Wanting to switch my 1967 L71 to manifold vacuum

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

I believe L-71 centrifugal starts at 900 and maximum is 30 @ 3800, however rather than relying on my memory, suggest you check the specification section of your CSM.

Of course, you should verify this on the car with a timing light...suggest initial of 8, and full vacuum should add 16 for 24 deg. total idle advance at 900 or less.

If you find it idles satisfactorily below 900, that's okay. Set the final idle speed that is the lowest commensurate with what you consider acceptable idle quality, but it should not be so low as to pull less than about 13-14" manifold vacuum so the VAC stays locked at full advance at idle.

Duke

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Making progress! For those who choose to do this, don't transfer rubber sleeve from your original VAC (if there is one) to your new VAC, lesson learned. My 435 has always started and shut off with ease, it just got hot when at idle. Now it idles at 850-900 rpm with manifold vacuum at 14-15 and temperature stayed at 190 for a good 20 or more minutes in my garage!!! St. Louis was wet yesterday, so I have not driven it yet, but I am very optimistic! I ran out of time, so I have not checked the timing yet, but as well as it runs, I can't imagine its too far off. The VAC I removed from the car shows a MS 201 15, not sure what that means?

Very happy so far, Chris Covinsky

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

That is likey the OE VAC. The first three digits are the last three digits of the GM part number and the last two digits are the maximum crankshaft degrees. All OE VACs follow this format.

Hang on to it because the 201 15 was used on a number of OE engines back to at least '63 and up to the seventies, but it was discontinued long ago.

Duke

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

I have yet to read any negatives about this conversion. Since I have yet to drive it after this conversion, are there any concerns I should be looking for? Are there any drawbacks to changing to manifold vacuum with the B26 VAC? What is the reason for setting the idle for 900 vs the CSM setting of 750?

Chris

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

No.

No.

Reread post #31.

Download and study the following pdf:

http://www.metroli.org/pdf/2012%20Nationals%20tuningseminar.pdf

Duke

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Thanks for all your help! Can't wait to drive it!!!

Chris

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Finally a beautiful day here in St Louis! I have not touched the timing since I purchased this 435. My initial idle timing (600 rpm) shows 16 degrees with the VAC disconnected and plugged. At 4000 rpm with the VAC disconnected, total timing shows 36 degrees, so this indicates the centrifugal advance only being 20 degrees. With the vac connected, initial idle timing (800 rpm with 15" of vacuum) shows 32 degrees. At 4000 rpm with the VAC connected, total timing shows 52 degrees. I read in the CSM the distributor should start at 900 rpm and have total advance of 30 degrees by 3800 rpm. The cars starts, stops, idles, runs smoother and cooler, however I am concerned having the wrong amount of centrifugal advance of only 20 degrees. The timing must have been set at 36 degrees total advance with no concern for initial advance. Is it safe to drive the car like this, can this set up do any damage??? I assume what needs to be done is for the distributor to be removed and professionally set up to factory specs. The car runs fantastic, but I just want to be cautious not to damage anything.

Thanks Chris Covinsky

[Timothy B.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Chris,

I think it will be fine, the factory advance curves are conservative. If there is any part throttle ping try the 12* vacuum advance control but I bet the 15* control will be good to go.

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

I'm not aware that you can buy an off-the-shelf 12 @ 12" VAC, so concentrate on the centrifugal. It's possible someone made and ill-conceived modification sometime in the past.

With the OE centrifugal curve the initial should be about 8, but you should reverify where it starts and stops, which is easy to do with a dial back light. Bring revs down as low as possible (VAC signal line disconnected/plugged), then increase and note the RPM where the timing begins to advance. This is the start point. Then rev the engine until it no longer advances, note the RPM and blip the throttle to higher revs to verify that it no longer advances.

Since the centrifugal is considerably out of factory spec, you should investigate. Remove the cap and rotor and inspect the centrifugal mechanism for free movement. GREASE IS A NO NO! If it has grease, remove the weights/springs and thoroughly clean off all the grease and just wipe down the cleaned up parts with WD-40. Note any numbers on the weights and post a photo of the mechanism.

If the current setup does not detonate, then it might not need to be fixed, but you may not have much margin. Maybe the CR was lowered sometime in the past. The limiting issue on how aggressive the centrifugal can be is detonation.

The 30 @ 3800 is actually pretty aggressive for an OE big block. (The L-72 is 30 @ 5000.) The OE centrifugal might be able to tolerate lighter springs to bring full centrifugal in at less than 3800, but since detonation is primarily a low rev issue, I like the 30 centrifugal along with about 8 initial, so there is moderate total advance at low revs/low manifold vacuum.

Many have made this conversion, and all were very satisfied and reported the same improvements, which considerably reduced overheating anxiety in traffic on a hot day.

Duke

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Thanks again to all who contribute to this forum! I have learned so much and continue to learn more about my Corvette!

"Moderate total advance at low revs/low manifold vacuum." I assume that I would be better off having the distributor performing as per the CSM, with 8 degree initial and 30 degrees centrifugal as opposed to what I currently have which is 16 initial and 20 degrees centrifugal??? The car drove very well yesterday with no detonation that I could notice (factory sidepipe car), however I am thinking the 16 degrees initial advance with the 16 degrees from VAC = 32 degrees at 800 rpm is too much?

Chris

[Duke W.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Moderate to high overlap cams require high 20s to low 30s total idle advance to idle with minimum fuel consumption and minimum EGT.

Google duke williams san diego corvette, and download the pdf of my 2012 National Convenstion presentation to understand how to set up a spark advance map.

Your engine may tolerate the current map, especially if compression has been lowered, but with side pipes you may not be able to hear light to moderate detonation.

Get into the distributor and see if you can find out why the centrifugal is limited to 20 rather than the OE 30 degrees. That is the next step.

Duke

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

I am reviving this thread because I up till now everything has been perfect. Now that it has been much hotter out and I have been driving it much more, I am noticing a light detonation around 1500 - 2000 rpm with moderate gas pedal pressure. I am trying to find someone in St. Louis to set up my distributor to stock specs. I am pretty sure that 16 degrees initial advance plus 20 degrees mechanical advance (should be 30 + 6 initial) in addition to 16 degrees full vacuum advance is causing this. What else could I do to stop the light detonation until I can have the distributor set up to stock specs on a sun machine??? Should have listened to Duke at the end of April and fixed the distributor then, now its two week before Bloomington Gold and I am running out of time. Thanks Chris

[Joe R.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Chris,

You could take out some of the initial timing, maybe two degrees at a time. See how it drives after each change.

Joe

[Chris C.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Yes, at this point the only two options I can think of are to reduce initial timing or try a different vacuum can (possibly modify the b26 can to have less advance???).

[Joe R.]

Re: Wanting to switch my 1967 L71 to manifold vacuum

Chris,

I am also from St. Louis. The other thought I had was to take the car to Dyno Bob Stiegemeier and run it on the chassis dyno. Bring a variety pack of centrifugal advance springs and carb jets with you. Assuming he can start the dyno pulls below 1500 RPM, Bob will sort it out for you. Plus you'll know how much HP and torque you're really making! Check with Duke for more tips on how to get the most from a dyno session. I assume you're running the Duke recommended vacuum can and you have verified it is working properly. 16 degrees of initial advance sounds like an awful lot to me.

Here's a link to Dyno Bob:

http://stiegemeier.com/services/dyno-tuning/

Also, are you a St. Louis chapter member? There is a meeting at JJ's in St. Charles tonight at 6:30. I am sure you would get lots of good advice there.

Good luck!!!
Joe