Let me ask this a different way. . . When you fill your tire with air pressure, do you think of the tire as a restriction?

When filling a pool with water, is the pool a restriction?

If you are running 20 psi of boost in the plenum, and you have 20 psi in the cylinder at IVC, do you think of boost as a measure of restriction?

There's a difference between filling a volume with compressed air and forcing air through through a hole using pressure. We are doing the former.

PV=nRT
You want to maximize the number of moles of gas in the cylinder. So, n=PV/RT. R is constant. So, you want to maximize Pressure and Volume, while minimizing Temperature. The Pressure here is the Pressure in the cylinder. So, to maximize mass trapped in the cylinder, you want to maximize pressure in the cylinder. If you go by the theory that "boost is a measure of restriction", then you would be making efforts to minimize the Pressure, which would reduce the mass in the cylinder. To put it another way, we all agree that we want a high density of charge. Density is a function of pressure. Reducing pressure reduces density.

Again, we are not cramming air through an orifice. That equation is more like Q=C(dP)^n. For this, yes, more dP OR flow area will increase mass flow. Put in a huge orifice, and the dP can approach zero and flow will be maximized.

 

YES!!! Air is going in but not coming out...its a 100% restriction...if you keep pumping in air, its going to keep increasing pressure...you are typically using a compressor that has a holding tank of ~120 psi...do the tires go to 120psi immediately??? NO!. The engine is doing the same thing when the Intake opens...its just doing it in a dynamic sense...I doubt there's equalization of pressure across the valve in the limited time its open since the volume is constantly changing...

a pool is not the same thing...its an open cavity...

 

so engineer mike, if you spun a turbo to say 105000 rpms and put an open pipe on the outlet(lets say a 2.5" pipe, one end is connected to the turbo, the other end is connected to atmosphere)

certainly, in this case there is no restriction, so if you put a boost gauge on that pipe will there be any boost showing in that pipe? my thought is that it will show boost but I might be wrong

 

Yes, which is why imo, boost pressure is not a measure of restriction but a value of atmosphere available for the engine to consume.

I suppose you could call it a measure of unused air but I don't find that accurate or useful. Lets say you have a 91mm turbo on a stock 6.0 at 20psi boost making 1000hp and the same turbo on a solid roller 427 at 20psi boost making 1500hp. Both are running 20psi boost but engine A is consuming 100lbs/min and engine B is consuming 150lbs/min airflow. So if 20psi is a measure of unused air why does it have no correlation to air flow?

On a naturally aspirated engine you can achieve VE over 100% due to inertial supercharging. Obviously the inertia of the intake charge plays a big part in cylinder filling otherwise you would have a big problem closing the intake valve at +/- 50deg abdc.

I see no reason the same wouldn't occur in a turbo engine which would mean cylinder pressure just before IVC should be the same or greater than manifold pressure.

I have dyno tested many many different cams in turbo combinations, and to generalize, the profiles that more closely resemble a traditional naturally aspirated cam made the broadest power and most peak power, even on combinations that had backpressure measured at 2x boost pressure.

 

A volume IS NOT the same thing as a restriction! There's the fundamental logic problem.

The restriction in the air hose is a different issue. Yes, the hose (likened to the runner/port/valve) is a restriction. However, the hose (runner/port/valve) can be readily improved such that they are no longer a restriction, but that doesn't change the fact that you want 35 psi positive pressure in the tire (cylinder). Remove the tire and you would get more flow. But flow is not the goal. . . positive pressure inside is.

 

Well said.

 

It depends on which direction the port on the gauge is facing, but in most cases it will show no pressure.

 

Why did GM put the 3.8l cam on a 106 and why don't we see cams of this theory used?

 

In my thinking, this would make sense.

 

You do...At least when Boosteds heads/cam combo was done by chris and the cam spec'd by ray. it was on a 108...

BTW, I still have your TB. Its in the trunk of the honda. Next day I work is Thursday night. Where you want me to leave it? Daniels or what? text me.

 

I disagree as well. An engine can only flow X CFM at a certain RPM. The turbo can flow Y CFM. If Y is greater that X at the time, you have boost. The actual CFM flow in the head will not change, even during boost, it only gets more dense. The ONLY way to change X CFM, is to change RPM, or modify the VE (heads/cam/Intake/Exhaust, etc). If X CFM goes up, (and Y CFM is the same) you have less boost, but the same mass flow. Tell me how Boost is not a measurement of restriction?


I do agree with you that If you have 15PSIG of Boost and 15 PSIG of backpressure, an N/A cam would make the most power. Problem is (from the simulations I've ran) is the the Boost to Backpressure ratio does not stay the same througout the rev range. It generally increases at a fairly linear rate as RPM increases.

 

Using my Comp LSR cam of 243/247 .624/.624 on a 116, my plan is to use a T6 housing of 1.25AR-1.32AR to keep back pressure to a minimum and hopefully make some good power. Will it work? Maybe. Can't hurt to try since I already have the cam in the car. I'm also going to use nitrous to help it spool. If it doesn't do what I want it's only $400 for a new cam. No sense to change based on what if before I see how it works.

 

My opinion is if you are turboing your current motor, don't waste time/money on changing the cam, its always free to try! If you don't like it, you can always change it later.

 

You guys are arguing semantics. You guys are pulling out your fancy scientific calculators and 3d imaging software for nothing. It's real simple. If you unhook all the pipes from the turbo/blower cold side and screw a MAP sensor into the compressor housing, you will likely see about 105-110 kpa no matter how fast and hard that bad boy is working. Everything from the front (combustion chamber side) of the exh valve back to the turbo/blower is a restriction causing the MAP sensor to read boost.

As for an NA cam working best for a turbo application with no back pressure, I'll agree except in the real world, turbo's have exh backpressure. Turbos without exh backpressure are called blowers. Which goes back to my other post about a blower cam being similar to a nitrous cam.

 

+1....

 

In a turbo air cooled VW application I have used an Engle FK87 cam. Which is 320 advertised duration, 276 @ .050, .561 lift w/ 1.5 rockers, on a 108 lobe sep. Wicked 4000-8000 powerband that pulls like hell.

 

yeah i definitely enjoyed reading this. learned a lot, and actually kinda reinforces some of the things ive been thinking about.. Its cool reading the engineering side and the real world side.

 

I don't see that as relevant as the turbo can only function in closed loop. By removing the cold side you've essentially removed the engines ability to drive the turbine (exhaust volume).

Again, in my experience (and still generalizing) overlap = power, even on combinations that have far higher exhaust backpressure than boost pressure.

 

See, that's precisely where the problem is. If you totally remove all restriction between the outlet of the compressor and the cylinder, you WILL STILL have pressure because you are filling a volume with air. I'm running out of ways to explain this. Think of it like filling a tank with pressure. Even if there is no restriction going into the tank, you still build pressure (and thus, mass) in the tank.

 

If you improve to the best heads/cam/intake/exhaust in the world, you still have boost pressure because you are filling a closed-ended cylinder. You want that cylinder as full as possible. So, getting 20 psi boost from the plenum totally and 100% into the cylinder (NO restriction at all) gets you 20 psi in the cylinder. Note that you have ZERO restriction, but still have boost pressure.