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Discussion Starter #1
Been shopping around the compressor map world, trying to decide on a new turbo. In my calculations, I pulled a volumetric efficiency of 75% out of my butt for the stock 6M.

Think I'm being conservative? It would be nice to have a VE of 80% or more to broaden my acceptable choices for a compressor wheel.

BTW, I can't access CS.com forums cause it's filtered here in Qatar.

DS
 

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I think you're right that 75% is a bit conservative. My guess is 90% would be a good # to plug in if you're calculating maximum flow for a compressor map at moderate boost, with a turbo significantly bigger than a CT26 and a decent flowing exhaust.

Peak VE (at torque peak) may even be higher than 90%, but peak airflow would be at power peak, where VE starts to fall off a bit.

One reason that it's easy to kind of low-ball the VE # is that when measuring VE in reference to manifold pressure, higher boost pressure yields higher VE. Like, VE at 5000rpm at 12psi will be significantly higher than VE at 5000rpm at 6psi. My intuition was to assume it would be roughly the same across boost levels, but I found out I was totally wrong. The difference was quite substantial moving from 6psi to 10psi

What turbo are you thinking of going with ??

Shiva
 

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Discussion Starter #5
I was thinking of the T4 (O trim?)/T04E 50 trim; I'm still a little undecided about my choice. This is my first time actually sizing turbos, so I hope I'm going about it the right way. I'm basing most of my calculations and graphing technique from Corky Bell's book and this site: vg30et compressor map overlays
 

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RRev Motorsports
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If you have a power curve for the non turbo engine you could get a much better idea of how the VE curve is for the turbo engine. Based on several assumptions:
Density=4.3 E-5 lb/in^3
A/F ratio=12:1
BSFC=0.5 lb/(HP*Hr)
Frictional loss through Transmission=15%
Displacement = 170.87 in^3

VE=[(wheel Power)*(BSFC)*(A/F ratio)] / [(% engine power)*(30)*(RPM)*(Density)*(Displacement)]

VE=[(wheel power)*(0.5)*(12)] / [(.85)*(30)*(RPM)*(4.3E-5)*(170.87)]

VE=[(Wheel Power)*6] / [(RPM)*0.187]

VE=(wheel Power)/(RPM) *(32.09)

So if you have a dyno graph for a non-turbo supra with the wheel power at various engine speeds, you can determine the VE curve of the non-turbo engine, which will give you a better idea of the turbo engines VE
 

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with a compressor as small as a t04e/50 trim, it might be worth looking into a T3 turbine, like a stage 3 or a stage v (t350).

What latteboost described is what I first tried with my VE table. It didn't really work too well. It was off-scale, in addition to having a significantly differently shaped curve. There was like a blip at 3800 then it dipped down then went back up and did some weird stuff. So, if you use it, use it as a very very approximate baseline.

Shiva
 

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Discussion Starter #8
Wow, I've got a lot to learn. What exactly are the BFSC and density figures? Is there some tutorial of further reading on the VE calculations? Luckily I have dynoed N/A and with the CT26-H3 at different boost levels; good reference material.

I'm finally grasping some of these techniques of proper turbo sizing, I just have some details to iron out.

I saw the T3 stage 3 turbines vs. the T4 O trim; there's definitely an abundance of T3/T04 combos out there. I'll most likely getting the T3/T04E-50, unless my future calculations direct me otherwise.
 

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RRev Motorsports
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BSFC is brake specific fuel consumption, it is a measurement of the amount of fuel flow used to make a certain amount of power in (lb of fuel)/(Horsepower Hours). Density is just the density of air at around 75 deg F and 1 atm. Basically your VE curve shape should look identical to your torque curve for a NA engine. So if you have a sudden spike in torque, the VE curve will spike as well. With a turbocharged engine the VE curve begins to change due to pressure differences across the engine (boost pressure and exhaust backpressure caused by the exhaust and turbine). The NA engine VE curve will make a much better starting point that simply assuming a constant VE ( like 90%).
The real difficult part in sizing a turbo for an engine is that most of the time the turbo companies dont provide the turbine map for a giving turbocharger. This makes it so that we dont have an accurate idea of the lug lines for the engine so we have to rely on very crude methods like the ones described in Corkey Bell's book.
 

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cheappower82, I think a t3/t04e 50 trim sounds pretty cool for a mild power 6m! Sounds like you're going to get more out of a turbo 6m than anyone else has.

Latteboost, what you are describing _seems_ like a really good idea. But I don't think it is-- that is exactly what I did and it did not work out nearly as well as I had hoped!! I just want to post this so people know to be extremely cautious and not to assume the curve will be the same when they're setting up their first few VE tables 'cause it could be dangerous (it was for me). cheappower82 is way past this stage already but for everyone else...

The VE curve shape will not look identical to the torque curve for the NA engine, even with manifold pressure at atmospheric pressure (and yeah, it's much different at boost pressures). I thought the shape would be the same, but unfortunately, it did not end up being that way. Adding a turbo with a puny turbine and housing like the CT26 will do it, I guess.

You won't f* anything up using the NA curve as a baseline for your non-boost entries in the VE table, but for boost, I'd really be careful assuming that the curve is the same. The first time I hit it I ran way leaner than I should have and I thought my entries, based off a 6m NA dyno, were conservative. So, I ended up increasing them until I was a bit too rich (misfiring), then I backed it down from there. Just be careful!

Shiva
 

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RRev Motorsports
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Shiva,

It seems to me that we might be speaking about VE for two different applications. I am talking about the VE curve that can be used for matching a turbocharger to an engine to determine if the turbo will be sized properly. It seems to me that you are talking about the VE tables used for engine management computers, correct me if I am wrong. If you are talking about the VE tables for the computer you are absolutely right that the VE will vary immensely based on all of the systems components. I have seen that by simply changing the exhaust manifold, the actual VE curve that could be used for a compute was vastly different from manifold to manifold.
 

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Discussion Starter #12
The VE figures I was referring to is for the airflow calculation used in determining compressor wheel efficiency {(RPM*.5*CID*VE / 1728)*PR}

On a side note, I do need to read the MSnS help pages a bit more; there are some important notes on VE for the tuning side.

Thanks for all the insight; I'm sure others will want to refer to this discussion if they are a newbie to turbo selection. I find that most believe bigger is better (usually my Honda people).
 

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Oops, my bad guys. I was thrown off by BuddyJs post.

But yeah-- just remember that boost helps fill the cylinders. Assume a higher VE than you might otherwise going off an NA dyno. So if you feel like you're hugging the surge line, don't worry too much.

Usually that's the way people go with turbos... bigger rather than smaller. The exhaust wheel should have the largest impact on spooling the turbo, but few people use t3 hot sides on 3.0liter motors, so it's kind of hard to decide whether its worth using something that small.

Shiva
 
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