Why do we need so much darned initial static timing?

[davepl]

The BBC in my truck wants, according to the tuneup sticker, (I think) 4 degrees initial with an auto and maybe 0 degrees with a manual. But whatever the actual numbers, 4 is the ballpark. And it'll barely run like that.

Over the on Chevelle forums, where there's a bit more big block stuff going on, the consensus seems to be that most are finding 16 degrees initial is required for best idle, and you then wind up pulling out total so you don't overshoot.

If that's true, great. But as much as old timers complain about "today's fuel", I still don't really know the how and why of us now needing so much more advance. It's not like nitro (very slow burning)!

[kwmech]

I've got the big block in my 68 set up with an initial of 4btc. Runs great, does not ping. My small block 400 in my 69 flat bed is set at 6btc and runs as well as the BB, plenty of power and no pinging. If I take the timing up in either truck........pings like hell and does not want to start after it warms up, I get that hard starter spin

[donut]

More initial can give you better throttle response, if your truck tolerates it.
Chevy specs, IMO, were to get the customer through the warranty period and pass emissions.
My smallblock liked 14-16, BB 16-18. In my case, I would get starter backlash before it would ping. (running 87 octane)

[BossHogg69]

My BBC took 18 to idle cleanly with good throttle response. I have a Performance Distributors 'D.U.I.' distributor that they custom curved for me. They recommended 12* initial as a baseline and to go from there. I know that doesn't tell you the 'why' but it is what it is.

[Marv D]

The 'why' is more about the 'RATE OF BURN' in milliseconds and the expanding gasses exerting pressure on the piston for xx° after TDC. (low speed torque and throttle response) and the RATE OF COMPRESSION and the Max PSI of the fuel mix before it detonates.

There is a LOT of flys in the ointment tho...
like,
As RPM's increase you have to initiate the burn earlier, to have the gasses put as much USABLE pressure on the piston, for as many crank degrees possible. As the piston races away from TDC, it will make the effect of the expanding gasses 'less effective'.
Just for example lets say gasoline will take 10milliseconds to do a 80% burn at 150PSI.
And lets say it takes 20 milliseconds for the crank to sweep 90 degrees at 1000rpm
All very dependant on stroke and rod length it may only take 5milliseconds to sweep 90 degrees at 6000rpm. We better start the burn EARLIER to have the best use of the fuel.

Then too it may take WAY longer in a dished piston, large chamber low compression motor to complete a 80% burn, than it will a 12:1 motor.

Not that there is NO pressure after xx°, just that it is not enough to create any really usable torque. i.e the exhaust valve opens on a Comp 284H at 100° (crank degrees) ater TDC,, the pressure is so negligible all it's good for is making a sonic boom in the exhaust pipe,, not any real 'torque'.

The whole 'rate of burn' and how it is effected by compression, and the RATE of COMPRESSION is the science custom cam grinders deal with. If I know all that I'd be selling you cams LOL

[snj8198]

milliseconds? sweep? The? Let's say I get another cup of coffee

[TJ's Chevy]

Set the timing to how the engine runs best and call it good. Factory specs are going to be right for some engines and wrong for others because no engine is built Exactly the same. Some engines might like 8 degrees advanced while others may like 16 degrees advanced. Time it to how it performs the best and call it good, but don't advance it to far or you risk detonation.

[RIDE-RED 350r]

Funny you bring this up...

My truck has a 350 in it. All of the following were installed by the PO.

-Full length headers and 2.5" true duals
-Edelbrock intake
-Edelbrock 1406 carb
-"Mild" cam claimed by PO but I have not confirmed.
-Some sort of fancy red distributor

I was doing some maintenance a couple of weeks ago and found my base timing at 16 deg.

This engine shows the outward signs of an out-of-chassis rebuild not too long ago.

I have had the truck a little over a year and it runs great. Low end seems pretty strong but it seems to really pull best between 2,000 and 3,000. Guess that can be attributed to the smaller CFM carb?? One thing I have learned though, it does prefer 89 octane fuel over 87. With 87 it would spark knock a little bit if I lugged it under 1200 rpm in 4th gear.

I was wondering if I should back the timing down, but I talked to a fellow who is very knowledgeable with this stuff who told me that much timing is pretty common for the way my engine is set up.. So I left it as-is and run 89oct in it.

[b454rat]

Bulb blew in the timing light years ago, never needed it. I set it close, drive it and see how it does. Advance it till it pings, back it off. Hard hot start, same thing. My f'n malibu has a 454, 10:1, iron heads and runs 93. Starts just as easy hot as cold. Doesn't ping, instant throttle, (too much and your sideways) so I'd say it's pretty good.

[Hollow65]

I've never done anything less than 12. I start with 12* and tune from there. I'm currently at 16 initial and 36 all in. Chevelle tech is a great site. Lots of smarts over there also. Ddennis, is super knowledgable.

[cableguy0]

Scratch the factory sticker off of the core support. That sticker is there for emissions purposes. No engine runs best at the factory settings. If it did hot rodding wouldn't exist. The first thing guys did in the old days was reset the timing,recurve the distributor and rejet the carburetor. 16 isn't ridiculous on advance by any stretch.

[crazy longhorn]

Originally Posted by davepl

The BBC in my truck wants, according to the tuneup sticker, (I think) 4 degrees initial with an auto and maybe 0 degrees with a manual. But whatever the actual numbers, 4 is the ballpark. And it'll barely run like that.

Over the on Chevelle forums, where there's a bit more big block stuff going on, the consensus seems to be that most are finding 16 degrees initial is required for best idle, and you then wind up pulling out total so you don't overshoot.

If that's true, great. But as much as old timers complain about "today's fuel", I still don't really know the how and why of us now needing so much more advance. It's not like nitro (very slow burning)!

Forget about the initial...set the total @ around 36 degrees, @ 2800-3200....where does the initial fall? Add 8-10 degrees to the vac can (at the crank)_, & you should be damn close I agree with Marv....after a couple more glasses of bourbon, we would have fun conversation! Honestly, that mill will be running/ pulling max mechanical advance @ 3000 rpms.....on a lite throttle "cruise', you may see 46-50 degrees, with the vac can involved? Figure , that most HEI dist, have about 20 degrees in the mechanical advance....I would add 14 to 16 intial to that. He!! we run 18/38 on the track in little bros truck...no vac advance, cut @ 2600 Longhorn

[Marv D]

Yeah I guess I read too much into Dave saying he was curious of he How and Why of needing significant advance to make our 40 year old motors happy. I do that sometimes LOL

CL would love to have that conversation some time

[crazy longhorn]

Originally Posted by Marv D

Yeah I guess I read too much into Dave saying he was curious of he How and Why of needing significant advance to make our 40 year old motors happy. I do that sometimes LOL

CL would love to have that conversation some time

We will kick er, back/forth, one nite when my head is a little more clear! Trust me, you have raced for many yrs...I trust you, & will bet that I can learn a bunch from you longhorn

[Hollow65]

Do you have a vacuum guage? And vac advance?