i would have said the opposite.
Low inertia will help with transient response (i.e. once you are above the boost threshold point) but it shouldn't really effect boost threshold rpm too much

 

A high backpressure/ low inertia wheel will decelerate quicker than a high inertia/low back pressure wheel which will maintain higher shaft speeds in the face of high back pressure.

We've had this discussion before Mike, haven't we? I figured that you would bite at this one.

 

I think you may be forgetting the compressor wheel and the effect of airflow dynamics with it when trying to determine which will maintain spool better. Yes, more inertia while maintain a higher shaft speed for a longer period but the turbo is going to be out of it's efficiency range almost as soon as you stop feeding the turbine side. The turbine will no longer be "powering the shaft" however the compressor is still trying to draw air in and compress it into the UICP, regardless of how fast the BOV opens there will still be some initial slowing caused by back pressure in the charge piping as well. The EFR turbo has an advantage there in that the integrated BOV is used to help maintain shaft speed by reducing the effects of the "draw" at the compressor wheel.

Josh

 

You are right. I always tend to focus on reducing exhaust backpressure via larger turbine wheels, for any given compressor wheel design. So, I was limiting my comments strictly to hotside flow dynamics for a given compressor wheel, independently of the compressor's dynamic envelope. Thus, I was thinking exclusively of the hotside of the rotating assembly.

Let me give you my personal preference. Once I find a compressor wheel that flows the amount of air that I might desire...then I prefer to mate it to a high inertia/low backpressure turbine wheel, as opposed to a low mass, high back pressure wheel. This is just my personal preference. I am not saying that it is the best option for anyone else.

For example, the HTA Green has a state of the art compressor wheel design. But, if I were to order an HTA Green for my own purpose, I would prefer it to be configured with the larger turbine wheel from an FP Red/Black.

Of course, I don't think that FP would configure a Green in that particular way. But, that is how I would prefer a Green to be configured. It would exhibit reduced exhaust backpressure eventhough it would increase turbine inertia.

So, I am happy with my FP Red as it has a higher inertia/lower back pressure turbine wheel than an HTA Green. The Red shares the relatively large turbine wheel from the FP Black. Thus, in a sense the Red has a relatively lower backpressure exhaust side than a Green. I just wish that FP would pair the Red's larger turbine wheel with the Green's smaller/lower mass compressor wheel.

 

Wouldn't this only be the case if you were running the exact same power and with the same boost?
And if thats the case, then that wouldn't happen because of the different flow characteristics.
I understand what you're saying though.
Like a 3076 vs a 3071.
Or HTA3582 vs a HTA3586.

After driving a Titanium ally turbo (Evo7 RS turbo) over a stock evo 6 turbo, i would take low inertia all day every day...

 

I am with you as i drove around with my Ti/Al JDM VIII RS turbo with a 9.8 hs and then I fitted the 10.5 cm hotside. This must have been pretty much the same turbo as the VII JDM RS turbo to which you are refering. With advanced cam timing(+2/0) this is pretty instant spoolup.

 

Yeah, thanks. This is kinda what I mean. I realize that there is a tradeoff but it is a personal choice. I would prefer the 3076 over the 3071 anyday. The 86 is my dream turbo and, yes I would prefer it over the 82. Thanks for taking the time to understand an old geezer.

 

ok now you've lost me lol.

you said you want the smaller compressor green on the red/black exhaust wheel.

but then you said you would have a 3076 over a 3071... thats a larger compressor wheel!

 

Some of this turbine sizing theorising sounds nice but it can backfire so to speak in the real world .

I had a Garrett GT2554R modified once because I though the 76 trim NS111 turbine from a GT2860RS would work better by producing less restriction on the hot side . Garrett do actually make a version of the GT2554R with a GT28 turbine (GT2854R) only they use a 62 trim version of the GT28 turbine not the 76 trim one like I had fitted .
Needless to say the lack of "trapping" efficiency of the turbine ment the compressor didn't spin up fast enough early enough so it really was a failed concept .
You can create a situation where the turbos rotating group speed is never really high enough compared to the engines speed so yes while you get low hot side restriction you don't generate enough airflow to make good torque .

I think it is possible to have a win with reasonably big diameter turbines provided the turbine trim is small enough to force the speed of the rotating group up so that you generate sufficient air capacity to give adequate boost pressure .

I'm not sure why some of you think that a turbine has to be either "high innertia" - heavy and low "back pressure" or vice versa .
For example an identical turbine made from Inconnel or Titanium Alloy shouldn't make any difference "back pressure" wise , I actually prefer the term turbine inlet pressure here because thats what it is .

The basic facts are that the turbine has to generate sufficient shaft torque to accelerate a given compressor fast enough to make a head of pressure into an expanding volume ie the accelerating engine . As the engine speed rises the compressor speed has to as well otherwise the inlet manifold pressure will drop .

Petrol performance turbo engines are hard to develop because its not like your boosting a truck engine pulling a heavy load where neither the truck or its engine is accelerating but the extra torque allows it to pull higher gears and maintain speed in the hills .

Personally I like the idea of lightweight internals like the TiAl turbine in a 10.5 cm turbine housing and the std 16G6 compressor wheel .
The turbines mass is a bit less and the compressors pumping capacity pretty well matched to a 4G63 so winding up the twin scroll turbo is not hard . I think you just have to accept that you need to pump the boost up with units like this to make strong early torque and it will fall off higher in the engines rev range . Rotary turbines and compressors don't have linear performance which is why turbos don't give linear engine performance . Imo the ways to make an engine have closer to linear performance is to have reasonably high CR / twin scroll manifolds and T housings and also try hard to find a port shrouded compressor housing . Low friction ball bearings would be the icing on the cake .
My gut feeling is that if you cobbled together a 10.5cm unit with the light turbine and a port shrouded early size compressor housing the response would be pretty good .
One of my thoeries is that if turbine/boost threshold performance is no 1 priority you could probably forsake the 9 type cartridge and larger diffuser diameter compressor housing . It may be ok on a MiVec engine where less low rev valve overlap helps with cylinder trapping efficiency but not all engines have it and so having the "MiVec compressor housing" may be a backwards step on earlier engines .
Making a port shrouded compressor housing snout for the 4-8 housing isn't really rocket science , it just involves welding a larger diameter thick walled tube on the front and milling/slotting to suit .

I'd like to think that firms like FP/CBRD etc were developing a generic replacement ball bearing cartridge that could take a few variations of the Mitsubishi TD05 and TD06 turbine families . Doing the compressor sides is a walk in the park and then provided they have provision for the OE turbine housing style and plumbing fittings it all has bolt up potential .

For me the ultimate toss up is an 8MR/TiAl/10.5/BB/Port shrouded comp hsg unit or a 9 based unit with all the same features . And if it STILL boosted a bit early maybe the 10.8cm TS turbine housing that GT Pumps sells here in Oz .

Cheers A .