How come BHP figures vary so much with same boost?

Sorry if these seems an ignorant question, but I'm puzzled. I recently saw a post where a guy has achieved 450FWHP on hybrids at 1.4 bar.

But at the same time, there are singles at 1.4 bar that achieve over 450RWHP and around 550 at the fly.

If these cars are running the same boost and have sufficient fueling, how can the horse power figures be so very different?

Sorry if this is dumb...

air temperature (and hence density) can make a difference. Different rolling roads read differently too.

Mike

Single turbo's flow a lot more air than standard twins at the same boost.

Sorry if these seems an ignorant question, but I'm puzzled. I recently saw a post where a guy has achieved 450FWHP on hybrids at 1.4 bar.

 

But at the same time, there are singles at 1.4 bar that achieve over 450RWHP and around 550 at the fly.

 

If these cars are running the same boost and have sufficient fueling, how can the horse power figures be so very different?

 

Sorry if this is dumb...

Its not the boost you run, its the amount of air the turbos can flow, bigger turbo can flow more air at last boost, where as a smaller turbo set up like mine flows less air at higher boosts, its all about the size of the turbo

Sometimes its not the pressure but the amount of air thats flowed

Sometimes its a dodgy dyno too

Did someone mention its the amount of air

Thanks guys. Now I realise I was being thick.

I started thinking... If I was blowing through a straw, the air pressure would be high, but the air flow would be low. But if I was blowing through a two-inch pipe, the pressure would be low but the air flow would be high.

So it's all about pressure and capacity for air flow.

P.S. You'd think I'd know all this already, given that I had a lung collapse on me a couple of months ago.

also smaller turbos tend to run hotter, so the intake charge is higher too = less horsepower.

Another good point.

I've never understood the "more air" explanation either.

If the boost pressure is the same, and the cylinders are the same size, and the pipework between the turbo and the cylinders is the same volume, how can there be MORE air at the same pressure?

I've never understood the "more air" explanation either.

 

If the boost pressure is the same, and the cylinders are the same size, and the pipework between the turbo and the cylinders is the same volume, how can there be MORE air at the same pressure?

 

Glad I'm not the only one that sees it like this

If the air pressure is at a constant level at the intake manifold, then surely the air supply to the cylinder is the same, regardless of what is making the air pressurised

Perhaps as Mike mentions it's just down to charge tempurature? I know that my car is losing masses of power due to high intake temps - ignition had to be retarded up to 4 degrees in some areas (Hope to solve that this weekend after a chat today with Ryan)

Surely when we're talking about the amount of air it's flowing, then it's how fast it's flowing the air. So a small turbo will push out less air per minute (cfm say) at 1.2bar than a large turbo will at 1.2bar?

Surely when we're talking about the amount of air it's flowing, then it's how fast it's flowing the air. So a small turbo will push out less air per minute (cfm say) at 1.2bar than a large turbo will at 1.2bar?

A turbo surely can't flow more air into the size same container unless the air is at a higher pressure - can it?

It's all about the charge temps and air speed. Some law of thermodynamics somewhere says that compressing air heats it up. Heating air up increases the pressure.

A more efficient big turbo will heat the air less than a small one. This means if it flowed the same amount of air molecules the boost pressure would be lower. However it supplies the same amount of boost pressure instead and the only was of doing this is to pack in more air molecules - and more air = bigger bang.

Yes intercooling helps the small turbos but once you lower the temps back down you have lowered the pressure. If the temps are coming down a lot, then the pressure is too. That means the little turbo is working harder than you think - boost pressure pre-intercooler will be way higher than 1.4bar So the pressure drops and airflow speeds fall as well because of this...

Which takes us to the second aspect - I think a bigger turbo also shoves the air out faster or something, so more gets through the plumbing and valves and into the engine per 4 stroke cycle. Up to a point, which I believe is the mythical beast known as the surge limit, where the air backs up and bad things happen at the turbo end.

That's my semi-unfounded thoughts on the matter

-Ian

Reminds me of this thread a bit. The stuff CW posted there helped me understand things a bit better (although I'm still not sure I have it all right!)

The way I see it, 'more flow' can only refer to the turbo itself (and subsequent replaced pipework for FMIC etc). So the advantage once the air gets to the intake is that it's cooler. But it's not a lot denser - air density is relative to absolute zero (-273 deg C), so even a 30 deg decrease in temperate is only about 10% more dense. So I think that's part of the power increase, but that most of it comes from being able to advance the timing. Am I even close to 'getting it' yet?

As I see it and to summerize:

A small turbo/twins can't flow air very quickly so to get the needed air it needs to compress the air more.

A large single can flow air quicker and therefor doesnt need the high pressure.

This has two sides. The higher pressure of the small turbo(s) as well as the physics of air running through them (high speed blades and so) result in higher temperature air and you have a point (stock tubbies=1.4 bar?) where the air gets less dense @ higher pressure than at lower.

So a larger turbo will not heat the air as much and can feed more O2 molecules to the cylinder due to being able to move the air quicker, where the small turbo needs a higher pressure to move the same amount of molecules, but the small turbo is limited due to the rise in temperature @ the higher pressuere which leads to fewer molecules.

But why is it that if you go single (maybe except for the smalles of single) you need a larger IC? If the boosting of the single creates a lower temperature air (should be lower if the pressure is lower) or the same (if pressure is the same) why need the larger IC? Shouldn't the need for a larger IC be dependant of the temperature (high pressure) and thereby a IC is more needed with hybrids running 1.4 bar than a larger turbo running 1.1?

Now I'm as puzzled as when I first initiated the thread.

I thought the airflow explanation was a good one, and my analogy with the straw seemed to make sense, but only if we assume that the turbo itself and the supporting piping is actually the limiting factor that is generating the pressure. But clearly this is a bad assumption, as I think someone intimated later. The pressure, ultimately, is derived from forcing the air into the engine.

Pressure is nothing more than a certain mass of air in a given volume. Since the volume is constant, then the mass of air forced in (per unit time) by hybrids at 1.4 bar, or a single at 1.4 bar, to my mind, should be the same.

But from Boyle's Law, we know that a given mass of air exerts more pressure at higher temperature. So, as others have already discussed, a big single will heat the air far less, so an equal amount of pressure (compared to twins) will result in the flow of more air. But can temperature alone account for FWHP differences of nearly 100bhp?

Does anyone know the true temperature differences downstream of the turbos, say, between twin tubbies and big singles? I presume someone must have run the equations at some time.

Maybe I'm just having a stupid moment. When I was a geeky student, this stuff might have made more sense to me. But working in IT for nearly 10 years has caused my brain to shrivel into a useless lump.

Depends were the boost reading comes from then, where does a boost guage take the boost reading from?

I've never understood the "more air" explanation either.

 

If the boost pressure is the same, and the cylinders are the same size, and the pipework between the turbo and the cylinders is the same volume, how can there be MORE air at the same pressure?

 

Isnt part of the Bigger turbo work changing this piping to larger diameters? wouldnt the boost guage read 1.4 bar before it goes into the pistons, therefore the boost pressure when in the cylinders could be alot more than 1.4 with a single?

I've never understood the "more air" explanation either.

 

If the boost pressure is the same, and the cylinders are the same size, and the pipework between the turbo and the cylinders is the same volume, how can there be MORE air at the same pressure?

 

We've had all this before dude, remember ?

Think about the engine sucking in all the air. Because the little turbos aren't flowing much air, the engine only sucks in so much, leaving the pressure high, say 1.4bar.

With the big single, the engine is munching and munching away eating up tons and tons of air, and still the pressure is kept high. If the engine was able to munch this much air with small turbos, it'd be sucking air through the little diddy turbos and there'd be no pressure any more, so it can't happen.

I think that's somewhere close anyway. I always hit brainfuck when I try to think about this properly, so I leave it..

Hmm. Unless you change the cams, the engine is gonna munch the same volume in each case. I reckon the only variables left are pressure and temperature...

 

Pressure is nothing more than a certain mass of air in a given volume.

Nope it's a given mass at a given temperature in a given volume As you sort of agree with later in your post

I still think velocity plays a part, not just pressure. But that's a guess

-Ian

Yes, that was in what the next paragraph was for. (I was endeavouring, and apparently failing, to be systematic.)

I can only draw one conclusion from all of this... The question wasn't that stupid afterall!

Let's hope some experts on fluid dynamics can come along and give us some PV=nRT action, with solid figures!

Hmm. Unless you change the cams, the engine is gonna munch the same volume in each case. I reckon the only variables left are pressure and temperature...

Hmm. I was envisaging the engine rising through the revs faster, but that can't be right can it, as the power comes at a static rpm point.