Can someone tell me??

[gixxer750]

I know this is technical, but its kind of general too, as it applies to all turbocharged cars.

Can someone tell me exactly why a larger turbo at the same boost makes more power. All the reasons for this are welcome. I want a very technical detailed reason, because me and a friend were argueing over it, and I couldn't explain why this is, only that this is the way it is. Mathmatical formulas are welcome also

 

[Ckanderson]

more pounds of air per psi.


also more efficient

/thread.

 

[gixxer750]

not /thread..... need to be more detailed..

 

[Ckanderson]

lol its flows MORE air per psi.. it also does it without heating the air up as much... what else do you want?

 

[jimi87-t]

Well you don't need a lot of technical info to explain it. It's rather simple, a larger "pump" moves more air. Think of the PSI+flow this way, the water faucet in your house is under the same PSI as the fire hydrant on the corner, open both fully, and the the hydrant will well out flow the faucet at the same PSI.
More air (+ fuel) equals more power. The larger turbo "pumps" a larger volume of air at a given PSI.

 

[gixxer750]

the problem with that is his argument. He agrees that the bigger turbo is more effecient and you get some power from the cooler air charge.

The problem comes in with the boost pressure vs intake valves. Think of the intake ports and valves as a calibrated orfice. A calibrated orfice will flow a certain amount per amount of pressure put on it. So, if you have 15 psi in the intake manifold from a stock CT26, what is the difference between that and 15 psi from an SP61GT or a GT3540R? 15 psi is 15 psi, and without increasing pressure, the calibrated orfice should flow the same amount into the cylinders. You have your pressure tank(intake manifold), and the exit to the tank(intake valves and ports) How does more air get into the cylinders at the same boost pressure.

See what I'm saying, I do have to get technical. I know its about flow, but I don't understand the prinicple of why the larger turbo can flow more into the cylinders. I know that a lot of the power increase comes from the cooler intake charge, and the reduced backpressure against the exhuast ports from the bigger hot side....

But what about the flow, how does that come into play?

 

[GrimJack]

Heh - you are on the right track, to say the least. I'll help you a little here.

Simplified a bit, your engine can only flow as much as the most restrictive part in the flow from the intake to the exhaust. This isn't entirely true, but it helps demonstrate the point.

On a stock car, increasing the boost pressure really doesn't do much in the way of adding extra HP at the wheels. Why? Because the intake and the exhaust are flow restrictions that prevent you from pushing a significantly larger amount of air through the engine. Replace those, and all of a sudden more boost pressure turns into dramatically more HP.

Next comes the IC piping and IC... as they are the next serious restrictions to flow.

Then the turbo, then the intake manifold, etc, etc.

Eventually you will get to the point of having to change your valves to get more flow through - but as has been proven many times, our head design outflows our turbos well in excess of double, so the stock valvetrain doesn't turn into a significant flow restriction until you are at some really big numbers. Long past the point where you have replaced the turbo, in any case.

Does that help?

I could go into some nasty calculations, but in reality they are only theoretical, and never seem to get *that* close to calculating flow... which explains why we have flow test benches, so that we can test how well our theories match up to reality.

In reality, every flow restriction counts towards the total - so replacing your valvetrain with a better one even on a stock car would create measureable results - but if you could acertain that the throttle body, for instance, was 4.5% of the total restriction, and you increased the size on that alone until it wasn't a restriction at all, you don't get 4.5% more power.

 

[cjsupra90]

its really simple. you comment about the calibrated orifce is somewhat correct. the problem here is this calibrated orfice is controling the CFM ( as we know cubic feet per minute) and yes we are limited to that number of CFM. the advantage to a larger more efficent compressor is that we can there for compact more lbs per min for the same CFM by increasing the density of the air charge which is in simple technical terms more oxygen molicules per cubic foot. the number of oxygen molicules that we stuff into the motor is what really makes the power that the motor produces. This is why a motor will produce a noticable amount more power on say a 45*F day then it will on say a 95*F day. On the 45* day the air is far more dense then it is on the 95* day (i.e. more oxygen molicules per given cubic foot)

this is why all good compressor maps 3 well really 4 veriables or points of interest. 1st is the PR (pressure ratio), 2nd is the lbs/min (pounds per minute) 3rd is the efficiency rating and 4th (but not entirely needed) is the compressor RPM.... Looking at the two compressor maps below lets assume that we are running a PR of 1.80 (11.76psi) from both turbos, and running them both at peek effiecenty of 74%, so in the first one (T3 60trim) if we follow the 1.80 PR line to the 73% eff. Island and then go straight down, we can see that we are getting just about 15 lb/min of air (ruffly 150 hp worth of air) now if we follow the same points (1.80 PR line to the 74% eff. point) on the second map (TO4B H-3trim) and then drop down we get pretty much 25 lb/min worth of air (or ruffly 250 hp worth of air). now inorder to get 25 lb/min out of the first compressor (T3 60trim) we would need to push it up to nearly a 2.00 PR (or nearly 14.7psi) inorder to maintain the 74% effiecenty but in doing so, we have also induced a noticable amount more heat which as we know is BAD and does push us back wards a slight bit and then we have to have the compressor work a slight bit harder which puts us into a slightly lower eff. range and there for more heat. as you can see from this, using a smaller turbo is kinda like a dog chasing its tail.