Dave Stribling
March 7, 2017

To Undercoat or Not

Some of my Mustang friends say I shouldn’t undercoat my convertible, but I plan on driving it. What is the drawback to undercoating on the car?

Norm D.
Greenwood, Indiana

There is a reason Mustang floorpans rust out so quickly; Ford didn’t apply much protection to them. Undercoating is good for the car, though it is not “correct” for some restorations. Two points about undercoating: I’ve had two cars that were undercoated while new back in the ’60s, and those two cars only needed the original floorpans cleaned up and detailed with no floor work. The undercoat saved the car. Second, when I and most of my snobby friends crawl under a car, and we spot large globs of undercoating, a yellow flag goes up. We assume its cover up work for other problems like rust. Most of the time, that is the case. We like to see nice paint drips on the ridges running down the floor pans, no MIG wire sticking out, and no evidence of flanged floor repair. Undercoating can cover up a multitude of mistakes, but then you’re stuck.

Undercoating doesn’t have to be applied sloppily, though. Depending on the product and gun, it can be laid down smoothly. In some cases, it can be sanded smooth. A 3M undercoating gun puts out a textured finish. Guns with a 2.5 needle can be used to create an even smoother finish. Most products, such as Lizard Skin (www.lizardskin.com), have a system approach that will lay down the coating smooth. And today’s coatings add more than just rust protection. Most can be used to improve sound deadening and heat dissipation.

If you are building to sell, you may want to steer away from undercoating or apply it very smoothly. If you plan on keeping it and driving it, don’t worry about it. Lay it down nice and enjoy your floors long after the other guys have replaced theirs.


The Fox-era Mustang 5.0L EFI, which is a mass airflow-controlled system, is a great budget swap. Many EFI conversion fans use the 1994-1995 intake seen on this swap to help induction hose and filter routing.
The Edelbrock Pro-Flo series is a direct port speed density-based EFI with a central carb like throttle body that can still wear a round or oval air cleaner to generate a near-stock look.

EFI Conversions

I am thinking about upgrading to EFI on my 1967 289 hardtop. It currently wears an Edelbrock intake and a Holley 570-cfm street avenger. What is better, mass airflow or speed density?

Rob R.
Salinas, California

Depending on your eventual goal for the car and what kind of look you want, determines which system you should choose. First, the difference between mass airflow and speed density (they both work just fine). Ford has used mass airflow meters since mid-1988, Chrysler still uses speed density. A mass airflow system uses a mass airflow meter (MAF) to measure the temperature and density of the air entering the intake and adjusts the fuel accordingly. Speed density uses a BAP (Barometric Air Pressure) sensor to measure the amount of air pressure in the intake, and along with the rpm of the engine, uses a series of internal fuel curve tables to determine the amount of fuel to inject. Mass airflow, in my opinion, is more efficient, and speed density is much more tunable in performance applications. That is why most aftermarket systems are still speed density. (Critics start your letter writing now).

I do recommend that you go with a multiport system, which is a system with an individual injector over each intake runner. I am not a big fan of central fuel injection. For mass airflow, you really can’t go wrong with the original Ford 5.0L setup, as used in the Mustang, Explorer, and a few other applications, on a street car. It has been done long enough that all the bugs are worked out, and there is enough aftermarket support to help you succeed. Plus, most of the parts can be obtained used, so if you are on a budget, this may be the best choice. If you want to stick with an original look under your hood, companies like Edelbrock have systems in which the throttle body is positioned like the carburetor you are replacing and are multiport. The Edelbrock Pro-Flo, shown here (http://edelbrock.com/automotive/mc/efi/pf3-intro.shtml), works great, looks good, and is speed density. For applications that require a little more hp, look at systems like the FAST XFI. I have used it on big hp engines, and it has worked great. Before you choose, check out Mark Houlahan’s EFI Buyer’s Guide online at www.mustang-360.com/parts/mdmp-1111-efi-buyers-guide. Good luck with your conversion!