turbine a/r won't affect boosthreshold

[Silverfc88]

"turbine a/r won't affect boosthreshold"




Ok its late at night and I'm doing some research on turbo's and this quote popped up on one of my threads and I don't quite understand it. LoL..



So what do you think it means? I thought it meant something like max boost using pump gas before preignition or detonation. Maybe max boost because of the exhaust energy from the engine. So even if you use a .68 a/r versus a 1.15 a/r turbine you will still get the same amount of boost pressure but only change when it comes online. I don't know, I'm just rambling and searching other posts and seeking different meanings.

[sisforsurfing]

Where did you get that quote? area/radius is the difference between the area of a turbine housing inlet divided by the distance from the center of the housing to the center of the inlet area. Boost threshold is at what RPM there is positive pressure coming from the compressor.
Usually a smaller A/R means faster spool with more backpressure on the top end, larger A/R having less response with more top end flow.

Boost threshold is different than lag. Turbos with larger A/R's lag more, but boost threshold isn't affected. (that's wrong)

So, that quote means a turbo's A/R won't affect at what RPM boost comes on (boost threshold), but how long it takes to reach full boost at that RPM when suddenly going WOT (boost lag.)



As always, take any tech info on forums with a grain of salt when not coming from a professional, but I'm pretty sure that's all good. :)

[Silverfc88]

Ok I've always understood bigger A/R slower spool, more top end and yada yada. I guess I've never really looked into boost threshold before. I've read the term before, but not as much as I have lately looking through different posts and different setup' and turbo's. Thanks your post helped a little.

[vetteracer79]

On the Money sisforsurfing. I'm a motor head and by george you've got it. BTW if you're worried about lag on your car w/ a larger a/r. It shouldn't be a problem assuming you've got the right turbo for your app. wankels have a tremendous amount of air that they exhaust. Besides you can always use ANTI-LAG. Bang-Bang!!

[Loserkidwac]

Remember we are talking about turbine a/r and not compressor a/r, turbo apps are compressor based, when you look at compressor maps to properly size a turbo you aren't accounting for the turbine size, the turbine will play a part on how each turbo performs ex. spool time and can also effect the torque...this based on what I have resaerched and personal experience so correct me if i am worng...

[Dave]

On the Money sisforsurfing. I'm a motor head and by george you've got it. BTW if you're worried about lag on your car w/ a larger a/r. It shouldn't be a problem assuming you've got the right turbo for your app. wankels have a tremendous amount of air that they exhaust. Besides you can always use ANTI-LAG. Bang-Bang!!

hmm i know a place thats selling a bee*r rev limiter/misfiring system. actually im planning on buying one ;)

[Action Park]

Boost threshold is different than lag. Turbos with larger A/R's lag more, but boost threshold isn't affected.

So, that quote means a turbo's A/R won't affect at what RPM boost comes on (boost threshold), but how long it takes to reach full boost at that RPM when suddenly going WOT (boost lag.)

Respectfully, these turbo terms have always caused confusion and the original quote in question is misleading, if not completely wrong. Boost threshold is the point where positive manifold pressure begins, 0 psi, out of vacuum. Lag is the amount of time it takes to reach boost threshold. Spool can be taken to mean the same thing(e.g. my turbo takes forever to spool, yo!), or it can be the time taken from boost threshold to the desired psi.

Now, beyond this debacle of words, a larger a/r hotside will most definatly take longer to reach boost threshold on the same engine and conditions. For example, suppose a .58 p-trim will attain b.t. @ 3.5k wot in third gear. The owner switches to a .81 p-trim and b.t now occurs @ 5k wot in 3rd. What I think the person meant by saying "Turbine a/r wont affect boost threshold" is that the time from b.t. to the max psi is unaltered, which is more proper. The only thing the cold side cares about, in relation to the turbine, is the rate at which it is spinning. When the comp. is spinning fast enough to provide positive mani. pressure, it is at no odds which turbine a/r is on the other side. Once this compressor rpm is attained(and remember, the engine rpm will be different with different a/r's), the amount of time it takes to reach the rpm that will provide full boost is not altered by a different turby a/r. Well, it might, but only because the v/e of the engine is changed at the different boost onset rpm's.
Of course, the other component of the turbine, the wheel, has an effect on boost onset(same as boost threshold) and ramp up as well. One can see how the larger diameter, and surface area of a larger wheel can cause the point of b.t. to become higher in the engine rpm range. Yet, when a larger turbine wheel is spinning at a the same pace as a smaller wheel, there is a larger amount of exhaust gas spinning it, and the greater surface area of the big wheel imposes more torque upon the compressor wheel. This can actually create a faster spoolup(0psi to x psi) once a suitable rpm is reached, though of course, this might be a matter of milliseconds. Think in terms of gearing for further elucidation in the matter of the effect of a turbine wheel. Then theres that darn v/e again.
I'd just say to not try and rediscover the wheel, what works well will work well on a similar engine under similar conditions. And yes, rotary engines, like acvw engines though to a greater degree, can use rather large turbines due to exhaust flow. Run as unrestrictive an exhaust setup as possible if there is a question if the turby is too big.

[sisforsurfing]

Respectfully, these turbo terms have always caused confusion and the original quote in question is misleading, if not completely wrong. Boost threshold is the point where positive manifold pressure begins, 0 psi, out of vacuum. Lag is the amount of time it takes to reach boost threshold. Spool can be taken to mean the same thing(e.g. my turbo takes forever to spool, yo!), or it can be the time taken from boost threshold to the desired psi.

Boost threshold is (like you said) the point where positive mani. pressure begins, but lag isn't the amount of time it takes to reach B.T.
Lag is (quoted from Street Turbocharging by Mark Warner, a book everyone should read), "the delay, when the engine is in an rpm range where boost can be created, between the time the throttle plate is fully opened and noticeable boost is measured. Lag is affected by the rotational intertia of the spinning turbocharger components, and also by the A/R ratio of the turbine housing."

Lots of people think lag is "my turbo doesn't start making boost until 4500 RPM," when that is really boost threshold. Lag is how long it takes to make boost when you're cruising at 5000 RPM and go WOT.


but... after reading the quote again I realize it, and part of my last post, is wrong.
For example, suppose a .58 p-trim will attain b.t. @ 3.5k wot in third gear. The owner switches to a .81 p-trim and b.t now occurs @ 5k wot in 3rd. What I think the person meant by saying "Turbine a/r wont affect boost threshold" is that the time from b.t. to the max psi is unaltered, which is more proper.

This is what I should have said :) Usually larger area ratio makes for a slower spool AND less response, but also flows more on the top end.