BB ingnition timing

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360duster

360duster

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Hi Guys, i have no experience with the mopar big blocks, just built SB´s up til now.....have got a question.

My buddy has got a coronet station wagon with 440cui engine, Engine built by a shop here. They left him alone with some problems (heavy pinging under load), i´d like to help him out as far as possible.

So i think these engine is built with too much compression. It´s a pretty stock build, cam seems mild to me, stock cast iron heads etc...i think they put wrong pistons in it and pushed the CR to high - he´s got no clue what´s inside. He has to run race gas to keep i from pinging - no fun at all if you have a big engine and cannot use it...

So i´d like to check the timing first, maybe the dizzy is advancing too much or too early. I´m positive that we can get a better situation with a little tweaking, so what do you think about some timing numbers to start with? What numbers do these engines like?

thanks for your help!

Michael
 
start at 32 and see if that works out if it does go up one or two degrees until the pinging starts again then go back to where it didn't.
if you can't do that start out at 6 and do the same thing.
 
Can you get cylinder pressure readings from it? What ignition is it? Has he set up the timing curve or carburetor yet?
 
ok, thanks for this info. I´ll try to get a baseline, will check the numbers initial / fully advanced and when it´s fully in.

Moper: it´s the stock mopar electronic ignition with a orange box. To me it seems that the engine builder did not take the time to take care about the timing events / dizzy setup.....as said before i will check the actual numbers to go from there.

I did no cylinder pressure tests up til now - is there a "safe" number? have to get a gauge for that, then we´ll do this test, too.

the Carb is a EDE 1407 / 750 cfm. I´ll check if it´s leaning out, maybe it´s better with 8"HG step up springs....we´ll see, we have a date this weekend.

I´ll report what i find!

Michael
 
got some free time this afternoon, decided to do some test today:

initial is set at 15° btdc, fully advanced it´s around 45°....i think the problem is that no one took care of this dizzy. I retarded the timing to around 10° at idle......little less power, but the pinging is gone! ok, that was easy.....

So it seems that the distributor needs a limiter kit for the mechanical advance, maybe i try welding / filing the slots until i reach the goal. Can anyone supply the length of the slot for 20° mechanical advance?

My buddy found the build sheet from the engine shop: cranking pressure is between 185 and 195 psi according to the sheet (warm engine). Cyl. chambers are 88cc, i found that KB237 Pistons are used (.040"), should be little below 10:1 compression (i don´t know the head gasket compressed height). Should work, what do you think? The shop told him he´s over 11:1 compression with this pistons....i think they are totally wrong.

It seems to run good if the full advance is limited to 35-40° (no more pinging..). It runs a little better if the initial is higher, will do some more testing even with the vacuum canister and limiting the total advance. Thanks for your tips!

Michael
 
I haven't checked if they work for both B / SB but FBO has a limiter plate

Slot length: Don't give me credit, found it elsewhere. BEAR IN MIND these figures are distributor degrees, so you want something in the 10- 11* range (example) to get 20 -22 at the crank

Dimensions for Mopar distributor slots, in DISTRIBUTOR degrees

Modifying advance slots
degrees / slot size
6.............. .340
7................355
8................375
9................390
10...............405
11.5 ...........420
12...............435
13...............445
14...............460
15...............475
16...............490
17...............505
18...............520
 
before you go modifing the slot try a heavier spring in place of the light one. that will slow down your rate of advance and will keep the total in check. shortening the slot is for if you want the total in sooner and you don't need that, it already pings.
 
No, that is not the way to handle a distributor. The short slots have nothing to do with how quick it advances.
 
You have the two issues without getting into the carb tuning yet: the rate of advance, and the amount of centrifugal advance. You should have one heavy spring with an elongated end and one a little lighter with two ends that are exactly the same. What you need to do is remove the stiff one with the elongated end and replace it with a Mr. Gasket "light" advance weight spring. Leave the factory lighter one in place. Then weld up the slots to get the centrifugal number you want. Then reset the initial to what you want. Then you'll want to tune the carb. If the initial change helped a lot, the mods above will fix it.
 
Thanks guys,

i´m also positive that some mods to the dizzy curve will give better results. I appreciate your input very much!

Will modify the distributor internals next week, we´ll see. I also have some lighter springs from curving my own so i can try these.

Michael
 
ok guys, got something done to this dizzy. welded the slots and opened up to around 20° crank advance....went a little too far, now it´s around 22° fully advanced.

I also changed the heavy black spring with a light silver one that was left from a kit. I adjusted it to 15° @ idle, is making 37-38° fully advanced.

The pinging is 95% gone, only thing is when i floor it in first til 5000 rpm (no pinging) and shift into second then a slight short ping is there. Could it be that the light spring is not able to pull the weight back for less advance? I´ll try the stock spring again, if it does not work i´ll have to go trough it again to adjust less advance.

Michael
 
bOb shingler said:
before you go modifing the slot try a heavier spring in place of the light one. that will slow down your rate of advance and will keep the total in check. shortening the slot is for if you want the total in sooner and you don't need that, it already pings.
Click to expand...

bob knows his stuff... pay close attention.

probably go 35-36° fully advanced. if you have a track day fill with race gas and then run 40° if you want

for just weekend cruising you will be fine without any ping :D
 
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