69 vacuum advance for L71

[Kenneth K.]

[Joseph W.]

Re: 69 vacuum advance for L71

201 15 is what is used for a manual.
I would call Dave Fiedler on Monday and ask him.
Guaranteed he would know and possibly have one. I know he told me he can rebuild them if you have your old one.

[Duke W.]

Re: 69 vacuum advance for L71

Tell us the data stamped on the currently installed VAC mounting bracket. You may have to disengage the cap retainers and move the cap to see the data. Also tell us what the VAC specs are from either your '69 service manual or the AMA specs that is a free download from the GM heritage Web site. These above data will allow determination of the OE VAC part number. Original GM manufactured VACs are long gone, but functional aftermarket equivalents that look the same to casual observation are available.

Since your engine is emission controlled, vacuum advance is ported, so no vacuum advance at idle. This will tend to make it run hot in low speed traffic especially in hot weather. I recommend you convert to full time vacuum advance.

You can tee into the choke vacuum break hose for a full time manifold vacuum source for the VAC or to tee in a vacuum gage to measure manifold vacuum.

Measure idle vacuum in DRIVE, and report it at xx" Hg @ yyy RPM. Make sure you fully engage the parking brake and chock at least two wheels or have someone in the driver's seat with their foot on the brake.

Then use the measured idle vacuum in DRIVE and the Two-Inch Rule to select which of the three VACs that provide full advance at 8", 12", or 15" is the best fit.

I suggest you look at the end of the sticky section, open the restoration documents thread and download and read the tuning seminar, and pay particular attention to the page on emission controlled engines. It will explain why all the above is necessary to find the best functional VAC.

Duke

[Joe L.]

Re: 69 vacuum advance for L71

My vacuum advance needs replaced for my 1969 L71 435 with the TI and automatic transmission Does anyone know what the correct vacuum advance GM part number is? The corvette supplies give several different numbers so I am not sure which one I need…


Thanks,
Ken

Ken------


The original vacuum control used in PRODUCTION for your application was a GM #1116201. However, that vacuum control was discontinued from SERVICE in January, 1965 and replaced by GM #1116163. The latter was discontinued about 20 years ago and replaced by GM #19138050 which was a Standard Motor Parts manufactured vacuum control. It was stamped B1. The 19138050 was discontinued without supercession several years ago. I believe the Standard Motor Parts equivalent, VC-24A and stamped B1, remains available, though.

However, notwithstanding the above, for a better running engine I'd recommend doing as Duke advises.

[Duke W.]

Re: 69 vacuum advance for L71

The ...163 (SMP B1) was foisted upon us decades ago by an ill-conceived parts consolidation by GM, and the aftermarket followed suite. The B1 may take 18+ inches Hg to pull to the limit, so it won't even meet the Two-Inch Rule for a base small block. I call it a "boat anchor".

Maybe you can tell us Joe, but I believe the ...201 was only available in service parts for a short time before it was replaced with the ...163 even though its production use went as far back as at least '62 and up to the late sixties/early seventies in the L-71 and LT-1.

Duke

[Joe L.]

Re: 69 vacuum advance for L71

The ...163 (SMP B1) was foisted upon us decades ago by an ill-conceived parts consolidation by GM, and the aftermarket followed suite. The B1 may take 18+ inches Hg to pull to the limit, so it won't even meet the Two-Inch Rule for a base small block. I call it a "boat anchor".

Maybe you can tell us Joe, but I believe the ...201 was only available in service parts for a short time before it was replaced with the ...163 even though its production use went as far back as at least '62 and up to the late sixties/early seventies in the L-71 and LT-1.

Duke

Duke------


The earliest PRODUCTION use of the GM #1116201 that I can find is the 1960 model year for various Chevrolet applications but it might have been used prior to that for other GM carline applications. The earliest SERVICE availability appears to be late 1961 or 1962. As I mentioned, it was discontinued from SERVICE in January, 1965 and replaced by the GM #1116163.

[Mark E.]

Re: 69 vacuum advance for L71

Take a look at a variable VAC unit; Accel is one brand. It allows you to dial in the amount of VA that works for your car and fuel. It won't pass judging, but makes a noticeable difference in throttle response.

[Duke W.]

Re: 69 vacuum advance for L71

I've never been a fan of so-called adjustable vacuum advances because you can't adjust both total advance and start/stop points.

There are only three OE/OE replacement VACs for single point/mag pulse distributors to choose from that are a "best fit" for OE or modified engines that have full time vacuum advance. All provide 16 degrees of advance at 8, 12, and 15 inches Hg, and the start points for all three are about half the vacuum it takes to achieve the full 16 degrees.

For any engine configuration, select the least aggressive VAC that meets the Two-Inch Rule. No adjustment needed. It's a no-braiiner!

Duke

[Kenneth K.]

[Mark E.]

Re: 69 vacuum advance for L71

I've never been a fan of so-called adjustable vacuum advances because you can't adjust both total advance and start/stop points.

There are only three OE/OE replacement VACs for single point/mag pulse distributors to choose from that are a "best fit" for OE or modified engines that have full time vacuum advance. All provide 16 degrees of advance at 8, 12, and 15 inches Hg, and the start points for all three are about half the vacuum it takes to achieve the full 16 degrees.

For any engine configuration, select the least aggressive VAC that meets the Two-Inch Rule. No adjustment needed. It's a no-braiiner!

Duke

Below is the advance chart for the Accel variable VAC. As it's adjusted, it varies the vacuum point for zero advance, maximum advance, and the amount of maximum advance. Minimum vacuum for maximum advance can be adjusted between about 8" to 11" Hg. So using the 2" rule, this VAC can accommodate engines with idle vacuum as low as 10" Hg.

After setting centrifugal advance for optimal WOT performance, I adjust the VAC for maximum advance without detonation during part throttle. I keep the adjustment wrench in the car so I can fine tune it for varying fuel and weather conditions. It provides an additional way to fine tune the ignition.

I've used adjustable VACs with good results on several engines including L48, L82, LS5, '70 L34 Chevelle, '71 LS3 Chevelle, '69 350/250hp Impala, '69 Cadillac 472, 350 and 455 Olds V8s. All of these are high vacuum engines.

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[Duke W.]

Re: 69 vacuum advance for L71

The 201 15 is the OE part number, so your VAC may be original. Have you checked that it meets spec?

With only 9"@ 750 in Drive even the B28 VAC doesn't pass the Two-Inch Rule, but it is the most aggressive VAC available, so in your case call it the One-Inch Rule.

Go the the end of the sticky section, open the restoration documents thread and download the "tuning seminar". The Rule is explained there, plus a lot of other information that you should find useful. Failure to meet The Rule will cause idle instability that leads to engine stalling because the VAC will not be "locked" at full advance. As vacuum drops below the point of maximum advance idle advance is lost that results in lower vacuum and further loss of idle advance until the engine stalls.

The B28 VAC is getting hard to find. As of now it's my understanding that the only available "brand" (all major brand names including Delco buy this part from Standard Motor Products) is Airtex, part no. 4V1053, but there are reports that some don't meet spec. I suggest you order and take delivery in person from a local parts store that can order Airtex. Bring your Mighty Vac and test that it meets nominal spec, start @ 4", max (16 crank deg) @ 8". If it maxes out a little less than 8" that is a good thing.

Also buy a couple of feet of 1/8" black vacuum tubing and a 1/8" tee. Remove the choke vacuum break hose, then fabricate a new harness with the tee, one outlet hose to the short VAC pipe attached to a manifold bolt and the other to the choke vacuum break diaphragm nipple. The inlet is the same as the OE choke vacuum break hose, which is a full time manifold vacuum source.

The additional idle advance will increase idle speed several hundred RPM, so you need to perform the idle speed-mixture adjustment procedure after you've made the mod.

Let us know how it works out. With the air cleaner installed no one will ever see the change, and it is easy to reverse if you want to have your car judged.

Also, since the 201 15 VAC has not been available in service parts since 1965 don't throw it away unless it doesn't work properly.

Duke

[Kenneth K.]

Re: 69 vacuum advance for L71

I have a 4V1053 on the way.

I bought my L71 in 1978 so it is almost certain that the VAC is original. Unfortunately it does not work.

Duke- thanks for directing me to the restoration files on the sticky page. I never really paid much attention to those documents until now. What a wealth of information. I really got a lot out of your Technical Information Bulletin on the VAC.

So based on what I read I will set my initial timing at 12 deg (which it already is at). Then when I attach my new 4V1053 VAC to the manifold vacuum tee’d off the vacuum choke break hose I should see 16 deg (after adjusting the idle speed back to 750 RPM).

I also could increase my idle speed to 800 RPM to increase my idle vacuum so it would be closer to the 2” rule. I am setting my idle speed now based on my mechanical tach so it may not be that accurate. For some reason my old tach/dwell meter doesn’t want to work with the TI. I also tried using a friend’s tach/dwell meter and it doesn’t want to work with the TI either. That is why I have to use my mechanical tach to set the idle speed.

I was going to remove my original VAC while the distributor is in the car but not sure if I can. The screw that holds the advance arm on the distributor is partially covered by the magnetic pickup coil. Do you know if I need to take the magnetic pickup off the distributor to change the VAC?

On one of your seminar slides you mention that most OE engines should run fine on ethanol free premium. Does this apply to the L71? I mix 50% av gas with 50% ethanol free premium. If I can run just ethanol premium that would be a huge convenience…

Thanks,
Ken

[Duke W.]

Re: 69 vacuum advance for L71

Look at the centrifugal advance specs in your '69 CSM or the GM Heritage center info package. It should be start @ 900, 30 @ 3800 max. So to achieve the best WOT power advance of 36-40 degrees, you should set the initial at 6-10 at less than 900, or with a dial back light set it at 36-40 at about 4000. Since it's quite possible that the centrifugal has been altered somewhere along the line verify that it meets spec, and if not, document what it is and go from there.

As you should be aware the above MUST be done with the VAC disconnected and the hose plugged with a golf tee or equivalent.

Once you've done the above connect the VAC, and at idle in Drive you should see the sum of initial and full vacuum advance, 22-26 degrees, and it should be stable.

The tuning seminar says that most engines will operate on 91-93 PON pump gas. Whether it has ethanol or not is irrelevant. In CA all we can get is 91 PON E10, and big blocks are more sensitive to octane than small blocks.

Since OE compression ratios are usually lower than advertised, and many engines have been rebuilt with "low compression" it takes experimentation to determine the minimum octane to keep the engine out of detonation. When the tank is near empty pump in about three gallons of your highest available pump premium, and perform detonation tests. Load up the engine at low speed. Repeat this test three times to dilute the existing avgas. If it detonates reduce initial timing in 2 degree increments and continue testing and adjusting timing as required until the detonation ceases.

About a year ago I converted a friends's '67 L-71 to full time advance vacuum advance. This car has 16K original miles and the heads have never been off, so it has the original CR, probably somewhat less than 11:1. He had been using 100 LL avgas, but it was EXPENSIVE and inconvenient.

Because the best pump premium in CA is only 91 we had to reduce initial to 2 degrees (OE recommendation is 4), so it lost a little power, but he rarely demands WOT from it. With 93 PON pump premium, I'm sure that we could have run at least 6 initial to at least get into the lower end of best power total spark advance range.

The VAC is easy to swap out with either a mag pulse or single point distributor. The trick it to pump down the VAC to full advance, then remove the two screws with suitable screwdriver(s) and carefully wiggle the pin free from the stationary pole piece (or breaker plate on single point distributors). Then pump down the new VAC to install in the reverse order.

If a single point distributor, don't forget to secure the breaker plate ground wire to one of the VAC retaining screws.

Since I have grown to HATE TYPING, request you use that thing known as a TELEPHONE that most people don't use anymore to CALL ME if you have any other questions or issues that arise.

Duke

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[Kenneth K.]

Re: 69 vacuum advance for L71

Duke-

I will give you a call if I run into any problems. I think I got it. I am going to start with initial timing at 8 which is the midpoint of the 6 -10 that you recommend. I have my vacuum tee’d and connected to the manifold vacuum at the choke break hose and just waiting for my new VAC 4V1053.

I rechecked my original 201 15 VAC on my TI distributor and it does work. It rotates the magnetic coil about 1/4” when I fully pump it down. Seems to start opening at around 11” Hg and is fully open at 15” Hg. I will keep it in my spare parts since I won’t need it any more after I get the new VAC installed. I am going to remove the distributor so I can replace the VAC on the bench. It looks to me like I will need to remove the magnetic coil to get to the one screw out that is partially covered beneath the coil. I will be able to take a closer look at it when I have it on the bench.

Thanks for all the great advice and information you posted on the technical sticky pages. I learned a lot about the VAC. I am hopeful that if I can get 24 total advance at idle that my engine will run cooler so I can parade the car again.

Ken

[Duke W.]

Re: 69 vacuum advance for L71

Be sure to check the that the new VAC meets nominal specs before you install, start at 4", pulled to the limit at 8". There are reports that some are WAAAAAAy off spec. The 201 should start at about 8".

As I explained in post #13 there is no need to remove the distributor to swap out the VAC. But it would not be a bad idea to remove the distributor, disassemble, clean, check for any problems, and lubricate the upper bushing grease well, which is probably long dried out unless you have records that it's been done.

Removing and reinstalling the distributor is easy to screw up if you have no experience at this task. I've explained it a zillion times here and on the Corvette Forum, but if you have never seen my explanation, call me and I'll explain.

Duke