Marley Says
OKAY DAVID, let’s tackle these one by one: Now, your first question seems, on the surface, a pretty simple one. But you would not believe how many phone calls I made trying to find somebody old enough to remember TC Cortina diffs who is still in the industry. But the general consensus seems to be that the four-cylinder TC Cortina used a physically smaller, 23-spline diff, while the six-cylinder Corty, with its gruntier engine, was fitted with a bigger, 25-spline diff.
I don’t have a problem with this theory, because it kind of stands to reason. The four-cylinder TC was the Brit-spec thing while there was no six-banger TC sold anywhere but here (and maybe South Africa or somewhere). Point being it was engineered by Ford Oz to accept the Falcon’s six-potter. And that being the case, there’s a fair chance it got a narrowed Falcon diff in the process. Which would explain why the two versions of the Cortina have different diffs. The bottom line being that, no, you can’t just swap one for the other. What you might be able to do, however, is either find a different ratio that will fit your four-cylinder diff, or graft the whole TC six-cylinder axle into your Marlin. Then, you’d have a choice of ratios because any six-cylinder Falcon diff-centre from that era should bolt in.
Next: If you do manage to fit a diff centre with a 3.23:1 ratio, it’s a simple (I hope) set of calculations to work out your revs at a particular road speed. Since 3.23 (the new ratio) is 86 per cent of 3.75 (the old ratio) it shakes out that you’ll then be doing 86 per cent of your old revs at the same speed. Which means your engine speed at 100km/h drops from 3250rpm to 2795rpm. And that’d be a worthwhile gain, yes? Actually, I completely made that methodology up – the percentage thing – but it seems a logical approach to me. So if I’m wrong, can somebody please let me know.
You will lose some overtaking flexibility, particularly as you’re almost at peak torque at 100km/h with the set-up you have. But the Pinto four-cylinder is a fairly grunty little gadget, so you should still be able to overtake in fourth. And shifting back to third for max thrust? Well, that’s what a gearbox is for, isn’t it. But here’s where it gets murky again, because my info says the TC Cortina with the 3.3-litre six used a 2.92:1 diff ratio, while the 4.1-litre version had an even taller 2.77:1 diff. No mention of a 3.23:1. But, as we’ve already discussed, that might be academic unless you’re prepared to swap out the whole drum-to-drum assembly. By the way, David, I reckon you’re pretty game revving a stock Pinto to 6300rpm. And there’s probably not much to be gained as I’d have thought the stock two-litre’s power peak was closer to 5500rpm (or maybe even 5200rpm) rather than the 6300rpm you’re quoting. Unless we’re talking a heavily modified
engine at which point all bets are off.
Now, 12.4 litres per 100km on the highway does sound a fair bit of fuel for a plastic-bodied, four-cylinder Alfa replica to be using. Changing to a taller diff should theoretically improve this, provided you don’t go crazy and over-gear the thing, forcing you to use more throttle to keep the engine on the boil. That said, there’s a whole lot more to fuel consumption than just diff ratios. Aerodynamics come into it, tyre pressures, kerb mass and even (in fact, especially) the way you drive the thing will all have an effect on this number. But yeah, 12.4 litres sounds high and, in fact, only works out to 23 miles per gallon in the old money. Meanwhile, trying to predict fuel economy before a proposed diff change is really stabbing in the dark.
Just as poor fuel economy has many causes, so does an exhaust pop on the overrun. What’s much more likely than incorrectly stamped timing marks is a leak somewhere in the exhaust that’s letting air into the system and igniting unburned fuel. A dribbly carburettor will contribute to this, making the problem worse. In fact, in the last of the Commodore SSs, the 6.2-litre version, the computer deliberately injects a small amount of fuel into the engine when the throttle shuts so that this snap, crackle and pop process can occur. All in the name of aural satisfaction.
Now, which dizzy is which. The best way would be to look at the part number stamped somewhere on the distributor. But my cruel mate, Goober, has a simpler method. His advice is: crank the engine. If it fires, you have a Bosch dizzy, if it doesn’t, you have a Lucas. Harsh, but somehow fair.