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Vacuum Advance Diaphragm 6.5X or 10?
https://www.slantsix.org/forum/viewtopic.php?t=51345		 Page 1 of 1
Author:  StephenShawScamp [ Sat Mar 09, 2013 11:27 am ]
Post subject:  Vacuum Advance Diaphragm 6.5X or 10?
I have a '72 225 Offe 4bbl with Holley 390. Just realized my vac advance diaphragm was shot. The old one which was for the factory 1 bbl has a 6.5X stamped on it and is easier to pull. I have one off my '64 V8 that has a 10 on it and has a tougher spring. If I am running a larger carb should I use the 10 or try to match the old 6.5X diaphragm? I see a lot of threads here with super technical info which I don’t have the ability to get without help.
Author:  DusterIdiot [ Sat Mar 09, 2013 6:56 pm ]
Post subject:  More can be better, if...
You can use a 9.5 or 10 off a small block distributor that if adjusted correctly could net you better highway mileage. If you are running a hi-po engine with lots of compression, then the 6.5x may be needed if the engine pings with lots of vacc. advance.

Note that this isn't plug and play, you will need to check your points with the longer throw vacc. advance with a vacc. pump to make sure when it is at max throw that the points are still actuated by the lobes on the governor.

If you don't have the tools to check and make the adjustments (or a buddy that does), then it would just be easier to use a standard replacement for the 6.5x pod.

-D.Idiot
Author:  StephenShawScamp [ Sun Mar 10, 2013 12:58 pm ]
Post subject: 
Author:  DusterIdiot [ Sun Mar 10, 2013 1:22 pm ]
Post subject:  Yes...
Quote:
Is the difference between the 6.5 and the 10 the arm throw or is it the spring resistance?
The arm is limited by the stop cut into the end of the arm at the can..the deeper the notch the more timing it will allow, what does happen on the points arms since they attach to the outside hole on the pickup plate compared to the inside hole on the EI plates is that the bigger can geometrically moved the plate enough to cause some misalignment with the points and the cam can totally be out of contact with the points at max advance. This may not be as problematic with the pertronix, but you will need to use a vacuum hand pump and apply full vacuum to the advance and make sure that the gap doesn't open up too much and you get a 'no fire' on a plug or two.

Spring tension is based on the application so the spring may be heftier in the other pod because it needs the advance to come on when there is high vacuum at the carb port (cruise/off idle-no load). This can be adjusted with an allen key in the hose port on the vacuum can to get it to come in sooner (advance starts at lower vacc readings).

All thing said, make sure to map your timing and the vacc advance to make sure your are getting enough (or hopefully not too much) advance mechanically and vacc.

-D.idiot
Author:  StephenShawScamp [ Sun Mar 10, 2013 4:19 pm ]
Post subject: 
I do have a vac pump, timing light and tach. Define too much advance.
Author:  StephenShawScamp [ Sun Mar 10, 2013 4:20 pm ]
Post subject: 
On the vac port allen key adjustment, can they be loosened too much and fall apart?
Author:  DusterIdiot [ Sun Mar 10, 2013 9:43 pm ]
Post subject:  Best rough in...
Quote:
I do have a vac pump, timing light and tach. Define too much advance.
That's subjective based on your motor build, but the "ideal" rough in numbers for mopars is 30 degrees of mech+initial when all out just about 2-300 rpm above your typical highway rpm....with Vacc advance target of 47-52(with initial and mechanical) is good (also depending on vehicle and driving). Too much timing is well above those numbers, or when the engine pings (then you need to back off 2 degrees and test again).

-D.idiot
Author:  StephenShawScamp [ Wed Mar 20, 2013 2:52 pm ]
Post subject: 
OK I managed to get some numbers but am confuzed about what you said. Not sure if I was to add to what the inital setting was or not. Here is what I have - At idle both mech only and vac connected = 20 BTDC;
2000 RPM both mech only and vac connected = 40 BTDC;
3000 RPM mech only was at 40 BTDC and with vac connected jumped to 50.
Are these within tolerance on a stock build or should I add idle=20 BTDC to your 47-52 and be looking for 70+ at 3000 RMP?
Thanks for your help. I think we are close to resolving this.
SS
Author:  DusterIdiot [ Wed Mar 20, 2013 4:24 pm ]
Post subject:  Looks good...
Quote:
3000 RPM mech only was at 40 BTDC and with vac connected jumped to 50.
With mech+ vacc at this rpm, your total advance looks good (you might get away with 1-2 more degrees but not needed).

I'd be happy with those numbers depending on how aggressive the mechanical advance was.

-D.Idiot
Author:  Aggressive Ted [ Wed Mar 20, 2013 6:10 pm ]
Post subject: 
SS,

What is your idle rpm?
At 750 rpm which my 74 sticker states for idle I am at 10 degrees. Initial only, since the ported vacuum line is at zero inches.
Quote:
On the vac port allen key adjustment, can they be loosened too much and fall apart?
I have never had that happen. It will stop or get pretty snug at both ends of travel.
Author:  DusterIdiot [ Wed Mar 20, 2013 6:58 pm ]
Post subject:  Yes...
Quote:
I have never had that happen. It will stop or get pretty snug at both ends of travel.
I have... unfortunately one VC-184 and another VC-168 at 4 turns out from "seated" rattle like the mechanism came off the screw or adjuster bolt, so keep that in mind if using a 'new' SMP can... (FYI, 5 full turns is the 'max out' on the older SMP cans.)

-D.Idiot
Author:  StephenShawScamp [ Wed Mar 20, 2013 8:06 pm ]
Post subject: 
DI - Great news on the advance. Thanks for the input.

Now on the allen key adjustment for the vac diapragm, loosening it makes the advance kick in sooner, correct?
Author:  Aggressive Ted [ Wed Mar 20, 2013 8:15 pm ]
Post subject: 
Read this...http://www.dutra.org/doug/draft-webpage ... ibutor.htm
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