.?????........
Difference in Vaccum Readings
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Difference in Vaccum Readings
Something I've never done - Wonder why.
Anyway, has anyone actually tested and compared the vacuum reading difference when swapping from one ratio rocker arm set to another?
Say, for example, check the vacuum reading on an idling, closed loop engine and with no other change than going from 1.5s to 1.6s test the vacuum reading again.
How much of a difference? Actual numbers rather than guesses and opinions would be best.
Thanks.
Jake
Anyway, has anyone actually tested and compared the vacuum reading difference when swapping from one ratio rocker arm set to another?
Say, for example, check the vacuum reading on an idling, closed loop engine and with no other change than going from 1.5s to 1.6s test the vacuum reading again.
How much of a difference? Actual numbers rather than guesses and opinions would be best.
Thanks.
Jake
Le Mans Master
Joined: Aug 2007
Posts: 8,735
Likes: 412
From: WV
Burning Brakes
Joined: Jul 2006
Posts: 1,080
Likes: 2
From: Pittsburgh PA
Not the answer you are looking for but I doubt you will see any significant measureable difference. You will see more of a difference in varying ignition timing than rocker ratios IMO. You can change several inches of Hg at idle with varying timing.
Le Mans Master
Joined: Aug 2007
Posts: 8,735
Likes: 412
From: WV
Jake, this is not what you ask for but I will tell you anyway..
..With the old stock engine I had in my 89 the vacuam pressure was 17 psi..When I put my new engine in that had 1.6 rockers the new engine also has 17 psi..I know this isn't comparing apples to apples but it looks like 350 engines with TPI intakes and a somewhat mild cam tend to be around 17 psi. I do know for a fact the psi drops with the bigger cams, some so much they have to put boosters on so the power brakes will work....WW
..With the old stock engine I had in my 89 the vacuam pressure was 17 psi..When I put my new engine in that had 1.6 rockers the new engine also has 17 psi..I know this isn't comparing apples to apples but it looks like 350 engines with TPI intakes and a somewhat mild cam tend to be around 17 psi. I do know for a fact the psi drops with the bigger cams, some so much they have to put boosters on so the power brakes will work....WW
Last edited by WW7; Aug 3, 2010 at 10:00 AM.
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
What peaked my interest in this were posts on another thread where it was being kicked around how much "effective" duration changes when rockers are changed from 1.5 to 1.6 or 1.7s.
Since it's generally reported (CompCams and others) that a one ratio change results in a 2-3 degree "effective" duration increase; going to 1.7s even more I began to wonder about the effect on vacuum. So my thinking is the vacuum reading may (or may not) be effected and, if so, by how much.
Seems that no one has done that test though.
Jake
Since it's generally reported (CompCams and others) that a one ratio change results in a 2-3 degree "effective" duration increase; going to 1.7s even more I began to wonder about the effect on vacuum. So my thinking is the vacuum reading may (or may not) be effected and, if so, by how much.
Seems that no one has done that test though.
Jake
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
I'm not being smart, but probably the reason no one has been able to answer you is that no one cares about the possible vacuum difference using 1.6 vs 1.7 rockers. Even if there were a .45 hg vacuum difference, knowing this would not change anything as far as tuning or whatever. We're not using power valves in carburators anymore. Why would anyone go through the trouble of changing the rocker ratios just to come up with a number that in the real world means nothing. How many of us know what the vacuum our motors are putting out now? Again, I'm not demeaning you or being a smart axx, but who cares?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
I'm not being smart, but probably the reason no one has been able to answer you is that no one cares about the possible vacuum difference using 1.6 vs 1.7 rockers. Even if there were a .45 hg vacuum difference, knowing this would not change anything as far as tuning or whatever. We're not using power valves in carburators anymore. Why would anyone go through the trouble of changing the rocker ratios just to come up with a number that in the real world means nothing. How many of us know what the vacuum our motors are putting out now? Again, I'm not demeaning you or being a smart axx, but who cares?
Jake
Intermediate
Joined: Jan 2010
Posts: 45
Likes: 1
Once I get my replacement alternator (hopefully this week) I might be able to give you an idea if it makes a big enough change to notice. I was in the process of tuning AFR's vs time to accelerate from 4000 to 6500 rpm when my alternator died. I was running 1.6 on intake, 1.5 on exhaust. I have changed to 1.7/1.6. When I run the car again I will take a datalog without making any other changes. Since I am still running open loop I probably will have to take a 2nd datalog after changing enrichment to get the same AFR #'s.
I have changed back and forth with the ratios before but made other changes at the same time so there was no way of getting any real data.
I have changed back and forth with the ratios before but made other changes at the same time so there was no way of getting any real data.
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Once I get my replacement alternator (hopefully this week) I might be able to give you an idea if it makes a big enough change to notice. I was in the process of tuning AFR's vs time to accelerate from 4000 to 6500 rpm when my alternator died. I was running 1.6 on intake, 1.5 on exhaust. I have changed to 1.7/1.6. When I run the car again I will take a datalog without making any other changes. Since I am still running open loop I probably will have to take a 2nd datalog after changing enrichment to get the same AFR #'s.
I have changed back and forth with the ratios before but made other changes at the same time so there was no way of getting any real data.
I have changed back and forth with the ratios before but made other changes at the same time so there was no way of getting any real data.
Could be that that'll help anyone with marginal power brake issues due to low vacuum. I've just been reading posts by a member who's considering changing lifters to address his low vacuum brake problem.
Then, since many of these theads and responses linger on the web seemingly forever, others - over time - could benefit as well.
Thanks.
Jake
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
Once I get my replacement alternator (hopefully this week) I might be able to give you an idea if it makes a big enough change to notice. I was in the process of tuning AFR's vs time to accelerate from 4000 to 6500 rpm when my alternator died. I was running 1.6 on intake, 1.5 on exhaust. I have changed to 1.7/1.6. When I run the car again I will take a datalog without making any other changes. Since I am still running open loop I probably will have to take a 2nd datalog after changing enrichment to get the same AFR #'s.
I have changed back and forth with the ratios before but made other changes at the same time so there was no way of getting any real data.
I have changed back and forth with the ratios before but made other changes at the same time so there was no way of getting any real data.
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Jake
Intermediate
Joined: Jan 2010
Posts: 45
Likes: 1
I have not tried 1.6/1.6 so I can not say if the split ratios work better for my application. 1.6/1.5 is what was recommended to me for my old head/ cam combination. I bought 8 - 1.7 this year to see if it would make any difference.
So as far as the vacuum difference well apparently changing rocker ratio and an alternator destroys your fuel pump. The short period I got to drive before my 1 year old pump quit showed a change from around 50 kPa with 1.6/1.5 rockers to 53 kPa with 1.7/1.6 rockers at about 900 rpm. So a difference of about 3 kPa or .9 inches at most, probably less. My logger doesn't record actual barometric pressure and the temperature was about 20 degrees lower for the 1.7's so that could be a couple of factors affecting the results. The idle did drop about 50 rpm.
Once I get a new fuel pump installed I'll post if I could see a difference in acceleration.
Luckily it stopped on the way home from work so some of my co-workers either passed me on the shoulder (some waved) or passed the tow truck with the car on the deck. I am really looking forward to coffee time on Monday.
So as far as the vacuum difference well apparently changing rocker ratio and an alternator destroys your fuel pump. The short period I got to drive before my 1 year old pump quit showed a change from around 50 kPa with 1.6/1.5 rockers to 53 kPa with 1.7/1.6 rockers at about 900 rpm. So a difference of about 3 kPa or .9 inches at most, probably less. My logger doesn't record actual barometric pressure and the temperature was about 20 degrees lower for the 1.7's so that could be a couple of factors affecting the results. The idle did drop about 50 rpm.
Once I get a new fuel pump installed I'll post if I could see a difference in acceleration.
Luckily it stopped on the way home from work so some of my co-workers either passed me on the shoulder (some waved) or passed the tow truck with the car on the deck. I am really looking forward to coffee time on Monday.
Thread Starter
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
I have not tried 1.6/1.6 so I can not say if the split ratios work better for my application. 1.6/1.5 is what was recommended to me for my old head/ cam combination. I bought 8 - 1.7 this year to see if it would make any difference.
So as far as the vacuum difference well apparently changing rocker ratio and an alternator destroys your fuel pump. The short period I got to drive before my 1 year old pump quit showed a change from around 50 kPa with 1.6/1.5 rockers to 53 kPa with 1.7/1.6 rockers at about 900 rpm. So a difference of about 3 kPa or .9 inches at most, probably less. My logger doesn't record actual barometric pressure and the temperature was about 20 degrees lower for the 1.7's so that could be a couple of factors affecting the results. The idle did drop about 50 rpm.
Once I get a new fuel pump installed I'll post if I could see a difference in acceleration.
Luckily it stopped on the way home from work so some of my co-workers either passed me on the shoulder (some waved) or passed the tow truck with the car on the deck. I am really looking forward to coffee time on Monday.
So as far as the vacuum difference well apparently changing rocker ratio and an alternator destroys your fuel pump. The short period I got to drive before my 1 year old pump quit showed a change from around 50 kPa with 1.6/1.5 rockers to 53 kPa with 1.7/1.6 rockers at about 900 rpm. So a difference of about 3 kPa or .9 inches at most, probably less. My logger doesn't record actual barometric pressure and the temperature was about 20 degrees lower for the 1.7's so that could be a couple of factors affecting the results. The idle did drop about 50 rpm.
Once I get a new fuel pump installed I'll post if I could see a difference in acceleration.
Luckily it stopped on the way home from work so some of my co-workers either passed me on the shoulder (some waved) or passed the tow truck with the car on the deck. I am really looking forward to coffee time on Monday.
Thanks much for that info. Now I have a ballpark idea instead of a blank slate.
Jake
Thanks much for that info
