So have been following this - I try to read up on ignition timing as much as I can. A long while back, I tried the vacuum method, and it certainly can work, But if I recall, you may need to be careful how far you advance. And keep in mind that every motor is different, and I only mess with mine, so this is only from my limited experience.

Then again, I think I'm still learning, so maybe I just need some constructive coaching in this area. Given where I'm at now, I don't see bondoboy's questions (as many of us are just trying to learn here) as out of line.

 

I thought the purpose of this forum was to share information and help others out, not to run others down for asking questions. I can repair just about anything on any vehicle I've ever owned. I've never had anything really high performance that needed special tuning. I'm asking questions to learn something new. All I asked was how much difference in timing settings have some guys seen between timing by light and by vacuum. I know all engines are different. I was just wondering how much different the numbers were when using both techniques on the same engine. Just to see how much or little difference there was depending on the engine. That's it, simple as that. No specifics on compression, cam, plug type, fuel type, or what color underwear the driver was wearing. Just wanted to see everyone's results. I try to help other members when I can. I've done a lot of fiberglass and paint work professionally and as a hobby. I try to answer people's questions before they have a mess on their hands, not belittling them. Not to be a d!ck here, but there is getting to be more attitude and namecalling on here. Let's try to remember why we're here and help each other out when we can.
Also, I realize than I am not the best writer and I sometimes have trouble explaining what I mean in print opposed to speaking in person. Sorry for any confusion.

 

bondoboy. I don't have an in depth knowledge of tuning or physics either and I don't think your question is out of line. In the past I've set my timing with a light and set the mixture screws with a vacuum gauge.
So since no one has answered your question I guess we will have to time it with a light and then see where it ends up doing it with a vacuum gauge.

 

Wow, bondoboy, I don't know what came over me earlier, I'm sorry I was so rude to you. Seriously, I apologize. I don't know what came over me, I'm usually pretty even-tempered. It's just that the answers to your questions are pretty self explanatory, if you have a good knowledge of the subject.

The literal answer to all your questions is: It depends....Yes, you will always advance the timing, except when it's too advanced, then you retard it.

After setting it by vacuum, how much more advance do you end up with than setting it by the experts' guidelines or factory settings? Well...it depends...there's really no way to tell until you do it. You could take a guess, I suppose.

See what I mean? Think of vacuum tuning your ignition timing at idle like adjusting your idle mixture screws. Using a vacuum gauge, you adjust them to where the engine likes it. Same with timing, use a vacuum gauge to set the timing where the engine likes it, then adjust your mechanical advance so it comes in where you want it. Sorry, but there are no easy answers here, you have to understand the subject at hand in order to understand the answers. In my prior post I provided a link that should help you out.

Scott

 

I merely disagreed with the first sentence of your post.

I also took the time and interest to reply to most of the questions you posed in that post. Feel free to point out those you disagree with.

I've also spent a ton of professional and hobby time dealing with and designing engine controls, along with my share of hearing damage in engine dynos. I'm speaking from my experience in this area, and certainly not posting anything here just to "be a dick here".

 

I apologize for any problems. I don't mean to cause any hard feelings. Thanks for the replies. I was just curious as to how this works. I guess what I'm really asking is there a time when you have it adjusted to the highest vacuum, that it's actually too far advanced that it will knock? Or will the vacuum reading start dropping when you get to the point of detonation? How do you determine when it's enough? I'd love to try this technique, but I currently have 2 cylinders that have bad valves and I cannot get a steady reading. Next time I have heads rebuilt I will specify "no valve guide knurling." Just didn't hold up. Again, thanks for the help, and sorry about getting aggravated. Just trying to learn new tricks.

 

First off- If you tune by vacuum and then go back and check with a timing light you find that you are normally within a few degrees of the recommended initial timing.

example if the cam guys say between 12-14 with this cam (based on lift and duration and an educated guess) and you use a vacuum gage the check it with the light you might be at say 13 or 15.

The difference is that you are at a place the motor likes. IE your cam, heads, headers, intake and carb combo like this setting.

You are normally very close to the experts guess but this eliminates the one size fits all generality.

You just need to be careful not to go too far.

As far as advance vs retard you basically figure that out on the fly based on where you are on the curve. Normally you advance until peak and back up. If it has too much in it (timing) already the you gotta back up first (retard) and the advance to peak.

You also gotta monkey with Carb to keep idle constant or you will screw yourself (figuratively and literally)

Hope this helps

If you have specific questions send me a PM. I am a technical trainer by trade and will try and help you as much as possible.

 

 

Thank you. That answers my question very well.

 

Ok, think what you want about my level of knowledge but I'm going to throw this out there because I want to learn. As a high compression, high lift, long overlap small block owner I've chased these questions for years (not my choice of cams). I understand using vacuum to analyze what an engine likes and doesn't like and from a fundamental level it all makes perfect sense. Most of this discussion seems to focus on conditions at idle, which for every engine is the starting point, and for every engine its slightly different, and any specs are an approximation, it all makes sense. But, what about top end? If every engine is different should we all be using the same max timing? Is there a way to determine what your total timing should be? Detonation certainly isn't a reliable indicator since at 5500rpm with free flowing exhaust and the wind in your hair you can't hear much of anything. Plus if you can hear it, you're probably already in trouble. Further, if you're running 100 octane fuel or better is pinging even a concern?

And then there is advance. Obviously advance is simply the difference between initial and total but how about range in between? In other words, can you determine the best curve using vacuum? I don't mean to be critical, but it seems to me if vacuum is the best way to determine optimal timing then it should be the best way to determine the optimal advance curve. Lastly, does load affect any of this? Load certainly affects carb settings, does it affect determining the optimal timing?

Granted, dynos are great for figuring all of this out but for those of us that won't being doing any dyno tuning soon timing tricks can be great information. As for who should be the ultimate authority on all of this? I welcome comments from anyone with an advanced level of knowledge. As for who is right? That could probably only be determined by a head to head competition, and in the end the contenders would probably be so close nobody would be a clear winner, and yet any results would for sure be better than I could do. Bottom line, there's more than one way to skin a cat.

I certainly hope all of our resident experts weigh in, its an opportunity for all of us to learn and make our own decisions based on our own level of knowledge, for better or for worse.

 

1. i listen to Fox News.
2. all engines are different.
3. all fuel is locally blended
4. altitudes are different.
5. three of those 4 affect the way an engine runs and vacuum is affected by it, vacuum is a tuning tool and should be used in concert with other methods.

 

Good info! Always glad to try something different. With the auto trans, be sure to stop the wheels and hold with brake pedal before shifting from Drive into Park... easy way to tear up trans internals (don't ask me how I know).

 

 

 

oh look someone from california

 

There is a lot of misunderstanding and misinterpretation here. Timing is, initial, mechanical, vacuum, along with the rate of mechanical and the rate and amount of vacuum advance. All have to be adjusted to what your engine wants and work together correctly and in unison, especially a high performance engine. Vacuum is an ideal way to establish correct initial, idle timing. That said if you set it that way your likely too high once the mechanical is all in. Vacuum reading is also needed to establish correct vacuum advance can. If you set initial with your vacuum gauge, establish which vacuum can you need, attach it and plug the line to it, then hook up your timing light and rev until mechanical is all in. If your too high (over 34-36) depending on your heads limit the mechanical by filling the slot. Then adjust your rate of mechanical advance by using different strength springs. 2500 to 3000 all in is best for optimal performance. The higher number for a car that is high compression-high DCR lower for a car that doesn't absolutely need premium. Then hook up the vacuum advance which should never exceed 52-54 degrees with initial, mechanical and vacuum advance in neutral @ 3000 RPM. So what I conclude is a vacuum gauge has its uses in setting up a precision timing curve but a timing light is also critical to do it properly and get the best results possible, especially if starting from scratch on a performance or unknown spec build. The final test of the timing curve setup is to drive the car and load it in higher gears checking for ping. If it pings drop 2 degrees timing and try again if it still pings you may need to drop some vacuum advance by changing cans or increase your all in mechanical RPM. This is my opinion based on my experience since the early 70's on a wide variety of SBC engines from stock to 500+ HP. No 2 engines need exactly the same curve and optimal for yours will not be optimal for the next guy. The object is to obtain optimal efficiency throughout the entire operating range with no ping. Proper timing curve throughout the entire operating range and use of the car is the very best time you can spend on tuning any car. Let the flaming begin..........

 

Mako just clarified my #1 question: Setting timing based on manifold vacuum an ideal way to establish idle timing. But other events and loads require other techniques.

 

No flaming here. I believe we're in agreement.

I've always thought that setting the timing with only a light and a generic curve was quick and easy, but inefficient regarding overall engine performance (horsepower, fuel economy, drivability).
An engine dyno is obviously the most precise way to dial in the timing curve, but dyno time is incredibly expensive, and not terribly convenient for most of us. There's a couple other ways to load the engine at different speeds and loads, but they're not easily available to the average enthusiast.
A vacuum gauge will give some actual data/information to help you find the ignition timing "sweet spot" in your tune. Obviously a vacuum gauge will only give you useful data in the engine load points where there's still vacuum, but that's still a majority of the engine's speed/load curve.