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CJ Renegade 06-24-2005 10:42 AM

Re: vacuum advance question
I went through this a couple months back. RRich has it right. His rpms and °advance are just about what I got after I switched from manifold to ported vacuum, and after I adjusted the new vacuum advance. I used all stock parts and the engine has never run better.
Sorry - I dont have the NAPA part # for my '86 258 vacuum advance, but as I recall it was about $30 and yes, a special order. It only took two days to get it. It installed easy and adjusted with an allen wrench even easier.
After going round and round about why the vac adv was plumbed to manifold vac...I even checked a second CJ, an '84, and it was plumbed the same way...I came to the conclusion that the computer must have some control over the advance.
But it works better now than ever, again, follow RRich's advice and you'll see for yourself.

LEVE 06-24-2005 10:47 AM

Re: vacuum advance question
...I even checked a second CJ, an '84, and it was plumbed the same way...I came to the conclusion that the computer must have some control over the advance....

[/ QUOTE ]

The BBD Feedback ECM does control the timing via the knock sensor...about + or - 3 degrees.

rogerscj7 06-24-2005 10:49 AM

Re: vacuum advance question
Hers's the Crane part # CRN-99600-1 $22.88 plus shipping.
Works great..

RRich 06-24-2005 11:10 AM

Re: vacuum advance question
Yes, the computer controls the advance - when equipped.
There are 2? maybe even 3 solenoids that the computer uses to switch the vacuum in and out.
Yes, base, initial timing is set mid range. They want it above idle, else it'll be set wrong.
The vacuum is manifold vacuum - used strictly as a power source, not for the vacuum curve characteristics.

So - when the computer "feels it's time" for some advance it opens the solenoids, letting the strong vacuum through to the distributer, advancing timing.

Not sure, but probaby the parameters for advance (open solenoids) are:
At operating temperature
At higher RPM
At High vacuum - indicating cruise condition
In high gear.
If all conditions are met, the solenoids open and the vacuum pulls it forward.
It may even always be open at idle too.

And criteria may also include -
When the engine is overheated
and possibly even during deceleration.

The focus of attenton with the design was emissions. Chrysler was already in trouble with the ARB and the Feds at the time.

Those are my guesses - haven't verified it though. Those are common criteria for many vehicles. There may even be one or two more!

So the effect is there is no vacuum advance until the computer says so - then suddenly it "pops in." Sometimes when driving it you can feel it come in. It's not a slow transition, but a sudden "pop" as they open.
I've seen a vacuum delay valve added to the distributor line too - not sure if that's stock or not, but it would take out some of the "pop."

Using straight direct ported vacuum allows the advance to become a function of throttle position, not at the whim of the computer.

Yes, if the computer has been disabled, rewired, Nuttered etc, then those solenoids are not going to be operated correctly by the computer. You will have no advance at all.
Rerouting to use straight manifold vacuum all the time will only cause other troubles, poor performance, ping, poor mileage etc.

Using straight manifold - as soon as the engine starts the advance will slam full and stay there. Acceleraton will be poor, probably will ping on acceleration - because timing is way too far ahead.
But that can be compensated by lowering the initial timing to 8 or so. Then acceleration will be better, still not good, but "less worse", ping will probably be inaudible (still destructive though) and poor gas mileage.
That's why some folks will say it works for them, they just don't know how good it could be.

The idea behind using ported vacuum -
Burn time in the cylinder is relativly constant, no matter what the RPM.
You want the maximum cylinder pressure to occur between 32-38 degrees AFTER TDC. That requires firing the plug with enough lead time to get it - thus timing advance.
The faster the engine turns, the more lead time needed.
There are two "ideal" curves - one for maximum power, one for maximum efficiency.
Mechanical advance follows the power curve.
Vacuum advance follows the efficiency curve.
Ported vacuum has the correct curve to provide the needed advance for the distributor.

Don't eliminate the solenoids or computer, simply reroute the vacuum hose to ported. You may need to convert back someday to pass smog inspection.

While you are at it - Disconnecting the EGR - plug the line to it, makes a big difference too.
Set initial timing to 8, no vacuum.

That's things you can do for less than $1.

Sorry, gotta do this - my disclaimer - these are not street legal modifications - off road use only.

He He -- Can I disclaim my disclaimer?

Using the right vacuum in combination with a properly jetted carb, and an ignition system worth having (Go HEI) - the little underpowered 258 really comes alive!!!

Now add some breathing modifications, like a better air intake -- that stupid restricting filter is a real joke. Try it on a test drive without the air cleaner - stay out of the dirt though. You'll see how much difference that makes too!
And - use the stock original plugs the heads were designed to use - ignore advertising bull!

It's all in the "knowing what to do," not the big $$$.

Every one I've done the owner gets that grin from ear to ear!

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