I know I’m missing something simple, so please forgive my question.

I’m wondering at what intake airflow on a roots or screw S/C application does the 90mm stock LS3 throttle body begin to severely restrict airflow drawn into the engine?

Also, can there be mass air flow conditions where pressure drop across the throttle body is great enough to cause a vacuum condition at the SC inlet and cause the bypass actuator to open the bypass valve and cut boost?

Looking at the pressure drop across a standard LS3 throttle body (28”Hg at 1360cfm)...it drops quite allot of pressure when you reach airflows north of 600HP.

For the Whipple the manual states the bypass valve opens when the vacuum is >3 inches.

My MAF shows I’m flowing around 4750lb/hr at peak RPM and power, or around 1050SCFM.

I estimate the 90mm LS3 throttle body is dropping more than 8psi at that 1050 flowrate.

So it would seem to put the pressure at the SC inlet close to 14.7-8=6.7psi (actually less pressure if we account for MAF and airbox/filter/airtube drops).

In any case, the vacuum at the SC inlet should be way deeper than the quoted opening value of the bypass actuator....but I know its not opening, because I develop boost just fine.

So I am confused and wondering what keeps the bypass closed under WOT when there is significant vacuum condition due to that high rate of flow across the intake apparatus?

 

Found this article that helped with allot of the questions I had...

http://www.superchevy.com/how-to/tra...wer-gettin-air

Still don’t understand what keeps the bypass valve from opening up under high flow and throttle body pressure drop....

 

The bypass valve uses intake manifold pressure as a reference to open and close. Boost pressure pushes the diaphragm on top of the actuator to close the bypass valve, allowing it to build boost, while engine vacuum pulls on the actuator to open the valve and let air bypass the rotors.

 

Thanks for the article link, definitely a good read and informative. I’ve always known there was restrictions in front of the supercharger but I didn’t realize how much.

With the bypass valve it’s the boost pressure being applied to the diaphragm of the actuator that keeps it closed even though there is vacuum in front of the valve that would normally cause it to want to open. Without the boost pressure from after the supercharger it would open.

 

Thanks for the input.. I did also notice a huge flaw in my original post. At rated flow of around 1350scfm, the LS3 throttle body drops 28 inches of H2O not Mercury. So at rated flow it only drops around 1psi. I also looked at some pictures I took the last time I had the throttle body off the car, and it does look like the actuator vacuum line is connected to the SC case underneath the bypass butterfly, which means the actuator is controlled by manifold pressure as Remnant-of-VII says (thank you).

I also was looking at an LS3 Camaro Whipple chart of pulleys, pressure ratios, etc for the Whipple by Stage 3 Motorsports, and it looks like whether they used a 4.25" pulley all the way up to a 2.53" pulley on 7.5" crank pulley with a pressure ratio of 1.88 to 2.73 and making boost of 7.5 up to 19psi @6500RPM, the SC inlet was between 12.35 to 12.92 psia. Lets just say 12.5psia to make it simple, which means the air intake and TB combined dropped about 14.7-12.5=2.2psi

The LS3 90mm throttle body is rated 1350scfm @ 1psi of drop. My car flows about 1050scfm at 6500RPM.

If we say the 90mm on my car is dropping 1psi x 1050^2/1350^2 = 0.6psi (its less than 1psi because I'm flowing less than rated flow of 1350 where it drops 1psi).

What kind of gain should I expect from a 102mm? Well the 102mm is rated around 1550scfm @ 1psi of drop, at my flow of 1050scfm the larger body would drop 1psi x 1050^2 / 1550^2 = 0.458. To me this says, all things being equal I might gain about 0.6-0.458 = 0.146psi at the SC inlet by upgrading the throttle body... or since I'm running a 3.625" pulley for a 2.07 pressure ratio, I'm looking at 0.146 x 2.07 = 0.3psi of additional boost in the manifold by upgrading from 90mm to 102mm tb.... not very significant.

I suppose on the other hand the air cleaner + MAF, etc is dropping 2.2-0.146=2.054psi .. I wonder how much of that approximate 2psi of drop can be eliminated by going with something like the Synergy 4.5" setup with the LS7 MAF?

I intend to hook up a vacuum gauge tomorrow to one of those large nipples on the SC inlet nozzle between the throttle body and the SC screws and see how much the intake pressure actually drops against that pulley chart.

Has anyone already measured the pressure there on a WOT run for a Whippled LS3 with stock 90mm throttlebody?