I apologize ahead of time if this is not the appropriate section to post in. I figured that since a lot of n00bs (including myself) read the "Noobie Questions" section regularly, this post will grab their attention because it is catered to them. I hope to educate some people and learn a few things myself through this thread. If you know the basics, skip to the big bold sentence at the bottom. Back to it.
"How much boost can I run on my xx turbo? How much psi can the stock sleeves/rods/pistons handle?" Are a couple of questions asked very often in this thread.
Some of the standard answers you get from experienced boostmongers are
"Its whp that matters", "Psi doesn't mean anything unless you're talking about a specific turbo." And they're right. But lets go into a little detail to clear up the issue.
A turbocharger essentially compresses the air that goes into the combustion chambers of an engine. Extra fuel is added in a proportionate amount and the whole mixture makes a bigger explosion that gives you more power. We've all heard that explanation before.
How does this translate into power for your car? When you up the boost, the bigger explosion shoves the pistons down even harder, which in turn pushes on the rods to spin the crank that eventually drives the wheels = more power.
Lets say this car was making 200hp at the crank. Now lets say you were a miniature superman and you decided to get into the combustion chamber and run the engine yourself. How do you make 200hp at the crank? You simple push the piston down really hard; that piston pushes on the rod and turns the crank. So you push down hard enough to make 200hp.
The constant variable here is the force pushing the piston down. Whether its superman or the explosion of an air-fuel mixture. Here's the key point of all this rambling: Keeping some variables constant (you can never model anything perfectly),
No matter turbo, nitrous, or N/A if you're making the same power you have the same (peak) cylinder pressure pushing down on your piston in order to generate the same amount of horsepower. I am speaking in the context of any particular one engine (not sure how this applies between engines of different bore, stroke, etc.).
So its the cylinder pressure (at the very basic level) that wears your ringlands, makes your rods punch holes in the block, and cracks your sleeves. All these components have limits. When people say whp not psi matters, this is why.
Wheel horsepower is simply crank horsepower minus the drivetrain loss and is proportional to crank hp. For example, a 220whp limit on a stock engine given 15% drivetrain loss, is 220/0.85 = 259 crank hp limit.
Practically speaking, on a D-series its the pistons and rods that usually fail first and its a good idea to upgrade these parts if you're aiming for over 220whp. Sleeves have been known to handle 400+whp. If you're attempting to build your car there are numerous other threads that will help you out better. I just wanted to explain the fundamental concept here.
Btw, if you want a more detailed and undoubtedly correct understanding of these concepts I'd suggest you read Corky Bell's "Maximum Boost". It was the best book I have ever read about turbocharging. Oh, and there are some good tech articles at:
http://www.turbobygarrett.com/turbobyga ... enter.htmlPlease feel free to ask questions, clarify, or correct me if I'm wrong.
Final Point:When running two different turbochargers, it doesn't matter how much boost you are running on each one if you make the same power. A 14b @ 12 psi producing 200whp is still creating the same peak cylinder pressure as a t3/t4 60 trim 0.83 AR @ 6 psi producing 200whp (just throwing out #s, smaller turbo vs. larger one). And the reason you would choose one turbo over another is (a) powerband and (b) efficiency of the turbocharger at your power level.
Wanting a certain amount of psi is completely irrelevant and it is just plain stupid to base your goals on a certain psi level.
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