Mazda (MX-5) MZR engine data and rod ratio.

While doing researching on the MZR/Duratec I4 engine i collected some data from different sources for the most common variants of the MZR L engine used in the MX-5 (factory and engine swap). I was especially interested in the rod ratio which says a lot about the characteristics of the engine.

The data is based on the following sources:

CR for 2.5 is said to be 9.5 but wiki says 9.7.  1.8 is not confirmed.

Engine code Displacement Bore Stroke Rod C-C Rod ratio Mean Piston speed 7K / 8K RPM CR VVT
L8-DE 1,799 cc 83.0 mm 83.1 mm 146.25 mm 1.76:1 19.4 / 22.2 m/sec 9.7:1 ? No
LF-VE 1,999 cc 87.5 mm 83.1 mm 146.25 mm 1.76:1 19.4 / 22.2 m/sec 10.0:1 Yes
LF-VD 1,999 cc 87.5 mm 83.1 mm 161.xx mm 1.92:1   N/A Yes
L3-VD ~2.3 cc 87.5 mm N/A N/A N/A   N/A Yes
L3-VDT (DISI) 2,260 cc 87.5 mm 94.0 mm 150.55 mm 1.60:1 21.9 / 25.1 m/sec 9.7:1 Yes
L5-VE 2,488 cc 89.0 mm 100.0 mm 151.79 mm 1.52:1 23.3 / 26.7 m/sec 9.7:1 Yes

Some observations

  • The 1.8 has a square bore and stroke. As I have said before: This engine with aggressive cams should be a screamer. It’s ironic that it comes with a 6800 redline and a super sloppy intake cam on the MX-5
  • The 2.0 is a bored out 1.8 that shares the same crank and rods but a VVT head with proper cams.
  • The 2.3 is a “stroked” 2.0 with a stronger crank and rods
  • The 2.5 is both stroked and bored compared to the 2.3

About rod ratio:

Executive summary:

  • Rod ratio = rod length center to center/ piston stroke
  • Low ratio = high rod angle, increased piston speed, more friction, good torque down low due to less dwell around TDC. Not good for high rpm. Compact engine due to short rods.
  • High ratio = Better VE at high RPM due to more dwell at TDC, lower piston speed, less friction and side loading of the piston. Less torque on low rpms.
  • 1.7:1 good compromise for sporty car engines. 14K rpm Sport bike around 2:1 , 17K rpm F1 2.5:1 or more.

Looking at the table the stroked 2.3 and 2.5 has a lower rod ratio than the 1.8/2.0. The 2.5 is way down with 1.52:1 which explains it’s low 6800 rpm limit from the factory. The 2.5 block is reported to be 14 mm higher which fits with the longer rod (+5.54 mm) and the increased stroke (+16.9/2 = 8.45 mm) . It could be fixed with longer rods and even higher block but that would add weight and hard to fit under the hood on a fwd.

The rod ratio greatly affects mean piston speed (MPS). The 1.8/2.0 MZR has less MPS @ 8K then the 2.5 @ 7K. The 2.5 @ 8K is 26m/sec where as 25m/sec is what goes for being max recommended MPS. Note the MAX piston speed is much higher.

After learning about the rod ratio on the MZR 2.5 I will not go more than 7000 rpm on mine. If I still had a 1.8 or 2.0 I would see the point of going with stronger rods, cams and valve springs that can handles 8K+. I would think the 2.5 should be very good for turbo as both the VVT and rod ratio is good for spool. It would probably spin a GT(X)35xx turbo with acceptable lag/boost threshold. Thats already proven on the 2.3l MS3


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