whats a safe PSI to run?

At what PSI for turbos is it safe to run,i was talkin to a kid with a jetta and he runs 24 PSI,also how exactly does a supercharger work?..it runs off the belt right?

That depends on the turbo, the engine, the fuel system, and quite a few other factors. If you want to know how a super-charger works look at the sticky at the top.

like polygon said, it depends on a lot of factors. But in general, small displacement engines can run more psi than larger displacement engines due to the simple fact that smaller engines draw less cfm than bigger engines. It requires more psi to push the same amount of cfm as a larger displacement engine. FWIW, more boost is not always better. There is a limit to every turbo to where it can keep making more boost, but the amount of heat generated actually decreases hp. Believe it or not, I actually know some subura wrx guys that are boosting their turbos beyond its efficiency.

Also, factors such as compression, what size turbo, whether the system is intercooled, altitude, weather conditions, what octane gas, and how well the car has been tuned all play a major role in boost.

thanks taht all helps..what is cfm?..cubic fuel per minute?idk anyway by the way that contour looks pretty crazy.

Believe it or not, I actually know some subura wrx guys that are boosting their turbos beyond its efficiency.

Ain't it funny how hard they try.
Even at 15psi (pressure ratio = 2) the efficiency is on the way out. I've heard of clowns trying to run these at 20psi.

Some turbos can efficiently boost to well over 30psi, but they aren't very common in passenger cars.
http://users.actrix.co.nz/dougal.ellen/vf22_map.gif

like polygon said, it depends on a lot of factors. But in general, small displacement engines can run more psi than larger displacement engines due to the simple fact that smaller engines draw less cfm than bigger engines. It requires more psi to push the same amount of cfm as a larger displacement engine.

this is only kinda true. The size of the engine doesn't change how the turbo works. The real issue is with smaller engines, they are more effective due to the smaller combustion chambers.

A properly sized turbo will not be any different on a 1.0L engine, or a 12.0L engine. They both will boost efficiently (reasonable charge temp vs psi). Turbo efficiency doesn't go down just because you are flowing more air...well, I should say, its not that simple anyway.

Anyone that runs 24psi on stock internals without some very carefully thought out engine management system designing and fuel type considerations, is sitting on a very short lived time bomb. But then, you didn't discribe the details of the motor and system. If you spend enough money on a motor, you can make it live through some pretty amazing things. But I assume you're not looking to spend that kind of money

All the info i have on the car is its a 4 door jetta,he mentioned something about a computer chip where he can control the boost,im guessing a boost controller,and other than that I dont think he has any internals done or fuel but I will ask him.

Another major factor is the compression of the engine. If you have a high compression engine, forcing extra air can be dangerous. Take for instance my Maxima engine. I've read that unless I want to do some major engine work I shouldn't push any more then 6 PSI. Which may not seam like a lot, but in my engine it is. Anything over 6 I chance some major damage to the engine.

I hear about a lot of honda moders that blow their engins very quickly for this very reason.

All the info i have on the car is its a 4 door jetta,he mentioned something about a computer chip where he can control the boost,im guessing a boost controller,and other than that I dont think he has any internals done or fuel but I will ask him.

So, I assume he has a 1.8t? If that is the case then he is full of shit. The stock fuel system can't keep up, and the stock turbo can't provide that much boost without damaging itself.

So, I assume he has a 1.8t? If that is the case then he is full of shit. The stock fuel system can't keep up, and the stock turbo can't provide that much boost without damaging itself.

Or dammaging whats around it. The wiring harness cant withstand the type of heat that would produce. My brother had the stock turbo maxed out and fried the 1500 dollar harness. You would not only have to mod the engine, but shield all the components in the enginebay if you wanted to run that much boost. Thoes VW's are touchy, anything goes wrong, the whole system shuts down.

Ill find this out from him as soon as i can,also compression ratio I know is measured in the cylinders?..could someone explain this CR to me in details please?

The compression ratio is between the volume of a combustion chamber and cylinder, when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detination more likely.

You make horsepower by how much air you move through the motor. A high compression 10:1 engine is more efficient than a 7:1 engine, so the 10:1 engine gives you more bang for the buck because the lower compression is not as efficient, it has the ability move more air through it. So, with boost, the 7:1 engine could get a higher compression ratio, which will not cause detonation, and be moving extra air, making more horsepower than the 10:1 engine. The 10:1 engine could be in self-destruct mode and have detonation if you push the same amout of boost through it as you do the 7:1.

It's a touchy process. You have to know what you are doing to make sure you dont blow the engine. Often times mods are required to help handle the higher compression ratio from a turbo or super charger.

Ok,I understand that but I hear detonation alot,is that when the internals go and the motor blows up?

detonation is also refered to as pinging, knocking, pre-ignition. Its when something causes the air/fuel mixture in the combustion chamber to ignite before the ignition spark.

What causes it is heat, fuel vapor will ignite at a certain temp, if there is too much heat (a hot spot or what have you) in the chamber, then it will light it off too early. When you increase boost and/or compression ratio, both of those things will add heat. When you ignite it too early, the piston is still traveling upward, and the flame front collides with the piston. This creates extremely high cylinder pressures and things can break....in quite a spectacular fashion.

Detonation is very bad in a boosted engine, because there is so much more force behind the explosion, it can severely damage the motor very quickly. Unlike most naturally asperated engines, which can suffer some occasional detonation with no noticable side effects.

It can break con rods, melt pistons and shatter ring lands. Its the #1 enemy to building a turbo'd engine, and its the limiting factor to what boost you can run.

Well this guy who lives near me always claims that his lifters are "knocking"..is this a different "knocking" from what you just explained?

The compression ratio is between the volume of a combustion chamber and cylinder, when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detination more likely.

You make horsepower by how much air you move through the motor. A high compression 10:1 engine is more efficient than a 7:1 engine, so the 10:1 engine gives you more bang for the buck because the lower compression is not as efficient, it has the ability move more air through it. So, with boost, the 7:1 engine could get a higher compression ratio, which will not cause detonation, and be moving extra air, making more horsepower than the 10:1 engine. The 10:1 engine could be in self-destruct mode and have detonation if you push the same amout of boost through it as you do the 7:1.

It's a touchy process. You have to know what you are doing to make sure you dont blow the engine. Often times mods are required to help handle the higher compression ratio from a turbo or super charger.

I should add:
A more efficient engine will produce more power for the same air and fuel flow.

Fuel burning in air provides power, the more efficient your engine is the more power it can extract from the burning fuel.
The rest of the wasted power goes out your exhaust and radiator as heat.

I know a guy who turbo'd a 1600cc Toyota with 15psi boost. It was a non turbo engine with high compression.
He said it went really well for about a week. After that it was molten scrap.

Detonation knocking is different to mechanical knocking.

Alright thanks for clearing that up.

This has all been good info, but don't let it convince you that you cant turbo on a stock high compression car, you just have to get a cold air charge (low psi on a big frame turbo) in order to make safe power. Generally what seems to happen for Honda's anyway, is that people will take a bigger turbo (gt35r or similar size) and run 11psi or less, but even at that pressure, you're making 340whp or so...i'm boosted on stock 10:1 and the numbers are in the sig...

The difference on low comp and high comp (especially aftermarket) is that the lower compression, the higher boost you can run w/o detonation, but there is a tradeoff. The lower compression you run, the worse your off-boost throttle response is. The turbo will spool at the same rate because its the same airflow, but the power up to that point is so much less than if you were to run higher compression. That is why most turbo builds (on Hondas anyway) are in the 8.5:1 - 9.5:1 range with 9:1 being the most common....

you just have to get a cold air charge (low psi on a big frame turbo) in order to make safe power

uh....no. Doesn't work that way. Bigger turbo's are not more effecient (and thereby colder intake charge at a given psi) unless you are comparing it to a very undersized turbo that is being pushed well beyond its effeciency range.

If you have too big of a turbo, you'll only be touching the very edge of the efficency range for the turbo, which is usally not very effecient. So depending on the specific example, it could be quite the opposite.

uh....no. Doesn't work that way. Bigger turbo's are not more effecient (and thereby colder intake charge at a given psi) unless you are comparing it to a very undersized turbo that is being pushed well beyond its effeciency range.

What he may be referring to, is that a bigger turbo will often run less backpressure for the same boost.
As a result you'd get less exhaust retained in the cylinders which would make for a cooler cylinder full of air.

thats a fine line of too small of a turbo. I guess it depends on what pressure ratio from exhaust to intake is considered "a lot"....but a bigger turbo still wouldn't be needed....only a larger AR exhaust housing.

Either way, I'd still call it "sizing the turbo properly"

you all just confused the hell out of me lol,I kinda understand some of the things you guys said then I just got lost when you got real technical.

thats what happens when you ask a bunch of know-it-alls a simple question :grinyes:

We can't keep it simple....

What you really need to do is educate yourself about engines in general, and then turbo's on top of it. I'd highly recommend some good books on the subject. Theres a hell of a lot of info to absorb on these subjects....

Yea im doing my best,I could tell you most general info about an engine and then some but I got alot of reading /research to do.

If you really want to make a boosted engine here are some tips
1) decrease you CR. the cheap way to do this involves a thick copper headgasket. Or if you are going to get forged internals getting reduced CR ones is even better)
2)Get higher octaine fuel, the genral rule is one octaine per psi of boost, so if your engine runs fine on 87 then running 93 will comfortably allow you to run 6psi of boost. Of course 126(eq) octaine methanol is awsome!!
3) Get a good (and I mean GOOD) water/alcohol injection kit prefrable one that is intagrated into your ECU. use a 50/50 mix and the alcohol will add octaine to your fuel + ritchen your mixture up a bit, and the evaporating water will cool all your hotspots and steam clean the inside of your cylinders.
4) BOOST CONTROL (turbo only) too much stuff to go into here but this is bigtime important, basicly keeps your turbo from overboosting, keeps your engine pressures in check, and helps lower your intake charge (with bypass valve)
5) Common sence, if you are trying to extract three times the power out of an engine that is already unreliable, chances are that the best setup in the world will not keep it from blowing to bits. Now get an engine like the 2jz-GTE or the RB26dett and you are in buisness :)

I'm going to clear a few things up,
1) the stock turbo on a 2002+ 1.8t jetta can safely boost 24 pounds, now peaking 24 and holding 24 are 2 different things, if he is holding 24 he will blow to much hot air, if he is peaking 24 he is fine.
2) Boosting 24 in a stock 1.8t motor will not damage the internals, this motor can hold up to 350whp without changing the internals of the car.
3) yes, if he is infact boosting the 24 he does need to up the fueling system as I have done on mine. Using a 4 bar Fuel pressure regulator as well as changing the injectors for larger ones will help alot, also tuning the ecu so nothing goes boom will help to.

Its funny you brought this up,I jsut talked to him the other day..he is now running 28 PSI...and he said he re-mapped his fuel.

Its funny you brought this up,I jsut talked to him the other day..he is now running 28 PSI...and he said he re-mapped his fuel.

again: not a stock setup, not on pump gas.

not with two brain cells to rub together anyway

I've seen guys take junkyard motors and put 350hp NOS shots to it, its "feasible" sure. But they didn't last long, let me tell you.

[QUOTE=UncleBob]again: not a stock setup, not on pump gas.QUOTE]



So are you saying he should not be on pump gas with a setup like this? I dont fully understand what you're trying to say.

So are you saying he should not be on pump gas with a setup like this? I dont fully understand what you're trying to say.
What he is saying is that he shouldn't be running pump gas while running 28PSI. He had better be using at least C23. Otherwise he will get some nasty pre-detonation.

So are you saying he should not be on pump gas with a setup like this? I dont fully understand what you're trying to say.
what I'm saying is KABOOM!! Very soon. Its not "impossible" to run 28psi on pump gas, but it takes a very thoroughly engineered engine, and IMO, it REQUIRES at the least, alcohol injection. I personally don't believe its feasible with pump gas (92 octane) by iteself, even if you're running 6:1 compression ratio and 5 degree's total timing.

OK, it would probably be feasible then, but it would drive like utter crap.

With alcohol injection, its a whole nother story, but then, its no longer "pump gas" either.

even if he "remapped" his fuel,he shouldnt be running pump gas?

Ain't it funny how hard they try.
Even at 15psi (pressure ratio = 2) the efficiency is on the way out. I've heard of clowns trying to run these at 20psi.

Some turbos can efficiently boost to well over 30psi, but they aren't very common in passenger cars.[/IMG]

From the turbochargers point of view, high boost pressures aren't that much an efficiency problem like it is an turbo speed problem. Sure, when you run very high boost pressures you lose a bit of efficiency, but lack of efficiency is more an issue related to too high flow rates. Of course, if you keep increase the boost the impeller inducer will eventually go supersonic which is followed by a reduction in efficiency. But the first problem with high boosts tend to be turbocharger speed. In most maps, like the one you posted, the highest rpm line is the maximum safe rpm. Above that point, we usually have the third critical speed for the turbocharger rotor, and if it spend much time at that speed it will break. Another problem is the centrifugal forces generated by the high speed, and together with the higher temperatures the stress will be great on the impeller. Most low boost turbos have impeller wheels made out of a simpler aluminum alloy, and these tend to lose strength at about 150 degC, the temperature you typically reach at about PR 2.5.

Compressor designs for higher boost pressures, tend to be made out of a high strength, heat resistant aluminum alloy, or a titanium alloy, preferbly billet machined from a forging. They also have a different design that makes it possible to build a higher boost at a lower turbocharger speed, however, that usually comes at the cost of map width. Some compressors for high pressure ratios also have a transonic impeller design to overcome the normal losses related to impeller speed going supersonic.

In general a turbocharger should be used at a pressure ratio where it offers the greatest map width; that will result in the greatest usable rpm range for the engine. In the map you posted, that is at about PR 2.1, but a higher pressure ratio can be used at lower speeds for more low end power.

Some turbochargers can run with very high pressure ratios, up to 4 or 5 even. However, most small turbochargers found on cars work best at around 2 or so. Bigger diesel turbos tend to favor ratios around 3, but some use 4 or 5. Big diesel engines, such as ship engines also tend to favor very high pressure ratios.

In many cases tough, the turbocharger isn't the component limiting the boost.

Whats a "pressure ratio"...?

atmospheric pressure at sea level is 14.5psi absolute.

"guage" pressure, at sea level is 0psi. If you run 14.5psi on the gauge, you have double the atmospheric pressure, or you have a ratio of 2.

3 pressure ratio would be 14.5 x 2 = 29psi. The psi for each ratio changes as you change altitude, so it can be a bit confusing when you are talking about ratio's, absolute, gauge pressures.

But to keep it simple, a ratio of 2 = roughly 15psi of boost on your boost gauge.

Whats a "pressure ratio"...?

For a compressor

pressure after compressor / pressure before compressor = pressure ratio (aka compression ratio)

For a turbine

pressure after turbine / pressure before turbine = pressure ratio (aka expansion ratio)

For a compressor the PR is always above 1, while for a turbine the value is always below 1.

Atmospheric pressure is typically around 1 bar, so a compressor running a pressure ratio of 3 will create a pressure of 1*3 = 3 bar given there are no pressure loses before the compressor. That pressure is usually described as 2 bar boost, since that is given in pressure above atmospheric. More imporantly though, imagine that we use a two stage compressor, each running at a pressure ratio of 3, the pressure in the system would increase to 1*3*3 = 9 bar or 8 bar boost.