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Project RX-8 - Idle Hands Are The Devil’s Workshop - Tech

With a low-compression Renesis under the hood, winter madness leads to an extreme solution

David Pratte
Mar 10, 2011

As the saying goes, Idle hands are the devil’s workshop. In other words, boredom can be your worst enemy, and as Project RX-8 sat accumulating snow this winter, our idle brains couldn’t help but cook up some pretty extreme plans to solve the lack of top-end power production that’s been plaguing the project up until now.

Modp 1105 01 o+project rx8+snow Photo 2/12   |   Project RX-8 - Idle Hands Are The Devil’s Workshop - Tech

But before we reveal our Winter Madness plan for the 8, let us finally explain why our 50,000-mile Renesis has been underperforming on the dyno (particularly in the top 2000 rpm). After exhausting every option we could to uncork the top end, we finally took the 8 to the local Mazda dealership and had them do a proper compression check on the engine. Most dealerships have the standard rotary compression tester, but the dealership we used has a custom tester that connects directly to a diagnostic computer station. The beauty of this setup is that the compression readings are recorded in KPa (easily converted to psi), rather than the 110 score displayed on the usual rotary compression test tool. This allowed us to accurately cross-reference the compression test data to the factory limits listed in the shop manual, rather than just having a tech say, Your engine tested at 6s across the board, so everything is OK.

We started by just testing rotor 1, and much to our surprise, chamber 3 showed zero compression. This was a stunner, since the engine should’ve been running much rougher than it was if it was truly making zero compression on that chamber. On the other hand, that would certainly explain the lower-than-expected power output from our Renesis. However, once the tech hooked up a second line so that both rotors 1 and 2 could be tested at the same time, the second round of testing resulted in compression figures more in line with what we expected.

Modp 1105 02 o+project rx8+computer Photo 3/12   |   Having tried everything we could to revive the top-end power of our Renesis, a compression test revealed numbers well below the factory standard of 830 KPa.

As you can see, compression was at the low end of the minimum factory limit, where 830 KPa or 120 psi is considered the standard for a healthy Renesis and 680 KPa or 98.6 psi is the minimum acceptable before the engine is considered for warranty replacement. The test standards also look for no more than 150 KPa difference between chambers and 100 KPa between rotors.

Modp 1105 03 o+project rx8+rotory block Photo 4/12   |   We sourced this low-mileage Renesis out of a wrecked RX-8 from well-known rotary builder Joe Ferguson, who just so happens to be handling the rebuild for us.

Unfortunately for us, the engine had just enough compression not to qualify for a factory replacement, which left us with a couple of options. Option A was to clean the engine and hope that by removing any carbon build up on the apex seals that the compression would come back up to somewhere near the 830 KPa standard (reviving top end power output in the process).

Pursuing this option is relatively painless. We used the Mazda rotary engine cleaner (awesomely labeled Zoom Power Engine Cleaner), but Seafoam is also a popular method of cleaning these engines. After performing the cleaning procedure as outlined in the shop manual we retested the compression, and although it came up a bit, the numbers were still far too low for our liking.

Modp 1105 04 o+project rx8+timing chain Photo 5/12   |   After dropping the oil pan, Joe unbolted the front cover, revealing the eccentric shaft and chain-driven oil pump.

This led to option B, which is to replace or rebuild the engine. A Mazda remanufactured Renesis will run you about $2,500 (not including the labor to remove the old engine and install the new one), but there’s no guarantee a refurbished factory engine will have optimized compression. In fact, word on the street is that these engines tend to have pretty wildly varying compression even when brand new, so the only way to be certain of maximum compression is to rebuild the engine ourselves, or more precisely, we’d have to find us a kick-ass rotary engine builder.

Modp 1105 05 o+project rx8+crank Photo 6/12   |   Once the flywheel is removed, the 18 tension bolts that hold the iron and rotor housing sections together were removed.

For this we turned to Joe Ferguson, a very well-respected rotary engine builder who’s built some pretty innovative street and race 13Bs in his day, including a FC 13B equipped with the lighter Renesis rotors but machined to accepted the bigger FC apex seals. As luck would have it, Joe happened to have a complete Renesis engine and transmission out of a salvaged ’05 RX-8, so for $1,000 we now have what should be a perfect engine to rebuild, not to mention a transmission we can refresh as a final solution to the fourth gear grind problem.

And this is where the Winter Madness part kicks in. Rather than just blueprinting and balancing our newly acquired Renesis with factory parts for optimum OEM performance, an approach that would allow us to continue to pursue the STX class Solo 2 build concept, we’ve decided to step things up a notch or two by building the engine for a much more significant boost in performance. This will mean competing in a higher Solo 2 class (most likely Street Modified, where a forced induction RX-8 could be quite competitive if built right and driven well), but it also means we’ll have a far more competitive Street class time attack car on our hands.

Modp 1105 06 o+project rx8+gasket Photo 7/12   |   Removing the first iron section, you can see the thin rubberband-like three-layer gasket that seals it to the adjacent rotor housing falling off.

But before we get down to picking the parts for the engine build and selecting what we consider the best forced induction kit for our goals, first we needed to tear down the donor Renesis to ensure it’s a suitable core for the project. For this we spent a few hours at Joe’s secret hideaway, where he’s building all kinds of wild machines from a 20B-powered FB RX-7, to a FC with a SR20DET swap under the hood.

Having seen pictures of disassembled rotary engines and having researched and even written stories about the history of Wankel’s brainchild, seeing our spare Renesis come apart in person provided a whole new level of respect for its unique and ingenious design. The irons, rotors and rotor housings all met with Joe’s seal of approval, as expected given the relatively low mileage of the engine (around 50K miles).

With the engine bits now being given a good ultrasonic bath to thoroughly clean them, we can start planning the build, which will certainly include some porting (using Racing Beat’s templates for the Renesis) and high-performance seals that’ll stand up to the stresses provided by 10 or so psi of boost.

Modp 1105 10 o+project rx8+engine disassembled Photo 11/12   |   After that, Joe just kept peeling back the layers until our Renesis was fully disassembled and laid out neatly on the bench (top). Soon we will start the rebuild, including port work and some high-performance seals, to name just a few of the tricks we have up our sleeve.

While we sort out the exact specifications of our Winter Madness Renesis rebuild, expect to see some lightweight seats being bolted to the floor of Project RX-8 next time, along with a safe and street-legal harness that’ll hold the driver more securely in place during those high-g corners and heavy braking zones.

Modp 1105 12 o+project rx8+mazda stamp Photo 12/12   |   Project RX-8 - Idle Hands Are The Devil’s Workshop - Tech

Joe Ferguson
Rotary Engine Builder

By David Pratte
216 Articles



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