Firstgens.com

I know the basics for setting timing, but I want as much as I can get without hurting it.
Last year a very experienced friend of mine set my timing by ear. The throttle response was instant. The power was great. But another friend I trust as well said he could hear some detonation when I was getting happy with my right foot. ( I did not hear it) So I backed the initial off a few degrees to be safe. It still has nads but not what it did. I have a Flowmaster American Thunder exhaust so it is pretty loud. I have a fully machined 4 bolt main, forged crank and rods with hypereutectic pistons at 9.75 to 1 with Performer RPM everything. Everything on the engine has less than 2000 miles on it except the carb. What do I listen for or look for to tell I am pushing it too hard? I have the most common gauges, oil pressure, water temp, trans temp, alt., tach etc. Do I just set it for a set number at 3k, say 38-40 degrees? Knock sensors and the like are not in the buget this year. Lot's of experience here, what can you guys tell me?

When one has the.opitum adv at a given rpm and load the car goes well and also economically
as one starts to retard, the power and economy drops off very quick and noticable
When on advances above opitmal, the power drops of very slowly, but the sound of the engine changes into a more deeper 'powerful' tone....this tone effect is so strong it gives the car a more powerful 'feel'

The VA does throw a spanner into the above...so take the above at WOT when the VA is not operatable

At cruise and light load when the VA is operating..this is when the most damage over time is done to the engine....not just detonation, but at cruise
If there is too much advance either in cent and/or VA at a given rpm, what is happening is the pistons are still coming up at the piont of max explosion, which loads the piston, gudgens, rod, bearings, crank.

This effect can be seen historically in Petrol converted LPG and CNG engines
These require a curve to come off real fast because of the hi octane, but the total advance very much lower than petrol.....installers would advance the intial so the custome feels very little diff in power in lower rpm ranges, which advances the whole curve up
So instead of So lets take a basically stock petrol engine, depending on final ratios weight of the car, cam CR etc etc total advance can be anywhere from 40 degrees to low 50s at cruise.
LPG is generally between 27 and 38
So the LPG installer ups the intal from say 8 degs to say 16 degs without changing the springs or amount of advance and the car cruises down the highway antwhere between 48 and 58 degs
WHEN it should be over 20 degs lower!!!!!!
Then engine failure is blamed on the fuel...

When an engine is over advanced one gets increases in NOx emmissions...
Ideally an engine needs to run lean, which requires more advance which increases NOx ...hence why monitored engines now require cats.

The simplest way to establish your all in under load (unmonitored carb engines) is measure the NOx emissions and or do several runs over a measured distance....start at a piont under advanced .....one will see where there is a sudden change down in times then a very slow increase
Then decrease advance down till times drop off quick
Then use that data to establish the piont where advance decreaes and times decrease.

Another rule of thumb is if one can just hear detonation at a given rpms..be it at cruise or WOT drop that piont back 3 or 4 degrees...

So lets say under very lite load one hears a detonation at 2600 to 2800 rpms..
this may mean the VA has to much advance in it or it is working in the wrong vaccuum range
OR the cent curve is coming off a biit to fast or the intial is too high.

A dizzy curve is made up of
cent the rate the weights come off and the amount of advance in the mechanisum
PLUS
VA which has the amount of degs in it AND the engine vac it operates under.
Thats 4 variables.

Don, the problem with what your friend did (the first one) is all he did was twist the distributor.

The additional timing at low rpm was exactly what the engine needed and wanted, but

He also increased the timing under any and all other imaginable conditions.

You need to map your timing. A degreed balancer (or timing tape) and a pointer (or just the zero mark on your timing tab) Disconnect the vacuum advance, and check the timing at 500 rpm intervals from idle to 4000 rpm and write em down or graph em on graph paper.

Then do the same thing with your vacuum can, you can do this at idle, using a mitivac to supply varying amounts of vacuum.

Then just analyze the curves.

Remember:
No SBC wants more than 36 degrees max advance (not counting vacuum) and many want less, as little as 32 perhaps.
These days, we want that at 3000-3500 rpm, not before. it ain't the 60's and we don't got no sumoco 260 gas.

Optimum at idle will be around 18. It might sound better at 24, but like steps said, it's just noise.

So, you get an "advance curve kit" from summit or jegs that has weights and springs and limiters for your distributor, and you set this up.

And then you plug your vacuum advance back in, to full manifold vacuum, and the motor wakes up and comes alive and is happy happy.

Steps that's a bit better than I have heard it explained before. Jim I like the simple plan to get it very close in the garage. I have a degreed balancer but I made a bone head mistake and tried to quickly clean an oil spill off with carb cleaner, cleaned the higher painted degree marks off as well. But I have a dial back light so I'll use that. Then when I can get her out when the salt is gone I can try Steps "real world" method.
Now I have to figure out where I put that spring kit 6 or 7 years ago. I'm off next Saturday (first in 9 days) and this sounds like a fun way to spend it! Anyone up for some cold ones and a few hours in the garage?

I degree my own balancer...
With a taylors tape measure the circum use mm metric
Divide by 360 and thats the distance per deg
Then mark off each five and 10 deg and stamp a L for 10 degs a dot for 5 degs measuye out to at least 45 deg.
Then wipe with some white paint,,,the paint getcaught in the stamps.

Always measure from the 0 for each mark, do not measure from the last mark to the next.

Maping Cent
As Jim says above.
maping VA pull the top off the cizzy put your fingure on the VA arm and with a hand vac pump ( you guys must call it a mitivac) and feel at what piont it starts to move and point it stops at in vac
this is the VA vac range
Then idle engine at 1000 rpms no vac note the advance....pull full vaccuum on the pump and reset idle to 1000 rpms and note the advance
this is the VA advance range.

Higher the octane of the fuel the more advance needed (higher octane means a slower flash across the chamber.)
Higher the Compression the less advance needed...now this depends very much on the cam
A low duration /overlap cam (stock) increases dynamic compression at low rpms
A 'performace' cam is designed to work in high rpms and creates low DCR at low rpms

So setting a curve is very much a balancing act between these variables

To change the amount of cent degs in a pionts or GM hei
increase ...one files a round about 1 to 2 mm off the tail of the counter weight
caution file too much and the counter weightwill fall off the plate and stick open...this can be fixed if the round plates the weights slide on are replaced with home made larger ones epoxed onto the mechanism
To limit..decrease cent advance....on the plate there is a small hole...tap ..thread the hole, then use a small screw and shape the head into a cam...the position of the screw cam can be adjusted to what is rewuired and locktited into place

For tuning in the VA you need to get familar with the vac range your engine rans at on the street with an in car vaccuum gauge Ted off the VA.
But the Cent has to be tuned in 1st.
know what the engine vac is at ilde, at 30 mph cruise at 30 mph light load..up a slope....at 60 mph cruise and 60 mph at up a slope...no VA
So lets say you have an idle vac of 18"
Cruise at 30 mph of 18" and on the slope 15"
Cruise at 60 mph 22" on the slope 16"
the VA hast to be all in at idle (if working off manifold vac)
and part in at 15"
So the Vac range would be something like 12" to 16"

I disagree with Jim on the high imtials at ildle this should be set with an exhaust anyliser measureing NOx
And a balance between a stable idle...and also adjusting the mixture screws and sec butterflys.
Also too much advance..and I refer back to the LPG above and installers upping the intial for performace, causes extra loads on the starter motor, this heats the soldered joints in the armeture causing dry joints...when the engine is cold the starter works fine...when warm from the latent engine heat the starter motor just clicks till it cools down...
Dry joints can be checked by using a growler on the armeture...IF you can find a auto shop that has one these days.

Pre emiisions dizzys (early 70s) tend to have more cent advance in them, post less, and post VA are calibrated for ported vac not full manifold.

I mapped my curve with the help of my lovely assistant Chris and I have 15 @1000 25 @ 1500 30 @ 2000 32 @ 2500 33 @ 3000 and 35 @ 3500. With the va hooked up I have 46 @ 1000. Looks good to me. Maybe 1 or 2 more initial. Any opinions?
Performer RPM everything, air gap manifold, hopped up HEI, 650 Holley vacuum secondaries.

OK lets compare the cent to an LPG curve..which comes off far faster than a petrol engine and less all in.
Typical LPG 10 @ 1000 25@ 2500 32@ 2800 to 3000
And add another 6 to 8 degs for VA.

To get a good LPG curve on a pionts or HEI GM dizzy it is touch and go with light springs as to having the weights 'bouncing' at idle.

Put the above over your graph...since LPG has a faster curve than petrol where does that put your curve?

So Step you think I'm coming in too fast? Or am I reading you wrong? She seems to be happy, maybe want more even.

Which springs do you have in the dizzy now Don and what options do you have beyond those?

Based on your map it increases 10* in the first 500rpm over your 1000rpm idle speed and then another 10* over the next 2000rpm. Starts out fast on the slows down. Your total increase due to mechanical advance = 20* which is not too bad which is confirmed by your intial of 15* and your all in at 35*. I think many out there like to see the total all in by 3000 for performance apps but every engine combo has different needs.

Unless your having problems with detonation I think your pretty good where you are for the street. When you go to the track and if you run some higher octane fuel you could work the timing for a little more initial which of course will increase your total.....the 1-2* you mentioned would be good.

LPG has a almost straight up to all in
Petrol is more like top left 1/4 end of an elipse

Think of it like this you dont want to be cruising down the road say 2500 rpms at 60 mph with the VA all in and the cent to far advanced...you will not here or feel the detonation with no load, and those pistons will still be coming up and firing before they hit TDC.
It is this that kills more engines slowly than anything else at cruise.

Sure good reasonably quick andvance , so long as it is not too quick she pulls well, but at the end of the day 99% of the engines work is cruising down the road.