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(Post Link) post #1 of 9 Old 18-03-12 Thread Starter
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Calculating ve ?

Spent all morning going through search results (on phone takes ages)but can't find how to calculate post, I think it was in zbrsk post.
I found I need rpm/ maf / map / iat,
I have calculated I have 117cid
Would like to compare original jtdm turbo ve and hybrid turbo ve.
Thanks in advance.
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No magic in it - get iat, map, air humidity, altitude, cylinder displacement, calculate maximum theoretical airflow and compare with logged maf value at given rpm.

Engine VE values for 8V engine is here ... https://www.alfaowner.com/Forum/tunin...ml#post4431424
On 16V it was from 96 to 94% in that zbrsk post, if I remember right.

Last edited by yan.ko; 18-03-12 at 14:03.
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o.k. back on lap top now found the calculation example
quote from luis
I "took" that information doing the proper calculations!

All it takes is the MAF vs RPM value, using the IAT and MAP values as well.

I'm trying to find a 4 year old log to re-make those calculations so I can have real numbers, but I don't seem to find it in this PC ...

Ok, here it is ... @4078rpms (260bhp), MAP=1.98, MAF=25.35, IAT=65C

@4078, with 100% of VE the engine would take 1912cm3/2*4078 = 3898568cm3 = 3.9m3 of air; PV=nRT; so, 101325 (atm pressure) * (1+1.98) * 3.9 = m * 288 * (65+273) ; m = 1177599.15/288/338 ; m = 12.1Kg/min ; m=26.88lb/min. In other words, @4078rpms, 1.98bar (relative pressure) of boost, 65C of intake pressure, a 1912cm3 engine would "suck up" 26.88lb/min. As we know the engine "sucks" 25.38lb/min of air, the calculated VE should be 25.35/26.88=94.3% ...

VE = 94.3% @ 4078rpms ... quite impressive figures aren't they?, either that, or one of the input variables is wrong ... not to sure if our MAF sensors can read reliably at these amount of flow rates, but if the 25.35lb/min are correct, so should the 94% be ...

Regards,

Luis

dont know if its just me but i cant see whats going on in this equation some numbers just appear and some dont add up?

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Yes this is about right... my measurements are a few percent less... but still over 90% at 4000rpm.

When you have the data you can play with this:
Not2Fast: Turbo Calculator
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if i give you these logs
log rpm maf map iat
1749 smic 3178 1206 2536 42.16
1749 fmic 3090 1241 2416 26.56

hybrid 3117 1190 2420 15.96
hybrid 3350 1230 2270 15.56

can you show one example being worked out, i can work other values then but luis formula has too many numbers not explained?
also is volume 1912cc or 1910cc (82.0 bore x 90.4 stroke =1910)
thanks again
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Don't confuse turbocharger and engine efficiency. That calculation above is engine volumetric efficiency. Turbocharger efficiency is given by turbocharger rpm and iat at the same boost pressure.
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Hmmm, now I see what sussexa would like to do...

But you cant call that VE, VE is volumetric efficiency and that is calculated for the engine, and that does not include any information about turbocharger.
Calculating turbocharger efficiency is much harder job. You will not be able to do it with the data that you logged from the ECU. ECU does not cover all the needed sensors for that calculation.

Rough comparison can be made by comparing intake temperatures (if you keep all other parameters the same, ambient temp and humidity, boost pressure, ...) temperatures will be lower if turbo efficiency is higher...
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sort of understand but is the turbocharger not part of the engine inlet system and therefore will directly affect breathing/ ve ? increase of exhaust bottleneck and inlet bottleneck should surely raise ve, just as the extra 2 valves in the 16v do.
so a jtdm with std turbo will have same engine ve as when fitted with gtb2260?
basically wanted to work out at what map pressure efficiency peaks or the sweet spot for my set up which will help when remapping, which should give max mpg / hp.
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Benefit of bigger compressor wheel comes by lower turbocharger rpms and thus lower boost temperature and pressure at the same airflow (you know colder air has higher density).

And because you put that new turbocharger boost pressure in the 100%VE calculation formula, it will not show you the VE increase over the older turbocharger. You can just cross compare them.

Btw. that PV=nRT is equation of state of a hypothetical ideal gas, where n is number of moles and R universal, gas constant (product of Boltzmann's a Avogadro's constants).

In my VE calculations I usually take air density for given altitude, temperature, humidity and base pressure from chemistry/physic tables (its calculation is beyond this post) and do not use this equation. You don't even need to mess with engine rpm for VE calculation as ECU value comes already in per stroke amount.

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Last edited by yan.ko; 19-03-12 at 09:29.
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