1969 L46. Dwell some how went to 35 degrees. How would that effect performance? Presently mechanical timing set to all in (36 Degrees) at 3200 rpm. Appreciate only replys from those who have knowledge and experience.

Thanks you,

John

 

Dwell is the number of degrees of camshaft travel (distributor degrees) that the points remain closed. Each one degree change in dwell changes timing by one degree. If the points are remaining closed longer, it means they are opening later. Since the spark to the plugs occurs when the points open, an increase in dwell will cause retarded timing and a decrease in performance.

The biggest contributor to increased dwell is wear of the points rubbing block. Since this is normal wear, the system is designed so that timing retards, rather than advances, as normal rubbing block wear occurs. An increase in timing with wear would produce a potentially damaging effect to the engine with over-advanced timing, so the system is designed to retard timing with an increase in dwell.

Lars

 

Wow.

 

what else could be said...

 

Lars, Thank you for the reply. Based on what you said, increased dwell will cause plugs to fire while the pistons are on the down stroke, thus taking away performance. My senerio, took her for a ride today and she pulled good at lower rpms but lost that pull at 3000 to 6000 rpms.

Put her in the garage and checked dwell and it was at 35 degrees. Set it to 31. Weather permitting, Will run her tomorrow.

John

 

Not quite. The plugs are still firing before top dead centre, but not as far as they should be. Reset the dwell first, then adjust the timing before you head out.

 

John,

Do you know what the dwell was when you set the timing to 36 degrees at 3200 RPM? Keeping in mind what Lars posted, if dwell wasn't 31 degrees when you set the timing, then you are not at 36 degrees for timing now.

 

A 4-degree increase in dwell will retard timing by 4 degrees. If your total timing was 36 before, it will be 32 with the dwell change. If you had about 16 degrees of initial timing before, it would have retarded to about 12. It would not have retarded the timing enough to cause the spark to occur on the pistons' downstroke unless it were changed by at least 12-18 degrees, and the spark occurring on the downstroke would only occur at idle prior to centrifugal advance pulling in more timing.

I just pulled out some of my dyno sheets from various engines that I've run and documented timing changes on. From my actual dyno numbers, a 4-degree change in timing will typically change horsepower by 5 to 10 horsepower, depending on rpm. In a car as heavy as a Vette you usually can't tell a power change less than about 15 hp in the seat-of-your-pants.

Lars

 

Lars rebuilt the OEM points-style distributor in my '69 L46. He tested it (if I recall correctly) in his '65 convertible vette in early Spring weather (read 'cold'). I never had a single issue with the distributor once Lars rebuilt it. He went through the entire unit and identified/replaced/fixed what needed fixing. I can't praise his work enough. He also built the qjet on the same car (and my '80 L48). Both carbs have performed flawlessly (I sold the '69 L46 in 2008).

 

It's funny how "points" get about the same bad rap and misconception as Q-Jet carbs from a certain crowd (tends to be the same crowd for both misconceptions, funny how that works). A good set of points actually runs incredibly well: A good quality set will spin through 7,000 rpm with no bounce, and will not require adjustment for years of operation. And in spite of clever marketing done by the aftermarket electronic ignition industry, getting rid of the points in favor of magnetic or optic breaker systems will not produce a power increase, unless the points were not running correctly to start with.

To put this theory to the test, I actually ran a dyno session on a 400-horse small block Ford (302) with then-editor-in-chief of Hot Rod Magazine, Matt King, at Westech Performance in L.A. Steve Brule was the dyno operator and was witness to the test results. We first ran the engine with an MSD distributor, triggering a Crane Hyfire CD box. After optimizing the electronic ignition, we went out and pulled the old Motorcraft points distributor out of King's Falcon in the parking lot, ran down to the parts store, picked up a new set of good points, and then we curved the Motorcraft distributor to match the curve on the MSD. The Motorcraft distributor with the points performed exactly the same as the fancy electronic setup. Exactly. Not even 2 hp difference anywhere on the rpm curve.

A bad set of points, and a poorly set up or worn out stock distributor, will not perform well. But there is no need to trash the stock setup or the points to get an outstanding level of performance: If you rebuild and correctly set up what you have, there are few aftermarket systems that will actually gain you a significant advantage on the typical street-modified engine or on a near-stock vehicle.

Lars

 

Quick update. Set the dwell to 31 yesterday. GM says 30 but 31 is better at high RPMS that I have been told. Today attached variable timing light and assured mechanical total timing pegs out at 36. One gold spring only installed allowed total mechanical advance at 2500 rpm. Locked in the dist. and took her for a short ride. Chilly in MA. No pinging but know she could do more. Will experiment. Like to close this post for the time being.

I want to thank you for your comments and suggestions.

John

 

Pleased to announce that I installed the second gold spring, left dwell at 31 and advanced timing by 4 degrees. Total timing comes in just after 3000 RPM. Remembered to hook up vacuum advance this time and she runs very good. No Pinging or detonation that I could detect. Any harm if I advance the timing another few degrees and check for pinging or detonation. Thanks again.

John

 

On a smallblock you usually don't have anything to gain by advancing total timing beyond 36 degrees - the torque will start dropping off after 36 degrees total. Larger bore engines, and engines being run at high altitudes, can often benefit from total timing up to 40 degrees, but I've never seen a smallblock wanting any more than 36. My 407, with good heads, produces peak power between 32 and 34. If it's running good at 36, I wouldn't go any further.

Lars

 

Thank you Lars. Will leave well enough alone.

 

What brand do you recommend Lars as I'm still using the points set up in my L79........ and have no intention of ever changing to electronic

 

On a stock or near-stock car where rpm is held below 6,000 rpm, I recommend using the NAPA/Echlin CS786 points with the RR175 condenser. This is a very good, reliable setup that will not float or bounce, and they will maintain their dwell setting for a very long time. For engines seeing higher rpm I use the Mallory 102X points. These easily spin to 7,000 rpm, but the stiffer spring tension tends to make the rubbing block wear a little quicker than the CS786 points, and this will require checking the dwell once a year.

Lars

 

I did not know lars would talk this much. I have a carb question its just not car related. I have a 75 honda twin when i rebuilt the carbs the muffler some how fell off on the ground so i changed the main jet from 100 to 110 (10 percent) i'm at 550 ft elevation, but im taking it to moab utah 4800/5200 elevation. would i be better off going back to the 100 jet ?

 

Thanks Lars......

 

Guys, I'm a bit rusty regarding points systems (I converted my '69 to electronic ignition in 1974, and haven't had to think about points in a long time) but I was always under the understanding that dwell is cam/distributor degrees, not crankshaft degrees. A nominal 30 cam degrees will allow a decent amount of time (the key ingredient) to charge the coil at most engine speeds, but 30 crank degrees gets pretty short for the typical high inductance/ballast resistive coil circuit. Dividing the (distributor) cam into 8 equal segments gets you 45 (cam) degrees per segment. Using 30 of these degrees to charge the coil allows you 15 degrees to fire the plug and discharge the total energy in the coil. Not nominal, but a decent compromise given that the points require the ramps to provide acceptable mechanical switching forces.
Also, given that the points dwell is cam degrees, not crank degrees, increasing the points dwell one degree will change the crank dwell angle two degrees. This shows up as the dwell starting one degree earlier, and ending one degree later, giving the impression that changing the dwell angle only changes the point where the points open, resulting in retarded timing changes when the dwell is increased.

 

I think I got this one from Lars also, and it sounds like you did exactly this, so go ahead and run up the RPMs and check with your light as you do. Assuming you did everything else Lars recommends (vacuum advance can proper for your motor, etc...he has a great paper on that topic) get it all in and then set the distributor so you get the magic 36 when the advance stops coming in (2500-3000 RPMS by your reports). Make adjustments to your distributor with car at idle. Rev it again and when you see it all in and stopping at 36 you are done. The idle spec should match up correctly. Drive and enjoy!