Vacuum readings - The 1947 - Present Chevrolet & GMC Truck Message Board Network

85Silveradoin04 · 11-09-2004, 09:29 PM

So I did a ported to manifold comparison.

At idle (1000RPM) with the dizzy on ported vac, I'm running 19 - 19 1/2"
At idle in gear (750RPM) with the dizzy on ported vac, I'm running 15 - 15 1/2". With the needle resting more on 15".

At idle (1000RPM) with the dizzy on manifold vac, I'm running 19 3/4 - 20"
At idle in gear (750RPM) with the dizzy on manifold vac, I'm running 16 - 16 1/2". With the needle resting more on 16".

Is this good for a freshly rebuilt ZZ4? One thing I noticed when changing from ported to manifold vacuum, the idle increased to 1300RPM. I had to lower it to a grand to get a better comparison. Whats the reason for this? Does this affect driveability?

While ported, the truck idles very well. Matter of fact, a buddy of mine has a '03 vette and its very similar.

With manifold, it seems to be more choppy. Hmm

**swervin ervin** · 11-10-2004, 07:09 PM

Manifold vacuum advances the timing because it has vacuum at idle. Ported is above idle so no advance of timing.

Leave it on manifold vacuum for best economy and idle. Plus it helps on engine temps too. Have you checked to see how much advance you are getting from the vacuum canister at idle with manifold vacuum?

85Silveradoin04 · 11-10-2004, 10:40 PM

Quote:

Originally Posted by swervin ervin

Manifold vacuum advances the timing because it has vacuum at idle. Ported is above idle so no advance of timing.

Leave it on manifold vacuum for best economy and idle. Plus it helps on engine temps too. Have you checked to see how much advance you are getting from the vacuum canister at idle with manifold vacuum?

swervin ervin, Thanks for the reply. No not sure about the cannister. I know I running 12 initial and I believe 36 total. Do you think I'm pulling good vac? One thing I'm gonna do is check my power valves. Seems to be running a little rich.

**swervin ervin** · 11-11-2004, 12:31 AM

Yeah, vacuum seems pretty good to me. Anything around 19-20 is good. A lot depends on what cam you have. It's all I can do to get 15-16.

PHOENIX · 11-11-2004, 11:57 AM

I have tried going from ported to manifold a few times now after Swervin recommended it.
I also had rpm increase at idle and had to adjust for it.
Its seems my idle gets rougher too. More then I would prefer.
Also during the test drive it seems pretty much the same, but at higher rpm's the engine doesnt rev as fast as the ported vacuum.
I ended up keeping it on ported every time I tried switching it.

Swervin what kind of performance differences did you notice from ported to manifold?

**swervin ervin** · 11-11-2004, 06:15 PM

Remember, vacuum advance isn't for performance. It's an economy thing only.

Eddie, I couldn't tell you. In my entire life, anything I've setup or built, I've never run ported vacuum. It's not the way it's supposed to be, at all. The only reason GM ever thought it might be a good idea on the vehicles they did use it on was to keep spark knocking to a minimum. If they would have built a better engine to begin with, they wouldn't need it.

It's all a matter of experimenting to find what works best for you. Believe me, manifold vacuum is the way to go. If you didn't get good results with it, you have other problems that need to be addressed instead of trying to mask it with ported vacuum.

11-09-2004, 09:29 PM	#1
85Silveradoin04 Registered User Join Date: Jun 2004 Posts: 64	Vacuum readings So I did a ported to manifold comparison. At idle (1000RPM) with the dizzy on ported vac, I'm running 19 - 19 1/2" At idle in gear (750RPM) with the dizzy on ported vac, I'm running 15 - 15 1/2". With the needle resting more on 15". At idle (1000RPM) with the dizzy on manifold vac, I'm running 19 3/4 - 20" At idle in gear (750RPM) with the dizzy on manifold vac, I'm running 16 - 16 1/2". With the needle resting more on 16". Is this good for a freshly rebuilt ZZ4? One thing I noticed when changing from ported to manifold vacuum, the idle increased to 1300RPM. I had to lower it to a grand to get a better comparison. Whats the reason for this? Does this affect driveability? While ported, the truck idles very well. Matter of fact, a buddy of mine has a '03 vette and its very similar. With manifold, it seems to be more choppy. Hmm Last edited by 85Silveradoin04; 11-10-2004 at 09:39 AM.

11-10-2004, 07:09 PM	#2
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	Manifold vacuum advances the timing because it has vacuum at idle. Ported is above idle so no advance of timing. Leave it on manifold vacuum for best economy and idle. Plus it helps on engine temps too. Have you checked to see how much advance you are getting from the vacuum canister at idle with manifold vacuum? __________________ Mike 1985 Chevy C-10

11-11-2004, 11:57 AM	#5
PHOENIX Registered User Join Date: Oct 2002 Location: Phoenix, Arizona Posts: 4,703	I have tried going from ported to manifold a few times now after Swervin recommended it. I also had rpm increase at idle and had to adjust for it. Its seems my idle gets rougher too. More then I would prefer. Also during the test drive it seems pretty much the same, but at higher rpm's the engine doesnt rev as fast as the ported vacuum. I ended up keeping it on ported every time I tried switching it. Swervin what kind of performance differences did you notice from ported to manifold? __________________ * AVOID: LOPER'S PERMORANCE / LOPERSPEED.COM OF PHOENIX, AZ & COTTMAN TRANSMISSION *

11-11-2004, 12:31 AM	#4
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	Yeah, vacuum seems pretty good to me. Anything around 19-20 is good. A lot depends on what cam you have. It's all I can do to get 15-16.

11-11-2004, 06:15 PM	#6
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	Remember, vacuum advance isn't for performance. It's an economy thing only. Eddie, I couldn't tell you. In my entire life, anything I've setup or built, I've never run ported vacuum. It's not the way it's supposed to be, at all. The only reason GM ever thought it might be a good idea on the vehicles they did use it on was to keep spark knocking to a minimum. If they would have built a better engine to begin with, they wouldn't need it. It's all a matter of experimenting to find what works best for you. Believe me, manifold vacuum is the way to go. If you didn't get good results with it, you have other problems that need to be addressed instead of trying to mask it with ported vacuum.



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