now that, i dunno. But i don't think the reason is what u said.

 

you should compare at same boost, same rpm...
considering the same boost for both cars, the difference in temp should go bigger as rpm increases...

 

30psi on turbo'd motor is one or more of the following:
*Upgraded turbo + intercooler
*stock turbo + methanol
*stock turbo + race gas + cams
*stock turbo + race gas
*stock turbo + fuel pump+injectors + intercooler

best option would be 1 /2...anything lower than the first two are a waste of time.

 

There is a reason for the newbie section. Its so that threads like this do not spill over into the general section.

 

Looks like you're one of the ones that didn't graduate... j/k

At the same RPM,air pressure and temperature, the same amount of air flows regardless of which turbo is used. Your engine can't take more than it can take regardless of your turbo being rated 1000cfm or 500cfm at 30psi.

 

Ok let's boil it down to a simple question....

Example car is nice Evo with cams, intake manifold, intercooler, headwork, etc and a good tune. We run a stock turbo at 30 psi and make great power. We then bolt on a nice 35R kit and run 30 psi and make a good chunk more power.

Specifically, why is the car making more power with the 35R?

My guess is two and only two reasons. One is because we have decreased exhaust back pressure, which means the engine "breathes" better. And then second reason is, we decrease charge air temperatures because the turbo is more "efficient".

Are there any other reasons why we made more power besides those two? Also, what physically is happening when you say the new turbo is more "efficient".

 

EE - you seem like a smart guy and are adding good info.

crcain - you are finally asking questions instead of regurgitating info you read on the intArweb

I will be happy to help answer your questions.

The main theory revolved around compressed air is the Ideal Gas Law.

PV=nRT

P - pressure
V - volume
n - number of air molecules
R - constant
T - temperature

Now lets keep everything the same between a stock turbo and a 35R. P=30psi and R is a constant, so we can ignore that.

On a 35R, n is going to go way up because the 35R has the ability to compress a lot more molecules of air than the stock turbo.

T is also probably going to go down on the 35R compared to a stock turbo at 30psi due to the efficiency of the 35R at that boost level. Put them at 20psi and T will probably be about the same because the stock turbo is still effiecient at that boost pressure.

So since n has gone way up, T has only gone down a little and P and R have stayed the same, the V that a 35R puts out is way more than the V of a stock turbo. More volume of air means more fuel needed for the correct AFR, which means more power.

 

All an engine is is a big air pump. The faster you can get the air in and OUT of the engine, the more effiecient it is. So by having a smaller backside on the turbo, you are creating a blockage in the air pump's system.

So that blockage doesn't allow as much exhaust gas to get out of the engine during the exhaust stroke. Therefore exhaust gasses are still in the engine when the intake cam opens....well that room in the combustion chamber for fresh air is being taken up by exhuast gasses.

So now you're not allowing as much air into the engine and it gets stuck in the IC and IC pipes. Now the turbo has to work a little harder due to the extra build up of air in the IC pipes/IC which creates additional heat in the system.


For example....my old STI. Had a stock head/cam and a 35R on it. Made 510whp at 33psi. P&P the heads and put big cams in and made 530whp at 29psi. The heads were a restriction and created backpressure in the system. Yes its not the same as a turbo size difference but its the same principle. The faster you get the air in and out of the engine, the easier the turbo has to work to make the same power.

 

+1 I cant wait for mine!!

this thread should have been locked right after DB posted or atleast moved to Newbie's section

I am officially less intelligent after reading some of the responses on here

 

The big chunk of power comes from the volume flowing into the engine. To put it simply its easier for a big turbo to flow more CFM than a smaller turbo. The smaller compressor wheel on the small turbo has to work much harder to flow the same CFM of air a much larger compressor wheel could. Even though the pressure is the same the demands of the engine at this pressure for CFM do not remain the same. As the engine RPM raises so does the volume of air it consumes give it more pressure and the volume it consumes increases per stroke.

Eventually whats going to happen is the small turbo is going to run into a problem where it simply can't spin any faster because its reaching a speed where the turbine blades no longer can effectively provide boost. At this point the CFM of the turbo is going to reach a limit as where a larger compressor can keep on going. In the world of horsepower providing the motor with more torque at higher RPM's means more horsepower we are creating this torque by providing more volume of air with a larger turbo in this case at the higher RPM.

This is not to say a compressor wheel alone will provide more flow you also need to power that compressor wheel with a matched turbine wheel which will provide the torque needed to move said compressor wheel.

Hope thats making sense.

 

Thank you RoadSpike and SloRice for your info.

SloRice, I'm very new (1 year really) to understanding internal combustion and forced induction fundamentals, but I've read 3 thick books (a. bell and c. bell), and I'm offended you think I'm just regurgitating info. If you look at my answers in this thread, I said from the very beginning (with full understanding)... that exhaust back pressure and cooler charge air are the reasons. Both seem to be the exact right answers.

After reading all replies since then, it seems something I've not gotten a handle on really... is that of course the wheel size and design is an important aspect. I do understand that larger wheels can flow more air. It seems as you guys say, if your not exceeding the flow potential of the wheels, it's sort of irrelevant. But once you push the boundaries of the flow limits, then I guess it comes into play when doing these turbo to turbo comparisons at fixed boost pressures.

Anyway, all your replies are good food for thought for myself and I thank you for it.

Final question... if we said 20 psi was the fixed boost pressure, would it be safe to say the primary reason the 35R makes more power is because of a reduction in exhaust back pressure?

 

I'm just as relatively new to forced induction as you, and here's my thought.

Reduced backpressure is only part of the equation and not the primary reason.
I THINK the primary reason for making power is because you're burning more air molecules (more specifically oxygen molecules) due to wheel size of the larger compressor. What good is reduced back pressure if you can only cram in so much oxygen molecules in the cylinder?

I could be wrong. And here's an interesting read from rx7 forum.



Why bigger turbos make more HP at the same PSI....

--------------------------------------------------------------------------------

My explanation: Not to be taken as gospel ;o)

I struggled to wrap my mind around this for a while.

example thought process: You have an engine which rotates at a given RPM, let's say 6000 rotations per minute for grins. now at 6000 rpm the intake port is open for the same amount of time for both a t78 and a stock twin setup. So the time intervals are fixed by the engines rotation. Assuming that both setups are creating 10 PSI of pressure measures from the intake manifold (which has a fixed internal volume) then it seems impossible for one turbo to make more power at 10 PSI than another because both the volume of the engine and the intake manifold are fixed. And since the volumes are fixed that means the same net force is exerted onto the intake charge in both scenarios.

this is the point where we scratch our heads ^^^

Most of you know this but I'll say it anyway:
1PSI = 1LB force per square inch (not Pounds of air per square inch!) - think about it, a square inch is a unit of area, not volume. 10 PSI = ten pound of force exerted exerted on every square inch of internal surface area of the intake manifold and intake ports = says nothing about how much air is in the intake/engine ( if it did it would be per cubic inch) just how much force the air is exerting as it gets force fed from the turbo's compressor.

With that being said, one can calculate the air density based on how much pressure is exerted, but PSI is not a measure of volume in and of itself.

Now, "Cubic feet per minute (CFM) is a non-SI unit of measurement of gas-flow (most often air-flow) that indicates how many cubic feet of gas (most often air) pass by a stationary point in one minute. In other words, it is a unit for measuring the rate of flow of a gas or air volume into or out of a space."
-wikipedia

OK, so we accept that a large turbo expells air at a higher velocity and therefore has a higher CFM. Now lets try to understand why:

One easy way to understand how larger turbos move air at a higher velocity it is useful to think about a neck in a river and the way that water flow through it.
The wheel size and outlet volume of a T78 turbo compressor is much larger than that of the twin setup. This large volume of air leaves the turbo and enters the bottleneck which is the intake tract speeding up just as water speeds up in a river bottleneck.
The air flowing from the small twin compressors on the other hand are flowing into the same river, but this time the river is large in relation to the charge volume so the air just creeps along.

OK so we established the following:

At a given RPM the intake ports are open for a defined period of time.
A large turbo moves air more rapidly through an intake tract with a fixed volume.
Next: Realise that air velocity has nothing to do with PSI. PSI is only a measure of how much force is exerted on the manifold and intake ports.

The tricky part:
At 10PSI (assuming the same intake temps) the T78 is cramming the same amount of air into the intake manifold as the twin setup. So what gives?

Here's the key point to understand:
The boost that you are reading at the intake manifold is not telling you how much air actually makes it into the intake ports during their short open interval. It's only a measurement of force exerted on the intake plenum.

Key to understanding:
The air coming from the twins will surge forward into the intake ports with a lower velocity than that from the T78 for the reasons that we established in our "river bottlnecK" example earlier.

So while both turbos are exerting the same amount of force on the intake ports the air from the T78 is approaching the intake ports at a higher velocity and therefore more will get in before the port closes.

Lastly: The boost reading that you see on your gauge is not taken from within the engine, it is taken from your intake plenum and should not be confused with an internal measurement. Understand these points and you will have the problem cracked.

I may be wrong but that's my logic ;o) Hope that helps someone.

 

Well, now you sound like you know what the hell you're talking about. Much better than this:
... which really needs to be reworded better to make any kine sense (since I can see your point in it).

Cheers.

 

Nice post evilbada!

For sure more power means more air. But reduce back pressure, and that means exhaust gasses can get out of the cylinder quicker, and because there is overlap of intake and exhaust valves, that means you can pull in more intake air. They call this scavenging.

But then your point, and the RX-7 guy's post... I never thought of how a larger compressor housing/wheel flowing into a small IC pipe could increase velocity. This seems like a valid point and something I had not thought of. Good stuff!

 

just get meth