help with pulley vs RPM
11-03-2009, 06:49 AM
#1
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help with pulley vs RPM I plan to start pulling my motor this weekend and when i put it back in im going to be doing the "chevychad" mod on my procharger, what im wanting to see is how small of a head unit pulley can i use before it becomes to had to drive on the street?
I have 3:73 gears and a 4L80 in my car with a tight 3800 stall, how small can i go before it will want to make boost at 70 MPH?
I have a 3.70 in the HU now and it does not even come close to making boost at 70, im thinking something like a 3.1-3.0 but im not sure if i can do that? oh yeah and i have a 7.25 cranks pulley
Thanks Guys Ricky
I have 3:73 gears and a 4L80 in my car with a tight 3800 stall, how small can i go before it will want to make boost at 70 MPH?
I have a 3.70 in the HU now and it does not even come close to making boost at 70, im thinking something like a 3.1-3.0 but im not sure if i can do that? oh yeah and i have a 7.25 cranks pulley
Thanks Guys Ricky
11-04-2009, 06:52 AM
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That's right Chad !!
guys i dont want to make boost at 70 but im thinking that once you get to such a small head unit pulley that it will be making boost all the time just driving it around? or at least trying to?
guys i dont want to make boost at 70 but im thinking that once you get to such a small head unit pulley that it will be making boost all the time just driving it around? or at least trying to?
11-04-2009, 07:08 AM
#5
its not the MPH, but the rpm-i have a 3.73 rear, 28" tire, 4L80E trans,
D1SC with a 4.25 DP, and a 7.53 crank pulley-i see around 2 lbs cruising around 1800 rpm, goes up from there a little-you would have to drop back to third at 70 to see much boost, but the carb or throttle body has gotta be open to really get boost, like when you hammer it, otherwise at cruise your just not gonna see a lot
smaller pulley you will prob see a little more boost, the blowers spinning faster, but still trying to push thru a small opening untill WOT
D1SC with a 4.25 DP, and a 7.53 crank pulley-i see around 2 lbs cruising around 1800 rpm, goes up from there a little-you would have to drop back to third at 70 to see much boost, but the carb or throttle body has gotta be open to really get boost, like when you hammer it, otherwise at cruise your just not gonna see a lot
smaller pulley you will prob see a little more boost, the blowers spinning faster, but still trying to push thru a small opening untill WOT
11-04-2009, 07:47 AM
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so when you are just driveing around at 1800 your makeing booost? is that noe bad on the motor and crazt hard on gas??
on a side note how do you like running the 28 in tire with 3:73s do you still 60 good?
on a side note how do you like running the 28 in tire with 3:73s do you still 60 good?
11-04-2009, 03:08 PM
#7
You shouldnt see ANY boost cruising around no matter what the pulley sizes are. You should be well into vacuum. I am at about 15-20in-Hg of vacuum while cruising. Your bypass valve should be venting ALL excess air preventing you from creating any boost until you start to really press the throttle down hard to accelerate.
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11-04-2009, 04:28 PM
#8
You shouldnt see ANY boost cruising around no matter what the pulley sizes are. You should be well into vacuum. I am at about 15-20in-Hg of vacuum while cruising. Your bypass valve should be venting ALL excess air preventing you from creating any boost until you start to really press the throttle down hard to accelerate.
Same here, any time the pedal isn't in the floor I'm at 12-15in-hg of vac. Put the pedal to the floor boost starts to build.
11-05-2009, 04:55 AM
#11
Ricky, basically the blower is always making boost as it is belt driven. This boost is always being relieved through the bypass valve until you go close to wide open throttle. You won't see any boost until that valve sees close to zero vacuum (it has a vacuum line hooked to it) which occurs close to wide open throttle. Bob
11-05-2009, 07:06 AM
#12
i run a carb, and take my meas in the carb hat, prob why i see a little boost, the rest. from the carb hat to the throttle blades is prob why i see it-i am not fully moderenized like you guys yet, 1 step at a time
as i remember when i did a dyno test, we mont. boost before the i/c, and below the carb., their was around 2 lbs loss, which was expected, as a little is in the 3 core i/c, and usually a little thru the carb-if i would have read it below the carb around 1800, maybe no boost, as my map sen. still shows vac.
like said, soon as the belt turns, there is boost (trust me, you dont pull a site plug out of the fuel bowl with the engine running, lol)
as i remember when i did a dyno test, we mont. boost before the i/c, and below the carb., their was around 2 lbs loss, which was expected, as a little is in the 3 core i/c, and usually a little thru the carb-if i would have read it below the carb around 1800, maybe no boost, as my map sen. still shows vac.
like said, soon as the belt turns, there is boost (trust me, you dont pull a site plug out of the fuel bowl with the engine running, lol)
11-05-2009, 10:35 AM
#15
i run a carb, and take my meas in the carb hat, prob why i see a little boost, the rest. from the carb hat to the throttle blades is prob why i see it-i am not fully moderenized like you guys yet, 1 step at a time
as i remember when i did a dyno test, we mont. boost before the i/c, and below the carb., their was around 2 lbs loss, which was expected, as a little is in the 3 core i/c, and usually a little thru the carb-if i would have read it below the carb around 1800, maybe no boost, as my map sen. still shows vac.
like said, soon as the belt turns, there is boost (trust me, you dont pull a site plug out of the fuel bowl with the engine running, lol)
as i remember when i did a dyno test, we mont. boost before the i/c, and below the carb., their was around 2 lbs loss, which was expected, as a little is in the 3 core i/c, and usually a little thru the carb-if i would have read it below the carb around 1800, maybe no boost, as my map sen. still shows vac.
like said, soon as the belt turns, there is boost (trust me, you dont pull a site plug out of the fuel bowl with the engine running, lol)
