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Old 08-23-2008, 01:21 PM
GMR GMR is offline
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vacuum gauge readings

I have built a sbc 350 and it's on a test stand. It has a Crane hydraulic cam 278* advertised duration, 220* at .050, 110* LSA.

I expected the vacuum to be lower than stock but don't know about what it should be. My vacuum gauge fluctuates between 15" and 16" and my tach fluctuates from maybe 900 to 1100 RPM.

Is this typical of a cam of this type?
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Old 08-23-2008, 03:38 PM
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The idle shouldn`t move that much. I would check it for vacuum leaks. Also check the idle mixture, you can set them with the vacuum gauge, adjust one at a time to the highest vacuum reading. Also check the mechanical advance in the distributor for looseness. You may also want to check the valve adjustment.
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Old 08-23-2008, 07:57 PM
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Thanks DoubleVision, I'll follow up.
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Old 08-23-2008, 08:13 PM
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Seems about the right vacuum for that cam, maybe a little on the high side.

Set idle timing to about 16 degrees and adjust idle mixture screws for best idle (highest vacuum).

If you have a Holley carb, check the power valve.

I would also recommend that you get an adjustable vacuum advance canister. Set to 12 degree advance and use it on manifold vacuum so it will idle at 28 degrees (16 + 12). This will allow a leaner idle mixture and will increase idle vacuum by 1 inhg. This also helps keep the carb in the idle circuit during idle (needs less throttle with more timing), because it sounds like your carb might be past the idle slots (throttle opened to far)

Finally you will need about 38 total degrees mechanical all in by 3000 which added to the 12 vacuum advance will make 50 degrees of timing at light load (high vacuum) going down the highway.
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Old 08-24-2008, 08:33 AM
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454C10, thanks, that helps me a lot. Maybe you can straighten me out on something.

I know vac adv should be used on the street. But let's say I have 2 distributors - one with vac adv and one without. Would I tune the motor (initial, total, mixture, idle speed) the same for each distributor? If not, what part of the tune would I expect to be different?

Right now I think I would tune the same and I would just have to limit the vac adv on that distributor to avoid detonation (as you mentioned using like 12* vac adv).
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Old 08-24-2008, 10:03 AM
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14-15Hg at idle is plenty of carb signal to make a good A/F quality mix and enough velocity to keep it in suspension till it gets into chamber and not foul the plugs at idle....

agree with 454's timing recommendation, 14-16base and 10-12vac is ballpark for that cam depending on the car weight and gears....(a performance cam needs more base to make more Hg)

road testing will tell you what the practical max can be for how fast the cent comes in determined by rpms and how much vac adv is applied depending on the Hg amount present....

vac units are all calibrated different so understand this pic' graph is just to show you how the degrees added slope and on/off start points change depending on total degrees added....(multiple the chart *'s by 2 for crankshaft degrees)
you can "plot" your cent adv graph with pad and pen by (plug and disconnect the vac adv) just increasing the rpms in 500rpms increments with the idle adjust screw and read the timing with your light till the cent is all in (at 3,000?)

needle bounce at idle is most likely idle screws balance,,,,,but can be a minor vac leak or a mis-fire from a loose/almost bad plug wire wire

edit add:
my favorite ign timing explained link:

http://www.gnetworks.com/v4files/bar...withimages.pdf
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Old 08-24-2008, 07:59 PM
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Thanks red65mustang, very helpful, appreciate the attachments as well - good reading.
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Old 08-25-2008, 08:07 AM
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if you use the vacuum advance on manifold vacuum you will have around 30 degrees of timing at idle. This makes the idle speed increase and makes the engine want a leaner idle mixture. So, turn down the throttle blades and lean the idle mixture to get best idle (highest vacuum).

Here is a copy and paste from a timing article.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
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Old 08-25-2008, 10:33 AM
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Thankyou 454C10, that sure helps too.
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Old 08-25-2008, 11:04 AM
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454,
your "article" is just a members post....
(there are way better articles to explain the benefits of manifold vac)

there are multiple pluses and minuses for using either manifold or ported/timed vac source....

which port has the most/best pluses depends on the particular build and car/gears combo as this article example points out:

http://www.highperformancepontiac.co...ech/index.html

confim your dist will work correct with either, try both....

LOL, my car is on ported just for more downshift engine braking....I've got manual drums and I'm too lazy to step hard on the brake to stop the car!

GMR, your welcome and thanks (from everybody) for saying thank you....most don't
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Old 08-25-2008, 11:23 AM
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I agree 100%, every engine/car setup is different. Needs to try both ways.

My mild 454 sure likes the extra timing at idle (manifold vacuum for advance). Exhaust smells much cleaner (even through I'm sure the emissions are worst) and throttle response right off idle is much better.

tech article or member's post, it's still a good one. But thanks for the clarification.
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Old 08-25-2008, 12:03 PM
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"Your article is just a member post?" Looks like a little more than that to me.

It seems the timing 101 article was written by John Hinckley. He has written many others tech articles for a Corvette magazine. I guess he may also be a member on this site, but the vette site seems to be his bigger concern.

Quote, "Many of the articles by John Hinckley are courtesy of Corvette Enthusiast Magazine. Please visit them here: www.corvetteenthusiast.com"
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