Hi.

I wonder about what is best for normal use of the camaro : crusing, mpg and performance.

I am running ported vaccum advance because the car start easely that way. I think i'd need to remove total advance if i were running full manifold vacuum advance. When i plug the hose there at the manifold without changing anything else, at idle the engine almost stalls!

Do you think i should try to run manifold vacuum and re-tune my advance?
by the ears because i have no lamps to halp me do it.

Please tell me the differences between both set ups on the point of drivability and mpg.

Thanks

Marc

Well i searched for info on previous topics.
Seems like manifold vacuum should help the car start better??
well i 'll check what set up i have now, i set it last year.
Anyway you can tell me your minds about either ported or manifold vacuum influence on drivebility.

Also to help me set at the first time the timing correctly, what should be the initial mechanical advance (at idle) and full total advance (at 3000 RPM) for my engine ? it is a 327ci with 9.5 compression ratio and 218/218@0.050 cam.

Thanks

IMO, you should run ported vacuum to the advance can/diaphram. Set initial timing at 6* BTDC.

Measure the circumference of the harmonic balancer and mark on it in 5* increments to the right of the TDC mark up to 50*.

Now you can use a standard timing light, unless you have a dial timing light, to measure the amount of advance.

The book I have here Chiltons, 1973, states manual trans gets TDC intial timing and an auto trans gets 4*BTDC. No specs given for vacuum & mechanical. So, your looking for total timing of 32*-34* at say 3500 rpm. The vacuum cans were set pretty tight if OEM.

Running the engine at oper temp, and with light hooked up, check initial timing. Disconnect vacuum hose at vac can and plug hose. Run engine up and check timing, this is mechanical timing.

Connect a vac gauge to vacuum port of carb ported vacuum. Run engine up and note the amount of vacuum and rpm it maxed.

Reconnect vac hose to vac can, run engine up to last rpm and check timing. This reading will show mech, vac (ported), and add initial. This is total timing.

Mechanical should be 15-20*, vacuum may be 10-12*, and initial s/b 4-6*. Make sure mechanical weights are free to move and springs back and vac diaphram has no hole in it.

This may call for a readjustment of idle mixture screws and curb idle. Set mixture screws for best max vacuum reading.

The final result is an engine willing to hot start without kickback against the starter and an engine with good pickup from cruise rpm without pinging at WOT. If it pings, back off initial timing 2* until it goes away.

If the engine tries to stall when you unplug your vacuum advance line you are most likely connected to manifold vacuum, not ported.

Ported advance works independently from mechanical advance -- you need both in a street car.

The correct port for vac advance only provides a vacuum signal at part throttle -- nothing at idle and nothing at WOT. This way, there is no vac advance added at idle or at WOT. You only need the advance at part throttle when your cylinders are only partially filling with air & fuel. The "partial fill" burns slower so the extra ignition lead helps get the most power out of a partial charge.

If you use manifold vac advance, the engine gets extra advance when at part throttle AND when at idle. You need the extra advance at idle IMO when you are running an engine with a really big cam that won't idle well. There are differuing opinions on this, but I personally prefer ported vac advance.

mechanical advance is directly related to engine RPM and not vacuum. So as engine RPM increases so does the mechanical advance. You can change the "curve" by changing springs in the mechanical advance mechanism. This mechanism is most important for max acceleration and power at WOT.

Most performance oriented Chevy 350 engines like a mechanical advance curve that provides no advance at idle (base timing at idle of around 14-16 degrees) and 20-22 degrees of mechanical advance all in by 2500-3000 RPM -- base timing of 14* + 22* mechanical = 36* total advance at WOT.

Hope that makes sense.

I've always run full manifold vacuam advance.... That way, I can use less than 10* initial and it cranks easy hot or cold. At idle, gets full advance from the vacuam.... when you crack the throttle, the vacuam advance drops... doesn't ping. Ported vacuam gives lower emissions, but either idles like crap or won't crank, depanding on where the initial is set. And yes, if you're really serious... you need just the right amount of advance WOT high RPM, that's initial plus mechanical. If initial is 6 and you want 38, mech needs to be 32...

I think a lot of people are forgetting that in the pre-emissions days all cars used to run full manifold vacuum advance. I think its the best way to go (and so did car manufactorers before the smog police showed up) ...My car starts just as easy either way and idles twice as good with manifold vacuum.

There are a lot of reasons to use either one. Whatever your engine likes is what you should use.

I always had trouble getting my engines to idle on manifold vac advance. For some reason the idle RPM would be unstable because changes in manifold vacuum would effect the timing. Then the fluctuation in timing would make the engine rpm change.

Like Oger said, whatever works best for you.

Yep, it really depends on what what the engine likes.

You'll find that some/most later small blocks use manifold vacuum. I myself prefer manifold vacuum with just about any non-stock camshaft. For a mild cam, I set the initial to 8-10*, then give it 8-10* vacuum advance. You get a easy starting engine, great throttle response, and the ability to expose the right amount of the transition slot in the carb. For my 12:1 motor, I give it 16* initial, and 5* vacuum advance also hooked up to manifold vacuum. This is so I can close the throttle blades the correct amount.

What combo are you running? Maybe we all can give you suggestions of what we think would work well (ported & manifold advance), and you can try for yourself to see what works the best.

I know with my large duration cam direct manifold vac is the ticket. Giving me more advance at idle for improved idle quality. Dave Ray from www.davessmallbodyheis.com (http://www.davessmallbodyheis.com) swears by this and recurved my distributor with adjustable crane vac can just for this purpose and made a world of difference.SS

scroll down to the post by JohnZ in the link, i always wondered, this explained it all to me.
http://www.camaros.net/cgi-bin/forum/ultimatebb.cgi?ubb=get_topic;f=4;t=006587

Originally posted by railing68:
I know with my large duration cam direct manifold vac is the ticket. Giving me more advance at idle for improved idle quality. Dave Ray from www.davessmallbodyheis.com (http://www.davessmallbodyheis.com) swears by this and recurved my distributor with adjustable crane vac can just for this purpose and made a world of difference.SS Right on. graemlins/thumbsup.gif

thank you for the inputs!
Then i think i already run manifold vacuum, but i'll check next week. The camaro isn't home...
For setting the advance, when you talk about setting 38 total @ 3500 RPM, is it with the vacuum advance hose plugged in?
Total is mechanical+initial+vacuum?
or only mech+initial?

Thanks.

Usually when you're talking total advance, you're referring to base timing plus mechanical.

I think Dave Ray has a way he sets up his distributors with manifold vacuum that works really well. He does something different to his that seems to work better with manifold vacuum than your typical GM HEI -- can't say exactly what.

I have played with one of Dave Ray's distributors and will say I was impressed. My old HEI for some reason always wanted to die when I stepped on the brakes hard (power brakes).

ps. If all you want is lots of initial timing just modify your stop plate to get what you want -- or use the 18* bushing in your MSD.

Oddly enough I just did this job this past weekend. I had my Road Demon at 8* initial and the advance hooked to the ported and the car ran like dog sh*t swapped the advance to manifold vacuum and the car instantly woke up. It was a big difference. It will definatley differ from car to car and you should just hook it up to whatever source helps your car run better. Good luck.

The vacuum source (ported vs. full manifold) has no effect on ease of starting - there's no vacuum advance while cranking - only once it starts running. Hard cranking due to excess advance is strictly related to excessive initial (base) timing, as neither centrifugal nor vacuum advance are functional when cranking. graemlins/thumbsup.gif

Your right John, but in an instance such as my situation, I can reduce my initial timing and make up for it by the vacuum advance using and manifold source. Hence the easier starting like you mentioned.

I have the same type of issue with the new stroker that I just installed. I am having big problems setting idle and keeping the engine running. I have base timing at around 18 and max around 40 at 3500 rpm. It does not make a difference with the vacuum ported or manifold. The idle (and timing) will fluctuate from around 700-1100 rpm just sitting there with the vacuum pulled and plugged. After reattaching the vacuum to ported I can drive it about 1/2 mile and the idle will stay around 1100 rpm without dropping. The car runs good and seems to have plenty of power, but the idle won't calm down. I show about 5-10 inHg at idle (yeah, I know) and it also seems to fluctuate with the idle. Of course as the idle changes so does the base timing. Any suggestions would be greatly appreciated. Here's the specs...




1974 Camaro LT Z28
383 Stroker 10.5:1 CR
64cc Trick Flow Heads
Crane 246°/254° duration at .050, .558/.558 in lift
Edlebrock AirGap Manifold
Jet Streetmaster Q-Jet 750CFM Carb
MSD Digital 6 & Mallory Unilite
Centerforce Dual Friction Clutch
Tremec TKO 5 spd
3.73:1 10 bolt stock rear

Sounds like you're idling on your mechanical advance. First, install the heaviest springs to "lock out" the mech advance at idle. Then, set your initial to about 18*. Then install lighter springs until it starts to advance just off idle (say, 1500RPM or so) and plateaus at about 2500-3000RPM. You'll want about 32-36 total. You can plug in the manifold vacuum to an adjustable canister and tune it to about 25-30 at idle. Perf motors typically like 12-16* vacuum advance, but it varies.

For a big cam, like others have said, IMO manifold vac is the only way to go. My 388, with a 250@.050 solid lifter cam and a relatively low 10.3-1 compression, idles WAY better with manifold vac vs. ported. In my experience, the best use for ported vacuum is when using 76cc smog type heads on a small block with low compression. These heads like a lot of timing, so you can really crank up the initial advance for much better low end response. With manifold vac on this type application, you would generally have too much advance at low speeds and it hurts throttle response even if it doesn't ping. Running less initial timing with manifold vac also reduces throttle response because vac drops when you open the throttle...so in this case you don't have enough advance.
Your engine will tell you what it likes. You can run less intial timing with manifold vac, and you will have less total timing as a result. Or you can run more initial timing with ported vac, with more total timing as a result. The total advance in your distributor will play a big part in this too. Many stock distributors have too much total timing in them even for mild performance applications, so to compensate you can run manifold vac and less initial timing. I find this the case with most '70's HEI distributors.