Is 51° total OK?

[Walter R. Malik]

The MVA/PVA topic is a regular feature on many forums. Unless you are using locked or fixed timing, MVA will ALWAYS help idle quality, idle cooling & tip in response.
Even when MVA was phased out & we got the USELESS PVA, manufacturers retained MVA as a back up, usually via a thermal switch. If engine temp got too high, PVA was switched to MVA which increased idle rpm, turned the pump & fan faster, cooled down the engine. Think about why the rpm increased by just adding idle timing....
.

Yes ... Coolant temperature vacuum switches are certainly available to do that without any electronics needed.

[Geoff2]

Dave Mc,
If MVA is properly dialled in, your chug-a-lug scenario never happens. If you want to PM me your email address, I will send you my MVA dial in procedure. You won't read about it any books or magazines...

[nickpohlaandp]

Dave Mc,
If MVA is properly dialled in, your chug-a-lug scenario never happens. If you want to PM me your email address, I will send you my MVA dial in procedure. You won't read about it any books or magazines...

Why not post it for all to see? I'm curious to know. If anyone has anything shitty to say tell em to piss off.

[RevTheory]

Dave Mc,
If MVA is properly dialled in, your chug-a-lug scenario never happens. If you want to PM me your email address, I will send you my MVA dial in procedure. You won't read about it any books or magazines...

I've found on larger cammed engines, it has to have enough initial advance to idle on its own without any help from vacuum advance so it doesn't hiccup and start the downward spiral.

Once the initial advance is dialed in, MVA becomes a bonus but not a requirement to idle. Learned that one the hard way, of course

[Belgian1979]

Without a dyno there is no good way to actually tell what best idle advance is. You can be in a 'good' range but that's about as far as you can get.

On my efi engine I just did it with vacuum and rpm in mind : crank timing way up to see what the lowest vacuum is, then adjust downward untill you see it drop. Set it like that. I started with 30° and had to adjust downward to 22-24° where it is now.

The issue is that at startup and when cold, the engine tends to run slower and when you have lets say 600 rpm instead of 1100 what my normal range would be you have way to much timing even at 22°. I solved that by creating a 600 rpm row with 12° which in an efi system is a no brainer. On a regular carb about the only thing you can do is use the choke. However on my old carb the engine always died when closing the choke some

[Geoff2]

Rev theory,
With all due respect, if you are using MVA [ Man connected Vac Adv ] to improve idle quality, then it IS part of the idle timing.

You could use 10* init for a large cammed engine, but it might need to idle at 1500 rpm so that it kept running. If MVA was used to add 30* [ which is the maximum the Crane adj vac adv unit, the best, can add ], then with 40* of idle timing, idle rpm will be able to be lowered & idle will be smoother, & with more vacuum.

I have seen idle rpm increase by as much as 300 rpm, just from rotating the dist with the engine idling to advance timing. And I have seen it more than once.....

You do not need a dyno to determine best idle timing.


On MVA, I think our friend David Vizard said it best in PHR magazine, Nov 2004, p. 77: [ Emphasis is mine ]

" At idle & low speed operation, the amount of advance required to most effectively uitilize the air & fuel entering the engine can be as much as 50 to 55 degrees. This is handled by a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen up & listen up good. A functional vac adv is the single most effective camshaft timer you can get.
By taking the time to hook up the vac adv to a manifold source you can get a big cam to idle as if it were 20 degrees smaller. Conversley, if you are looking for a decent idle, the use of vac adv will allow you to use a cam, at the very least, 5 degrees more duration/overlap than would otherwise be the case."

[Truckedup]

On a Chevy I used manifold vacuum and limited it to 12 crank degrees...The mechanical curve was 10 initial and 34 total , all advanced by 2800... With a moderate cam and 9.2 compression and older style heads this gave a good idle, sharp throttle response and excellent fuel mileage ....The added benefit of the manifold vacuum increased idle speed so the carb idle adjustment without vacuum was only 500 rpm and this eliminated the tendency for run on when the ignition was shut off...

[Geoff2]

Trucked,
Yep, not surprised with your results, It's all plus, plus, plus...

[Schurkey]

Chevy 357 ci gen1, ported Vortec heads 062's, regular HEI dist, Edelbrock Performer intake for Vortec's, Edelbrock 750 carb. (1407) CR 9.8. Just played with the timing yesterday, installed a MSD vacuum advance stop plate, doesn't advance anymore than 10° now. Mechinal advance stops at 10° also with 12° initial for 32° at idle. Vac advance is connected to the ported side. Rev it up to 2500-3000 I get 51° w/vac adv. connected. Went for a drive in this 98° Texas heat an It drove surprisedly well, responsed well, I was surprises as well!! I'm just a bit worried but not to much about 51° total. Should it be lower?

Part of this problem is terminology.

"Total advance" DOES NOT INCLUDE VACUUM ADVANCE. You have 12 degrees initial, plus 10 degrees (?) centrifugal, for 22 degrees total. This doesn't make sense. 12 degrees initial, plus 10 degrees (camshaft) of centrifugal advance which would be 20 degrees (crankshaft,) of centrifugal advance or 32 degrees total. Now we're making sense. Then you add vacuum advance, which moves the advance to 51 degrees. Either you have more than 20 degrees of centrifugal, or you have more than 10 degrees of vacuum advance. Is your "10 degrees" of vacuum advance camshaft degrees? That would be 20 crankshaft degrees, added to 32 total degrees of advance, for 52 degrees of total + vacuum. Very close to the 51 degrees you're claiming.

Are you SURE the centrifugal is all done advancing at 3000 rpm?

51/52 degrees of total + vacuum advance is probably a little high for Vortec heads. See how the engine responds to knocking a couple of degrees from the centrifugal advance, and knocking about five degrees from the vacuum advance. ~30 degrees total, ~45 total + vacuum.

Ported vs. manifold vacuum--whichever the engine "likes" more.

[Truckedup]

Trucked,
Yep, not surprised with your results, It's all plus, plus, plus...

Yes...And the manifold vacuum at idle allowed a leaner mixture just off idle without hesitation or need for excessive accelerator pump squirt...

[MadBill]

I find that as the idle + vac. spark is advanced, a point is reached where the idle speed and manifold vacuum starts to get erratic. I then back it off enough to stabilize the idle, which usually drops the vacuum a couple of "Hg.

This works great with a manual transmission, but with a low stall automatic, especially if the vacuum can is not fully advanced at in-gear idle, it can make the engine quite load sensitive, so enough idle speed to keep it running in gear leads to excessive neutral idle and a lurch when shifted into gear. In this case I find a little less advance reduces the idle speed load sensitivity.

[ZEOHSIX]

51 degrees adv is a lot of timing for Vortec heads.

30-36* (and usually closer to 30*) is what I find works.

I agree with this....

With crappy open chamber heads I've had SBC's running 42 degrees total.....I have found it's hard to detect slight detonation at highway cruise speeds....I think you need to as someone has already said look at cruise AFR ratios....I had well tuned engine combos that flat refuse to run well at low RPMs and idle with vacuum cans hooked up.....look at a Chevy L-88 distributor....the vacuum can is a "dummy can" with no nipple on it....I guess the Factory engineers weren't too talented back in those days

[Truckedup]

51 degrees adv is a lot of timing for Vortec heads.

30-36* (and usually closer to 30*) is what I find works.

I agree with this....

With crappy open chamber heads I've had SBC's running 42 degrees total.....I have found it's hard to detect slight detonation at highway cruise speeds....I think you need to as someone has already said look at cruise AFR ratios....I had well tuned engine combos that flat refuse to run well at low RPMs and idle with vacuum cans hooked up.....look at a Chevy L-88 distributor....the vacuum can is a "dummy can" with no nipple on it....I guess the Factory engineers weren't too talented back in those days

At light throttle cruising ,45 degrees advance will usually give better fuel mileage and the engine will respond better to slight changes in throttle openings especially when the mixture is a bit leaner for economy. When the throttle is open a lot the vacuum falls off and the engine runs on the power spark advance of 30-40 degrees. On a engine jacked up with a lot of cam and richer fuel mixtures like an L88 doesn't need the all the spark lead at cruising and fuel mileage is not a concern ...
Heavy trucks and boats generally don't have a vacuum advance since they are expected to run at low vacuum /heavy loads...Just like the L88

[Walter R. Malik]

If it is not simply a race engine, then take the time to get it right and you will have a big smile while driving.

[Belgian1979]

If it is not simply a race engine, then take the time to get it right and you will have a big smile while driving.

Agreed, but outside a dyno there is no much you can do than guess.