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So I was going to go down and fight with my end links, and then wash my car. Then I smashed my big toe on the steps and have resorted to another pot of coffee. Anyhow, I've been reading a lot lately. A lot of what I've read has been counter-intuitive to most internet speak, so I thought I'd post up some various arguments to get people's heads thinking a bit.


Everybody has seen people boast about a tune... 22psi at 3200 rpm... Holds 18psi strong through redline... Hits peak boost by 1500 rpm. However you put it, its really not useful information in my opinion. Lets turn the argument around for a minute, and think of boost as a restriction, not a measure of power 'injection' like it is normally conveyed.

To start, its important to realize that an engine isn't just a garbage bag that is pumped full of air... The engine acts as air pump. Meaning it has to take air in, AND expel it. The more air, the larger the displacement.

So lets look at a few situations to get a grasp on the matter. Lets put a small turbo on an engine, and slap a poor tune on it. For the sake of the situation, lets say the tune is trying to get the most 'boost' possible out of a given turbo. Wouldn't be too hard. Just keep the intake valves closed almost the entire time. If you don't allow any time for the pressurized volume to enter the engine, it'll stay pressurized. It's like spraying an aerosol can - the pressure in the can won't drop unless you hold the button down for a while. In turn, the turbo can keep spinning out of its efficiency range, generating hot air, and attempting to push the air into closed valves. End result - high boost... low displacement. The valves only let in so much air in a short amount of time

Counter situation. Same turbo, better tune. Now, the tuner doesn't care about peak boost, but he does want to extract the most power out of the turbo. In this case, he would leave the valves open for the ideal time. During this time, since the valves are open and volume is flowing, boost may drop depending on if the turbo can keep up with the air the engine is sucking in through the intake valves. End result: The tune would be limited by the AMOUNT of air the turbo can flow in that time (CFM... not psi).

So just to emphasis, it's very easy to boast a high psi number. If tuned correctly, however, that number may be lower, with a much better power output. Really, the larger the psi number on the gauge, the more air your engine isn't breathing in!

Again, not too in depth really, but something to get your minds turning. There was a similar thread about this, and not everybody really grasped it. Lets take another go on it.

Discussion?
 

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That's actually very enlightening, and makes a lot of sense. It's all a matter of allowing all the air you're pressurizing with the turbo to flow into the engine, instead of just up in the intake pressurizing even more. On the other hand, the more pressurized the air, the faster it'll flow into the engine once the valve opens because it would have more potential energy, right? So a higher psi would allow you to have the valves open for a shorter ammount of time, to get the same ammount of air in as a somewhat lower psi and longer open valve time.
 

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Discussion Starter #3
That's actually very enlightening, and makes a lot of sense. It's all a matter of allowing all the air you're pressurizing with the turbo to flow into the engine, instead of just up in the intake pressurizing even more. On the other hand, the more pressurized the air, the faster it'll flow into the engine once the valve opens because it would have more potential energy, right? So a higher psi would allow you to have the valves open for a shorter ammount of time, to get the same ammount of air in as a somewhat lower psi and longer open valve time.
Yes, the higher the pressure, the 'faster' the air will enter - true. In that same thought, however, if your turbo is not up to the CFM, the pressure difference will dissipate during that time until the valve closes and the turbo can build the pressure back up.

A good way of thinking about the trade off - for a given waste gate duty cycle on your turbo (a given amount of time the turbo is pumping air)... if you can tune the valve timing or cam adjustment to lower the psi... but maintain the same duty cycle / cfm... then you are just making life easier on your turbo. In turn you'll get cooler, denser air entering the valves - which may compensate for the higher volume if you were to have your turbo building up a large pressure differential.

Basically, you are normally limited on the amount of air you can flow from your turbo. You should do everything you can to use all of it efficiently, not just shoot for a high pressure.
 

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It depends greatly on the compressor map and efficiency range of your turbo. If your turbo is at peak efficiency at 40psi, then I doubt you will make more power at 18psi than you would at 24psi.
 

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Discussion Starter #5
It depends greatly on the compressor map and efficiency range of your turbo. If your turbo is at peak efficiency at 40psi, then I doubt you will make more power at 18psi than you would at 24psi.
I agree.

That being said, your 40psi turbo could be putting out 40psi at various CFM ratings... depending on your engine tuning. You'll still want to maximize your turbos CFM, not psi as you know. If your turbo can not make 40psi through the power range you intend with correct engine tuning... and you have to compensate by opening the intake valves late, or closing them too early (I am just using that as an easy to grasp concept - I know there are many ways to do this), you probably do not have a turbo that matches your engine well.

With a good tune, this should be taken into account already. I just am trying to look at it from another way to show that life isn't about the gauge reading that you see. It's easy to crank that psi number up, but somebody with a much lower number on that gauge may have a much more reliable, quicker, and more enjoyable to drive car. Even with the same turbo.
 

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Right, there's lots of variables to tuning. And lots of different methods. Some tuners like high boost, some like more timing. It would be fairly easy to make the same numbers on the same car with different boost targets. The key is finding that sweet spot for the turbo, heads, engine, and everything. Simply adding more boost is rarely the answer.
 

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Discussion Starter #9
Right, there's lots of variables to tuning. And lots of different methods. Some tuners like high boost, some like more timing. It would be fairly easy to make the same numbers on the same car with different boost targets. The key is finding that sweet spot for the turbo, heads, engine, and everything. Simply adding more boost is rarely the answer.
Exactly. A lot of people out there, especially ones that came from muscle cars, revolve around the thought that X psi = 2.534x the current hp. It's worth a shot out now and then that psi does not necessarily equate to hp. In fact, psi rarely is indicative of hp one way or another.

Were you huffing out of whatever balloons you didn't have left over from chucking them at the Swat team? :p
The tube of non-filled water balloons were given to the cop who searched our apartment - no joke. He took them as 'evidence'.


Lol.I remember that thread.Whatever happened in the end?
I took a rubber shell from a guy's shotgun that repelled down the side of our building. Also, the guy we had in charge of reloading our bucket was slacking and we had to slow down our firing rate.

...Ended up having to pay a lawyer $500 each so she'd yell at the cop and drop it to a summary charge. So I have a summary charge. Better than projectile missles / obstruction of justice / public drunkenness / assault... There were like 3 misdemeanors and 3 or 4 summaries the guy had charged me with. Basically ended up with a rolling credit card bill the last two months of college. Bleh. I even offered the lady a chair and some coffee who was guarding my apartment and asked about the keurig box in our kitchen. Can't win sometimes.
 
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