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Discussion Starter #1
I have a question on the functionality of the blower. Whereas I am a noob to the CTS-V, I am not a noob to force feed engines. My question is concerning the boost gauge on the car. I thought this was a supercharged engine, and the boost would be the same no matter what is going on.

I learned a lot when I installed a turbo on my Harley. The volumetric efficiency of the naturally aspirated Harley engine is about the same as a Chevrolet V8. That is about 75%. I think the similar size of piston, stroke and valves must play a part in the similar numbers. The V.E. on a turbo engine varies, depending on the boost. When under full boost the V.E. reaches into the 90's% area. The top number is most likely dependent on the max boost. One big difference between a turbo engine and a supercharged engine is that the volumetric efficiency of a supercharged does not vary, and whatever it is, it is. Low RPM V.E. is the same as high RPM V.E.

Now I drive my V, and when I dare to glance at the gauge under WOT, the needle is moving. Why? Shouldn’t the boost with a blower be the same no matter what I am doing with the engine? Isn’t the blower a roots type screw that is increasing in RPM the same as the motor? What is telling the boost to increase? It is certainly not exhaust gasses, but then what is it?

Sorry if this is a noob question. Just curious. Thank you for enlightening me.
 

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A roots blower moves a specific amount of air per revolution. As you increase the motor RPM, the blower RPM increases. Pretty simple concept. The thing that isn't simple is how the engine ingests the air. As you valve timing varies, you will be pushing against those closed intake valves more at higher RPM. This is what causes the boost gauge to move. It shouldn't be significant, but it does happen.
 

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I have a feeling this thread will lead into a cam install discussion. I think understanding the above mentioned topic will help a lot of owners in terms of modifications and how they improve the overall performance.
 

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Discussion Starter #4
Thank you for helping me understand what is going on with the blower. You mentioned the cam, which leads to even more questions. On the old turbo Harley, the recommendation was to install a cam with zero overlap. Makes sense since we are forcing so much air in the intake runners, we do not want all that fuel air mixture blowing right past out the exhaust. The resulting idle on the motor is very tame, even boring.

Fast forward to a video I watched on this site of a modified V with a new cam. The engine sounded like it was slobbering all over itself at idle. Read misfire. It sounded like some big cam with plenty of overlap, so much so that it has a hard time idling. Don't get me wrong, I love that sound, but it is counter intuitive to a force fed motor.

What am I missing?
 

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This is why the LS9 is popular for an LSA upgrade. Smooth idle , -24.5 overlap to help meet emission and not blow fuel out the tailpipe. BTR stage 1 & 2 positive displacement blower cams are -10 deg too. Still streetable.
 

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This is why the LS9 is popular for an LSA upgrade. Smooth idle , -24.5 overlap to help meet emission and not blow fuel out the tailpipe. BTR stage 1 & 2 positive displacement blower cams are -10 deg too. Still streetable.
Thank you for reviving an eight year old thread.


Sent from my iPhone using Tapatalk
 

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Gotta be a record.
 

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Here is some info that might help one understand what Volumetric Efficiency (VE%) is.

Trapped Ve% is the Measured CFM minus the Ring Blow-By CFM, minus the CFM
lost during the OverLap Period, then divided by the theoretical CFM

Theoretical CFM @ 100 % Ve = CID * RPM * .000289352.

For those that prefer to divide instead of multiplying using the
above decimal value, the reciprocal of .000289352 is 2.4.

TrappedVe = ( MeasuredCFM - ( BlowbyCFM + OverLapCFM )) / TheoreticalCFM

Increasing VE% also means more fuel can be added helping to make more power.
However, fuel takes up space within the combustion chamber.

This then leads to an increase in your 'Dynamic Compression Ratio'.
which is also more accurately described / called 'Trapped Compression.

These engines, the stock LSA, have very low static compression ratios.

So as we spin the little 1.9L blower harder, it moves more air, we add
more fuel, and the Trapped Cr increases.

One more however..lol

The 'Burn Angle' increases considerably relative to an NA Engine
having a higher geometric or static Cr.

This is because the higher the static compression,
the more efficient the engine will be.

The above can be referenced either as the 'Otto Cycle' or the
'Percentage of Air Standard Efficiency' (%ASE).

So, if we take a low compression engine, and apply 2-BAR of boost,
we will (unassumingly) also be filling the cylinder with a larger amount of fuel,
which will take much longer to burn, hence . . . . 'The Burn Angle' will be
longer as referenced to time, in order too burn all of the fuel.

The above issue leads to a very different Exhaust Lobe requirement than
would be the case for an NA Engine, making the same fwHP, but having an
higher Static / Geometric Cr.

Using an oxygenated fuel also helps, as many on this forum do when using E-85,
as an oxygenated fuel burns quicker, hence a lower numerical value, given in degrees,
will be seen for the burn angle.

The above referenced (lower value) burn angle, associated with the oxygenated fuel
will also be of an lower / similar value for an engine using a higher static Cr.

Finally. . . . .
Please do not pay attention to articles ill written on the Internet regarding 'Dynamic Compression Ratios'.

A dynamic compression ratio of, lets say. . .8.0:1, is a simple scalar value,
which does not change. The term 'Dynamic Means. . . It is a Changing Value.

If your 'Static Compression Ratio' is 8.0:1 and your able to achieve 125% VE
as the intake valve closes moving from the intake cycle to the compression
cycle, then your 'Trapped Compression / Dynamic Compression' will be. . . .

=> (8.0 * 1.25)= 10.0:1.

Cheers,
RD
 
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