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CFM Flow Rates for RB26


nocab72

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Can't you just assume a certian VE. Use thoes and your known displacement and max RPM and you can caluclate air flow rate for a given boost pressure. This boost pressure (compression ratio on the compressor maps) and flow rate will put you at a certian point on the compressor flow maps.

 

Or better yet you could find the compressor flow maps for your turbos and find a stock dyno chart. Use this and known or assume driveterian loss (will give you engine power). Assume a BSFC. Assuming a certian boost pressure (stock ECU can probably change it slightly) you could work backwards and calculate VE.

 

These methods can be extended across the rpm range to make sure your not just sizing the turbo for max RPM but so that you don't run too ineficient down or hit compressor surge no-no land down low. Or if your calculating VE to calculate it across the RPM range as it might (will) change due to intake tract, exhaust tract, head and cam flow dynamics (as flow rate increases air speed will also increase, but differently obviously for different orfice sizes, then you reach super sonic and everything reverses, you go turblant and your power starts to drop off).

 

I guess I'm kind of confused. Knowing your displacement, at a given RPM and manifold pressure, assuming a VE, there should be a given CFM (air flow rate). (If I'm remembering right, been a while since I brushed up on my engine/turbo theory, can someone back me up here). There are equations to do all of this.

 

p.s. don't take any of this to heart till I go read up on my Croky Bell.

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Guest stevil

2.6L

157 ci

7500 rpms

10 psi

 

Perfect NA CFM would be 340 CFM ((CI x RPMS)/3456). Using a VE like 90%, it would be less, like a realistic 307 CFM. Now add boost, and factor in various temps... is it 75 degrees out or 40? Using an intercooler? Numbers can vary so much based on temps...

 

If you had an awesome water to air intercooler that cooled the boost way back down to 75 degrees ambient, you would use 516 CFM at 10 psi. (307 CFM x 1.68 density ratio). Looks like about the most CFM possible, or 35.6 lbs/min

 

Other hand, if you had no intercooler at all, at 75 degrees, 10 pounds of boost and assuming 70% turbo efficiency, you would use 420 CFM (307 CFM x 1.37 DR), 28.9 lbs/min.

 

If you had a front mount that was 50% efficient on a 75 degree day, same turbo efficiency, you would use 463 CFM (307 CFM x 1.5 DR), 31.9 lbs/min.

 

For compressor maps, 10 psi would be 1.68 pressure ratio. CFM x .069 = lbs/min. Looks like you want something that flows good around 30-35 lbs/min.

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