Timing and spring differences between NA auto and manual specs

Picked up a Rods manual yesterday, was having a read through and noticed some interesting (well I thought so) differences between the auto and manual car specs.

Amongst softer cams and lower compression on the auto which I already knew about, it shows the timing is more advanced on the manual engine. 9-11º BTDC compared with 7-9º on the auto. I need to replace the CAS o-ring on my auto so does this difference in factory timing indicate any reason why I shouldn’t advance to 10-12° at the same time?

The auto also has stiffer springs, presumably to accommodate the extra weight of the auto box. When I do the manual conversion will the car ride noticeably higher or handle differently as a result of keeping the auto spec springs on?

I presume yours is a 1600cc?
Mine is the 1840cc whose engine is identical to a manual, but I’m intrigued about the different spring rates. my 93’ suspension was the same as a manual…as far as I’m aware anyway.

Auto springs are softer, not firmer. The difference is in free length.

This is for Miata. No reason why they are different to other markets.

Grainger doesn’t get everything correct.


Yeah 1600cc, 1991 V-Special.

Here’s the differences listed in the workshop manual:

Compression ratio:
Manual 9.4:1
Auto 9.0:1

Manual 116 @ 6,500 rpm
Auto 105 @ 6,000 rpm

Manual 100 lb ft @ 5,500 rpm
Auto 100 lb ft @ 4,000 rpm

Ignition timing (during self test):
Manual 9-11º BTDC
Auto 7-9º BTDC

Valve timing (manual):
Intake opens at 5º BTDC
Intake closes at 51º ABDC
Exhaust opens at 53º BBDC
Exhaust closes at 15º ATDC

Valve timing (auto):
Intake opens at 5º BTDC
Intake closes at 40º ABDC
Exhaust opens at 55º BBDC
Exhaust closes at 5º ATDC

Cam height (manual):
Intake 40.888mm
Exhaust 40.889mm

Cam height (auto):
Intake 39.984mm
Exhaust 40.888mm

Spark plugs - standard fitment:
Manual NGK BKR6E-11 or ND K20PR-U11
Auto ND K20PR-U11

Alternator output:
Manual 12-60 V-A
Auto 12-65 V-A

Front suspension:
Coil spring colour code:
Manual Red
Auto White

Coil wire diameter:
Manual 10.8mm
Auto 11mm

Free length:
Manual 282.5mm
Auto 292.5mm

Number of coils:
Manual 5.91
Auto 6.32

Rear suspension:
Coil spring colour code:
Manual Blue
Auto Orange

Coil wire diameter:
Manual 10.1mm
Auto 10.2mm

Free length:
Manual 339.5mm
Auto 347.5mm

Number of coils:
Manual 7.68
Auto 7.96