I've used a timing light to set the timing. Recently, I've heard that using a vacuum gauge gives a more true actual time for the set up of each individual engine.
From what I see, you rotate the dist to the peak vacuum and then back an inch or two. Then you set the idle mixture to the highest vacuum, adjusting the idle speed back-and-forth until it settles at peak vaccum at the right idle speed.
Anyone with real knowledge and experience know about this?

 

How is that going to give you the correct timing under load at various engine speeds, makes no sense to me.

 

Yes - I've watched people do it, and then I have checked their timing in the end: It typically results in the timing being off by as much as 20 degrees. You cannot set timing without reference to crankshaft position. Best vacuum at idle may give you an idea of where initial timing needs to be, when used in conjunction with a timing light, and can give you information on how long your centrifugal curve needs to be. Peak vacuum at idle has no relevance whatsoever to best timing at wide open throttle or at cruise, so it is irrelevant to overall timing curve setup.

Lars

 

Vacuum gage can help with engine diagnostics. Link below is to a Holley Carburetor you tube post. Unfortunately, that link is cut short and does not include all engine problem vacuum characteristics, but it serves as an example.

 

I used a vacuum gauge to set the idle mixture. I didn't mess with the timing, though, which was already dialed-in per @lars instructions.

Perhaps you are mixing two sets of instructions?

 

you are also assuming none of the zillions of vacuum hoses leak at all.

 

You CANNOT set timing by:
Ear
Vacuum
What your neighbors Grandpa said
Initial

One of the reasons I have backed off this hobby so much is the sheer amount of misinformation and the amount of idiots that support this misinformation. It is similar to reading the "NEWS" now......

I have seen 1 degree of timing be worth 80 horsepower on a dyno.......on a 632 2X4 setup......now, the idiots will say "But that is a race motor....not the same!", of course it is the same......it functions on the same basic principles.....

I used to have a tuning special at my small backyard work shop in Michigan.........it made me somewhat of a local hero as out of the 100 cars I worked on over the course of 5 years.......about 8 or 9 had correct timing specs. Less than 10%......and some of these cars had $8000-9000 engines in them......

I am not sure exactly why the average person just refuses to listen to the straight answer and is compelled to find "magic" in these old engines.......

Jebby

 

Amen!
Owner of the garage near me uses "the ear" method.
When I heard him say that I told the wife that she should get her DD serviced somewhere else.

 

It does not help that the correct answer happens to go against what GM puts in their Shop Manual, and what the State of California (and possibly other places) insist is the correct way to set timing (initial).

Even one of the best car movie scenes ever perpetuates the wrong answer (1:47).

 

no, that was the correct timing spec as far as GM was concerned. they cared about the cars running well enough to leave the dealership and pass emissions. they didn't even care how close to estimated fuel mileage they got.

 

100% fact.

Jebby

 

True. But they cared MORE that setting timing somewhat retarded would also drastically reduce their warranty costs. Damage to a well-tuned engine happens more frequently than to a "stock" tuned engine.

Setting timing to a vacuum gauge (max reading at idle) makes sense if ALL you care about is idle quality. But performance will SUCK! (pun intended)

 

There are three elements of ignition timing: initial, total (initial plus mechanical timing combined) and curve (how rapidly the timing reaches total and at what RPM). Initial is just that; initial. A starting place to get the engine running smoothly at idle. The rest has to be performed under load and acceleration. Once you understand these principals you can successfully time your engine. Two methods: chassis dyno or butt dyno. Chassis dyno is most precise but expansive. Second method takes a lot of passes, a system of timing and your ability to discern the differences.

 

Lars,
I just finished reading your Technical Information Bulletin: How to Set Your Timing for Peak Performance.
​​Many comments have made reference to timing under load.
I don't see any reference to timing with the engine under load in your bulletin. Can you comment on this aspect of the timing procedure. Thank you

 

.
Under load is no different than not under load. Timing is RPM and vacuum signal driven.

In other words your total timing is derived by the vacuum being disconnected (and plugged) from the distributor and observing the timing at an RPM where it stops advancing. This would be your WOT under load timing.
Timing at cruise would be your total timing plus what ever the vacuum advance adds. This can be observed with the vacuum connected while using the timing light and RPM'S where the timing stops advancing.
Your mechanical advance is RPM driven. So it can be observed by noting the difference at idle with vacuum disconnected and plugged to a point in the RPM range where the timing stops advancing.
So if at idle you see 10* then rev the engine to 3000 and higher RPM's don't advance it further and you have a reading of 30* then your mechanical advance is 20*.
If you use the same procedure but with vacuum attached and you see 50* then vacuum advance is adding an additional 20*.

 

Nothing wrong with using a vacuum gauge to set you idle timing. The vacuum reading will give you an indication of the combustion efficiency at your designated idle speed. Adjusting the idle timing and fuel mixture for best combustion/thermal efficiency will result in a cooler and smoother running engine.

There are a couple additional items to keep in mind, though. Setting your idle timing and fuel mixture at one ambient temperature will cause it to be slightly off at another ambient temperature. Without ECM control, the timing and fuel misture will always be a compromise to work in differing conditions.

Also, you now only have one part of the timing curve measured (low RPM, low load). The other end/extreme (higher RPM/WOT) is more difficult/expensive. You can spend some time and money on a dyno tune, or go with the numbers that experience from the collective here has found to work well (34-38* at 3000-3500 RPM), depending on your engine combination and the ambient conditions in your area.

The complicating thing now happens, merging the low speed and high speed timing needs of your engine. Most stock timing curves are long and slow, so it requires some reworking of the distributor advance mechanisms to get the new curve "installed". This is easy if you can locate someone who does distributor work, and who can rework your distributor to match the curve specs you worked out.

That's basically the basics. I applaud you for your interest in using additional equipment to get actual data about your engine.

 

The "total timing" (initial plus full centrifugal) is the timing under load. Under load, there is no vacuum, so the additional timing from the vacuum advance does not come into play. The only difference between "timing under load" and other conditions is that the vacuum advance does not come into play under load.

Lars

 

Lars statement is correct as stated in post #3 above. BUT...refers/is in reference to the total timing curve vs rpm etc (if I read the post correctly)
I've been using a vacuum gauge to set idle ignition timing for 50+ years. On all kinds of carbs cars not just GM or American hardware
No it is NOT a performance tune (vac at idle)
no it is NOT the only measurement I use to set idle

Ask any ol' timer mechanic about this. The object of vac to set idle is to compensate for ancient wear, weak/aged dist springs, dist gear/cam slop, etc etc
when you cannot state un-equivacally the real state of all the operating parameters, this does give you best curve possible with a mucked up/aged dist.
AND ONLY under these conditions. You still need to set idle properly and deal with how much initial idle advance you end up with vs the max your system will deliver THAT IS ACTUALLY USEABLE. for the rpm range in question.
NO this is useless on electronic ignition ...obviously.
One of my toys runs 24° advance at an idle of 850 rpm. Buy by 1200 rpm it's right back at 12° and progresses to 54° at 4500rpm (a non linear curve)
but that is a programmable FI system using a spreadsheet as an advance curve
A 1992 IROC.... doesn't even have a curve (or specs you'll find published anywhere.) You set an idle timing..... period! Below a set rpm (980rpm) ALL timing is max'ed out and only retarded when the knock sensor kicks in (how whacked is that) So even there a vac gauge does work to set the smoothest idle AT the proper idle rpm.

...flame suit on.
ptegler

 

Unless someone has reprogrammed the ECM in that IROC, the factory tune has a timing curve over all RPMs, loads, and temperatures, There's NWIH to meet emissions and mileage requirements with a locked timing curve that's "max'ed out" at idle speed.

 

he he .... nope! no tables. Full schematics and hex source code in hand here... can personally attest to the archaic nature of the beast. No cats, no air pumps, . no crank sensor....etc etc on a '91 Also the reason(s) it was binned in favor of a home built fully programmable ECU using the norms of MAP, IAT, water temp, crank trigger etc

Aaahhh....just re-read your response...you bolded a portion.... my bad.I mis stated/ mis typed .... Below a set timing (980rpm) timing is fixed, above, all timing is maxed out and only retarded....

The dist HEI module handled the 'limp' mode to at least get you off the road when the ecu died.

ptegler