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I've got a '76 Triumph TR7 and I was wondering this after I got its carbs tuned after its restoration. As many of you may know, old engines are initially not capable of running as lean as modern engines, due to less (local)cooling capacity, and older and maybe softer valves and valve seats. My manual originally prescribes a (idle )CO% of 2.5-4.5% which equals to approx. lambda 0.87-0.93 or 12.8-13.6 AFR.

The point is, when I rebuild the engine, I put hardened valves and valve seats in it so I could use unleaded fuel. And those modern valves and seats may be more heat resistent than the oldies. Also, modern coolant and engine oil may have higher cooling capacity and better heat transfer. Lastly, modern fuel is different from fuel from those days.

If I sum up those things, I would say one can run this engine leaner than prescribed because the engine benefits from modern parts and fluids. However, I'm not sure if it's true in practice, and how much leaner you can go before you burn your valves or other problems might occur. Does anybody have experience in these matters? Tips, info are welcome.

Cheers

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  • Surely the main reason you can run a modern engine lean is that it has a knock sensor so will adjust the timing and fueling if it detects a problem.
    – Steve Matthews
    Oct 19, 2016 at 11:18
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    @SteveMatthews You can closed-loop run as lean as possible with a knock sensor, but even with a knock sensor, an old engine could not run as lean. There's obviously more problems than pinging alone when running lean. Pinging is audible so i don't really fear for that, burning your valves is not audible though, so that's what i'm especially scared of. You also can't measure valve temperature, which is what makes this matter a though one, so i asked here.
    – Bart
    Oct 19, 2016 at 15:23
  • Pinking is your first audible warning of impending engine meltdown. Without a knock sensor and the software that will adjust the fueling, things will get very hot and start to melt.
    – Steve Matthews
    Oct 20, 2016 at 8:28
  • Yes right, so if i create a AFR curve from which i experimentally know that it doesn't cause pinging i'm safe for pinging and detonation. But even then i could still burn my valves.(which is my only fear) I don't see the relation between the two, except the fact that they both can be caused by a lean mixture. For the rest they don't have a link as far as i know. Or am i wrong?
    – Bart
    Oct 20, 2016 at 8:40
  • Pinking, Pre-Ignition or Detonation is the fuel mixture burning too early, i.e. when the inlet valves or still open so combustion occurs across the valve face and into the inlet manifold. This is the reason that the valves get hot / burned.
    – Steve Matthews
    Oct 20, 2016 at 8:56

1 Answer 1

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I wouldn't go too far with it, for risk of pinking, which could damage your pistons - plus, as I'm sure you well know, the Triumph slant 4 isn't exactly known for it's cooling abilities!

You'll probably need to run it on premium fuel - 97 octane or similar to match the 4-star it was designed for (incidentally, they run fine on unleaded without needing any conversion, as the alloy head has hardened valve seats from factory)

So no, I don't think you could go much leaner than originally intended...

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  • P.S. Good to see another Triumph fan on here, welcome!
    – Nick C ♦
    Oct 19, 2016 at 10:49
  • Yeah i read about cooling problems, although i havent had the engine the slightest bit above working temperature, while i regularly drive it very sportively. Besides, i could throw in a better radiator and fan if i get cooling problems. If there's pinging it might be eliminated by retarding the ignition a bit. The point is. i use this car as daily driver,(1500km/month) so a better mileage is of great importance for me, and running leaner gets me that. I run 95RON now by the way and it doesn't ping, so there's room for that if i go to 98RON.
    – Bart
    Oct 19, 2016 at 10:59
  • yeah they're awesome cars aren't they, you keep getting surprised about the unconventional solutions those engineers came up with. Like a directly from shaft driven waterpump, separated from the oil circuit only by a rubber o-ring..
    – Bart
    Oct 19, 2016 at 11:03
  • @NickC, I thought it was pinging, not pinking... :) Good answer, BTW.
    – anonymous2
    Oct 19, 2016 at 11:44
  • @Bart Keep an eye on that water pump - the seals don't last as long as they ought to - watch out for water dribbling out from the slot under the intake manifold...
    – Nick C ♦
    Oct 19, 2016 at 12:12

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