78 C3 Vacuum readings
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Joined: Jun 2012
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From: Dayton Nevada
78 C3 Vacuum readings
I have a 350 ci /300 hp with a cam that gives it a bit of a lop to it.
I am trying to fine tune the areas I could not get to while it was sitting. One concern was the Vacuum inches needed.
I read where 18-20 inches of vacuum at the manifold is healthy for 350 / 220 motor, and if there is a drop of 2-3 inches in a system line (headlight, heater, etc.) then a possible leak exists somewhere in that line.
I started at Ideal
1,000 rpm 13.5 " of Vac. at Manifold.
1,500 " 15 " "
2,000 " 16 "
2,500 " 16 "
3000 " 15.5 "
I think this is a good thing. considering it is a 350 / 300 motor with a little higher cam to get the lop.
Is this to low to run the vacuum systems on the 78 C3?
Would added vacuum canisters help?
The above reading were made at the power brake port, Not the manifold port , so they are wrong. see below.
I am trying to fine tune the areas I could not get to while it was sitting. One concern was the Vacuum inches needed.
I read where 18-20 inches of vacuum at the manifold is healthy for 350 / 220 motor, and if there is a drop of 2-3 inches in a system line (headlight, heater, etc.) then a possible leak exists somewhere in that line.
I started at Ideal
1,000 rpm 13.5 " of Vac. at Manifold.
1,500 " 15 " "
2,000 " 16 "
2,500 " 16 "
3000 " 15.5 "
I think this is a good thing. considering it is a 350 / 300 motor with a little higher cam to get the lop.
Is this to low to run the vacuum systems on the 78 C3?
Would added vacuum canisters help?
The above reading were made at the power brake port, Not the manifold port , so they are wrong. see below.
Last edited by Douglas Brown; Jan 12, 2018 at 10:17 AM. Reason: New information
Le Mans Master
Joined: Jul 2000
Posts: 6,664
Likes: 162
From: columbia city in
I think you are a little low on vacuum.I would plug off all vac lines and check again.if still low ,adj timing.If no improvement use a can of starting fluid and spray around intake.base of carb .If after all of this and it still is low I would say its just the cam.If your brakes are good and the headlights work okay ,just drive it.
Melting Slicks
Joined: Feb 2017
Posts: 3,232
Likes: 1,232
From: PHX AZ
normal vacuum
I have a 350 ci /300 hp with a cam that gives it a bit of a lop to it.
I am trying to fine tune the areas I could not get to while it was sitting. One concern was the Vacuum inches needed.
I read where 18-20 inches of vacuum at the manifold is healthy for 350 / 220 motor, and if there is a drop of 2-3 inches in a system line (headlight, heater, etc.) then a possible leak exists somewhere in that line.
I started at Ideal
1,000 rpm 13.5 " of Vac. at Manifold.
1,500 " 15 " "
2,000 " 16 "
2,500 " 16 "
3000 " 15.5 "
I think this is a good thing. considering it is a 350 / 300 motor with a little higher cam to get the lop.
Is this to low to run the vacuum systems on the 78 C3?
Would added vacuum canisters help?
Any in out????
I am trying to fine tune the areas I could not get to while it was sitting. One concern was the Vacuum inches needed.
I read where 18-20 inches of vacuum at the manifold is healthy for 350 / 220 motor, and if there is a drop of 2-3 inches in a system line (headlight, heater, etc.) then a possible leak exists somewhere in that line.
I started at Ideal
1,000 rpm 13.5 " of Vac. at Manifold.
1,500 " 15 " "
2,000 " 16 "
2,500 " 16 "
3000 " 15.5 "
I think this is a good thing. considering it is a 350 / 300 motor with a little higher cam to get the lop.
Is this to low to run the vacuum systems on the 78 C3?
Would added vacuum canisters help?
Any in out????
Last edited by L-46man; Oct 4, 2017 at 06:56 PM.
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Joined: Jun 2012
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From: Dayton Nevada
Does the cam make a difference??? IF so how much.?? Ok it depends on what lift and duration, I don't know that it was build to have a bit of a lope at ideal.
18-20" is what I read for a healthy 350/220, only 3" hg range
The 17 - 21" you mention would give a 5" hg range.
My 350/300 is new, less than 150 miles on it. It has a 13-16" 3" hg range
I'll check hose connections for loose fit and put clamps where there are none.
and check for leaks around carb. base.
Head lights work good, not slow up or down, Brakes are good.
If 13 - 16 " hg is all it is, then that is all it is. I'll work with it, if I need
extra Vac. assist. then I will deal with it then.
Thanks guys, always grateful for your help.
DB
18-20" is what I read for a healthy 350/220, only 3" hg range
The 17 - 21" you mention would give a 5" hg range.
My 350/300 is new, less than 150 miles on it. It has a 13-16" 3" hg range
I'll check hose connections for loose fit and put clamps where there are none.
and check for leaks around carb. base.
Head lights work good, not slow up or down, Brakes are good.
If 13 - 16 " hg is all it is, then that is all it is. I'll work with it, if I need
extra Vac. assist. then I will deal with it then.
Thanks guys, always grateful for your help.
DB
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Joined: Jun 2012
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From: Dayton Nevada
I picker up some smaller tubing and tube clamps. spent some time replacing everything that was loose or questionable. I have driven about 300 miles on this new motor. Fine tuning/adjusting the systems will take some time. Like the fan belts (getting the right size), steering box (was to loose, all over the road, better now) Alignment (Done, not pulling to the left now) rollup windows are next( gap at top needs to be closer), A/C is good now. (Vacuum system under dash??)
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
You are asking the right questions and going the right direction toward evaluating your engine's capability to make vacuum. But, to make a proper assessment, you need to disconnect ALL vacuum systems from your engine, FIRST!
What you want to know is how much vacuum your engine is CAPABLE of making, rather than what is it doing 'as it sits'. The various vacuum systems in your car could have leakages within them which will lower that 'potential' vacuum level. Also, timing adjustments can have a pronounced effect on an engine's vacuum level.
You first need to know what vacuum your engine COULD produce; then re-connect all the vacuum systems to determine how much leakage is present (by how much the vacuum level drops because of those reconnections). Then, if you need to diagnose and fix any vacuum system leaks, you can do so.
What you want to know is how much vacuum your engine is CAPABLE of making, rather than what is it doing 'as it sits'. The various vacuum systems in your car could have leakages within them which will lower that 'potential' vacuum level. Also, timing adjustments can have a pronounced effect on an engine's vacuum level.
You first need to know what vacuum your engine COULD produce; then re-connect all the vacuum systems to determine how much leakage is present (by how much the vacuum level drops because of those reconnections). Then, if you need to diagnose and fix any vacuum system leaks, you can do so.
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Joined: Jun 2012
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From: Dayton Nevada
You are asking the right questions and going the right direction toward evaluating your engine's capability to make vacuum. But, to make a proper assessment, you need to disconnect ALL vacuum systems from your engine, FIRST!
What you want to know is how much vacuum your engine is CAPABLE of making, rather than what is it doing 'as it sits'. The various vacuum systems in your car could have leakages within them which will lower that 'potential' vacuum level. Also, timing adjustments can have a pronounced effect on an engine's vacuum level.
You first need to know what vacuum your engine COULD produce; then re-connect all the vacuum systems to determine how much leakage is present (by how much the vacuum level drops because of those reconnections). Then, if you need to diagnose and fix any vacuum system leaks, you can do so.
What you want to know is how much vacuum your engine is CAPABLE of making, rather than what is it doing 'as it sits'. The various vacuum systems in your car could have leakages within them which will lower that 'potential' vacuum level. Also, timing adjustments can have a pronounced effect on an engine's vacuum level.
You first need to know what vacuum your engine COULD produce; then re-connect all the vacuum systems to determine how much leakage is present (by how much the vacuum level drops because of those reconnections). Then, if you need to diagnose and fix any vacuum system leaks, you can do so.
I'd like to Dyno-Tune it just to have a good starting point. But finding a shop that works on the older stuff is hard here.
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Melting Slicks
Joined: Oct 2012
Posts: 2,156
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From: Dunedin NZ.
16" is about what I get with a mild cam in my 350 (it was 17" with the std cam). If you run your engine then turn it off and can still raise and lower the headlights after a couple of minutes with the engine off your vac system is not leaking or not enough to affect the vac measurement JMPO.
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Joined: Jun 2012
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From: Dayton Nevada
16" is about what I get with a mild cam in my 350 (it was 17" with the std cam). If you run your engine then turn it off and can still raise and lower the headlights after a couple of minutes with the engine off your vac system is not leaking or not enough to affect the vac measurement JMPO.
Drifting
Joined: Apr 2017
Posts: 1,377
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From: Bedford New York
On my '72 LT-1 (bone stock), I'm getting 20.5" of vacuum measured at the PCV port of the carb base, and also at the intake manifold fitting for the vac tank. Same readings.
If I measure it at the small port of the check valve (that feeds the interior switches and the vac tank), I get 9". I'm assuming there is a restrictor in that check valve fitting of some sort?
So make sure you are checking at the source: the carb base or right at the manifold fitting.
If I measure it at the small port of the check valve (that feeds the interior switches and the vac tank), I get 9". I'm assuming there is a restrictor in that check valve fitting of some sort?
So make sure you are checking at the source: the carb base or right at the manifold fitting.
Burning Brakes
Joined: Oct 2012
Posts: 1,152
Likes: 252
17-20 inches is for a stock cammed engine. The camshaft used will affect idle vacuum readings.
as stated above, take a reading with all else disconnected (including PCV). A healthy engine will have a solid reading.
If you have an aftermarket cam I would expect lower numbers than 17-20 but contact the manufacturer and find out what the idle vacuum should be with their cam.
as stated above, take a reading with all else disconnected (including PCV). A healthy engine will have a solid reading.
If you have an aftermarket cam I would expect lower numbers than 17-20 but contact the manufacturer and find out what the idle vacuum should be with their cam.
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From: Dayton Nevada
What I have learned on this lesson.
1st. I need 17-22 inches of vacuum for this car to operate properly at sea level.
2nd. I am at 4,400 ft. Elevation. Vacuum adjustment is 4 inches per. every 1,000 ft = an adjustment of 4.25 inches or 12.75-16.75 inches of vacuum.
3rd. The Edelbrock Carb #1400 has main jets of .95. Edelbrock says they reduce (lean) that out. at 6% per. every 1,500 ft. of elevation = 6%x3= 24% resulting in a jet metered to .98
4th. I have a Mild Comp Cam which will affect Vacuum/Timing some what.
After disconnecting Vac. lines that would cause poor reading. I check the Vacuum gauge, it reads 8 " hg, (for a 12deg. setting) moving the Distr. to get the highest vacuum read possible I can get 10-11 inches of hg.
The timing light shows the timing mark above the timing gauge at the damper wheel. (that's of no use)
What dose this mean. Well.. I am short vacuum by 1.75 " just to bring me to 12.75 ' hg. I am told the vacuum for this car has to be higher than that, Cruise Control 14" hg., ( I assume that is sea level) which -4.25 " elev. adjust. = 9.75 " hg. for Cruise Control operation, which is where I am.
Any input??
I will include PVC as well. when re-confirming readings. Maybe that will pick up the need 1.75 "hg I need.
1st. I need 17-22 inches of vacuum for this car to operate properly at sea level.
2nd. I am at 4,400 ft. Elevation. Vacuum adjustment is 4 inches per. every 1,000 ft = an adjustment of 4.25 inches or 12.75-16.75 inches of vacuum.
3rd. The Edelbrock Carb #1400 has main jets of .95. Edelbrock says they reduce (lean) that out. at 6% per. every 1,500 ft. of elevation = 6%x3= 24% resulting in a jet metered to .98
4th. I have a Mild Comp Cam which will affect Vacuum/Timing some what.
After disconnecting Vac. lines that would cause poor reading. I check the Vacuum gauge, it reads 8 " hg, (for a 12deg. setting) moving the Distr. to get the highest vacuum read possible I can get 10-11 inches of hg.
The timing light shows the timing mark above the timing gauge at the damper wheel. (that's of no use)
What dose this mean. Well.. I am short vacuum by 1.75 " just to bring me to 12.75 ' hg. I am told the vacuum for this car has to be higher than that, Cruise Control 14" hg., ( I assume that is sea level) which -4.25 " elev. adjust. = 9.75 " hg. for Cruise Control operation, which is where I am.
Any input??
I will include PVC as well. when re-confirming readings. Maybe that will pick up the need 1.75 "hg I need.
Last edited by Douglas Brown; Jan 12, 2018 at 12:00 PM.
Melting Slicks
Joined: Feb 2017
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From: PHX AZ
Doug; This may or may not be germaine.
I am the Pres of the VW Scirocco Club. The newest of these was 89, the oldest is 74.
Many of the components in the car are (like our Vettes) run by vacuum. Idle stabilizer,HVAC, emissions etc.
VW ALWAYS provided a vacuum accumulator, sort of an egg crate looking device and a small metering/check valve to the vacuum operated components.
When the car is on 'overrun' =MAX V=vacuum at it's highest, the engine would 'pull' about 25-26 inches. This would 'charge' the accumulator. Then each metering check valve would provide adequate vac to the component and also damp out the vacuum sine wave.
Anyway...good luck. 'Vacuum SUCKS' lol
I am the Pres of the VW Scirocco Club. The newest of these was 89, the oldest is 74.
Many of the components in the car are (like our Vettes) run by vacuum. Idle stabilizer,HVAC, emissions etc.
VW ALWAYS provided a vacuum accumulator, sort of an egg crate looking device and a small metering/check valve to the vacuum operated components.
When the car is on 'overrun' =MAX V=vacuum at it's highest, the engine would 'pull' about 25-26 inches. This would 'charge' the accumulator. Then each metering check valve would provide adequate vac to the component and also damp out the vacuum sine wave.
Anyway...good luck. 'Vacuum SUCKS' lol
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Apparently, you still have not run a vacuum "baseline" test with all vacuum systems removed from the engine. Without doing this, you are 'flying blind'. Lots of folks can give 'advice', opinions, etc; but no one can give you real direction until you know what vacuum level your engine is CAPABLE of making.
As it is, you are just 'chasing your tail'....
As it is, you are just 'chasing your tail'....
Melting Slicks
Joined: Feb 2017
Posts: 3,232
Likes: 1,232
From: PHX AZ
Doug...so all the vacuum ports are capped except for the gauge.
Rev engine to 1000/2000/3000/4000, yellow line for your engine.
record.
Let off throttle abruptly...record peak vac at each RPM....report!
Good luck. UnckaHal
Rev engine to 1000/2000/3000/4000, yellow line for your engine.
record.
Let off throttle abruptly...record peak vac at each RPM....report!
Good luck. UnckaHal
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Pro
Joined: Jun 2012
Posts: 603
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From: Dayton Nevada
Apparently, you still have not run a vacuum "baseline" test with all vacuum systems removed from the engine. Without doing this, you are 'flying blind'. Lots of folks can give 'advice', opinions, etc; but no one can give you real direction until you know what vacuum level your engine is CAPABLE of making.
As it is, you are just 'chasing your tail'....
As it is, you are just 'chasing your tail'....
With out any vac connections to the carb, except the fuel line running from the fuel reservoir to tank for sucking off excess fumes and the Power brake line..
Vacuum is 12" hg at 800-865 rpm.
Re-connect vacuum lines, adjust Rpm and mix screws 840-872 rpm I get 11-12 " hg.
put on the air cleaner and I get 10-11 " hg at 750 rpm when adjusted.
with RPM at about 900 I got 14" hg.
Thread Starter
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Joined: Jun 2012
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From: Dayton Nevada
I think what your looking for is the increase in vacuum at each level.??
And here are the numbers... RPM at " hg upon release
1,000 @ 12" hg 9" hg
2,000 @ 12" hg 19" hg
3,000 @ 14" hg 20" hg
4,000 @ 11" hg 21" hg
Vacuum returned to 10-11 " hg. each time before next increase.
is that what your looking for???
Last edited by Douglas Brown; Jan 12, 2018 at 05:51 PM.
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Joined: Jun 2012
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From: Dayton Nevada
Doug; This may or may not be germaine.
I am the Pres of the VW Scirocco Club. The newest of these was 89, the oldest is 74.
Many of the components in the car are (like our Vettes) run by vacuum. Idle stabilizer,HVAC, emissions etc.
VW ALWAYS provided a vacuum accumulator, sort of an egg crate looking device and a small metering/check valve to the vacuum operated components.
When the car is on 'overrun' =MAX V=vacuum at it's highest, the engine would 'pull' about 25-26 inches. This would 'charge' the accumulator. Then each metering check valve would provide adequate vac to the component and also damp out the vacuum sine wave.
Anyway...good luck. 'Vacuum SUCKS' lol
I am the Pres of the VW Scirocco Club. The newest of these was 89, the oldest is 74.
Many of the components in the car are (like our Vettes) run by vacuum. Idle stabilizer,HVAC, emissions etc.
VW ALWAYS provided a vacuum accumulator, sort of an egg crate looking device and a small metering/check valve to the vacuum operated components.
When the car is on 'overrun' =MAX V=vacuum at it's highest, the engine would 'pull' about 25-26 inches. This would 'charge' the accumulator. Then each metering check valve would provide adequate vac to the component and also damp out the vacuum sine wave.
Anyway...good luck. 'Vacuum SUCKS' lol