davepl

Here's something I have never understood that perhaps one of the more advanced gearheads can explain to me:

When you "floor" a car with a naturally aspirated motor like a 6.2L LS3, it makes 400hp. All that "flooring" the throttle does is open it fully so that the cylinder intakes can be filled by atmospheric pressure.

Now in your LT4 you can roll into the throttle until you get to 0 boost/0 vacuum. But it's nowhere near the power output of an LS3 at full sail.

So, put another way, why isn't an LT4 at 0 vacuum the same as an LS3?

Why doesn't a blower motor with zero boost feel the same as the NA motor?

ASTROCREEP

Compression Ratio
Cam timing
Direct injection
Different heads
Different gearing
Parasitic drag of supercharger
Current weather pattern

Take your pick

Perf n Restore

Let me add my understanding...

When you "floor" a car with a naturally aspirated motor like a 6.2L LS3, it makes 400hp. All that "flooring" the throttle does is open it fully so that the cylinder intakes can be filled by atmospheric pressure. Well....yes, "filled by atmospheric" BUT the LS3 is pulling air/fuel in during each intake stroke. If the LS3 didn't suck in air/fuel, atomospheric pressure alone would not do much. The LT4, with forced induction is assisted by the supercharger. Hard to imagine, but the LT4 at 0" is already being boosted with very little throttle. "Boost" from the supercharger, as the throttle blade opens gets the vacuum to 0" very quickly.


So, put another way, why isn't an LT4 at 0 vacuum the same as an LS3?
Because the LT4 at 0" with the help of the supercharger only requires very little throttle to get to 0".
With forced induction, there the air/fuel is just waiting on the "outside" of the throttle blade to be "pressured" into the intake manifold as the throttle blade starts to open.

Help this adds a little to your excellent question....

3 Z06ZR1

I hope I have this right
The blower motor is 0 boost till it sees it sees 0 Vacuum then the boost valve opens. The ls3 is 0 vacuum long before wide open. Right when you give it gas it does the same thing. Lt4 feels way stronger than the Ls3 at any speed.

spoolin98

I'm new to superchargers, I thought I understood how they worked. Maybe I don't? I don't want to step on any toes here, this doesn't make much sense to me...
I know turbo systems very well, maybe I'm far off on superchargers.

spoolin98

What is a "boost valve" and where is it located?

Pacembellum

Pull the blower belt and it will feel like an ls3. At 0 vacuum you are actually getting some boost.

spoolin98

See that makes sense to me, even though you're at zero you built enough boost to take you out of vacuum.

Higgs Boson

The LS3 requires 100% throttle to get to or near 0 vaccum.

The LT4 requires very little.

It is therefore missing all the other things associated with full throttle like fueling, spark, RPMs, fully open throttle body, etc etc.

Vacuum/boost does not equal horsepower. CFM equals horsepower.

dar02081961

All the others here have explained it well. I will try to put a little more meat on the bone to make it easier to grasp the concept.

The simple answer is at 0 vacuum the LS3 is at WOT and the LT4 is not.
The LT4 can achieve 0 vacuum at very little throttle opening due to the supercharger if the bypass is closed.

In a NA engine 0 vacuum simply means there is no restriction in the intake track from the throttle blade being partially closed and the manifold is at 1 atmosphere. (or whatever the barometric pressure is at that location). This can only be achieved in a NA engine when the throttle is at or very near WOT.

A FI engine (especially supercharged engines) have the ability to achieve 1 atmosphere with a restriction in the intake track. In other words the throttle blade doesn't have to be at or near WOT to create 1 atmosphere in the intake manifold.

In order to understand this we have to understand that 0 vacuum and 0 boost pressure are the same thing.

However vacuum is a measure of the unused volume/capacity of the intake manifold and engine behind the throttle blade. In other words vacuum is the measure of restriction the engine is seeing versus if it were had no throttle blade at all.

Keep in mind the throttle limits (restricts) the intake manifolds and internal engines access to the atmosphere by varying the effective size of an orifice. If the size of this orifice is less than "wide open" to the atmosphere (0 vacuum) it creates a restriction measured as vacuum.

I an FI engine (LT4) the supercharger can fill this vacuum or overcome the restriction of the smaller than WOT orifice because it can create pressure. We must understand the supercharger (or Turbo) job is to create additional atmosphere. Therefore they can fill and must fill this "vacuum" before it can create additional atmosphere (pressure/boost). This is important to understand because this means superchargers and Turbos are working and providing benefits before you see them on the boost gauge as positive boost.

Therefore a NA LS3 has to be at WOT to achieve 1 atmosphere in the intake manifold.
Or said another way, the LS3 must be attempting to produce maximum power to achieve 0 vacuum.

The FI engines can produce 0 vacuum at practically any power setting because they have a compressor capable of creating additional atmoshere.

So yes an LS3 is much stronger at 0 vacuum than an LT4 because the LT4 can achieve 0 vacuum at part throttle.

spoolin98

Pacembellum

You need to think through it all.

By itself doesn't equal MORE power. I.e. minimal boost doesn't really do much, but its boost.

silver74vette

There are some very good statements on this thread, let me see if this works.

In the LT4 if you have the throttle set to show 0 (1 atm) on the guage you are still accelerating pretty well, but I agree not to LS3 levels.

The main things that are detracting from the performance include:
- Heat generated by the supercharger - hot air has the same pressure at less density.
- Driving the supercharger - this is a considerable drag on the engine.
- Valve and spark timing optimized for higher boost levels, the LS3 is optimized for this manifold pressure, the LT4 is less efficient here for making power.

If you take an LS3 and put a supercharger on it, you will see the same thing.

dar02081961

I think Higgs comment is generic and is made with the assumption that boost in and of itself means or equals backpressure.

Meaning that boost itself isnt what creates power. Or said another way you can have boost and not create additional power if you dont have increased flow (CFM) AND the accompanying fuel to take advantage of that additional oxygen created by compressing the charge.

So boost without flow and matching fuel create zero to little extra power. All you have without these elements is back pressure.

That being said you are correct if matching fuel is there keeping the AFR acceptable (which it should be) boost will always create additional power up until the limit of heat or airflow (CFM) of the engine and exhaust system.

Higgs Boson

pretty much, yes....

sort of like adding headers, removing cats, adding a high flow cat back, porting the heads, installing a bigger camshaft and losing 3 lbs of boost as a result but picking up 100 hp.....

boost in and of itself (would you call that boost in a vacuum? hahaha I'm so funny) is just a measure of restriction.

dar02081961

I know what you meant by this but it isnt 100% correct.

For example depending on the gear and the terrain in the LT4 there are conditions where we can hold the boost/vacuum at 0 and not accelerate or in extreme cases still lose speed/momentum simply because there isn't enough horsepower being produced to overcome the load factor. So we cant always use acceleration to delineate.

At the low loads required to replace manifold vacuum supercharger efficiency (heat) is not generally a factor either. Heat is a byproduct of compression and is a factor in superchargers and turbos however this heat factor occurs when we compress the charge. Replacing vacuum doesn't create much heat simply because we haven't compressed the charge until boost is achieved(above 0 boost/vacuum).

Driving the supercharger does put considerable drag on the engine. However this drag isn't a set figure. examples given below are to aid the conversation and not actual hp equivalents. But follow the logic.
HP required to drive the Supercharger.

1. Minimal. It is much less when the bypass valve is open during cruise and there is vacuum. lets say the drag is equivalent to the hp it takes to run the power steering pump.

2. It is slightly more when the bypass valve is closed and we are in vacuum but less than 0 PSI boost. Lets say this hp (drag) is equivalent to the power required to run the AC compressor and the alternator.

3. When we are creating boost (positive PSI). This is where the significant drag to run a supercharger comes in. It increases as boost PSI increases and is said to be as much as 200 hp on an LT4 at 9.4 PSI at max power RPM.

So here again at cruise and small throttle openings where we are just replenishing vacuum there isn't that much of a price to pay in parasitic hp for the supercharger.

The LT4 will always have better (more optimized) valve timing than an LS3 because it has variable valve timing. (compare an LT1 to the LS3)

Spark timing is a function of cylinder pressure, engine speed and fuel quality. Both engine computers will always run optimum timing for the above conditions. Neither has an advantage.

The LS3 supposedly has a slight compression advantage (10.7 vs 10.0) but if we consider variable valve timing, less intake duration of the LT4 cam and the supercharger, I am not sure at any rpm above idle the Ls3 ever effectively produces higher cylinder pressures than an LT4 (ditto for the LT1). Regardless the additional .7-1.0 CR advantage of the LS3 and LT1 over the LT4 isn't what folks will have you believe.

spoolin98

spoolin98

silver74vette

Questions and comments below,

So are you saying that the OP is incorrect in his observation and the LT4 produces more power than the LS3 at 1 atm (at the intake valves)?