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How come BHP figures vary so much with same boost?


Dash Rendar

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Is it really that simple?

 

So, you're saying that the increased air temperature can actually account for the 20-25% reduction in air density that would be required to give these bhp discrepancies?

 

(And this assumes a linear relationship between boost and FWHP, and since we know machines aren't 100% efficient, this implies that the air density must be dropping by even more than this.)

 

I'm certainly happy to accept this as an explanation, although it means that the smaller tubbies run *much* hotter (compared to big singles) than I had thought.

 

It's the "increased air flow" argument that I just can't buy into.

 

A few minutes ago, I asked if anyone around the office knows anything about fluid dynamics, and it turns out a guy just three desks away has a PhD in fluid dynamics! (What are the chances...) I'm hoping he'll pop in and add his thoughts too.

 

Always lovin' a bit of a debate.

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As has been said, its in theory down to air density. Stop thinking of the engine as gobbling 3 litres of air every two revolutions and start thinking of it gobbling a certain mass of air and it becomes more clear. The mass of air you get in 3 litres is dependant upon the density, and the density depends on the pressure and temperature. Heat the air less and you'll be able to get more mass in for a given pressure. Hence bigger turbos working in their efficiency island work better than small ones being forced to flow more air than they were designed to.

 

However, I reckon that a lot of the variations we see on here can be attributed to discrepancies between dyno figures. The only way to compare engine performance is using an engine dyno.

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Along those lines of thinking; heat/pressure/density then it just goes to show how important getting the temp of your charge down helps...

 

I had a massive boost of ower when I swapped to an fmic on a bpu.

 

That said it would be interesting to see a chart of intake temperatures along side a dyno graph then.

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I cant buy that the only reason you get better performance from a bigger turbo is because they create less heat

 

Surely it depends were the boost guage is reading from. If its just after the turbo then 1.4 bar on a smaller turbo is not going to be flowing the same amount of air as a bigger turbo. As for the pressure entering the the pistons then that might be alot higer with the bigger turbo than the smaller turbo but at the point the air leaves leaves the turbo then the pressure will be alot different for the two. Im sure it will change as it reaches larger dia and smaller dia sections of the system.

 

So... where do you plumb a boost guage in?

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tinly little blades of a small turbo chop the air up so much at high pressures they will put more heat in there, nice big ham fists of a 71mm turbo just scoop up buckets.. but not until your engine is already cranking... its all in the compressor maps.. no?

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Thanks Digsy, is 100% clear now :)

 

I cant buy that the only reason you get better performance from a bigger turbo is because they create less heat

 

Surely it depends were the boost guage is reading from. If its just after the turbo then 1.4 bar on a smaller turbo is not going to be flowing the same amount of air as a bigger turbo. As for the pressure entering the the pistons then that might be alot higer with the bigger turbo than the smaller turbo but at the point the air leaves leaves the turbo then the pressure will be alot different for the two. Im sure it will change as it reaches larger dia and smaller dia sections of the system.

 

So... where do you plumb a boost guage in?

 

The boost pressure will be the same, but the air density will be different.

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The only place to put a boost sensor is in the plenum. If you put it after the turbo you won't see the pressure drop across the intercooler pipework.

 

Is this a concensous then? Only in the plenum for a true reading?? as Terry rigged mine from a tap on the compressor housing. I'd be interested to see the difference running of the wastegate spring alone.

 

Also I read in some intercooler text that some intercoolers proudly claim to have a limited pressure drop.. but infact you want the biggest pressure drop possible, as this should show it's cooled as much air as possible...?

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Guest TeCKis300

No ones quite got it right...

 

It all has to do with volumetric efficiency!! That is the PRIMARY reason for differenct hp level at a given boost level. More specifically, how the turbine (A/R) affects the breathing ability of the motor through its RPM range. HP is made at high rpms. (remember hp = rpm*torque*5252). Even more specifically is how constricting the turbine is at high rpm/high flow because that is where you make hp. Better VE at higher rpm will make you more hp given the same psi.

 

The secondary reason is the efficiency of the turbo at different boost ranges and how it affects air density on the compressor/intake side. A larger turbo will heat up the air less at high psi/CFM resulting in denser charge given the same psi.

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The VE for turbocharged motors is a little bit different story...

As I've posted earlier - the most notably for VE is the size/time of the valve openings compared to the volume of the cylinder and streamlining the ports. Engines with higher volumetric efficiency will generally be able to run at higher RPM and produce more overall power. So - the main difference will be if we will use "agressive" camshafts. If we have 1.5Bar in INTAKE MANIFOLD at given RPM - there is no difference how it is created (large single or twins)... Difference between turbos will be just in resulting air temp/density. But this is not so important like cams efficiency...

 

PS: Sorry for possible grammar mistakes :)

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I think the VE argument is a bit of a moot point. Of course there are going to be differences if people are running different cams and different timing. You may as well ask "why doesnt a 2.2 litre engine at 1.4 bar make the same power as a 2.0 litre engine at 1.4 bar". If you want to compare power for a given boost level using two different turbos then making sure that everything else is the same is pretty fundamental. Otherwise the question should have been "How come BHP figures vary so much with the same boost but a load of other differences?"

 

Incidentally, I believe that the highest VE occurs at the maximum torque point, not the maximum power point, but yes, you still want to make sure the engine can breathe higher up the rev range.

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Guest TeCKis300
The VE for turbocharged motors is a little bit different story...

As I've posted earlier - the most notably for VE is the size/time of the valve openings compared to the volume of the cylinder and streamlining the ports. Engines with higher volumetric efficiency will generally be able to run at higher RPM and produce more overall power. So - the main difference will be if we will use "agressive" camshafts. If we have 1.5Bar in INTAKE MANIFOLD at given RPM - there is no difference how it is created (large single or twins)... Difference between turbos will be just in resulting air temp/density. But this is not so important like cams efficiency...

 

PS: Sorry for possible grammar mistakes :)

 

Agreed on most points but it all comes back to VE. Yes, camshafts greatly and directly effect VE. I was assuming stock head with different turbos, but aftermarket camshafts will be a huge factor as well as turbine housings.

 

VE applies to turbo motors just as much as NA. VE is not a static number. VE is a curve. The VE of a NA motor is exactly it's torque curve. The VE on a FI motor is a product of it's resulting NA torque curve, turbo response, and turbo efficiency.

 

Camshafts don't in themselves make power. They shift and modify the VE curve. They always give a little to gain elsewhere.

 

The trick comes back to the hp equation: hp = rpm*torque*5252. While it is better to make more torque everywhere, shifting the torque peak (VE peak) higher in the rpm band will make you more hp.

 

To make more hp for a given boost pressure, you will need to analyze the VE mods that move the torque curve higher in the rev band (while usually giving up VE (torque) lower in the power band).

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Is this a concensous then? Only in the plenum for a true reading?? as Terry rigged mine from a tap on the compressor housing. I'd be interested to see the difference running of the wastegate spring alone.

 

Also I read in some intercooler text that some intercoolers proudly claim to have a limited pressure drop.. but infact you want the biggest pressure drop possible, as this should show it's cooled as much air as possible...?

 

You'd want to read the pressure at as close a point as possible to the engine, so you see what it sees. So yes, plenum is the place.

 

Intercooler pressure drop values are based on the restriction factor, not the cooling factor, if that helps :D

 

-Ian

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You can fool wastegates on cars with the pressure take off on the scroll by re drilling and tapping nearer the eye of the scroll, where the pressure is less, thus delaying the wastegate opening and raising boost. It was a common cheat years ago in some race series. So a pressure gauge take off on the scroll MAY show readings lower than at the plenum, even allowing for pressure drop across the IC.

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