calwldlife

first, i have a rudimentary grasp of what, why, how
a bump stick can and cannot improve performance.
mostly thanks to members here.

a have some practical experience but nothing
on the level of members here.

one thing that is curious is base timing.
i get the base+centrifugal+vacuum total timing

what i would like to know is what aspect of the cam
determines this base timing
taking that the cam is installed straight up, no adv or ret
on the timing set,
seems the dist curve and vacuum throw of the original
dist setup controls the base timing?

given that dist weights and vac pods can be changed
just where and what cam aspect will determine this?

stock gm 350s are anywhere from 0 deg timing up to 14 adv.

because of smog laws, i am locked into 6 deg plus or minus 2 deg.
this is half of what a performance motor is usually set to.
12-14 being common?

hope this thread helps others.
thanks

corvgreg

On a cam there is lobe separation, the higher numbers help the idle, such as 112 or 114 degrees. Duration @.050 will set the valve overlap and the rpm powet range will move higher as overlap increases. Lift is total valve opening for flow.

427Hotrod

Advancing the timing increases cylinder pressure. You start the mixture burning earlier. Higher performance cams typically have longer duration which tends to reduce cylinder pressure and vacuum at low speeds until RPM is up and the cylinders get fully filled. So advancing the ignition timing helps build pressure/power/throttle response etc. Just about everything improves as long as you don't go too far. Race engines will lock the timing at say 36* even at idle. I routinely idle and run low speeds at 25-36* depending on the engine. Many engines have vacuum advance at idle and light throttle that might throw 10-20* on top of the base timing with non-ported vacuum.

That low timing reduces cylinder pressure/heat which helps some aspects of emissions. As soon as you pass your smog test no reason you can't crank it up so it will run much better.

JIM

REELAV8R

It has to do with cylinder pressure, intake reversion and charge contamination, and how that affects fuel burn for idle conditions which is what initial timing is primarily for.

As the overlap period increases either by way of LSA or duration or valve lift the cylinder pressure below peak torque rpm drops. intake reversion also increases as does charge contamination below peak torque.

As these factors are increased the fuel burns less effeciently and more slowly and peak cylinder pressure is reduced. This being the case it takes more ignition lead to get the fuel to burn adequately to produce torque. Intake reversion and charge contamination won’t change but more of the fuel will be burned to produce power due to there being enough time for it to burn.

So at idle as the cylinder pressure is dropped via the cam specs then the fuel burns less effeciently giving you a rougher idle and a slower rate of acceleration if insufficient timing is not available to get peak cylinder pressure somewhere around 12* ATDC. If it occurs before or after this point torque is lost.

cardo0

Discussing a camshaft is one piece of equipment itself but ignition timing is controlled by the distributor another equipment piece. Now you want to optimize ignition timing with camshaft parameters somehow? No thats not how it works. I think you are confused with what each of those do. You can read the features of each of a cam specifications nearly everywhere and no sense regurgitating it here. And you can set the timing to what ever you what to control the engine but most will try and optimize it for performance and the modern way that is done is on a dynomometer.

I guess what Im saying is there is no mathematical formula for relating camshaft parameters (like duration and lift and lobe centerline) to optimize ignition timing. Its done empirically by testing.

resdoggie

Hot rodders throw away GM timing specs because they are meant more for emissions and not so much for performance. I've found that the guys with the high HP engines are running initial timing north of 18* and total timing of 32-34* w/alum heads and ~36* w/iron all in at ~3000 rpm. Vacuum advance is cruising speeds and that's why some guys don't bother with it - not concerned with fuel economy and mostly a track car. The trick is finding the base timing for the engine build. A vacuum gauge will establish the initial timing or get it pretty close to optimum. A modded engine will be in the upper teens or higher. Now the problem is reducing, depending upon initial, the mechanical advance for a total of, lets say 32*, at 3000 rpm. Best, easiest and most accurate way of creating a timing curve is with a programmable ignition system. Create whatever curve works for your engine combo and get about 95% of the engines performance potential without spending money for dyno time. And your cam and compression has everything to do with initial timing.

stingr69

The cam profile determines where the torque will be in the rpm range.

The ignition timing is adjusted to maximize performance for a given combination at RPM below the peak torque. Start the fire early so by the time the explosion is fully developed, your piston is ready in position to put the pressure to good use. You want the max cylinder pressure that you can get at any given RPM without creating high pressure when the piston is coming up before top dead center. That condition works against you and can cause detonation/engine damage.

Vacuum advance is ignored for most competition applications because it is only used under part throttle conditions. The general guideline is Initial + Centrifugal add up to about 36 degrees for a old school combustion chamber and a little less at say 32 degrees for a modern style, faster burn combustion chamber generally works best. If your engine tolerates/needs more ignition advance at idle, you modify the mechanical advance mechanism in the distributor to reduce the available centrifugal advance. Typically, the need to reduce advance is pretty much gone by the time you get to around 3000 RPM so you want to modify the curve to provide the max centrifugal + initial from about 3000 RPM and up.

Best way to set your ignition timing is to remove and plug the vacuum advance, hook up the timing light and rev the engine up until the timing advance stops advancing. Set it to 36 (or your target) and lock it down. Hook up the vacuum advance and see how it runs. You can reduce the centrifugal advance from there if you need more initial. You will need a dial back timing light or if you do not have that, you can make a 36 degrees mark on the balancer yourself. Wrap a fabric sewing tape measure (or a string) around the balancer and measure the circumference. 1/10th of the dimension is 36 degrees on the balancer. Mark it with a sharpie and use that mark to set the timing.

calwldlife

interesting,
i appreciate the feedback.
not sure what i am after except
everything
you know;
100 mpg
1000 hp
1000 tq
idles like it ain't running
vacuum to suck the stink off crap
usual stuff

i am guessin a cam maker could grind in some
advance to offset the low initial setting need?

stingr69

What you want is not a cam You just want a bigger engine.

calwldlife

yup
more more more

just trying to get more info and knowledge.
maybe help some others with the bump stick
and what to set a motor to when
going away from stock.

jim-81

That was sort of what I got out of the question. You are talking about two different types of 'timimg'. Ignition (spark) timing and cam (valve events) timing. Not sure if I have anything else to add but I was sort of confused with the question.

calwldlife

yes, confusing/ why i brought it up.
yes, spark timing and valves timing.

but, cam lift and duration along with comp ratio
is not a one size fits all when spark time is
factored in.

dyno? yeah ok.

with all the depth of knowledge about cams and effect on power when comp ratio
come into it, seems an adjustment to where timing spark
produces best burn.

like i said, a guy has say a L48 and wants a better cam.
after the , need more CR, need better flow, need better
duration, the owner is then generally told to
go for 36 deg all in by 3k rpm.
then has to do math for base timing.

while a dyno would tell the story, not all of us have the money.
seems enough other people have dynoed or at least companies making
cams could dyno to make a specific base time, centrifugal by rpm and then vacuum level for choosing vac adv can.

so this guy with a L48 has a stock timing of say 4 deg adv.
never seen a cam come with what the grinder states is now base timing.
depends on may things.

and when a state smog guy says, no no no=you are locked in a 6 deg.

anywho. some answers where close to what i am after.

stingr69

Cam timing sets RPM range of torque peak. You need compression for efficiency (power). Ignition timing modifications are like fine tuning your setup for operation below peak torque. Fuel quality is the limiting factor. Recommendations come from experience. Any more than that and you will need to reinvent the otto cycle.

You move the torque peak around the RPM range mostly by changing the cam (or cam timing). The combustion efficiency goes down at low RPM when you move the torque higher in the RPM range - There aint no free lunch. Less efficient combustion at idle means you want to try to start the fire earlier in the combustion cycle so the engine will run better. That requires the spark timing to occur sooner at idle than it did before you made changes. The higher RPM cammed engine will tolerate (and enjoy) an earlier spark now but only at low RPM. The ignition timing requirement at the peak torque is not affected much because it is still closer to optimal efficiency at that (torque peak) RPM.

gkull

what state are you in that a smog person actually checks your car with a timing light

jim-81

The more I learn about cams the more I realize I don't know. Aint that true about most things! Just recently I learned about the correlation between duration and compression. Someone posted a link to a table of general recommendations between them. And the light went on. That was sort of the missing link for me as far as where to start when spec'ing a cam.

gkull

http://www.crankshaftcoalition.com/w..._compatibility

jim-81

That's it! Lots of good stuff in there.

Big2Bird

California.

calwldlife

looking for some of that experience
california.
but the info to help get efficiency for low cr motors should be the same.
yup, thus the thrad for some info for us that need more understanding
nice.

which will bring me back to the amount of spark time and
how to increase per engine rpm.

seems there has always been the "dist recurve" guru around
that people just had over the specs to.

MelWff

There seems to be a lot of confusion in this thread by the OP between cam timing and ignition timing. So dealing purely with ignition timing, you set your mechanical advance in the distributor to 36 degrees total since you have iron heads and it should hit 36 degrees no higher than 3,000 rpm. With the spring kits comes a brass advance stop that you must install to replace the missing/broken stock advance stop. You make note of what the initial timing is after you have obtained the total timing of 36 degrees. When you go for inspection, assuming they actually check with a light, retard the initial timing back to 6 degrees or whatever they want. After the inspection reset the initial timing back to the number you noted when setting the total timing. You will also need to make sure you are using a vacuum advance can that only gives you an additional 10 to 12 degrees.