morris

iTrader: (2)

We were talking about the difference in boost between a supercharger and a turbo. can someone explain to us common folk how 10lbs of turbo boost is different from 10lbs of supercharger boost?

bags

Well, let's start with the obvious..

A turbo should reach FULL boost before redline. Why is that important?

It's called volumetric efficiency.

What is volumetric efficiency?

" In a four-stroke naturally aspirated engine, the theoretical maximum amount of air that each cylinder can ingest during the intake cycle is equal to the swept volume (displacement) of that cylinder (0.7854 x bore x bore x stroke). Since each cylinder has one intake stroke every two revolutions of the crankshaft, then the theoretical maximum volume of air it can ingest during each rotation of the crankshaft is equal to one-half its displacement. The actual amount of air the engine ingests compared to the theoretical maximum is called volumetric efficiency (VE). "

Another

"Volumetric efficiency is the measurement of how close the actual volumetric flow rate is to the theoretical volumetric flow rate. A engine has a set volume (displacement) that can be calculated. However, your engine will not use the full volume (100%) it has available because of friction losses, leaks, and the fact that a mass produced engine can only be so good before the money out weighs the benefits (the point of diminishing returns)."

Speaking of diminishing returns, AFTER PEAK VE is reached, VE goes DOWN. So, VE goes UP as you reach peak TQ and then back down after. Just like most TQ curves.

morris

iTrader: (2)

Man I feel dumb, becuase that doesnt mean a thing to me, I was clueless when I read it on the google search and I am just as clueless after reading it 4 times here

morris

iTrader: (2)

I know what volume is, it equals the space in the cylinder. But where does the efficiency part come in. And how does it relate to turbos or superchargers?

bags

ok, let's say your engine at 5500 rpmCAN hold 1 litre of air in the cylinder. BUT, actually at 5500 rpm it HAS .9 litre's of air in that cylinder. Therefore, you have 90% of the amount of air that your cylinder CAN hold.

So, you have a VE of 90% at 5500 rpm.


This is when your intake, cams, exhaust, and other various componets, are working ALL TOGETHER to flow the maximum amount of air possible into and out of the engine.

Now, using the same numbers above, EXCEPT at 2500 RPM, your cylinder may only hold .5 litre's of air, so that's 50% VE. And it increases as RPM increase.


Well, how it applies to turbos and superchragers is tuning and the fact you are CHANGING your airflow into the cylinder, so your changing your VE. But, I know there are other things to cover. But, this one is overlooked very often. Again, at peak TQ/VE, your cylinder pressure is at it's highest point. By forcing more air than a SC, your forcing more pressure into the cylinder. At some point the pressure is going to be to high and something is going to give.

Here's 2 dyno's from 2 maxima's running almost the same amount of boost


SUPERCHARGED 9 psi 3inch exhaust




TURBO 9.7 psi 3 inch exhaust







You see the TQ difference.. and how fast my TQ curve goes up? I'm giving the engine more air at lower rpm, therefore the VE is increasing FASTER than the SC'd maxima.

Does this help at all?

Neo

iTrader: (3)

This thread is going to be fun reading.

NoRyceGS4

LOL i like your second definition using economics. LOL gave me a hearty chuckle. bah im a nerd

Lextreme

Bag,

You are making it too complex for Morris. Just think of this way. Which is heavier? 10 pounds of cotton or 10 pounds of steel?

RMMGS4

iTrader: (5)

HAHA

C'mon, everyone knows it's the steel.

morris

iTrader: (2)

I knew this was going to be interesting but I think it is coming together. THE VE is higher in the turbo than the SC at lower rpms becuase the turbo reaches full boost a lot sooner in the rpm scale. This results in a flatter torque and HP level throughout the rpm band, primarilyh due to the fact that there is no constant ramp up of boost in a turb like a SC. The Turbo engine gets full boost and highest VE at the point it reaches full spool, right?

bags

Not just lower RPM, the entire rpm range. And your correct.


hmm.. I'm gonna try and not confuse you... The turbo reaches full boost at a given RPM. It does not matter to the Turbo what the VE is, is just needs "X" amount of exhaust gas velocity, to reach full boost

The engine's highest VE WILL be at peak TQ. * all things being equal*

To make it short and sweet, if your turbo reaches it's 10 psi at 3500 rpm and your engine's highest VE is at 4500 rpm, you should still have your highest VE at 4500 rpm.

*DISCLAIMER*
The above statement is with the N/A engines peak VE/TQ. The turbo increases airflow into the engine, so it's possible the highest VE/TQ will change after the turbo is on there.

Make sense?

bags

Not my definition... That's why I used ""...

I got the joke, I just did not want to take credit for the definition.


Lex, Cotton, and I know which one has more static volume

Lextreme

back to the answer of 10 lbs of cottom or 10 lbs of steel. Of course they are the same weight. This principle also apply to FI. 10 psi from SC and 10 psi from TC is the same. However, how much energy it cost to make 10 psi is another factor. Secondly, when is 10 psi arrive. Supercharger will take to redline vs. TC is somewhere about 1/3 of the redline. For example, my cobra will take 6500 to make 17.5 psi. My LS400 will take about 3500 rpm to make 17.5 psi.

Earlier boost, means earlier power. So, if a SC GS400 make 10 psi and a TC GS400 make 10 psi, the TC GS400 will reach earlier power. The total amount of power is pretty much the same for both engines at 10 psi, but TC GS400 will get full power earlier. Instead of redline, the TC GS400 will get 10 psi at around 2600.

SCV8

Another key difference between the two is that the turbo uses wasted exhaust heat for the pressure, where the supercharger uses perfectly good crankshaft horsepower, not as efficient.

morris

iTrader: (2)

Yea I am definitely clear on the lack of parasitic losses with a turbo, I think it is upwards of 50hp on a SC "in general"