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Reading Comp Flow Maps

Guest INASNT

By Guest INASNT,
in Engines & Forced Induction

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Steve Grand Master

Steve

Posted
Posted

Million dollar question:)

Depends on alot of things, capacity of your engine, manifold design, head design, cams, timing etc not to mention turbine AR ratio, wheel trim, exhaust back pressure - and probably a dozen plus other things.

The compressor map only shows what the compressor is capable of doing at certain speeds. As Sydneykid alluded to, careful selection of turbine etc.

This is why I am leaning toward a small turbine housing to start with. The other thing you could do is look at what other people are running, and taking into account all of the above - make an educated guesstamate - then be prepared to change a few things once its on if its not quite what you were expecting or after.

EDIT: why do you think that performance companies like to keep their specs so secret? You are going to be up for a bit of R&D - and the more research, the less likely to need developement!

Warpspeed Experienced

Warpspeed

Posted
Posted

Hi Inasnt.

If you plan to run 1.0 bar boost, looking at that compressor map, that is going to be at about 90,000 compressor RPM.

Now, what you need is an exhaust turbine and housing a/r combination that will spin up the compressor to the required RPM. The tip speed of the exhaust turbine is just about the same as the exhaust gas velocity inside the exhaust housing scroll.

Now comes the compromise part. You can reach the required exhaust velocity by forcing the exhaust through a small exhaust housing, BUT the exhaust back pressure is going to be very high.

So a small a/r is going to boost early, but it will have high back pressure.

Once it reaches 1.0 Bar, the wastegate will open and hold the shaft speed close to that required 90,000 RPM. It will also hold the exhaust back pressure at about the same level required to reach that 90,000 RPM right up to redline.

Or you can run a larger a/r and reach the required exhaust velocity at a higher engine RPM with less exhaust back pressure. The lower back pressure will produce more engine power at the same boost level.

So you get a choice, lots of low RPM boost and reduced top end power, or lots of lag, and huge top end power. Take your pick.

One factor though, if that special exhaust housing you have, turns out to be not what you want, you might be stuffed finding another one with a T3 flange and more suitable a/r.

The cams you run depend mostly on exhaust back pressure. Small cams with small exhaust turbine housing, big cams with big exhaust turbine housing. Try to keep some proportionality between valve overlap and exhaust back pressure for best results.

It really all depends on how peaky you want it to be.

GiJOr33 Experienced

GiJOr33

Posted
Posted

go the 0.84 A/R... cos the feeling of the car when it just keeps pulling hard all the time is awesome!

my old skyline fell off in the top revs and it loses the excitement... my new skyline is AWESOME!!!! and i think i have a large exhaust A/R (not exactly sure) and a 0.70 A/R Compressor.

Much fun to be had!!! :)

Sydneykid Grand Master

Sydneykid

Posted
Posted

Hi INASNT, what you gunna do about the internals? This turbo will produce too much combustion pressure for standard RB25DET internals. You could turn down the boost or limit the shaft RPM wiht a small turbine and cover, but that's a band aid. You would be better off with a turbo that more closely matches the power potential.

Hope that clarifies.

Guest INASNT

Guest INASNT

Posted
Posted

sydneykid!

i dont plan on running more than 1 bar boost with this turbo, and hey if it blows i will put in forged pistons, short penned rods etc. I was goona do that anyway but i thought let it be, and if it blows then change it!

At 1 bar i should be right shouldnt i?

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