Uncle Jon's agony page
7th July 1998 From Ben Couture - I noticed in your web page that you had special half-shaft but made no mention of changing the rear-end/differential. Are you able to use the stock piece or have you installed a special differential to handle the engine's power? Thanks for your help; I am looking to use my '68 Mk III for racing
The car has a standard (what you American guys call stock) differential casing from a MKIV/1500 spitfire (the one fitted to the Mk I-III has small diameter quarter shafts which easily snap) with the crown wheel and pinion from an Standard Atlas Van of 4.55:1 ratio. Also there is a Quaiffe torque-sense limited slip differential unit with a 400bhp rating. This has lasted 60 races but broke last weekend at Mallory!! We have not had the engineer's report yet so we don't know what caused the failure.
6th July 1998 Thomas Moenstermann of Germany asked me about cam profiles - we tend to use the Kent 264 profile witha 1.63:1 ratio roller rocker assembly. The car is still drivable on the road provided you don't go silly on the exhaust system. The 272 profile has too steep a ramp on the opening side of the lobe which means the cam is using the edge of the follower during the lift cycle. This leads to a very short cam life.
Kent Cam Data (taken directly from a Kent Cams data sheet)
| Cam | Type | Power band | Cam lift | Valve lift in/ex | Duration in/ex | Timing figures | Inlet timing at full lift | Valve clearance in/ex |
|---|---|---|---|---|---|---|---|---|
| TH2 | Fast road | 2500-6500 | 0.261 | 0.382 / 0.378 | 295 | 39/76 76/39 | 108 | 0.014 / 0.018 |
| TH3 | Road rally | 2500-6800 | 0.261 | 0.412 / 0.412 | 284 | 34/70 70/34 | 108 | 0.016 |
| TH5 | Fast road | 2200-7000 | 0.282 | 0.423 / 0.421 | 280 | 37/63 73/27 | 103 | 0.022 / 0.024 |
| TH6 | Rally | 3000-7500 | 0.309 | 0.447 / 0.445 | 290 | 42/68 78/32 | 103 | 0.022 / 0.024 |
| TH7 | Race | 3750-8000 | 0.324 | 0.470 / 0.468 | 300 | 47/73 83/37 | 103 | 0.022 / 0.024 |
| 264 | Race | 5000-9000 | 0.493 / 0.491 | 310 | 52/78 88/42 | 0.022 / 0.024 | ||
| 272 | Race | 5000-9300 | 0.505 / 0.505 | 314 | 54/80 90/44 | 0.022 / 0.024 | ||
| Cam lift based on 1.52:1 rocker ratio | ||||||||
| Measurements in thousandths of an inch | ||||||||