the s5 is higher compression so i would think that it would use a little less right?

 

yea, thermodynamically it's efficiency should be higher, rotationally the rotors weigh less, so the S5 engine should be better on gas. however, S5's are also heavier, so you need more energy to haul it around

 

Basically, it gets worse fuel economy for the reasons topless stated (weight > compression). I don't know the EPA numbers though. Mazda didn't need to save gas anymore when they made the s5, while the s4 has massive quantities of aluminum for that very reason. The s5 is probably close, though.

 

so engine to engine the s5 uses more gas than the s4?

 

I got better mileage from my S5 than my S4 with the same mods. The S5 was overall in better shape. On long drives with a tailwind the S5 saw 25 MPG. Ocassionally 26 MPG. that was a rare though.

 

how much gain in power did you feel from swapping your s5 motor into your 88 (doing the same thing, rebuilt 91 motor into my blue 88 gxl)?

 

zero
now im trying to figure out why its just running so god damn rich

 

Here's the EPA ratings for the S4 and the "heavy weight" S5 as some like to say:

1989 S5

RX-7 1.3 L, Man(5), (FFS), Regular 17 25
RX-7 1.3 L, Auto(4), (FFS), Regular 17 23
RX-7 1.3 L, Man(5), (FFS,TRBO) Regular 16 24

1988 S4

RX-7 1.3 L, Man(5), (FFS), Regular 17 24
RX-7 1.3 L, Auto(4), (FFS), Regular 17 23
RX-7 1.3 L, Man(5), (FFS,TRBO), Regular 17 23

 

yea, it makes sense that the S4 would do better in the city while the S5 would do better on the highway. All the acceleration in the city would make the S5's heavier weight more of a factor, while the S5's more efficient engine would make a difference in steady-state cruising on the highway

 

all the EPA ratings are listed in the standard features and options sticky thread found at the top of this section... please read that before creating threads like this

 

start with the basics, tps, idle speed, vacuum leaks.

then work on the suspect list; 02 sensor (key for mileage), engine coolant temp sensor, plugs, wires, filters etc. the updated s5 afms are much better, but they did fail a lot back in the day.