Even though LS1 engines have a very efficient chamber design that requires only about 27* at WOT, they idle 22*, an electronic form of MVA.
Or, an electronic version of a proper mechanical advance curve, which allows for a good idle, due to having the proper amount of initial advance, without the undesirable effects that manifold vac on the can causes.
WildBill's post should have ended this silly argument.
MadBill wrote:One potential issue with manifold vacuum:
o An engine with a long-ish cam is very load-sensitive when idling with a lot of vacuum advance. Set to say an 800 RPM idle in gear it may flare to a bothersome 12-1400 in neutral and cause a big lurch when shifted into gear.
o Exacerbating the situation is the fact that for best economy, vacuum cans may have as much as 30° advance.
Not to mention the engine being more apt to stall due to the manifold vac falling off, which pulls the timing back momentarily, after a letting the clutch out, or after a little blip on the throttle with an automatic.
dave brode wrote:Not to mention the engine being more apt to stall due to the manifold vac falling off, which pulls the timing back momentarily, after a letting the clutch out, or after a little blip on the throttle with an automatic.
Exactly why I use ported. Smoother when pulling away from a stop sign is smoother when compared to a manifold source.
Something else to remember:
Unless I am sitting at curb idle, the net result with ported and manifold is EXACTLY the same driving down the road. As soon as the throttle is moved just a little bit, the ported source is exposed to manifold vacuum, so the vacuum can sees the same signal it would from a manifold source.
dave brode wrote:Not to mention the engine being more apt to stall due to the manifold vac falling off, which pulls the timing back momentarily, after a letting the clutch out, or after a little blip on the throttle with an automatic.
Exactly why I use ported. Smoother when pulling away from a stop sign is smoother when compared to a manifold source.
Something else to remember:
Unless I am sitting at curb idle, the net result with ported and manifold is EXACTLY the same driving down the road. As soon as the throttle is moved just a little bit, the ported source is exposed to manifold vacuum, so the vacuum can sees the same signal it would from a manifold source.
Exactly novadude, i use the ported vacuum for good idle and smoother pulls. I tried the manifold vacuum, just idled way to high. I set my carb for proper slot transition, i wasnt going to mess with that. So i tired the ported vacuum source, works great.
dave brode wrote:Not to mention the engine being more apt to stall due to the manifold vac falling off, which pulls the timing back momentarily, after a letting the clutch out, or after a little blip on the throttle with an automatic.
Exactly why I use ported. Smoother when pulling away from a stop sign is smoother when compared to a manifold source.
Something else to remember:
Unless I am sitting at curb idle, the net result with ported and manifold is EXACTLY the same driving down the road. As soon as the throttle is moved just a little bit, the ported source is exposed to manifold vacuum, so the vacuum can sees the same signal it would from a manifold source.
That comes from not having the proper mech advance curve. you should see around 20 at idle, 40+ at cruise and when you stab the throttle it will drop to mid 30's and climbs to your peak setting 36/38... just like modern fuel injected engines do.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
dave brode wrote:Not to mention the engine being more apt to stall due to the manifold vac falling off, which pulls the timing back momentarily, after a letting the clutch out, or after a little blip on the throttle with an automatic.
Exactly why I use ported. Smoother when pulling away from a stop sign is smoother when compared to a manifold source.
Something else to remember:
Unless I am sitting at curb idle, the net result with ported and manifold is EXACTLY the same driving down the road. As soon as the throttle is moved just a little bit, the ported source is exposed to manifold vacuum, so the vacuum can sees the same signal it would from a manifold source.
That comes from not having the proper mech advance curve. you should see around 20 at idle, 40+ at cruise and when you stab the throttle it will drop to mid 30's and climbs to your peak setting 36/38... just like modern fuel injected engines do.
Why do I need 20 deg at idle when my car has a clean 750 rpm idle and 15" hg with 12 degrees BTDC initial and ported vacuum? How come many cars in the pre-emissions era were not designed to use 20+ deg at idle? Did the engineers just not know any better?
Why do I need 20 deg at idle when my car has a clean 750 rpm idle and 15" hg with 12 degrees BTDC initial and ported vacuum? How come many cars in the pre-emissions era were not designed to use 20+ deg at idle? Did the engineers just not know any better?
I guess they didn't because they haven't done it that way in years. I did my timing the old way for years because that what the book said but after learning the benefits of doing a performance curve I will never go back. more initial timing will allow less throttle opening to achieve the same rpm so you use less fuel and if dialed in correctly it will be more responsive and will tame down the lope of a big cam.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
GARY C wrote:more initial timing will allow less throttle opening to achieve the same rpm so you use less fuel
More throttle opening at a given RPM allows for reduced pumping losses. Running lots of initial and a fast curve for too much advance at slow speed, light throttle can cause a surge at cruise speed in many cars.
No "one-size-fits-all" answer. Lots of initial, curve all in at 2500 rpm, and manifold vacuum isn't the way to go for every application.
GARY C wrote:more initial timing will allow less throttle opening to achieve the same rpm so you use less fuel
More throttle opening at a given RPM allows for reduced pumping losses....
True, but that is not always a net benefit. If achieved via tall gearing or lean mixtures, the reduced pumping losses help F.E., but optimum vs. retarded SA reduces throttle opening but is a net plus for F.E.
GARY C wrote:more initial timing will allow less throttle opening to achieve the same rpm so you use less fuel
More throttle opening at a given RPM allows for reduced pumping losses. Running lots of initial and a fast curve for too much advance at slow speed, light throttle can cause a surge at cruise speed in many cars.
No "one-size-fits-all" answer. Lots of initial, curve all in at 2500 rpm, and manifold vacuum isn't the way to go for every application.
Although that's not what I said, it tells me a lot.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBERS AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!
GARY C wrote:more initial timing will allow less throttle opening to achieve the same rpm so you use less fuel
More throttle opening at a given RPM allows for reduced pumping losses. Running lots of initial and a fast curve for too much advance at slow speed, light throttle can cause a surge at cruise speed in many cars.
No "one-size-fits-all" answer. Lots of initial, curve all in at 2500 rpm, and manifold vacuum isn't the way to go for every application.
Although that's not what I said, it tells me a lot.
Not sure what you mean by that, but I stand by what I said. Lots of initial isn't necessary in many cases.
I've tried it many different ways. My car likes a slower curve, and ported vacuum. It works for me.
Hands up anybody who can tell me of a detailed procedure to dial in MVA. The instructions with the Crane [ & other ] adj vac adv units are useless. Anybody ever see any?
If I want to dial in my Holley, I can buy any number of Holley books which will tell me how to do it.
Folks spend hours, days tinkering with their carb to get it right. They also spend time on the dyno getting the WOT timing correct for max HP.
But MVA? Often, the rubber hose is connected to any old vac adv unit & that is MVA [ or PVA ] dialed in!! When it doesn't work as expected, they blame MVA!! It's like the bloke who builds an engine with 12:1 CR, runs it on 87 octane & holes a few pistons, & then says, 'High compression ratios don't work'.
Over 20 years ago I developed a procedure to dial in MVA, & the number ONE rule is never use non-adj vac adv & the number TWO rule is use the Crane adj vac adv [ Some supposedly 'adjustable' units like Accel & Mr. Gasket have limiting sleeve inside the spring; you can get a situation with these where it locks solid, you get NO vac adv! I'll bet a few people used these not realising this & blamed the MVA concept for MVA not working....]. I have set up dozens & dozens of cars with MVA, never had one that did not benefit from MVA, with every driver driving away with a smile...
It takes time & some experimentation to get it right. Sometimes the centri curve &/or init timing will have to be changed slightly to get the overall curve correct. But don't blame the concept of MVA, it works on every engine, subject to the qualifications I mentioned in my earlier post. PVA is a waste of a vac adv unit.
Dave Brode/Madbill. The drop off in vacuum &/or large drop in rpm going from N to D happens because the VA unit is not adjusted properly. My engine drops a mere 30 rpm, but about 100-150 rpm is more common. You need to check that the timing is the same in N as it is in D. If the timing has dropped going into gear it means [a] VA spring adjustment is too stiff or your centri curve may have been activating in N. It may also be not enough MVA, see below.
I really shake my head when I see/hear comments like 'My idle is too high with MVA'. That is because the engine LIKES & WANTS the extra timing; it is making MORE HORSEPOWER, hence the rpm increase! The fix is to keep MVA & readjust idle speed & idle mixture.
Surging. Yep, total timing [ init + centri + VA ] can cause surging. The fix is not to get rid of VA, but to readjust the interaction of init, cetri & VA so that at cruise there is a few less degrees of timing. 50-55* total cruise timing is common, & on occasion 60* might be reqd for a low compression engine with large cam, big carb etc. Another cause of surge, & more likely, is lean mixture. Because VA uses less throttle opening on account of it's efficiency, there may be insufficient cruise fuel; the fix is to increase the size of the IFR &/or IDC, NOT cut back on the timing.
David Vizard, highly respected on this forum, says in his latest Holley book that a short cammed street engine typically requires 35-40* of idle timing & a street/strip engine as much as 50* [ Yes,50*!! ]. My engine idles with 48*. So important does DV regard the interaction of idle timing/carb calibration, that he mentions it several times in the book so that the message sinks in...
Also from DV, in PHR magazine Nov 2004:
"At IDLE & low speed operation, the amount of advance required to most effectively utilize the air & fuel entering the engine can be as much as 50 to 55*. This is handled by the vac adv; a function many hotrodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen up & listen good. A functional vacuum advance is the single most effective camshaft tamer you can get. By taking the time to hook up the vacuum advance to a manifold vacuum source, you can get a big cam to idle as if it were 20* less than it actually is. Conversely, if you are looking for a decent idle, the use of vac adv will allow you to use a cam of, at the very least, 5* more duration/overlap than would otherwise be the case."
Another from DV, PHR Aug 2004:
A 485 HP 383 Chev engine: "At this point our project engine was proving itself to be truly streetable. With vac adv hooked up, the combination of compression, cam, carb & ign allowed a very smooth 650 rpm idle."
The guys afraid to get into the carbs don't get it.. PVCR, IJ height, Emulsion package, bleeds, etc.. make all the difference in the world.. tune it!
Some of the GM Vac cans add as much as 15*(30* at the crank) but don't come in until 15" vac.. the old L88 vac can intended for the solid cam BBC limit the advance to about 8*, but are made to come in closer to 8" vac.. so about 52* cruise with a WOT 36* FWIW
Excellent summary Geoff!
One other consideration can further complicate matters, particularly if the total idle SA is less than optimum: A big cam engine often has less vacuum at curb idle than off-idle. That can mean that if you have any VA at idle, SA will increase if the revs pick up for any reason, which in turn increases vac, which further raises RPM, etc., leading to a runaway idle speed. Similarly any momentary drop in revs results in a downward spiral of SA and idle speed, provoking a stall. Effectively, this puts the engine on a knife edge rather than the desired nice flat 'plateau' for idle stability.
I know what you are saying & 100% correct. However, the method I use to dial in MVA eliminates that problem.
Folks, sorry for the large bold section in the middle of my last post. I don't know what happened, obviously pressed the wrong key. I am better at setting up MVA than using a PC.....
The guys afraid to get into the carbs don't get it.. PVCR, IJ height, Emulsion package, bleeds, etc.. make all the difference in the world.. tune it!