lr172

I have completed the rebuild of my 496. My cam is 255/263 @ .050 and I have a holley 850 DP. I purchased the Vac. cam that Lars recommends (I think it is 1684). This gives full advance at 8" of vacuum. Unfortunately, this cam only gives me about 8" of vacuum at 1000 RPM. If I get the idles set just right, it will hold. However, if the vac falls to about 7", I begin to lose advance and then starts a cycle (reduced vac retards timing which drops RPM) that results in the engine shutting down.

The engine idles fine at 900-1000 RPM, assuming the vac doesn't fall off. I hate to run the idle at 1100+ if I don't need to. It seems like my options are to find a can that will pull the advance at 6-7" instead of 8" or to run the advance off the ported vac (no advance during idle) and retune for the reduced timing. I am running 20* intial, 36* at 2500 and 16* vac adv.

Does anyone have a recommendation for a vac adv can that will give full adv. at 6-7 degrees.

Thanks for your assistance here.

lars

What type of distributor do you have (HEI-style big unit, or points-type small distributor)?

Stroker-427

For my experience a big cammed engine will havo more vacuum off idle then at idle.
For this reaso I had more luck connecting the canister to the timed port of the carb.

You will have a more stable idle and a lot less risk of stall the engine loosing some vacuum (i.e. in drive...).

just my 2 cents....

lr172

It is a stock GM HEI distributor. The stock mechanical advance gives me 18 degrees.

Also, it is 4 speed so I don't have to worry about the idle drop in drive.

olescarb

I am a fan of ported vacuum for a engine with a big cam / low vac at idle because if you use a vac can that gives full advance at 6 to 8 inches you may have advance from the vac advance at a part throttle acceleration and thus have a ping or detonation problem from the vacuum advance. a cam with over 250 duration @ 0.050 lift should have a initial of about 18 degrees (source: barry grant demon selection guide) when you add to that 16 degrees of advance from the vaccum i think you have too much spark advance at idle. I would also recommend limiting the vac advance to 10 to 12 degrees of advance for the gasoline of today because it burns faster than the leaded gasoline that was in use when most vacuum advance units were designed.

I know that the ported vs manifold vaccum source for a vacuum advance debate is like taking about religion to many but that is my opinion.

Henry @ olescarb

lr172

Thanks for the insight. As a follow up, I am using this following cans :

VC1862 AR31 starts at 2-4" 8*(16* @ crank) @ 6-8"

is this too agressive? I don't have enough time on the engine to tell if it will ping. I have lugged it a few times and have heard nothing on 93 octane.

Is this can bringing in the advance too soon for this engine in load situations, such as part throttle? If so, do you have a recommendation for a can/configuration that will work well here?

Thanks.

hugie82

I have a 427 w/ 260 duration and I did not have a problem with my Idle or vac over 800 RPMs.(under 800 it loped too much but still idled) This might sound crazy but you may have your valve a little too tight. Try backing off a 1/4 turn and you may have a better chance of a good idle

olescarb

I tend to use the B-26 can that we limit the travel on (advance degrees) to 10 to 12 degrees, I almost always use a ported vac source unless the factory used a manifold source or that is what the customer wants. My reasons for this is with the formulation of the fuel here in Calif. i can read and hear a missfire problem with our exhaust gas analyzer when we combine the 12 to 20 degrees of initial timing (based on the cams duration @ 0.050") that works best for the formulation of todays gasoline plus 16 degrees of vacuum based advance (28 to 36 degrees of spark advance at idle). I know tuners that will only use manifold vaccum while others like myself and Cliff Ruggles prefer ported vacuum but again that debate is like talking about politics or religion

in my mind i would not want any vacuum based spark advance untill the engine vac is above 6 inches of vacuum to avoid detonation. we have done tests with a 5-gas exhaust gas analyzer using a high NOx reading as a indicator of detonation (excessive heat early in the combustion process), during these tests we could watch the NOx reading rise quite high while pulling a grade when we used a vac advance that gave us advance below 6 inches of vac and/or when the vac based advance was more than 14 degrees (note the engine did have a aggressive mechanical advance curve).

you can not always hear the ping/detonation problem that could damage your engine with the reformulated gasoline of today like you could with leaded gas of years past unless the the spark advance is very excessive.

I hope this helps Henry @ olescarb

lars

Henry and I use the same parts and tuning techniques in general. The B-26 is a great, versatile unit, but it's for the points-type distributors and won't fit the HEI.

The VC1862 is one of the "softest" units you can get for the HEI. In deciding to run ported or manifold vacuum, here are some things to consider:

Although I generally recommend running manifold vacuum, this is not a hard rule. The real rule is: Tuning involves giving the engine what it wants. Many engines run exceptionally well on straight manifold vacuum. There are other cases where the ported vacuum source offers an advantage. On any engine, I advocate trying both methods and seeing how the engine responds and feels.

If you decide to use manifold vacuum, it is important that you use a vacuum advance that "pegs out" its advance at idle. If it's in the middle of its range, you get the timing fluctuatiuons that you are seeing, and this results in unstable idle rpm and quality. Also, if you use manifold vacuum, you have to make sure you stay out of the detonation zone as Henry notes.

If your idle vacuum is so poor that a vacuum advance unit such as the AR31 does not fully retract, you are often better off using ported vacuum in order to stabilize idle timing. But when you do this, you want to make sure you have enough initial timing for the cam: A lumpy cam pulling poor manifold vacuum at idle will want 18 to 22 degrees of initial timing at idle. To do this, you may need to shorten your centrifugal curve to allow the high initial timing while limiting the total to 36 or less.

Note that many of the vacuum advance cans produce more advance than their advertised spec - I'm seeing many of the 16-degree units that produce 18 degrees or more of advance. I agree with Henry that today's fuels will not support a lot of vacuum advance, and many applications will require that you shorten the vacuum advance curve in order to avoid detonation, especially if you run a lot of initial advance with a big cam.

The tuning process is finding the "sweet spot" with these variables...

olescarb

[QUOTE=lars;1565541536]Henry and I use the same parts and tuning techniques in general. The B-26 is a great, versatile unit, but it's for the points-type distributors and won't fit the HEI.

Opps, i forgot about HEI, i use a AC/Delco # D1370a vac advance for HEI units it gives 10 degrees of advance @ about 10 inches of vac it starts advancing at about 5 to 6 inches of vac.

Henry @ olescarb

lr172

Guys,

Thanks so much for sharing your experience. I will bump up the idle to 1100 and play with that a bit. Then I will move the same can (1862)to the ported vacuum and see if the engine still tolerates less timing at idle(18*). If so, I will swap to the VC1843 (on ported vac) to get a few more inches before the advance comes in and avoid detonation. If it doesn't like it the retarded timing at idle, I will stick with the 1862 and run the idle at 1100.

Either way, I will limit the total vac adv to 12*

Thanks again for your help.

BarryK

lr172
Not to contridict Lars advice by ANY means but since this wasn't mentioned in this thread yet I'd thought i'd throw it out.......
something to keep in mind is that an idle of 1000rpm is already fairly high and increasing it to 1100rpm just makes it higher. One issue you may run into at this point is the centrifigal advance already starting to come in while at idle.
Correct set up would be to NOT have the centrifigal advance coming in at the idle speed and only start to come in a few hundred rpm's above idle.
also, by having a high idle speed are your throttle plates opening up at all which would cause your Ported vacuum to start sneaking in while still at idle? If you want to switch from full manifold vacuum to ported to try to see if it makes an improvement you aren't going to have a proper test of that if the high idle is causing Ported vacuum to come in while still at idle rpm.

Stroker-427

My experience:

-Small Block 427
-13:1 compression (the only way to have some driveability with big cams.... for my opinion)
-Dart PRO1 227cc CNC heads
-Victor Jr
-Migthy Demon 850 CFM annular boosters
-Solid roller Crane Cam with 256° - 264° @ 0.050" - 106° LSA
-Crane Shaft Rockers 1:1.6 Int - 1:1.55 Exh

Idle @ 850 RPM dropping @ 750 RPM in drive (2800-3000 RPM stall converter)
Vacuum @ idle 4.5 Hg
Timing @ idle 18°
Full advance 32° @ 3000RPM
MAX Vacuum advance 8° @ 7 Hg
Vacuum advance 0° @ 4.5 Hg

By the way I use this very usefull unit from MSD:

http://www.msdignition.com/tc_11.htm

The 8977 unit have a boost retard feature that can be use as a vacuum advance system!

Very nice!!!!!

You can drive your car with the laptop on the passenger seat and check everythink...... changing the advance curves in seconds!!!!!!

Read the features..... it is very well designed for me!!!!!

just my two cents....

lr172

Thanks for the recommendations. I have both my throttle plates at or below the point of "Squared" transfer slots, so I shouldn't be into the ported vac. However, I will need to add some throttle to keep the idle up after getting rid of the advance that I had while on the manifold vac. However, I don't think it will be enough to get into the ported vac. Also, if I am not fighting the vacuum level with the manifold vac, I should be able to drop the idle down to at least 900 or less.

I know I am a bit into my mechanical advance now. I wont be able to get that right untill the new weight pins show up later this week. That will also require a bit more throttle to compensate.

It should just require some good old fashioned trial and error to see what works best.

Thanks again for all of the ideas.

73, Dark Blue 454

Seems you'd have more vacuum in a big stroker with that cam than 6-8 hg at idle. Thinking out loud.

Also, I'm sure you've investigated one of the adjustable cans from MSD or somebody.

lr172

Yea, I really thought I would get closer to 10. It holds 8" pretty well at 900-1000 RPM. However, a drop of just an inch sets off the downward cycle because that is where the can begins to drop a little advance. As the advance drops, so does the RPM.

I played with an adjustable can last year and struggled with it. It would seem that a spring with a set rate for a specific purpose would do better than one that is adjusted by tension, the same way that you can't change your suspension by tighten up the spring. The spring rate is the spring rate and it is set by metallurgy and winding.

BarryK

wrong can than. That's the reason why you want a vac adv can that is pegged out at least 2"hg BELOW what your vacuum level is at idle.

I agree though, only 8" vacuum seems pretty low. are you 100% SURE you don't have a vacuum leak somewhere in the system? or that the carb is set up correctly?
If you double check and find that the 8" vacuum is accurate than you need a can that is all in somewhere in the range of 4-6".

if it was ME (and again, not to contradict any advice Lars or even Henry has offered) I'd start back at scratch:

disconnect the vac. adv can and plug the hose
set for correct idle speed of about 800rpm and set timing. (By the way, you mentioned your stock HEI is only giving 18º centrifigal advance - most stock HEI's were set for a standard advance of 20º so that may be telling you what I was saying previously about the advance coming in at idle because of your high idle speed)
Make sure there is no centrifugal advance coming in at idle.
As it's been mentioned, with your bigger cam you may need to bump up the initial timing but if you do that than make sure your total timing still doesn't exceed 36º.
once that is set than put a vacuum gauge on your carb to make sure your idle fuel mixture is correct - adjust it for the highest steady vacuum reading. you may have to readjust your idle speed while you do that to keep it at the correct level.
Once you get to that point, you will know your idle speed is correct, timing is set up correctly, and that new vacuum reading than you will have is accurate and you can use it to select the proper vac adv can your motor needs. At that point you can install the proper adv can and experiment to see what works best between the full manifold vacuum source and a ported vacuum source although in most cases full manifold vacuum works better and gives you numerous benefits over ported vacuum. (I know Henry won't agree on this last point - sorry Henry! )

lr172

BarryK,

Good advice here. Sometimes good to go back and establish a clean starting point. I agree that I need to have my initial and mechanical just right before I proceed down this new path. Thanks for the nudge.

I had all the vac lines plugged when I did my checking, so if I have a leak, it is in the manifold or carb. When I bought the cam, the cam designer indicated that I wouldn't get a lot of idle vacuum and encouraged a 112 LSA if I wanted to be certain I had vac for power brakes. I chose the 110 for power.

olescarb

in most cases full manifold vacuum works better and gives you numerous benefits over ported vacuum. (I know Henry won't agree on this last point - sorry Henry! )[/QUOTE]


But in many cases excessive spark advance from 16 degrees of vac advance at idle plus 12 to 20 degrees of initial timing will increase engine misfire or the HC (unburnt hydrocarbons) and increase the NOx (a indicator of excessive heat early in the combustion process) content of the exhaust.

A properly tuned engine with the correct advance curves (and correct a/f mixtures) will pollute the air less. I want it all, maximum power and clean air!! If we can keep the exhaust emmisions of these "older" cars as low as possible maybe the EPA will leave us alone and not bring the older cars back into the smog program.



Henry @ olescarb

lr172

I did some experimenting over the weekend. No vac leaks, unless they are coming from the manifold. Not sure if carb is set up correctly. It is pretty much from the factory Holley 850 DP. I have set the mixture.

In order to get it to idle with only 18* of advance (initial only), I have to open the butterflies to the point that I am a bit passed "square" on the transfer slots. This also seemed to create problems for the timed vac port. Sometimes it would be getting some vac from this port and the advance wouldn't fully drop off for a bit. It was hard to adjust and it didn't always drop right back to 1000. In order to run this config, I think that I would need to drill the butterfiles.

I welded on a plate to limit the advance to 12*. I hooked up to manifold vac and set the initial to 18*. I am able to get a pretty steady idle at 900-1000 and pull 8". It will occassionally drop a bit and require a blip of the throttle, but I should be able to get it to be stable at 1000 with a bit of adjusting.