1997 Mazda MX-5 Miata: Rotaries Are Good For Something After All
November 27, 2012
We're not swapping a rotary into Project Miata, but we are in a very small way taking advantage of its bigger brother's infamous thirst.
If your Miata is turbocharged or supercharged, its fuel system is flowing a significantly higher fuel flow rate than stock. Fuel flow rate scales (roughly) proportionally to power -- increase the power by 50% and you increase the fuel flow rate by 50%. And all of it passes through the filter. From a filter design standpoint, higher fuel flow rates require more filtration media, which are those pleats of paper wadded up inside every filter canister. Manufacturers don't like to put any more filtration media into a filter than they need to because, hey, that stuff costs money.
So what happens when you put 171 hp worth of fuel through a 100-hp filter? Hard to say without measuring, but its a good guess that the pressure drop of the fuel across the filter increases significantly. Higher pressure drop means the fuel pump has to work harder than it really needs to.
I have no idea when the previous owner of Project Miata last replaced the filter, which means it was time to replace the filter. In doing so, I wanted to find a filter that at least matches our modified car's horsepower and looked like it can be made to fit with minimal effort. Effort sucks. Being savvy with part interchanges is awesome.
Top: Filter for a 1993-1995 Mazda RX-7. Bottom: Filter for a 1990-1997 Mazda Miata
In this case, the fuel filter for the 1993-'95 Mazda RX-7 is externally very similar to the Miata filter. Yet the RX-7 not only generated 255 hp, it did it poorly. Rotaries are inefficient, so they require more fuel for each horsepower than a piston engine. Cynics like to say that a rotary consumes fuel like a piston engine of twice its horsepower. That's probably a stretch, but 255 rotary horses are like at least 300 piston horses. And its filter has to support all that fuel flow. It's more than enough for a juiced-up Miata.
The RX-7 filter bolts up nearly the same as stock. It's a bit heavier than the Miata filter despite being a hair smaller in diameter, suggesting it is indeed packed with more filtration media. But who knows. Since the RX-7 filter's inlet nipple is a bit shorter than the one on the Miata filter, I replaced the little segment of fuel hose on our car with another piece that was about an inch longer. Make sure to use high-pressure "EFI" hose (SAE J30R9 or better) here, not the low-pressure hose intended for carbureted cars.
Is it any better? Logic suggests yes, but even if it's not better it certainly can't be worse. Plus it's an easy peasy swap and -- bonus -- the RX-7 filter is couple bucks cheaper. Woohoo.
Jason Kavanagh, Engineering Editor