I've been reading some older threads that talk about getting boost spikes and boost creep as you start making mods to your 7. I can understand how you would get boost spiking as you free up the intake and exhaust, but what I don't get is if you control you boost down to stock levels, say 10 psi, how is it that you still get more HP? Aren't you still putting in the same amount of air if you keep boost down to 10 psi? Can someone clue me in?

 

Its all a matter of flow. Its a bit easier to think of if you compaire a big single to the stock twins.

Say at 15 PSI on the stock twins makes 350 RWHP. And say a T-78 at 15 PSI make 425 RWHP. You may ask why because they are both running at 15PSI???? It all comes back to flow. For example you have a 1" piece of pipe and a 3" piece of pipe. Now pressurize each one to 15PSI, now you tell me which one is going to be flowing more air.


By freeing up the intake and exhaust you are just increasing the flow capabilities of the motor. Make sense?

 

Thanks for the reply, it kind of makes sense. Let me ask you this now, to see if I'm getting this. Say I add a cold air intake, a downpipe and a cat-back. Then I limit boost to 10 psi, will I run lean because I am flowing more air even though boost is kept at 10 psi? Thanks for being patient and I hope I'm not annoying you.

 

Definitely wait till another w/ more experience responds as well BUT I believe you are correct. Thats why everyone will tell you 2-3 mods to the intake/exhaust will call for an ECU change (to resolve that lean issue).

 

you will not run lean enough to do anything bad. If you had engine ported that woudl be differnt story( in regards to your post this would be the same as increasing boost, but generating less heat), but I will assume you don't. THe main reason stuf like efini y-pip, cat back, intake add power is becasue the result in more boost, quicker spooling, etc. freeing up the exhaust and intaek makes it easier for your engine to complete it cycle, limiting it to 10psi, will still give you more power, but well within the range of the fuel system to compensate, and not as much if you didn't limit boost. regardless of boost, exhaust mods an intaek allow engien to flow better consume more, etc. However, this is really marginal, not a big deal. With a ported engine you are really increasing the volatility and size of combustion. Hence large timming adjustments, etc. THe all go toegther, say you are running twelve lbs of boost, then the benefit of an exhaust becomes much more prevelant, and beneficial. As you consume more air, it must be expelled more quickly as well.

I think I forgot the point, I will stop now.

 

You got it. The higher the boost pressure(amount of air forced into the engine), the more the need for fuel to keep the air/fuel ratio the same for clean combustion. More fuel is needed to compensate for more air being force-fed into the engine. The stock ECU can't adjust the rich/lean mixture beyond 12 psi of boost. You can reduce boost levels to compensate for free-flow mods and still gain hp, but at one point the power gains will drop off when the boost is decreased. I guess you could summarize with "The more free-flow mods, the less boost you need to maintain the same horsepower, and the more psi you run the more fuel needed to prevent lean running and detonation."

Too bad you can't just turn a jet screw like on a $200 carbuerator, huh?

 

Thanks for your replies. So if I run only a DP and a cat-back and I keep the Boost to 10 psi I'll be running more lean, but I'll be o.k. right?