expected idle?

[Virg464]

Virg,
I suspect your engine will be happiest with about 40ish degrees at idle. [ Advance the timing to 40* at idle & you will see what I mean ]. A 440 I built with slightly smaller cam [ idle timing is all about the cam.....] idles at 42-44*. This is probably too much for WOT, where 32-38* is reqd. The fix is quite easy. You use a dist with an adjustable vac adv. Get rid of the useless MSD dist & get a cheap Mopar dist from Summit that has an adj vac adv. The vac adv is connected to manifold vacuum [ MVA ]; that means, it is active at idle. The VA is load dependent, & drops to zero @ WOT. So if the VA unit adds 22*, you would set the initial [ static ] to 20-22*.
The Mopar dist can be used to trigger the 6AL.
Until you get the idle timing correct, you will never get optimum idle quality, vacuum & tip in performance.
I could give heaps of MVA success stories, here is one...

View attachment 1741348

Thanks, I appreciate the info. This is my first build at this level and these numbers are a little uncomfortable for me.

 

[Virg464]

It's auto trans or manual? Sometime a 727 Auto wants a slightly higher idle if the torque converter is not loose enough with high enough stall.

727 trans, FTI converter 3600 stall (advertised)

 

[Virg464]

What’s the vacuum at idle? And yes that cam will probably not be happy with a stock converter. Chrysler idled some engines at 900,it works well for me.

20* static timing, 38* all in at 2500 rpm. Pulls 8" vac at 950 rpm idle.

 

[Geoff 2]

Bump your idle timing up to about 40* [ not critical ].

Three things will happen:
- idle rpm increases [ engine making more hp ]
- vac increases
- idle gets smoother.

Now, how do I know this....

 

[Virg464]

Bump your idle timing up to about 40* [ not critical ].

Three things will happen:
- idle rpm increases [ engine making more hp ]
- vac increases
- idle gets smoother.

Now, how do I know this....

That will be tomorrows project, as well as hooking up the shifter to see what the idle does in gear. Appreciate the info and advise.

 

[Virg464]

Well, have to order a new shifter cable and some heat shield sleeve before I can see what the idle does when its in gear. I didn't pay close enough attention to routing of the cable and it got too close to one of the header tubes. The outer cover melted and the cable is locked up tight! New spring kit for the mech advance on the way and plug wire brackets so I don't burn one of them.

 

[furious70]

I understand what's being recommended for him. How do you set it up if it was a street car and you wanted 55* at light cruise. Would the vac advance pull enough more at light cruise vs idle when hooked up to manifold vs ported?

 

[Geoff 2]

No. The amount [ number of degrees ] of vac adv added at idle.......will be the same as at cruise. The vac at cruise will be higher than it is at idle........which is why it so important to ensure that the vac adv plunger pulls all the way in at idle. When I hear comments like 'MVA didn't work for me', the above is often the reason.

 

[furious70]

I've read about this many times over the last 20 yrs but never tried it. At what duration or idle vac reading does switching to MVA work better then ported?

 

[Joske2]

I've read about this many times over the last 20 yrs but never tried it. At what duration or idle vac reading does switching to MVA work better then ported?

Its all about the engine, and what the engine likes. But generaly speaking , the more cam overlap, the more timing it likes.
I have a 68 Charger in the shop, 383 with dome pistons, 53°overlap, needs almost 60° at idle to run the best.
I have a client with a 68 Mustang where i build a 347 stroker, 51° overlap, needs 35° at idle, more and it sounds rough.
Another client with a 69 El Camino 396, 4 speed, 48° overlap, runs on ported vacuum, more then 20° at idle and it sounds rough.

Don't be afraid to try, the engine will tel you when its not happy.

 

[Geoff 2]

Furious,
Even a stock Chrys engine that came with 0-12* BTDC initial timing will benefit with MVA. Chrys missed the boat on MVA, simple as that.
Stock Pontiacs idle with 26*: 6* initial + 20* of MVA. Chevs, about 24*.

 

[furious70]

My apologies for side tracking a bit more - 35yrs in the hobby and I've only ever really worked on Mopars. Are we the only ones using ported?

 

[Geoff 2]

Not sure what Ferd did. MVA disappeared in the late 60s, to be replaced by the useless PVA. Because of emissions. Chrys DID in fact use MVA through the 70s as a back up, as did the other car makers. If the engine got too hot, a temp sw switched from PVA to MVA. This increased idle rpm, which in turn increased water pump & fan speed to cool the engine.

 

[Virg464]

New shifter cable showed up along with springs and bushings set for the distributor, and cams for the carb accel pump. Should be ready to try this again in the next day or two. Going to adv the timing first and go from there.