Wanting to switch my 1967 L71 to manifold vacuum

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Joe,

Thanks for the information. I am not a member of the St. Louis chapter, however I purchased the car from Randy Rapp and have many friends who are members. I do have the B26 can that Duke recommends and it is rated for 16 degrees vacuum advance and that works perfect with 8 degrees initial advance for a total of 24 degrees advance at 800 rpm idle speed. My distributor mechanical advance is set up for only 20 degrees (factory should be 30 degrees) thus requiring 16 degrees initial advance. My focus currently is finding a way to modify my B26 can or another one better suited to limit the vacuum advance to 10 degrees. With the vacuum advance disconnected, the car performs flawlessly to redline with no detonation. Just trying to eliminate the light detonation between 1500 - 2500 rpm with light gas pedal pressure.

Thanks Chris

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

I have to question whether you have measured total WOT advance correctly. Disconnect and plug the VAC signal line, hook up a dial-back timing light and rev the engine until it stops advancing, record that engine speed, then blip the throttle to be sure it doesn't advance any more.

Without the VAC connected it may not want to idle, so use the idle speed screw to set idle at about 1200. At this speed there should be a few degrees centrifugal, so drop the idle speed until the advance stop retarding. You need to find the start point for the centrifugal (OE spec is 900), and if you choose to set initial advance rather that total WOT advance at high revs, you MUST set it below the engine speed that the centrifugal starts.

The OE maximum centrifugal is 30 @ 3800, so with 16 initial you would see 46 at over 3800, which is way to much. I question why anyone would limit centrifugal to 20, but you never know. In any event, do more testing and make sure you are reading the data correctly. Before testing, remove the cap and rotor and verify that the centrifugal advance mechanism moves freely with and without the weights and springs installed. Also, get everything squeaky clean and wipe down the parts with WD-40. NO GREASE OR OIL!!!

If you are absolutely, positively certain that max centrifugal is only 20, back the initial down four degrees, check total WOT advance again, and test for detonation. If it continues to detonate back down initial in two degree increments until the detonation stops.

The reason it's detonating is too much advance at low revs, and if the centrifugal is really 20, you will have to back down on initial enough to eliminate the detonation, and in the longer run, overhaul the distributor and get the 30 centrifugal back, and set initial at 8-10. There's nothing wrong with the VAC. The problem is either insufficient centrifugal/too much initial or you have not measured the centrifugal properly, think it's only 20 rather than something closer to 30 and are running too much initial.

Once the spark advance map is dialed in and with the vacuum advance connected, go through the idle speed/mixture adjustment procedure with a target idle speed of 900.

Duke

[Joe R.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Chris,

You're welcome. I'm sure it's a very nice car if you bought it from Randy. Maybe I asked the wrong question. You don't need to be a chapter member to come to a chapter meeting. You just need to be thinking about joining if you're not already a member. Come on by tonight if you can swing it!

Joe

[Mark E.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

There is also a way to achieve optimal (i.e. more advanced) timing at idle while using ported vacuum. The issue is that Chevy V8s with a close-to-stock cam will idle and perform much better (and cooler) with 10-12 degrees of initial (@ idle) and 36-38 degrees total centrifugal advance (WOT @ 0" vacuum).

There's (at least) two ways of achieving this:

Option 1- Run the VA with manifold vacuum. This requires modest (4 deg) initial centrifugal advance since most VA units add about 8 degrees at idle (this requires choosing a VA unit that creates the targeted advance with the vacuum your engine produces at idle). To achieve 36-38 deg total centrifugal, this requires 32-34 from the distributor's centrifugal advance.

Option 2- Run the VA through ported vacuum. This requires 10-12 BTDC initial centrifugal advance since there is no VA at idle. To achieve 36-38 deg total centrifugal, this requires 24-28 from the distributor's centrifugal advance.

Option 2 typically requires reducing the amount of centrifugal advance from the distributor's stock advance curve, which is why you'll sometimes see the "sleeve" mentioned in an earlier post... somebody limited centrifugal so they could add initial while using ported vacuum.

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Optimum total idle advance is in the range of low 20s to low 30s. This is achieved with the sum of initial and full vacuum advance. Also, the higher the valve overlap, the higher in the above range total idle advance should be.

The OE L-71 spark advance map is pretty good as is, although the OE 201 15 VAC (0 @ 8", 15 @ 15.5") won't work with full time advance because it doesn't pass the Two-Inch Rule, but the B26 does since all 16 degrees of advance is in at 12".

The OE centrifugal of 30 @ 3800 is a good starting point. Initial of only 8-10 will yield 38-40 total WOT advance, (and 24-26 total idle advance), but not too much at low revs, which might cause detonation. The only thing to do with the centrifugal is try some lighter springs to get it in earlier with low speed detonation being the limiting factor on how quick a centrifugal curve the engine can tolerate.

Ported vacuum advance is an emission control strategy (increases EGT to promote HC/CO oxidation in the exhaust system at the expense of fuel consumption and higher heat rejection to the cooling system) and is best to avoid unless the car requires field emission testing.

Duke

[Mark E.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Duke,

We should swap notes sometime. I've had some cars tolerate as much as low-20s for initial advance, but for many engines, that much or more initial advance can cause a rough idle and/or hard cranking.

Ported vacuum was born because of emissions. But ported VA along with more initial centrifugal can also be used to tune a car with an efficient WOT timing curve. Think about it:

- Idle to WOT with manifold VAC- Begins with a little centrifugal plus significant vacuum advance at idle. Then when you floor it, vacuum goes to zero which momentarily retards the timing... then timing advances again with RPM due to centrifugal.

- Idle to WOT with ported VAC- Begins with significant centrifugal timing only at idle (no VA). Then when you floor it, the vacuum signal to the VAC remains zero resulting in a strictly centrifugal advance curve. This avoids having the advance retard momentarily when the accelerator is floored and vacuum goes to zero.

When tuning a car, I'll typically try both manifold and ported, matched with different VAC and total centrifugal advance to see which the engine likes best.

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

I would never, ever use ported vacuum advance to set up a spark advance map unless it was necessary to pass field emission testing.

Please go to the link in post #35 and study the document, carefully. Talk about notes...!!!

Duke

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Mark,

I have 16 initial, 20 centrifugal, and B26 with 16 degrees in at 12" with manifold vacuum. This causes detonation 1500 - 2500 rpm with light gas pedal pressure. If I switch it back to ported vacuum, is it ok to leave the B26 in place or replace it with the original 201 15 VAC? Is 16 initial degrees enough to allow the engine to idle without overheating when stuck in traffic on a 90 degree day? My intent was to help cool this 435 at idle in traffic. Or should I just have the distributor blueprint rebuilt to original (30 degrees centrifugal) specs and continue with manifold vacuum? I assume 8 initial with 16 VAC = 24 idle advance will idle cooler than 16 initial alone? I am really trying hard to understand all the teachings and concepts from all who contribute on this forum, so thanks for all your contributions!!!

Chris Covinsky

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

I'm still skeptical of your data. Are you sure the centrifugal stops at 4000? Did you rev it to 5000 to confirm?

If you're sure the centrifugal is only 20 @ 4000 and does not advance further with further revs, remove the cap and rotor and check the centrifugal advance mechanism for cleanliness and proper operation. Further, I recommend you remove the springs, weights, snap a good close up photo and post it here. We can probably determine if they are OE or something else.

With the springs weights removed, you can then more closely inspect the autocam mechanism and possibly determine why is does not have the OE maximum 30 degrees centrifugal advance.

You're asking a bunch of speculative questions that can only be answered by the guy who takes accurate, confirmed data, and inspects the inside of the distributor if the data confirms a problem. The guy closest to the situation is you, but take Joe up on his offer to attend the chapter meeting.

I bet there will be someone there who can figure it out.

The setup I recommend is:

1. B26 VAC connected to a full time vacuum source.

2. The OE 30 degrees maximum centrifugal at 3800 or sooner with lighter springs if it doesn't detonate.

3. 8-10 initial for total initial plus centrifugal = 38-40

Probably at least a dozen guys with L-71s have set up their spark advance map this way with excellent results. Find out why you can't get the 30 degrees OE centrifugal and fix that problem. If you can't solve it by the time you have to drive to Bloominton, take initial out of it until it stops detonating and leave the B26 installed, connected to full time manifold vacuum, until you or someone else figures out and solves the problem.

Duke

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Distributor should arrive Monday to Dave Fiedler for a complete blueprint rebuild. He will make sure that everything is to factory specs. He did mention that some of his 435 customers request to set the centrifugal advance to 28 degrees instead of 30 (factory specs). I guess some want a couple of extra degrees of initial advance. I am leaning toward just having it restored to factory specs, as I certainly don't want to risk any detonation issues in the future. I still plan on using manifold vacuum to relieve hot idling concerns. Thanks again for everyone who takes the time to help mechanically disabled Corvette enthusiast such as myself!

Chris Covinsky

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Get the centrifugal set up as close to OE as possible - start at 900-1000, 30 @ 3800 although factory tolerance on max advance is about +/-2 degrees. Since SHP big blocks have more propensity to detonate at low revs you want more total centrifugal than a SHP small block, which requires less initial to achieve 38-40 total WOT advance. On a SHP small blocks, 24 centrifugal coming in at under 3000 revs with 14 initial will usually be okay.

Get the spring kit to experiment with how quickly you can bring in all the centrifugal without detonation. You can only do this on the car, and it has to be done on each individual car if you want to optimize performance and fuel economy.

Make sure he understands that you want the B26 VAC installed because you want to run full time vacuum advance, and an OE equivalent will not pass the Two-Inch-Rule. He may not understand that, so you will have to explain it to him.

Ask him to explain why the total centrifugal was less than spec and let us know why in this thread. There may be a lesson.

Duke

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Got bad new from Dave Fiedler today. The distributor out of my 435 that I sent him is a 1971 small block TI distributor, not good! He states that I need to have a 1967 big block distributor because the housing is specific to 1967 and earlier big blocks? Something to do with oiling passages. I do want the car to operate correctly, however purchasing a 1967 435 distributor is sounding painful!!! Is it true that I should not run a small block distributor in a 1967 big block? Or can the small block distributor be set to 30 degrees centrifugal and work just as well? Just wanting to do the right thing without breaking the bank!

Thanks Chris

[Michael C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Chris, I have a L-71 that was ported vacuum from the factory. At first I did what Duke recommended and tied into the choke hose from under the front of the center carb. (Full vacuum) The car idled and ran so much better after that. I am having my car flight judged and was not happy with non factory appearance of the tee connection. I didn't want the lose points for that. I looked into the modification needed to switch the correct nipple over to full vacuum. You just need to move the hole that feeds the side port to below the throttle blades where you have full vacuum at idle.
I removed the center carb and then removed the base plate. Looking closely at the side port you can trace its route thru the plate and see that it ends in a small hole above the throttle blade. This will not get full vacuum at idle. You have to drill a small hole in the base plate
at an angle that will exit below the plate. See the image attached that has been posted on here many times before. It shows the angle of the new hole. I used a dab of JB Weld to plug the stock hole above the plate that is not needed now. Note this can be reversed if needed. I hope that this helps you out like it did me.

Mike C.


Convert Vac.jpg

[Donald A.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

You just need to move the hole that feeds the side port to below the throttle blades where you have full vacuum at idle. I removed the center carb and then removed the base plate. Looking closely at the side port you can trace its route thru the plate and see that it ends in a small hole above the throttle blade. This will not get full vacuum at idle. You have to drill a small hole in the base plate at an angle that will exit below the plate...

I also did this mod, is a very easy fix and totally invisible on inspection of the carb and vacuum lines. Better idle, cooler idle especially in hot stop/go traffic. Photo below of my carb just after drilling the new hole. Old hole closed with JB Weld.

Carb Vacuum Mod.jpg

[Donald A.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Got bad new from Dave Fiedler today. The distributor out of my 435 that I sent him is a 1971 small block TI distributor, not good! He states that I need to have a 1967 big block distributor because the housing is specific to 1967 and earlier big blocks? Something to do with oiling passages. I do want the car to operate correctly, however purchasing a 1967 435 distributor is sounding painful!!! Is it true that I should not run a small block distributor in a 1967 big block? Or can the small block distributor be set to 30 degrees centrifugal and work just as well? Just wanting to do the right thing without breaking the bank!

1111258 restored correct orig tag dizzy around 2K, check Driveline, here, eBay etc. I had mine redone by Mike Zamora do a search to look him up here. Now here is the other message: NOT BAD NEWS to have a 71 small block TI dizzy in that the partial annulus big block 65-67 dizzy story has been well written up and described here and the other forum many many times and the consensus, see multiple posts by JohnZ (John Hinckley) that basically say that Chevy distributors are 100% interchangeable between small block and ANY standard deck height big block - the parts books show the same distributor (360* annulus) for all 57-74 Corvette SB and BB applications. The danger is using one of the partial (interrupted) annulus big block distributors and orienting it incorrectly which will cut off oil flow to the right side lifter gallery. SO your small block dizzy is fine as a TI housing but would need rebuilt with correct springs, weights etc for the L71 application. This is just my opinion but I researched this thoroughly and discussed with others and I definitely recommend you review JohnZ's posts on this and take his opinion as gospel!
So... I know Dave Fiedler is the TI guru but on this question I don't think he has given you the full story. You can definitely use your SB dizzy if it is setup correctly, housing will work just fine. Good luck with your car!