What is the effect of the dwell setting?

[ntheyer]

I understand that dwell is the rotational angle that the points remain open or closed?<br /><br />On my engine (87 Merc 470) the spec calls for 28-34 degrees and 4 degrees timing advance.<br /><br />What happens if I set the dwell to 27.8 or so... It's such a pain opening and closing the cap and starting and shutting off the engine to do so, so I want to leave it at 27.2 - 27.8. Will I hurt something? What happens when you have more dwell or less dwell than needed?<br /><br />I adjusted the timing to 4 degrees after setting the dwell.<br /><br />I know that everyone will tell me to get a Pertronix electronic ignition. I will eventually get it but for now just want to run the boat as is. I am also unsure as to which Pertronix kit to get for my application. When I go to their website, there are too many choices to make for me to feel comfortable that I would get the right one.<br /><br />This was more just a theory and understanding question as to how dwell affects things.<br /><br />Thanks.

 

[JAL51974]

Re: What is the effect of the dwell setting?

when you do go the Pertronix route, you will find that their techs are knowledgeable and helpful in getting you the correct one for your application. mine will be coming in a few days, I will let you know how it goes.

 

[KaGee]

Re: What is the effect of the dwell setting?

Why do you need the cap on to set the dwell? You should be able to get a rough reading turning the engine over, cap off.

 

[seahorse5]

Re: What is the effect of the dwell setting?

Since the motor has a few years under its belt and surely the distributor bushings are worn some, so set the dwell at 34° while cranking the motor and I'll bet you will see about 29° to 31° dwell when it is running. You will have to reset timing then.

 

[ntheyer]

Re: What is the effect of the dwell setting?

This is something new to me...! How do you read the dwell without the engine running? I have always adjusted points and then put the cap back on, fired it up and read with my nultimeter/dwell meter off the negative side of the coil and a engine ground?<br /><br />How do you measure dwell otherwise?<br /><br />Thanks.

 

[imported_Dave_C]

Re: What is the effect of the dwell setting?

Dwell is the number of degrees of distributor shaft rotation that the points are closed.<br />1 or 2 degrees off from specs. won't hurt except on Volvo 4 cylinder engines. They are reported to be pretty critical.<br />The trick to setting points with a dwell meter is to ground the high tension coil wire to the block while cranking the engine and setting the points. This will reduce the error in the meter caused by open circuit high voltage, and the lower normal running spark voltage.

 

[Boatist]

Re: What is the effect of the dwell setting?

The dwell setting is just to get the best possible spark. At the proper setting condensor charges just enough and the discharge will get you the best spark but it does not have to be perfect. I set mine on the cam lobe then adjust to .018 inches and it will be right on.<br /><br />I am one that thinks points are better than pertronix. I herd of no improvement in HP or Mileage. I herd of many stranded because they left the key on. My 1980 boat has never missed at all and runs perfect. I changed the points and condenser only twice. Just figure should change every 15 years or so. Keep both old sets in tool box and carry a spare new set just in case. If it should fail I could change my Points and be back under way it less than 10 minutes. I know I could change with just a screwdriver and eye ball and be under way. I have all the tools on board also.<br /><br />If the electonic gained you 20 HP or better mileage then I would change this week.<br /><br />If you do change make sure you get the model II so it does not blow up if you leave the key on.

 

[Dunaruna]

Re: What is the effect of the dwell setting?

Do it once, do it right. Warm motor, idling.

 

[seahorse5]

Re: What is the effect of the dwell setting?

Originally posted by ntheyer:<br /> This is something new to me...! How do you read the dwell without the engine running? I have always adjusted points and then put the cap back on, fired it up ...

Remove the cap and rotor and either hook up a remote starting switch attached to the starter solenoid, or have someone crank the motor over while you check the dwell reading. Of course, first remove the coil wire and ground it.<br /><br />At cranking speeds the distributor shaft does not "whip around" as much as it does during running, that is why you set the dwell at the top of the specification (points close together).<br />When running, and with worn bushings, the dwell will be close to the middle or lower end of the spec. <br /><br />You can see how much the shaft will move the points by checking the point gap while pushing the shaft away from the point rubbing block, then remeausuring while pushing the shaft towards the rubbing block. On a used engine you will see several thousandths difference.

 

[KaGee]

Re: What is the effect of the dwell setting?

Boatist...<br /><br />In my personal experience, there was an immediate difference in start up after installing Pertronix. Especially on a cold engine. Engine idles much smoother. Any added mileage or HP performance I have no evidence there. <br /><br />As for the key-on scenario, I always shut the key off. All other electrics are run off of a separate circuit. IMO, that's the way it should be anyway.

 

[ntheyer]

Re: What is the effect of the dwell setting?

Thanks for the advice gentlemen. It seems like 27.2 or so is at the bottom of the spec and will only get me a lower number as they wear, so I will readjust more towards the 34 degree end of things. Thanks.

 

[newport dave]

Re: What is the effect of the dwell setting?

I would leave it 27. Wear of the cam follower on the points will increase dwell, not decrease.<br /><br />Dave

 

[Buttanic]

Re: What is the effect of the dwell setting?

The plug fires when the points open. The dwell time is the time or number of degrees of distributor shaft rotation the points are closed. The longer they are closed the more time the voltage has to buld up in the coil's primary windings. The more voltage in the primary windings = more voltage out the secondary windings to the spark plugs. A V8 engine fires 8 times in one 360 rotation of the distributor which equals one cylinder firing every 45 degrees. If the dwell is 32 degrees, the points are closed for 32 degrees and open for 13 degrees of each 45 degrees of rotation in the firing order. The best you could do is have an ignition system with 44.999 degrees of dwell. With points the mechanical system of a cam and points limits how fast points can be opened and closed. Electronic ignitions are not so mechaniclly limited so they can operate with a greater dwell but still not reach the optimum. On a point system closing the point gap increases dwell but much beyond 34 degrees arcing between the point contacts becomes a problem. Less dwell than 26 degees and the coil does not have enough time to build up voltage to bridge the gap at the spark plug.