Vacuum advance help

[Robert E.]

I Don't want to beat a dead horse, but I also need some assistance decoding the can on my '71 LT-1. I recently had my motor rebuilt and upped the CR to approx 10.25:1 and am having some detonation and overheating problems.

I have read through the archives about the subject and realize the potential problem maybe the vacuum advance can. I realize taht Duke Williams recommends the NAPA/Echlin #VC1810 for my application.

I haven't actually checked to make sure my advance can is operational at this time, but have retreived the numbers off it. It is 437 15.

Can someone tell me if at least this can is appropriate for my application?

Thanks in advance

Robert
41801

[William C.]

Re: Vacuum advance help

What is the proper distributor number for you car? I'm a little light on '71 parts numbers.

[Robert E.]

Re: Vacuum advance help

Bill,

It's a T.I. 1112038.

Thanks

Robert

[William C.]

Re: Vacuum advance help

Specs are : centrifugal-0-4 degrees @ 1300 rpm, 14-20 degrees at 2400 rpm, 22-26 degrees at 4800 and above. Proper vac adv is the 1973437, starts at 7-9 inches of vacuum, full advance at 16-18 inches, 15 degrees crank.

[Duke W.]

Re: Vacuum advance help

The detonation is probably because the engine has ported vacuum advance, which heats up the combustion chamber boundaries and causes hot spot preigntion that leads to detonation.

Suggest you convert to full time vacuum advance (You can tee into the choke vacuum break line for full time vacuum) and install the VC1810 vacuum can. These two changes MUST go together. Do not do one and not the other.

Even though the VC1810 is more aggressive, detonation should be less because the internal engine temperatures will be cooler.

Set the initial timing at 10. With full time VC1810 advance the total idle timing will be 26, which should make the engine run cooler and eliminate the detonation. It will also use less fuel and be more responsive.

If the detonation goes away you can then try quickening the centrifugal, which should yield further improvements in throttle response and low end torque.

Most SHP/FI 327s will run the OE igntion map without significant detonation, but they don't have maps that cause the engine to run hot that can cause detonation.

Duke

[Patrick H.]

Re: Vacuum advance help

Sounds like he still has his original can, then.

Don't throw it away, Robert! You'll still need it for judging someday.
Well, someone will....

Patrick

[Robert E.]

Re: Vacuum advance help

Gentlemen,

Thank you very much for the information. As Patrick has stated, the can may be original, I have to wonder if it's still opperational? I will certainly take Duke's advice and will change the vacuum can and convert to full time vacuum advance.

Robert

[Verle R.]

Re: Vacuum advance help

Is there and equivalent to the VC 1810 for an HEI distributor?

Thank you,

Verle

[Duke W.]

Re: Vacuum advance help

Not exactly. All the HEI cans are set up for emission control, so they typically have more advance than pre-emission set ups, but at much higher vacuum.

The late seventies L-82 vacuum can is closest. The specs are 0@4", 10@8". The Delco number is D-1356, but I haven't been able to cross this to an aftermarket number, and I don't know if Delco has any remaining stock.

I use this can on my Cosworth Vega (with full time vacuum to the can), which didn't have OE vacuum advance. It considerably improved idle quality, low speed driveability, reduced coolant temperatures in hot weather low speed driving, and I was able to reduce idle speed from 1600 to 1200 with 12" vacuum. After I reindexed the cams (retarded the inlet 8 deg. and advanced the exhaust 8 deg.) to eliminate most of the valve overlap it idles butter smooth, 18"@900 (any lower and the alternator won't charge with the lights on); 80 percent torque bandwidth is 1900 to 7200 with one HP/CI at the rear wheels at 7000 on regular unleaded , and it pulls smoothly from below idle speed.

From being the least flexible engine I have ever owned, it is now the MOST flexible.

Duke

[Duke W.]

Re: Vacuum advance help PS

I have Lars' listing of Delco vacuum cans for single point, TI, and HEI. If anyone wants it e-mail me.

The list is comprehensive, but probably not 100 percent complete. Be sure to READ THE INSTRUCTIONS, particularly the part about the specs. Advance is listed in DISTRIBUTOR degrees, so you have to double this number to determine crankshaft degrees.

Duke

[Chris D.]

Re: Vacuum advance help

Interesting that there is such a large spread between your 18" idle vacuum and the 8" level where your favored can tops out. I take it the rule of thumb that suggests a can limit 2" lower than idle is simply a starting point. Is there something inherent to your setup that drives this or did it just work out that way?

[Duke W.]

Re: Vacuum advance help

The 2" "rule of thumb" is the minimum to ensure that the vacuum can is pulled to the limit at idle with the desired total idle timing of 24-32 degrees for Chevy V-8s. If idle vacuum will not pull the can to the limit it can "dither" and cause loss of idle timing,which leads to idle instablility. Been there with my '63 L-76. It only pulls 12"@900, but the OE can was 16". This first year that Chevrolet put vacuum cans on SHP/FI engines, they didn't get it right! I figured this out in my sophomore year of my undergraduate engineering studies and upgraded my distributor to '64-'65 SHP/FI specs, which solved my idle problems. That was about 40 years ago.

I don't recommend the VC 1810 for all engines. For mild SBs a 16" can is a good starting point, 12" can for all Corvette BBs, and 8" for most SHP/FI SBs.

Since L-79s pull 14-15" at idle, a 12" can should be okay even though the OE can is 8". If a L-79 exhibits part throttle detonation, a 12" can should eliminate or at least mitigate it.

My Cosworth Vega did not have an OE vacuum can, and the OE recommended idle speed is 1600, and it only pulled about 16". This is because of the excessive OE valve overlap to control NOx. It would not idle any lower without becoming unstable, so I added a 8" can to get a lower idle speed with acceptable stability and was able to achieve 1200@12", The 8" can was about right.

After reindexing the cams to eliminate most of the overlap, the 8" can is more aggressive than necessary for the 18" idle vacuum, but it doesn't detonate at part throttle, so there is no need to swap it out for a less aggressive can. NOx doubled, but it's still only half the California limit.

Also there is the "problem" that most HEI cans are set up for emission control V-8s, so they have way more vacuum advance than the CV needs. Since it only needs about 30-32 degrees of maximum WOT timing due to its small chamber and centrally located spark plug, it doesn't need as much vacuum advance as a SB either, so the 10 deg @ 8" was about the only choice I had that delivered a modest amount of advance at relatively low vacuum.

So it's set up with 14 degrees initial and the vacuum advance brings total idle timing to 25 degrees (the vacuum can actually adds 11 deg.- production tolerance) and the 18 degrees of centrifugal is all in at 2000, which yields total WOT timing of 32 and total cruise timing of 43.

One of my "demonstration tests" is to use the brake to drag the CV down to 200 revs in first gear where it chugs like a little diesel, then floor the throttle. It just takes off without a hiccup or protest, and the speed-density analog EFI system certainly contributes to this flexibility. Conventional carburetion would not achieve this feat. Since it's only a 2-liter engine, it obviously has limited torque, but third gear is usable in normal driving to 1000 revs and fifth to 1500. The OE gearing yields 3000@60 in fifth - about the same as a Corvette with 4.11s.

Duke