1. What oil do I need and what do those ratings mean?
There are several ratings assigned to motor oils by the American Petroleum Institute (API). The newest ratings (SM and SN) always supplant older ones and are backward-compatible for engines, meaning they’re approved for use in anything that was covered by the previous generation of oils. If your engine uses a roller cam, no sweat, you’re fine with the modern oil of your preference.
The issues for rodders arise with flat-tappet cams as modern oils have much lower levels of ZDDP (zinc dialkyl dithio phosphate), which is the key component for cam and lifter wear prevention on flat-tappet cams. Modern oils use roughly one quarter of the pre-1990 level of zinc and phosphorus to meet the EPA’s emissions standards and requirement of 100,000-mile catalytic converters. That amount of ZDDP doesn’t allow for much of a sacrificial coating to prevent metal-to-metal contact. Guys with modest engines with small cams and low spring pressure may get away with it; guys making real power likely won’t.
Pages could be written on this topic, but we know all you really want to know is what goo to pour in. Unfortunately, it’s not as easy as looking for generic “racing,” heavy-duty, or diesel oil, since that labeling doesn’t specifically mean they have the additive levels needed. The simplest answer? Buy oil from the guys who hang their hats on racing and produce oil specifically designed for flat-tappet engines with aggressive lobe designs and high seat pressure. You may not find it at Wal-Mart, but there are plenty of options; AMSOIL, Royal Purple, COMP Cams, Valvoline, Kendall, Brad Penn, and Joe Gibbs Driven all have oils available that provide ideal levels of zinc and phosphorus for flat-tappet engines. AMSOIL’s new Z-Rod oil, for example, is specifically formulated with high zinc content for flat-tappet engines.
Alternatively, a specific additive like ZDDP Plus can bring just about any modern oil to safe levels, though it obviously can’t offer the benefits of a specifically blended additive package designed to work with a specific base oil.
One more tip for flat tappers: opt for COMP Cams’ Pro Plasma Nitriding surface treatment on your cam. Nitriding increases the cam’s hardness by about 15 points on the Rockwell scale, which will help significantly during break-in and wear.
2. Wiring: When is it time to replace?
The first and most obvious thing to look for here is the general condition of the insulation and the wire itself. If the insulation is dry, cracked, or broken, or so petrified that bending it snaps it, well, that means the wire is likely done on two levels. Broken insulation obviously results in the increased chance of short circuits and fire, but it also means that the wire itself may have been exposed to moisture-causing corrosion. Corrosion is sneaky too; once it gets started it can travel up wires, even under the insulation. That results in less electrical conductivity, more resistance, and probably more heat—none of which are good.
The second thing to consider is: How much electrical equipment are you going to add? EFI, gauges, power accessories, and other various modern electronics were never considered in anything pre 1980s, so the odds are the capacity (physically and electrically) of the fusebox in anything older isn’t up to the challenge. There likely won’t be extra fused slots available, so you’ll end up stringing new wires and building out auxiliary circuits with their own external fuses and relays to safely add on some of your upgrades. You’ll quickly end up creating as much work for yourself as just replacing everything from the get-go. Fortunately the aftermarket has kept up with the pace, and model-specific kits are available from American Autowire and Painless Performance for many popular vintage cars that make the process much less intimidating.
3. Tires and wheels: How much can I fit, what offset looks right?
Looks are very subjective when it comes to stance, but generally speaking wider is better, and the more meat the merrier. That means getting the wheels as close to the edge of the body with as much tire as practically possible. Historically, wheels were only available in limited sizes and offsets, so rodders were much more constrained on what they could package, but modern
multipiece wheels have made that obsolete. Now you can have wheels spec’d to what your individual car needs.
The basics to consider here are the intended ride height, suspension travel, inner fender shape, and turning radius. For example, a wide tire that clears at stock ride height may end up hitting the wheel lip or inner fender when the car is lowered. It may also run into the fender, control arm, or even subframe when turned to full lock. The first step is to establish what the car’s ride height will be since everything else will have to work around that. Now possible issues with upward suspension travel become more obvious.
For increasing wheel and tire width, a great cheap way to simulate the new package and dial-in what will and won’t work is with a wheel and tire fitment tool like the Wheel Rite from Percy’s High Performance. These tools are cheap through Summit Racing and can simulate wheel widths from 6 to 11 inches and diameters from 15 to 30 inches so you can see where issues will arise before you order wheels. That’s actually how we spec’d out the wheel clearance for Project EcoNova. (Look for the web exclusive on www.popularhotrodding.com.)
4. Tires: What do all those numbers on the sidewall mean?
There is a plethora of information on the side of tires, but we’ll just touch on the ones of top interest to hot rodders. Modern street car tire sizes are typically denoted by three numbers (example: 275/40R17); this is called P-metric sizing. The first number is the section width (measured at the overall widest point, not at the tread) of the tire measured in millimeters, 275 mm in the above example.
Followed by that is a number that denotes the tire’s aspect ratio, or sidewall height from bead to tread, given as a percentage of the width. For our 275/40 series tire; 275 × .40 = 110 mm. The last number indicates the diameter of the wheel the tire is intended to be mounted on in inches. If there is an “R” directly preceding the diameter that means the tire is a radial. If there is a “P,” for “passenger car,” preceding the section width, that means the tire uses the P-metric load and inflation tables. No “P”? That means it uses the Euro-Metric table.
Want to know how much speed your tire is capable of sustaining? The code directly following the size specs of the tire will tell exactly what speed the tire has been tested to withstand.
|Common Speed Rating
||168 (usually combined with a Z or ZR)
||168+ (usually combined with a Z or ZR)
||186 (usually combined with a Z or ZR)
||186+ (usually combined with a Z or ZR)
The treadwear number indicates the tire’s wear rate. The higher the number is, the longer it should take for the tread to wear down. For example, a tire graded 400 should last twice as long as a tire graded 200. This is also typically tied to the tire’s intended use, as higher performance and race tires will invariably have a lower treadwear because of their softer compounds.
This indicates a tire’s ability to stop on wet pavement and is graded highest to lowest as “AA”, “A”, “B”, and “C”. An “A” tire should stop in a shorter distance in wet weather than a “B” lower grade. These grades are usually tied to performance as well; ultra-high performance tires will almost always have a higher grade than all-season tires.
Graded "A", "B", or "C," this letter indicates a tire’s resistance to heat when properly inflated and not overloaded. High speed, under inflation, or overloading increases heat buildup and the chances of tire failure.
5. When are my tires too old to be safe?
Just because you only drive your rod on weekends and only rack up a couple thousand miles a year doesn’t necessarily mean those tires will last longer than the ones on your daily driver. That’s because tires can “age out” before they wear out.
General consensus in the tire industry is that about six years is the maximum safe range for most tires that are used/abused in less than ideal driving conditions, which is all of them. Sun exposure, dry or hot climate, rain, chemicals from road grime, and even extended parking all accelerate aging. Makes sense; no tire we know of is warranted beyond six years regardless of mileage. Actually, Ford once appealed to the Federal Government to impose a general six-year expiration date for most tires, and the British Rubber Manufacturers Association (BRMA) once recommended that “even unused tires should not be put into service if they are over six years old.”
If you work your tires hard, count on much less lifespan; heat cycling degrades rubber compounds and the more aggressive and softer the rubber compound, the faster the wear and aging process. Ultra-High Performance and competition tires age the quickest. If you like running your car in road race or track events, know that many events have an age rule; tires often must be under 2 years old, regardless of condition, or you don’t race.
So how do you know how old your tires are? Excluded obvious issues like cracks, dry rot, and bulges, visual inspection won’t usually tell what you need to know now, but the date coding on the sidewall will. Since 2000, the week and year the tire was produced can be found in the last four digits of the Tire Identification Number. The first two digits identify the week; the last two identify the year; i.e. 4205 would be the 42nd week of 2005. If your tires are pre-2000 the coding differs, the first two numbers signify the week of production, a single digit represents the year; i.e. 326 would be the 32nd week of 1996.
6. What’s the right size carb for my engine?
The basic formula for carburetor sizing and cfm consumed is cfm = ci × rpm × VE ÷ 3456). The main variable here is the Volumetric Efficiency, which is essentially a percentage of how much air actually is pumped into the cylinder when running versus off. With the piston at bottom dead center (BDC), atmospheric pressure at 14.7 psi will fill it 100 percent, however, multiple restrictions from the intake to the heads will prevent most engines from ever seeing that while running. In most cases a ballpark guess of the VE will suffice; figure on stock or very mildly modified cars being in the 75-85 percent range, well-built performance engines in the 85-95 percent range, and dialed-in race motors in the 95-105 percent range.
Of course, that’s assuming your engine is actually providing the required vacuum signal that will allow the carb to flow at its max cfm. Carburetor cfm ratings are assigned using an assumed vacuum drop of 1.5 in-hg for four-barrel carbs, but it’s not uncommon for engines to fall short of that ideal scenario even at wide-open throttle. Actually fully optimized high-performance or race engines ideally won’t see more than .75 to .5 pound of vacuum at wide-open throttle since the higher the vacuum, the lower the air density.
Basically that means your 650-cfm carb may not be delivering 650 cfm of air to your particular engine. However, that may or may not actually be a problem depending on the combination you’re running, how it’ll be used (cruising versus racing), operating range, compression ratio, how heavy the car is, and what kind of transmission and gearing. There’s a whole lot to consider if you want the perfect carb, so ideally you should work with a tech guy from the manufacturer of your choice to pick the right package for your particular application. Since a little bit too much is better than too little, our general rule of thumb for hot street cars is to use the formula for a starting point, then step up to the next logical cfm size and tune within an inch of its life with the aid of a chassis dyno shop.
7. What’s the easiest way to figure out my gear ratio?
If you’re lucky enough to still have the original rearend in your car and there happens to be a data tag on it, simply decoding that based on the manufacturer’s conventions will tell you what the vehicle was originally equipped with. Of course that’s assuming it hasn’t been altered at some point.
The quick and easy way to figure out what ratio you have now is to jack up the rearend of the car and securely place a jackstand at each end of the axle (assuming solid axle) that provides at least an inch of ground clearance for both rear tires. Using chalk, make corresponding marks on the rearend flange and the housing itself. Now make a mark on the tire that’s referenced to a mark on the ground, or somewhere on the body or chassis. All you have to do now is put the trans in Neutral, slowly spin the tire (or driveshaft), and count how many times the driveshaft rotates for each full rotation of the tire using the flange and housing mark. Obviously this works best with a friend to help turn the tire while you lay under the car and count, but with good placement of the marks it could be a one man job.
By the way, now you also know if you have an open or limited-slip (posi) style rearend; if the rear wheels spin in opposite directions, it’s an open diff. If they spin the same way, it’s limited slip.
8. When do I need a rollbar?
If you’re looking to construct a rollbar or rollcage that meets NHRA tech specs, it’s imperative that you get a current year NHRA Rulebook, and preferably an SFI Foundation one as well. That said, we can give you a few basic specs.
A rollbar is required in any convertible running 13.49 or quicker in the quarter-mile, and in other cars beginning at 11.49. An approved rollbar is accepted in vehicles running as quick as 10.00 (10.99 for convertibles) provided that the stock firewall and floorboard is intact and unaltered, with the exception of wheeltubs. The rollbar must be constructed of minimum 1.75-inch OD x .11-inch wall mild steel tubing, or 1.34-inch x .083-inch chrome-moly tubing.
If the floor and/or firewall has been modified, then a full rollcage is required beginning at 10.99. A full rollcage is required in any vehicle running 9.99 seconds or quicker and any vehicle running 135 mph or faster (regardless of e.t.). The rollcage must be constructed of minimum 1.625-inch OD .118-inch mild steel tubing, or 1.625-inch x .083-inch chrome-moly tubing. Additionally, the rollcage of any vehicle running 9.99 or quicker, or 135 mph or faster, must also be certified by NHRA every three years and have a serialized sticker.
Most autocross events will not have rollbar requirements, though most will encourage convertibles to have one. Open road race and track events vary with the speed of the car and track type, but most will require convertibles to have at least a rollbar.
9. What’s the best vacuum vs. mechanical advance for the street?
Though you can get away with running either, in general carbed cars that spend most of their time on the street will want a vacuum advance distributor. That’s because a vacuum advance adjusts timing based on the engine’s load while a mechanical advance reacts only to a change in rpm.
Vacuum advance was invented to provide optimal spark advance in reference to engine load and changing operating conditions mostly to deal with varying lean and rich fuel conditions during driving. Lean mixtures, like at idle and steady cruise when the butterflies are mostly closed, take longer to burn than rich mixtures. To compensate, during lean times timing needs to be advanced to create an earlier spark giving the fuel more time to burn. The opposite is true with fast-burning rich conditions, such as acceleration when the butterflies are open. At or near full throttle, vacuum advance completely drops out of the equation and the engine is responding to the initial static timing plus the mechanical advance. That’s why dedicated drag cars often don’t run them; they don’t have the same part-throttle concerns.
That said, if you’re running a big cam on the street that makes little vacuum at idle, your vacuum canister needs to be adjusted accordingly. Stock GM units aren’t fully activated until they see about 15 in-hg, and many aggressive cams don’t make that at idle. Swap to a unit that is fully activated with a few inches less vacuum and the engine’s street manners will improve.
10. How do I find TDC on an assembled engine?
The easiest and most accurate way to find true TDC is to use a piston stop (like the ones offered by COMP Cams and Crane Cams) in cylinder number one along with a degree wheel on the crankshaft. Both work in the same fashion. Use your thumb over the spark plug hole to find the compression stroke then thread in the piston stop. Rotate the crankshaft clockwise until the piston hits the stop and record the number indicated on the degree wheel. Next, rotate the crankshaft counterclockwise until the piston hits the stop from the other direction and record the new number on the degree wheel. The midpoint of the two marks is what you want, so add the two points on the degree wheel together then divide the sum by two. Rotate the crankshaft to this number and you have TDC for cylinder number one.
Prior to the introduction of High Energy Ignition (HEI), more powerful coils, and capacitive ignition boxes like MSD, stock plug gaps on most muscle cars were in the .035 range. Higher energy coils, such as an MSD Blaster which yields roughly double the voltage and significantly more amperage per spark, allow opening up the gap, but there is a point of diminishing returns. Finding the optimum plug gap for your application is best determined by experimentation, because there are so many engine variables to consider. MSD, for example, recommends starting at the original gap spec then increasing by .005 to .010 followed by repeated testing and tuning.
So what’s a generally recommended safe gap for naturally aspirated hot street engines? Experienced racer and dyno operator Eric Weinreich at Dyno-Motive in Placentia, California, says, .44 is the gap that seems to work best for a wide range of naturally aspirated high-performance and race engines with strong ignition systems. That’s the gap we dyno test the majority of our engines at, including the one for Max Effort in this issue.
Late-model muscle cars are already equipped with high-energy ignition systems so stick with the manufacturer’s recommendation unless you’re adding forced induction; that’s an entirely different scenario that may require tightening up the gap to maintain spark.
12. What is the max compression ratio for pump gas?
The important thing is to make sure you’re thinking about the correct compression ratio, because it’s the dynamic compression ratio (DCR), not the often cited static compression ratio (SCR) that makes the difference. SCR is the swept volume of the cylinder using the full crank stroke (BDC to TDC). The swept volume for the DCR is determined by the position of the piston when the intake valve is closed. That’s important because compression doesn’t begin until the valve is closed and a great deal of compression can be bled off before then—that’s why the DCR number is always lower than the SCR.
The easiest way to figure out the intake closing point is to use the camshaft’s advertised duration number. Obviously this means that the Lobe Separation Angle (LSA) has an effect, as well as advancing or retarding the cam, since that alters the intake closing point. Now you know why cam degreeing wheels are so useful and one of an engine builder’s best tools to alter an engine’s characteristics; it can adjust the DCR an engine sees. That’s how some guys get away with outrageous-sounding SCR like 12:1 on the street while still running pump fuel, and why you’ll often see manufacturer’s SCR recommendations accompanying cams with lots of intake duration.
Figuring out where the piston is at valve close isn’t quite so easy since it requires a few potentially confusing equations to correctly measure the difference in effective rod length. The good news is that the folks at United Engine and Machine (makers of KB, Silv-O-Lite, and Icon pistons) have a very useful calculator on their website that will calculate both SCR and DCR for you. (Find it at www.KB-Silvolite.com.) As for the max recommended DCR on pump fuel, shoot for around 8.6:1.
13. Antifreeze mix: What kind, what ratio, and when is it time to change?
All antifreeze solutions are glycol based, either ethylene glycol or propylene glycol, with ethylene being the most common. As far as performance, they’re roughly equivalent since it’s the additives that really determine effectiveness. Neither works well in pure form in a cooling system, though.
Regardless of type, antifreeze has three main functions: heat transfer, corrosion resistance, and freeze/boil-over prevention. Water by itself is a great conductor of heat for cooling, but it offers zero protection for corrosion or freezing temperatures. Adding glycol actually proportionally decreases the ability of water to transfer heat since its heat capacity is about one half that of water, which is why running 100 percent is a bad idea. In the right ratio window of 40-70 percent, it’s not enough to be an issue in a properly functioning cooling system and adds the benefit of raising the boiling point into the 225-degree F range for a standard 50-50 ratio.
On the other end of the spectrum, water mixed with glycol dramatically lowers the freeze point. That same 40-70 percent window works here as well, though the maximum freeze protection for ethylene glycol is at 67 percent and resists freezing down to -84 degrees F (pure ethylene glycol freezes at 8 degrees F). Propylene has no freezing point; it only exhibits a phenomenon known as supercooling, but the same 40-60 percent window is typically recommended.
As for corrosion, all antifreeze types have a chemical package that’s designed to mitigate the varying issues presented by the standard mix of electrochemically incompatible metals found in engines. All of those inhibitors require water in the proper ratio to function, however. Speaking of water, deionized water or distilled water works best, but there’s no real issue with just using good tap water.
As far as lifespan, most antifreezes have a recommended interval for change (usually two to three years or 30,000 miles for the standard green stuff), but the only true way to know is to test it. The easiest way we’ve found is using special antifreeze test strips that react to the pH of the coolant and change color.
14. Fuel: How long can it sit?
Gasoline does degrade over time, usually first by losing volatility. The chemical components added to gasoline during refining to promote vaporization and combustion can evaporate, especially in vintage cars with fuel systems that aren’t tightly sealed. That can result in a loss of power and increased emissions, but the good news is that topping off the tank is usually sufficient to bring the fuel back up to par.
The bigger issue is when fuel sits around long enough to begin to oxidize. That’s where the stereotypical gums and deposits that can clog carbs and filters come from. Let fuel sit long enough and you can expect to be going though the system from engine to tank. If there’s a question about whether the fuel is starting to oxidize, eyeballing it can actually tell you quite a bit; oxidized fuel is usually darker and has a stale smell. Compare the questionable fuel to a fresh sample; if the fresh one looks like “pale ale” and the aged more closely resembles an “amber,” that’s good evidence of oxidized fuel. If you really want to get precise, a gasoline hydrometer like the one offered by VP Racing Fuels is the way to go.
On top of that, most vintage cars have vented metal tanks. If they’re only partially filled, temperature swings can cause moisture to be drawn in and create condensation, which will not only water down the gas, but also promote rust in the tank.
Unfortunately there’s no single answer for how long gas can sit around since it all depends on the type and quality of the fuel, the type of fuel system, and its storage environment. Some can’t be counted on for more than a few months, while Rockett Racing says theirs is good for a solid two years due to lower levels of olefins and higher levels of additives to resist gum formation. Overall, a good rule of thumb is to add fuel stabilizer for any storage longer than several months. That can buy you roughly 12 to 15 months. Anything longer than that and you’re better off draining the system.
15. How do I know the internal condition of an engine without pulling it apart?
There are lots of tips and tricks for checking out used engines or junkyard finds, from those obvious ones like inspecting the ports and popping the valve covers off, to the more in-depth like inspecting a main bearing or sliding a borescope into the cylinders. But what if it’s in a running car you’re considering buying and you don’t have time or luxury of digging in?
If you really want to know what’s going on inside an engine and how it has been maintained, it’s the oil that can actually tell you just about everything you’d want to know. The oil can reveal the state of wear in the engine, fuel or contamination, whether the filtration is effective, and even whether or not the oil is still working or needs to be changed. That’s what oil analysis can do, and it’s cheap and easy. AMSOIL, Wix, and Blackstone Labs all have inexpensive test kits available that allow you to take a small sample of engine oil and mail it in for full analysis by their technicians. How useful can it be? Check out the sample reports on Oil Analyzers Inc. (the lab that handles AMSOIL’s testing) at www.oaitesting.com/ and also from Blackstone Labs at www.blackstone-labs.com.
16. What’s the cheapest and easiest way to go faster?
There are tons of quick bolt-ons that can yield significant bumps in horsepower and torque which will in turn make a car quicker, but from an all-around standpoint the king of them is still dieting. Take that as you like, but every pound that either you or the car drops is not only less weight to be propelled forward, but also less weight to stop or carry through a corner. In essence, you can make your car do everything more quickly by reducing curb weight and thereby reducing the forces working against you, even if horsepower stays the same.
The whole idea is to improve the power-to-weight ratio of the car and everything you can safely remove counts: carpet, sound deadening, passenger seat, A/C, stereo, and the like. Beyond that, swapping for lighter wheels and fiberglass, aluminum, or carbon-fiber body parts can all yield significant results.
The formulas for figuring out the gains through a corner or braking are complicated, but if you want to quickly check how much your weight loss is improving the car’s power-to-weight ratio, all you need is the weight of the car minus driver and cargo, and the horsepower of the engine: PW = Vehicle weight ÷ Horsepower. For example, for a C6 Corvette Z06: 3,133 pounds ÷ 505 hp = 6.2 lb-hp. Accuracy counts, and ideally you want to know flywheel horsepower, but even the chassis dyno results will give you something to compare as you lose pounds.
17. What is heal-toe shifting and why does it help?
It’s really all about rev-matching to decrease driveline shock, especially if you’re also braking hard. (Imagine yanking on the parking brake while turning and you’ll get the picture.) While it’s typically considered a performance driving practice, we actually try to practice it daily since rev-matching also decreases stress and wear and tear on the clutch and entire driveline.
So how do you choreograph it?
- Lift your right foot from the throttle and press the brake
- Clutch in with your left
- While still braking, roll the side of you right foot (or use your heel if more comfortable) and blip the throttle to bring the engine up to the rpm necessary to match the engine to the speed of your tires
- Downshift the trans
- Release the clutch and rotate your right foot off the gas simultaneously
- Continue trail braking through corner as much as necessary (usually up to the apex)
- Foot on the gas to accelerate out of corner
18. Quick vs. Fast: Are they interchangeable?
These two words often get used interchangeably, but they’re really two different things. Quick is a measure of time; fast is a measure of speed. They’re also both a bit relative depending on what is being referenced. For example, 190 mph in a street car is fast, and so is a top speed of 415 mph across the Bonneville Salt Flats. On the quarter-mile cars with low elapsed times, 9.50s for example, are quick.
That also explains why quick cars aren’t necessarily fast, and fast cars aren’t necessarily quick; a car geared to run over 200 mph won’t accelerate as quickly as one geared to cover the quarter-mile as rapidly as possible.
19. Does cutting a coil spring increase its rate?
In a word, yes. Things get tricky in springs with differing wire diameters or inside diameters, but in the case of standard consistent coil springs like found in vintage cars, the rate of a spring is determined by the amount of steel used. Simply stated, rate is the amount of weight required to deflect a spring 1 inch. Assuming a set group of parameters, the longer the wire is, the lower the spring rate. As the wire gets shorter (as when cutting coils) the spring rate increases correspondingly. For complete explanations, check out the spring gurus at Eaton Detroit Springs (www.eatonsprings.com). So yes, you will increase rate by cutting a spring. Even time, however, can’t do the opposite. Coil springs never lose their rate. Even if it sags and loses its load height, the spring’s rate is still the same as when it was new.
20. How do I get the best mpg?
This can be a tricky question since many variables can be thrown into the equation, but assuming a nice flat road with little elevation change the maximum mpg per rpm is at the lowest possible rpm with the lightest throttle pressure in the highest possible gear that does not lug the engine. That’s true for any gear, though the obvious problem is that you may not be traveling at the speed you’d like. Without changing gearing or overall tire diameter, cruising rpm versus vehicle speed is a constant, so part of the consideration therefore becomes how fast you want/need to drive and how do you maximize that condition.
At that point it all becomes about driving habits and keeping the load light on the engine. Driving in Sixth at low rpm isn’t efficient if you’ve got your foot halfway to the floor to accelerate. You’ve heard this before: slow acceleration, light and steady pedal pressure. The easiest and most foolproof way to keep yourself in the zone that will return the best mpg for your particular car is still the good old manifold vacuum gauge. At full throttle, the gauge will read 0 Hg, but could be as high as 24 Hg when coasting. The key is to train yourself to drive in a manner that keeps the vacuum pressure as high as possible because the higher and more consistent you can keep the vacuum, the leaner and less voluminous the fuel mixture being burnt, and in turn, the better the mileage. Typically, this is in the 10-17Hg range for engines with stock to moderate cams.
All that being said, the ideal scenario for theoretical max fuel economy is to have the very smallest engine displacement possible that can overcome weight, rolling resistance, and aerodynamic drag at the target speed with uncambered airflow that minimizes pumping losses from a partially closed throttle blade and restrictive exhaust. That’s the theory behind variable displacement cylinder deactivation: make a larger engine cruising inefficiently effectively smaller and open up the throttle to maintain required power.