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Discussion Starter · #1 ·
is 1bar of boost from stock turbos *exactly* the same as 1bar of boost from a pair of twin T78 or some other mammoth setup? Or does one have to factor in turbo type when comparing boost levels from car to car?
 

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Discussion Starter · #3 ·
two thoughts:

where is the boost pressure measured? if it's a fixed volume area like the intake plenum, then more air would equal higher pressures, and conversely, equal pressures would mean equal air (given the same temperature of course).

if indeed 1.2bar on GT-SS is completely different from 1.2bar on a single T88, then is there an equivalency chart, like 1.4bar on N1s equals .8bar on 2530s, or something like that?
 

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Pressure is the result of restricted flow.

In a Turbo charged 4 stroke, this includes the restriction from the exhaust housings in the turbine section of a Turbo. Therefore small restrictive housings from small turbo's will show a higher 'boost' pressure at a lower airflow volume.

So no boost is not boost.

There is no conversion charts, because it will vary according to engine design, cam timing, etc.
 

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The only difference is that bigger turbo's can deliver the 1 bar more efficiently and so delivering 1 bar of cooler air. And can make a little bit more power. I don't know how efficient the stock turbo's are @ 1bar but the difference wouldn't be too big at that pressure.

When the manifold is filled with 1 bar off pressure it shouldn't matter which turbo deliverd that pressure, but how effiecient the turbo(s) deliverd that pressure. The efficiency of the turbo makes the charge temperature hotter or less hot.

If you improve the inlet chanel, like piping, filters, and do a port job youb can make more power with the same boost pressure. That's because more air actualy gets into the cilinders.
 

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The only difference is that bigger turbo's can deliver the 1 bar more efficiently and so delivering 1 bar of cooler air. And can make a little bit more power. I don't know how efficient the stock turbo's are @ 1bar but the difference wouldn't be too big at that pressure.

When the manifold is filled with 1 bar off pressure it shouldn't matter which turbo deliverd that pressure, but how effiecient the turbo(s) deliverd that pressure. The efficiency of the turbo makes the charge temperature hotter or less hot.

If you improve the inlet chanel, like piping, filters, and do a port job youb can make more power with the same boost pressure. That's because more air actualy gets into the cilinders.
If you have small turbine housings, you will have more pressure in the exhaust manifold, which will require more mechanical effort from the crank to expel the ex. gas = lower crank hp.
Also, because the residual pressure in each cylinder will be higher when the ex. manifold pressure is higher, there will be less pressure differential between the inlet and the cylinder residual pressure. That means less airflow and less power.

It's not just about turbo map efficiencies. A small turbo may be in a higher eff island at 1 bar than a big monster turbo at 1 bar.

The key factor is total pressure differential across the engine, and a bigger Turbo will have a lower ex manifold pressure, therefore more pressure differential.
 

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Discussion Starter · #10 ·
It is all about volume flow, for the same pressure a larger turbo will flow more air.
so in essence, the volume of air moving from turbos to intake is the main factor, not the pressure of the air itself. And a bigger pressure differential between the exhaust manifold and intake will result in more air volume flow at a given pressure? ah, I feel more enlightened :)
 

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For example, modern Indy cars run/ran something silly like 2psi boost due to that being the rule limit, and still get 900bhp+ from 2.6litres or whatever they are these days.

Pressure means nothing in terms of power unless you know the rest of the situation.
 

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It is all about volume flow, for the same pressure a larger turbo will flow more air.
The restriction in the intake chanel, and the volume efficiency of the engine, and the engine RPM determine the volume flow of air trough the intake chanel.

To build up pressure you must exeed the flow of the engine,
and for a certain boost pressure you need a certain air flow to reach that pressure. Lets say to reach 1 bar there's 35LB/MIN needed, then would it matter what turbo deleiverd that flow? If it was just the flow I think not.
If its about power then yes, couse the efficiency is the difference between the compressors. Of course there are things like turbine housing that are restricting power at the top end, but lets leave that aside for now. But if you compair the different turbo's at the same pressure they will have a different RPM and a different efficiency.


You can see that in a compressor map:

look at 35LB/MIN PR 2
GT42 compressor map


GT2860R-5 compressor map:
( look at 17,5 LB/MIN) PR 2
 

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Discussion Starter · #13 ·
therefore, when Skyline owners talk about how much boost they're running, it can't really be comparative unless both cars have the same turbos, similar intakes, etc?
 

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therefore, when Skyline owners talk about how much boost they're running, it can't really be comparative unless both cars have the same turbos, similar intakes, etc?


think the best answer of the power of the car and how good the engine is, is when the power output says 600bhp @0.8bar instead of 550bhp @ 1.6bar. both cars have larger turbos and better fueling but one is standard engine and one is mega modified internally.
 

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therefore, when Skyline owners talk about how much boost they're running, it can't really be comparative unless both cars have the same turbos, similar intakes, etc?
Its mainly turbos, and head/cam spec. If they wildly different then its a pointless comparison.

The other little things make little differences.
 

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look up 'volumetric efficiency' somewhere(google it??)....... its all about the pressure at the exhaust housing and its knock on effects.


kev
 
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