will two different turbos set at 1.2 bar produce the same result?

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Perrin21

Member
This question may seem silly to some but its something ive always wondered about. Would the standard turbo set up at 1.2 bar produce the same result as a 2871R set at the same boost pressure? I understand the bigger turbo would take longer to spool up but is the flow rate the same at the same pressure?
 
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PobodY

PobodY

Moderators
Staff member
In short, no. The bigger frame moves a larger volume of air at the same pressure (or conversely, the same volume of air at a lower pressure)

Without meaning to make it too complex:
pV=nRT The pressure x volume = number of mols (just read that as "amount") x the ideal gas constant x the temperature.
If we assume that temperature is constant (and R is already a constant), then p(1)V(1)=p(2)V(2) --> so if you double the volume, the same ammount of gas (air) has half the pressure. Therefore if you keep the pressure the same (e.g. 1.2bar), you cannot possibly have the same amount of air moved by two different sized turbos.

I don't think I did a great job with that, but I hope you can see why it's not always about the boost figure; it's really about the amount of air you move. - Pressure is about squashing more into the volume you've got.
 
Trondelond

Trondelond

Active Member
A bigger turbo will produce more power at the same pressure as a smaller turbo. This seems to be a concept many people struggle with (no offence intended). An example may be the rotary engine, where they run huge turbos at relatively low pressures, but put down tons of power.

Also - the standard turbo will produce around 330bhp at 1.2 bar, whereas a 3071 will give you close to 400. Roughly.

Edit : PobodY beat me to it with his rocket science reply.
 
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Perrin21

Member
i get what your saying and its helped me understand better. So if i had a larger Turbo (same examples as above) at a lower boost pressure to create say 345 BHP , would running the lower boost on the bigger turbo be easier on the engine? or is there no difference??
 
campbellju

campbellju

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Staff member
Even boost pressure on the same turbo is quite misleading . If you have a 2.5" exhaust that is restrictive you can run 1.8bar but generate less horsepower than a car with a 3" system runing 1.6bar. You need to make sure your engine can breath properly before sticking on a big turbo.

Air pressure is an indicator of air flow but heat and volumetric efficiency have a big part to play too.

Matching your turbo and boost pressure to the car is important as well. If you use a bigger turbo designed to run high boost and then run it at low boost you'll find your boost controller is waiting for the turbo to spool up and then doing it's best to hold it back all the time. My 3071r was sub-optimal below 1.4Bar and only really comes alive above 1.6bar when the turbo is operating in it's design window.

First decide how much hp you want, this will dictate how much air the turbo has to flow, then the forum can tell you which turbo at which boost pressure is best for the job.
 
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Perrin21

Member
campbellju said:
Even boost pressure on the same turbo is quite misleading . If you have a 2.5" exhaust that is restrictive you can run 1.8bar but generate less horsepower than a car with a 3" system runing 1.6bar. You need to make sure your engine can breath properly before sticking on a big turbo.

Air pressure is an indicator of air flow but heat and volumetric efficiency have a big part to play too.

Matching your turbo and boost pressure to the car is important as well. If you use a bigger turbo designed to run high boost and then run it at low boost you'll find your boost controller is waiting for the turbo to spool up and then doing it's best to hold it back all the time. My 3071r was sub-optimal below 1.4Bar and only really comes alive above 1.6bar when the turbo is operating in it's design window.

First decide how much hp you want, this will dictate how much air the turbo has to flow, then the forum can tell you which turbo at which boost pressure is best for the job.
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im looking for low lag upto 375hp max. not going forged internals
 
campbellju

campbellju

Moderators
Staff member
Too slow on posting! In theory running a lower boost pressure is better for your engine but in practice it makes bugger all difference nowadays.

You want to pick the turbo that is operating at it's most efficient be that 1.2bar or 1.6bar. At this point you'll get the most compression for these least amount of heat generated.

Once the compressed charge is in the cylinder then the piston will compress the air far more than turbo can. The effective compression is dicated by your engine timing. If you run too much timing then the exhaust gases become too hot.

I won't get on to mapping but if you choose a turbo that is running in it's target operating window then you control the timing and fueling then it doesn't matter. For on the road performance you are better to fit the smallest turbo you can get away with.

At 345hp a 2860r is optimal.
 
campbellju

campbellju

Moderators
Staff member
I'm still one post behind :)

2871r is better then. However, before running a bigger turbo that needs a lot of additional supporting mods, get a decent 3" exhaust system first that will give you free hp in the short term and you'll need for >350hp.
 
Fast Guy

Fast Guy

Moderators
Staff member
Yes, you can do anything you want without a remap, but the chances are you'll blow your engine. So don't do it.
If you fit a bigger turbo get it remapped.
 
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skiddusmarkus

Active Member
It will run lean if you get a bigger turbo at 1.2bar and won't make the power it should be doing with a proper remap.The figure of 375bhp seems right in gt2871 territory.
 
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Perrin21

Member
the plan would be to get the car mapped very soon after, within the week, however i would have to drive about 200 miles to Edd. Maybe im best getting him to fit the turbo on the day. Hence my question. is he reasonably priced for the turbo?
 
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skiddusmarkus

Active Member
Driving about normally won't hurt it, just don't boost it all over the shop and drive steady and it will be fine.I am in the same situation as you as I have got to drive to his with a bigger turbo on and ecu not mapped for it.
 
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Perrin21

Member
skiddusmarkus said:
Driving about normally won't hurt it, just don't boost it all over the shop and drive steady and it will be fine.I am in the same situation as you as I have got to drive to his with a bigger turbo on and ecu not mapped for it.
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so will it be ok at motorway speeds for 200 miles?
 
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markwashington

Guest
You can't just throw a 2871R at your car and expect it to make 375bhp without supporting mods. If I was you the first thing I'd get would be a 3 inch exhaust system, forget about a new turbo and you'll find that on the standard set-up you should make 320-330bhp once it's properly mapped. I can't emphasise just how much of a difference my 3 inch system made, the low down torque is brilliant and the boost comes on so much quicker. I genuinely think that after the brakes and tyres the exhaust mod should be the next on the list for any modifying, that way you've already got a base should you want to pursue more power in the future and it'll greatly increase the desirability of the car if you come to sell. I think Pipeworx is up in your neck of the woods and they're very highly rated by people here. It's a false economy to cut corners with an R, I've learnt that the hard way. Always buy the best you can afford and use items that add to the car not just make it faster/more powerful. It's been said before and I've no doubt it'll be said again, a well set up R running 320-330 bhp on a virtually standard set up that breathes well will feel considerably better than a badly set up cobbled together car running 350+ and while more expensive in the short term will give you so much more reward in the long run.

Just my 2 cents worth!
 
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