525sc dominator 1050 carb question
06-18-2012, 10:37 AM
#21
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In order to properly tune the PV opening point, your vacuum readings need to be taken at the point at which the PV chamber is referencing vacuum. In a non boost referenced application, this would be at the base of the carburetor, above the supercharger... anything else, whether it works to a certain extent or not, could be a costly mistake. While using a wideband to monitor your tune is an excellent measure, it's not nearly as difinitive of a PV tuning measure as monitoring vacuum at the same source where the PV is referenced, and it does leave room for error.
The issue (and the entire basis behind externally referencing power valves in the first place) is that at higher supercharger speeds, low pressure (vacuum) begins to build at the base of the carburetor(s), below the throttle plates (where the PV chamber gets it's signal). Should this low pressure build to a point in excess of the power valve opening value, the power valve will close... I'm assuming no one here needs a lesson on how a closed power valve while under a load of boost typically turns out...
That being said, this is often a non-issue in a small blower marine application... smaller supercharger volume, relatively low boost levels, and relatively low engine speed keep the amount of low pressure that builds below the throttle plates to a level at which there is no danger of it ever pulling the power valve closed. Thus the reason why most applications such as the 525SC never used an externally referenced PV... it's simply not necessary.
Every roots supercharged engine I dyno is done so with a vacuum gauge attached at the base of the carburetor, above the supercharger... in a low boost environment, rarely will you see even two inches on the gauge at WOT. Bigger cube engines, big blower, higher RPM, higher supercharger speed... it can become an issue, and if PV's are even being utilized in such an application, they'd better be tuned correctly... on both ends of the spectrum... opening at the correct time during engine acceleration, and not closing back up at higher engine speed/boost level. Also, I check every PV with a hand vacuum pump and checking chamber... it's important to know that the thing opens at whatever value is stamped in the side. You'd be surprised at how many vary from their rating by as much as two inches of vacuum.
Not getting into the pissing contest here... just putting some facts out there on how it all works.
The issue (and the entire basis behind externally referencing power valves in the first place) is that at higher supercharger speeds, low pressure (vacuum) begins to build at the base of the carburetor(s), below the throttle plates (where the PV chamber gets it's signal). Should this low pressure build to a point in excess of the power valve opening value, the power valve will close... I'm assuming no one here needs a lesson on how a closed power valve while under a load of boost typically turns out...
That being said, this is often a non-issue in a small blower marine application... smaller supercharger volume, relatively low boost levels, and relatively low engine speed keep the amount of low pressure that builds below the throttle plates to a level at which there is no danger of it ever pulling the power valve closed. Thus the reason why most applications such as the 525SC never used an externally referenced PV... it's simply not necessary.
Every roots supercharged engine I dyno is done so with a vacuum gauge attached at the base of the carburetor, above the supercharger... in a low boost environment, rarely will you see even two inches on the gauge at WOT. Bigger cube engines, big blower, higher RPM, higher supercharger speed... it can become an issue, and if PV's are even being utilized in such an application, they'd better be tuned correctly... on both ends of the spectrum... opening at the correct time during engine acceleration, and not closing back up at higher engine speed/boost level. Also, I check every PV with a hand vacuum pump and checking chamber... it's important to know that the thing opens at whatever value is stamped in the side. You'd be surprised at how many vary from their rating by as much as two inches of vacuum.
Not getting into the pissing contest here... just putting some facts out there on how it all works.
06-18-2012, 03:54 PM
#23
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Ok guy i have not test my vacum this week end but i have tested my A/F. The A/F that i read at cruise is 11.2 just on the 4 barel opening got 10,5 and WOT 11.5. I am jeting 104 4 corner and i have 6.5 pv (stock 525sc). I think that i have the right pv but litle rich on the primary side at cruise.
06-18-2012, 07:52 PM
#24
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In order to properly tune the PV opening point, your vacuum readings need to be taken at the point at which the PV chamber is referencing vacuum. In a non boost referenced application, this would be at the base of the carburetor, above the supercharger... anything else, whether it works to a certain extent or not, could be a costly mistake. While using a wideband to monitor your tune is an excellent measure, it's not nearly as difinitive of a PV tuning measure as monitoring vacuum at the same source where the PV is referenced, and it does leave room for error.
The issue (and the entire basis behind externally referencing power valves in the first place) is that at higher supercharger speeds, low pressure (vacuum) begins to build at the base of the carburetor(s), below the throttle plates (where the PV chamber gets it's signal). Should this low pressure build to a point in excess of the power valve opening value, the power valve will close... I'm assuming no one here needs a lesson on how a closed power valve while under a load of boost typically turns out...
That being said, this is often a non-issue in a small blower marine application... smaller supercharger volume, relatively low boost levels, and relatively low engine speed keep the amount of low pressure that builds below the throttle plates to a level at which there is no danger of it ever pulling the power valve closed. Thus the reason why most applications such as the 525SC never used an externally referenced PV... it's simply not necessary.
Every roots supercharged engine I dyno is done so with a vacuum gauge attached at the base of the carburetor, above the supercharger... in a low boost environment, rarely will you see even two inches on the gauge at WOT. Bigger cube engines, big blower, higher RPM, higher supercharger speed... it can become an issue, and if PV's are even being utilized in such an application, they'd better be tuned correctly... on both ends of the spectrum... opening at the correct time during engine acceleration, and not closing back up at higher engine speed/boost level. Also, I check every PV with a hand vacuum pump and checking chamber... it's important to know that the thing opens at whatever value is stamped in the side. You'd be surprised at how many vary from their rating by as much as two inches of vacuum.
Not getting into the pissing contest here... just putting some facts out there on how it all works.
The issue (and the entire basis behind externally referencing power valves in the first place) is that at higher supercharger speeds, low pressure (vacuum) begins to build at the base of the carburetor(s), below the throttle plates (where the PV chamber gets it's signal). Should this low pressure build to a point in excess of the power valve opening value, the power valve will close... I'm assuming no one here needs a lesson on how a closed power valve while under a load of boost typically turns out...
That being said, this is often a non-issue in a small blower marine application... smaller supercharger volume, relatively low boost levels, and relatively low engine speed keep the amount of low pressure that builds below the throttle plates to a level at which there is no danger of it ever pulling the power valve closed. Thus the reason why most applications such as the 525SC never used an externally referenced PV... it's simply not necessary.
Every roots supercharged engine I dyno is done so with a vacuum gauge attached at the base of the carburetor, above the supercharger... in a low boost environment, rarely will you see even two inches on the gauge at WOT. Bigger cube engines, big blower, higher RPM, higher supercharger speed... it can become an issue, and if PV's are even being utilized in such an application, they'd better be tuned correctly... on both ends of the spectrum... opening at the correct time during engine acceleration, and not closing back up at higher engine speed/boost level. Also, I check every PV with a hand vacuum pump and checking chamber... it's important to know that the thing opens at whatever value is stamped in the side. You'd be surprised at how many vary from their rating by as much as two inches of vacuum.
Not getting into the pissing contest here... just putting some facts out there on how it all works.
