Re: Turbo & Boost Question
[Re: Michael Lee]
#331107
May 21, 2010 10:27 pm UTC
May 21, 2010 10:27 pm UTC
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Joined: Jul 2007
Posts: 828 Mississauga, ON
Ju Chen
Serious Member
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Serious Member
Joined: Jul 2007
Posts: 828
Mississauga, ON
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air temperature = 70F, density at 20psi = 0.177 lbs per cubic foot
air temperature = 150F, density at 20psi = 0.154 lbs per cubic foot
air temperature = 200F, density at 20psi = 0.142 lbs per cubic foot
air temperature = 400F, density at 20psi = 0.109 lbs per cubic foot
The higher the boost (pressure in psi) the hotter the air will be exiting the turbo. Quantity of air is determined (ignoring flow in CFM for now) by it's density which is directly related to temperature. A larger turbo can produce compressed air at a lower temperature than a smaller one.
Here is an example. You take a 16g turbo and set it at 10psi. Dyno the car from 5000rpm to 6000rpm. You then take the FP3065 and do the same. I bet the power difference is negligible. That's because they're both well within their efficiency range.
Now do it again at 25psi. At this point, the 16g's air will be MUCH hotter than the FP3065, so at the same boost, the air going in will be much denser for the FP3065, thus netting higher HP and more MASS flow of air.
Of course there is more to it than that. Engine RPM will also determine the mass air flow the turbo is required to output along with other factors. My example is to show how much temperature matters, otherwise there will be no use for a large FMIC.
As for the boost gauge issue. Mechanical boost gauges are what's called a Bourdon Tube gauge. Fluctuations/oscillations will only net an average reading which is good enough.
Last edited by Ju Chen; May 21, 2010 10:28 pm UTC.
'02 BMW ///M3 '95 TSi AWD - Sold
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Re: Turbo & Boost Question
[Re: Ju Chen]
#331124
May 22, 2010 04:17 am UTC
May 22, 2010 04:17 am UTC
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Joined: Jul 2008
Posts: 61 Mississauga, Ontario
Rob Martin
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Regular Member
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Mississauga, Ontario
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Ok so given that there may be a difference in temperature due to different efficiencies, I still doubt that the change in temperature due to a slight difference in efficiency would make a huge difference compared to the overall temperature change caused by actually compressing the air. So I think that the two turbos, both working around 70% efficiency will have similar outlet temperatures. I could be wrong about that though. There are more fundamental differences in the flow with the two different compressors. For example if you take a look at the flow map for the 14B http://www.stealth316.com/images/td05-14b-raw.gif compared to the 20G http://lovehorsepower.com/MR2_Docs/compressor_flow_maps.htm (first map) you can see that the 20G can flow up to 640CFM at a pressure ratio of 2, while the 14B can flow only around 420CFM (0.2m^3/s) and I believe these are actually "corrected" airflows so they are actually mass flows, with density taken into account. Since we know that the impeller on the 20g is larger diameter than the 14B, that means that if both are spinning at the same speed. The outer tips of the 20g will be moving faster, so the air coming off of them will be moving faster as well. The compressor is able to increase the pressure by slowing down the air in the casing. The only thing I can assume then is that in the 20g, the air is coming off much faster, which allows it to be slowed down slightly to increase the pressure by 15psi say. Meanwhile on the 14B the flow would have to be slowed down by the same amount, which then has it end up at a lower velocity, and therefore lower flow rate. Of course this whole scenario assumes that they're rotating at the same speed, which could be totally incorrect what with the compressor being powered off the exhaust...
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Re: Turbo & Boost Question
[Re: Rob Martin]
#331125
May 22, 2010 04:37 am UTC
May 22, 2010 04:37 am UTC
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Joined: Jul 2008
Posts: 61 Mississauga, Ontario
Rob Martin
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Mississauga, Ontario
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Ok since I didn't want to talk out of my ass too much i decided to do some quick calculations on the temperature rise.
First of all, there will be an inherent temperature rise from increasing the pressure. This temperature rise is dependant solely on the pressure ratio, P2/P1 which for my example will be 2, corresponding to a manifold pressure of 15psi. This will determine the temperature ratio. So regardless of what kind of compressor you are using, the temperature will increase by this amount.
Assuming air comes in at 25C, or 298K the air will exit at 363K or 90C. Now if we take efficiencies into account we'll have to look at the power that is imparted into the fluid. This is simplified as pressure rise times flow rate. Using a flow rate of 0.2m^3/s and a pressure rise of 15psi (103kpa) I calculated power to be 20.7kW into the fluid. At this example, the 14B is running at 60% efficiency, and the 20G is running at 77% efficiency. This means that there will be 6.2kW of heat added by the 20g and 13.8kw added by the 14B. Dividing by specific heat (1.005kJ/kgK) and mass flow rate (0.2m^3/s*1.2kg/m^3) we get that the temperature rise for the 20g is 25.6C and the temperature rise for the 14B is 57C.
So perhaps i wasn't exactly correct in my assumption that temperature wasn't that different. However the final temperatures for the two compressors are 115C and 147C. Which corresponds to densities of 0.909kg/m^3 and 0.835kg/m^3 and these values are not that far off when it comes down to it.
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Re: Turbo & Boost Question
[Re: Rob Martin]
#331137
May 22, 2010 02:23 pm UTC
May 22, 2010 02:23 pm UTC
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Joined: May 2000
Posts: 2,637 NiagaraFalls, ON
Paul Bratina
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NiagaraFalls, ON
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But I guess without putting pressure and temperature sensors throughout the intake, it would be impossible to tell.
I think that's the bottom line. Unless pressures, temperatures, etc are known fairly precisely and over very short time frames, in many different areas of the intake, etc, it's largely guesswork. It's probably why most engine builders, etc would probably just rely on observed results and not worry a great deal on the entire explanation in all it's complicated minutia. 'Course it does still make for an interesting discussion. Certainly there's no harm (or a waste of time) in attempting to understand the physics behind it all.
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Re: Turbo & Boost Question
[Re: Paul Bratina]
#331147
May 22, 2010 05:51 pm UTC
May 22, 2010 05:51 pm UTC
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Joined: Jul 2008
Posts: 61 Mississauga, Ontario
Rob Martin
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Posts: 61
Mississauga, Ontario
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It's probably why most engine builders, etc would probably just rely on observed results and not worry a great deal on the entire explanation in all it's complicated minutia. 'Course it does still make for an interesting discussion. Certainly there's no harm (or a waste of time) in attempting to understand the physics behind it all. Yeah, I guess that's mostly the way everything works. Test it in real life don't worry about the theory, usually theory isn't entirely accurate anyway... But thinking about this stuff is all super interesting to me. Attempting to apply my schooling to something interesting. Although I had to laugh at myself for being home doing math for entertainment on a Friday night...
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