Moss Motors, Ltd

I tried asking this question about a year or so ago but probably because of my lack of understanding wasn't very articulate with it and never got a complete answer so I'll try again. 

I have in my TR4A engine a camshaft (Moss part # 851-051) the timing of which is inlet 31-67, exhaust 67-31.  Duration 278 deg.  The standard camshaft that was put in by the factory is inlet 17-57 exhaust 57-17.  Duration 254 deg.

Standard ignition timing on this engine is 4 deg BTDC.  It is my understanding that ignition timing should be advanced to accomodate the inlet valve opening 14 deg sooner and the increased overlap.  Question: Is this correct and by how much?

Second part of question....I live at 6000  ft above sea level.  My workshop manual and other sources tell me I should advance ignition timing 2 deg for every 1000 ft so that means with the standard cam I would be at 16 deg BTDC ie 4 + (2 x 6)

Right now my engine idles happily at 20 deg BTDC and peaks at around 40.  Although I never get any pinking under load something tells me this advance is a bit too severe and I certainly don't want to break anything.  Obviously there is a limit to how much you can advance the spark but I don't know where that point is.   If I retard it a bit the idle becomes more lumpy but there seems to be better power at the top end. 

I've been trying to find the optimum by hit and miss but somewhere there must be a more scientific calculated answer.

 Thanks,

 John

 

 

 

John,

When it comes right down to it, the true optimal ignition timing changes with the weather, among a host of other factors. Modern fuel injected cars have a jillion sensors which the computer reads to determine fuel timing and quantity, let alone ignition timing for each individual cylinder on each compression stroke.

By theory, everything you state makes sense. However, there is a sort of law of diminishing returns when it comes to advancing the timing, and 40 degrees, on a street car, pretty much gets right next to breaking that law in a dramatic way.

We're here at 5,000 feet, and we don't go as far on a TR-4 as 14 initial, more like twelve

You really want to avoid detonation in one of these engines. It's not pretty.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

I live in Peru and we do go up high. 15,000 ft does'nt get you over the top. I have a 59 TR3, original configuration (the SUs are new) and have my timing set at 8º BTDC all of the time (live at sea level). When we go up to the mountains all I do is close the jets very little (1/6 to 1/3 turn). I have the big liners (2,138 cc) and it is a torquey engine. It will rev up to the limit without problems. Compression is 160 psi for all 4.

I went to School of Mines until 1974 and regret not having met motorbill. I had a 1968 Lotus Cortina Mk2 and could have used some help and advise back then. Fantastic mountain roads to enjoy the cars. Miss Colorado very much.

Thanks for the good perspective, Motorbill.  I knew that you would jump on this one.  One thing I've learned about 50 year old British machinery is that nothing is an exact science.  I will follow your advice and dial back to twelve degrees and go from there.

 

John

Hi Alfonso,

 Thanks for the response.  I assume you use eight degrees not only at sea level but also higher altitudes regardless with the only adjustment being the mixture.  Interesting.   Motorbill is a great resource isn't he.  I went to his shop once several years ago......real neat place but it's 120 miles from me.  I also had a regular Ford Cortina (without the Lotus bits) in England many years ago ....awful car.  I swapped a Mini for it (insane move)because the family was growing and nearly killed myself at the first corner.

John 

 

Yes, 8 degrees at all altitudes. It didn't make that much difference advancing the timing and the engine has so much torque that I don't bother fiddling with it. 

I have a 45 D distributor. It is not the original. I got it cheap here from very old stock, now our 4 cars have the same distibutor. No Pertonix yet. Enjoy your TR in thos mountain roads.

 

Alfonso