VAC Ported to Manifold Vacuum Modification Results

[Chris H.]

[Duke W.]

Re: VAC Ported to Manifold Vacuum Modification Results

You're Welcome! I take it this is a L-71 and you replaced the OE 201 15 VAC with a B26, then went through the per-emission idle speed/mixture adjustment procedure. Correct? If you keep accurate fuel consumption records, you should see an improvement.

[Chris H.]

Re: VAC Ported to Manifold Vacuum Modification Results

Duke, yes, L71, replaced the OE 201 15 VAC with the B26 and went through the idle speed / mixture adjustments. Next up I'll swap in the lighter centrifugal advance springs. Chris

[Gene M.]

Re: VAC Ported to Manifold Vacuum Modification Results

Duke, yes, L71, replaced the OE 201 15 VAC with the B26 and went through the idle speed / mixture adjustments. Next up I'll swap in the lighter centrifugal advance springs. Chris

Chris, don't go too light as you have to stay out of centrifugal so it does not start affecting the idle timing. If you don't have a sun machine you can still check on the car. Start with one spring change and work into it.

[Chris H.]

Re: VAC Ported to Manifold Vacuum Modification Results

Gene thanks for the tip. Duke, in your paper why do you mention weight pivot bushings? Is it because they disappear? Or the stock bushing is not optimal? Just curious thanks.

[Scott P.]

Re: VAC Ported to Manifold Vacuum Modification Results

Duke, my search ability seems to be nearly zero. Would you please point me to the Paper that Chris is referring to. Thanks Scott

[Richard M.]

Re: VAC Ported to Manifold Vacuum Modification Results

Scott, I've attached Duke's White paper below.

Duke, and others, were instrumental in helping me with my first L71 Engine Build about 6 years ago.....Here.

Since then I have converted several ported vacuum BB VAC systems to full time vacuum with excellent results. As Chris has done, I simply add a Tee to the choke pull-off vacuum line, tie the B26 VAC to that and plug the ported carb fitting.

Rich

[Duke W.]

Re: VAC Ported to Manifold Vacuum Modification Results

Gene thanks for the tip. Duke, in your paper why do you mention weight pivot bushings? Is it because they disappear? Or the stock bushing is not optimal? Just curious thanks.

The centrifugal weight bushings only apply to HEI... not used on point type or TI distributors.

[Duke W.]

Re: VAC Ported to Manifold Vacuum Modification Results

Next up I'll swap in the lighter centrifugal advance springs. Chris

All year L-71s have a fairly aggressive OE centrifugal curve, start @ 900, 30 @ 3800 (L-72 is 30 @ 5000), but initial timing is specified as only 4 deg. so total WOT advance is below the optimum range of 36-40.

Start by advancing initial to the 8-10 range. You should be able to feel this SOTP at low revs, but check for detonation by loading up the engine at low to medium revs.

If it doesn't detonate you can then experiment with lighter springs until you find the detonation limit.

The last L-71 I worked on actually needed a slower centrifugal curve due to detonation on 93 PON fuel, but we knew that the CR was probably higher than when it left Tonawanda.

It's not a big deal if the centrifugal starts below idle speed, This is the case with 365/375 HP 327 that have a very aggressive OE centrifugal curve, start @ 700, 24 @ 2350 because it's tough to get a 30-30 cam engine to idle much below 900 with acceptable quality.

As you're finding out, it doesn't take a lot of time or funds to optimize the L-71 spark advance map, and the return on investment in terms of time spent and cost is excellent. In fact, I can say that for pretty much all OE engine configurations.

Duke

[Edward B.]

Re: VAC Ported to Manifold Vacuum Modification Results

Duke, do you have a timing paper for dummies? (And I mean DUMMIES!! I've NEVER been able to understand the ins and outs of timing.)

No matter what I do, when I change my 69 L36 4 speed from ported vacuum to manifold vacuum, the car runs like crap! I don't know about the others, but in my case I need a VERY SIMPLE step by step tutorial guide. A Youtube step by step video would be even better! I'm REALLY clueless when it comes to this.

If you don't have one, you're more than welcome to come over to my house and use my car as the Guinea pig!!! I'm in Murrieta.

Ed

[Chris H.]

Re: VAC Ported to Manifold Vacuum Modification Results

My current vacuum hose arrangement to the VAC is only temporary. I will eventually drill the carb base plate so the setup will look completely stock. Photo courtesy of Timothy Barbieri.

[Jeffrey S.]

Re: VAC Ported to Manifold Vacuum Modification Results

My current vacuum hose arrangement to the VAC is only temporary. I will eventually drill the carb base plate so the setup will look completely stock. Photo courtesy of Timothy Barbieri.

Chris,
You beat me to the punch! This is exactly how I have my 69 Quadrajet set up to run full time advance. I did this many years ago and the good thing is that it appears completely stock and if you want to go back to ported (why would you) some JB Weld in the hole reverses the modification. Thanks for posting the picture.
Jeff

[Duke W.]

Re: VAC Ported to Manifold Vacuum Modification Results

Duke, do you have a timing paper for dummies? (And I mean DUMMIES!! I've NEVER been able to understand the ins and outs of timing.)

No matter what I do, when I change my 69 L36 4 speed from ported vacuum to manifold vacuum, the car runs like crap! I don't know about the others, but in my case I need a VERY SIMPLE step by step tutorial guide. A Youtube step by step video would be even better! I'm REALLY clueless when it comes to this.

If you don't have one, you're more than welcome to come over to my house and use my car as the Guinea pig!!! I'm in Murrieta.

Ed

I don't make house calls, but you're welcome to drive up to my place in Redondo Beach for a day, and we can probably get your car squared away, or we could hook up at a chapter meet.

I evaluated a '69 L-36 TH400 earlier this month. It has some issues that we weren't able to resolve that day, but the owner is making progress, and at least, now, I know a better source for the timing light power on a C3.

The thread discussing the issues is here:

https://www.forums.ncrs.org/showthre...311#post805311


As far as a detailed step by step procedure of what I do is concerned, it doesn't exist, but the best way to learn is on-the-job training.

Duke

[Edward B.]

Re: VAC Ported to Manifold Vacuum Modification Results

I may take you up on that Duke! Although I rarely drive the car anymore, I'd still like to get it so it doesn't run so hot. I purchased a new Dewitts radiator a few months ago, but haven't had the time to install it! I'm hoping that with the new radiator and changing to manifold vacuum, I can keep the car under 200 degrees!

Out here in Murrieta/Temecula, it's not unusual for the temperatures to reach 120 degrees at the hottest part of the summer, and the car is basically undriveable during those times. (It's supposed to be in the low 100's today and the rest of the week!) I typically see over 210 on the gauge, and even 220 in stop and go traffic.

The car has K66, so does that make any difference?

Ed

[Duke W.]

Re: VAC Ported to Manifold Vacuum Modification Results

Single point and TI distributors use the same family of VACs, centrifugal weights/springs, and they are easy to swap without removing the distributor. So as long as the distributor doesn't exhibit any internal problems that require removal and disassembly to correct, getting the spark advance map optimized is pretty easy.

Duke