Timing and spring differences between NA auto and manual specs

Technical Area Engine, Transmission, Exhaust etc
1

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?

2

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.

3

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

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

Grainger doesn’t get everything correct.

2 Likes
4

Yeah 1600cc, 1991 V-Special.

Here’s the differences listed in the workshop manual:

Compression ratio:
Manual 9.4:1
Auto 9.0:1

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

Torque:
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