351w vacuum and engine health - Vintage Mustang Forums
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post #1 of 17 (permalink) Old 07-10-2019, 01:48 PM Thread Starter
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351w vacuum and engine health

Hi,
Just working out the kinks now that the 66 is finally moving after a 1 year build.

I'm getting a vacuum reading of 11 at idle in park.

I bought this car "in progress" so here is what I know:

351w circa 1978
rebuilt 1969 heads (PO mentioned he did this to raise compression numbers)
Original pistons, new rings and bearings. Rings were cast, he mentioned, for a quick break in period.
Edelbrock dual plane manifold (think it's performer 351w..)
Holley 1850 / 4160 vacuum secondaries 600 cfm
Comp cams XE268H-10 duration 268 intake 280 exhaust 110 separation .51 lift intake .512 lift exhaust 35-242-3 - Xtreme Energy? Hydraulic Flat Tappet Camshafts
I don't know if it was degreed when installed
I did not re-adjust the valves. They seemed right by feel, I could just spin the pushrods when not on a lobe, know that is not a certainty.

I've put maybe 20 miles on this thing max, maybe rings are still seating, don't know exactly if he honed the cylinders, assume so he seemed to know what he was doing.

I do get some light smoke coming out of the breather while it is running.

Moved timing around from 12 - 16, which moved the idle not a huge impact on vacuum reading.
I've adjusted the idle mixture in until it starts to stumble a little then back out until relatively smooth
it idles well, little lumpy if I drop it too low, 800rpm in park 550 in gear.
Vacuum reading from full manifold port on the carb (where the trans modulator line was hooked) pcv not installed yet, that port is plugged on the carb, no power brakes or other vacuum attached except disty advance.

So, do I have issues? Seems to run pretty darn good, it's fast.

Thanks, sorry for long post, wanted as much detail as I could include.
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post #2 of 17 (permalink) Old 07-10-2019, 02:08 PM
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bigger cams mean lower vacuum. I would expect a vac of 17-20in hg with a stock cam....couldnt say for that particular cam...but its probably about right. what vacuum are you getting at cruise?
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post #3 of 17 (permalink) Old 07-10-2019, 02:22 PM Thread Starter
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will have to rig the gauge to see while driving, will report back.

what would be a good reading while cruising?

Forgot to mention disty is using timed port from the carb for the vacuum advance and it was hooked up while taking the reading.
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post #4 of 17 (permalink) Old 07-10-2019, 02:27 PM
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I would expect at cruise that you would pull a normal 17-22, depending on RPM
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post #5 of 17 (permalink) Old 07-10-2019, 02:36 PM Thread Starter
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great

and after reading many other threads (know this has been covered extensively) I may try hooking the monstrous distributor to full manifold vacuum for kicks. So do I plug it up, set base timing to 14-16, then plug in the vacuum advance hose and tweak idle mix as needed?
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post #6 of 17 (permalink) Old 07-20-2019, 01:43 PM Thread Starter
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Ok vacuum is now up bouncing around 14-15 might have had a small leak. I added a pcv also. Anyway tried switching to manifold vacuum but couldn’t get it dialed in right initial timing was way high and reviving it want off the charts way over 40. I put it back for now but will try again later, need a good procedure for this.


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post #7 of 17 (permalink) Old 09-01-2019, 06:26 PM Thread Starter
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Well I’ve gone between ported and manifold vacuum several times now. Seems like I want to stick with manifold vacuum for the advance (idles cooler and seems responsive) but maybe this hei has a stock vacuum advance that comes in too late vacuum wise. Idles great in park and have initial set at 15 with advance disconnected and plugged but when I hook it up and put it into gear it drops and won’t idle worth a darn, likes to stall.

Probably my procedure is off. Using a vacuum pump I can see the advance starts to move at 7.5 and keeps advancing to about 17” hg. Any good rules of thumb here? I think it is a bigger cam than stock but not crazy either.


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post #8 of 17 (permalink) Old 09-16-2019, 05:16 PM Thread Starter
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just to close the thread on this, I swapped in a vacuum canister that is all in by 12" of vacuum and am now back set on manifold vacuum. It is running very well. (the previous one kept advancing until I hit 17" vacuum)

still not sure if I had the right procedure but here is what I did:

Disconnect / plug vacuum.
Initial timing at idle set to 16*
I can see it go up to maybe 38* via mechanical advance
Connect Vacuum - idle goes up
adjust idle back down and readjust idle mixture
final fine tune of idle level so it is still at least ~700 in gear
this revs clear above 50* with no load but I have no pinging I can hear in various driving situations.

Thanks!
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post #9 of 17 (permalink) Old 09-16-2019, 11:01 PM
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Originally Posted by green_giant71 View Post
just to close the thread on this, I swapped in a vacuum canister that is all in by 12" of vacuum and am now back set on manifold vacuum. It is running very well. (the previous one kept advancing until I hit 17" vacuum)

still not sure if I had the right procedure but here is what I did:

Disconnect / plug vacuum.
Initial timing at idle set to 16*
I can see it go up to maybe 38* via mechanical advance
Connect Vacuum - idle goes up
adjust idle back down and readjust idle mixture
final fine tune of idle level so it is still at least ~700 in gear
this revs clear above 50* with no load but I have no pinging I can hear in various driving situations.

Thanks!
As far as I can tell, this all seems pretty good. I have about the same total numbers (less initial, more mechanical) on a stock 302 with 10:1 static compression ratio. And lumpy cams which have a lower dynamic compression ratio than stock (assuming the same static compression ratio) are more forgiving.

BTW: Just curious... Where did you find a vac adv canister that is all in at 12" of vac?
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post #10 of 17 (permalink) Old 09-16-2019, 11:18 PM Thread Starter
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I found this article and in part 3 there are a whole load of vacuum canister options.

Yes it’s all gm and I have the shame of an hei distributor but that’s how I found the can, mine is from a 75 skylark.


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post #11 of 17 (permalink) Old 09-17-2019, 07:42 AM
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You probably want to look up your dizzy and find a way to limit the degrees of vac advance. With that much timing at cruise, your combustion chamber is sure going to be hot, and you'll be down on power (until you step on the gas and vacuum drops, ditching the extra timing). Idle would have issues too, unless you put it on ported, but that's just a bandaid. Better to get it something around 12-13 degrees of additional timing from the vac advance, and say 14 mechanical. That way you're at about 26-27ish (but this will change based on what your engine likes, and cam has a lot to do with that).


Remember: Your mechanical timing is what your engine wants only at WOT. No ping, best power. It won't run perfectly smooth at idle with the vacuum advance disconnected, because your engine NEEDS more timing when it's getting very tiny amounts of air and fuel. This is why ported vacuum is a stopgap measure for cars that have such a rough idle they can't run properly on manifold vacuum. It's possible that you have so little vacuum and enough lope that you're having trouble, but honestly, a 268H on a 351 is just not that wild and radical! I'd expect you to have a bit more vacuum when you get your timing set up right.

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post #12 of 17 (permalink) Old 09-17-2019, 12:12 PM
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Originally Posted by green_giant71 View Post
I found this article and in part 3 there are a whole load of vacuum canister options.

Yes it’s all gm and I have the shame of an hei distributor but that’s how I found the can, mine is from a 75 skylark.


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Re: "...this article..."

Was there supposed to be a link to the article in your reply?
... if so, it seems to be missing...
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post #13 of 17 (permalink) Old 09-17-2019, 12:26 PM
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Originally Posted by Grimbrand View Post
You probably want to look up your dizzy and find a way to limit the degrees of vac advance. With that much timing at cruise, your combustion chamber is sure going to be hot, and you'll be down on power (until you step on the gas and vacuum drops, ditching the extra timing)...
From what I've read having 50* advance timing (when including the vac advance) during cruise is not an issue. Here is the reference:

http://www.camaros.org/pdf/timing101.pdf

And the excerpt:
"The same thing occurs under steady highway cruise conditions. The mixture is lean, takes longer to burn, the load on the engine is low (it takes only about 40 horsepower to cruise at 50mph) and the manifold vacuum is high, so the vacuum advance unit is again deployed and adds 15 degrees of spark advance over and above whatever the distributor centrifugal advance mechanism is providing at that engine rpm. If you had a timing light connected so you could see it as you cruise down the highway, you’d see about 45-50 degrees of spark advance – your fixed initial advance of 10 degrees, 20-25 degrees provided by the centrifugal advance mechanism, and the 15 degrees added by the vacuum advance unit."
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post #14 of 17 (permalink) Old 09-17-2019, 01:58 PM
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P-51, if you're 'cruising' at over 3k RPMs, then 50 degrees total could possibly make sense, you're right. I think I misread this a bit, and thought he was getting 50 at idle. =)


Carry on!

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post #15 of 17 (permalink) Old 09-17-2019, 05:13 PM
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P-51, if you're 'cruising' at over 3k RPMs, then 50 degrees total could possibly make sense, you're right. I think I misread this a bit, and thought he was getting 50 at idle. =)


Carry on!
I think what he is doing is rev'ing the engine while the car is stationary and seeing what max advance it hits with initial + mech adv + vac advance. That sorta gives one an idea of what it will be doing during cruise.

Yeah, 50* @idle RPM... that would be kinda bad ;-) :-)
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