Vacuum Advance For 1111258 TI

**Wayne M.** · January 2, 2011, 09:18 AM

Re: Vacuum Advance For 1111258 TI

Rich -- I don't know what the '67 TIM&JG says, but for the 1111258 distrib, the Delco Remy Test Specifications book DR-324S-2 calls for the 1116201 vac advance, which would be stamped MS 201 15.

**Richard M.** · January 2, 2011, 10:05 AM

Re: Vacuum Advance For 1111258 TI

Wayne,

Yes I've seen that referenced in one of the discussions above. There was much talk about the MS 163 16 and that it was equivalent and may have been used as a alternate part. That first thread I linked to in the opening post has a lot of info about it all. Someone there asked for info on other 258's and what they had for advance units.

The latest '67 JG makes no mention of any particular part# for the BB cars, only says...."except L88 430 hp, is cadmium or zinc plated and has a black hose....".

Rich

**Duke W.** · January 2, 2011, 12:21 PM

Re: Vacuum Advance For 1111258 TI

The B20 VAC with full time or ported vacuum advance converted to full time is the best functional VAC for all big blocks.

The VAC is worth three points - two for originality and one for condition.

When I judge mechanical at local chapter meets I'll deduct one point if the VAC doesn't have the correct OE stamped data AND the correct data is called out in the JG. If the stamped data is not called out and it's a "B-number" replacement with plating in good condition I take a one-point deduction for OE replacement.

Often the replacement VACs are not a good functional match for the engine, whether or not their specs match OE or not (some OE VACs are not a good match), in which case I will recommend the B-number that will provide the best function during owner debriefing.

There are a whopping total of three B-number VACs that are a good match for all vintage Corvette engines to include those that originally had full time vacuum advance and those that were converted to full time from ported.

It's just a matter of selecting the right one, which has been explained here many times and in papers that are on the Web as you referenced.

The 163 16 is a replacement VAC that showed up in parts books after GM consolidated a bunch of part numbers decades ago. It requires up to 18" manifold vacuum to deliver full advance, and thus, is not a good match for ANY vintage engine because it doesn't meet the "Two-Inch Rule" even for base small blocks; and I don't think it was OE on ANY vintage Corvette engine.

Based on a 1967 issue of Corvette News with tune-up specs, the L-71 VAC data matches the 201 15 specs, and this along with other data (see Wayne's post) yields a preponderance of evidence that the 201 15 was the OE VAC. It was also OE on all 1963 engines, but it was replaced by the 163 16 service part in the late sixties.

Given the points value of the VAC you are much better off to have a functionally correct VAC with full time vacuum advance. The idle/low speed driving behavior and fuel economy benefits far outweigh the one point loss for having the "wrong" VAC.

Duke

**Timothy B.** · January 2, 2011, 12:31 PM

Re: Vacuum Advance For 1111258 TI

To add some to this post, if the Holley (ported) vacuum advance port is in the side of the metering block the base can be modified so this port sees manifold vacuum and no one will be the wiser.

**Michael H.** · January 2, 2011, 02:20 PM

Re: Vacuum Advance For 1111258 TI

I've owned several unrestored 67 435 cars that had a #163 vacuum advance unit. I still see quite a few used 258 distributors that appear to have their original 163 unit.

I've driven 435 HP cars all over this country with their original vacuum advance system and never had an issue with overheating. If a restored 435 overheats, there's most likely something else causing it.

**Richard M.** · January 3, 2011, 06:43 AM

Re: Vacuum Advance For 1111258 TI

Originally posted by Timothy Barbieri (6542)

To add some to this post, if the Holley (ported) vacuum advance port is in the side of the metering block the base can be modified so this port sees manifold vacuum and no one will be the wiser.

That's sounds clever Tim. What exactly is the mod? Does the manifold get drilled, along with the carb base?

Rich

**Richard M.** · January 3, 2011, 06:49 AM

Re: Vacuum Advance For 1111258 TI

Originally posted by Michael Hanson (4067)

I've owned several unrestored 67 435 cars that had a #163 vacuum advance unit. I still see quite a few used 258 distributors that appear to have their original 163 unit.

I've driven 435 HP cars all over this country with their original vacuum advance system and never had an issue with overheating. If a restored 435 overheats, there's most likely something else causing it.

Michael, right, that's why I posted pics of this 258 with it's original(IMO), 163. Lars and Dukes articles mentioned the minute differences between the 201 and the 163, as does Joe Lucia mention his opinions about the interchange possibilities back then. This was in the referenced thread in the first post.

I'm just not using the 163 that's on it because it's old. I'd feel more comfortable with fresh, new functionality in the build I'm undertaking. It's my first BB experience and I need to rule out as much as possible before my first run-in exercise.

Rich

**Duke W.** · January 3, 2011, 09:41 AM

Re: Vacuum Advance For 1111258 TI

Originally posted by Richard Mozzetta (13499)

That's sounds clever Tim. What exactly is the mod? Does the manifold get drilled, along with the carb base?

Rich

I know at least one person who has done this. It only requires modifying the carb base. One hole has to be plugged and one hole has to be drilled to effectively move the vacuum advance signal port to below the throttle plate. No changes to the manifold.

It's difficult to explain without pictures, and it would make a good article for the Corvette Restorer.

If someone has an available Holley, a camera, and wants to make the change, you take the photos and I'll write the text.

The quick and easy way to implement full time advance on a Holley is to tee the vacuum advance signal line into the choke vacuum break hose, but modifiying the carb base as above makes the change totally invisible.

Duke

**Timothy B.** · January 3, 2011, 11:26 AM

Re: Vacuum Advance For 1111258 TI

Rich,

The modification is exactly as Duke states with only the base modified.

Only problem is with a 163 vacuum advance control according to my measurments it takes 16" vacuum to pull to the stop. If you change the can to a better suited part, it's not original.

I can't figure out why the advance is ported on this engine when the low hp engine has full manifold vacuum to the VAC.

**Michael H.** · January 3, 2011, 11:33 AM

Re: Vacuum Advance For 1111258 TI

Originally posted by Timothy Barbieri (6542)

Rich,

The modification is exactly as Duke states with only the base modified.

Only problem is with a 163 vacuum advance control according to my measurments it takes 16" vacuum to pull to the stop. If you change the can to a better suited part, it's not original.

I can't figure out why the advance is ported on this engine when the low hp engine has full manifold vacuum to the VAC.

I still don't understand what your trying to gain by changing the vacuum source. Is there some issue with the way the engine runs?

**Duke W.** · January 3, 2011, 12:30 PM

Re: Vacuum Advance For 1111258 TI

Ported vacuum advance yields greater EGT, which is good for emissions, but it transfers more heat to the cooling system, resulting in higher idle/low speed driving operating temperatures and increased around town fuel consumption.

Everyone I can recall who has made this modification including the late Rich Snow has confirmed the above effects, and all were very pleased with the change.

L-71 for California required K-19. The center carb was set up for ported vacuum advance and the same center carb was also used for 49-state versions without K-19. L-72 also used just one carburetor number set up for ported vacuum advance.

Ported vacuum advance is nearly universal on emission controlled engines.

It was also the configuration of the '63 FI engine - again, for obscure reasons. It was the first time Chevrolet used vacuum advance on Duntov cam engines, and they didn't get it right. The 201 15 VAC used on all '63 distributors didn't meet the Two-Inch-Rule for the 340/360 HP engines.

It was not a particular problem with the FI engine because it had ported vacuum advance, but my 340 HP engine, which is full time vacuum advance suffered from major idle stability/stalling problems until I replaced the OE VAC with the 8" 1116236 that was installed on the updated 1964 SHP/FI distributors, and the '64 FI engine was full time vacuum advance.

With full time vacuum advance, L72/71 should pull about 14" idling in neutral at 900, so a 12" VAC meets the Two-Inch Rule, making the B20 or B26 a perfect fit.

Duke

**Timothy B.** · January 3, 2011, 03:19 PM

Re: Vacuum Advance For 1111258 TI

Michael,

I can only speak to my low hp cars with full manifold vacuum to the VAC. The engine runs very efficient and cool and it likes the timing very much at idle.

I realize too much timing at idle and the engine will want a leaner A/F ratio and as a result you have to gas the throttle to get it moving. In contrast, a richer A/F ratio because of a less efficient idle, (less timing) and the car will launch without much effort.

I am sure the ported was done for a reason but why the SHP big block and not the other engines, the only reason I can figure is my thinking above.

**Duke W.** · January 3, 2011, 04:05 PM

Re: Vacuum Advance For 1111258 TI

At idle, even with low overlap cams, there is considerable exhaust gas dilution because exhaust pressure is essentially atmospheric and inlet pressure is only about half an atmosphere. So there is considerable leakage of exhaust gas into the inlet port during the overlap period. It has two effects.

1. It reduces the ignitability of the mixture unless it is very rich

2. It slows flame propagation speed.

Typical best idle speed mixture is about 12.5:1, which corresponds to about six percent CO.

After converting from ported to full time vacuum advance, the idle speed/mixture procedure must be accomplished using the pre-emission procedure.

Set idle speed at the target, the idle mixture screws at the initial position called out in the service manual, and then adjust them to achieve maximum speed/manifold vacuum. Readjust the idle speed back to the target and tweak the mixture screws to see if you can get more speed/vacuum. Repeat the procedure until adjusting the mixture won't increase the idle speed anymore.

Best idle spark advance on vintage Corvette engines is low twenties to low thirties. Low overlap cams are okay at the lower end of this range, but high overlap cams are best at the top end of the range, because the more more exhaust gas dilution, the slower the flame propagation speed, and high overlap cams cause the highest exhaust gas dilution, even with their higher required idle speeds. If idle speed is set too low, exhaust gas dilution is extreme and idle can be unstable to the point where the engine can stall after idling for a short period.

For most engines, the proper idle spark advance will be in the correct range with the sum of initial timing and full vacuum advance, and if you have both the mixture and idle spark advance in the correct range, shooting the manifolds with an IR gun should show not more than about 500 deg. F.

Ported spark advance engines will show much higher exhaust manifold temperature.

Duke

**Timothy B.** · January 3, 2011, 04:21 PM

Re: Vacuum Advance For 1111258 TI

What I don't understand is if emissions was such a big concern for the 435 engine (ported advance) then why not on the other small blocks & big blocks for 1967.

Regardless, I am sure the engine will run much better with pre emission advance set-up.

Vacuum Advance For 1111258 TI

Vacuum Advance For 1111258 TI

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