Detonation - vacuum adjustment

[Ross Wooldridge]

Hi All - I'd like to get rid of a detonation problem my car has.

440 in my 66 T&C - runs beautifully (now that the no start-run issue was dealt with)
Pertronix setup.

I experience detonation on mild to medium heavy acceleration and would like to reduce or eliminate it.

I know that I can adjust the vacuum advance canister with an allen key, and that all this adjust does is increase or decrease the amount of vacuum needed to allow advancement to occur, and that it does not adjust the amount of total advance, or change the curve.

So - this is a place to start. My thoughts are that I need to INCREASE the amount of vacuum needed to allow the diz to advance, so that the mild vacuum increase created during mild acceleration is not enough to advance the diz as much, and thereby reduce detonation.

Is that correct?

As well, if I'm on the right track, which way do I turn it to reduce the detonation? Clockwise or counter?

TIA!

 

[Ripinator]

Hi All - I'd like to get rid of a detonation problem my car has.

440 in my 66 T&C - runs beautifully (now that the no start-run issue was dealt with)
Pertronix setup.

I experience detonation on mild to medium heavy acceleration and would like to reduce or eliminate it.

I know that I can adjust the vacuum advance canister with an allen key, and that all this adjust does is increase or decrease the amount of vacuum needed to allow advancement to occur, and that it does not adjust the amount of total advance, or change the curve.

So - this is a place to start. My thoughts are that I need to INCREASE the amount of vacuum needed to allow the diz to advance, so that the mild vacuum increase created during mild acceleration is not enough to advance the diz as much, and thereby reduce detonation.

Is that correct?

As well, if I'm on the right track, which way do I turn it to reduce the detonation? Clockwise or counter?

TIA!

Ross: Just pull the dizzy and send it to Ray at Halifax Hops (@halifaxhops). He went through the distributor from my '66 300 440 TNT, and now the engine runs great with terrific throttle response and no pinging.

 

[Xenon]

Ross: Just pull the dizzy and send it to Ray at Halifax Hops (@halifaxhops). He went through the distributor from my '66 300 440 TNT, and now the engine runs great with terrific throttle response and no pinging.

And to be helpful to anyone who may be thinking of doing the same thing,
what was the cost of having this done?

 

[Ripinator]

And to be helpful to anyone who may be thinking of doing the same thing,
what was the cost of having this done?

I don't remember. . . But I know it wasn't out of line though. Check with Ray.

 

[Davea Lux]

To answer the original question, turn the hex key counter clockwise to increase the vacuum needed to advance the distributor.

See www.forabodiesonly.com/mopar/ams/how-to-limit-and-adjust-chrysler-vacuum-advance-cans.183/

Dave

 

[Ross Wooldridge]

Thanks Dave! Any idea on how sensitive the adjustment is? Would I typically need to a 1/4, 1/2 or full turn to notice a difference?

 

[Davea Lux]

Thanks Dave! Any idea on how sensitive the adjustment is? Would I typically need to a 1/4, 1/2 or full turn to notice a difference?

I would try it in 1/2 turn increments, if that proves to be too much of a tweak, you can always go to 1/4 turn. Record the total number of turns or partial turns so if the car runs like crap after the adjustments, you can return to a fixed starting point and start over.

Dave

 

[Ross Wooldridge]

Thanks!

 

[furious70]

What carb? What PV if Holley or springs if Carter? Maybe a little quicker enrichment is in order to stop the ping and improve performance

 

[Ross Wooldridge]

Stock Carter AFB 625 CFM.

I'm going to take it step by step - I'm going to try adjusting the vacuum advance, and then go from there if there's more improvement required that's not available by adjustment of the vacuum.

 

[CBODY67]

Are we talking "trace rattle" or "DETONATION"? Is it gone at WOT, but not part-throttle? What rpms, generally? Something that's been there a while, or just started? Might be a fuel issue, presuming it's the stock 383 10.0 CR?

With 93 Pump Octane super-unleaded, if it's trace rattle, then the vacuum advance tweak might do it. If it's DETONATION, then it might take a retarded initial timing setting to do it, provided no springs in the distributor are broken.

The next question might be what initial timing and if the distributor has been "messed with", meaning lighter advance springs OR a broken one?

Just curious,
CBODY67

 

[Ross Wooldridge]

Good questions. It's not a trace rattle, and is usually gone or becomes just a trace at WOT.

Diz has not been messed with, and timing I believe is at stock settings (12.5 BTDC). If I run Shell with no Ethanol, I can only get 91 Octane, and the issue gets worse. Sunoco (PetroCan up here) 93 lessens the issue, but there's Ethanol in it. I will be checking the initial timing as well as tweaking the advance can.

 

[twostick]

Sounds like it might be a tad lean at part throttle.

Kevin

 

[The Goose]

And to be helpful to anyone who may be thinking of doing the same thing,
what was the cost of having this done?

About a buck forty. I just got a rebuild from him and I’m getting ready to send in my worn out original that the new (old rebuilt) one replaced for the same treatment. Yes they’re that good...

 

[halifaxhops]

Thanks guys for the plug. Yes usually to clean them up and work per factory specs about 150 ish for a typical aluminum one, really depends what it needs. Just did a Cardone for a guy now that was a mess needed just about everything to make it run right for them.

 

[Ross Wooldridge]

Sounds like it might be a tad lean at part throttle.

Kevin

Interesting - something to keep in mind as I delve into this.

 

[RagTop66]

Thanks guys for the plug. Yes usually to clean them up and work per factory specs about 150 ish for a typical aluminum one, really depends what it needs. Just did a Cardone for a guy now that was a mess needed just about everything to make it run right for them.

May be a stupid question, but are these adjustments needed with a dizzy that’s electronic ignition or is this just a points thing?

 

[Davea Lux]

May be a stupid question, but are these adjustments needed with a dizzy that’s electronic ignition or is this just a points thing?

If the distributor has a vacuum advance, the adjustments are about the same. Units with electronic advance controls do not usually have a vacuum advance. 1st generation lean burn systems had a vacuum advance on the distributor which was gotten rid of fairly quickly on the mid '77 and up units. Some of the retro conversion kits are a drop in module in a point type distributor that retain the vacuum advance. Early mopar electronic ignitions starting in '70 still retained a vacuum advance as did the conversion distributors that were sold as a conversion
kit.

The older engines running a 10-1 comp ratio take some tweaking to cure the detonation issues caused by modern moonshine blend fuels which do not flow or burn the same as vintage gasoline blends.

Dave

 

[Ross Wooldridge]

This particular diz has the popular Pertronix conversion in it which retains the vacuum advance. As well, this advance can is the one known to have the adjustment capability. Some are not adjustable, but I'm not sure which ones.

 

[CBODY67]

In thinking about it, I suspect that after about the middle '80s or earlier, after ANY OEM-application emissions regulations would be "long gone" for the B/RB engines, then most probably came from the same contractor and were adjustable. Just that we didn't know about the Allen-wrench adjustments back then, replacing the Bradded connection between the metal backing plate and the internal spring.

Where the "max travel" stop on the vac advance "arm" is and the spring tension would be the two areas where calibrations occur, I suspect. Some had a number stamped on the arm, for the amount of distributor degrees allowed by the partiular unit, but some did not.

I suspect that if you look at the vac advance specs in the respective FSMs, you might discover that, as with the centrifugal advance, the starting vacuum levels will be similar, as will the max advance-reached vacuum levels. End result, a pretty common internal spring that could be customized a bit by the Allen wrench adjustment of the spring's tension.

Similarly, the centrifugal advance start points have a range of rpms, but generally with a smaller window of median values. From there, it's spring tension and advance spring weights, in addition to the shape of the weight which determines the ultimate advance curve.

These things can be the reason that the reman distributors can fit so many different applications/model years of engines. "Will fit/work" rather than "application specific" to meet a particular set of emissions regulations, but a bit more "median value" in orientation. In some cases, the difference in driving feel and power might only be really apparent on the dyno, I suspect. Which is another reason the vac advance would be adjustable, so that WE might better tune the item to work better with our particular/individual applications. IF we knew that such tuning was possible!

On the centrifugal advance issue, there are instructions deep in the FSM about bending the tab the heavy spring attaches to, or even removing it (stated in the Direct Connection Race Manual for B/RB engines, possibly A/LA too) for a faster advance curve. Considering that you have to basically disassemble the Chrysler distributor to get to those weights/springs, unlike the Delco-style distributors with those things in plain sight.

Enjoy!
CBODY67