This question is just out of curiosity. If an intake usually works off vacuum and a blower adds boost, what is going on when you go part throttle and the gauge reads “0”? No vacuum, no boost, wouldn’t this mean no airflow? Or is this the happy medium whre air is being pushed in just enough to not be a vacuum but not quite enough to boost air levels? And if so, wouldn't this be the most efficient level for the engine?

This question is just out of curiosity. If an intake usually works off vacuum and a blower adds boost, what is going on when you go part throttle and the gauge reads “0”? No vacuum, no boost, wouldn’t this mean no airflow? Or is this the happy medium whre air is being pushed in just enough to not be a vacuum but not quite enough to boost air levels? And if so, wouldn't this be the most efficient level for the engine?
0 would just mean there is no intake restriction. Anytime there is a vacuum reading there is an intake restriction. If you had a car with too small a carb or throttle body it would go to 0 when you mashed the gas but would slowly creep back to the vacuum side when the rpm's got higher, but usually only 1-2". Boost just means that you are supplying more air than the engine can consume at normal atmospheric pressure, so it has to compress it to use it all. Almost all naturally aspirated engines will operate at 0 vacuum while under WOT as long as the induction system doesnt pose a bad restriction.

yes it is the happy medium where the intake presure is the same as atmospheric presure. normaly on a NA set up there will always be a little vacume so you are geting some extra hp from the blower even at 0psi.

often people see great gains from very low boost levels. something like 50hp from 2psi kind of thing but you are really seeing more of a diference that 2psi because there was some vacume that was overcome too.

i dont really know when the max efficency would be but i dont see any reason why it would be right at 0psi. it has more to do with the compresor, heads, cam, stuff like that.

Thanks guys. That's kinda of what I was thinking but wanted some other ideas. Heres a question for ya that is sure to have people arguing.
What is the most boost I can run on stock internals with a good/dyno tune? I'm wanting to go down from a 3.4" pulley to, say...a 3.25" and try to get 9 pounds or so. I know 10 pounds would be pushing my luck. What is the limiting factor? Rockers, springs, push rods?

I guess this is why you get better mileage from a turbo set-up at normal driving speeds because of the help the turbo gives you. The engine does not have to suck air, it is being given by the turbo instead?

yes it is the happy medium where the intake presure is the same as atmospheric presure. normaly on a NA set up there will always be a little vacume so you are geting some extra hp from the blower even at 0psi.


You could just use the blower to get you to 100% VE without boost, which could be 0 vacuum, but I guess that would depend on the heads, cam and such. You could deffinetly get closer to 100% VE without any boost and just using the blower to force air. I would think that the valves and cam would make some restriction that might require some boost to overcome. But who would do that? I have a hard time not putting on the absolute smallest pulley I can find just to see what my stock motor will take. :bomb:

Thanks guys. That's kinda of what I was thinking but wanted some other ideas. Heres a question for ya that is sure to have people arguing.
What is the most boost I can run on stock internals with a good/dyno tune? I'm wanting to go down from a 3.4" pulley to, say...a 3.25" and try to get 9 pounds or so. I know 10 pounds would be pushing my luck. What is the limiting factor? Rockers, springs, push rods?
I dont think 10-11 PSI is all that much if you are not beating on the truck constantlyand have a safe tune. I might try 13-14, I am at 10 now. Its still under warranty anyway. :D

Thanks guys. That's kinda of what I was thinking but wanted some other ideas. Heres a question for ya that is sure to have people arguing.
What is the most boost I can run on stock internals with a good/dyno tune? I'm wanting to go down from a 3.4" pulley to, say...a 3.25" and try to get 9 pounds or so. I know 10 pounds would be pushing my luck. What is the limiting factor? Rockers, springs, push rods?


the limit is your fuel and tuning. tranny wont be happy either.

about 6-7psi seems to be the limit on non intercooled set ups with pump gas, i would think an intercooler would add 2psi or so but i am not up on what the limit for a radix on pump gas is. there is a point when you have to run it soo rich and pull so much timing to run the higher boost and stay out of knock that you end up gaining no power.

I dont think 10-11 PSI is all that much if you are not beating on the truck constantlyand have a safe tune. I might try 13-14, I am at 10 now. Its still under warranty anyway. :D

a radix and a procharger are going to have significantly diferent max boost limits. the radix will see max boost at 2000rpm and the mid range is where the knock will set in. the procharger wont see the high boost till higher rpms and knock seems less likely to set in up there.

From what you guys are saying I think I could run 9 #'s safely. I have an intercooler, I run 93 octane fuel and plan on getting a dyno tune when I go with the smaller pulley. I just wanted to make sure my rods, springs and etc...would take 9 #'s. I see about 7 at wot right now. Mine however is Not under warranty. :ripped: Actually it is under extended warranty but the dealership won't even look at my truck with the Radix on it. I haven't fought that battle yet.

That's one good thing about the Procharger, I can have it off in a couple of hours, and the last time I put it on it only took about 3 hours. Practice I guess, seeing as how I had to remove it for the trans warranty. I think the bottom end is pretty tough, but if you beat on it often you will obviously shorten its life.

9.5 lbs on a stock magnacharger tune for my 4.8. I'm doing it with a 3.25 pulley. It bumps 10 psi at the top of 2nd and 3rd. I have had no problems so far and have only heard the valves rattle a couple of times going up a steep hill in 5th gear. Very light though. The only other mods I have is Corsa cat-back.

0 is relevant to the current atmospheric pressure, whenever and wherever you happen to be at the time. Like said it means the ambient air pressure cannot "force" more air in and/or the engine cannot "suck" more air in. Basic definition of an internal combustion motor = air pump. Forced induction just that, forcing more air than the engine or atmosphere could do alone. It will alter your pressure/vac readings at most any throttle position other than idle/closed. The mention of VE ( volumetric efficiency ) is a good description of non-WOT readings.
On the sturdiness of the GenIII motors, a couple of folks for the fun of it have pushed the limits of N2O or boost, with decent tuning, to 500+ whp with no intenal failures. Rod bolts and pushrods seem to be the parts replaced first. I agree that tuning is your only limitation.

I'm getting a dyno tune no matter what. And from what I hear from friends who have gone to this guy, he is really good. So I figured I go ahead and go to the smaller pulley for the extra couple of pounds of boost and see if I will feel the diff. in the seat of the pants. What do you guys think? 6-7 vs. 9-10, am I gonna notice the diff or will it just help a couple of tenths in the 1/4?

Each 2psi increase on the turbo was translating to 50hp or so on the dyno. Following the 10 hp = 0.1 second improvement in 1/4 mile times both your timeslips and butt dyno should see improvement! I know I felt the power with a 2psi bump on the STS.

Butt Dyno :jest: Well hopefully I can find time to get this done in the next few weeks and see.