Vacumn at WOT?
#1
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Vacumn at WOT?
Another question from a long time FI guy. I'm used to a good fuel injection manifold having almost no engine vacumn at WOT. If not, your throttle is too small.
I realize that with a carb, it's always a compromise, but I'm wondering what is considered "good" for a high performance application where Idle is still important? I'm getting just under 1 in/hg at WOT (from a calibrated sensor). I figure that's about a 15-20hp loss. Is that about the best I can expect?
I realize that with a carb, it's always a compromise, but I'm wondering what is considered "good" for a high performance application where Idle is still important? I'm getting just under 1 in/hg at WOT (from a calibrated sensor). I figure that's about a 15-20hp loss. Is that about the best I can expect?
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not sure about the logic... your premise is essentially correct. i.e. if the annular opening is large enough there should be near zero vaccum. what i am not quite getting is your suggestion that , somehow this costs you power.... if the motor pulls a 1 atm "vacuum" on the inlet stroke and the atmosphere is 1 atm, then the motor gets all the air that a normally aspirated motor will... so you get what you get and make the power you are going to make with it... in theory... obviously if its a carb it needs sufficient signal to actually function like a carb but making that assumption, i.e. that the carb does actually work,
where would you lose power ?
where would you lose power ?
#4
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Haha! That's exactly what my buddy said
Steve: A two barrel with 5in/hg depression at WOT would provide an EXCELLENT signal to the carb, but would be down 20-30% in power output vs. a good 4 barrel. A fuel injection system needs no vacumn signal going through the carb and hence can big big enough that there is almost no depression in the manifold at WOT. In a perfect world, a perfect carb with a perfect venturi would also recover 100% of the pressure loss accross the carb at WOT.
What I am wondering is what is considered reasonable. At some point, running a bigger carb would have very little difference on the pressure loss and would only show reduced part throttle performance
Steve: A two barrel with 5in/hg depression at WOT would provide an EXCELLENT signal to the carb, but would be down 20-30% in power output vs. a good 4 barrel. A fuel injection system needs no vacumn signal going through the carb and hence can big big enough that there is almost no depression in the manifold at WOT. In a perfect world, a perfect carb with a perfect venturi would also recover 100% of the pressure loss accross the carb at WOT.
What I am wondering is what is considered reasonable. At some point, running a bigger carb would have very little difference on the pressure loss and would only show reduced part throttle performance
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of course. but that wasn't your premise... you said "or your throttle is too small" which is of course, correct... and again which, with a 2 barrel, it is. the answer to this question is simple. dennis moore answers it on page 147 of his book. it gives the arithmatic for calculating precisely the carb you need for a given displacement at given revs. anything smaller than that and you leave something on the table. anything bigger doesn't help you.
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of course. but that wasn't your premise... you said "or your throttle is too small" which is of course, correct... and again which, with a 2 barrel, it is. the answer to this question is simple. dennis moore answers it on page 147 of his book. it gives the arithmatic for calculating precisely the carb you need for a given displacement at given revs. anything smaller than that and you leave something on the table. anything bigger doesn't help you.
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If your looking for great performance be careful with those Holley calculators and suggestions. They tend to be very conservative.
For someone that wants to just bolt a carb on and go usually anywhere from 1.0"Hg-1.5"Hg will work pretty well and give decent performance.
For those that can tune carbs or have a good carb tuner than many will shoot for .7"Hg-.9"Hg. Some people will go lower, but they need to really understand how to tune the carb and ignition. Tune does not mean just jets and PV's.
BTW: When testing this way test with flame arrestor on and off. You may find your arrestor is choking you.
For someone that wants to just bolt a carb on and go usually anywhere from 1.0"Hg-1.5"Hg will work pretty well and give decent performance.
For those that can tune carbs or have a good carb tuner than many will shoot for .7"Hg-.9"Hg. Some people will go lower, but they need to really understand how to tune the carb and ignition. Tune does not mean just jets and PV's.
BTW: When testing this way test with flame arrestor on and off. You may find your arrestor is choking you.