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NESpeed -> RE: A Turbo MAP or graph (3/31/2006 6:23:16 PM)
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"Too often I've heard the 'bigger is better no matter what' philosophy around here, and I used to think along these lines until I was painfully corrected by a few people who know much more about turbos than I probably will ever know (and I've read a good portion of the literature out there on turbos, have read Corky Bell's book a couple times cover-to-cover)...
I'd like to suggest that bigger turbos will make more power at the same pressure due to a higher flow rating of the turbo, but only to a degree. And, that on the same engine, it is very possible that at the same peak boost level, a smaller turbo will created more area under the power curve on a dyno, and therefore give a better-performing car overall...
If you have an engine that flows 450 cfm at 15 psi (iirc around what a 16T flows?), then if you're boosting within the main "island" with a smaller turbo, i.e. 14-15 psi with a 16T (does someone know where some mitsu boost maps are so I can verify this?), upgrading the turbo to something bigger would be a pointless exercise...
If the turbo is flowing 200 cfm more than the engine can flow, where does that air go in a closed system?
Yes, heat is created by the friction generated by the turbo spinning, so as rpm's of a turbo go higher, there will be more heat created in general, which lowers the thermal efficiency of a turbo... However I think that thermal efficiency is secondary to several other factors (unless you're running way outside of ideal boost levels, i.e. superheating the intake charge).
Let's run through some numbers for choosing a turbo for an 850... We'll assume a few variables such as volumetric efficiency (90%? ) and barometric pressure (29in hg).
First, we'll get a flow rating for the engine at 0 boost and redline rpm's (I know the 850's probably don't have a VE of 90% here, but bear with me).
2.3*61 = 140.3 ci
convert to cubic feet:
140.3/1728 = ~.0812 cubic feet
Multiply this by rpm / 2:
.0812 x (6000/2) = 243.6 cfm
Factor in VE:
243.6 x .9 = 219.24 cfm at 6000 rpms and 90% VE
There's your baseline engine flow rating... Now we'll find the engine's apx flow rating under a couple different boost levels.
first we need the pressure ratios at a couple different boost levels... (the 14.7 is atmospheric pressure..):
12 psi:
(14.7 + 12)/14.7 = 1.82
15 psi:
(14.7 + 15)/14.7 = 2.02
So now with these pressure ratios, we can find the flow rate of the engine at those boost pressures (notice that this doesn't change from turbo to turbo):
219.24 x 1.82 = 399.02 cfm
219.24 x 2.02 = 442.86 cfm
To finalize the calculations, we really need the compressor flow maps for the two turbos we are comparing... And I can't find a map of a 15G or 16T, the turbos we need to disprove to justify upgrading... Anyone have these?
But here's a similar situation posted a while back at rice cop by SemiHemi, and links to the two compressor maps needed to really observe the point he's making:
[quote:c727634e97]
We have a 2.0L Eagle engine. First we convert the displacement to cubic feet.
2*61 = 122ci
122ci/1728 = 0.0706 Cubic Feet
Next we calculate the ideal volume flow through that engine at it's ideal maximum speed, say 6500rpm (I don't know where it's powerband is really, this is an example).
0.0706*(6500/2) = 229.45 CFM
That would be ideal, except that an engine is not perfect. You must factor in it's volumetric efficiency, which we'll just say is 90%, for example.
229.45*.9 = 206.505 CFM
Next we convert our ideal intake psi to in. Hg, by multiplying by 2.03. And calculate the pressure ratio.
((15*2.03)+29)/29 = 2.05 pressure ratio
We then figure for the ideal temperature rise. Pulling it straight from the Y tables and multiplying it by the ambient temp in degrees Rankine(530). Then divide by the compressor's efficiency(74% for the 14b) to get the actual temperature rise.
0.2252*530 = 119.356F
119.356/0.74 = 161.291F Temp rise
161.291+70 = 231.291F actual intake air temp after compressor
We then use this to find the acutal air density ratio between the intake and outlet. Figured in degrees Rankine.
(530/(231.291+460))*(((15*2.03)+29)/29) = 1.571 density ratio
Then we use the density ratio to figure the compressor inlet flow.
206.505cfm*1.571 = 324.4 cfm
On the compressor maps follow the 325 cfm mark up to where it meets the 2.0 mark. Notice that at that pressure ratio both compressors are in their "sweet spot". Also note that the 14b has a 74% efficiency there while the 16G only has a 71% efficiency. On an engine operating at that boost level, rpm, and volumetric efficiency the 14b will make more actual power then the 16G - because it will be slightly more efficient, giving it a slightly higher air density. Where the flow advantage would come in on a 16G is if you also modified the engine at the same time. Let's say that you got a better valvetrain and rotating assembly that let you rev it 2000rpm more, and that also increased your VE to 95%. Perhaps at that point your engine now flows 500 cfm(example only) at the same boost level(15psi). The 14b can't flow 500 cfm at 15psi, you would hit the turbo's "choke point" at 430cfm and would not be able to make more power after that point. Upgrading to a 16G would allow your engine to continue to make power up to it's new 500cfm max flow rate"
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