Mapping adv curve 300hp 65

[Jim V.]

After a year-long project I finally got the ole-L75 fired up after a complete rebuild. After a little rework on chasing leaks and resetting the distributer I got around to mapping the advance curve.

I took JohnH approach and set initial @ 6 with the weights banded. Then connected the vacuum advance control and popped up to the expected 22 degrees advance. Then went on with the mechanical weights freed-up to map the mechanical advance curve.

It appears the mechanical advance comes in early and finishes short of spec. Here are my results
RPM TotalAdv Mech(tot-22) Spec diff
600 29 7 0 +7
1500 40 18 15 +3
2000 43 21 -- --
2500 43 21 -- --
4100 44 21 26 -5

How can I teak the springs and stop to achieve the spec and ultimately best performance. BTW...my idle vac is around 18 inches.

Thanks

[Timothy B.]

Re: Mapping adv curve 300hp 65

Jim, Looks like the curve stops around 2000RPM. Someone may have installed a larger stop bushing on the mainshaft pin or changed the weight base cam or the distributor may just need to be cleaned/lubed. Probably a good idea after you work the bugs out of your engine rebuild to take the distributor apart and verify the parts within then clean/lube and assemble.

I know you don't want to pull the distributor out after all your work so if you can verify total centrifugal advance at 22 and vacuum advance around 15/16* then set inital timing at 12* with vacuum advance disconnected. You will notice a big improvement in throttle response and engine temp. Just make sure you don't have more than 36* +- total centrifugal advance (this includes the inital timing setting)

[Jim V.]

Re: Mapping adv curve 300hp 65

Thanks Tim! I cleaned up, re-shimmed, and lubed the dist prior to re-assembly. All new small parts including new VAC (ECHVC1802 15inch can). Note that this is a OE 300hp rebuild with OE cam and SCR=9.9:1.

The mech advance stop pin/bushing looked all original, so I didn't mess with it.

Plan1: I understand upping the initial from 6 to 12 would bump my idle advance to 35, my WOT advance to 28, and cruise to 50. I would be all in at around 2000rpm. This may be a bit aggressive for my high dynamic compression ratio so I fear detonation might be a limiting factor.

Plan2: Another option would be to fiddle with the mechanical adv curve...different springs and the mechanical advance limiter bushing. Here I would looking to extend the top end advance from 22 to 26 and raise the starting point from around 500 to 900 rpm. Can anybody recommend where I can source these items?

Hmmmm...pick a curve...plan 1 or 2??

Plan 1 (Dist. as-is with advanced initial from 6 - 12. Note VAC=16 degrees)
Initial=12
Idle=35 (12 Init +16 VAC +7 mech)
WOT=34 (12+0+22)
Cruise=50 (12+16+22 Note: all in at 2000rpm)

Plan2 (extend mech from 22 - 26 & push curve up)
Initial=6
Idle=22 (6+16+0)
WOT=32 (6+0+26)
Cruise=48 (6+16+26 Note: all in at 2500rpm)

Thanks

[William C.]

Re: Mapping adv curve 300hp 65

Plan 3, use stronger springs to slow the curve to all in at about 3000, move the initial timing to 10-12 degrees, and be very happy with the result.

[Duke W.]

Re: Mapping adv curve 300hp 65

What Bill said...

The OE centrifugal curve is 0@750, 26@4100, so something is amiss with the total in addition to the curve being way too aggressive.

The good starting point curve for a 300 HP engine is 0@700, 28@3000 with initial timing in the 6-10 range, whatever it will take without detonation.

You should test the centrifugal curve with the VAC disconnected to ensure that it has zero influence.

Total cruise timing is not critical. What's important is to get a combination of initial and centrifugal that's aggressive enough to be on the ragged edge of detonation at WOT, all engine speeds.

Total cruise timing will usually end up at something between 44 and 54 somewhere in the range of 2500-3500. Anything in or near this range is okay.

Mr. Gasket and Moroso make spring kits that are available from the usual hot rod parts vendors. Don't use the weights - just the springs in some combination to achieve the above.

Duke

[Jim V.]

Re: Mapping adv curve 300hp 65

Thanks Duke. Just ordered up the spring Mr. G. spring kit from Summit. I understand you can mix different spring weight to dial-in the curve.

I just installed some L79 springs I had and bumped the initial to 8. This resolved the idle early-in problem but the curve may still be too aggressive.

0@600 matches OE spec
21@1500 versus the OE spec of 15@1500
24@4100 versus the OE spec of 26@4100

[Duke W.]

Re: Mapping adv curve 300hp 65

Those kits usually have three sets of springs, but I would only mix two different springs that are next to each other in force, not a lightest with a heaviest, so you have five combinations to choose from.

light-light
light- medium
medium-medium
medium- heavy
heavy-heavy

I suspect the springs you found in your distributor are not OE.

My recommendation is to file the slot to achieve 28 degrees. You can measure 14 deg. with a protractor or make a 14 degree template as an aid to filing the correct amount.

You should also check if the weights are OE. Most GM weights have a three digit number stamped on them, but I'm not sure what your year/engine should have. I am also supect of the entire auto-cam assembly - whether it has the OE parts; 0@600 with 21@1500 and 24@4100 is a very odd advance curve.

Duke

[Brian Monticello]

Re: Mapping adv curve 300hp 65

Something to think about... the aftermarket weights on my distributor had an odd shape that prevented them for fully returning which limited my total advance range. We put on new properly shaped weights and the advance range increased. My initial inclination was to file out the slot - glad I didn't do this.

brian

[Clem Z.]

a lot of after market weight are not hardened

and wear away causing a wear problem with the fixed advance plate on the shaft. hit them with a file to see if they are hard and if they are not dump them. i have found a quick way to speed up the advance curve is to remove one spring and put a "E" ring on the weight post to prevent the weight from coming off

[Jim V.]

Springs.....

Just got the Mr.Gasket 928G springs....As you can see the ends are off-center whereas the OE are on the center line.....

[Jim V.]

Thanks Clem...I suspect their not hardened

for the five bucks I spent. I wish I was trying to move up my curve I would try the e-clip approach. Wouldn't ya think a single spring would cause a wobble or out-of-balance situation and result in excessive shaft bushing wear?

In mycase my curve is too quick. Very odd given I tested with the original L75 springs and a set of GM Restoration parts L79 springs. Both tested with too aggressive curves...basically both were practically all-in around 2500 rpm.

Turns out the springs are in order of resistance Black, Silver, Gold. I will test various combinations and report back in a new thread.

[Clem Z.]

Re: Springs.....

they make them the cheapest way.