Author Topic: Carbureted: Using vacuum gauge to help dial in DTT ignition  (Read 346 times)

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Online tmwmoose

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Using vacuum gauge to help dial in DTT ignition
« on: October 26, 2017, 11:45:03 AM »
Yeah had a brilliant brain storm why not use the gauge on my Mitivac and hook it up to the manifold . Well its hooked up but somethings to simple for me to understand it reads backwards so to speak. It shows a higher number at idle and cruise with lower numbers for wot. Is it because the tool uses it differently?.

Offline convert1967

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #1 on: October 26, 2017, 12:26:00 PM »
Sounds right to me.  At idle and cruise your vacuum will be higher than at WOT.

Online tmwmoose

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #2 on: October 26, 2017, 12:30:01 PM »
OK reason I'm confused is the DTT ignition map shows 30 in hg for wot and then lower numbers for cruise on down to small throttle openings

Online wolf_59

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #3 on: October 26, 2017, 12:59:07 PM »
I was just researching this same thing this morning and found this thread where it was kind of discussed
http://harleytechtalk.com/index.php?topic=97779.msg1141285#msg1141285

Online tmwmoose

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #4 on: October 26, 2017, 01:33:48 PM »
I was just researching this same thing this morning and found this thread where it was kind of discussed
http://harleytechtalk.com/index.php?topic=97779.msg1141285#msg1141285


Cool I have that bookmarked also and is good info although when watching a gauge on the road and opening the throttle wide open it goes down toward zero which is low vacuum right! so why does DTT show it just the opposite as high vacuum? I guess a phone call is in order.

Online wolf_59

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #5 on: October 26, 2017, 01:56:04 PM »
Seems like it was the same in my old blazer that I installed a vacuum gauge in, Idle was high vacuum, cruise was little lower and WOT was no vacuum. From my memory 25 years ago tuning info was to advance timing at idle to achieve highest vacuum adjust idle then back timing off a little to get vacuum to drop 1-2 in hg, The higher the vacuum while driving was best fuel milage. Apparently vacuum is opposite from in hg or MAP in the timing maps
Sorry couldn't be of more help   

Offline convert1967

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #6 on: October 26, 2017, 02:14:26 PM »
%            psia,          Inches          Inches          kPa
Vacuum   (lb/in2      Mercury         Mercury          abs
            abs)          Absolute         ON Gauge   
0           14.7          29.92                  0             101.4
1.3        14.5          29.5                    0.42        99.9
1.9        14.2          28.9                    1.02        97.7
7.9        13.5          27.6                    2.32        93.5
21         11.6          23.6                    6.32        79.9
34           9.7          19.7                   10.22       66.7
50           7.3          15                      14.92       50.8
61           5.8          11.8                   18.12       40
74           3.9          7.85                   22.07       26.6
88           1.74        3.54                   26.38       12
92.1        1.16        2.36                   27.56       8
96.1        0.58        1.18                   28.74       4
99           0.147      0.299                 29.62       1
100           0           0                       29.92       0

Online 1FSTRK

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #7 on: October 26, 2017, 05:12:02 PM »
I was just researching this same thing this morning and found this thread where it was kind of discussed
http://harleytechtalk.com/index.php?topic=97779.msg1141285#msg1141285


Cool I have that bookmarked also and is good info although when watching a gauge on the road and opening the throttle wide open it goes down toward zero which is low vacuum right! so why does DTT show it just the opposite as high vacuum? I guess a phone call is in order.


The DTT is Pressure scaled not vacuum scaled. High pressure is low vacuum.

Online tmwmoose

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #8 on: October 26, 2017, 05:28:43 PM »
I was just researching this same thing this morning and found this thread where it was kind of discussed
http://harleytechtalk.com/index.php?topic=97779.msg1141285#msg1141285


Cool I have that bookmarked also and is good info although when watching a gauge on the road and opening the throttle wide open it goes down toward zero which is low vacuum right! so why does DTT show it just the opposite as high vacuum? I guess a phone call is in order.


The DTT is Pressure scaled not vacuum scaled. High pressure is low vacuum.



Thanks ,I saw the in hg designation on the DTT maps so I thought it was vacuum. So what I want to do is have a gauge that reads the same as the DTT maps read that I can use on the bike . I want to be able to run the bike at say 18-20 on the gauge  and say theres some ping at 25-3k  I can go to the software and make changes no guessing no converting . Would this be what I see sold as a boost gauge?

Online 1FSTRK

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #9 on: October 26, 2017, 06:29:03 PM »
I was just researching this same thing this morning and found this thread where it was kind of discussed
http://harleytechtalk.com/index.php?topic=97779.msg1141285#msg1141285


Cool I have that bookmarked also and is good info although when watching a gauge on the road and opening the throttle wide open it goes down toward zero which is low vacuum right! so why does DTT show it just the opposite as high vacuum? I guess a phone call is in order.


The DTT is Pressure scaled not vacuum scaled. High pressure is low vacuum.



Thanks ,I saw the in hg designation on the DTT maps so I thought it was vacuum. So what I want to do is have a gauge that reads the same as the DTT maps read that I can use on the bike . I want to be able to run the bike at say 18-20 on the gauge  and say theres some ping at 25-3k  I can go to the software and make changes no guessing no converting . Would this be what I see sold as a boost gauge?


No a boost gauge will still read backwards. The DTT uses Absolute Pressure and the gauges will read oposite as shown in convert1967's post


%            psia,          Inches          Inches          kPa
Vacuum   (lb/in2      Mercury         Mercury          abs
            abs)          Absolute         ON Gauge   
0           14.7          29.92                  0             101.4
1.3        14.5          29.5                    0.42        99.9
1.9        14.2          28.9                    1.02        97.7
7.9        13.5          27.6                    2.32        93.5
21         11.6          23.6                    6.32        79.9
34           9.7          19.7                   10.22       66.7
50           7.3          15                      14.92       50.8
61           5.8          11.8                   18.12       40
74           3.9          7.85                   22.07       26.6
88           1.74        3.54                   26.38       12
92.1        1.16        2.36                   27.56       8
96.1        0.58        1.18                   28.74       4
99           0.147      0.299                 29.62       1
100           0           0                       29.92       0

Online tmwmoose

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #10 on: October 26, 2017, 07:54:42 PM »
I was just researching this same thing this morning and found this thread where it was kind of discussed
http://harleytechtalk.com/index.php?topic=97779.msg1141285#msg1141285


Cool I have that bookmarked also and is good info although when watching a gauge on the road and opening the throttle wide open it goes down toward zero which is low vacuum right! so why does DTT show it just the opposite as high vacuum? I guess a phone call is in order.


The DTT is Pressure scaled not vacuum scaled. High pressure is low vacuum.



Thanks ,I saw the in hg designation on the DTT maps so I thought it was vacuum. So what I want to do is have a gauge that reads the same as the DTT maps read that I can use on the bike . I want to be able to run the bike at say 18-20 on the gauge  and say theres some ping at 25-3k  I can go to the software and make changes no guessing no converting . Would this be what I see sold as a boost gauge?


No a boost gauge will still read backwards. The DTT uses Absolute Pressure and the gauges will read oposite as shown in convert1967's post


%            psia,          Inches          Inches          kPa
Vacuum   (lb/in2      Mercury         Mercury          abs
            abs)          Absolute         ON Gauge   
0           14.7          29.92                  0             101.4
1.3        14.5          29.5                    0.42        99.9
1.9        14.2          28.9                    1.02        97.7
7.9        13.5          27.6                    2.32        93.5
21         11.6          23.6                    6.32        79.9
34           9.7          19.7                   10.22       66.7
50           7.3          15                      14.92       50.8
61           5.8          11.8                   18.12       40
74           3.9          7.85                   22.07       26.6
88           1.74        3.54                   26.38       12
92.1        1.16        2.36                   27.56       8
96.1        0.58        1.18                   28.74       4
99           0.147      0.299                 29.62       1
100           0           0                       29.92       0




OK so bottom line is there's no gauge available to do what I'm looking for correct?

Offline speedzter

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #11 on: October 26, 2017, 08:33:23 PM »
There appears to be some around that would work?
Do a search for "economy gauge"



https://www.amazon.com/Auto-Meter-2610-Mechanical-Vacuum/dp/B00062YW9M

Edit : maybe not, its hard to get your head around absolute pressure, maybe re-label one ??
« Last Edit: October 26, 2017, 08:41:12 PM by speedzter »

Offline HD Street Performance

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Re: Using vacuum gauge to help dial in DTT ignition
« Reply #12 on: October 26, 2017, 09:47:43 PM »
Be careful with the unit of measure.
Inches of h2o is very different from inches of hg.