Chevrolet Cruze Forums banner

1 - 13 of 13 Posts

·
Premium Member
Joined
·
1,283 Posts
Discussion Starter #1 (Edited)
Simple question, but the overwhelming majority of "car enthusiasts" cannot answer it. Let me explain.

Driving a big turbo car on the street for years (my old Talon), one of my biggest pet peeves was when people would pull up at a light or a car meet and ask me how much boost i was running. Why is that annoying? Because the answer, without other information to qualify it, is completely irrelevant.

Boost pressure tells you nothing. Zero, zilch, nada.

You cannot determine how fast a car is based on boost. You cannot determine power, or airflow, or efficiency, or anything else that is at all relevant to performance, based on how much pressure is in the intake manifold of a car. In fact, you can't even get a ballpark without other information.

I can show you 2.0L hondas that will suck up a ZR1 on 18psi, and i can show you 2.0L VWs that can't trap 95mph on 24psi...yet without fail if you drive a car with a big FMIC, you will endlessly answer the question "hey man, how much boost you running?".

Boost pressure can best be defined as byproduct of restriction. While not a direct measure of restriction, it is a dependent variable directly determined by restriction. For instance, if you add a decent cam (or set of cams) to a car that had stock cams, you will make more power at the same boost level. The engine is less restrictive, therefore air flows more easily into it, therefore it takes less pressure to force the same volume of air into the engine.

If we expand this to turbos, it gets more interesting. As a hypothetical, we have a 4L engine with a GT35R. On 10psi the engine makes 300whp. We take that turbo off and add a GT45R, and then dial it in at the same 10psi...and the result with no other changes is 400whp. Well, the boost pressure going into the engine hasn't changed, but the power output has changed 33%. Why?

The answer is simple, the airflow changed 33%. At the same pressure, the larger turbo is flowing a lot more air. Engines are air pumps, essentially. Put more air in, get more power out. That doesn't mean put more pressure in, get more power out.

Pressure is not, in any way, related to flow.

To prove that point, look at supercharged cars. A given pulley ratio will deliver a given airflow (assuming air density remains constant). If you take a blower car that was on 10psi, and add a nice head/cam/intake package, you might only see 6psi, but the power level will be basically identical to what the engine previously produced at 10psi. Lower restriction in the system means the engine will swallow more air with less force needed.


This is why it drives me insane when people say so called "rule of thumb" things like "For every 15psi you double an engines horsepower". Um...No. I can run 20psi on one setup and fail to double an engines horsepower, or run 10psi on another setup and succeed.

You can't determine how much boost an engine can take before failure. Statements like "You can only run 20psi on stock internals" are the biggest crock of crap i've ever heard, because 20psi from one turbo is the same amount of air as 10psi from another.

Then there's the people who claim things like "you can't run more than 25psi on pump gas"....I always loved that. That would mean 26psi would be the limit of detonation, which is a function of how much volume is compressed in the cylinder, which cannot be determined by boost pressure....so essentially that statement means "i'm an idiot with no clue what i'm talking about"...and generally the proof was sitting right in front of them, as i typically ran about 30psi on 93 octane, occasionally more.

So to wrap this up, boost pressure only matters when you can frame it. If you know the turbo size, the head/cam/intake setup, and the boost pressure, and you know what you're looking at, you can get an estimate of airflow from the pressure, and an estimate of power from that airflow. Knowing boost pressure alone is like asking someone you've never met what they weigh, and deciding how healthy they are based only on that information. If someone tells you they're 350lbs and you know nothing else, you have no idea if you're talking to a morbidly obese 5'1" person, or an Olympic power-lifter.

Best,

Hoon
 

·
Premium Member
Joined
·
906 Posts
:sigh: Amen Brotha!!
 

·
Premium Member
Joined
·
4,339 Posts
Just about as (ir)relevant as the older jingle: "...that have a HEMI in it?"
 

·
Premium Member
Joined
·
1,283 Posts
Discussion Starter #4

·
Administrator, Resident Tater Salad
Joined
·
16,777 Posts
Yep! Said like a boss.

The TD04-16G on my old car only put out 10 PSI (could be tuned for a lot more), but people running the 18 and 19Gs could push a lot more air at 10 psi, and yield a faster car.

But the spool times...man...and those are considered "small" turbos in the Mitsubishi lineup. The cars gearing really didn't help getting it quickly to the 2500-3000 RPM spool point.

Meanwhile, the Cruze runs what, 16 PSI, but the turbo is the size of a hamster wheel and doesn't move a whole lot of air.


Sent from AutoGuide.com App
 

·
Registered
Joined
·
308 Posts
my cruze runs 10psi mostly wont go any higher than say 12psi unless in 5 or 6 gear then about 15-16psi. Mine is 2012 eco MT with intake and catback otherwise stock. is that normal? I live in Central Ontario so no mountains or any weird air. I just hooked my boost gauge up couple weeks ago. Shows lots of vacuum.
 

·
Super Moderator
Joined
·
5,017 Posts
I always loved people in the 80's and before asking: Do you have a cam in that thing? My reply: Yea I like my valves to open and close! Stupid question. Of course I knew what they were asking but.......
 

·
Premium Member
Joined
·
1,283 Posts
Discussion Starter #10
Amazing read. I did not know this, yet i never asked how much boost, since to me it never meant anything anyway. I always ask "What do you have in it".
Thanks. If it's a boosted car I generally ask what turbo someone is on, then go from there.

Yep! Said like a boss.

The TD04-16G on my old car only put out 10 PSI (could be tuned for a lot more), but people running the 18 and 19Gs could push a lot more air at 10 psi, and yield a faster car.

But the spool times...man...and those are considered "small" turbos in the Mitsubishi lineup. The cars gearing really didn't help getting it quickly to the 2500-3000 RPM spool point.
Funny you mention the old Mitsu turbos. They're good, cheap turbos and no one really uses them. Outside the Mitsu world, they're a well kept secret.

Im well into putting an Evo III 16G (TD05H) on my bike. The turbo is cheap, efficient, flows 2.5x more than the cruze turbo, and at low boost levels it will last forever. Plus it's ugly so i can downplay it very easily..."It's just a stock motor 750 with a junkyard turbo".... Busa owners will never see it coming, lol.

my cruze runs 10psi mostly wont go any higher than say 12psi unless in 5 or 6 gear then about 15-16psi. Mine is 2012 eco MT with intake and catback otherwise stock. is that normal? I live in Central Ontario so no mountains or any weird air. I just hooked my boost gauge up couple weeks ago. Shows lots of vacuum.
The cruze ECU varies boost to maintain the same amount of torque output, so altitude and atmospheric conditions determine how much boost is needed to make the requested torque output.

On a cold night i might not see more than 8.x psi, because the air is dense. On a hot summer day i might see 12+psi because the atmosphere is thin, so it takes more PSI to make the same power output.
 

·
Premium Member
Joined
·
4,339 Posts
Hey, iKermit...nice "tire inflator" there, do they come in redhead models too (wink,wink)?
 
  • Like
Reactions: iKermit

·
Registered
Joined
·
1 Posts
Simple question, but the overwhelming majority of "car enthusiasts" cannot answer it. Let me explain.

Driving a big turbo car on the street for years (my old Talon), one of my biggest pet peeves was when people would pull up at a light or a car meet and ask me how much boost i was running. Why is that annoying? Because the answer, without other information to qualify it, is completely irrelevant.

Boost pressure tells you nothing. Zero, zilch, nada.

You cannot determine how fast a car is based on boost. You cannot determine power, or airflow, or efficiency, or anything else that is at all relevant to performance, based on how much pressure is in the intake manifold of a car. In fact, you can't even get a ballpark without other information.

I can show you 2.0L hondas that will suck up a ZR1 on 18psi, and i can show you 2.0L VWs that can't trap 95mph on 24psi...yet without fail if you drive a car with a big FMIC, you will endlessly answer the question "hey man, how much boost you running?".

Boost pressure can best be defined as byproduct of restriction. While not a direct measure of restriction, it is a dependent variable directly determined by restriction. For instance, if you add a decent cam (or set of cams) to a car that had stock cams, you will make more power at the same boost level. The engine is less restrictive, therefore air flows more easily into it, therefore it takes less pressure to force the same volume of air into the engine.

If we expand this to turbos, it gets more interesting. As a hypothetical, we have a 4L engine with a GT35R. On 10psi the engine makes 300whp. We take that turbo off and add a GT45R, and then dial it in at the same 10psi...and the result with no other changes is 400whp. Well, the boost pressure going into the engine hasn't changed, but the power output has changed 33%. Why?

The answer is simple, the airflow changed 33%. At the same pressure, the larger turbo is flowing a lot more air. Engines are air pumps, essentially. Put more air in, get more power out. That doesn't mean put more pressure in, get more power out.

Pressure is not, in any way, related to flow.

To prove that point, look at supercharged cars. A given pulley ratio will deliver a given airflow (assuming air density remains constant). If you take a blower car that was on 10psi, and add a nice head/cam/intake package, you might only see 6psi, but the power level will be basically identical to what the engine previously produced at 10psi. Lower restriction in the system means the engine will swallow more air with less force needed.


This is why it drives me insane when people say so called "rule of thumb" things like "For every 15psi you double an engines horsepower". Um...No. I can run 20psi on one setup and fail to double an engines horsepower, or run 10psi on another setup and succeed.

You can't determine how much boost an engine can take before failure. Statements like "You can only run 20psi on stock internals" are the biggest crock of crap i've ever heard, because 20psi from one turbo is the same amount of air as 10psi from another.

Then there's the people who claim things like "you can't run more than 25psi on pump gas"....I always loved that. That would mean 26psi would be the limit of detonation, which is a function of how much volume is compressed in the cylinder, which cannot be determined by boost pressure....so essentially that statement means "i'm an idiot with no clue what i'm talking about"...and generally the proof was sitting right in front of them, as i typically ran about 30psi on 93 octane, occasionally more.

So to wrap this up, boost pressure only matters when you can frame it. If you know the turbo size, the head/cam/intake setup, and the boost pressure, and you know what you're looking at, you can get an estimate of airflow from the pressure, and an estimate of power from that airflow. Knowing boost pressure alone is like asking someone you've never met what they weigh, and deciding how healthy they are based only on that information. If someone tells you they're 350lbs and you know nothing else, you have no idea if you're talking to a morbidly obese 5'1" person, or an Olympic power-lifter.

Best,

Hoon
Hey Hoon. Big fan, I have a 2012 chevy Cruze, LS 1.8L, best things to upgrade and everything for putting a turbo on there? Everything's stock minus the Kn&n Cold air, and the magniflow exhaust. Tia.
 

·
Super Moderator
2014 LT program car, Pull Me Over Red
Joined
·
8,821 Posts
Hey Hoon. Big fan, I have a 2012 chevy Cruze, LS 1.8L, best things to upgrade and everything for putting a turbo on there? Everything's stock minus the Kn&n Cold air, and the magniflow exhaust. Tia.
Welcome Aboard!(y)

He has not been online since 2017. You may want to watch Crewz though. Project 1.8 Turbo (Turdbo)

Don't forget to introduce yourself and your Cruze here.
 
1 - 13 of 13 Posts
Top