Macadoo

Yeah, I know, another thread about timing. But what I'm wondering is; where do you folks land on the initial vs mechanical percentage issue. Why do some guys lock out their mechanical (racers) and have it all at initial? Why is it better to have as much initial as you can, without the starter kicking back? I just read an article that said 24-26* initial makes for a killer street engine. My 455 would never turn over with that setting, and it would only leave me with 6-8* of curve (manipulating the total in the distributor).
If I understand the flame-front, combustion chamber shape, spark location equation (which I probably don't, really), then taking out most of the mechanical (moving it to initial) also removes most of the timing curve, yes? How is that a benefit?

A while back I wanted to check my steady cruise state vacuum numbers and had the wife text them to me as I read them off. But then I got busy and forgot about them. Looking back through the texts it looks like I'm pulling 35-40" at around 2k rpm steady cruise. So all the vac can advance is in play with a little mechanical thrown in. I need to get all these actual numbers, I realize, but that sounds like a lot of timing. I think I have 8* in my adjustable can right now. So at lower rpms, wouldn't we need less timing, not more?

My 455 does seem to like a lot of initial for idle purposes. Right now I'm at 19 initial and 34* all in by 3k (I need to verify the actual rpm) but I'm still getting some detonation at WOT so I need to back that down to 32*. Do I take it out of initial or mechanical?

So, to sum up; what is the advantage of having the most initial advance the starter can handle and does that have a detrimental effect at increasing RPMs because of the lack of curve?

oldcutlass

That can't be, the maximum vacuum theoretically is 29.5"hg. There is no way your pulling 35-40. Have you gotten an A/F sensor and gauge?

cutlassefi

So you’ve verified that your air/fuel ratio is fine at that point?

Rocketbrian

following

Macadoo

No AF gauge yet. Still saving up the $$. And yes, Mark, I haven't made any changes to the car since that last thread. All I have time for right now is working things out on paper. But before I get back at the carb, I want to make sure my ignition is copacetic.
So the wife didn't know how to read the gauge. Not her fault, I didn't really show her. But anything over 7 or 8*, I'd be pulling in the vac advance. Any opinions on initial vs mechanical balance? Or is it just about idle quality?

Run to Rund

The factory brought in the timing by 4200 for stick W30s and around 3000 for autos in some years. Even with a very big cam, I found 12 deg initial to be best with a quick curve for a total of 36 at 2400. The curve does start coming in early, so at 1000 the total mechanical advance is about 20 deg. Those who lock the timing at 36 generally use an ignition box like MSD that puts in 25 deg. retard for starting.

Vintage Chief

(1) To be 100% absolutely clear on one specific item. One atmosphere @ sea level @ standard ambient temperature of 59*F (15*C) = 29.92" Hg (14.7 psi). This is not a theorem - it is fact.
It is impossible to have a vacuum higher than one atmosphere [29.92" Hg] (@ sea level @ 59*F).
(2) An engine does not pull a vacuum. The vacuum measured on an engine is the difference between one atmosphere (29.92" Hg) and the intake manifold's air pressure (stated differently - the lesser of one atmosphere of pressure). Therefore, if you read 19.92" Hg on a vacuum gauge, you have a reading (vacuum) of 10.00" below one atmosphere of pressure (29.92" Hg). When there is ABSOLUTELY no vacuum applied to an engine, you cannot achieve a vacuum higher than one atmosphere of pressure (@ sea level @ 59*F). IMO, vacuum (as it applies to an engine - unless we were to specifically refer to a 'venturi-vacuum' [Bernoulli's Principle]) is an oxymoron or at best a misnomer in the automotive industry.

EDIT: Well, actually I'll sorta/kinda regress at tad. If you were situated in a HIGH PRESSURE system it would be possible to have a slightly higher reading of 29.92" Hg (but for our purposes this is a non-sequitur).

cutlassefi

I understand the financial stuff, really. But you’re asking questions about a scenario that may not hold water. Just wait and get a wideband, then we can all help you work out the rest when needed.

coppercutlass

So here is my take. My car is working good with 14 initial 22 mech and a medium speed curve. IM stuck with that for the time being because my idle is high naturally due to the engine. But it comes in about 1200 and shoots up. Locking out the timing will lean out the AFR at idle. the biggest benefit is the engine will spin a little faster ( which is why guys with higher initial say ( killer engine its more responsive ) but for those guys they get a cleaner idle too., with more timing at idle it helps with engines with big cams with a lot of overlap get a cleaner burn. For me Mac. Im running more cam than you with a small block and im running 14 initial and my idle is fairly clean..... ( will touch back on that later ) It comes down to what the combination wants and what works best. My car isn't loosing out on anything with my timing set up. If I was to guess you engine likes a lot of initial because the idle AFR is rich . More initial leans it up and cleans up the idle and steady's up the RPM naturally, You have a heavy car , with a lot of initial , underload this spikes cylinder pressures as the engine is seeing a very high pressure situation trying to move the car. High cylinder PSI with more initial with a heavy car going to a FAST WOT transition will do that as there is excessive cylinder pressure which = heat and pre ignition with the engine already trying to run ahead of the rest of the car . Now say you have 14 initial and a steadier timing curve coming in by 3k it gives the engine the ability to spin up to par with the rest of the car and not create a lot of heat by it having to fight the engine RPM vs the rest of the car.

Edit:/ad …. Lean = hot so if you fuel mixture is lean and you have high cylinder pressures and bunch of initial timing that don't help anything.

Now here is my story on AFR gauges. Now despite the fact I never changed my stoichiometric settings on the gauge from real gas to 10 percent e blend which I run. the difference isn't much compared to the spectrums im running on. I was targeting 12.5 12.0 I had it set to that and the car ran fairly well. BUT!!!!!! I had only raced the car in really hot weather. ( keep that one in mind) A month or so ago I did some exhaust work and had to cut out my o2 bung. Long story short I didn't get to weld it back in and was hitting the track once more. First pass was DEAD lean I mean back firing out the carb bad under load off the line. Since I had NO AFR . I capped the exhaust. Which fattened up the mixture car ran well. pulled into the pits and went up 4 jet sizes front and rear. Car went back to normal. Went up 2 more jet sizes on the primary and car clicked off damn near the same ET as my fastest. Prior to this my AFR said 12.5 on the big end and all across the board exc. idle to WOT. I haven't put my AFR back on but my idle went from pretty clean to smelling really rich. But my plugs are clean . If I was to guess I would say my AFR is closer to the low 11's but I will find out when I put the o2 bung back in. Why does this matter. I have been tuning all year with an AFR and its GREAT !!! but one of my buddies told me the AFR is not the end all be all. My car ran a 11.89 on an almost 90 degree day because it was actually lean . Keep in mind I did have some carb issues which could have been fattening up my mixture but the only time my AFR ever changed was going from idle to WOT which the issues I found mainly affected. My idle smells PIG RICH but my plugs are clean.

Just my 2 cents and my current experience.

Fun71

Most of those folks have either manual transmissions or a very high stall converter, so their RPM flashes to a very high level (so full mechanical advance) immediately after putting the pedal to the floor. The higher initial timing also helps the idle quality with a large camshaft.

Macadoo

Thanks for the write-up, Copper. and I was actually able to follow it, for the most part, lol. I'll get the gauge as soon as I can and get it installed. And won't do any serious tuning until then. But I still don't understand the theory. As the revs go up, the piston is moving faster. Combustion takes the same amount of time so the spark needs to happen earlier to give the burn the time it needs. I get that. But moving a lot of the timing to initial would start the burn too early at low RPMs so what's the advantage? Is it for guys that go straight from idle to 6k with nothing in between so no need for a curve? Maybe? And the article that said 24-26* initial for a killer street motor? It just doesn't jibe.
As for the AFR gauge, my exhaust isn't welded, just U-bolts after the collectors. Will that effect the accuracy? I think I have some Gun Gum stashed somewhere.

coppercutlass

The sooner the timing the more energy than can be compressed in the cylinder . The benefits of more initial are usually a cleaner burn , better idle with big cams and " crisper throttle response" . The fuel helps cool the cylinder so retarding the burn say from 24 initial to 14 intial gives it that much more time for the fuel to stay in the cylinder prior to igniting and less cyl pressure to an extent as you are compressing that combustion later killing some of that momentum which is why that whole early timing and heavyloads and excessive cylinder pressures tend to detonate with more initial timing under the right circumstances and this is where the AFR comes into play. Keep in mind the sooner the combustion starts the more pressure your build up on the way to TDC so keeping it stable between that oeriod is critical since pressure = heat. Most race cars tend to be on the rich side anything over 12.5 is considered lean or so that's what I have been taught. Big cams used in racing apps where you will find most situations where timing is locked out usually tend to idle rich so the earlier timing leans the idle AFR which is why it works for race engines and most race cars usually have high stalls that let the engine rev up along with numerically taller gears it helps it all move without any hiccups.

That's just what i have gathered up and makes sense to me. I could be wrong.

OLDSter Ralph

[QUOTE=Macadoo;1198393]Thanks for the write-up, Copper. and I was actually able to follow it, for the most part, lol. I'll get the gauge as soon as I can and get it installed. And won't do any serious tuning until then. But I still don't understand the theory. As the revs go up, the piston is moving faster. Combustion takes the same amount of time so the spark needs to happen earlier to give the burn the time it needs. I get that. But moving a lot of the timing to initial would start the burn too early at low RPMs so what's the advantage? Is it for guys that go straight from idle to 6k with nothing in between so no need for a curve? Maybe? And the article that said 24-26* initial for a killer street motor? It just doesn't jibe.
As for the AFR gauge, my exhaust isn't welded, just U-bolts after the collectors. Will that effect the accuracy? I think I have some Gun Gum stashed somewhere.[/QUOTE
Mac, I think you are looking for a simple one sentence answer and a number for a complex set of circumstances. Obviously, if "spark" occurs at "0" the burning air/fuel mixture will will start expanding after the piston has started down the hole.....making very little power. So, some ignition advance is needed.
As RPM's increase, you need more advance to make power. The mechanical advance does this with the use of springs and counter weights. Vacuum advance does this based on engine vacuum, but has no idea what the engine RPM's are.
MY simple opinion: To much initial timing at idle will cause the air/fuel mixture to expand to the max while the piston is still coming up ? How much is "to much initial timing" ? Shorter duration/smaller overlap cams can't tolerate to much over advance at low RPM's. Longer duration/larger valve open overlap can tolerate more "over advance" at low RPM's. This is assuming you have the right air/fuel mixture.
As suggested by Cutlassefi, get a wideband ! Until you get the A/F ratio right, you'll be chasing your tail in circles.
.........Just my two cents worth.

cutlassefi

Advancing the idle timing past best vacuum is useless, and potentially damaging.
Anyone want to guess at what point of rotation max cyl pressure occurs on a properly tuned piston engine? I’ll tell you, it’s typically about 12-14 degrees AFTER TDC. So correct ignition timing should be set to achieve that goal in a running engine as there is obviously a time delay between spark, burn/flame front and max cyl pressure. That’s why you need advance. But more is not necessarily better, lots of factors play into that. However why can you get by with more advance at light throttle and loads without detonation? Anyone?

Remember they don’t call detonation the “silent killer” for nothing.

oddball

I'll throw my questionable knowledge on the pile as well. You get what you pay for.
I'm sure Mark and Joe will correct a lot of this.

MY tuning method is to figure out best max mechanical tuning - usually in the neighborhood of 34deg. Best figured out on a dyno. If no dyno, just assume 34deg ("safe enough"... and the variations are usually small)
The engine will tell you what idle timing it wants. The specific combo will determine what works best. E.g., my 350 with a fairly mild cam loved 12deg initial. My current 403 doesn't live until ~30deg initial! A ton of initial is hard on the starter - a good aftermarket starter helps, as well as good wiring. Mine starts just fine. It'll burp a bit when hot. You can ignore total timing for the moment. Just drive around town seeing how the engine reacts from idle to 3,000 in various situations. Don't go WOT.

Now you have initial and total. Modify the distributor to get the right amount of mechanical sweep and springs that keep timing solid at idle and all-in by 3000.

Now vacuum advance for a driver. Hook up the vacuum and lug it. Keep introducing more vacuum advance until it pings. Now back off several degrees.

You can also experiment with ported vs manifold vacuum. An engine that likes lots of idle timing can use manifold to make life better - during cranking the vacuum advance will be at 0, so you just have the base advance in the distributor. Once the engine's running, the vacuum can pulls in.

Race engines get by with locked distributors because they don't spend much time under 3,000 RPM, especially under load. So locking out timing is fine - detonation is less likely when the engine isn't loaded. Locking out timing also removes a whole set of things that could break during the race and prevents any ignition timing wobble. Well, any wobble from the advance mechanisms at least.

Macadoo

Uh oh, Mark's gonna' come after you for the magic 34* number

What you guys are saying makes a lot of sense but I still don't see how a lot of initial (like 30*, good god oddball! Lol) is any different than too much total. But it's cool, I may have surpassed my ability to understand

"However why can you get by with more advance at light throttle and loads without detonation? Anyone?"

Hmmm......less atomized fuel in the cylinder, which isn't compressible, allowing more compressible air in the mix which lowers cylinder pressure?

Fun71

A while back I read an article about timing and the burn rate in the combustion chamber. Typical ignition advance causes the air-fuel mixture ignition to occur sooner as RPM increases, but at some point the burn rate in the combustion chamber increases significantly due to turbulence and no further ignition advance is required (it actually becomes detrimental after that point). This is why you can achieve maximum ignition advance way before you reach the maximum engine RPM. It's kinda like a built-in advance. If I can find that article I'll post a link later.

Macadoo

Aha, that would make a difference. Everything I've read said combustion always takes the same amount of time. Hence my confusion.

Troys Toy 70

Mac,

I feel your pain. I understand the idea of initial, advance, and total timing; but the timing of the advance of the timing is a science that is beyond me.

Here is where I am at: Dyno’d the best at 32 degrees total (cam is 4 degrees advanced) all in at 2500 mechanical advance. Car runs rich at idle, and probably throughout. Mark recommended that I slow down the timing advance to be all in at 3500, and cut the jets back. AND get a wide band.

That being said, after 5 years it is on the road. Drives great, is very responsive, no bog at all. Twitch your toe at any speed where it is above 2000 rpm and it is an automatic 5 mph jump on the Speedo.

At some point in the spring, I will play with the timing and jets and get a wide band, but for now. It sure is fun!

I think as others have said it becomes very situational. Race track/round town driving, hot weather/ cool weather, etc etc.

Macadoo

Hey Troy, your ride is looking sharp, man!

I don't know why I want a deeper understanding of timing than is necessary. Sometimes I get tunnel vision, lol. But I'm with you, I think an AFR gauge would improve my game quite a bit. I'm tired of making changes, running down the road, making changes, running down the road... Heck, the gauge should pay for itself in saved gas money

I know I have more power than I'm tapping into. But my tuning has been a bit helter-skelter, I'm hoping the gauge will help. What stall converter are you using, or do you have a standard trans? Part of my issue might be the 200-4r. Pressures are lower than I had hoped and it shifts a little too early still. I have a GN style governor I still need to test but probably not before the snow comes. The engine runs great with just me but put another person next to me, especially a larger person, and it feels like a turd.
Okay, now I'm just rambling. It's been a long week, lol.

Fpcopo

Run to Rund. Was that your 442 at the Vegas track on Goblin Works Garage show tonight?

Troys Toy 70

Mac,

i had a little bit of advantage in so much I had my engine dynoed. So, I am close, I think. However, the dyno shop did not have anything for Olds. Mine was the first he had ever done in 20 years of business. So, we had nothing that allowed him to attach his O2 sensors in. This, we were flying blind to a point. I can tell you that my BSFC was high. We did nothing with the internals to the distributor.

Real life- I have about 2000 miles on it now. When I first started driving it, I got 6-7ish mpg. Thus, the mixture was more of a concern. Now, I am getting 9-10 mpg. That increase started about 500 miles No changes to anything, just driving.

I am running a Th400, that was built by a friend of mine who has dragged raced all his life. He basically build it as if I was going to the track every weekend. I have a Coen 2800 stall converter. 3:23 rear end. I am very happy with this set up.

Thus, I am indecisive about changing anything. As my friend said:” lean is mean, but fat is where it’s at. “ Who knows, that’s why I’ll play with it this spring.

Run to Rund

I don't think so, but haven't seen the show.

Fpcopo

It said GTO Go to Olds on the back and said Rund in red on the side I think and it was white

Macadoo

Uhhh, maybe I'm happier with my 200-4r than I thought. I'm getting 16 highway with 3.73 rear gears. 70 MPH and I'm turning maybe 2200. I'm also running a 2500 stall. But it shifts too quickly unless stabbing the go pedal. My TV cable is set correctly, it's just the governor and the way the trans was built.