If we know the motor wants 37 degrees total timing at full throttle, is there
a method to set the timing curve with mechanical and vacuum advance
without risking a knock situtation?

When under part throttle, the vacuum advance will engage, and while in a certain
RPM range, the mechanical will also begin to advance.

If the base timing is set to 10 degrees; the vacuum adds 15 degrees, and the
mechanical advances 12 more that will be 37 degrees at part throttle (if all
circuits happen to be fully advanced).

At wide open throttle, the vacuum is eliminated and the resulting total is 25 degrees.

To have a total of 37, I'd have to set my base (inital) timing to 25 degrees
with a mechanical of 12 degrees.

But...

If the vacuum advance kicks in while the mechanical is fully engaged, too much
timing will be the result (~42-45 total)

My point is simply, with vacuum advance connected, the initial timing must be
considered

IE: 10 base + 15 vacuum + 12 mechanical = 37 total at part throttle, or 25 at W.O.T.

I have always disconnected my vacuum advance to run 25 base + 12 mechanical with a curve kit to bring in all my timing by 3000 RPM.

Is there a better way? Do you recommed connecting the vacuum advance
for better mid and top RPM throttle response?

From what I can see, it's a big pain to tune vacuum and mechanical to hit
your desired WOT advance while preventing an "over advance" issue at part
throttle.

Sorry for the redundancy, I just want to make the question clear.

 

The LT1 computer stock timing curve provides as much as 46-degrees advance under low load "cruise" (high vacuum) conditions - for high fuel economy - then drops to the range of 32-37 degrees at WOT (low vacuum). That would appear consistant with the base-25, mechanical-12, vacuum-10 approach that you dismiss as "too much timing". Obviously, each engine combo is going to be different in terms of required advance, but I wouldn't rule out the base + mech + vacuum combo that you dislike. Perhaps a base-10, mech-25, vacuum-10 approach, depending on how much advance the engine likes at idle, what produces the best fuel economy at cruise, and the max advance that can be tolerated at WOT. Not suggesting these specifically, because I don't know the specifics of your setup - just suggesting a practical approach.

 

This is a Gen 1 L98 Iron motor.

With a dyno tune, and some ignition tweaking we found the full advance to
make best power @ 37 degrees.

The average curve looked best with a mechanical entry of 1200, fully engaged
by 3000 RPM.

I'm not sure how much the engine could take with a vacuum advance connected
and 25 degrees base setting.

By 3000 RPM, if any vacuum advance was to be added in the amount of 5-10
degrees, I was afraid the timing would drop the output power,or even cause
knock (from early ignition).

I was shown to remove the vac. for performance type apps. From all the
engine setups I have seen (referring to track only), there is no sign of vacuum
on the distributor.

IE: Since the pedal is mashed, the vacuum advance doesn't really account
for much on the strip.

I thought this was partially because the base timing was so far advanced to
begin with, the combination of mechanical and vacuum at part throttle would
cause problems.

 

You ALWAYS ALWAYS set initial then mechanical...vacuum advance is last.

get your 12 base and 25 mechanical....do not run any more then 20 degrees base unless you have an aftermarket starter...the factory POS isnt worth its weight in gold starting a hot motor with that much initial timing..

theres 37 degress...Then tune in as much vacuum advance as u can without knock during tip in

 

I have to disagree with those comments.

I'm running 24 degrees initial timing with 13 degree mechanical - no vacuum.
Also using a rebuilt GM starter.

24* initial yielded the best power curve on the dyno using a Moroso curve kit
for my engine.

I don't think it's proper to limit timing to 12 base for every motor.

 

How do you know 42-45* is too much?

Consider that vacuum advance is only present when the engine has vacuum- under light throttle conditions. Light throttle, low-density air-fuel mixes light off more slowly than the dense WOT mixes, and as such, REQUIRE more advance. 45-55* is typical for a Gen I SBC, running at 3000*, with a typical cylinder head. Go toward the low end of that range for a high VE setup, with a fast-burn style combustion chamber (e.g. Vortec), and toward the high end with a more old-school open 76cc head.

Without vacuum advance, you leave part-throttle response and fuel economy on the table. For RACE apps, vac advance isn't used, because the engine is used at basically full throttle, all the time. For STREET apps, 95% of your time is spent is the area where vac advance is useful and prudent. Late timing puts heat down into the cylinder walls, and into the exhaust system, and your headers will likely be several HUNDRED degrees hotter without it, than with it, when simply cruising around. And, you could be leaving upwards of 4mpg on the table, depending on your specific setup.

Todd

 

Back in the "OLD DAYS" we used to adjust timing by loading the motor and advancing the distributor until it pinged. Then backing off until the ping stopped. With an auto power braking, with a manual driving about 20mph and then shifting to fourth and putting it to the floor. But that was a long long time ago.

 

Actually the timing and the cranking compression are the problems. When you get high DCR's then you will also have these problems with the motors. The timing curve IMHO should be set to work with the motor at WOT so the low end might be really high or it might be cut back a ton so the thing doesn't detonate.

Bret

 

I think many of you are overlooking my point.

Specifically with my setup, connecting the vacuum advance while having such
an aggressive timing curve causes some issues.

I'd have to drop my base timing, or alter the mechanical timing curve to add
vacuum advance.

With my springs and weights fully engaged by 3000 RPM, there is 37 degrees
of advance under part throttle and full throttle regardless.

My base timing is high enough that it's more or less accouting for some of the
light load vacuum advance.

Among other tips I have picked up from some of the replies, the fact that
a less dense mixture combusts at a slower rate is a surprise to me.

My thinking was that less mass (less fuel and air) would combust quicker,
which led me to believe less advance would place the peak of the pressure
as the piston approached TDC.

I've also considered the piston speed. If the piston moves from BDC to TDC
at a slower rate @ 2000 RPM, I figured less advance would prevent early
ignition issues (ping,knock,etc.)

As for fuel economy, it's not really a concern, but it would be nice to recover
some mileage while crusing in town. The heat is an interesting point which
has never been a problem, but improvements are always a "plus".

 

First part false, second part true.

Burn rate is affected by density. Lower density, slower burn. Burn rate is NOT substantially affected by piston speed, which is why we have mechanical advance in the first place.

Let's stop speaking in the abstract, and begin speaking of your problem. What is your problem exactly? I cannot decipher it, from what you've written. Vacuum advance might be YOUR problem, but in general, it is NOT a problem to run a small block at 45-55* under light load. So, we'll need to get to the bottom of WHY that is, with yours.

What are you using to add vacuum advance? What can, specifically?

 

If you are looking for your mechanical advance total to be at a higher RPM, then the weight springs need to be stronger. A distributor advance machine would be a big help.

 

I really don't think there's a problem now that I've learned a couple of key points.

I dont' want to bring in timing any later, or it will drop the output power
from 2500-3000 RPM. By using the spring combo on the mechanical advance,
I will able to get best average power and best peak power using a total of 37 degrees
and initial of 24 degrees (mechanical advances by 1200 RPM and full by 3000 RPM).

I guess I'm stuck with the best of ONE world. It's either all for track, or modify
to help out with fuel economy in town.

I'll have to play around with the springs and vacuum, as well as initial timing
for street use.

 

I still don't see an answer.

Are you pinging at 42-45* of timing? Do you KNOW that the timing is at 42-45* when you are pinging (i.e. you've put the light on it), or are you doing math to come up with that number?

Do you have the stock vacuum can?

I'm not trying to bust your *****. I'm really confused. I've never seen an engine that doesn't ping at 37* under WOT, but does at light throttle just 5* more. And, your listed combo on your webpage isn't that unusual.

 

I don't know what the total timing is when it pings. I can only assume its more
than 37 degrees at part throttle @ 3000 RPM.

You are correct, I'm just figuring mathematics knowing it's somewhere in that
range based on timing light measurements with the vacuum connected.

If the manifold pressure changes with load, I can't assume it will be the same
with a timing light by revving the motor in neutral correct?

Even with the vacuum can adjustment in the lowest position, the motor feels weak, and even pings at times (sounds like marbles in a jar under acceleration).

The distributor must have a stock can installed. It's an OEM replacement
with modified cap and internals for MSD.

I'm toying with the idea of connecting the vacuum for street use without
killing my track setup. At this point, I'm having a tough time getting it right
without changing my base timing and timing curve (best setting according to dyno).

 

Now we're getting somewhere.

You likely do have too much timing with the can connected. A typical late-70s-80 can adds a full 22* of advance when vacuum is high. Add that to your total of 37*, and you've got 59*. Too much, for any engine, really. If you rev the engine and hold 3000rpm or so, you'd be able to tell. Light throttle at 3000rpm will give you full advance. It's tough to say where your can is really at, because with that cam, your vacuum may be low enough that the can never pulls all the way in. If your balancer isn't so equipped, put a timing tape on it (strip of tape, has * marks all the way around), and you'll be able to get exact numbers with your light that way (if you don't have a dial-back light).

Here's what you might try- get yourself an adjustable can. Something like the Crane piece. It has two adjustments- one is a hard stop, to limit how much advance the can adds, and the other is the allen adjustment inside the can, that controls when (in terms of vacuum), the can starts pulling off.

The base setting seems awful high for a full throttle setting. Was this an engine dyno, or chassis dyno? I'm guessing the pull was started above 2000rpm, which is why the big initial "looks" right. At that point, much of your timing should be in, anyway, so the big initial might be band-aiding a curve that isn't optimum.
I bring this up because with my T56, my engine spends most of its time under 2000rpm, so I've spent a lot of time tuning the curve in that area.

Todd