Horsepower doesn’t come cheaply or easily in Spec Miata. Sometimes it doesn’t come at all. First, the engines are tiny, measuring either 1.6 or 1.8 liters in displacement. For an imperial reference, the 1.8 is 110 cubic inches. The 1.6 is even smaller at about 98 c.i.d. I’ve seen bigger engines in Harleys.
Unless you have the budget to pay top dollar for a “pro motor,” you have to scrounge for power everywhere you can find it. That means it doesn’t come easily. One way to get more power is by installing an adjustable fuel pressure regulator. Here’s why.
When a Miata engine hits about 4,200 rpm — the 1.6 and 1.8 — it begins dumping fuel to lower combustion temperatures to keep the cylinders cool. By the time you hit 6,000 rpm, the air-fuel ratio is down to 11:1 — and you still have nearly 1,000 rpm to go before you shift. For reference, a good rule-of-thumb AFR for making power in forced-induction engines is 12:1. Naturally aspirated engines fall somewhere around 13:1. Now you know why the back bumper of a Spec Miata is usually stained with carbon deposits just above the tailpipe.
In a laboratory, the optimal mixture of air and pump gas is around 14.7 parts of air for every one part of fuel, for an air/fuel ratio of 14.7:1. At this ratio, and under the right conditions, all gasoline and oxygen can burn, leaving nothing but the combustion byproducts. This is called the “stoichiometric” ratio, but as we know, conditions under the hood often do not mirror those in the laboratory.
One of the few permitted engine modifications in Spec Miata is the addition of an adjustable fuel pressure regulator, which leans the engine out at higher rpm — well, all rpm — to help it create more power. Racers also are allowed to add an air-fuel ratio gauge to keep an eye on things, although a dyno shop’s instruments are more precise and reliable for tuning purposes.
The good news is that the project isn’t too difficult. I requires no special tools — unless you think a test light is exotic — and can be done in one day if you don’t have a wife, kids, a job, weekend chores, a life outside racing, etc. If you have all that, it takes a bit longer, but it’s an easy project to pick up wherever you leave off if you do it right.
I went with an AEM air-fuel gauge because the company has been in that field for so long. I chose the modified fuel-pressure regulator from 5X Racing and the inline fuel-pressure gauge from Weekend-Racer.com because of their simple approach. It’s also interesting to note the principals of all those companies are NASA racers.
Essentially, you have two projects. One is installing the AFR gauge. The other is installing the regulator, but we’ll treat it as one for the purposes of this story. There will be another story to follow this one on using your new equipment to find more power in the mighty Miata motor, but that’s a feature unto itself.
For now, let’s focus on installing the air-fuel gauge and pressure regulator without hurting yourself or burning down the garage.