71dustar
Well-Known Member
my engine builder set my total timing at 34 degrees at 2000rpm. does this sound correct?
Not always and if the motor is on the edge of detonation using the vacuum advance could blow a hole in the piston.
That's just not true. At least not with an adjustable can. That's why it's adjustable so that you can limit the amount of advance the vacuum can has because they always have too much. TOTAL timing means mechanical PLUS vacuum advance, so as long as you set total in that way, you'll not have a problem.
my engine builder set my total timing at 34 degrees at 2000rpm. does this sound correct?
you sound more like you're suspicious and don't trust the builder.
why not ask him why he's doing it and see what he says.
Literally total timing would mean the sum of initial timing, mechanical advance, and vacuum advance, but most everybody refers to total timing as initial and mechanical only as the vacuum is removed from the equation (plugged) when setting the total timing. An example of 34* total timing would be 17* initial and another 17* mechanical. Vacuum advance could add another 17* for a total of 51*. Total timing on many vehicles with vacuum advance in the circuit can easily run in the 50`s depending on the needs of the engine. Most everybody running their cars on the strip will plug the vacuum advance since the only throttle position the car sees is wide open. I agree with Stroker Scamp on the use of the vacuum advance around town where most of your driving is below 3000 rpm. The car will have smoother throttle response and get slightly better gas mileage. Detonation can be a problem with the gas that`s available today but with moderate compression and several high gear/low rpm acceleration tests, you should be able to find a timing setting that you and the car can live with. As already mentioned both the mechanical and vacuum advances are adjustable so you can fine tune where and how much advance you add to the initial timing.
Gotta say Daves post is pretty thorough for the do it your selfer
LOL. You're preachin to the CHOIR and don't even know it. Let me break it down for you. "I" don't worry about initial timing. It's irrelevent. All you need is enough for the engine to run so you can set the rest. I remove the vacuum hose from the can and run an extension to a vacuum gauge taped to the windshield. I note my vacuum reading in high gear at 2000 RPM while just cruising. It is at this point that the timing should be all in.....at least for "me". Then, I go back to the driveway, disconnect the hose and reconnect to the vacuum can and take a timing reading at whatever reading I got on the vacuum gauge. It will of course be at a higher rpm, since we are now static in the driveway and not under load. This is why you don't use a tach to set timing. I let the mechanical advance in the distributor alone. I simply limit the vacuum can to whatever I want the total timing to be at at whatever given vacuum reading I got on the test drive. This is actually the most accurate way. While it's true you can play around with the mechanical advance some, I have found through the years, very little performance advantage in doing so on a street car. Of course, if using only a mechanical advance distributor, you must address adjustment, since that is your only means of advance. I have only on one or two occasions in better than thirty years found it necessary to adjust mechanical advance in a distributor using a vacuum can. I've read all the books. I've spent time at the tracks. I've set it to the inth degree as described above, but this method has worked best for me. It will on any street car, too. It's a good shortcut that will get you so close if not right on top of the technical guy's timing, it ain't funny.
Mine comes in 35 at 2500 rpm. To get that though my base timing is 18. Seems high but car does not ping. Should I bring lower to about 10-12 base and recurve to 35 @ 2500 rpm. stock 360, 904
Your post is confusing and could tend to mislead folks trying to set up a distributor on a significantly modified engine.
The mechanical advance is rpm dependent and vacuum advance is load dependent. Together they do a pretty good job of providing the timing the engine needs over it's operating range. For full out performance the mechanical advance is the only thing that matters because in these situations the throttle is open and there is little or no manifold vacuum and the vacuum advance does not work.
If your engine is close to stock the way the distributor is set up from the factory is close and what you describe is OK. But if significant modifications have been made then you need to take structured approach to dialing it in; initial, mechanical, rate and vacuum will all need to be addressed. The process I described in my post will minimize the interaction between these and minimize the back and forth fiddling.
To say initial timing is irrelevant is simply wrong. If you don't care about a stinky rich idle that you can't adjust out, or having to set the idle in park of your auto car at ~1200 rpm to keep it idling in drive at 700, or you don't care about sluggish response off idle then by all means ignore initial timing. Of course if your engine is basically stock setting the initial to the factory number or reving the engine until the mechanical stops and set the total timing you are close. But if you have made significant changes to the engine especially in cam duration/overlap then it just won't run as well as it can if you don't.
If you have significantly changed the initial timing then you need to address the total mechanical timing or in most cases you will have more timing than the engine needs to make max power and can also create detonation problems. Also, if you have replaced your old school heads with modern heads with efficient chamber designs you will need to address total mechanical since in these heads typically require less than the old school heads.
The rate at which the timing comes in is a place where the factory made significant compromises to address the bad driving habits of the people that purchased these cars. They gave up performance to protect the engine from detonation folks using the wrong fuel or lugging the engine or general poor care. Speeding up the rate with lighter springs even on a stock engine will give you noticeable improvements in the performance.
After all these items have been addresses you can adjust the vacuum advance.
Im lost #-oI know how to set stock timing on engines,but I am lost on timing the modified 360 on my Duster.And non of this advancing at 30 deg at 2000 rpms is really making sense.I need to find a video on how to do this.lol