alexandervdr

My centrifugal on a standard AC distributor adds about 28°. What is the best way to reduce that?

Jackfit

I don't think you want to reduce it , you may want to increase the RPM s that it come in by the use of different strength springs....One Middle one light, one middle one heavy , one light one heavy. Middle and Middle etc.

The distributor will have built in limiter to 36 deg centrifugal . You control the timing with the vac can and initial advance so that too much timing is not coming in too soon. You vac can may be putting in too much...I use an adjustable one to dial in my advance...and then use the springs too control total advance at all speeds.

My car is happy at 21-25 at idle...with VC and Int.....in.....very little centrifugal at idle....10* int plus about 12-14 VC....in an L-79....

At 3,000 rpm I am about 48-50 total * 10 + 12 + 30 ish centrifugal + 48-52, with VC dropping as rpms increase....allowing full 30+* centrifugal....I think.....lol

Jack

alexandervdr

I have a very low CR crate (8.5-9), and those (according to spec sheet) require only 22°. Just want to try that...

Jackfit

Just use the heavy springs on the weights ...you test by plugging off VC subtracting int....from total at 3,000 rpms and keep changing springs until you get 22* at desired rpm.... 3,000 + using a dial back light.....

Total Reading at 3,000 - 6* I think for your engine..


Jack

L78racer

you can get a bushing kit to limit advance.

alexandervdr

that does screw up the rpm when total gets in, so I really want to reduce centrifugal impact. You can't resist trying to stop me working on my car , I just love it

I just discovered there is a bushing for that...(I was typing this while L78Racer was posting ;-) )

Jackfit

I was going to mention it......

Have fun.....I take the car out of the oil tank next week....first time in decades that I have not use it during the winter.

Jack

Jackfit

Just curious , is the car surging or detonating at 28* or you just want to dial it in....what is your total now at 3,000? , before you pull the distributor out......but then again.....why wash dishes or house chores when you can be out in the garage.....

Jack

DansYellow66

The bushing is the best approach on a GM although it will only do so much. But if you don't have a bushing now, it may take enough out to satisfy you. After that a bit of light welding to close up the slot in the shaft plate can limit it further but takes someone with a deft touch with a welder to not make a mess of it (unlike me).

Frankie the Fink

Those 'spec sheets' are usually quite conservative to cover a variety of driving styles, available fuels, altitudes and traffic conditions. I wouldn't take that as gospel for one minute, they can almost invariably be improved on.....if you're not having problems now (trailer hitching/detonation) why would you 'detune' your car ?

alexandervdr

Just experimenting, you are right ,may be better or worse, the point is I want to learn and understand as much as I can, just love it. There is little if any expertise over here to go ask, I am on my own...

alexandervdr

I am at 38° (no Vac) mechanical and 10 initial advance.

alexandervdr

I checked, there is no metal but a rubber bushing installed in my (new) distributor. Diameter about similar, so that would not make a big difference. I had some mail/post exchange recently with Lars and Duke that made me rethink my settings ...

Frankie the Fink

Lars and Duke are the 'guys' for this effort....

My experience is that with initial and centrifugal 'all in' at about 36*-38* as quick as your engine can take it (2500 RPM -3000 RPM) is about ideal. With vacuum advance on top of that 48* is good but I've pushed some cars to 52*.

wmf62

that is the best way to reduce advance

Bill

alexandervdr

with a 16° vacuum advance on top of the 38° one ends up (as I do..) at 54° All this advance stuff is chasing tails ....

JohnZ

Remember that in the real world, under most normal driving situations, initial and centrifugal maximum advance are not additive. At high rpm and WOT, the vacuum advance is inoperative, so the engine's only working at 38* under those conditions.

DansYellow66

Go ahead and install the largest metal bushing that will fit and slide in the slot - it will make at least a bit of difference over the rubber one.

alexandervdr

I know, still, what is then the relevance of arguing total advance (it's used all the time in discussions...)

Jackfit

Here goes my simple explanation. At your garage, setting the specs (48 or so, adding int, centrifical, & vac, at most cruising speeds, you will only be at 36 or and safe as JohnZ stated. At idle you need the VC & int to run well....at high speeds WOT VC cuts out , no denonation from too much advance.

Now, all I said could be wrong, but I sure have my car running...but again, it’s good to be in the garage . Maybe something is lost in translation....lol...my hello to the sane one in the family.

Jack

Jack