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Turbo boost discussion

4K views 43 replies 13 participants last post by  Unobtainium 
#1 · (Edited)
I was discussing turbos/power and boost today with a mate and he made the following statement:

"boost pressure measured at the inlet manifold, regardless of turbo, gives the same power"

For example, all things being equal, if you took a 4g63 engine with an 80 series on, giving 300bhp at 1 bar, and then swapped that turbo and nothing else for a 3586hta, the bigger turbo (at 1 bar inlet manifold pressure) would not produce any more power than the 80 series.

I totally disagree, based on experience...so how can I convince him otherwise?
 
#7 ·
I can't help but think this thread is bit of a wind up, it is from Kenny after all :lol:

Surely everyone knows the difference between volume & pressure :confused:

Time to name and shame :lol::lol:
 
#8 ·
Devils advocate - different turbos both giving 1 bar at a given revs, say 3000 revs. So both giving 1bar at 3k revs, will 1 produce more power than the other at that 3k revs, if so then why??

I get peak power increases due to increased flow, the bigger turbo can flow more volume to maintain higher pressure at higher revs, just as spool can be affected too, but at lower revs when flow isnt an issue on the smaller turbo, assuming both can flow sufficient to maintain 1 bar at 3k revs, will the bigger turbo produce more power?

Stu
 
#9 ·
Devils advocate - different turbos both giving 1 bar at a given revs, say 3000 revs. So both giving 1bar at 3k revs, will 1 produce more power than the other at that 3k revs, if so then why??

I get peak power increases due to increased flow, the bigger turbo can flow more volume to maintain higher pressure at higher revs, just as spool can be affected too, but at lower revs when flow isnt an issue on the smaller turbo, assuming both can flow sufficient to maintain 1 bar at 3k revs, will the bigger turbo produce more power?

Stu
evo 7 germany answered this one..

larger turbo means, within reason, more eficciency, which will mean less air temp at turbo outlet. This means denser air with more oxygen.
Also, larger turbo will, usually, have larger exhaust side, which means less exhaust restriction and backpressure, hence better VE of the engine as a whole..
 
#11 ·
Power is essentially derived from the mass of fuel you burn in the engine. That fuel requires a certain mass of air to be able to generate the most amount of power.

That mass of air is sent to the engine from the turbo.

Using the ideal gas law, mass = (Pressure x Volume) / Temperature

If you have the same pressure, volume and temperature you have the same mass of air regardless of what turbo you are running.

So provided that the pressure and temperature in the manifold are the same and the volumetric efficiency of the engine hasn't changed then your mate is correct that you will develop the same power for the same manifold pressure.

If the manifold temperature is different (likely as the turbos will be operating at different thermodynamic efficiencies) or the VE changes due to the different turbo then you can get different power outputs.

Andy
 
#13 ·
So provided that the pressure and temperature in the manifold are the same and the volumetric efficiency of the engine hasn't changed then your mate is correct that you will develop the same power for the same manifold pressure.

Andy
Lets say my car does around 500bhp on the rollers with a 3586hta at 1 bar.

I swap the 35 for a standard Evo 5 turbo and do a run on 1 bar.

Pressure is taken from the inlet manifold which is where my map sensor is attached to.

The standard Evo 5 turbo does 500bhp at 1 bar does it?
 
#12 ·
Moving this discussion on a bit - assuming exactly the same engine configuration, but one with an 80 Series and the other with a GT30, at 1bar boost at, say, 5000 rpm, the real world situation is that there would be difference in performance. Is this mostly going to be down to air temperature differences or VE (which I don't profess to understand) or are there other things too?
 
#14 ·
VE, Volumetric eficciency basically means how much air can an engine ingest and exhale during one cycle..

VE depends on varous factors, such as cams, intake, intake ports, valves, but also the restriction in the exhaust, which can be something basic as exhaust size, or something more complex as the size of the turbo attached to it. Of course, VE is not a fixed value, it changes trought the rev range.. for an engine with constant intake pressure, highest VE is at the point of highest tq.
 
#16 ·
My understanding is and I apologise as it’s very basic and don’t have any of the experience that some of you guys have.

The pressure is measured by having a restriction in the pipewor which opposes the flow of air in the system where by the boost pressure measured.

If a smaller turbo is able to create a bar of boost in the pipework producing a certain volume of air and the larger turbo will produce the same pressure all things being equal but the volume of air at that pressure will be larger?

Hence larger volume of air would require more fuel and would produce more power in very simple terms?
 
#20 ·
I have quite clear experience actually. I used to run a TT S214b turbo on my stroker engine, when it was mapped by APT at 1.8 bar it produced 504 bph on their dyno. I've recently gone to a GT3076HTA turbo, which produces 552 bhp at 1.8 bar on the same dyno and the same mapper. Nothing else changed in the engine.
 
#21 ·
but at what revs was the peak power. There are still too many variables in your situation, compared to the theoretical question, but the new turbo is either holding the 1.8 higher up the rev range, or producing less temperature to achieve the same pressure. Also there is the efficiency of the new turbo vs an older turbo

Stu
 
#30 · (Edited)
^^
Plus: and the Engine VE aka Turbo Hotside stays the same ( Same Shaft / Wheel and Housing ) and any component related to Engine VE.

If you run an GT35R with the 0.63 hotside you get better Spool but less Power running high boost (2 bars)
Same Turbo, on the same engine, with the same BOOST but with an 1.06 Hotside makes MORE Power but spools way later....because it flows more air due to less Backpressure.

At lower boost ( maybe 1 BAR ) they will be more close together powerwise because Backpressure would not be an issue....

you cant never compare apples with oranges.... without understanding the Physics behind ANYTHING involved

If you will run an Stock Turbo in an evo with an Airflow meter AND boost sensor, you will see that the Airflow drops ( LOAD ) but the Boost is holding ( i.E. Load shows 270 at 1.7 Bars at 4000 RPM, but at 7000 RPM the load is 230 at 1.7 Bars....)
Using an FP Green instead, Load will show 270 across the whole range at 1.7 Bar.... no drop in Flow ... more Power.
 
#33 ·
Something else I was thinking about how to word on my way home from work, but still not sure how to word it anyway is this...

Modestly sized turbos generate a good boost profile by having a restricted sized turbine inlet. This effectively restricts the mass flow upstream, but creates a faster flow through the turbine to extract the energy better to spin the compressor. Ultimately, that limits the mass flow through the entire compressor>cylinders>turbine system. Good spool, good response, but smothering the top end.
 
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#40 ·
Isn't the point of this thread that saying simply that my Evo is running 1bar boost and doing a lap time of xxxxx of no relevance whatsoever unless you know a lot of other things?
No, the point of this thread is to prove your statement wrong by getting very clever people to contribute to proving you wrong.

If anyone says the above...like i did when I did a 33 at cadwell on 1 bar...it is simply stating the car has a lot more potential in it.
 
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