AZGASSER

Quick question with maybe a not so easy answer. At what engine RPM does the Z start making boost? At what supercharger RPM? Just looking at the relationship between the two and how all the pulley changes work. I have the 2.3 CPR pulley and I know I am spinning the blower faster so in theory I should be making boost earlier in the RPMs, correct?

T/A KID

If I get time later I'll pull a log and do some math for ya!!

But yes, faster blower speed will indeed increase rotor speed and deliver boost earlier..

davepl

Actually they make boost at idle if the throttle were WOT and the bypass closed. Not as much as up top, because the blower's not nearly as efficient at that low of a speed, but still in boost.

It's not like a turbo or centrifugal that has to get to a certain impeller speed to make boost. Because:

1) the blower is driven directly off the crank
2) the blower (as geared) displaces more than the engine

therefore

3) except for internal leaking and inefficiency, it ALWAYS makes some boost

But your basic conjecture is correct - pulleying up the blower will make -more- boost down low than before.

silver74vette

I agree with Dave, RPM is not the critical factor. The position of the bypass and throttle valves are.

How are you defining making boost? Increasing pressure across the blower or having post blower pressure above atmospheric?

Since you are running your supercharger faster you will create more pressure (boost) for a given set of conditions.

Many folks talk about boost and have very different definitions, especially when comparing turbos and superchargers (due the the compressor being before or after the throttle valve).

My Wife's TT Cadillac has a "boost gauge" that shows pressure before the throttle valve which is cool but does not correlate to engine power the way the manifold pressure gauge in the Z06 does.

Z06NJ

Is anyone factoring in load into the equation?


It's possible to gently press on the gas and increase the RPMs slowly until you are at 6K, but at the same time not be in boost at all, is that correct?

silver74vette

You can keep manifold pressure in the negative all the way to redline. It depends on what you call making boost.
For a supercharger if the bypass valve is shut you will have a pressure differential across the compressor, I consider this boost. Some people do not call it boost until you exceed atmospheric pressure.

Z06NJ

To me, I consider boost when I can feel it

silver74vette

Not gonna argue with that one!

dar02081961

At 70% throttle (on the way to WOT) and 1120 rpm, I was producing 4.2 psi in 1st gear in a stock M7.

In cruise mode in 7th gear at 1200 or 1300ish rpm if I go 75% plus throttle I often see 1.5-2 psi in my logs.

This is why I say folks complaining about the small supercharger aren't telling the whole story for the daily Z06 driver. Its also why an LT1 is never ever stronger than an LT4 regardless of where you are on the power curve. And finally its evidence those saying a M7 Z06 can't accelerate in 7th at 70 mph are being disingenuous.

Wrap your head around that fellows. 4.2 psi at 1200 rpm! Daaamn!
GM deserves a pat on the back!!

I will try to post a screenshot later for the non believing C7 Z06 haters.

dar02081961

Not sure where you are heading with this but don't forget about the E92 ECM being torque and driver demand table based.

Just because your engine can produce more boost and more power/tq down low doesn't mean you can get it. If you reach a torque limit in these tables your output tq will be limited by throttle or spark to the max torque predicted or allowed in these tables. Complicated stuff but if you aren't tuned its possible you may not notice much of an improvement for this reason.

AZGASSER

I am looking at what blower RPM with the bypass closed you surpass atmospheric pressure, i.e. "making boost". I know the blower spins in proportion to the RPMs of the engine and the bypass valves open and close based on PCM inputs and manifold vacuum pressures. My question is, what blower RPM does the internal supercharger pressure exceed atmospheric pressure with the bypass closed?

silver74vette

Your best answer will come from T/A Kid or someone who can give you a datalog. One of the tuners on the site may be able to provide some good data.

Higgs Boson

As stated above, this type of blower can always make boost, and is therefore not dependent on RPM, but throttle position. It will make boost at idle RPM (not idle of course because technically Idle is foot off throttle). If you lay into the throttle at idle you will get boost (without other limitations like the computer or bypass). Maybe best tested in 7th at 800 RPM if you wanted to put that much load on the motor....it's just going to have no spark lead anyways.

So then, if you wanted to know the blower RPM, just multiply your engine RPM where you first see boost by the pulley ratio (crank and snout), which is about 3 or so on the LT4, so say 1000 RPM X 3 = 3000 blower RPMs. 6500 * 3 = 19,500 RPMs at redline.

dar02081961

An example of what Higgs, others and I are saying.
In this example the accelerator is at 55.7% and the throttle blade is at 62.7%. and as you can see at 828 rpm we have 2.4 psi of boost available already.

dar02081961

Absolutely. The ratio on the LT4 is 3.1 for 20,150 rpm at 6500 rpm.

Thomasmoto

I've been wondering when this might come up and ask a question or find an answer. Has anyone over ridden the by-pass valve yet? Either mechanically or electrically? Sometime the IAT temps get high enough that it dumps boost in 3rd gear or possibly others as well. My first concern would be the timing. It might not be a good idea to run as much boost as mechanically possible under certain circumstances. Any other ideas? Also I get the "reduced engine power " in the DIC.

I'm not trying to high jack the thread I thought it might be relevent to the post.