IW STi Forum banner
Cancel
Create thread New Garage Forums

How much psi is too much for a stock motor?

Jump to Latest
1 - 20 of 23 Posts
W

wrxrick

· Registered
Joined
·
186 Posts
Discussion starter · #1 ·
My 2005 STi has lasted me since 05' with no problems. The car has 36,000 miles right now. Its been pro-tuned almost all its life. I am about to go with a bigger turbo and alcohol injection.

I want some good power but I also want my internals to last (rings, pistons, etc.)

I know plenty of people (probably many of you all reading this) push about 17 to 19psi on a daily basis and their motors are fine (at least the 2004 and 2005 years). So how much psi is too much for the internals? 22psi? 25psi? No more than 20psi on the stock block? FYI I'm going with a Green turbo.

Thanks.
-Rick
 
Discussion starter · #3 ·
Another thing. I understand that using the alcohol injection significantly reduces the inlet temperature. Though one can make a leaner map and still be safer as opposed to just running 93octane and no alcohol/methanol (meaning almost no chance for detonation) I'm sure that the rings and pistons will still take a beating when more air is pushed to them. This is why I am concerned with how much is too much.
 
Discussion starter · #4 ·
snorky said:
four...

Kidding. It's not boost dependent. Its hp/torque/tune/etc. dependent.
Click to expand...
To an extent. I'm sure running 40psi will cause too much stress for a stock motor as far as internals are concerned. It must be boost dependent to an extent. Although I'm sure what your saying is that the TUNE is what really matters most. And a bad tune will mess things up quicker than the longevity of Subaru's internal parts/build quality.
 
#5 ·
wrxrick said:
My 2005 STi has lasted me since 05' with no problems. The car has 36,000 miles right now. Its been pro-tuned almost all its life. I am about to go with a bigger turbo and alcohol injection.

I want some good power but I also want my internals to last (rings, pistons, etc.)

I know plenty of people (probably many of you all reading this) push about 17 to 19psi on a daily basis and their motors are fine (at least the 2004 and 2005 years). So how much psi is too much for the internals? 22psi? 25psi? No more than 20psi on the stock block? FYI I'm going with a Green turbo.

Thanks.
-Rick
Click to expand...

There is alot of variables with this how you beat your car, is it a tracked car etc, how you have maintaned it.. etc.what kind of power your gonna be pushing.. You have been protuned at what boost?. With the Green your gonna want to run more than 20psi of boost which is totally fine for the stock block. Its all in the tune, you can be tuned for 16psi and have your motor blow you know what I mean its not a boost issue. So what you need to do is talk to your tuner its up to you wether you want a conservative tune or a aggresive tune either way tuned right the motor can take the boost so dont worry about that. If your motor is healthy to begin with, do a compression test before the Turbo swap and get it tuned for your goals and your style of driving and have fun our motors can take it..
 
#6 ·
wrxrick said:
To an extent. I'm sure running 40psi will cause too much stress for a stock motor as far as internals are concerned. It must be boost dependent to an extent. Although I'm sure what your saying is that the TUNE is what really matters most. And a bad tune will mess things up quicker than the longevity of Subaru's internal parts/build quality.
Click to expand...
clearly theres a correlation but boost is not a measuring stick. there are other things to consider. flow rate (CFM) of the turbo is far more important than boost.

Edit: i would go into more detail but i'm lazy and no one seems to appreciate my insightful posts (maybe a little bitter).
 
#7 ·
It has to do with power being made(mainly the flow aspect of a turbo) not boost pressure. Take boost pressure(25psi) and compare it to the cylinder pressure during combustion(exponential to that) and see why boost pressure has nothing to do with grenading an engine. more flow = more air = more fuel able to be burned(stoichiometry anyone?) = more power = bigger boom = harder on engine. it has nothing to do with the pressure at which you shove the air in there, which is why if you run 22 psi on a vf39 and 22 psi on say a 35r, you get much much much more power from the 35r because of the flow rate.
 
Save
Reply Quote Like Like
#10 ·
snorky said:
clearly theres a correlation but boost is not a measuring stick. there are other things to consider. flow rate (CFM) of the turbo is far more important than boost.

Edit: i would go into more detail but i'm lazy and no one seems to appreciate my insightful posts (maybe a little bitter).
Click to expand...
Na man. You're on target on this one (not that I need to tell you that). Sometimes people just don't understand.

Simply put, it is totally possible that an STI making 15psi of boost could make more power than an STI making 23psi of boost. One might be running a 40R and the other a VF39.

t
 
#11 ·
WolfPlayer said:
Na man. You're on target on this one (not that I need to tell you that). Sometimes people just don't understand.

Simply put, it is totally possible that an STI making 15psi of boost could make more power than an STI making 23psi of boost. One might be running a 40R and the other a VF39.

t
Click to expand...

+1

and don't forget the intake temp as well. you might have a higher boost pressure and very hot on a small turbo and less boost pressure with much colder intake air due to very good intercooler.
 
#12 ·
shouldn't what matter is how much fuel is being burned in the combustion chamber? so even if you want to run 40 psi and you are not pushing enough fuel into the chamber, there won't be enough exhaust pressure to let the turbo be pushed up to 40 psi. Is this right?
 
#13 ·
never going against wolfplayer on this, (i agree with his post 100%) - but i believe high boost levels on the stock block will start stretching head bolts since they aren't super high strength like ARPs. your tune/tuner can make the car safe to run at any boost level, but at a certain level, the physical limitations of the "weakest link" in the motor will fail.

i don't have a specific boost number to say is the pressure we should stay away from.
 
#15 ·
RemlapaN said:
The real question that you need to be asking and thinking about is:

How much detonation is too much for a stock motor.

Boost does not kill motors, especially if you're running an FP-Green. Detonation kills motors.
Click to expand...
any detonation is too much for any motor. what you need to ask yourself is how much boost is too much and what turbo you should get besides an FP. Greens kill themselves. :lol:
 
#16 ·
WolfPlayer said:
Na man. You're on target on this one (not that I need to tell you that). Sometimes people just don't understand.

Simply put, it is totally possible that an STI making 15psi of boost could make more power than an STI making 23psi of boost. One might be running a 40R and the other a VF39.

t
Click to expand...
+11ty
People focus on boost, but its really a red herring. The key issue is airflow through the motor and the amount of air being crammed into each cylinder. When you do a head and cam swap on a FI car, you will see that your boost pressure LOWERS while HP increases. The reason being that the motor, with new parts, is flowing more air, so the snail that pushes a constant rate of air into the engine at a given speed is not creating as much intake tract back pressure (i.e. boost). This is why large turbos make so much more power then small ones at the same boost pressure, they are flowing more air.
 
Save
Reply Quote Like Like
#17 ·
Majin said:
any detonation is too much for any motor. what you need to ask yourself is how much boost is too much and what turbo you should get besides an FP. Greens kill themselves. :lol:
Click to expand...
+1 here too.
Detonation bad. If your motor pings, your tune is not adequately covering your fueling and timing needs.
 
Save
Reply Quote Like Like
1 - 20 of 23 Posts
About this Discussion
22 Replies
15 Participants
Last post:
JDM-STI
IW STi Forum
posts
4.1M
members
124K
Since
2001
A forum community dedicated to Subaru Impreza and STI owners and enthusiasts. Come join the discussion about performance, engine builds, exhaust, modifications, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
GD-General 2.5 Liter/Litre Factory Motor GD-Members' Journals General Subaru Discussions 2015+ STI Discussion