why is max boost limited?


Aug 14, 2008
with my mods in sig why do these cars only max boost out at like 16-17 psi from what i heard there will be alot of knock

any options besides alky where i can bump up the boost 16-17 psi still feels like doogish to me car moves but not nearly as much as i hoped for
Your tune is way out of wack. You need to get the tune nailed down to get the car to respond like you want.

15-16 psi on pump gas(91-94) is about it. Better octane and you can turn up the boost.
You need more octane with more boost. Adding boost is like adding compression to a non-turbo car. 16# of boost is like 11-1 compression and you couldn't run that with 87 octane fuel could ya?
im not comparing these cars i just wanted an explanation

what limits it on my 95 eagle talon tsi where it can run 25 plus psi

and srt4 neons with over 20 psi

with bolt ons and pump gas

why are we so limited to only 16-18 psi
its all in the state of tune that is going to decide the maximum amount of boost. nobody is saying that you cant run 25 or 50 lbs of boost... but the engine will fail at those levels if the tune, does not match. this applies to ALL engines.

17 to 18 lbs is the SAFE limit to the amount of boost with nuthing more than just pump in the tank. with the proper tune and better gas, higher boost can be achieved.

But boost pressure is not a realative number,

just because another car has higher boost dosent mean mean that it makes more power..... that particular design of an engine or its tune allows it to run more boost pressure simple as that. comparing boost pressure alone to different engines is like comparing apples and oranges.

so yes, you can crank on the adjuster on the actuator , or add a bleeder and you will get more boost.... but if there isnt enough octane in the gas,or too much timing in the tune, poor adjustment, the engine will DESTROY ITSELF VERY VERY QUICKLY.

does this help?

Well if you look at my setup im at 18 psi with no knock,i did run the car at 20 psi on 104 octane before.the only way to be safe with higher psi like 21,22,24ect your going to need help like alky and a well tune car.
Also like dr frankenstein said before, 17 to 18 is pretty much the limit that you want to be at with your setup until adding more octane and properly tunning your car.Once again if your goals are to run 20+ psi i would go an
spend the $500 to $600 on the Alcohol Injector System and call the day
Boost is a measure of pressure in the intake. Nothing more. If you put restrictors in the intake runners so the air couldn't make it to the cylinders you could run 80 psi on 87 octane with zero knock. Cylinder pressure causes detonation because the more you squeeze things (like air and vaporized fuel) the hotter they get. This is because as you compress stuff the molecules have less room to move and so the give off the energy they are no longer using as heat (basically). This is also why your air compressor gets hot when you run it. Back to your car. All things being constant if you turn up the boost you get more air in the cylinder. When this happens pressures go up as you compress the mixture and as pressures go up more heat is created. If it gets too hot the charge lights off all by itself. The plug still fires when it's supposed to so now you have two flame fronts moving through the combustion chamber. The flame fronts collide, pressures go through the roof and things come apart.

So, the better your heads, intake, and exhaust flow the less pressure (boost) you need to get the same amount of air into the cylinder and the lower your overall boost number for a given octane will be. Now, like I said before this is with all other things being constant. Often times with better flowing heads, turbo, better intercooler, ect. you can make more power on lower octane because your charge starts off cooler and so you can squeeze it more before it detonates.

If I hosed any of this up please feel free to correct me. I was just trying to give an overview to the OP.

No disrespect to the poster....This car is all out of whack. Return the car to a safe level of tune and start from there. You want a bunch of power immediately, and that isn't how it works. Can't just jump in and turn the boost up. Dr.F put that plainly - the engine will destroy itself.

20+ psi in a 4 banger is basically adding displacement where we have more displacement and do not need to run as much boost. I run 24-28 psi with a GT6131 turbo. Some guys run a much larger turbo and make more power with less boost!

with my mods in sig why do these cars only max boost out at like 16-17 psi from what i heard there will be alot of knock

any options besides alky where i can bump up the boost 16-17 psi still feels like doogish to me car moves but not nearly as much as i hoped for

Where's the exhaust system?You don't have one listed in your sig.If your running stock cat/exhaust system that could be causing your car to run like a turd
right now its an open dump with stock elbow and DP but got one from full t hrottle on the way 3.5

how else would i tune the car other than just buying a chip made to it

i heard the scan master has a learn mode?
I have been able to run 20psi (according to the FAST computer) on 93 octane hess fuel with out any knock.... but every car is different.

Honestly, if you are asking this question, you have A LOT of reading to do my friend.

Why did you purchase a 3.5" downpipe? You wont really gain much from it with your setup and will still be limited to 16psi. In my opinion you should have got something that will help tune your car better. It would have been money better spent.

But hey, its your money not mine! :biggrin:

Its all in the tune!
Buy a front mount, aluminum heads, have clean pistons, drop the timing.. you can run 20+ PSI on pump gas.

Your TSI wont run 25 PSI boost on pump gas as it came from Mitsubishi. More like 8-12 PSI. And you run 25 PSI on 87 octane.. it will pop instantly.

lastly, most DSM's that run really high boost on pump gas do it at 8-10 degree's timing.
They also have near hemi style combustion chambers with two quench areas - at least that is what a guy I know, that works with them, tells me about them.
why cant the car handle more than 16-17 psi?

It can handle more than that IF you have all your ducks in a row. Meaning that it's tuned right.

Key thing is to not have any knock. Otherwise you will blow something. Even at 16 PSI. Knock is very bad in any engine ESPECIALLY a forced induction engine.

These guys have given great info. At higher boost pressures you run into detonation rather easily. That's why you need either race fuel or premium 92 + fuel and alky.
the car only gets super and new chip is coming in ill see what happens but what do you mean tuning?

i have a scan master and the chip he is sending me is updated for the mods in my sig

what else can i manually tune after the chip