vacuum gauge question...
Thread Starter
Racer
Joined: May 2005
Posts: 399
Likes: 4
From: Suffield CT
vacuum gauge question...
I was reading this
http://www.carprogrammer.com/Z28/PCM...gauge_tips.txt
and wondered what the folks who have experience building engines thought about this statement ?
Restricted Exhaust (Catalytic Converter): When the engine is unable to exhale properly, a positive pressure will develop inside the cylinder each time the exhaust valve opens. This increases the pressure inside the intake manifold as the intake valve opens. The end result is lower manifold vacuum.
Run the engine at 1000 rpm and record the vacuum reading. Increase engine speed slowly to 2500 rpm. Exhaust backpressure, depending on the amount of restriction, will increase with engine rpm. If the vacuum reading at 2500 rpm should drop more than 3 in.Hg from the reading taken at 1000 rpm, the exhaust system is most likely restricted.
Based on this trouble shooting technique could a person do this as a base line and then put on a "performance" exhaust and re-test to see if the change if vacuum shows the exhaust system helped increase performance ???
-ALF out....
http://www.carprogrammer.com/Z28/PCM...gauge_tips.txt
and wondered what the folks who have experience building engines thought about this statement ?
Restricted Exhaust (Catalytic Converter): When the engine is unable to exhale properly, a positive pressure will develop inside the cylinder each time the exhaust valve opens. This increases the pressure inside the intake manifold as the intake valve opens. The end result is lower manifold vacuum.
Run the engine at 1000 rpm and record the vacuum reading. Increase engine speed slowly to 2500 rpm. Exhaust backpressure, depending on the amount of restriction, will increase with engine rpm. If the vacuum reading at 2500 rpm should drop more than 3 in.Hg from the reading taken at 1000 rpm, the exhaust system is most likely restricted.
Based on this trouble shooting technique could a person do this as a base line and then put on a "performance" exhaust and re-test to see if the change if vacuum shows the exhaust system helped increase performance ???
-ALF out....
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I couldn't say whether the "3 in. Hg drop" would indicate an exhaust restriction or not, but the concept the write-up describes is reasonable. And, if you are only changing the exhaust/CAT system, a pre-post measurement of vacuum would give you some indication of backpressure reduction. What a reduction of vacuum loss would indicate in terms of backpressure reduction is unknown...but it would indicate some improvement.
Racer
Joined: Nov 2004
Posts: 485
Likes: 1
From: Oakville ON
If the vacuum reading at 2500 rpm should drop more than 3 in.Hg from the reading taken at 1000 rpm, the exhaust system is most likely restricted.
The wording may be a bit confusing. You will see less manifold vacuum with a restricted exhaust. ie: if you have 18" of vacuum at 1000 rpm a restricted exhaust will give you less vacuum (depending as previous poster said), maybe only 15" of vacuum at 2500 rpm. Unrestricted, you should see a higher vacuum reading (maybe 25").
The wording may be a bit confusing. You will see less manifold vacuum with a restricted exhaust. ie: if you have 18" of vacuum at 1000 rpm a restricted exhaust will give you less vacuum (depending as previous poster said), maybe only 15" of vacuum at 2500 rpm. Unrestricted, you should see a higher vacuum reading (maybe 25").
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i use the vaccum gauge all the time as a diagnostic tool....not necessarily the gopsel but as a good pointer on where to look
