rage2

Alright, for all the turbo gurus, here's my question of the day...

With a smaller turbine housing (let's say, KKK#6) you'll get a lower rpm spool up as compared to a larger housing (let's say KKK #8). The problem with a small housing is, it limits top end power... why is that? Is it due to huge backpressure at high horsepower? With both housings, theoretically you should be able to reach the same turbo rpm speed... well, the smaller one theoretically should be able to spin even faster assuming the turbo can handle it.

If so, why can't we use a KKK#6, and a big wastegate, so the excess pressure gets vented out (reducing backpressure), which would give us quick low rpm spool along with a lot of top end?

Or am I way off base here as to why a smaller turbine housing limits top end...

Trying to learn turbo sizing hehe.

J Chen

Hi Rage,
I posted a similar question once.
At that time I was green about
turbos. I have now realize what
retrictive means. If you look at the
K26 hot housing, the width of the
housing is very narrow which
means that the volume in the volute
is very small. This is where a lot of
the back pressure comes from. I
have compare the K26 hot housing
& a T3 housing & come to the conclusion
that we should be able to atttain quick
spool with much low back pressure if
we go with the T3 hot housing.
I proberbly will get flame for making
this observation but I don't think the
K26 hot housing is optimum choice
for the 951 in the first place same
goes for the K26/8 hot housing.
This brings up another question.
Why are there no Tuners out there
that's coming up with a T3/T4 turbos
that can be bolted onto the 951 ?
Instead you guys are relying on the
KKK hybrids consisting of No.8 & 10
hot housings with either stage III
or V trim ? Why Why Why ?????
Oh & charging premium price as well.

DanD

Rage2,
I think Danno is working on getting the most out of a k26. Others have too.

J Chen,
I agree for the most part.

Realize that there are a couple turners out there developing the true Garrett. SFR has modified exhaust that will bolt up. Any good exhaust shop could modify your existing exhaust. I think the problem would be the oil lines in and out of the turbo. The in wouldn't be a big problem, the outlet is actually a part of the motor mount. I don't know how SFR solves this problem.

I'll give a comparison from my almost finished Garrett hybred vs my stock 951.

jimbo1111

Backpressure between the turbo hot side and the exhaust ports holds back the air being pumped into the motor creating higher boost pressure in the manifold. It's a major restriction if your running high boost. That part of the exhaust see's 3 times the pressure of the manifold with a #6 hot side. It's great for spool up but our car make peak HP close to red line.

rage2

So if we have a monsterous sized wastegate sitting between the turbo hot side and the exhaust ports, and it opens up to vent the excess pressure, wouldn't that relieve the major restriction on a small turbine housing but still allow enough pressure to go to the turbine and spin it up to create the high boost on the compressor side?

That's what I'm curious about... if there's a way to use a small housing but without the huge restriction at high hp levels.

Russ Murphy

If a wastegate already has enough flow capacity to prevent you from overboosting then isn't it a moot point if you make it bigger? The backpressure is related to the boost level (and the rpm) your running at the time isn't it?

J Chen

Hi Rage,
Yes, it's like what Tial says in their
website. If you're running high boost
you can get away with a small waste
gate. But if you're running low boost
than you've got to run a big wastegate.
Why, cause at high boost it is assume
that the engine will consume more air
& less wastage. Now if we look at the
K26/6 hotside, it is a known fact that
once you go beyond 15psi the back
pressure will be very high. With your
proposal of venting the excess gas of
is like to back to square one.
Luke has a hybrid 60-1hifi with stage
III trim with a #6 hot housing & he says
that power tails off at about 5500 rpm.
I think it would be a better to adopt a
T3 housing to get quick spool with the
added benefit of getting higher hp returns.

Dan, I don't see much problems in mounting
the turbo as most of the hybrids that is
being sold uses the CHRA from Garrett.
Am I missing something Dan ?

Duke

I'm interested to know if anyone tried a 10# hotside on a 2,5 liter.

When looking at turbo converted BMW's (there's a lot of them here) they have monstrous hotsides! Probably 2-3 times a KKK 8#, and although they must be pretty laggy they do have boost after 4000 rpm.

TurboTommy

Rage,
That is an excellent question; I have wondered about this myself. I've said this before, but this whole hotside thing is much more of a black art than the compressor side and it seems to be a closely kept secret among the turbo elite. Sometimes research needs to be done outside of the automotive industry to find out how things really work.

From the limited amount that I know, it has something to do with turbine effeciency (how fast can the rotating assembly turn with a given amount of exhaust energy). With a small hot section the law of diminishing returns comes into effect; it comes to a point where tons more exhaust energy is needed just to increase the turbine speed a little. So, in order to keep the boost up the wastegate needs to stay closed to funnel this high exhaust energy (which will produce back pressure) to the ineffecient turbine. So, I guess what's happening, in a nutshell, the wastegate needs to stay closed more because all the exhaust has to go through the turbine. A more efficient turbine (usually larger) needs more exhaust just to get it to do any appreciable work but the wastegate can vent off back pressure sooner when it has satisfied the demands of the compressor.

Something to that effect.

It would be great if anybody had some real hard info on this.

J Chen

Hi Guys,
I'm gonna try the combination of
a K26/6 hotside with a stage V
trim turbine soon. Will post results
when done. As the turbine trim
itself will contribute some back
pressure as well, maybe going
bigger trim will offset the drawback
a little. We shall see.

jimbo1111

Rage I think your confusing psi with cfm. By venting the exhaust energy to reduce psi in the crossover you would lose boost pressure in the intake. At higher rpms the motor need more air to consume to maintain the specified boost level. More air in the higher rpm's means more exhaust.
When selecting a turbo there is never a perfect combo for spoolup and top end power. If your looking for peek HP numbers you must concentrate on higher rpms. Witch means that you have to loose some low end power and spool up characteristics. I think many here confuse flow(cfm) with psi. Psi is a measurement of restriction. Cfm is the measurement for air moving through the motor. In a stock configuration someone that boost's 12 psi at the manifold will not flow the same cfm as someone with a free flow exhaust and a maf even though the manifold psi is the same.

rage2

Ok, so what you're saying is, in order to move X cfm of air on the compressor side, you need the same X cfm on the turbine side in order to spin the compressor side fast enough? But with a small turbine + big compressor mismatch, it would cause a lot higher psi on the turbine side (backpressure) than the compressor side (boost)?

I was always under the impression that you need much less cfm on the turbine side to spin the compressor side fast enough to make way more cfm on the compressor side... the smaller the turbine housing the lower the cfm required to reach peak turbo rpm speed. Maybe this is where I'm confused?

Tomas L

The volume flow is always greater on the exhaust side since the temperature is much higher.
In order to spin the compressor there is a certain power demand on the turbine. When the volume flow through the compressor increases the power demand also increases. The things that produce this power in the turbine are volume flow and pressure drop over the turbine. How much power the turbine produces from a certain flow and pressure drop is dependant on the turbine efficiency. A smaller turbine will have worse efficiency at higher flow rates. In order to produce sufficient power to the compressor this means that the backpressure, which is regulated by the wastegate trying to maintain a boost level, increases.
I also belive that a larger compressor, which is more efficient at higer flow rates, will consume less power and therefore keep the turbine within it's efficient range longer.

As Tommy said, this seems to be a Black art. I have not found any literature on this subject.

Tomas

jimbo1111

Posted by rage:
I was always under the impression that you need much less cfm on the turbine side to spin the compressor side fast enough to make way more cfm on the compressor side... the smaller the turbine housing the lower the cfm required to reach peak turbo rpm speed. Maybe this is where I'm confused?

This is where it gets tricky. A smaller passage in the hotside will create more velocity witch will get the turbine moving faster/sooner. To the point where the turbine will not spin any faster because of flow limitations. Flow limitations created by the housing. Now the same turbine with a larger housing will not spool as quickly because the velocity of the exhaust is reduced by the larger housing but will flow more in volume allowing it to increase speeds with higher flow. Maintaining that speed where it's vital for more HP in the higher rpm's
The added back pressure of the smaller housing is also holding back exhaust witch is restricting flow out of the motor in the upper rpms.

Pauerman

Are the KKK replica #8 turbine housings with whatever common trim wheel any more or less efficient/restrictive compared to the original design?