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Well I understood why it might look the way you described it (big intake runners, but more low rpm torque), but my view is that the engine behaves the opposite of that and it’s only the smaller primary turbo on the Cosmo 13BRE that gives it that initial low-end torque, followed by the larger HT-15 secondary turbo. Whereas the early FD3S 13BREW has identical twin HT-12 turbos.
Because you can see in the Mazda engine dyno documentation below that they both make 294 N-m peak torque output, but the Cosmo-RE peak is at 3500 rpm vs. 5000 rpm on the REW. Which sort of matches the big runner - low torque impression that was referenced. However, at 3500 rpm on the REW graph it’s still close to the peak value at ~285 N-m, where the Cosmo-RE torque is dropping off at the REW 5000 rpm peak to ~275 N-m. And dropping off even faster as rpm increases. The REW carries it’s torque output at higher rpm despite being strong at very low rpm, resulting in a much broader torque curve overall.
Cosmo graphs compliments of RX7Club forum member Akagis_white_comet
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. 1993 FD3S REW graph
As indicated though, the Cosmo-RE peak hp is only 230 PS vs 258 PS for the REW, both at 6500 rpm. So my feeling is that it has less to do with the intake runner size, but more with the turbo configuration/controls and intake resonance features. It wasn’t so much that they did one for the Cosmos luxury car vs the other for the RX7 sports car, but the Cosmo got what they had at the time and the new RX7 received the next evolution available when it came into being. The Cosmo also already had the 20B turbo option for increased low-end torque and power.
With the Eunos Cosmo nearing the end of it’s model life cycle in the 1995 production year, it didn’t make economical sense for Mazda to swap over to the more advanced FD3-13B REW twin turbo configuration even though it could have benefited from it on the base 2-rotor version.
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those are all very good points. for sure the Cosmo got what they had at the time, and the FD was the next better one.
you're probably right about the turbos being a big factor too. peak boost is probably lower, although i didn't see what it was in my Cosmo book
the 13B Cosmo uses a pair of HT-10's and the 20B is the HT15 and HT-10
that was way before my time/knowledge base, but multiple internet sources are saying this for the 13BRE, though they may just be copying each other incorrectly. Likely just copying Wikipedia since it states the same:
The 13B-REW got two big Hitachi HT-12 turbos. The second turbo receives a full share of exhaust flow and adds its boost with the primary turbo after 4,500 rpm. The 13B-RE from the JC Cosmo series has a large Hitachi HT-15 (57 mm diameter turbo) as primary with a smaller Hitachi HT-10 turbo (51 mm diameter). The small one provides the boost from 1,500 rpm up to 4,500 rpm where the bigger turbocharger designed for higher rpm starts boosting up to 8,000 rpm. This tandem minimizes the turbo lag very well.
09-21-23, 03:06 AM
