I need help deciding on a turbo....

[PanzerAce]

So now that I know that my L30 block is an N42, and I've got a line on a P90 in good condition for cheap, I need to start figuring out what I am going to do for the turbo itself. The problem is I have NO idea where to start. I keep reading about T3/T4 hybrids, A/R (or is it AR?), hot side, etc, and I have no idea what any of it means. Is there anywhere I can read up on it (I already have maximum boost, but that applies more to theory than practice), or does anyone care to post some help in this thread?

 

Thanks.

[rztmartini]

page 27 in Maximum Boost starts where you can put in numbers for engine size, etc to calculate flow, and from there you can look at different compressor maps and see where the boost (pressure ratio) you want to run and the airflow you want meet on the graph. page 30 has some examples of these compressor maps and where your line will lie in the efficiency islands. from there pick the turbo that has this point (or range of points depending on desired RPM range) in the most efficient "island" on the map. that is how to choose a compressor size. the turbine size (A/R and wheel) will depend on where in the RPM range you want your boost to come on in...there are tradeoffs so you cant have 25psi at 2500 RPM and pull all the way to redline. you can either have quick boost but gets choked up at upper RPM or have longer boost threshold but breathes freely at the upper range.

 

you can also see this website for specific L28 info.

http://www.mygen.com/users/dbruce/myz31/TurboMaps/L28ET%20Engine%20Air%20Flow%20and%20Turbo%20Compressor%20Maps.htm

 

hope this helps, anyone else correct me where im wrong...

[Pyro]

What are your intensions for the car?

 

Typically a T04E 50 trim compressor for the L28 is a good choice. Can boost at low rpms without surge and still can handle and good amount of rpms, high boost, and up to 450hp.

 

Use a stock T3 turbine if you plan on using the stock turbo cam.

 

If you plan on a longer duration cam, then have the turbo shop open up the stock T3 turbine for more flow (stage 1, 2, 3, 4, 5,) The better the turbine flows, the more lag the turbo will have. However, this is ok and required if the cam is bigger than stock. And, the bigger the cam, the more open the turbine should be.

 

longer duration cams will have more overlap. This makes the engine not like high exhaust manifold pressures. Because, when the exhaust manifold pressure gets higher than the intake manifold pressure and if both valves are open at the same time (exhaust and intake valve overlap), the exhaust pressure will push out the intake. This is avoided with free flowing turbines because exhaust pressures are kept relatively low.

 

When using a stock turbine, boost is very quick, but exhaust manifold pressure is very high. However, this isn't a problem with a stock cam since there is no overlap. The down side is power falls off at 5500 rpm. The up side is gobs of torque at 2800 rpms, which is fun on a street car.

[Pyro]

here are a few nice sites

 

http://www.turbobygarrett.com/turbobygarrett/tech_center/acronyms.html

 

http://www.squirrelpf.com/turbocalc/

[PanzerAce]

Thanks for the help guys. As for where I want my power, I'm already running a pretty wild cam, so I'm used to a higher range power ban.

 

Also, pyro, do you actually mean lag, or do you mean a higher boost threshold?

 

and FYI: I hate it when people talk about 'stage' stuff for engines, since there is NO consistency to it.

[Six_Shooter]

and FYI: I hate it when people talk about 'stage' stuff for engines, since there is NO consistency to it.

 

As do I....

 

BUT....

 

There actually is consistancy to it in reference to Garret T3 turbine wheels. This is how Garret decided to to refer to wheel sizes for this series. I have seen people use trims for the T3, but is less common. Igt seems to b e be only guys like me that would take the time to measure and calcuate the trim of something more commonly refered to by stage anyway.

[Pyro]

I mean lag. High flow turbines don't boost as low in the rpms as restrictive turbines do.

 

I do think my engine does have a little boost threshold problem. Boost does start to come on at 3000 but the engine doesn't seem to use it until 3500. Most likely due to the cam (msa stage 1, 260/250 114 lsa) and maybe less than ideal ignition timing (28 degrees total). However, the stock setup was the same way, boost would come on as early as 2200 in 4th gear but the engine wouldn't use it until around 2800. Again, could be an ignition timing issue. It would be nice to get an efi system with ignition timing control so I could test that theory.

 

Yes, I don't like using "stages" either for describing turbines or cams or anything else. But I forget the A/R on the turbine, but do remember stage 3. Got it from majestic turbo in dallas. I bet they would know how they set up their stage 3 turbines. It is a different wheel and a machined turbine housing. I do know full boost comes on about 700 rpms later than a stock turbine. And I can drive down the freeway at 80 mph with 10inhg of manifold pressure with the stage 3. My stock turbine was at 0 inhg at 80mph (almost boosting). I use a 3.90 gear and a later model NA 5 spd.

[(goldfish)]

High flow turbines don't boost as low in the rpms as restrictive turbines do.

 

 

This would be describing boost threshold. At what RPM the engine can make boost.

[Pyro]

This would be describing boost threshold. At what RPM the engine can make boost.

 

I don't think so. After recently researching "turbo terms" on the web, Lag is a no boost condition and Boost Threshold is when the turbo makes boost but the engine doesn't use it. I guess this would happen if the engine doesn't have enough rpms to be in the power band of the cam.

[Six_Shooter]

Boost threshold is the point where boost comes in. Usually said by RPM.

 

Lag is a time dependant variable, though many people seem to refer "boost threshold" as "lag".

 

I've never heard of an engine not being able to use the boost created, I'm not even sure what if that's possible, especially since boost, even boost threshold can and is determined by increased load on an engine.

[Brad-ManQ45]

I've never heard of an engine not being able to use the boost created

 

This is called surge, where the turbo is supplying more air than the engine can use at that rpm.

[Six_Shooter]

This is called surge, where the turbo is supplying more air than the engine can use at that rpm.

 

 

Not in my books.

 

Surge refers to the demand of air being greater than the supply, in otherwords out of the effeciancy range of the compressor, on the far left side if you look at the compressor map. This area of the map would be less flow than what the engine wants. The turbo will try to over spool, and won't be able to keep up.

 

Again, I just can't fathom how a turbo would supply more air than the engine can use, although by definition, "boost pressure" is simply a measurement of how much restriction to air flow there is in the intake tract.

[m4xwellmurd3r]

I was watching a dyno vid and you could hear when the turbo would surge. the engine rpm's keep rising, but the turbo sound (that nice turbo whine) started to stutter till the rpms got up a bit more, then the turbo would keep spooling.

 

boost lag is responsiveness. as in, how long it takes for the turbo spool to catch up to the engine rpm. if you mash the gas up to the redline and it takes a couple more seconds for boost to hit full boost, that's lag. the opposite would be when you mash the gas to the redline your boost comes to full almost instantly.

 

boost THRESHOLD on the other hand, is if you were to slowly ease on the throttle, and the turbo makes no boost until a certian rpm range. a turbo with a low threshold would be able to start making boost at a lower rpm range than a bigger turbo with a higher threshold. say, one turbo starts making boost at 3k, where a bigger one starts making boost at 4.5k

 

surge is where the turbo is trying to push too much air into an engine that cant use it all. what happens is there is now more pressure trying to slow the turbo, than there is behind it (from the exhaust) to push it. it causes the turbo to stall and makes nasty noises

http://videos.streetfire.net/video/True-Turbo-Compressor_163406.htm

like in that video

[Six_Shooter]

Have anyone read Maximum Boost, or Turbochargers (Hugh MacInnes) or any other (published) book that deals with turbocharging?

 

Threshold is not a matter of easing on or mashing the throttle, it is simply where boost will buid in refernce to RPM, Lag is how long something takes, or in this case how long it takes to build boost.

 

Describe exactly how there can be too much boost in the intake that the engine just can't use? That will just show up as high intake pressures.

 

In that video it clearly shows that they are trying to get more boost from the turbo (pressure and air flow) than it (the turbo) is capable of, causing it to overspool and gets into "surging" where it will speed up and slow down repetivly, trying to provide flow it can't.

It's not too much flow, it's a lack of ability to flow the needed air.

[m4xwellmurd3r]

Have anyone read Maximum Boost, or Turbochargers (Hugh MacInnes) or any other (published) book that deals with turbocharging?

 

Threshold is not a matter of easing on or mashing the throttle, it is simply where boost will buid in refernce to RPM, Lag is how long something takes, or in this case how long it takes to build boost.

 

Describe exactly how there can be too much boost in the intake that the engine just can't use? That will just show up as high intake pressures.

 

that's what i ment about if you were to ease on the throttle. you'd see where the boost starts to build up in reference to the rpm.

 

and there are plenty of cases where an engine can't use the boost from a turbo. if you suddenly let off the throttle with a stuck bov you'd get turbo surge, and the pressure would start to puke out the intake. and that high intake pressure, if is too high, without enough exhaust gases pushing the turbo, would cause it to surge as well. i can't remember the exact cause of that one, i think it can be a combination of things from an improperly sized turbo to a stuck wastegate.

 

oh, what you described about the video is what i was trying to explain sort of. but what i know for sure is that if manifold pressure>exhaust pressure, than the turbo will stall/surge/bark

 

anyways i'll be quiet now XD

 

the only time really where you'd see turbo surge is if you have a stuck bov or no bov at all, and when you let off the throttle you'll hear that "wub wub wub" of the pressurized gas escaping out of the turbo's intake.

[Challenger]

In that video you can see the hp numbers shoot from 320-380. Scary...

[Six_Shooter]

that's what i ment about if you were to ease on the throttle. you'd see where the boost starts to build up in reference to the rpm.

 

Threshold is measured where the boost pressure will build, at WOT, since that is a measure of matching the turbo to the engine, using anything less than WOT is irrelevant, when it comes to determining if a turbo is properly matched for the use, since it's really not needed at anything less than WOT. For all intents and purposes, a turbo could be bypassed completly until WOT, since all that would be needed to increase power output of the engine would be to open the throttle farther. (more air and fuel = more power put simply.) At WOT if you want to increase the power you would need to add more air (and fuel), this is done in a turbocharged engine by increasing the pressure in the intake. I don't think many people understand how this really increases flow, and again comes back to a pressure differential. When the intake valve opens the pressure in the intake is higher than in the cylinder, this is true of N/A or forced induction. It is this pressure differential that causes flow from the intake into the cylinder, by increasing the intake pressure, this causes a higher pressure differential and will increase the flow for the same given time (valve open time).

Where am I going with this? Just illistrating that boost pressure at anything less than WOT is irrelevant from a max power production point of view, it does factor into driveability and tuning however. This would open a whole 'nother can of worms that has load effecting this, that adds more complexity to the situation.

 

and there are plenty of cases where an engine can't use the boost from a turbo. if you suddenly let off the throttle with a stuck bov you'd get turbo surge, and the pressure would start to puke out the intake. and that high intake pressure, if is too high, without enough exhaust gases pushing the turbo, would cause it to surge as well. i can't remember the exact cause of that one, i think it can be a combination of things from an improperly sized turbo to a stuck wastegate.

 

 

That's NOT surge, that is "stall", where there is a lack of BOV, or stuck BOV, any pressure between the turbo and throttle plate has no where to vent, and goes for the lowest pressure area, back through the compressor (air pressure on the front side or inlet to the turbo) is lower than what is in the intake tract.

 

That is also not a case of the engine "not being able to use" the "boost" or air flow from the turbo, since well the throttle is closed, the pressure in the intake tract won't be "used", it needs to be vented.

 

Stall is also a very bad problem, just so that it doesn't seem like I'm making light of the compressor stall.

[Pyro]

mr six,

yes, you are right. I was most likely smoking crack that day with I looked up threshold. Threshold is the rpms when the engine starts making boost.

[Clifton]

Not in my books.

 

Surge refers to the demand of air being greater than the supply, in otherwords out of the effeciancy range of the compressor, on the far left side if you look at the compressor map. This area of the map would be less flow than what the engine wants. The turbo will try to over spool, and won't be able to keep up.

 

Again, I just can't fathom how a turbo would supply more air than the engine can use, although by definition, "boost pressure" is simply a measurement of how much restriction to air flow there is in the intake tract.

 

In everyone else's book that's choke. I'm not an airologist, but surge is basically when the air is moving to slow for the pressure ratio and stalls. Once the air stalls it picks up velocity until it happens again. I don't remember why it stalls though. Run a turbo into surge (easy with a larger compressor at low revs in 5th). You can watch the boost gauge bounce all over and never make full boost. makes evil noises too.

[Gollum]

I do know full boost comes on about 700 rpms later than a stock turbine. And I can drive down the freeway at 80 mph with 10inhg of manifold pressure with the stage 3. My stock turbine was at 0 inhg at 80mph (almost boosting). I use a 3.90 gear and a later model NA 5 spd.

 

Anyone else care to comment on this?

 

I have a stock engine/turbo with the same late 280ZX 5 speed with 3.545 rear gears, and I've gone well over 80 without getting near boost.

 

Does the RPM make that huge of a difference? Because the load/RPM ratio is different, and the amount of power required to propell the car at 80mph doesn't change no matter what RPM you're at.

 

So it comes down to volumetric efficiency. And to be at 0hg at 80mph I think you must be at a much lower VE than I've ever reached on the freeway...

 

I might have some of this backwards, I just found it odd stumbling upon a post like this, as I've only ever hit boost when putting the engine under load.

 

...maybe my wastegate is cracked open?