How much boost?

B

BadstuW

Registered User
I was wondering, with a 93 SC all stock except exhaust, what are supposed to be pulling for boost and what is the the vac suppose to be at in idle. thanks alot!
 
Idle vac should be between 18-20. For max boost it seems to very a lot depending on all kinds of things like where you live and whatnot, but most people say 12-14 psi I think.
 
You might get a mixed bag of responses here. I've only modified my exhaust so far. I have ported and coated manifolds, hi flow cats, Magnfow resonator and a Force II Flow Master exhaust and I lost about 2PSI of boost. I'm guessing the decrease would account for a faster exiting exhaust system although I don't feel any power loss.
I know of post here that have also stated the same.
 
i belive you arnt loosing 2lb its just how the stock gauge reads the boost? maybe someone that knows some more can chime in here
 
You did indeed lose 2 lb's of boost, but thier should be the same, if not greater amount of air flow. You could stick a sock in your manifold and get 20 psi, does this mean that your getting more horsies? nope.
 
Tickler said:
You did indeed lose 2 lb's of boost, but thier should be the same, if not greater amount of air flow. You could stick a sock in your manifold and get 20 psi, does this mean that your getting more horsies? nope.
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What are you talking about? Explain without the sock...
 
rickbtbird said:
What are you talking about? Explain without the sock...
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Having a free flowing exhaust system will create less back pressure, thus your boost level will drop, but will create more HP. If you have a restrictive stock exhaust system, you will make more boost but not necessary more HP. Boost is measure by back pressure!
 
Yup, they are right. By opening up your exhaust your gettin better flow and thus less boost. I dropped 2-3 psi when i did my exhaust. That is kinda how you can tell that your exhaust upgrade is working. I have done mine twice, the first time it was NFG and i dropped no boost. This time I dropped boost and it made a huge difference in performance. Way more top end...due to better flow. Hope this helps.
 
Adding a pulley

Thanks for all the information here. I'm assuming now that it's safe to overdrive the blower now with either an overdirve blower pulley or an underdrive jackshaft pulley. :D
 
BadstuW said:
I was wondering, with a 93 SC all stock except exhaust, what are supposed to be pulling for boost and what is the the vac suppose to be at in idle. thanks alot!
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Vac: 18-20
Boost: 10-12 stock
 
rickbtbird said:
Thanks for all the information here. I'm assuming now that it's safe to overdrive the blower now with either an overdirve blower pulley or an underdrive jackshaft pulley. :D
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Overdrive jackshaft pulley. Anytime you are driving something faster than engine speed you are overdriving. Basically you are just overdriving more than you were.

Jeff
 
J57ltr said:
Overdrive jackshaft pulley. Anytime you are driving something faster than engine speed you are overdriving. Basically you are just overdriving more than you were.

Jeff
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Overdrive doesn't quite mean that, as the stock SC pully rotates the SC faster than the engine even at idle and its considered "normal". If your overdriving an accessory means your rotating it faster at the same engine rpm by altering the pully size than what the accessory would rotate stock.

Frit
 
Yes it does. Anytime a drive ratio is higher than 1:1 it is overdriven. Why do you think 4th gear is called overdrive in an AOD or 5th gear in an M5R2, or 5th and 6th in a T-56? because the driveshaft rotates faster than the engine (2 times as fast in the T-56)

The 10% or 5% and so on just means that it is that % faster than it was before.

This refers to technical definition not marketing dribble.

Jeff
 
Stock Overdrive = 250%

fturner said:
Overdrive doesn't quite mean that, as the stock SC pully rotates the SC faster than the engine even at idle and its considered "normal". If your overdriving an accessory means your rotating it faster at the same engine rpm by altering the pully size than what the accessory would rotate stock. Frit
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The superchargers on the early model SC are overdriven 250%. That means that the SC spins 2.5 times for every revolution of the crank shaft.

The +5% or +10% pulleys are in ADDITION to the already 250% overdrive.

68COUGAR
 
68COUGAR said:
The superchargers on the early model SC are overdriven 250%. That means that the SC spins 2.5 times for every revolution of the crank shaft.

The +5% or +10% pulleys are in ADDITION to the already 250% overdrive.

68COUGAR
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duh..... like I didn't know it was spinning faster than the crank shaft............... :rolleyes:

What I was pointing out was the fact that when most "normal" folk around here say overdrive the sc means they are taking the stock "overdriven" ratio and want to "overdrive" the SC to spin faster.....

I pointed it out so as to keep the confusion down for some folks who perhaps would not realize that the sc was already overdriven in stock condition........

So Mr. Expert.... how should all members now ask about overdriving the SC? to be technically and grammatically correct of course.

Frit
 
rickbtbird said:
I'm assuming now that it's safe to overdrive the blower now with either an overdirve blower pulley or an underdrive jackshaft pulley. :D
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YES, it would be safe to do so if you have an improved exhaust system!
 
:confused: I live in the Denver, Colorado and only get 10 to 12 lbs of vacum at idle an the boost tops out at about 8 to 10 lbs. Is this from the altiude or should I be looking for leaks? I know that our 1995 Thunderbird 4.6 develops a lot more power at lower altiudes and actually shifts at a higher rpm at WOT. By the way my sc is a stock 1990 with an automatic.
 
You got it. If you search yahoo I think you can even find the calculator for altitude changes in relation to boost and vacuum. Atmospheric pressure is 14.7 at sea level, you are 5280ft higher than that.
 
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