quick question..
was having a lil debate with a friend...
If you have a big turbo, which is 20psi, will it put the same stress on the internals as a smaller turbo, which is also 20 psi? and also if the boost is too high, wont it mess up the gaskets/seals? thanks!

 

1) A big turbo at 20psi flows MUCH more air then a small turbo at 20psi... To say that a T25 at 14psi is the same stress as a T88 is an absurd comment. They flow different on the compressor side (20psi isnt 20psi isnt 20psi) and more then that, there is also the load on the exhaust side of your engine as well. The differences in turbo size may (or may not) mean cooler intake temps and more air- which means technically the larger turbo will have a cooler charge and less stress, but because it's cooler, more air and thusly more stress. It's a condition of detonation vs sheer load. Do you detonate and blow a ringland, or have so much power you bend a rod?

Such things that can happen when either you run too much boost, detonate, or both, include the following:
Blowing a hole in a piston
Splitting a piston in half
Blowing the rings or ringlands on a piston
Cracking your block
Blowing your head gaskets
Bending or Snapping a Rod
Twisting or Snapping the crank
Destroying pushrods
Destroying Valves
blowing the exhaust gaskets/seals
Blowing your bearings on any part of the motor
And many more things......

Too much boost (or detonation - you can run 6psi and blow apart an lt1 in a single second of a single blip of the throttle under load) may destroy your engine. It will do a lot more then mess up gaskets and seals. Blowing out a gasket is likely, but usually when the motor is built properly- a few psi on some gaskets isnt the issue - it's what that extra few psi means in fuel needs and in timing control. And even a stock motor will usually blow the internals before it blows all the gaskets and seals. But anything can happen when things go wrong.

 

Geoff, I disagree. If you have that T88 on the same motor as the T14 and the wastegate is set to 20psi on both turbos and the motor can spool both to that boost level, The stresses are going to be pretty close because the airflow at 20psi through the motor is going to be the same because that is determined by the intake tract, intercooler, heads, intake manifold and valves. The only real difference is the RPM of the turbo compressor wheels, the amount the air is heated, and the RPM at which you can spool the turbos up to that boost level.

 

thats exactly what i was thinking, turbo6

 

Actually guys, Geoff is correct. Take a T60-1 at 20psi ~500rwhp and a T88 at 20psi ~850rwhp, and I promise you the T88 is making WAY more cylinder pressure (hp) then the T25....hehe. It has to do with efficiency.

Jose

 

i agree with geoff and JZ...if all the different turbos made the same power at 20psi...there would be no reason to go bigger.

boost is a measure of resistance...my setup for example, i make 7psi at 6000.... 1000RPM will = XXX blower RPM, well, that means the blower will ALWAYS push the same amount of air (measured in cfm) at each engine rpm. but back to the point, if i go to a long tube header, which will create less back pressure(less resistance) my boost pressures will drop 1-2psi, but the blower is still flowing the same amount of air as before. i could lose 2psi, and gain 20-30rwhp. with the same blower setup i have right now, if i was to take my intake manifold off and block off 3/4 of each intake runner, then put it back on..having changed nothing else...my gauged boost would DRASTICALLY increase..

hope that kinda helps some understand it better...you should be more concerned with airflow rather than psi...





to answer the initial question, the larger turbo will produce more cyl. pressure, and do more damage if not properly tuned and set up. but that would also be true to the contrary...a too small of a turbo at 20psi (T25 for example) will blow nothing but hot air...and you would see minimal gains besides very quick spool, which would be traded off by the quickly increasing intake temps (hell, i hit 16psi by 3000rpm on a 2 litre with a t25, but would heatsoak the IC after 2 runs)

 

Efficiency and resistance make a huge difference, more then a lot of people realize. I first noticed this myself in my 1g DSM. Running 14psi on the stock 14b turbo and going up to a Super 16g. I noticed 14psi on the gauge, set it as such. Both turbos flowed 14psi. No problem. Nothing, literally, nothing else was changed on the car aside from the turbo. Even the oil lines were all braided before and after.

1) My injector pulse width changed, I was running more fuel now.
2) My intake temp at 14psi fell a few degrees (though still heat soaked the stock side mount)
3) I can assure you there was more power to be had by SOTP. Easily noticed. Less traction shifting between gears.

Now lets assume I was running 14psi from 4000-6000rpm - as both turbos are spooled by then and both would be pushing 14psi in the manifold.

Why was I running more fuel and having a faster car? Look at the compressor maps. Look at the pressure ratios and see the differences. The two flow vastly different amounts of air. This makes a huge difference, more then most people realize.

Hypothetical: Your turbo is running marginal efficiency to push 800cfm on your setup. It produces 14psi on whatever engine you have. Cool. It's running 75% efficiency. Now lets pull the intake tract apart, and without adding length, add like 4 sharp 90 degree bends. Leave the boost controller set to the same psi. Your turbo is now working harder to produce the same psi in that manifold because those bends add resistance, so they make the flow slower, more turbulent, and thusly less efficient! So now knowing this, you know your turbo is working harder to produce the same psi, thusly the intake temp probably just went up, and you just lost power. Oops! And that's the same turbo on the same engine running the same psi.

Air Flow in lb/min or lb/hr is good. Temp is also good. Psi IS NOT.
Lets say we have big metal can with holes in it (your intake manifold!).
Push 10/lb min of air in there are 200 degrees. This will net you a certin pressure inside the manifold.
Then push 20lb/min of air at 100 degrees. This (hypothetically) will net you nearly the same pressure in the manifold. Which of the two has more oxygen in the manifold, and thusly needs more fuel, and THUSLY makes MORE POWER? Psi was the same on both setups - psi didnt matter at all! It was lb/min and temp(efficiency of the setup) that mattered.

Also there's the concept of flow from another aspect additionall. Do you think that 14psi in your intake manifold equals 14psi of boost in the cylinders? Not on your life! You could have 14psi in the manifold, but have such a sloppy cam you're pushing half the boost past the valves because of huge overlap. If 14psi is 14psi like you say, then going from a 218/218 110LSA cam on a turbo car to a 218/218 116LSA cam wont gain power. Lets then say you run 14psi with a 230/230 116LSA cam. Still 14psi in the manifold!!! Can your turbo compensate for the increased needs for air???

Turbos are wonderfully complicated. 14psi is pretty much never 14psi. Psi doesnt matter when compared to lb/min and temp(efficiency). Because thats what tells you how much oxygen your motor is using where psi is just a pretty number to shout to your friends. And then changing the timing and stoich around will also change output because it will change the efficiency. Lots of variables. 14psi on one turbo or setup is not the same as 14psi on another turbo or setup... Ever. I know it doesnt make sense, but thats just how it is...

Psi is really just a flashy number to tell your friends, and isnt any judge of the setup or it's output.

 

where is the pressure being measured at? manifold or turbo?

 

Boost pressure is not your friend. It realy is just a flashy number and can't realy tell you any thing.
All you realy can tell is how inefficient your intake and heads are.
The turbos efficiency is why you will make more or less power when you swap turbos and keep the pressure the same.
A more efficenct turbo will give you cooler intake temps and less back pressure on the exhaust.
Theres a lot more to it than just that.
The boost gauge is connected to the intake manifold, some where.
I tell people all the time around here that thay should get there heads on there turbo car P&P or get bigger cam(s). They say why when I can just turn up the boost.
To them more power is a bigger turbo and intercooler. It's so sad.
The best part is when they mod there car till it runs like crap then get it half tuned up then the engine goes. I have seen it so many times.

 

supra guys come to mind. all they know is more boost


oil pan is right though. when people hear we only run 4psi they yell and scream "why do you only run 4 lbs of boost! in it!"

Well, its got about 6psi NA and when your heads/intake/cam are that good and its 7 liters- 4 psi is a lot of air.

 

It is the supra and skyline guys, the Rx7 guys want to coppy them to be cool and I have yet to see one not blow there engine.
It's not the cars fault it's the person.
Yes that is how I see it.
Spool is a relative term.
If you need 12psi to get your car realy moving and mine takes off with around 7 or 8 I will have less turbo lag and my car will be faster.

 

Lets break it down.
Assuming:
-The same engine
-The same turbine and housing
-The same exact piping and tune on the car

All we changed was the compressor.

This means that since the compressors have different characteristics one of them would have a greater efficiency at any given point. The one with the greater efficiency by definition will take less work to sping the turbo to pass the same amount of air, causing less back pressure and creating more horsepower. It also creates less heat for the IAT giving it more horsepower as well.

Once you start changing turbine housings it changes everything.

You need to define stress on the internals. Stress from what? Starting an engine without oil puts alot of stress on them.

 

I agree with this, but believe the difference will be relatively small - on the order of 50 hp max unless one is severely undersized. Probably not 300 hp, though.

 

should also point out that while alot of boost pressure isnt necessarily good, you need boost to make power, otherwise you've just got something fancy blocking the flow of air to the engine. its comparable to fuel pressure...pretend your injectors are your valves to the cyl...and pretend your blower/turbo is a fuel pressure regulator..the more pressure you add, the more you squeeze by when its open. but there is a happy medium, and the performance gains are not linear..after a certain point, things can cause you to lose more power than actually help you make.

 

It's true that it's lb/min that count!

As far as 200F taking twice as much air as 100F for the same mass flow, that's wrong. If you consider the ideal gas law, PV=nRT, you have to look at temperature in absolute terms, meaning 659 versus 559 (0F = 459R). So, 200F would take 18% more volume of air than 100F to have the same mass flow rate. You also have to consider that increased temperature at the same mass flow rate means higher pressure = more stress on engine parts.

As was correctly said, you have to have a tune (A/F ratio) to match. If you are running to lean, you WILL blow up your engine.

So if you are measuring Pressure in the intake manifold, 14 psi does equal 14 psi, IF the temperature is the same. So, a smaller turbo at the same pressure is better if the temperature is the same and can hold the boost to red line, because the smaller turbo will deliver power at lower rpms making your torque curve better = better acceleration over all conditions.

The trick with turbos seems to be exactly this, just enough turbo to meet the mass air flow demand. Anything more restricts your torque at lower rpms.

Torque at lower rpms may be irrelevant to your needs. Wanting 1/4 mile times is different than autox racing, where you want your torque curve to be near its peak sooner on the rpm's.