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.
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.
Having tried everything we could to revive the top-end power of our Renesis, a compression
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.
We sourced this low-mileage Renesis out of a wrecked RX-8 from well-known rotary builder J
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.
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).
After dropping the oil pan, Joe unbolted the front cover, revealing the eccentric shaft an
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.
Once the flywheel is removed, the 18 tension bolts that hold the iron and rotor housing se
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.
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.