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Timing and heat ported as opposed to full and effect temps

3K views 20 replies 11 participants last post by  Malibu ss 64 
#1 ·
I have another post that we were discussing heating and cooling, but was curious on the effect of running Ported vs Full vacum at idle and too much advance. Do these effect more heat at idle? I have read conflicting information - more timing creates more HP so more heat, but running full vacum adv at idle creates better burn to keep things cooler.

I am running a 383, new motor, all machine work and prep done properly, running 34 degrees total timing, the cam is 465/488 lift and duration of 279/289 with a 112 LS. Running full vacum to the distributer. Compression is at about 9.1. No pinging, no hard starting and pulls great when driving.

Just would like some thoughts or experiences of anyone who has tried running both, full vacum and ported and what the effect was on your temps at idle.

Not looking for advice on cooling suggestions, have that covered on the other post and I am trying a few things there, just looking for information on timing and heat effect at idle speeds.

Thks

Rich
 
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#2 ·
Try it and find out. Experimentation is a great way to learn.

First Guess: No big difference if the cooling system is adequate. Bigger difference if the cooling system can't keep up, or if there's additional reasons the engine might run warm--lean air-fuel mix, air conditioning problems, etc.
 
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#4 ·
I have also tested zero difference on my car If I run ported or non ported even had less initial with old hei distributorvand didnt saw a change.

I think Schurkeys guess is right.
 
#5 ·
Thks guys, Thankful for your feed back, quite honestly I don't want to run it on ported vacum, it runs so much better on full. I'll try it just for the heck of it, but will continue the path of different shroud, fan spacing etc.

Thks

Rich
 
#6 ·
There is more to it than that. First you need to make sure there is adequate vacuum advance at cruise, then move it over to full manifold and adjust carb a/f mixtures accordingly.. It does make a difference because leaner fuel mixtures require more advanced timing to run efficiently. It will drop temps and smooth the idle of a more radical cam.
 
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#9 ·
If you have a Leyland or a cam for a girl:D
For Us that use a more manly cam theres not enough vacuum in gear for manifold vacuum. Not even with your beloved Crane can. If you need help with your Chevelles cooling system just let us know if its your Leyland your talking about go to another forum. Any pics of your Chevelle yet or is your Camera still broken?
 
#10 ·
Thanks - How do you determine "correct" with the cam I mentioned and a stroker. My total is at 34, and no pinging and no hard starting, and runs nicely. I assume if anyone of those are an issue then I would have to back it down. I certanly can't set it at a stock application as there are no settings for it.
 
#11 ·
Rich, if it runs fine on manifold-leave it. I have never really noticed any variations of temps at idle between ported,manifold or no vacuum advance when the curve is right and it sounds like you are all set. Can't over think this stuff, its farm simple :thumbsup:
 
#12 ·
LESS advance usually runs warmer has always been my experience with these. It isnt as stinky either, marginally. With a lumpy cam there were overriding concerns so full went back on. Believe it was an emissions thing.
 
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#13 ·
LESS advance usually runs warmer has always been my experience with these.
That has been my experience. Back in the day when I lived in CT and my wife's bone stock 1977 Chevy V8 needed its annual sniff (which it almost always failed with "stock settings) I turned the distrib. to decrease the advance. Presto it passed. Every time. The trick was to get it sniffed and get outta "dodge" before it overheated. This was not a performance car with all the endless curve modifications and carb tuning and and and that we do with cars that are discussed here. It was just a DD that for whatever reason drove great with the stock timing and whatnot settings but always flunked the test.

Turn back distrib. get a Passed sticker for the window....turn distrib. back to normal advance setting and .......happy motoring until next year. Sometimes after testing I drove down the street from the test station pulled over within sight of the test station and bumped the timing right there on the side of the road. I had an index mark painted on the distrib. and the manifold. All ya needed was a distrib wrench to back off the clamp and retighten it after "tuning".
 
#14 ·
34* total is a little light, did you try 18 initial, 36-38 total to see if things change? I did not see where your initial was stated.
 
#15 ·
In the 70s after ported vac adv had been in use for a few years & emissions got tighter- & engines ran hotter- manufacturers incorporated a temp switch into the cooling so that once temps reached a critical temp, ported VA was switched to manifold VA to cool the engine. I doubt the manufacturers would have gone to this expense if this didn't work....

This is the description in my Motors Manual 1973-78 models, for GM cars. Others are similar. " ...the thermo vacuum switch is added to the system on some engines. The switch senses engine coolant temp & if temp reaches 220*, the switch valve moves to allow manifold vacuum to reach the dist & advance the timing to & allow the engine to run cooler."

The Crane adj vac adv unit will work down to as low as 5~6" of vacuum which will cover most peoples needs.
 
#16 ·
You cant compare a 70`s smog engine with a engine built for performance. Those 70`s smog engines didnt have enough initial timing to start with some had redicilous low initial like 2-4 deg and lazy curves so total wasnt all in before 4500rpm.

Most with a performance engine run 16-18 initial some like me even more Im running 25 initial. Some even uses looked out distributors.

There are GM cans that start as low as 2 hg” of vacuum.
I tried both them and the crane and none of them where fully activated at 8hg” that I have in D wich resulted in unsteady timing. You need a can that is fully activated atleast 2hg” lower than the hg” you have in gear on a car with aut transmission. Both a couple of GM and crane cans worked at idle in P since I had 10hg” in P put it in D and 150 rpm drop and 2hg” drop was enough to not get the vacuum can to work with manifold vacuum with my combination.

Same thing can happen if you use To light springs so your mechanical curve has started at idle in P but not at idle in D because of the rpm drop between P and D.

My car runs same temp with 16 deg initial as with 25 at idle in line up traffic no matter of time and outside temps of 90+ deg, used 16 deg initial with my Hei and now 25 deg initial with my msd pro billet both had a total of 35 deg all in by 2500. Still ran 175-180 deg in the head or 155-160 in the intake with a 160 deg thermostat with both settings.

So my experience is that if your cooling system is marginal from the begining it may have effect if you use ported vs manifold but if it has some extra cabability no difference to write home about.
 
#19 ·
Malibu,
I am very happy for you that your engine runs at the temp you are happy with. Not everyone is as happy as you are with their coolant temps, which was why the OP started this thread.

Vac adv connected to manifold vacuum is just another thing the OP can do to reduce idle temps, his choice if he wants to try it.

When I set up Crane VA units for low vacuum cams, I do the same as Dave Ray & move the stop to towards the diaphragm end of the unit. Works perfectly.
 
#20 ·
Goat good for you but it didnt work for me with the crane can, the stop plate is the best part you get when you buy a crane can and imho if everything in you cooling system is correct the difference in temp between ported or manifold vacuum isnt anytning you will notice.
I prefer manifold vacuum but have my own experience to help others and afaik op asked if there was big difference in engine temp ported vs manifold in this thread? And I Said no if your cooling system is setup to work correct. If its marginal from the begining it may be different. But is it the timing thats wrong than? Or something in the cooling system?

This is how I use a crane stop plate


But a piece of metall works just as good and is cheaper.
 

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#21 ·
To take it one step further gm and other very well respected engine builders clearly says in that instructions not to us vacuum advance at all. Gm even on very mild 350’s.
If gm thought they would overheat without vacuum advance I think they would change their instructions. No vacuum advance at all or ported is same at idle.

Ive installed a couple of those 350 and went against gm and used vacuum advance same with a zz502 I installed. If they not run cooler they get better mpg.
https://www.chevrolet.com/content/d...ate-engine-19201332-deluxe-specifications.pdf

Im not saying you shouldnt run vaccum advsnce at all or use ported instead of manifold just that the difference in idle temp if everything is as it should is minimal and in most cases something else cause it to run to hot if.
 
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