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How does the HF2 compare with a GT35R ?
Old 07-16-2008, 05:38 PM
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Hobie,
I'm confused too by your data.

Doesn't (V)olume stay constant between the turbo pressure side and the combustion chamber at a given moment in time?

The examples about the tires and straws are good, but I think in this case, the volume is fixed in time.

How quickly does your temp sensor cycle? If it takes 15 seconds to notice a change with big heat applied, that could be the issue. You could easily go between no boost to full spool and back in 15 seconds.

If you put an ice pack on the IC, how fast would that register at the temp sensor?

Doug
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Old 07-16-2008, 06:15 PM
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Originally Posted by DW SD View Post
Hobie,

Doesn't (V)olume stay constant between the turbo pressure side and the combustion chamber at a given moment in time?

The examples about the tires and straws are good, but I think in this case, the volume is fixed in time.
Doug
Doug, yes... that is why my original question: how does bigger turbo generate more hp. And we are talking 25-30% more so a simple 30degF drop in air temp won't cut it

But, if I understand it correctly now... pressure builds when the fixed volume is completely filled. For an engine, it can be: a) while the valves are open (then turbo fills air in chamber then the rest of the intake track), b) when valves are closed (then just the intake track).

I suspect a small turbo will not fill the chamber as well as big ones BEFORE the valves closed. Hence, the peak boost reading really comes AFTER the valves closed.

If not, I am back to asking: how?

BTW - I am certain my IAT sensor is working properly to a good degree & reacting fast enough. For reference - even coming off the track, the output side of my intercooler is stone cold. So, for temp to be stable at 15-20degF higher throughout the run is believable IMHO.
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Old 07-17-2008, 05:40 AM
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Quote:
Originally Posted by NathanUK View Post
if at WOT and say 6K RPM you have 1bar of boost measured at (inside) the inlet manifold and providing the charge intake temperature is the same temperature at (inside) the manifold, then how can one turbo make more power than another? Beats me anyway...
The only other explanation is if the smaller turbo is restricting the exhaust getting out of the engine as the bigger turbo will open up the wastegate more than the smaller turbo which needs all the exhaust it can get to maintain turbo speed and therefore cold side flow.
What about my 'theory'
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
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1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

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Old 07-17-2008, 09:26 AM
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Originally Posted by NathanUK View Post
What about my 'theory'
Nathan, not sure if I understand this completely: "if the smaller turbo is restricting the exhaust getting out of the engine as the bigger turbo will open up the wastegate more than the smaller turbo which needs all the exhaust it can get to maintain turbo speed and therefore cold side flow."

Bigger turbo tends to need more exhaust pressure/velocity to spin up than smaller ones I thought?

If bigger turbo can flow waaayy more air in same amount of time given a static volume to fill, then it will cause more pressure hence opens up the WG more/longer.

But neither directly relates to how more air actually got into the chamber at same pressure & time, unless (as I "discover" now - not sure if its right) that the small turbo didn't really fill the chamber as full as the big one before the valve closes.
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Old 07-17-2008, 11:56 AM
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I had my materials-scientist-PhD-friend and accredited tuner explain this to me again last night, and I showed him this thread.
A "turbo" is a centrifugal compressor, and to compress the air it has to accelerate the air by spinning it with the wheel. A selected turbo is chosen by its efficiency range, which is the amount of airflow (CFM) and pressure (bar, psi) this compressor works best. A turbo too big for the required application will take too long to spin up or not spin up at all, and a turbo too small will spin beyond it's ideal flow at higher engine rpms. When a centrifugal compressor is spun beyond this flow range it can continue to flow more air, but its heat output begins to dramatically negate the actual DENSITY of the product.
A bigger compressor wheel will flow more air density (at X psi) with less heat because it's having to accelerate that air less, and less violently. Partly because the wheel is spinning more slowly and partly because the housing collector is bigger.
Lastly, larger compressors are usually paired with larger hot sides. Larger hot sides usually flow exhaust gasses with correspondingly lower TIP (turbine inlet pressure) to MAP. This results in lower cylinder pressure, lower cylinder heat, all sorts of other good stuffs.
The fact remains, though our intercoolers can somewhat counteract the high heat production of a small compressor, a larger compressor will make more horsepower almost exclusively because of lower output temp for given CFM.

The one fact remains though, a slow-spooling turbo is a b*tch to drive, so we scale our turbo somewhere between ultimate WOT/redline efficiency and drivability.
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Old 07-17-2008, 12:33 PM
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Originally Posted by hobieboy View Post
Nathan, not sure if I understand this completely: "if the smaller turbo is restricting the exhaust getting out of the engine as the bigger turbo will open up the wastegate more than the smaller turbo which needs all the exhaust it can get to maintain turbo speed and therefore cold side flow."

Bigger turbo tends to need more exhaust pressure/velocity to spin up than smaller ones I thought?

If bigger turbo can flow waaayy more air in same amount of time given a static volume to fill, then it will cause more pressure hence opens up the WG more/longer.

But neither directly relates to how more air actually got into the chamber at same pressure & time, unless (as I "discover" now - not sure if its right) that the small turbo didn't really fill the chamber as full as the big one before the valve closes.
You quote in red was my point. If the WG is open then the turbos hot side is not strangling the engines out flow?
Unlike the smaller turbo which will have WG tightly shut.

This comes back to the air pump, getting air in & out of the engine = power.
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg

Last edited by NathanUK; 07-17-2008 at 01:16 PM..
Old 07-17-2008, 01:13 PM
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Quote:
Originally Posted by NathanUK View Post
You quote in red was my point. If the WG is open then the turbos hot side is not strangling the engines out flow?
Unlike the smaller turbo which will have WG tightly shut.

This comes back to the air pump, getting air in & out of the engine = power.
Xactly! You can do a few things with a Turbo change.

1) Get a More efficient Turbo that is a newer design and spools quicker and more linear, maybe more HP or not. This by itself may not be a big TQ or HP producer, but may make the car more tractable by itself.

2) Get a Turbo that moves your RPM band around with changing the Hot and cold housings characteristics. Give up low end for high end or vice-versa.

3) Or Bolt on a Turbo to suit current or planned mods to make a high Torque- High HP motor. This is the "Air Pump" principle. If you want more TQ and HP, it really needs to work with other mods that make the motor a better "air-pump" and the turbo as a facilitator, given equivalent boost levels when comparing before and after scenarios.

If your Turbo over feeds your air pump, you get more wastegate dump. If your Turbo under feeds your air pump you get less or no wastegate dump and may not make the required boost level.

If your Turbo can't make the required Flow and boost, that's a scenario where a Turbo change could get a Big HP and TQ increase.

A Turbo change can make a hell of a difference if you are using it to make lots more boost(That your current Turbo can't make), or are making lots of mods in reference to how your motor moves air in and out.

I think the thing that starts these type of threads are when you go to a PowerHaus page or whatever and you see the Phase I-IV kits and they make reference to how much HP the Different Turbo's make at each phase. In Isolation those Turbo's can't do that HP by themselves. They work with the CAM, Valves, Manifolds etc.
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Old 07-18-2008, 10:50 AM
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Two turbochargers delivering same amount of boost with same discharge temperature will move same mass of air.

But a smaller turbocharger out of it's efficiency island won't be able to compress air to same pressure as efficiently as the big one, thus discharge temperature will be higher. PV=nRT thus mass of air delivered by small turbocharger will be less.

OK you say, just attach the intercooler and bring discharge temperature to same? Nope...if you run 120 deg. C air to intercooler and then ask it to lower it to 50 deg. C you will introduce big pressure drop trough intercooler. You will have to run much more boost prior interooler than post intercooler, pushing the turbo even further out of efficiency range.

And last and most important: a small turbo trying to push 1.3 bar, heating it up a lot and then letting the intercooler try to lower the temp (and lowering pressure to 1.0 bar in the end, due to ideal gas law) will introduce immense exhaust backpressure. That backpressure will be lower on big turbo. Engine is just an air pump and letting it work against exhaust pressure will lower it's output.


So to round it off:

Boost != power.

Power is produced by moving highest mass flow trough engine. To move high mass of air you need low backpressure. You also need low air temperature.

Small turbo means high discharge temperature and high exhaust backpressure = low mass flow. (Plus additional strain on engine, hot heads and lot's of fuel wasted just so you can heat up the air and then cool it with intercooler).

Big turbo means lower discharge temperature and low exhaust backpressure = high mass flow.

So boost is by no means a measure of power. It's just a funny figure that doesn't mean much alone. I have a four cylinder track car that produces more power @ 0.5 bar that same car (with same turbo) produces at 0.9 bar. How? Well the usual hard way: hotter cams, less restrictive exhaust, more ignition advance, ported head etc.

Remember, BMW M3 E36 produces 326hp from 3.3L with zero boost


Regards,
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Old 07-19-2008, 04:01 AM
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Thanks for clarifying this Goran. That is exactly how I understood it.
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1981 UK 930. G50/01 shortened, 964 3.8RS Fibreglass Body Kit, 18" Alloys 8.5" F & 10" R, 225's F & 285's R, Special Colour Metallic Blue Paint, FIA Sparco Evo's, A/C and Air Pump removed, Electronic Boost Controller, GHL Headers, Tial46 WG.
Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg
Old 07-19-2008, 04:42 AM
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Look at the exhaust pressure (exhaust manifold) to generate a given boost pressure (intake manifold) at a given engine speed and WOT. The best hardware match for that given condition is one that produces the lowest exhaust pressure. This will take in all the considerations discussed the past three pages.
The answer.....it depends on your goals from the start.
The highest performing turbo(s) match would be undesirable to 90% of the street car drivers. The remaining 10% go for HP videos and top end.

Last edited by copbait73; 07-21-2008 at 11:42 AM..
Old 07-21-2008, 11:19 AM
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Inlet temp staying the same on boost and off boost doesn't sound possible. I know it's not the case on my engine.

With EFI, I'd say my car is more responsive with a HF2 and full bay IC than it was with my old 7200 and B&B IC.
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Old 07-21-2008, 11:59 AM
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Goran, thanks for the great explanation as usual... It really helped me to understand how a bigger turbo is better at producing more hp than a smaller one with a highly efficient

thanks!
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Old 07-21-2008, 06:52 PM
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Goran, thanks for the great explanation as usual... It really helped me to understand how a bigger turbo is better at producing more hp than a smaller one with a highly efficient

thanks!
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Old 07-21-2008, 06:52 PM
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