My battles with timing continue...I've got a Comp Cams XE274, and have experimented with a range (14-18 BTDC) of initial timing settings using a timing light.

Funny thing is, when I set the initial according to the vacuum gauge (max vac), or where the engine "sounds" best, the initial is more like 25-30 BTDC. Could this be right? I'm @ 850 RPMs, with vacuum advance disconnected. I'm just about ready to replace this HEI.

Let me know if you need more info.

You can't set initial timing with a vacuum gauge. You'll wind up with WAY too much advance unless you're running a locked-out distributor. Sure, the RPM and vacuum goes up with more advance. Then you have problems getting a low enough idle speed.

You need "about" 35 degrees maximum advance all in at "about" 3000 RPM.

Add "about" 10-15 degrees of vacuum advance. Use ported or manifold vacuum, whichever works best for your engine. (Probably manifold.)

Your initial timing needs to be "around" the 5-20 degree range, depending on the amount of total advance built into the distributor.

All of these figures are ballpark estimates, feel free to experiment.

Yep...
It will probably like 25-30 timing with the vacuum advance hooked up.

Hook up the vacuum advance, set engine for best idle, maybe back it off 2 degrees from there.

No unplug the vac advance. Take idle measurement.

This will tell you how much the vacuum advance is pulling it around at idle..

Now rev to 3000 rpm, observe how far it is off from desired total, i.e probably ~36 degrees. Good place to start anyway.

You may have to limit the mechanical advance weights to bring the mechanical advance to where you need it.

If you rev to 3000, set it at 36* with no vacuum advance, then plug the vacuum advance to manifold vacuum, it will get you close anyway, and you can tune from there.

I had to ressurrect this one from the depths. I am having a similar issue running the same XE274 cam in my 402. I am running an MSD distributor and 6AL and i have no vacuum advance... all mechanical. When I set the base timing to what it should be, the car idles "ok", but the more timing I give it the better it idles and more vacuum. I have been having issues with my power brakes and have had to install a vacuum pump running the car with 14-16 degrees of initial timing. Any help would be great.

With your cam 18deg base/18 deg mech = approx 36 total is good starting point for dialing in the ign timing curve.

Make sure motor is idling low like 600-650 rpm when setting base timing to ensure mech adv in dist isnt partially activated upsetting base timing setting. Vac adv needs tro also be plugged when setting/checking base/total timing.

If total goes over 36-38deg when reving above 3k rpms you need to get the mech adv in dist recurved for 18 deg all in by 2800-3k rpm ,then 18deg base + 18deg mech in dist = 36 total.

But dont just retartd the total timing if its over 38deg above 3k rpm to get it back down to 36-38 deg. Doing that will also retard the base timing 1 deg for every deg you retartd the total and that really hurts lower to mid range rpm perf/power/throttle resp/fuel economy. Thats why you need to have the mech adv in the dist recurved if the total is over 38 deg when reving the motor over 3k rpm when running 18 base timing.

Also run a vac adv thats fully activated at idle that gives you an additional 12-14 deg timing for better throttle resp & fuel economy when at low load part throttle cruise.

You will likely need to run 93 fuel with this perf ign curve & listen for ping .if you hear it back off the base timing a couple/2 deg to hepefully get rid of it.

Scott

I am definitely running 92 octane (highest we have around here) already. I guess I should really look into a vacuum advanced distributor that will work with my MSD box. Any suggestions?

I was hoping to be able to remove that vacuum pump for my brakes because it is loud and annoying, but it looks like that will need to stay from what I am reading.

BTW love those KZs!

36° total timing unless it is a large dome high compression motor, if so it could require up to 40° timing on race fuel and large piston domes

I wouldn't predict you would need a PB vac pump with a 274 in 400 inch engine. I recommend you fall back and regroup on the tune-up stuff. Do the drill on verifying TDC is correct on the timing marks. if you're using a dial back timing light try a different one. Some dial-back lights don't get along with MSD, especially at idle. If you can run 16 - 18 initial you shouldn't be having such low vacuum that the PB won't work. Were the PB tee'd off the PCV? Some other vacuum leak? Is the cam maybe installed retarded? Something is goofy about this deal somewhere.

I had a 67 396/325 HP that was all stock other than a 240/246 @.050 .517/.530 114 LSA Crower solid lifter cam,RPM intake,HEI and headers. For some reason,it loved 40* total..and it ran its best ET and MPH with that total (12.45 @ 108 and change)
We even checked the balancer out with one of those TDC tools that screw into the sparkplug hole.

I run 30* idle timing (16 base +14 vacuum). Any less it runs like @ss.

My 427 loves all the initial advance I can give it.....until it becomes difficult to turn over:sad:. Some tips by Scott and the initial number is now about 14 deg with 20 mech for street driving. When I am feeling naughty and have better fuel or mix...,,,,16-18 deg initial.
That 4 degs makes all the difference in the world so far as idle firmness, crispness, power. When I pull the distrib again, I'll yet again remove a bit of mech adv and put a bit more adv into the initial

Food for thought...

20-some odd years back I ran a 400 SB in a `75 Monza..186 heads, factory iron intake and a QJet, Crane HMV278 hydraulic cam, re-curved factory HEI distributor. Worked pretty well for no more than what it was, ran consistent low 13s at 105-ish through a stock converter, a dinky 2 1/4" dual exhaust and a 2.93 gear in the back.

During a T&T session at the track I decided to experiment with initial timing settings and advance curves. I baselined the car with 12° initial and 36° total all in by 4000 rpm. The only change I made was swapping the advance weights and springs to allow 16° initial and 36° total all in by 3000 rpm. Earlier "butt dyno" testing seemed to indicate the engine was much more responsive with the higher initial/quicker total and idled much nicer, so I wanted to see what it was worth at the track. I was rather surprised by the results:

The additional initial and quicker total actually slowed the car down by a full 4 tenths and 3 mph. Traction was not an issue, the car never spun the tires thanks to the stock torque converter.

I swapped the heavier springs and previous weights back in and the .4 and 3 mph came right back.

Just because it idled better and seemed more responsive didn't translate to better real-world performance.

My current 496 project uses a cam with 256°/264° @ .050, and it idles perfectly at 900 rpm with a mere 14° initial timing. Throttle response is razor-sharp as well.

Suffice it to say I'm not a subscriber to the theory of lots of initial timing unless it's a very serious combination, in which case a locked out distributor should be SOP.

Just my personal opinion based on my experiences, your results may vary. :thumbsup:

Eric

[QUOTE=Vintage Musclecar;1661312
Just because it idled better and seemed more responsive didn't translate to better real-world performance.

[/QUOTE]

Back when I was young and dumb (er), I found out the same thing. I was running ny `67 SS every week at the grudges. Playing with the timing, it felt stronger each time I gave it more advance. I was sure that my next runs would be my quickest yet but when I got to the strip I was almost .3 slower. Whem I took out most of what I'd added and my ets went back to normal.

Alot of your total timing can be be figured upon what combustion chambers and how big of a dome. 36°-38° for most but with our huge dome pistons the engine runs quicker at 42° advance.

The larger the dome the more timing you need 99% of the time

I had a 67 396/325 HP that was all stock other than a 240/246 @.050 .517/.530 114 LSA Crower solid lifter cam,RPM intake,HEI and headers. For some reason,it loved 40* total..and it ran its best ET and MPH with that total (12.45 @ 108 and change)
We even checked the balancer out with one of those TDC tools that screw into the sparkplug hole.

According to my MOTOR's manual, that motor called for :

4* initial @500rpm hot idle (700rpm depending on model)
32* mech adv starting @900rpm, all in by 5000rpm
Plus 8* @15" vacuum (vac advance)

Seems retarded all around no?

Incidentally, the 1965 396/325 motor (full size) in comparison is:

4* initial @500rpm hot idle
26* mech adv starting @600rpm, all in by 4100rpm
Plus 12* @13" vacuum (vac advance)

why two different ways to get 325hp?:confused:

I set mine to what SWHEATON said to and it was better, I then but total at 42* at 3000 rpm and it runs like a top, now for a new carb, I hate double pumpers.

I agree with Tom. The XE series of cams ramps are ground to produce more vacuum than say a Magnum. That cam has 230/236 @.50 so it should be plenty fine to run power brakes. I know its not comparing apples to apples but that cam in a 327 gives 14" of vacuum in my buddies 65 Elky and the small block version has a tad bit more duration.

According to my MOTOR's manual, that motor called for :

4* initial @500rpm hot idle (700rpm depending on model)
32* mech adv starting @900rpm, all in by 5000rpm
Plus 8* @15" vacuum (vac advance)

Seems retarded all around no?

Incidentally, the 1965 396/325 motor (full size) in comparison is:

4* initial @500rpm hot idle
26* mech adv starting @600rpm, all in by 4100rpm
Plus 12* @13" vacuum (vac advance)

why two different ways to get 325hp?:confused:
This is the situation I've encountered with my 66 396, 325 h.p. Anyone have any input?

Those stated timing specs are for stock gm factory cams only,any aftermarket per cam (even mild RV type cam) will generally like approx 16-18 deg base timing (36-38 total) because they have considerably more duration them stock gm camsdid in general.

Scott

I've been having the same issues, no vacuum, no PB, stalls when I put it in gear... I'm wondering if my ancient sears timing light is reading the #1 correctly as I have a 6a box, supercoil and msd distributor... According to my light I have 50 degrees of initial advance!