Yes, yes, again, technically correct. I will not say you are wrong. But, it's advanced hotrodding.This guy can barely figure out how to adjust his idle screws, he's new at this stuff, he has a long way to go before he needs to start worrying about that level of tuning. Lars' technique for setting timing is quick, easy, and will normally result in an ignition advance curve that will work very well.

ATIS, I am having trouble understanding your posts, is there any way you can explain things more clearly? Just start from the beginning and tell the story.


Keep the shiny side up!
Scott

 

Just a purely neighborly thought: In future posts if you feel the need to post "questionable" information (ie: little white lies for the noobie types) perhaps you could include a short phrase such as "don't worry about it for now" to alert the new guys that the information they're reading is incomplete, but basic enough to get them going. This should help the new guys distinguish between good basic information and shadetree wives' tales.
Genuinely not trying to be difficult here, but there's no shortage of BS information floating around, and I'm just trying to minimize the confusion for the noobs.

 

Ok, from the top, I grew up under the hood as my grandfather was an engineer for GM. If it wasnt for autoshop I would have failed highschool, with that said, I havent touch or seen a carb or non computer controlled dizzy in 20 years so all my old knowledge is fuzzy at best. Plus I have driving and working on Mustangs for the last 15 years.

So my original question was about the base timing, what btdc number was I looking for. At the time I wasnt looking for a performance tune but rather fix the timing as I bumped hee and she moved! Then I noticed that I had a fuel leak along the lower carb to manifold gasket so I pulled it and replaced all the seals. Got her running again but could smell the fuel and during a quick 10 mile test drive I went through 1/4 tank of gas and she was bogging down. Thats when I decided to tune the carb, along qith the fact that she was hard to cold start.

Hope that all makes sense. My current timing light lacks dewell and rpm sensors and my tach is currently broke.

 

Well, when operating without a dyno - to some extent, that is the approach

Not disagreeing with the overall observations, but for someone trying to get it "in the ballpark" we can be pretty confident that if get around 36 degrees of advance before 3,000 RPM we're in the ballpark for a GEN I SBC. If that leads to too much initial advance, then we can adjust with limit bushings, etc. But I think we'd agree that initial timing isn't actually all that critical, as it's just one component of the overall timing, including vacuum advance. If initial is between around 8-14 BTDC, chances are we're in decent shape. These very general parameters are all pretty well-established for the GEN I SBC.

"Optimum" is great, and a dyno tune and professionally recurved distributor is the right way to get it...but we can get "good", and that's good enough until we can get "optimum"...which I think was the point being made

 

Critical? Probably not. Very important? Yes.

 

OK, so let's quantify the importance - fuel economy? power? driveability? As an example, let's say for a particular engine optimal static timing is 11 degrees: what's the impact at either end of the range of 8 to 14 degrees?

Again, keeping in mind that at idle and low/part-throttle most of the advance is coming from vacuum advance.

 

Oh. Well, if that's all you're looking for, then just set it at 8 degrees for now. It'll be fine.


Keep the shiny side up!
Scott

 

Yes, but remember that the vacuum advance contribution, regardless of its magnitude, is added on top of the base/static timing. The same goes for the mechanical advance. It's all added together to get an ignition point (relative to crank angle) that hopefully allows the combustion process to reach its peak pressure at 15* ATDC (LPP, location of peak pressure). Regardless if we're looking for power or economy, we'd like to maximize the thermal efficiency of the combustion, and also the mechanical leverage of the cylinder pressure on the piston/rod/crank assembly. If the timing is off (too little or too much advance) then we've lost power or fuel mileage. With too little advance we never get the expected/theoretical cylinder pressure, and our horsepower is down, or our fuel economy takes a hit. With too much advance, the cylinder pressure curve and LPP is too early, and we waste (reduce) engine power because we're using power from other cylinders to push each succeeding piston up against the combustion pressure prior to TDC (resulting in negative work). This negative work reduces light load fuel efficiency also.
The impact of inaccurate timing is different for every engine combination. Wilder cams (w/long duration) are going to cause increased residual exhaust gas contamination in the cylinder at light loads and low RPMs. This requires additional advance to compensate for the slower combustion rate, compared to a similar engine with a milder cam. Other engine details cause differing timing requirements also.
How to quantify the horsepower hit? Without a dyno or time slip, it's just a guess. How about the efficiency/mileage loss? Well, a long drive and a fuel fillup will give an indication, or just twisting the distibutor at an idle will give you an indication if your base timing (plus VA) is off, and requiring a change to the static timing and a corresponding change to the mechanical curve to get to the 34-38* WOT timing that seems to be workable for most engines.
Essentially, I don't subscribe to the philosophy that the only important timing number is the over 3000 RPM WOT numbers. As I mentioned before, our engines spend 98-99% of their useful lives in non-WOT conditions, and I believe it's prudent to spend time optimizing the timing curve for these conditions too.

 

scott, thanks.

 

As 69427 indicates, total timing, by itself, is not the all-important number. The ignition system is exactly that: a system. So all parameters of the system must be working properly, to include initial timing, total timing, centrifugal advance, and vacuum advance. When everything is working together and properly set up, the engine will idle well, not overheat, have great throttle response, and develop maximum power.

One of the reasons I am an enthusiastic advocate of setting the "total" timing is that this simple process forces the engine tuner to actually observe the behavior of the ignition system. It amazes me how many of these old Vette ignition systems have non-functional centrifugal advance mechanisms, ruptured vacuum advance control units, and grossly inappropriate advance curves. When setting the engine for total timing, the tuner is forced to observe that the centrifugal advance system is, in fact, advancing. This won't be observed when simply setting the initial timing at idle with no other inspection or observation. Once total timing is set, the process then allows the tuner to observe what the resultant initial timing is: If the resultant initial timing is very low or very high, the tuner knows that corrections and repairs will need to be made to the system. Also, when setting the total, if the tuner realizes that the full centrifugal advance is not occurring at a reasonable rpm below 4000, or if it all comes in at 1500, there is an opportunity for improvement and correction. Finally, when the vacuum advance is plugged in, an opportunity is there to observe that it is working correctly and applying a reasonable amount of advance.

In this way, setting total timing is not just a way to simply set a certain number for WOT performance, but the process of setting that number affords the tuner a clear opportunity to inspect and correct the entire ignition advance system. This system-level approach and critical evaluation of what is observed is what I teach in my Tuning Tour Seminars, and is an effective approach to tuning for best performance and drivability.

The following section containing my opinions and observations about timing specs is contained in my "How to Set Your Timing" paper. You can get a copy of the complete paper by dropping me an e-mail request:

Lars
V8FastCars@msn.com


My Comments, Observations, and Opinions about Factors Affecting Optimum Timing Specs
I see a lot of opinion about what the “correct” initial and total timing should be, and I frequently get asked, “What is the best timing spec for my engine?”

Without actual dyno testing of the engine in question, it is virtually impossible to arbitrarily determine what the “best” numbers should be for any given engine, so some generalizations and “suggested” starting point specs are made in this paper. The 36-degree total timing spec is a generally-recognized and agreed-upon spec amongst almost all performance engine tuners and engine builders, but it can vary from one engine to the next as the “optimum” spec. I have done a significant amount of dyno testing of a wide variety of engines, and one of the tuning variables we always “play” with is the total timing. I have hard data obtained from dozens of dyno runs, making it possible to draw some general conclusions about “optimum” timing. Here are my observations based on years of testing:

• The 36-degree total timing number is a good mid-point number for most small block GM and Ford performance engines with factory-type heads. Most small blocks with stock head castings will take 34 to 38 degrees total timing for best power and drivability.
• A small block with good, efficient aftermarket heads with good combustion chambers will make better power with less total timing: I usually see best power with total timing in the 32 – 34 degree range on small blocks with good heads.
• Big block (large-bore) engines will typically take more timing. Big blocks with inefficient factory iron heads (like 455 Pontiacs with stock heads) will typically develop best power with total timing in the 38 – 41 degree range. Large-bore engines with good heads will typically take 36 – 38 total.
• “Optimizing” timing on the dyno from the 36-degree suggested starting point will often result in power gains in the 5 to 10 horsepower range. You cannot feel a 5 to 10 horsepower change in the “seat of your pants” in a typical street driven vehicle weighing over 3,000 lbs., so don’t worry about trying to optimize total timing within 2 degrees without the use of a dyno. If you’re at 36 total, and do not have detonation, you’re going to be making very good power.

Although I downplay initial timing, it is important that the initial timing hit a certain range in order for the engine to idle properly. Initial timing has a significant effect on emissions, if that’s a concern to you. It will also greatly affect idle quality and manifold vacuum (for proper carb metering signal) on engines with bigger cams. My generalized recommendations are:

• On mild or stock engines, initial timing (checked and set with vacuum advance disconnected) can be in the 8 to 12 degree range. This will provide a smooth idle, low emissions, and good drivability.
• On engines with an aftermarket performance camshaft, the initial timing should be raised (i.e., the centrifugal curve should be shorter). The 16 – 18 degree suggested spec is a good target range for any performance engine. This will produce better manifold vacuum at idle on an engine with a larger cam, thus allowing the carb to properly operate on its idle circuit (rather than into the transition circuit).
• Engines with big solid or roller cams, and engines not utilizing vacuum advance, should be run with initial timing in the 18 – 24 degree range.
• The engine will run cooler with more initial timing, up to a point. If you’re having overheating issues running in stop-n-go traffic, and your initial timing is in the 8-degree range, try shortening your advance curve and running 16 initial – it can make a dramatic effect on coolant temp while idling.

As for the transition from initial timing into total timing (the “quickness” of the centrifugal curve), this will depend on the weight of the vehicle, engine compression, and the quality of fuel used. Generally, the lighter the vehicle, the quicker the curve can be. Heavier cars generally will take a slightly slower curve in order to prevent detonation under load. For light street driven vehicles, I’ll bring in the total advance in the low-2000 range. For “normal” cars (Camaro, Vette, GTO), I’ll set the curve up for about 2800 as a target. Generally, something on the 2500-3000 range works well for most performance applications, without getting too aggressive.

 

Lars, you and I are on the same page.

 

The vette went to the painters today, get her back in a week and then its time to tune her and drive her...well drive her a little. I already put more miles on her in a week then she normally goes in a year. LOL

 

I think you and I have been doing this long enough that we've come to the same solutions and conclusions...
Lars

When you get it back, set that timing up, and then run the heck out of that thing and enjoy it!

Lars

 

Lars,
Thanks for the class on ignition timing.Could you explain about vacuum advance and advantages of using it.I also would like to know what is the best way to dial it in for your particular engine.Should we drive around with a vacuum gauge and take readings?....Thanks!

 

Note that the OP was asking about a stock engine.

We all argree with Lars

 

I have a complete paper dealing with the subject of vacuum advance. Just drop me an e-mail request for the "Vacuum Advance Paper."

Lars
V8FastCars@msn.com

 

I, for one, am certainly not going to be the one to argue with Lars. No offense, 69427, if Lars says you know what you're talking about, that's good enough for me. Sorry to have been so argumentative.

I have said it before, and I'll say it again: If Lars told me my car would go faster if I peed in the gas tank, I'd give it a try!

Man, how does Lars stay so down to earth with people like me saying stuff like that about him? I'd probably get a big, swelled head if people stated saying stuff like that about me! Fortunately for me, that hasn't been a problem I have had to deal with.........


Keep the shiny side up!
Scott