Don Terrill’s Speed-Talk

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?

I think I'd try it for a while and just see how it runs at part throttle overall. I don't think it'll hurt a thing as long as it is able to drop away quickly enough when you open the throttle/increase the load on the engine to avoid pinging. If it is too aggressive you could try adjusting the vacuum advance canister a turn or so and check the difference.

With a locked distributor using a vacuum advance I have seen almost 60 degrees be pretty common; especially in competition classes where a certain manifold vacuum is a rule.
At wide open throttle the vacuum part goes away so, as long as it does not have any detonation at part throttle, it should be OK.
Slow down the mechanical curve RATE if to much part throttle timing is present.

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

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

51 "total" not at WOT....at cruise is fine...

Two entirely different requirements....

Vacuum advance should be hooked to manifold vacuum, not ported vacuum.

I usually connect to manifold vac but have read that no vac advance is needed at idle.

Abbottracingheads wrote:Vacuum advance should be hooked to manifold vacuum, not ported vacuum.

The idle will be more consistent on some engines if you connect it to a ported source so that there is no vacuum advance at idle.

Common reason why it's not happy at 51 degrees: it's too rich.
Drop the primaries one size until you like it, get the WOT back with a different rod.

DaveMcLain wrote:

Abbottracingheads wrote:Vacuum advance should be hooked to manifold vacuum, not ported vacuum.

The idle will be more consistent on some engines if you connect it to a ported source so that there is no vacuum advance at idle.

In a thread I had going a while back when I was first breaking in a 390 I had just finished I learned that Ford went to a ported source whereas it was more common for a Chevy to go to a manifold source. I have a lot more experience with Chevy than Ford and so I had it hooked up to manifold. After having this hammered into my head I went and hooked it up to a ported source (no vacuum at idle) and the engine ran better. This phenomenon still confuses me a little bit because an engine doesn't know if it's a Ford or a Chevy, so why does the vacuum source differ from Ford to Chevy? It's been a very looooong time since I used a vacuum advance setup on a Chevy... or a distributor at all in that case, so I kinda had to go back and re-learn what I thought I knew. In the end, hooking the Ford up to ported vacuum produced a good idle and good overall performance on the street. There's my $0.02

It depends on what your engine idle timing requirements are on the one hand and the vacuum curve on the other hand. Let's assume that the Ford was set at 12° initial and you have a vacuum advance can pulling another 20° at full vacuum, then you end up with 32° at idle which may be way too much. If you then change it back to ported, there would be no vacuum advance so you would be running 12° which is more reasonable at idle.

If your chevy on the other hand has the same 12° initial, but the vacuum can only pulls 10° additional you would end up with 22° which could be required when running a higher overlap, long duration cam,

Just my .02$

Too much vac advance coming from manifold (rather than ported)makes them hard to start. They kick back and are hard on the starter motor.

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....

When I hear comments such as 'MVA didn't work on my engine', what it tells me is that it wasn't dialled in properly. Not that it doesn't work.
It takes time & effort to get it dialled in properly. Folks seem happy to spend hours tinkering with their carb to get it running perfectly, but then blame MVA when they hook up a vac hose to the vac adv & doesn't work as expected.....

You can have too much timing at idle, though too little is more common. You have to have the right amount of init timing & the right amount of VA added at idle. You also need to make sure the vac adv unit will deploy with the amount of available engine vacuum. Guess what happens when you try & use a VA unit that needs 12" of vac, but the engine only makes 10".......MVA gets blamed........'It didn't work on my engine'....

joe 90 wrote:Too much vac advance coming from manifold (rather than ported)makes them hard to start. They kick back and are hard on the starter motor.

I don't think that where the vacuum advance is connected would have any effect when starting. How would that be even remotely possible when there isn't any significant manifold vacuum until the engine starts?

Geoff2 wrote: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....

When I hear comments such as 'MVA didn't work on my engine', what it tells me is that it wasn't dialled in properly. Not that it doesn't work.
It takes time & effort to get it dialled in properly. Folks seem happy to spend hours tinkering with their carb to get it running perfectly, but then blame MVA when they hook up a vac hose to the vac adv & doesn't work as expected.....

You can have too much timing at idle, though too little is more common. You have to have the right amount of init timing & the right amount of VA added at idle. You also need to make sure the vac adv unit will deploy with the amount of available engine vacuum. Guess what happens when you try & use a VA unit that needs 12" of vac, but the engine only makes 10".......MVA gets blamed........'It didn't work on my engine'....

What will sometimes happen when using manifold vacuum at idle in an engine with some cam duration is this: The engine will be idling along with reasonable vacuum and thus additional timing which also tends to help increase the vacuum. Then after a while the engine will for whatever reason drop just a slight amount of manifold vacuum which then drops out timing which causes the manifold vacuum to drop more and then soon the engine starts to chug-a-lug.