Polar Jet

A very simple question to those in the know... to all those with a centri kit, what RPM does boost usually begin to rise? I hear 2,500 and 3,500 all the time. That 1,000 RPM difference could be the deciding factory on whether or not I go roots or centri to my existing 08 LS3 or on a newer Grand Sport.

Thanks!
Rob

68/70Vette

First of all...the main advantage of a centri blower is also is main disadvantage. When I drive my 08 in city street and freeway traffic, I really don't get much above 2000 rpms. At 70 mph, I'm turning 1700 rpms. At these engine speeds, the blower turbine just isn't doing anything. Also, I understand the blow off valve is open, so the big advantage here is that It's basically as if I was driving a stock engine and getting stock gasoline mileage. The roots blowers take mileage down significantly even if you're just driving mildly on the street. ............So the main disadvantage of a centri blower, is that it won't produce power at low rpms,...you have to downshift. The advantage is that the car is very tame and fuel economical....below 2000 rpm's. I feel comfortable about my 65 year old wife driving the car, I do warn her to keep the rpm's below 2000.

I have an A&A Vortec supercharger. Getting up to the 2500 rpm range, I can feel the extra power. Approaching 3000 rpm, the engine...as compared to stock.....just explodes. Get pushed back into the seat, and the nose of the car lifts up, and with traction control on, the car jerks from side to side as the traction control keeps the wheels from spinning. I have the dyno print out for my car. The HP curve only starts at 3000 rpm. I get 300 rear wheel hp at 3000 rpm and 580 rwhp at 6500 rpm. Every thing else with the car is completely stock.

Basically,,,,,,I like the astounding power I get with my centri, but I also like the stock performance and fuel economy for normal driving.
......................

The turbine spins at 7.5 times crank speed. At 2000 rpm, the turbine is spinning 15,000 rpm's. Apparently, It just doesn't pump much air at 15,000 rpms. At 3000 rpms, it's spinning 22,500 rpms and is definitely compressing air. At 6500 rpm engine speed, the turbine is 48,750 rpm and this is max power. I think I'm getting about 8 1/2 psi compression at 6500 engine rpms/48,750 blower rpms.

Polar Jet

Thanks for the reply.

My current 08 is an A6 with a 2800 stall and a 3.42 diff so climbing up to 2500 RPM is a breeze. That seems to be where I would like the supercharger to start to kick in.

I love the roots blowers and the fact that they are what is used on factory GM vehicles, but with my current set up, they might be too much for the tires to handle.

68/70Vette

The current ZR1, with it's roots blower, is rated at 21 mpg highway. My stock 08 is rated at 27 mpg highway, and I don't think I'm taking much of a mpg hit. (I just mention mpg as an interesting observable, I wouldn't care if my 08 got 15 mpg, etc) I was originally planning on putting on an Edelbrock Eaton Roots blower, which I'd guess is exactly the same as the GM ZR1 option. I drove my car to A&A Superchargers in Oxnard, Ca. Andy Green, installs the Edelbrock superchargers..and has installed a large number of them, but of course he is partial to his centrifugal installation.

About your tire comment... I think that the standard run flat tires, although they are a little soft, are nevertheless too hard for a lot of hp. You don't want to be driving at max power, throw a shift, and have the rear wheels break into a spin at 60, 70 mph (for example).

old motorhead

The modern roots blowers (TVS) add little if any drag while not producing boost. Might even be less than a centri depending on how it's set up. The main reason the LS9 gets such poor MPG is the 9:1 compression ratio that it employs. The blower has nothing to do with it.

I'd urge the OP to drive both and decide how he likes his boost delivered. I kinda like all of the boost I paid for available whenever I choose. If too much power avialable down low is problematic to you....just don't press the go pedal down quite so far. If you can't figure that out, get the centri

MARSC6

I would say around 3500. If you don't have boost your in the wrong gear.

Unreal

I make 0.5psi at 1700rpm. So you make boost early but I would say you can't feel it until 2500+. Blower and motor setup will effect that.

JoesC5

PD blowers(LS9, Maggie, E-Force) have an internal bypass valve. When crusing down the highway, it only takes about 1/3 horsepower to spin the rotors, as they are not compressing any air. Give it the gas, vacuum drops and the bypass valve closes, directing the air to the rotors, and you have boost. Fuel mileage does not decrease with a PD supercharger when in cruise mode.

My Factory PD supercharged Mercedes has a electic clutch(looks like the clutch setup on the A/C compressor) so when in cruise mode, the rotors are not spinning, thus not using any horsepower. When I give it the gas, the car's computer automatically puts 12 volts to the clutch and I have boost. No waiting for a centi to spool up.

chimazo

I have a Vortech on my GS, but nothing hits harder than a PD blower. Just for comparison, my old GT500 was already at 15psi by 2k rpm. Insane low end torque that no street tire could easily put down to the pavement. Fun, but not the best way to go fast. But definitely fun.

turbotuner20v

Here's a ProCharger D1SC with a 4.75" pulley and stock crank pulley on a stock car (boost is lowest line, one with filter, one without filter):

3 Z06ZR1

Here's a vortech si trim on a ls-3 mustang dyno.
Vortech si on Ls-7 (second dyno)! Making more RWTQ at less than 2500 than the Ls-7 makes at any RPM stock.
The Ls-7 has a Paxton 1500sl now.
Same with the first dyno, boost is there pretty early.
These boosted engines ABSOLUTELY DESTROY THE STOCK ENGINE IN ANY CONTEST!
THe si made 9.03 on the ls-3 and 8 psi on the ls-7

saplumr

Is that graph correct? A LS7@8 psi and only 585 whp?

CussCuss

In an isolated sense, assuming the supercharger can also flow enough air, boost comes on as a square of the impeller RPM and the impeller RPM is directly tied to the engine RPM in this case.

So assuming no other things like restrictor plates or overdriven supercharges with wastegates and a redline of 6500rpm giving max boost, you would hit 1/4 of max boost @ ~3250rpm and 1/2 of max boost @ ~4600rpm

You can see this is turbotuner20vs graph.

1SwtRide

I wish I could supply all the answers to your questions, but the best I can do at the moment is to give you the below dyno sheet that is not how I'd want it, but is what it is for the time being. This run was aborted because my ECS build smoked the tires on the dyno at or about 4000/4500 rpm. However, you can see that the torque is pretty stump pulling - roots or otherwise. One observation, low end torque is really fun for showing off laying rubber, but when you get to the serious power levels, a V-tech might very well beat you will your sucking smoke chevy rubber (not serious but not impossible )

mdchaser

I happen to have an 11 GS as well and planned on having an A&A kit installed sometime in the (hopefully) near future. How do you like the setup? Any suggestions you can make as far as upgrades? I'm new to FI (last vette was H/C) so any thoughts would be appreciated!

Kadorja

With a centri you are always going to be making boost but feeling it just depends on the pulley size and when the engine naturally makes power itself. My car really kicks in at 3-4k rpm where the car is already making 3-4+ psi and the engines torque kicks in naturally. The tires light up in 2nd and sometimes 3rd. A smaller pulley would mean the power band picks up sooner and is higher at the top end.

Basically if I go WOT in 4th at 50mph or so I would make a low amount of boost (maybe 1-2 psi) but it would still making power. You could always run a setup with a smaller pulley and wastegate or the new Procharger i-1 setup. Lot of power right off idle and it levels off where you limit it.

saplumr

A centri won't make boost at low RPM's due to the BOV.

Red Sleeper

I've been doing some testing on my roots blower E-Force and here is where the boost comes in (about 1000 rpm) - Also 40 kPa = 5.8 PSI for anyone interested.

red line = boost
blue line = rpm

http://i145.photobucket.com/albums/r...92/Capture.jpg

MARSC6

Those lines look like a good ricer kill

ShaneFap

So many variables to answer this question.

Depends on CR/Cam/Load among other things.

For example My procharged G8 could make boost on the line brake stalling it on the converter. .8-1.4 PSI to be exact.