray, the problem could be either an insufficient amount of fuel or dirt, irregular passages, and so on in the transition circuit.

Inspect the choke, which provides a richer mixture when the engine is cold to prevent stalling by reducing air supply. At the same time, a fast-idle cam holds the throttle open a pinch more to induce a faster idle.

Automatic chokes operate based on engine temperature. As the engine warms, the choke opens up, leaning the mixture. Cold stalling is an indication of lean choke and overly rich conditions if your engine is belching a lot of thick, black smoke from the tailpipe and idling rough. Most of the time, chokes don't come on enough, if at all, which causes a lean condition when cold as indicated by stalling.

The choke should come on approximately 3o4-closed for a good cold start. Every engine mandates something different that requires choke adjustment. Some need more choke than others. Much depends on ambient temperature and engine temperament, e.g., cam profile, carburetor, and so on.

Sometimes you do all you can for a carburetor, and the engine still won't run properly. Ford 170 and 200ci sixes, for example, suffer from poor idle quality and lackluster performance. The long intake manifold cast into the cylinder head is the main reason these engines struggle. Fuel distribution is poor because of the long, roughcast internals that interfere with air/fuel flow. Cylinders 1 and 6 get shortchanged most of the time, followed by cylinders 2 and 5.

Step By Step

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Mump_0605_13_z Vintage_ford_mustang_engine_tuning Autolite_single_point_distributor
Here's a simple Autolite single-point distributor. The vacuum-advance unit moves the breaker plate as the throttle is opened, and the vacuum advance receives a vacuum signal. The downside to contact points is irregularity. Point gap and dwell angle change constantly under the best of conditions, affecting performance regardless of how often you tune the distributor.
Mump_0605_14_z Vintage_ford_mustang_engine_tuning Rebuilding_an_autolite_distributor
Marvin recommends installing a PerTronix Ignitor II electronic-ignition retrofit kit, which eliminates the points and condenser, making a vintage Autolite distributor maintenance free. Marvin tunes mechanical advance first by using the right combination of springs and tension. Then, he tunes the vacuum advance. A vacuum advance should be tuned in a running engine while you're setting timing.
Mump_0605_15_z Vintage_ford_mustang_engine_tuning Checking_current_resistance_in_ingition_parts
It's the little things that can create performance issues. Marvin strongly recommends checking every place there's current flow for proper resistance. Distributor rotors, for example, are notorious for resistance issues that can cause a weak spark or misfire. Check the resistance across the rotor, primary wire, ground wire, rotor cap, and ignition wires with a digital ohmmeter, even when your engine is running well.
Mump_0605_16_z Vintage_ford_mustang_engine_tuning Aftermarket_vacuum_advance_adjustment
Aftermarket vacuum-advance units are adjusted with an Allen wrench. Turn clockwise to slow the rate of advance; turn counterclockwise to speed up the rate of advance.
Mump_0605_17_z Vintage_ford_mustang_engine_tuning Msd_mechanical_advance
This is the mechanical advance in an MSD distributor. Flyweights are affected by rotational speed and centrifugal force. As shaft speed increases, flyweights swing outward against spring pressure to advance the rotor timing. Spring tension is what determines advance curve. MSD offers different spring tensions depending on the desired curve. Ford did the same for Autolite and Motorcraft distributors.
Mump_0605_18_z Vintage_ford_mustang_engine_tuning Rebuild_parts_for_an_autolite_distributor
McAfee suggests an arsenal of top-quality ignition parts for your old Autolite distributor. A PerTronix Ignitor II retrofit is only the beginning. Opt for the gray cap because it offers the best resistance against crossfire. Go with either the factory, adjustable vacuum advance or this aftermarket, adjustable vacuum advance for precision tuning.

There are times when no matter what we do with a carburetor, performance will never improve because of invisible flaws. Carburetors are aluminum castings that don't always have clear passages and smooth machining tolerances. Heat and use cause distortion and core shift, making a good carburetor turn bad over time.

We sometimes blame jet size, a faulty power valve, a blown accelerator-pump diaphragm, clogged idle passages, or poor idle adjustment when it's simply a distorted or closed passage. When you have exhausted all efforts and checked everything thoroughly, sometimes you've no other choice but to replace the carburetor. Doing a temporary carburetor swap can solve a lot of mysteries. If idle quality or overall performance improves, you've solved the mystery. If nothing changes, your problem likely isn't the carburetor.

Troubleshooting should include elements around the carburetor; base gaskets and spacers, intake manifold gaskets, vacuum lines and hoses, and throttle linkage adjustment. For example, did you know that a vacuum leak just about anywhere can cause poor performance? A torn power-braken booster diaphragm, bad vacuum modulator (automatic transmission), or ruptured air-conditioningnvacuum motor can cause vacuum leakage and poor performance. Trouble-shooting should include eliminating these as problem sources before looking any further.

Ignition Tuning
Performance can be improved dramatically by fine-tuning a classic Autolite or Motorcraft ignition system. First, have your distributor rebuilt by a qualified professional who can install new parts; bushings and shaft, cam, breaker plate, electronic-ignition retrofit, high-quality rotor and cap, and drive gear. Ideally, your builder will know how to properly curve the mechanical advance for your application. A seasoned distributor-tuning pro knows how to dial in the mechanical advance (affected by engine speed), then tune the vacuum-advance unit to work hand-in-hand with the mechanical advance. The mechanical advance can be dialed in accurately on a distributor machine. However, vacuum advance needs to be tuned in a running engine for best results.

At idle, there should be no vacuum to the vacuum advance because the throttle plates are closed. Vacuum-advance units get ported vacuum, which means only when the throttle opens. As the throttle opens, a working vacuum advance begins to get ported vacuum, which enables it to move (advance) the breaker plate. Theoretically, as engine rpm increases, the vacuum advance should hand off its job to the mechanical advance, which is affected by increased engine speed. Spark advance is needed because the fuel/air mixture doesn't explode when the piston reaches top dead center (TDC). The fuel/air mix needs time to ignite in a quick-fire before the piston reaches TDC. When the spark plug fires before TDC, the fuel has time to ignite and get fully underway before the piston reaches TDC. When the piston reaches TDC, it can make full use of the heat and energy created by the spark/air/fuel combo.

To get it right, dial in mechanical advance first, then fine-tune the vacuum-advance unit's rate of advance. Ford was smart about this with its original vacuum-advance units. Inside were shims and a spring designed to control the advance rate. Add shims to slow the advance rate; use fewer shims to quicken the advance rate.

Aftermarket vacuum-advance units typically employ an Allen screw inside to control advance rate. Turn the screw clockwise to slow the advance rate; turn it counterclockwise to quicken advance rate. To understand the effect this tuning has, use a timing light and watch the timing mark as you open the throttle. Jon tells us if you're seasoned at this, you won't even need a timing light. He does it by ear and feel.

Ignition timing needs to be checked and tuned at least two different ways. First, static ignition timing should be checked with the vacuum-advance hose disconnected. Follow factory specifications for initial timing. Small-block Fords are typically anywhere from 6-12 degrees before top dead center (BTDC). Six-cylinder engines are 5-6 degrees BTDC.

This is why a timing light is a good idea, even if you're astute at tuning by ear. Timing marks allow us to see how timing events are unfolding. Watch them as you goose the throttle, which determines vacuum advance adjustment. Note how MCE Engines has marked this balancer: TDC, then idle timing at 12 degrees BTDC, then maximum advance at 34-36 degrees BTDC. This gives you a tuning window as a guide.

With the vacuum advance reconnected, check total timing, which can be done two ways. Total timing is checked at 3,500 rpm with the vacuum advance connected. Ideally, push for the maximum number of degrees before BTDC without detonation (pinging or spark knock). In most circumstances, total advance should be 34-36 degrees BTDC.

The rate of advance is checked by goosing the throttle to wide open and watching the timing mark with a timing light. If spark advance comes on too quickly, the engine will tend to break up (misfire) and ping (spark knock), which means you need to slow the rate of advance. If the engine noses over and doesn't rev quickly, speed up the rate of advance.

When the vacuum advance is dialed in just right, the engine revs smoothly and crisply. On the road, power comes on quickly without hesitation or spark knock. At sustained highway speeds, the engine should maintain a smooth demeanor without misfire. If you feel a slight misfire at cruising speeds or hear pinging under hard acceleration, you have too much timing.

Another area to check is manifold vacuum. Healthy manifold vacuum is normally 15-22 inches at idle. Checking manifold vacuum is just as important as checking compression and cylinder leakdown. It determines an engine's state of health. Note that manifold vacuum gets tricky when the engine is fitted with an aggressive camshaft or too much carburetor.

Good Carburetion
Pony Carburetors was founded in 1988 to meet the restoration needs of vintage Ford enthusiasts. Jon Enyeart founded Pony Carburetors as a natural extension of his skills and now practices his craft for fellow enthusiasts, continually improving his process through the years. A big part of what Pony Carburetors is all about is impeccable restorations and tuning classic Mustangs to run better. John has a good, in-depth understanding of how carburetors work and how to improve performance.

Dyno Test: '65 Six-Cylinder
Owner: Lucio Telles
200ci Six, C4 automatic
Lucio's '65 Mustang convertible ran very well when he drove it into Jon's shop. The six-cylinder droptop has long-tube headers and dual exhausts, which gives the 200ci six some advantage. It did have a rough idle, typical for most inline Ford sixes. Jon's objective was to make more power and clean up the idle. We did four dyno pulls. We use the term pull loosely because not all of them were wide-open throttle.

Pull 1: Baseline
Peak horsepower: 73.66 at 3,150 rpm
Peak torque: 124.07 lb-ft at 3,100 rpm
Air/fuel ratio: 13.4:1 at 70-mph cruise; wide-open throttle at 11.0:1: too rich
Autolite 1100 1V with 70 main metering jet for sea level operation

Pull 2
Peak horsepower: 55.88 at 3,100 rpm
Peak torque: 104.98 lb-ft at 2,750 rpm
Air/fuel ratio: 12.5:1 at wide-open throttle: some improvement
Same Autolite 1100 1V with smaller 69 jet to lean the mixture. A check of ignition timing was spot-on at 6 degrees BTDC at idle. With a jet swap, it's always a gamble which way power will go. We lost power with this jet swap, and Jon wasn't pleased with the result.

Pull 3
Peak horsepower: 58.21 at 3,400 rpm
Peak torque: 88.83 lb-ft at 3,400 rpm
Air/fuel ratio: 14.0:1 at 70-mph cruise: improvement
Smaller 68 jetting resulted in some horsepower improvement but with a significant loss in torque. Jon is as baffled by the numbers as we are. It's still quite rich at wide-open throttle (10.02:1).

Pull 4
Peak horsepower: 77.38 at 3,250 rpm
Peak torque: 124.86 lb-ft at 3,250 rpm
Air/fuel ratio: 14.3:1 at 70-mph cruise
Same Autolite 1100 1V with smaller 67 jetting. Jon took extra time double-checking everything this time around, coupled with concise communication with Lucio about how to manage the throttle.

Jon concluded the Autolite 1100 runs progressively richer at wide-open throttle and high rpm despite efforts to get it leaner. Regardless of the jet changes, our 200 six still runs fat, 10.43:1, with the throttle wide open. Jon was able to increase 4 hp by downsizing the jet to 67 from the original 70. This worked well in Las Cruces, New Mexico, which is at 4,000 feet. Sea level conditions or a higher elevation would have mandated different jet sizing in either direction.

We gained 4 hp with a simple jet swap but didn't gain torque. Torque is normally gained through a richer mixture, increasing jet size instead of downsizing. This proves there are no free lunches in the power game. Lean the mixture and gain horsepower along with cleaner emissions. Enrich the mixture, lose horsepower, gain torque, and pollute the atmosphere. In this case, Jon took the best approach: a leaner mixture, more horsepower, and cleaner emissions.

Step By Step

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Mump_0605_27_z Vintage_ford_mustang_engine_tuning Tuning_ford_289ci_engineMump_0605_28_z Vintage_ford_mustang_engine_tuning Light_tan_on_plugs_indicates_good_burn
A plug reading on all eight cylinders showed a light tan color, perfectly normal. It's a good fuel mixture and doesn't burn oil.
Mump_0605_29_z Vintage_ford_mustang_engine_tuning Poorly_adjusted_throttle
Here's one reason Doug's 289 didn't perform to potential: a throttle not opening fully. Jon adjusted the rod and fixed the problem.
Mump_0605_30_z Vintage_ford_mustang_engine_tuning Wrong_vacuum_hose
Our troubleshooting efforts turned up this vacuum hose between the oil-filler cap and the carburetor-base PCV port. Do you see the restrictor someone fabricated at the carburetor-base end? This is a vacuum leak serious enough to alter air/fuel ratio and cause poor performance.
Mump_0605_31_z Vintage_ford_mustang_engine_tuning Bad_carburetor_base_gasket
While we were tossing the improperly installed PCV valve hose, we decided to check carburetor-base and spacer gaskets. Both were deteriorated, causing vacuum leaks and poor performance.

Dyno Test: '65 289
Owner: Doug Holland
289-2V, C4 automatic
When Doug Holland arrived at Jon's shop, his Mustang was plagued with a definite misfire and rough idle. Doug's original 289-2V had received only one valve job in its 42-year history. We were convinced the misfire was ignition related or rooted in weak compression. Testing would prove us wrong on both theories. Jon decided to do the baseline test exactly the way the car came in.

Pull 1: Baseline
Peak horsepower: 98.27 at 3,500 rpm
Peak torque: 167.59 lb-ft at 2,650 rpm
Air/fuel ratio: 12.8:1 at 70-mph cruise; at wide-open throttle, 12.0:1

We began troubleshooting with a check of the timing and ignition system integrity. The Autolite single-point distributor had seen better days. However, it produced a healthy spark on all eight cylinders. One of the first moves to help this distributor should be installing a PerTronix Ignitor II and curving the advance units, which makes a significant contribution to performance and reliability. We weren't prepared to do that in the time available, so we continued with the same ignition system.

Jon wondered about compression considering the soft misfire he traced to the No. 2 cylinder. The compression check was remarkable with all eight cylinders coming in at 105-110 psi each. A cylinder leakdown test yielded a healthy 289. So why the misfire?

Pull 2
Peak horsepower: 87.65 at 3,250 rpm
Peak torque: 157.69 lb-ft at 2,600 rpm
Air/fuel ratio: 14.6:1 at 70-mph cruise; at wide-open throttle, 13.0:1
When Jon observed air/fuel mixture, he concluded that horsepower could be improved by reducing jet size from 52 to 48. Ironically, horsepower fell off a pinch to 87.65, but torque increased to 157.69. This tends to break all the rules because, theoretically, horsepower should have increased. There are other factors going on here, including a marginal ignition system. We still had the misfire.

Pull 3
Peak horsepower: 83.92 at 2,900 rpm
Peak torque: 151.87 lb-ft at 2,900 rpm
Air/fuel ratio: 14.6:1 at 70-mph cruise

When Jon decided to try another dyno flog using the same 48 jets and ignition timing, something caught his attention: the throttle linkage. It did not open the throttle fully with the accelerator pedal floored. Jon adjusted the throttle rod to allow the throttle to open all the way.

Pull 4
Peak horsepower: 136.7 at 3,500 rpm
Peak torque: 205.10 lb-ft at 3,500 rpm
Air/fuel ratio: 14.6:1 at 70-mph cruise

After Jon took care of the maladjusted throttle rod, faulty carburetor base gaskets, and improper PCV valve hose, positive changes began with the right air/fuel ratio and throttle positioning. These were simple fixes costing less than $5. Jon also swapped in larger 49 jets to richen the mixture and adjusted the idle mixture to help improve idle quality.

Despite Jon's best efforts, he was unable to get rid of the misfiring plaguing Doug's Mustang throughout our testing. Althought Jon checked virtually everything, compression, cylinder leakdown, spark quality, and timing, carburetor gaskets, and vacuum hoses, the misfire remained throughout our testing.

During each pull, we couldn't help but notice black smoke coming from the exhaust at wide-open throttle and deceleration. We sprayed carburetor cleaner around the intake ports and didn't observe a change in engine operation, which would indicate a vacuum leak on top. We concluded there was a vacuum leak at the No. 2 cylinder's intake port underneath, causing the affected cylinder to misfire for no apparent reason. We suggested Doug replace the intake manifold gasket, which should correct the vacuum leak and get rid of the misfire. We also advised Doug to invest in a rebuilt Autolite distributor, retrofitting it with a PerTronix Ignitor II to improve idle quality, cold starting, and drivability.

Dyno Test: '67 390
Owner: Jamie LaPage
390 High Performance, C6 automatic
Jamie LaPage arrived at Jon's shop with a rough idle and poor performance for a 390 High Performance V-8. Jamie's 390 has an aggressive hydraulic camshaft, which adversely affects idle quality. What's more, there's an Autolite 4100 on top of an Edelbrock Performer 390 dual-plane manifold. These modifications aren't a bad thing, but without proper tuning, they can adversely affect performance.

Pull 1: Baseline
Peak horsepower: 193.30 at 3,750 rpm
Peak torque: 270.66 lb-ft at 3,750 rpm
Air/fuel ratio: 12.8:1 at 70-mph cruise; at wide-open throttle, 11.0:1

Jon checked out Jamie's 4100 carburetor and concluded it was not calibrated for altitude but ran the 390 anyway to establish a baseline.

Pull 2
Peak horsepower: 198.0 at 4,700 rpm
Peak torque: 264.32 lb-ft at 3,650 rpm
Air/fuel ratio: 14.0:1 at 70-mph cruise; at wide-open throttle, 12.5:1

Jon pulled Jamie's 4100 and replaced it with a remanufactured 4100 from Pony Carburetors calibrated for the thinner air in Las Cruces. Horsepower went up but torque went down. However, we experienced a broader torque curve at high rpm with this simple swap to a carburetor jetted for 4,000 feet (49/66). Jon also retarded the ignition timing a couple of degrees to reduce the risk of detonation and improve the torque curve. Another solution would have been to tune the distributor and curve it for Jamie's manifold, carburetor, and camshaft combination.

Dyno Test: '69 351 Windsor
Owner: Rob Sharp
351W-4V, FMX automatic
Rob Sharp roared up in his Black Jade '69 Mach 1, which seemed to be the healthiest of our Mustangs. Rob had the advantage with a healthy 351W-4V engine with an aggressive hydraulic roller camshaft and 600-cfm Holley 1850 carburetor on top of an Edelbrock dual-plane manifold. However, Enyeart was about to do something no one expected: replace the Holley with an Autolite 4300.

Pull 1: BaselinePeak horsepower: 164.5 at 3,700 rpm
Peak torque: 256.06 lb-ft at 2,950 rpm
Air/fuel ratio: 11.0:1 at 70-mph cruise, too rich; at wide-open throttle, 10.0:1

Rob's 351W came right out of the chute with an attitude. It ran strong and never sputtered. The 600-cfm Holley 1850 delivered exceptional performance considering it was not fine-tuned beforehand.

Pull 2
Peak horsepower: 169.4 at 3,550 rpm
Peak torque: 262.49 lb-ft at 3,050 rpm
Air/fuel ratio: 12.8:1 at 70-mph cruise, leaner; at wide-open throttle, 12.0:1: still too rich.

With the factory 4300 carburetor, torque comes on strong at a low rpm. Horsepower is up as well. The 4300 yields a broader torque curve if you know how to tune it.

Pull 3
Peak horsepower: 172.5 at 4,100 rpm
Peak torque: 263.03 lb-ft at 3,150 rpm
Air/fuel ratio: 13.8:1 at 70-mph cruise; at wide-open throttle, 12.0:1

Enyeart tried a primary jet swap to 46s, .002-inch smaller to lean the mixture, which enabled the 351W to produce more horsepower and deliver a broader torque curve.