seandizzie

iTrader: (1)

Yes you can over spin the turbo, it will spin very fast when venting to atmosphere. if you do any rpm/throttle input that would normally spool the turbo, you are spinning the hell out of it.

desmond

did not know that. thanks for the info. good thing im done with emissions then

Montego

iTrader: (1)

In just because this one is a classic!

dude seriously read up on automotive 101 and make an intelligent, knowledgeable decision yourself.

wanklin

iTrader: (2)

Bolt on the twins as a ghetto exhaust and wait until you get the PFC and fuel.

NissanConvert

iTrader: (1)

I understand CFM (volume/time) but i don't see how at the same manifold pressure, with the same engine, same porting, etc. how one turbo will be getting the engine (otherwise unchanged) to ingest more air. (with the exception of the now better flowing exhaust manifold) the only thing i can think of is that if the turbocharger is more efficient then it won't heat the charge as much or the outlet is a larger diameter but then you're still restricted by the elbow and throttle body.

Please don't make any mistake: i don't think that he had a good idea.

Mahjik

iTrader: (1)

You just said it. Fast moving air means more/faster airflow at the same pressure. This is also why it's kind of pointless to say to some of the single turbo guys that you'll be faster at 10 PSI than their slow spooling turbos. Even at 10 PSI, a large single turbo will ingest a LOT more air, thus they'll be building more power, just at a lower PSI.

If CFM made no difference at all, then switching turbos would have no effect at the same boost settings as the stock twins (i.e. 10 PSI would result in x HP on all turbos given the same car/engine). But we all know that's not the case.

Mahjik

iTrader: (1)

Let's put it this way:

You have a few ways to get more power, but they both require air and fuel. For air, you have basically two things:

1. Larger turbo
2. Engine porting

The larger turbo is going to send more air faster. The intake ports are only open for a limited time. The more that can get into the intake ports, the more power you can make (given the rest of the equation is complete as well).

Engine porting helps by opening the intake ports so they are basically open longer. This is why you can make more power with the stock turbos and porting as the ports are open longer to bring in more air. However, they are still limited by the CFM which a large singe will still outflow them with or without porting.

NissanConvert

iTrader: (1)

But with all of the other restrictions in the system how is the single going to move more air? I don't deny that a single will move more air but if the air doesn't have anywhere to go the air isn't getting moved.

This is all a moot point anyhow. If he doesn't have a boost controller or his wg spring is set too high (i doubt it's set at 14lbs but even 10lbs would likely be too high) then one stab on the throttle might be a one way trip to a rebuild.

rynberg

Perhaps you should rethink your evaluation of the "restrictions" in the system.

Stock at 10 psi, I made 220 rwhp. After all bolt-ons, BNRs, streetport, I made 305-310 rwhp at 10 psi. What has changed? Volume of air is the only answer. Air is moving faster through my system than stock.

Mahjik

iTrader: (1)

The restriction really isn't in the exhaust. The restrictions are really in the combustion chamber and your rpms. No matter how big your exhaust porting is, you only have so much air the combustion chamber can hold. The faster your rpms, the more air can be taken and moved (i.e. why you make more power as your revs go up).

NissanConvert

iTrader: (1)

What bolt ons?

Ultimately yes the restriction is the combustion chamber volume. But if that was the only restriction no power would be gained from increasing exhaust or intake diameter.

wanklin

iTrader: (2)

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

Here's why it's tough to come to grips with. (the following will repeat some of what has already been said)

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 most people scratch their 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 gasflow (most often airflow) 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:

1. At a given RPM the intake ports are open for a defined period of time.
2. 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.

Mahjik

iTrader: (1)

I already explained the intake porting gains. Exhaust starts into other flow dynamics which is really outside of this conversation.

crazyrx7

Nissanconvert; you can get new 850 primaries with rail if you want but how are you going to tune the factory ecu and tell it that there are 850's in there instead of 550's? If you had a power fc you would understand how much setting up it takes to get the proper amount of fuel on startup. After startup the car has to stay running some how so there is a fuel map for that rpm. I can go on for hours but I already know this where as you should read up on it then you will understand that what you said will not work.

R.K.

wanklin

iTrader: (2)

I started a new thread for this flow discussion since it is probably driving this thread off track, if it even has an identifiable direction.... lol

https://www.rx7club.com/3rd-gen-archives-73/why-bigger-turbos-make-more-hp-same-psi-645551/

NissanConvert

iTrader: (1)

You're killing me...

I'm really getting tired of quoting myself.

I will be attending the fluid dynamics of exhaust flow discussion.

crazyrx7

Maybe I should of taken your advice and read what you wrote. Sorry man. Either way maybe I educated the original poster of this thread.

R.K.

Jason

iTrader: (5)

This thread is funny. The answer to his question is yes he can run that turbo on the stock ECU, but dont get into boost.

seandizzie

iTrader: (1)

But he doesn't understand how to keep it out of boost. I say we break out the beer and pizza and listen to this kid destroy his car...

wanklin

iTrader: (2)

Read his screen name....lol