Direct "Dear Dave" tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.
TemptationI have a '92 Miata with a dead engine, so I'm planning on having it swapped for either an SR20DET or an F20C. I've also seen a rotary swap before, but I'm worried about reliability issues. Which engine would be a better choice for my Miata? And what parts and procedures would be required for the swap?Chaiyaphruk ChotikasupaseraneeWaugullewutlekauh,* CA
Instead of answering your question I'm going to, as usual, explain what a bad idea it is to even ask it in the first place. It's no mystery why Miata engine swaps are so tempting. The car's handling is second to few, but the engine is completely lacking in nutsack. However, stuffing in non-Miata engines is notoriously tough.
Other than determining if the engine hole is big enough, the most fundamental issue with any rear-drive engine swap is making sure there is no interference between the oil pan, the crossmember, and the steering rack. There are very few swaps where this doesn't become an issue, and in most cases, this is where the most fundamental mistakes are made.
The Miata has a rear-sump engine, meaning the lowest part of the oil pan is toward the back of the engine. The bulk of the crossmember and the steering rack mounts sit under the shallower front portion of the oil pan. Both the SR20 and F20C are front-sumpers, meaning the deep part of the pan is right where the crossmember and rack want to be.
The smart thing to do here is convert to rear sump, either by making a new pan or figuring out how to mount the original pan backwards. A slightly easier solution is to modify the crossmember and move the steering rack. But since steering and handling were probably the main reasons you wanted to use a Miata chassis in the first place, don't ruin them by moving the steering rack.
The next problem is the powerplant frame. A Nissan Silvia (like most other rear-drivers) has two engine mounts, a transmission mount, a driveshaft carrier bearing mount, and four differential mounts.
The Miata, by contrast has just two engine mounts and two differential mounts. The transmission and differential are rigidly connected with a powerplant frame that both simplifies mounting and makes the entire drivetrain one rigid structure to give that crisp, responsive feel that makes so many people forgive the Miata's lack of power.
Neither the Nissan nor the Honda transmission has a tail housing designed to accommodate the powerplant frame, so you'll have to make a transmission crossmember and figure out how to mount the nose of the differential. This is not an insurmountable challenge and it's not as likely to ruin the car as moving the steering rack, but it will still soften one of the Miata's virtues.
In spite of your fears, Mazda's own rotary engine is better for these two mounting issues. The third-gen RX-7 has a similar rear-sump/front rack layout and it also uses a powerplant frame. The RX-7's frame is mounted on the driver's side of the driveshaft, though, versus the passenger's side for the Miata, so it still isn't easy enough to really be worth doing.
The reason none of these hurdles are worth jumping is that the Miata's engine is so easy to turbocharge. There are several good kits on the market and the engine is fundamentally robust and takes to turbocharging very well. With only 200 to 250bhp, a Miata will feel wildly powerful. Your best bet is to either swap in a less tired engine or do a simple rebuild on yours, then bolt that spinny thing to the exhaust. You won't be sorry.
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