What's your vacuum please check
Thread Starter
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
What's your vacuum please check
It is always interesting when a thread like Redman86's Cat/Leakdown thread comes up.Everyone starts looking at thier cars and wondering 
Anyway it seems to me that a vacuum measurement of the intake would have helped in that thread early on.So I pose this to you guys.What is your vacuum measurement at the intake.Mine is 20inHg and seems to be pretty strong and must indicate a strong flow out of the cylinders.If everyone would get a baseline measurement when their car is running right it could help when it does not.It is hard to tell if you have a vacuum leak if you don't know what it should be.I wish I had measured it before I replaced my bad EGR that was not holding a vacuum just to see how much lower it was then.Obviously if it is low you either have a leak or cat on the way out or other blockage.
So come on get the gauge out and post it here. I am especially interested in LT1 engines for comparision to my own.
If for some reason you disagree let me know why.Thanks
Anyway it seems to me that a vacuum measurement of the intake would have helped in that thread early on.So I pose this to you guys.What is your vacuum measurement at the intake.Mine is 20inHg and seems to be pretty strong and must indicate a strong flow out of the cylinders.If everyone would get a baseline measurement when their car is running right it could help when it does not.It is hard to tell if you have a vacuum leak if you don't know what it should be.I wish I had measured it before I replaced my bad EGR that was not holding a vacuum just to see how much lower it was then.Obviously if it is low you either have a leak or cat on the way out or other blockage.
So come on get the gauge out and post it here. I am especially interested in LT1 engines for comparision to my own.
If for some reason you disagree let me know why.Thanks
Racer
Joined: May 2004
Posts: 476
Likes: 0
From: DFW TX
Originally Posted by AKS Racing
I must confess, my is quite modded.I would hope that yours is modded as well to pull the lower vacuum at idle.
That 14" HG is at 870 idle.
That 14" HG is at 870 idle.
Going in for a tune next week, my idle is to low about 700, it needs to about 800 or a bit more. If I hold the idle at about a hair over 800 I'm getting a hair over 15" hg.
Safety Car
Joined: Apr 2001
Posts: 3,525
Likes: 2
From: Houston TX
I think the surprising part is that with a cam that is 248/254 and 0.670"ish, that these cams can pull that level of vacuum. Additionally, it pulls hard from just off idle to well past 6000 (I only run to 6600). It has a nice lope at 870RPM.
Aaron
Aaron
Racer
Joined: May 2004
Posts: 476
Likes: 0
From: DFW TX
Originally Posted by AKS Racing
I think the surprising part is that with a cam that is 248/254 and 0.670"ish, that these cams can pull that level of vacuum. Additionally, it pulls hard from just off idle to well past 6000 (I only run to 6600). It has a nice lope at 870RPM.
Aaron
Aaron
Your LS must be 114+......mine is at 112 LS.Sorry Redeasysport if I took the thread off track
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Safety Car
Joined: Dec 2004
Posts: 4,088
Likes: 0
From: Poop land
Mine is 18" but I dunno if that is from my m0dz... 
This was both on a cold start and later a hot start. I'm guessing large duration/low degree LSA cams cause a low vacuum. 18" of vacuum sounds like a stocker to me.
This was both on a cold start and later a hot start. I'm guessing large duration/low degree LSA cams cause a low vacuum. 18" of vacuum sounds like a stocker to me.
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
Any time a cam swap is made for a higher lift/more duration cam the vacuum will drop. This is also true when you add 1.6 or higher rockers. Mine holds 14 at 850 rpm. If you have a stock engine and low vacuum it's time to see why before you damage something.
Race Director
Joined: Feb 2004
Posts: 12,518
Likes: 19
I was told that altitude has an influence on vacuum reading as well. After A new motor is built and broken in, I am in the habit of doing a compression reading and a vacuum reading. This way I have the info to compare against if I ever have an unusual problem.
My 87 super rammed, 219 cammed, 10.6 compression makes 13 at stock prom idle. (650 or so) I am at approx 1,000 ft above sea level.
My 87 super rammed, 219 cammed, 10.6 compression makes 13 at stock prom idle. (650 or so) I am at approx 1,000 ft above sea level.
Team Owner
Joined: Sep 2002
Posts: 22,209
Likes: 12
From: OBAMA IS HITLER
I don't know the topography in my area vs. Pete's, so I don't know my relative altitude. See my bio in my profile for mods. Also, I have a 6 speed. At 850 rpm idle I'm set at, I've got 16" vacuum.
Racer
Joined: Oct 2005
Posts: 264
Likes: 1
From: La Grande Oregon
This has got me thinking,.....i know open mouth,.....insert foot. I used to time one of my cars with the vac gage, it's easy,...hook up the gage, (warm engine) take a reading so you know where you are before you start, then advance the timing to the point that it doesn't build any more vac. Then back it off one inch pound. this was done on a small block with about 650 lift, carbed,no computer. I don't know if this will work on a C-4, if someone trys it let us know what happens. ... 