360 V8, 270 cam, 1.6 rocker ratio and low idle vacuum

[femtnmax]

Idle vacuum is 8-9 inches at 750 rpm.  Vacuum increases to 15 inches at 1500 rpm.  Ignition timing 6*advance at idle, 36* total.  The car is street driven.   I'm running power brakes so I need vacuum, maybe 12-14 inches at idle for reliable power brakes is that correct?  Or is the 15 inches at 1500 rpm OK???
When I picked this cam, I thought the 1.6 rocker would not affect the 270 degree seat duration, so should not affect idle vacuum.  But...am I wrong??  Is it true that the rocker ratio increase is the problem?  I checked the vacuum gauge on a truck, had 14 inches at 600 rpm.

One other question.  I set the idle mixture for highest vacuum reading, at 750 rpm idle speed.  But..The engine is after-running a little.  The carb is a thermoquad;  the primary throttle plates are set to expose 1/2 of a "square" transfer slot as suggested by 'demonsizzler' thermoquad tuning website.  Demon said to have a full 'square' of the transfer slot exposed, and I backed off a little trying to reduce engine after run.    Why would it after run with such a low idle speed?  I'm going to check the idle mixture ratio with an 02 sensor, thinking maybe the mixture is too rich.

Any help with my questions would be appreciated.  The challenger is 5+ years in the making, the snow is melting, and I'm close to running it on the road at long last.  thanks.

[femtnmax]

My guess is the larger rocker arm ratio did not change the degrees of intake/exhaust overlap, but did change the area within the overlap triangle because the valves are being opened at faster rates.  So the idle vacuum would decrease.  The question then is how much to reduce the seat & .05 durations to have an acceptable idle vacuum.  Either a smaller cam and/or consider widening the lobe separation angle.  The 270 cam has 110*LSA, but the factory 340 cam had 114*LSA, at least the auto cars did, not sure about the 1 year only 4 speed cam.
The 270 cam degrees of overlap is:
Add intake & exhaust seat duration:  270+270=540
divide 540 by 4=135
subtract LSA=135-110=25
multiply by 2=25 x 2=50           So the 270 cam has 50 degrees overlap.
If I changed to 112* LSA, then the overlap is:  46 degrees

For the after-running, remembering the '68 440 GTX I used to have with those big open combustion chambers; it would after-run real bad if the gas tank had anything less than the very highest octane available.   So on the 360 I have now, it has .044 quench, but the gasoline is a mix of regular and mid-grade if I remember right.   The answer is to fill it up with premium and see how the engine acts then.

I would still appreciate some ideas and suggestions.  thanks.

[Challenger340]

Just a suggestion for your consideration ?
Maybe,
Try Advancing the 270/110lsa up to 105* atdc ?
It will increase "lope" or port reversion at idle, but will also increase Engine vacuum, by placing more lift earlier of the peak Cylinder "Pull" earlier for accessories.

I have no ideas for the engine run on problem ?
IMO,
it will be related to whatever mixture is present after shutoff ?

Alot of Thermoquads utilized an Idle Solenoid that closed the plates entirely after shutoff ? Does your have this ?

[firefighter3931]

Ignition timing 6*advance at idle, 36* total. 

That's your problem. Bump the base timing up to 18* at idle and limit the mechanical to 18* for a total of 36* 

That should increase the idle vacuum significantly and give it a LOT more snap ! 



Ron

[kamkuda]

Ignition timing 6*advance at idle, 36* total. 

That's your problem. Bump the base timing up to 18* at idle and limit the mechanical to 18* for a total of 36* 

That should increase the idle vacuum significantly and give it a LOT more snap ! 

Ron

I would agree with Ron.
I would adjust the timing up by had and see what the engine likes at idle.  If it is 14 than set the initial at 14 and total to 36
My 408 liked 18 and and a total of 36

Email me if you need the intructions on how to do it and I can email you an article

[kamkuda]

FYI

Quick reference for size of slots

here ya go i found the magazine,

dist. degrees X 2 + initial= total


dist. degrees / slot size
6.............. .340
7............... .355
8............... .375
9............... .390
10.............. .405
11.5 ........... .420
12.............. .435
13.............. .445
14.............. .460
15.............. .475
16.............. .490
17.............. .505
18.............. .520

[femtnmax]

Thanks for all the ideas.  I increased the idle timing to 18*, and ended up with 10.5-11 inches vacuum.  The amount of vacuum did steadily increase as the idle timing was advance.  The engine seemed to run a touch smoother at 16* degrees advance.  I readjusted the total timing to stay at 36*.  I set the idle speed at 800 rpm.

How do you decide which timing setting is more correct, 16 or 18 degrees advanced?  I had left it at 18* only because the vacuum was a little better, but as mentioned the engine seemed to run a little smoother at 16*.

So with the increased initial timing, the vacuum did increase, but not to 12 or 14 inches I was dreaming of.   When the snow melts some more, I can get this hot rod out the driveway without getting stuck, and can check out the brakes to see how they work.

With all this initial timing advance, I did not see much benefit of the vacuum advance, so I unhooked it.  I installed light mechanical advance springs for total advance about 2500 rpm.  The car has a 4 speed, 3.23 axle gears.

One benefit of increasing the idle timing was it eliminated the after-running. Does this suggest the extra timing advance at idle is actually a little early, thereby helping to stop piston motion when the ignition is shut off?  I don't mind that the end result is the engine stops when you turn it off:)

[greenpigs]

  I would go by max vacuum & if the car starts easy and does not denotate, then keep giving it advance untill you do not see any increase in vacuum & it starts easy & has no ping.
  It also depends on how it runs so if it runs better @ 16* with not much difference in vacuum from 18* go with 16*. Every combo is different so that may be your sweet spot and the only way to find out is doing what you are.
 
  Have you checked for vacuum leaks?

[femtnmax]

thanks for more tips.  I'm not finding any vacuum leaks, I'll spray some carb cleaner around and see if theres any increase in idle rpm.
Maybe my elevation is the cause of low vacuum readings.  I found this:
"Vacuum readings are also lower at higher altitudes; the rule of thumb is approximately 1 inch of mercury for every 1,000 feet of altitude gain".   
So at 5000 feet, my engine has lost 5 inches; it would have 15+ inches at sea level.

[greenpigs]

Since your not far off have you thought of getting a vacuum reservoir, not a pump.