The "Monster Miata" was the creation and product of Monster Motorsports, a defunct California company. The Monster was a Miata with a Ford 5-liter (actually 4.9) cast-iron block/heads OHV pushrod engine and a Borg-Warner T-5 transmission. The swap was easily accomplished - almost unbelievably the entire engine and transmission fit into the Miata as though it had been designed by Mazda for it (everything fits under the stock hood!). That and some appropriate springs, cooling, and exhaust (restrictive) made up the basic technical package and the cost started at several thousand dollars and the car. The much heavier Ford engine did add about 4 percentage points to the front/rear weight balance - and the car takes a corresponding amount of care to drive in turns.
Somewhere over 275 Monster Miatas were claimed to be built by Monster Motorsports. A couple of hundred unofficial conversions were made by other companies, and kits remain available from them to this day. RX-7 conversions are also possible since much of the chassis is shared.
Unique - but subtle - badging inside and out identified the car to anybody getting close enough to notice. The appearance of the entire car was very stealthy until you started it up: deaf "opponents" wouldn't know what hit them. Everybody else would get an aural shock.
The Monster was known to be over-geared since the Miata differential gearing was well over 4:1. Even the optional (and much stronger) RX-7 differential posed a problem in this regards. Since the 4.9L Ford was not a high-revving engine, this was an issue for cruising, and the gearing certainly wouldn't contribute to engine reliability either. Later cars offered the option of a Lincoln Mark VIII aluminum differential with much more appropriate gearing. Even later cars offered the entire MN-12 Thunderbird rear suspension, with very large flares to cover the added width - although only a few were built like this. Some of these cars were labelled as "Mega Monster" Miatas. Pricing could sky-rocket with options.
Like all good things, the "production" conversions were built for several years. Unfortunately, Monster Motorsports as a company appears to have folded - the original website was shut down some time ago.
Contrary to popular belief, the roughly 225 HP bone-stock Ford engine was the only standard conversion offering, since the car had to meet California emissions regulations. Monster Motorsports also offered some additional power-adders if California aftermarket certification was available for the year it was offered. Of course, given the popularity of the Ford engine, many different power-adding enhancements could be made by owners to take HP up considerably. But the car still had to meet any emissions regulations of wherever it was eventually licensed.
Conversions are an interesting consideration for the enthusiast. California (and most state) laws allow engine conversions if the engine is from the same production year of the recipient car or newer. 4-cam Acura B16 engines are a popular swap in earlier Civics, and even better is a B18 or even an R model engine. There are also some very narrow circumstances where JDM engines that were never certified for the U.S. are certifiable in certain situations - you'll note a number of people swap engines in their 240SXs for the 2 liter turbo offered in Japan. One enthusiast in Dallas built a V-8 powered Lexus IS300 - an option the factory considered but never offered. But by and large conversions are becoming a thing of the past for late model cars, the electronics are becoming too difficult and the emissions certifications are legally and technically impossible. Swaps such as a late-nineties Supra TT engine into an S2000 would be impossible to certify and register in most counties given the trend of emissions laws and testing. But there is another reason to do swaps - and that's for the technical challenge. That's one reason I can understand.
Enthusiasts should also consider the value of a classic car.. Would an classic 240Z, for example, maintain it's value as a classic with a Chevy V-8 under the hood? The answer is a resounding "NO!", and technically the chassis is so dated anyway that the net functional result is debatable. The slightly more modern early Miata has a better chassis, but the functionality is still limited. But there are so many Miatas on the road their value as a classic is small-to-nil. So if a conversion is a must, then "Monster" the Miata!
The following car is currently offered for sale on eBay . This is a great example and from the ad it appears to be clean. It has the Mark VIII differential and some odd aftermarket headlamps, as well as Ford Motorsports GT-40 intake (I would check to see if it also received the proper matching heads and an appropriate cam). It would apparently be a worthwhile collectible - although I also feel that a close-up inspection would be advised.
This example has an added GT-40 intake. Note my suggestion above. All Monster Miatas used an open air filter - there wasn't room for anything else. Some sort of partition would have been a good idea so that the engine doesn't have to suck up hot engine compartment air at idle. Note that the car has AC, but not power steering.
Exhaust routing was a major problem - there was only room for a single exhaust pipe past the cats, although for the sake of appearance it does split at the bumper. The muffler in this example appears to be a flowmaster - which likely wasn't the original equipment. If this is the case, then the use of the car should be looked into - it may have been used for drag racing. You can also see that the new differential required customer mounts and half-shafts that are different from the original Mark VIII mounting design (the rearmost bushing isn't used). Many of us are familiar with that differential from it's later use in 1999-2004 IRS-equipped Mustang Cobras and it would be plenty strong for this engine.
Badging is custom-sewn into the seats (either factory cloth or leather). There is also a nicely made external badge on the top edge of the front bumper. Additional off-the-shelf Ford badging is also available.
The Monster Miata is definitely collectable. An example built by Monster Motorsports would be much preferred. Look for the proper paperwork to guarantee authenticity.
For the performance driving enthusiast, a Monster might work in some autocross classes, although it would be a fully prepared class and the competition would be very tough.
For open track events it's be at a disadvantage because of the tiny Miata brakes. Some big-brakes kits are avialable and would definitely be needed.
In either case, durability is a open question: will the 4-cylinder chassis hold up to V-8 torque? Certainly the Miata differential, control arms, mounting points, and rear wheel bearings would be potential issues. Straight-lining the car would be a very poor choice.
For the Sunday cruiser, the car would probably be a good alternative to a Sunbeam Tiger. Brakes, transmission, and cooling are probably all superior. The Tiger is far more collectible, and owners should certainly take care to preserve their examples. But the Monster Miata is more easily repairable, and given the very large numbers of Miatas available standard Miata parts are not likely to be an issue. Some re-welding and fabrication may be needed at some point to keep the rear suspension together.
Collectors will find several road tests are available, including two in Road & Track that I remember. Monster Miatas are very rarely for sale, and any potential buyers should look the car over very carefully and also get any and all original literature from the seller.
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DrivingEnthusiast.net references:
