How to modify your car without ruining its value

It’s yours. The car you’ve dreamed about, researched, obsessed over, found in superior original condition, or maybe even spent years restoring to make it perfect. Maybe it has even won awards recognize its originality, the way experts agree it should be, down to the number of threads on the valve stem caps that hold that de-ionized air inside correct one-year-only tires.

But then you drove it. And as perfect as any of our cars may be, we can’t ignore that time and technology march on. The best performance car of 1965 can likely get its clock cleaned in a race against an average modern family sedan. Not to mention you’d probably survive an evasive maneuver in the new car. Not to be grim, but the odds are a lot lower in a classic.

As a result of these inherent flaws, you might not like driving your old car as much as you like the idea of driving it. And while many of us don’t view our cars as investments, we certainly aren’t out to needlessly destroy their value with modifications, even if they could help solve this problem. You’d have to be a fool to start monkeying around with a bone-stock collector car, right? Any modification that could make it safer and perform better would ruin the car, right? Not necessarily.

High level concours winning GT500KR with an aluminum driveshaft detailed to look like a factory original steel one. Only the lack of NVH gives it away.
High level concours winning GT500KR with an aluminum driveshaft detailed to look like a factory original steel one. Only the lack of NVH gives it away. Colin Comer

Nothing that can’t be undone

The truth is there are usually a number of non-invasive, easily reversible modifications that can transform the driving experience without hurting the value of the car. (That excludes the cost of the procedure and reversing it, if need be.) This list also includes anything that bolts on—and therefore could also bolt off—that doesn’t require drilling new holes or other modifications to any original components.

For example, a friend recently bought a super original 1966 Chrysler. After a few weeks I asked him how it was doing and he said, “Great, except when I jam on the brakes at 100 mph they pull like crazy!” I should mention that he is a professional race car driver, so I’m giving the obligatory “don’t try this at home” disclaimer. But clearly he’d found the limitation of 1966 four-wheel drum brakes trying to slow 4500 pounds at a speed and rate of deceleration they were never designed for. I suggested doing a front disc brake conversion and an upgrade to a dual-circuit hydraulic system to cure the problem.

Pertronix electronic ignition conversion hides under the distributor cap in place of the original points.
Pertronix electronic ignition conversion hides under the distributor cap in place of the original points. Colin Comer

His reaction was the same one I’ve heard a lot, and one we are trained to believe: “That would ruin the value,” of the car. I explained that there are kits that bolt onto factory components that don’t look out of place to the untrained eye. And should he ever wish to return the car to stock it would be a similarly simple process. I also may have mentioned that a 1966 Chrysler smashed, rolled, and on fire in the ditch would have a lower value than one with 1968 Chrysler front disc brakes bolted on. I do love to point out the obvious.

Other easily reversible upgrades are “drop-in” electronic ignition conversions to replace ignition points, modern Halogen headlight bulbs and plug-in relay kits to get them the voltage they need to shine brightly, radial tires in place of bias ply ones; upgraded suspension springs, shocks, and sway bars; and many other upgrades that are simple one-day projects. Some require no parts at all, such as having a professional properly tune the car on a chassis dyno or set up the chassis alignment to best take advantage of those radial tires and better underpinnings.

Take it from me

These are all things I have done to my own cars. For example, cruising in my 1968 Shelby GT500KR felt—to quote Lefty from Donnie Brasco—“like driving a waterbed” in its stock configuration. With a good alignment to more modern specs, a set of Koni shocks, Pirelli V-Rated tires, slightly stiffer (and shorter) front coil springs, and a larger-diameter power steering pump pulley from a 1967 GT350 to slow down the pump and calm down the over-assisted power steering, the car is now infinitely more enjoyable to drive.

Modern rubber tires
Modern rubber tires Colin Comer
Modern rubber tires on a GT500 KR
Modern rubber tires Colin Comer

Mind you it isn’t a new Ford GT, but now I have no concerns about throwing it around on a winding road. These few simple tweaks turned a car I didn’t want to drive into one I now enjoy using. And if I ever get the urge to return the car to its former life of a concours example, a couple of days swinging wrenches and it could be back to stock, no worse for the exercise.

Mods you can count on

Sometimes modifications have nothing to do with performance, but rather reliability. Because if your car won’t run it sure won’t make you want to drive it, especially if it has stranded you on the road enough times. My XK120 Jaguar was a notoriously poor starter when hot. The two, small, original 6V batteries that Jaguar placed behind the seat were wired in series to produce 12 volts for the electrical system. But by the time that power reached the front of the car, and that huge heat-soaked Lucas starter, there wasn’t always enough grunt to get things going.

After a little investigating I found that a first-gen Mazda Miata battery was roughly the same size as one original Lucas 6V battery. So I put one Miata battery in each 6V battery box, changed the wiring to run them in parallel to get double the cranking amps but not double the voltage, and added an additional ground strap to the chassis to make sure I would get all the juice available. Up front a modern gear reduction hi-torque starter replaced the Lucas unit. All told, for less than $500 I now have an old Jaguar that cranks over and starts like a modern car, with virtually no visual indication anything at all has been changed.

XK120 with dual Miata 12V batteries hidden in stock dual 6V boxes
XK120 with dual Miata 12V batteries hidden in stock dual 6V boxes Colin Comer
XK120 12V battery upgrade
XK120 with dual Miata 12V batteries hidden in stock dual 6V boxes Colin Comer

Modifying original parts

Of course, there is another level of more involved modifications that can still be done without negatively impacting the value of your car, but some may require modifying original parts in the process. For example, modern power steering can be added to many vintage cars, either by means of the new electronic systems that mount to the steering column or hydraulic systems that use modern steering boxes and pumps adapted to original mounting positions. If you ever want to switch them back you’ll be repairing or replacing that original steering column or shaft that had to be modified.

Some of my favorite modifications are completely hidden, done solely to increase performance: things like having manual transmission and differential gears micro-polished and/or lightened to reduce heat, noise, and friction. Similarly, a custom aluminum drive shaft takes many pounds of rotating mass out of the driveline for significantly less vibration and quicker acceleration, and when it is painted to look like the mild steel one it replaced, it’s pretty tough to notice. Relining brake pads and shoes with modern Carbon Kevlar material greatly increases stopping power, and having your brake rotors and drums precision balanced can often eliminate those annoying phantom vibrations and nibbling you feel through the steering wheel.

Installing a peel and stick sound-deadening product such as Dynamat under the carpet, inside the doors, and under the headliner allows you to enjoy the soundtrack from the exhaust and not the resonance of the big snare drum you’re riding in. Because if you can’t hear yourself think while driving along, it won’t be a fun ride.

These are just some of the tricks I’ve found that greatly improve the safety and enjoyment of old cars, and, when applied thoughtfully they offer no downside to value beyond the cost of doing them. After all, what is the point of having the car of your dreams if you don’t enjoy driving it?

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    Good information, thanks.
    Regarding modifications, my 1961 Jaguar Mk9 is not a concours example, but is also not far from being one.
    I have a full set of “period style” seat belts that I would like to install, which means holes will have to be drilled in the original floors. Will this reduce my car’s value?

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