Low vacuum
Thread Starter
Joined: Sep 2004
Posts: 2,552
Likes: 12
From: Visalia, California
Low vacuum I'm pulling my hair out (I'm bald! haha) fellas. I've been chasing lower vacuum on my gen4 4.8 for some time now, and ran across some threads talking about pushrod length.
I got an adjustable pushrod, and found I needed a 7.350" to get ~.050" preload on my Comp 850s instead of the 7.400 I had in it.
I just finished installing them and firing it up. My VE table is jacked up and way lean now, but still looks like lower-than-expected vacuum, despite switching to a shorter pushrod.
Attached is my cam card, and a video of it idling in park at 850rpm and 20* timing.
Please let me know your thoughts.
https://photos.app.goo.gl/jQyb7v44yC89eQry5
I got an adjustable pushrod, and found I needed a 7.350" to get ~.050" preload on my Comp 850s instead of the 7.400 I had in it.
I just finished installing them and firing it up. My VE table is jacked up and way lean now, but still looks like lower-than-expected vacuum, despite switching to a shorter pushrod.
Attached is my cam card, and a video of it idling in park at 850rpm and 20* timing.
Please let me know your thoughts.
https://photos.app.goo.gl/jQyb7v44yC89eQry5
Thread Starter
Joined: Sep 2004
Posts: 2,552
Likes: 12
From: Visalia, California
I've got a leak down kit coming today but it's supposed to be 106*, so may hold off until it cools down a bit.
Thanks for the response!
Thread Starter
Joined: Sep 2004
Posts: 2,552
Likes: 12
From: Visalia, California
Cylinder 1
Cylinder 3
Cylinder 5
Cylinder 7
Cylinder 2
Cylinder 4
Cylinder 6
Cylinder 8
TECH Enthusiast
Joined: Feb 2020
Posts: 734
Likes: 209
From the video I see 12-13 inches of vacuum on the pump gauge but single digit on the interior gauge, can't make out the laptop # . The 12-13 actually seems about right to me for that cam in a 4.8 . I run a slightly smaller cam in a 6L and get 16-17 . From the cam card your at a 112 int CL ( no advance ground in ) advancing the cam 2-4 degrees will help a bit - earlier closing of intake valve. Degreeing would be best to see where you are actually at and adjust from there . Also what ignition timing are you running at idle ? Can have a big effect on vacuum
Thread Starter
Joined: Sep 2004
Posts: 2,552
Likes: 12
From: Visalia, California
@GMCGreg thanks for taking time to respond. Yes, I noticed the eBoost2 shows a few inhale lower than the mechanical gauge for some reason, maybe something with the vacuum block, I’ll move its reference to another port to check. 3 bar map shows mid 60s in park, low to mid 70s in gear. 96.4kPa last night with the key off, and I’m at 295’ ASL.
My target is 20*. I see a gain of about an inch if I work it up closer to 30*, but it’s most stable at 20*, and 800-850rpm.
Thanks again!
My target is 20*. I see a gain of about an inch if I work it up closer to 30*, but it’s most stable at 20*, and 800-850rpm.
Thanks again!
TECH Enthusiast
Joined: Feb 2020
Posts: 734
Likes: 209
12.5 in hg is about 60 kpa , so doing OK there . Degreeing the cam would be best IMO but also a big PITA at this point . I idle mine at 725 rpm 22° timing, simple comparison of bigger cam , smaller engine I would try 24 ° or so , this may help more in gear . How's the converter ? Too tight ? I'd say if you can get it a little higher and power brakes are working OK then don't sweat it and tune away
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Thread Starter
Joined: Sep 2004
Posts: 2,552
Likes: 12
From: Visalia, California
12.5 in hg is about 60 kpa , so doing OK there . Degreeing the cam would be best IMO but also a big PITA at this point . I idle mine at 725 rpm 22° timing, simple comparison of bigger cam , smaller engine I would try 24 ° or so , this may help more in gear . How's the converter ? Too tight ? I'd say if you can get it a little higher and power brakes are working OK then don't sweat it and tune away
Thank you again!
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Don't forget about the valve springs.
I broke a valve spring a little while ago, didn't make any noise or anything but made 10-10.5 inches of vacuum when usually its up around 12-13.
I got a manual brake kit because I thought the booster failed since there was no other symptoms and when I pulled the driver side cover, I saw the busted spring.
Replaced the springs and idle vacuum came right back up.
I run around 32 degrees of timing at idle as well.
I broke a valve spring a little while ago, didn't make any noise or anything but made 10-10.5 inches of vacuum when usually its up around 12-13.
I got a manual brake kit because I thought the booster failed since there was no other symptoms and when I pulled the driver side cover, I saw the busted spring.
Replaced the springs and idle vacuum came right back up.
I run around 32 degrees of timing at idle as well.
