Mazda RX-7 Engine Swap

There are always purists that adamantly reject swapping engines. RX7's have received countless swaps, even though there are those who despise the idea. However, just because they're not built together, the RX7 may just deserve a new engine.

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Mazda RX-7 Engine Transplants.

     The following statement is a direct quotation, and it was made by a local (South African) specialist in all things Mazda, when he was asked about the possibility of transplanting a specific V8 engine into a second generation Mazda RX-7 in a local motoring publication: “Buy a car that was specifically made for such an engine. The amount of modification, and the cost involved would in my opinion not be worth the effort, and besides, the car would probably not handle well.”

     There is no doubt that this sentiment appealed hugely to RX-7 fans and purists everywhere, and the fact that many RX-7’s will outrun most V8’s in standard form only serves to strengthen the Mazda camp’s disdain for any sort of conversion of the RX-7. However, the fact is that the rotary engine is plagued by many shortcomings; not the least of which are its poor fuel economy, perceived (and real) reliability issues, the relative lack of replacement parts, and the severe turbo lag it can suffer when its boost characteristics are altered in major ways.

     Another truth is the fact that the design of the rotary engine is such that the RX body shell had to be designed with a huge transmission tunnel, which makes it possible to fit just about any engine and transmission into it. In fact, there is arguably a bigger variety of non-Mazda engines running in RX-7’s today than in any other platform; Nissan RB26, RB25, BMW inline sixes and V8’s of all capacities, Ford V8’s, Chevy V8’s, Lexus V8’s, Alfa-Romeo V6’s,  and even some Volkswagen V8’s among others, have all been successfully fitted to the RX-7 platform. Nevertheless, how, you may ask, is this possible? 

     You may well ask, but the list of possible engine transplants is too long to discuss each in any sort of detail, so we will focus on one specific engine that has proven to be the most popular choice- the mighty General Motors LS1.

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Why V8 Power in a RX-7?

     The simple answer is “Why not?” since the RX-7 body shell is perfectly capable of coping with the huge torsional effects of a V8, and for reasons that are outlined below:

• Mazda designed and built the RX-7 to be its flag bearer in the sports car category, and thus spared no expense in making the shell as strong and rigid as they possibly could. The result is a body shell that is not plagued by warped panels, cracked and popped-out windshields, or doors that suddenly do not fit as a result of the chassis flexing under higher than normal torsional loads.


• The peculiar, high-up location of the original transmission required a huge transmission tunnel, which adds to the overall rigidity and strength of the body shell. This rigidity is further enhanced in third generation RX-7’s by the use of additional reinforcing struts across the transmission tunnel.


• Even more strength is added by the doorsills, which are considerably deeper, and larger in cross section than the sills on almost any other car.


• The large radii in the door openings serve as reinforcing, which means there are no acute angles around which the shell can deform.


• The sub-assemblies that make up the complete body shell are all connected by reinforcing sections during initial construction in the factory, which work in concert to produce a shell that cannot deform unless it is driven off a cliff.

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Some More Good News…

     The standard rear axles and final drives of RX-7’s are extremely tough and durable, and it is highly unlikely that even a LS1 V8 will break it. The reason for this has to do with the fact that rotary engines have a low torque output, so to make it possible to launch a standard RX-7 without stalling the engine, the designers resorted to using a huge flywheel. Flywheels are energy storage devices, and the energy a RX-7 flywheel releases when it spins at 10 000-RPM boggles the mind-, which requires a rear axle that can absorb it without breaking anything. 

     This translates into a rear axle that can withstand anything a V8 throws at it, and even more so because a V8 develops most of its torque at relatively low RPM’s. In practice, this means that the standard RX-7 rear axle is not subjected to the severe shock-loads a rotary engine delivers during launch at high RPM’s. While there are many bolt-on rear axles available, most run at the same, or nearly the same ratios as the standard RX-7 units, and below are some standard RX-7 ratios for purposes of comparison.

• 1979-'83 All models:    3.909 : 1
• 1984-'85 All models, except the GSL-SE:    3.933 : 1
•  1984 GSL-SE:      4.083 : 1
•  1985 GSL-SE:      4.077 : 1
•  1986 -1991 All models,  except automatics:    4.100 : 1
•  1986 -1991 Auto, except, the convertible:       3.909 : 1
•  1988 -1991 Convertible:         4.100 : 1
•  1987 -1991 Turbo II / GTU:    4.100 : 1
•  1989 -1990 GTU:          4.300 : 1 
•  1993 -1995 Manual trans:   4.100 : 1
• 1993 -1995 Auto  trans:       3.900 : 1

     By way of illustration of the durability of standard RX-7 rear axles, consider the fact that many V8, and slick-equipped RX-7’s run mid to low 10-second quarter miles, which is something that needs at least 475-500 HP to accomplish.

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The Issue of Weight... 

     By quoting our Mazda expert again, “…and besides, the car would probably not handle well.” any one without actual experience of transplanting V8’s into RX-7’s might be forgiven for thinking that the weight of a huge V8 will completely ruin the weight distribution of the car. Nothing could be further from the truth; RX-7’s were designed to have close to a 50-50 weight distribution between front and rear axles, which goes a long way towards balancing the weight of any transplant. Again by way of illustration, below are the average weights for a few second-generation RX-7 models in standard trim (excluding a driver), that were obtained from official Mazda sources:

• Base models = about 2 700 lbs. 
• Automatic models = 2 735 lbs. 
• Turbo-fitted models = 2 850 lbs. 

     To put this into perspective, below are some details on the weight distribution of a recent and actual conversion of a second generation PX-7 to run a LS1 V8 mated to a T-56 transmission; these figures were obtained before any attempt was made to shift any weight around.

• 3 005 lbs total weight, which included a 215 lbs driver, and ¼ tank of gas. 
• 1585 lbs on the front axle
•  1420 lbs on the rear axle 

     This particular example, in which the car had no air conditioning or power steering, was balanced by merely filling the gas tank, and given the new LS1 engine’s HP rating, the power to weight ratio worked out at 8.12 lb/hp, which is better than the p/w ratio of a new $30 000+ 2014 Ford Mustang GT- and all for less than $12 000!

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Crunching the Numbers...

     The above-mentioned figures might seem too good to be true, but having a near-perfect weight distribution to begin with, makes it easier than you might think to balance a V8 equipped RX-7. For instance, the example used above had no power steering or A/C, but the interior is still standard, and the original cooling system was relocated. To move more weight to the back, the battery was moved to a position just in front of the taillights, which resulted in a slightly tail-heavy situation!

     However, depending on how the weight is shifted, and what equipment is retained or discarded, a LS1-equipped RX-7 could turn out to be lighter than a bog-standard rotary-equipped RX-7, by virtue of the fact that the LS1 engine has an aluminum block and cylinder heads. To illustrate the effect some engine components can have on final weight distribution, below are listed some actual weights, which will make the picture a little clearer:

• Aluminum intake manifolds are up to 28 lbs lighter than their cast-iron counter parts.
• Aluminum cylinder heads can be as 50 lbs lighter than cast iron heads. 
• An all-aluminum block could shave off anything between 55- and 95 lbs.
• On average, modern V6’s are only about 10 - 11% lighter than V8’s.
• Fitting an aluminum hood can lose around 30 lbs.
• Using headers can save another 20 lbs or so over cast-iron manifolds /head pipes.
• By discarding the typical RX-7 A/C system, another 55 lbs can be saved.

    As an alternative to Chevy V8 power, using a small block Ford V8 in a second generation RX-7 can make the conversion even lighter, with all-up weights in the region of  2 750 to 2 800 lbs being achieved routinely. 

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What is the Downside?

Most things in life have downsides, but in the case of fitting V8’s to second generation Mazda RX-7’s, there is none. V8 conversions are easy; there are fully developed conversion kits and even complete instruction manuals available, which leaves only one question unanswered- 

“When are you going to convert your RX-7 to run a proper engine?”

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