Proper vacuum routing for L68 full manifold vacuum?

[Douglas C.]

Hi. My L68 is set up for full manifold vacuum advance, currently I have a T fitting linked to the rear manifold / brake booster T fitting hose, so I am sharing vacuum so to speak with the head lights, etc. for the vacuum advance cannister.

Is there a proper routing for running full manifold vacuum, as I wonder if my erratic head light door / wiper door function is simply related to my current set up.

Thanks,

Doug

[William C.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Is it a '68 or '69? There may be a direct port on the carb you can hook to to feed the Distributor advance, been a while since I worked on one of those. The issue with lights and wiper doors is a symptom of low vacuum at the actuators or bad actuators, have you measured the vacuum at your source and checked all hoses for leaks?

[Douglas C.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Is it a '68 or '69? There may be a direct port on the carb you can hook to to feed the Distributor advance, been a while since I worked on one of those. The issue with lights and wiper doors is a symptom of low vacuum at the actuators or bad actuators, have you measured the vacuum at your source and checked all hoses for leaks?

69 L68. Yes, manifold vacuum at idle around 14", actuators fairly new, new hose kits complete, although I do have to trace everything to make sure all is in order.

I just wondered if my set up for the full manifold vacuum right now is not the best location, am I stealing vacuum from effecient function of headlights / wipers.

Thanks,

Doug

[Duke W.]

Re: Proper vacuum routing for L68 full manifold vacuum?

First, with 14" idle speed vacuum you need a 12" VAC to comply with the Two-Inch rule. The OE VAC is a 12-incher, and the best modern replacement is the B20 or B26.

The larger the volume that the VAC is connected to, the slower it will respond to changes in manifold vacuum. My recommendation is that if you cannot find a full time vacuum port on the carburetor, tee it into the choke vacuum break hose, which I believe the Quadrajet has, and it sees manifold vacuum at all times.

Very little volume moves through the VAC itself, so I doubt if it has anything to do with any vacuum motor operational issues.

Duke

[William C.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Duke isn't 14 a little low at idle for a BB? I'd expect more...

[Duke W.]

Re: Proper vacuum routing for L68 full manifold vacuum?

I don't recall measuring one personally, but form the anecdotal evidence I've picked up on the TDB - sometimes in response to my requests for an idle vacuum/speed measurment - I have the following results for manual transmissions idling in neutral.

L36/68 - 14" @ 600
L78/72/71 14" @ 900

The L36/68 cam is no slouch - 214/218 deg. @ 050" with a 115 deg. LSA. It's a lot closer to the L79 cam than the 300 HP cam. The SHP cam at 050" (above the tops of the clearance ramps, so it's measured at .062" above the base circle) is 231/231 deg. with a 114 deg. LSA

Duke

[Douglas C.]

Re: Proper vacuum routing for L68 full manifold vacuum?

First, with 14" idle speed vacuum you need a 12" VAC to comply with the Two-Inch rule. The OE VAC is a 12-incher, and the best modern replacement is the B20 or B26.

The larger the volume that the VAC is connected to, the slower it will respond to changes in manifold vacuum. My recommendation is that if you cannot find a full time vacuum port on the carburetor, tee it into the choke vacuum break hose, which I believe the Quadrajet has, and it sees manifold vacuum at all times.

Very little volume moves through the VAC itself, so I doubt if it has anything to do with any vacuum motor operational issues.

Duke

I have posted a few different posts about my car and working on getting it set up. So, I probably should have pointed out the details to date.

I bought the 1969 L68 a while ago from someone who had restored it about 90% of the way. He left it stock except for a Comp Cam 280 Magnum and although it has the stock distributor he had put a Pertronix in it with different advance springs. So, I am trying to dial it in knowing these changes are part of the engine. I upgraded to a Pertronix III and put in new advance springs to come all in at around 2,500 rpm's.

So, I started checking vacuum and have 14" at idle, set the mechanical timing all in at 34 degrees (12 initial / 22 centr.) @ around 2,500 rpm's. I just installed a B26 vacuum advance and will check to see if I do get 16 degrees advance if hooked to manifold vacuum. I will use the choke pulloff as suggested for the vacuum source. The car so far runs very cool so I will probably bump the timing to 36 all in + the 16 vacuum advance to equal 52 degrees cruising.

Now, it still runs rough at times, so I am now down to adding new ignition wires, found one bad wire, so the ignition is I hope then not an issue.

Next I will make sure the carb linkage is correct, meaning the front and rear carbs are really closed at idle. If still a problem I'm wondering if the previous owner put the center carb manifold gasket in the wrong position, meaning maybe the PCV slot is exposed and causing a vacuum leak.

With the Comp Cam with specs fairly close to a L71 I decided to use a CV 746 PCV as my reading indicated that would be correct for a L71.

So, that is it so far. I would appreciate comments as I think I am on the right track but others here I am sure have experience withnthese tri power big blocks.

Thanks,

Doug

[Jeffrey S.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Here's another approach to supplying full time vacuum to the vacuum advance. I drilled a small hole in the throttle body from underneath into the cavity that has the advance tube. This eliminates the ported vacuum slot being covered by the throttle plates at idle and shutting off the vacuum. I have a slightly more agressive cam which gave a slight lope at idle which I didn't know I would get and didn't like. This procedure smoothed the idle out. When I switch back to a stock cam this coming winter I will probably JB Weld the hole shut and return to ported advance.
Jeff

[Timothy B.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Jeffery,

Make sure you did not create a vacuum leak from above the throttle blade, you may need a smear of J&B weld to close off the ported hole if these are connected.

Douglas,

You may try to find the best inital timing by using a vacuum guage and set inital to best vacuum. Don't advance further than point of best vacuum, after that check it with a timing light and see what it reads.

Depending on the inital setting, you can determine if ported or manifold vacuum advance is best for you engine combo. Ported advance is not bad IF the inital and centrifugal is tuned properly. By that I mean don't use the factory settings as they are low so the spark is retarded for emissions reasons. If the inital timing the engine likes is too high the motor will kick back and the starter will not turn over well, that's where the manifold vacuum advance will serve you better.

I understand there is a general feeling that manifold vacuum advance is better on our cars but there are advantages of both ported and full manifold advance and they are both good depending on the motor combo..

[Duke W.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Here's another approach to supplying full time vacuum to the vacuum advance. I drilled a small hole in the throttle body from underneath into the cavity that has the advance tube. This eliminates the ported vacuum slot being covered by the throttle plates at idle and shutting off the vacuum. I have a slightly more agressive cam which gave a slight lope at idle which I didn't know I would get and didn't like. This procedure smoothed the idle out. When I switch back to a stock cam this coming winter I will probably JB Weld the hole shut and return to ported advance.
Jeff

That's a good way to convert a ported vacuum advance to full time. It can be done on most carbs and usually requires plugging an existing hole and drilling another small hole that places the vacuum sensing hole below the throttle plate.

Full time vacuum advance provides improved around town fuel economy and lowers the cooling system load. Ported vacuum advance is an emission control technique to increase EGT, and this increases heat transfer to the cooling system as the hotter exhaust gas passes through the exhaust port. This means that more of the fuel's energy is being thrown out the exhaust rather than being converted to useful mechanical energy.

Lowest idle fuel flow and the smoothest idle is achieved with low 20s to low 30s total idle timing, which is the sum of initial and full vacuum advance. High overlap cams have a lot of exhaust gas dilution, which slows combustion propagation, so they like total idle advance in the upper end of this range. Low overlap cams are okay in the lower half.

When you install the lower overlap cam, you might need a less aggressive VAC. You want to be as close to the Two-Inch Rule as possible to avoid transient and light acceleration detonation.

Douglas - you are on the right track, but your centrifugal curve is very short. The Chevrolet Power Manual recommends 36-40 total WOT advance for big blocks, and most had 28-30 centrifugal, so I recommend you extend the centrifugal to this amount and experiment with initial timing to get the 36-40, and if the engine will tolerate 40 without detonation, that's where you should run it.

Engines make the best across the range power and have the lowest fuel consumption when the spark advance map is set up on the ragged edge of detonation.

Duke

[Jeffrey S.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Timothy,
Thanks for the tip. It seems to me that it's not a vacuum leak since when the slot is covered at idle, the engine is "sucking" through the small hole and when it is off idle and the slot is open it is sucking through both but always through the vacuum advance tube; never to an outside, open source. At any rate I have never checked this and the engine runs extremely smooth and doesn't act like there is a leak. When I close down the idle mixture screws I get a good lean roll and can do a good adjustment.
Jeff
Jeff

[Timothy B.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Jeffery,

You could be right depending on where the ported hole is in relation to the throttle blade. Most I have seen are just above the closed blade so air would get sucked through the hole and into the motor through the lower hole.

I mentioned the ported timing to Douglas because some of these low hp hydraulic engines like the inital timing a bit more retarded than where it's at with full manifold advance. I am talking 12-16* not 5 or 6*.

[Douglas C.]

Re: Proper vacuum routing for L68 full manifold vacuum?

Thanks guys for the comments. As said, I am starting from a novice guy who threw things together, but all good parts I think.

So, first I have got the timing at a fairly good point. I agree with Duke, I'm now all in @ 36 degrees around 2,800 rpm's, but with initial @ 14 I'd rather have the 36 all in with a 18 initial and 18 centr. So, I can later install a 18 degree distributor stop bushing to accomplish this.

Still a stumble, and I did improve somewhat by adjusting the linkage, front rod direct into the lever hole and the rear carb rod 1/2 the rod diameter behind the lever hole, so front & rear carb throttle plates are closed at idle.

Now, I found the shooter in the center carb loose, leaking down fuel, so I tightened that. Man, how can someone be so goofed up to not do the basics.

Still a stumble, so next is new ignition wires, as I found one wire bad. And I have a good thick plastic cap & rotor to put in with brass contacts. If still a stumble I will pull the center carb, thinking maybe the gasket is backwards leaving the PCV slot open.

One concern though. I hear a buzzing / scratching sound from the distributor cap at idle at times. Pulled the cap and one post is scraped quite a bit. So, I wonder if the original distributor shaft is worn and has a slight wobble. Feels o/k with almost no play, but a concern.

If I need a distributor rebuild who is a good source?

My last step would be to send the carbs to Custom Rebuilt Carbs here in Jersey. Great work, knows his carbs, recolors and sets up based on engine combo / vacuum / etc.

I am determined to have a sweet idle and crisp throttle response, just takes one step at a time. I may not get as smooth as I would want since the Comp Cam 280 Magnum the engine has is a LSA of 110, and I think the factory, even the L71, had a 115 LSA, and that would help keep good vacuum and smooth idle.

Oh well, carry on I will. Thanks again for listening and the helpful information.

Doug