...which is why these engines have vacuum advance.

You admonished someone on another thread to "do their homework" - I would offer the same advice to you. Read the timing sticky and email Lars for his most current paper on performance timing for a GEN I SBC - this mechanical advance curve and initial advance guidance is very well validated.

 

OP, would you please advise what distributor you have? Can you provide a part number off of the silver band at the base, if there is one? In general, is it an HEI distributor? Does it have a tach drive?

HEI distributors were offered in 75 and I saw it written last night that they all have 20 degrees of mechanical advance. Since TI and points-type distributors may have varying amounts of mechanical advance, it would be important to know what the OP's distributor is.

And maybe the only way to know for sure what mechanical advance is present is to use a dial-back light and bring the rpm up to the point that the timing amount holds steady.

 

........................................ .............
All this back and forth in this topic with I know more then you about the finer details is just drunken babble ..

Look he needed to know how to set timing..I told him .. I did not get into deep routed theory of how the system works and what to do with the vac advance because if you read his first post the poor guy is clueless.

Set it as stated . plug the can in and drive her,, if she detonates make sure u are using 91 or higher octane and if it still detonates come on back and we can discuss deeper more detailed timing adjustments.

 

Bill, I expected better from you.

I'm quite well aware of why distributors have vacuum advance. But you seem to be ignoring the same issue that diehrd is. Ignoring the low speed timing curve specs leaves you with any range of timing numbers depending on what distributor you're using or stuck with, and not the specific curve numbers that your particular engine combination wants.

Regarding Lars, IIRC, he and I are roughly the same age. We've both worked for major engineering companies in our careers, and I suspect that Lars has also experienced the same slight hearing damage as me from time spent in engine dyno rooms. I respect his knowledge and experience as I know first hand how much of the automotive science can be learned from books, and how much can only be learned from hot smelly time working with a dyno.

I'm also particularly impressed with Super6's understanding of this thread topic.

 

Your like a ghost haunting with no apparent purpose . . . The OP needed basic info as he has basic knowledge. He was given that several times.

You apparently need to stroke yourself in public with all this glorying talk of your superior knowledge . Let lars speak for lars .. Let this topic die as the OP aint even replied and must think we are all nuts ..

 

In my experience you are correct sir.
My L-48 mildly modified with 54* of overlap on the cam experiences low cylinder pressures, exhaust contamination of the fresh charge and incomplete burning of fuel at idle.
To combat these items I am currently running 19* initial timing with manifold vacuum supplied vacuum advance for a total of 34* advance at idle.
That means I'm running 15* of vacuum advance. Fuel burns slowly and poorly at low pressures/low turbulance and high contamination vs higher pressures less contamination.
Off idle response is excellent and the cam will idle down to 400 rpm if I want it to. I currently idle at 650 in gear and 750 out of gear, Th-350 and it gets 19 MPG.
My stock HEI has 20* of mechanical advance. This with 19* of initial and 15* vacuum would result in 54* for cruise timing. That turns out is too much. so I modified the throw of the mechanical to give 16* of advance.
This gives me 35* WOT all in. 16* mech+19* initial=35* It also gives me 51* of timing at cruise. I know 19+16+15=50 but the timing light says 51*.
These numbers give no ping, excellent throttle response and good mileage.
I could have just reduced initial until total timing came to 52* or less but discovered instead that higher initial created a much nicer running engine at idle and off idle.

 

The OP's original post is 13 sentences long. In those sentences, he wrote the words "vacuum advance" 6 times. From the OP - "My question is, what is the required vacuum to advance the distributor at idle".

For some reason, I had the odd thought that the OP was interested in the vacuum advance.

You can't write 2 short posts like you did, not addressing his question, and give instructions to set the engine all-in with no explanation of what he needs to do and then call the guy "clueless". And then go after other people that are just trying to help the guy out.

 

No one is ignoring anything; I suspect you didn't follow your own advice and so didn't read the sticky and Lars' paper.

Let's net this out: we're not talking about many different engine types, we're 99.999% talking about GEN I small-block Chevy engines making anywhere from .5 to around 1.4 HP/CID. They've been around for 60+ years, and the appropriate timing curve has been proven millions of times over on the "hot, smelly dyno" and at the track. As much as Lars rightfully asks folks not to quote or post from his papers, I'll ask his indulgence on one sentence from his "How to Set Your Timing for Peak Performance" paper; the latest version of this paper can be requested directly at V8FastCars@msn.com

That's pretty unambiguous, and the key words there are "perform best". And this is where I think your contention is, on two points.

First, we're not tuning for fuel economy or other aspects at part-throttle operation - we're sacrificing to some extent those attributes for maximum performance when we ask for maximum performance from these types of engines. To some extent the vacuum advance mechanism compensates for this. There are many other aspects of this approach in performance engine design; for example, we may have a truly horrible idle and lousy off-idle throttle response in exchange for peak power at 6,000 RPM. But these are not Priuses (Prei-?), they are Corvettes - and when we push the go pedal we want go. Lots of it. All we can afford. To Hell with fuel economy.

Which brings us to the second point - how we practically buy and tune these systems. Your incredible knowledge of ignition system design frankly isn't really practically applicable in this domain. Posters here aren't doing computer-controlled ignition design, they're purchasing off-the-shelf parts and tuning them to the extent possible within the constraints of a 60+-year-old design, with some modern technology added. There's no question that each engine is somewhat different, but very few will go to the effort and expense of a dyno tune to find that "ideal" curve, and even those that do are somewhat limited in what they can achieve due to the distributor design - limited to weights, springs and limit sleeves (which allow us to adjust the initial timing if required). Even so, 99.999% of those will find themselves using the recommended curve within a few degrees of initial and total...because we know - know for sure, from "hot smelly time working with a dyno" - that the curve Lars and virtually everyone else on the planet recommends will get us within a percent or so of "optimal" for investment of a tuning kit and a couple of hours on a Saturday.

I recognize as an engineer you and others hate this concept The people in cool, dark rooms full of computers vs. those working on the "hot, smelly dyno" will always be at odds to some extent - as a former tool and die maker in a prototype shop, I understand this completely But your ideal engineering world simply doesn't play out in the real world of classic Corvettes. That said, if you have reasonable, practical guidance on how the regular guys here can and should tune their timing curve for optimal performance, then post it.

 

As educational and entertaining this thread may be, I think you scared off the thread starter.

Carry on!

 

That particular quote from Lars is in regard to WOT performance. I was not talking about the 1% of the time that the engine is in that performance envelope. My original comment is about the other 99% of the time that the engine is running (ie: cruising 100 miles round trip on the highway to do two miles of total WOT time at the dragstrip).(Yes I know the math in this example is closer to 98%.) I don't subscribe to the philosophy of setting the timing for most performance at high RPM WOT, and then ignoring what the timing curve is while you're driving that 100 mile trip on the highway. Perhaps an analogy. With a WBO2 sensor you adjust the WOT A/F ratio to a targeted 12.5:1 down the dragstrip. With the philosophy stated by some here that would be all you need to do and don't worry about if the drive home A/F ratio is a similar 12.5 or 15:1. I disagree with that cavalier philosophy. My point is to also spend time modifying the timing or A/f ratio at low/cruising speeds to improve the drivability and fuel mileage.
Change the timing at idle: does it improve the idle speed and quality? If yes then change the VA can or shorten the mechanical curve so you dial in more initial. Monitor a vacuum gauge while in steady state cruise. Will a change in base timing or curve shape allow that cruise RPM at a lower throttle setting and higher manifold vacuum? If yes, then modify the curve of the distributor to get this added advance without losing your 36* high RPM WOT timing objective. If not, then you at least know where you are regarding being close or closer to the timing curve your particular engine wants.
I am not saying that anyone needs to follow my philosophy, but I am saying that I strongly disagree with some members' philosophy of adjusting the timing for high RPM WOT and then thinking that the low speed/low load timing is likewise optimized. It's not.

 

And you've validated these changes on the dyno or the track? Before and after dyno or timing sheets, please. You can diddle all you want, but unless it's backed up by genuine increase in performance as validated on the dyno or the track...it's just diddling. Surely an engineer of your experience and expertise recognizes the importance of real-world validation of engineering expectations.

You don't have to - you just need to prove that Lars' and everyone else's approach doesn't work - and back it up with your own dyno or track results. Not your extensive professional and educational credentials, but genuine results. I asked for practical guidance for how this SHOULD be done from your perspective, and once again you've failed to provide practical, real-world advice. The purpose of engineering is to achieve real-world results - I am all ears


This claims that setting timing per Lars' approach for timing - both mechanical and vacuum advance - won't deliver drivability or fuel economy - while in fact virtually everyone I've seen using this approach claims dramatic improvements in both. Again, I'd like to see your proof that you can optimize well beyond what the proven curve and approach will deliver. I agree that it's not 100% either/or, but certainly for most Corvette and other high-performance car owners they place the emphasis on performance and are willing to compromise other attributes to get it. For example, the ride in my '03 Z06 is brutal from a daily driver perspective...but I put up with it for the couple of hours a year I get to really exploit it on the track.

Again, no practically applicable information or validation at all. Simply disagreement with what is known and proven to work. I notice that you conveniently focus on what "should" be done without providing guidance on "how" it should be done...which would be a genuine service to this community vs. pages upon pages of theory and unproven claims.

I would genuinely love to see a post from you with your detailed guidance on timing and A/F tuning....which may be the more appropriate place to continue the discussion

 

Bill, I gave a few suggestions on how regular guys like us can do some simple inexpensive underhood and road tests to see if their timing is obviously off the mark. You just replied with smartass comments. I mentioned in an earlier post that you are capable of better. Please don't prove me wrong on that assumption.
If you want to talk serious engine stuff I'm all ears. I genuinely don't need your smartass stuff as I get enough of that in the political discussions in PR&C.

 

I asked you to validate the claims that you were making and offer specific guidance. I don't know why you chose to take this personally rather than simply responding to the very reasonable questions posed, but that's your choice.

Fortunately, folks will continue to use the excellent guidance from Lars and achieve excellent results.

 

NO specific riming information in this post to help the original person posting. Just my thoughts on this issue.

MAN...OH...MAN!!!!!

One thing about this post is truly clear. There are several people who have strong views on this subject. I can appreciate the passion. And there is somewhat a lack of documented information that can prove or disprove a way of setting timing in a specific way....or at least it has not made its way onto this post.

For what it is worth...which can more than likely be agreed by those people who have done it also....that getting the best performance/fuel economy, emissions out of an engine when a car is on a chassis dyno is relative to that engine due to age, wear and numerous other variables that can come into play.

Like many of you...I have dyno'd numerous engines and have saved the information and gave it to the customer...but I never started to compile a data base. The only reason why, is that it would not help me in the next engine because I am for certain that the engine would not be exactly the same. Close...maybe....but often times that is only good when playing horseshoes. And any serious drag racer out there who may have traveled the circuit for years could..or should have... compiled a database. What was done internally in the engine can be recorded along with air temps, track temps and the data from the run that could be used the next year if the weather conditions were very close to what they have experienced in the past. It is a starting point for the "set-up" for qualifying...instead of beginning with nothing and hoping for the best. I have experienced this in the Nitro Funny car class and saw what happens when you have NOTHING to go off of.
So a person dealing with their engine and obtaining results for them is all that matters...and comparing is somewhat pointless in my opinion.


There are way too many variables ( which I hope we all can agree on) that would effect the end result if a person were to preform a specific procedure in timing their engine and EXPECTING the same end result as someone here on the forum that got a specific result that was documented. Which I know is NOT what was being asked....but some people go by NUMBERS on a sheet. I have seen it many times... and "they" would wonder how "so-and-so" got this result and I can not achieve what "they" got. And I can completely understand that if an engine was timed to a specific number and then record what it produces on a dyno...and then adjust the timing differently and record the results again...One could see if you were gaining or losing on the dyno....depending on what you are going for...power and performance, fuel economy or emissions.

In closing...I can appreciate the comments and questions and the debate at hand. I am not adding what the original person posting is wanting and I am truly sorry about that. I already commented and got a reply on my comment that was....what it was. And I know myself...when communication...if I allow my emotions to override my intellect...I lose every time. With this "brain trust" going at it so passionately...I am just trying to hopefully "switch gears" and have everyone try to employ and maintain the thoughts of "Peace, Joy and goodwill to all men".

If not......time will only tell.

DUB

 

This is more heated than a pulled vs. chopped pork pig BBQ thread

I will weigh in with my admittedly novice experience. When I bought my 79 L82, it was set up per the factory specs and wouldn't hardly get out of it's own way. Lars did my QJet and I set up my timing per his instructions. Different car as to how it idles and accelerates. Huge and very noticeable difference when the timing was set up for 36* all in around 2800 rpm.

Of course, YMMV, but I think it helped that, too

 

Dude, you can time your ignition any way you please, but pulled is the way to go with pork!

Scott

 

Capt.,

Yes, I think this thread has gotten out of hand and away from helping the OP. I hope it can recover.

I had gotten involved here because the situation the OP has is similar to mine. Although he has a small block and I'm just breaking in a big block, the common denominator for me is the 10" of vacuum at idle, which puts both our engines in the "non-standard" category. And I had hoped that a resolution for the OP would be something for me to try as well. I'm not questioning Lars' all-in method of setting the timing at all. What I am saying is that the job doesn't end there. There's the question of the initial timing in relation to the size of the cam and the choice of the vacuum advance can.

In my mind, the ideal arrangement would be very similar to what REELAV8R has done, where he has optimized all aspects of the timing and vacuum advance to suit his engine specifically. Is this what the OP should do to the letter? I don't know what he will do, but he should have the benefit of the information being presented to him for his review so he has the chance to evaluate it and decide what he thinks.

Lars' name has been thrown around in this thread and the "all-in" timing method where the initial-falls-where-it-falls has been presented as an inflexible rule so much that I thought I ought to see what Lars actually says in his latest papers. Lars sent me his timing and vacuum advance papers last night. And while I'm still digesting specifics that apply to my engine, it is clear that Lars has seen many situations that fall into the non-standard category and the methods and timing values are flexible. The timing paper is 5 plus pages long. One full page is devoted to changing the amount of mechanical advance to maybe shorten a too long curve, but also to allow for a higher initial timing value than the just-where-it-fell value. A second full page is devoted to factors affecting the timing set-up, including total timing and the initial timing as separate items. Large cams are a factor in the choice of initial timing and they required more initial. And the same applies to not running vacuum advance. Lars recommends 18 to 24 degrees of initial timing if you're not running vacuum advance.

I think the 2 camps in this thread recognize Lars as the authority on timing and tuning, but maybe we all aren't that familiar with his whole philosophy and the flexibility that he has allowed in his methods. I've learned quite a bit in just the quick read I've done on these papers. In any event, if we haven't done a good job for the OP, maybe the best advice we can give is to recommend that he contact Lars at V8FastCars@msn.com and ask for his timing and vacuum advance papers.

 

indeed

 

Super6, please don't speak for me - especially as you're substantially misrepresenting my position and my statements.

At no time did I state that Lars' approach was an "inflexible rule" - I simply noted two facts: 1) that most guys aren't going to take on a high level of tuning/optimizing and so as a rule of thumb it will get them in the ballpark within a percent or so. 2) that even a highly optimized tuning curve will very likely be within that curve within a degree or two.

There's always room for adjustments in the curve and A/F ratio...but the number of folks that are going down that route on their own or via a dyno session are few. I own and use an Innovate and a dial-back timing light, and I frequent the dyno that is just 20 miles away. It's the very best $250 ANYONE can spend to get their tuning truly optimized and should be part of every engine build or significant modification.

You note REELAV8R's "optimization"...and I would note again that if it's not done on the dyno, no one knows how "optimized" it is. The engine may "seem to like" this or "run better" with that...but only the dyno will tell if the engine is making the power it's capable of. All the "low cylinder pressure", etc. theory means nothing until validated.

Finally, I also referenced Lars' paper and suggested people simply download them. Overall, this was started by one contributor calling the Lars approach "Nonsense" and then failing to provide any proof of that perspective.

I'm sure that sadly the OP has since departed a contentious thread - it's unfortunate that someone so securely grounded in theory failed to recognize Lar's practical contribution to the community.

 

This post is great. This is more heated than a louisville vs kentucky basketball game. I do have the stock tach drive distributor that came with the 72. I took yalls advice and ordered a vacuum can that will work with my 10" of vacuum at idle. It comes in sunday. So ill let you know how it goes. I think some of my low vacuum readings are due to a not properly adjusted carb.