Dwell

[John Morrison]

Looked high and low for the dwell either angle of percentage for a TR4,

 

Anybody know what it is or where I can find it?

 

john

[bob-menhennett]

John

 

My Triumph Repair Operation Manual for a TR4 , post Comm No 17954, ( Lucas Distributor part no 40795 )

quotes 30 degrees, plus/minus 3 degrees.

 

"Pre- unleaded" days data of course..so maybe others....the racers and professionals... can input their experiences.

I shall be starting from this point , for the first time, soon .. so would be interested myself !!

 

Bob

[John Morrison]

Bob thanks for that,

so any advance on 30 degrees?

 

john

[Cinnobar]

Dwell would stay the same given the same distributor geometry as it's a function of the recovery time of the coil.

The timing however will vary given the differences in fuel.

[marvmul]

A dwell angle of 30° means that the distributor cam (and spindle) will turn 30° during from closing to opening position of the points. On a 4 cyl engine, the point are closed during 120° and open during 240° in one revolution of the distributor spindle. When the points are closed, current is flowing through the primary (low tnesion) windings of the coil, building energy in the electric/magnetic coil fields that will be released when the points are opened (>spark). Unfortunately the time that the points are closed is function of the rpm : shorter with rising rpm, the spark gets weaker at high rpm. Some electronic ignitions (i.e. Lumenition Performance) give a more or less constant 'dwell' time, instead of the constant dwell degrees : the spark will be strong at high rpm.

[jean]

For those who never understood, like me, the background of Auto Ignition have a look at the website: Faraday's Law and Auto Ignition

You will find it via Google Search engine. This is the most comprehensive explanation I found up to now.

[jonlar]

Hi John, I thought 4 cyl. engines were supposed to be around 55º dwell (Black Hawk auto meter data-sheet), but found in the TR6 manual that for the 6-cyl. engines the correct dwell is 35º ± 3º, ie 32º min., 38º max, points open 25º ± 3º, extrapolated from 60º to 90º would give 52.5º ± 4.5º dwell and 37.5º ± 4.5º open. This would tally with the BH info..

[John Morrison]

Thanks guys, but I am going back to a feeler guage!.

 

cheers,

 

John.

[jonlar]

Hi John, I'd suggest you set as accurately as you can with feeler gauges, then do a dwell meter check and post back here the results, the 30º quoted by Bob, doesn't sound anything like long enough for a 4-cyl., if it's 33 for 6-cyl.. The larger the points gap the lower the dwell angle, setting by dwell angle tends to be more accurate than feeler gauges as the points start to pit and peak with sparks almost instantly. I, for one, would be interested to know your findings.

[John Morrison]

Will do at the weekend.

 

john

[John Morrison]

Hi Guys sorry it took so long to get back.

 

Anyhow mesured the dwell both by degrees and percentage, Borowed a second meter.

 

Having set the points at 15 thou;-

 

percentage reads 60%

 

degrees read 52

 

The table which comes with the meter (That I 've now found) has a conversion, and gives

for a 4 com lobe 54 degrees as 60 % I guess that i'm quite plesed with my efforts, car certainly does go well.

 

Thanks for all the helpful replty.

 

john.

[jonlar]

Hi John, many thanks for posting the results, interesting for future reference - good to hear you're sorted.

[helicfii]

A little more about dwell:

 

 

The period of points closure is specified by the ignition system designer and is typically expressed as degrees of distributor rotation. In a four cylinder engine, the angle between each ignition cam lobe is 90° and the period of points closure or "DWELL" is usually a bit over 45° of distributor rotation. In a six cylinder engine, the lobes are 60° apart and the dwell time is 30° to 35°.

 

The dwell is adjusted by setting the points gap to a specified distance at maximum opening. A narrower gap gives more dwell and a wider gap gives less. Taking it to extremes, excessive dwell means that the points close too soon after opening, cutting off the magnetic field collapse before it delivers all its energy. Too little dwell gives the magnetic flux insufficient time to build up to the maximum.

 

Both conditions give a weak spark which gets even weaker as the engine RPM rises and produces misfiring at normal operating speeds. The dwell, as well as spark plug gap, do have an effect on ignition timing. The later the points open, the later the spark comes and retards the timing. The earlier the points open the sooner the spark comes and advances the timing. That is why timing is the last thing to be set in a tune-up.

 

A mechanical distributor has a fixed dwell-angle, sometimes resulting in overheated coils at low speeds, weaker sparks and misfiring at high speeds. Why? An ignition coil only needs a couple of milliseconds to charge, but with a fixed dwell-angle these are overcharged many times when idling, resulting in - overheated coils! At high speeds the fixed dwell-angle tends to leave too little time to fully charge the coil, resulting in - weaker sparks and misfiring...

 

That said, I would think that the proper dwell angle would vary from one 4 cyl engine to the next, taking into consideration the age of the ignition design, and the capacities and limitations of the installed ignition components, etc...

 

Just my .02