Why does boost kick in at say 3500 rpms on a vortech type blower when the roots type comes on strong at 500? Can it be lowered?

Also, I know I asked this in another thread but lost track of it.... What are the pros and cons of installing a lightened fly wheel when changing ones clutch?

Thanks!

 

You can lower your compression ratio but, of course, that is $$. I have a 9:1 but that was done when I forged my pistons.

You can always install a cam with an operating range down low. Would NOT do anything for the boost but would provide more power down low. I have one with a range of 1800-5000 RPM but with my new ratio I see good boost effects as low as 2000.

 

The difference is because Positive Displacement type superchargers (Roots and Twin Screw) are fundamentally different from centrifugal superchargers (Vortech / Paxton / Procharger). In a roots blower, air is trapped between the rotor lobes, and every single revolution of the rotors moves that fixed amount of air. E.G. On a TVS 1900 every turn of the rotors displaces 1900cc (1.9L). On a TVS 2300 that number is 2.3L.
Your engine, of course, is also a positive displacement device; for an LS2 (6.0L engine), every two revolutions (because its a 4 stroke engine) the engine will take in a theretical 6L of air. Real efficiency is more like 80% of the theoretical displacement but you get the idea.
Now, A Whipple 3L will move 3L per revolution. Lets say we stick one of those on top of an LS2, and pulley it down to a 2:1 ratio; if our engine is turning at 2000RPMs, the Whipple will be going at 4000RPM. Our engine is now sucking in ((2000 / 2 *6)/28.32) = 212CFM. But the Whipple is putting out ((4000*3)/28.32)=423CFM. Where does all that extra air go? It pressurizes your intake manifold (I.E. Builds boost). This example would see about 1ATM boost (14.7PSI). You can calculate how much power that works out even. A rough rule of thumb is 1.5cfm per hp. Knowing that, you can figure out how much HP a positive displacement blower is good for by looking at its displacement and the highest speed it can be spun up to.
And since the supercharger is being driven at a fixed multiple of the engine speed, as the engine speeds up it too speeds up maintaining that pressure ratio. The only time you are not making full boost with a positive displacement blower is if you have run out of efficiency (like the roots blowers are notorious for doing up top), or in the very low end (sub 1500RPM or so) where the volumetric efficiency of the blower is affected by leakage between the rotors.

Now lets look at the centrifual blower; as the name implies, it relies on centrifugal force to move air; there is no hard seal between the inlet and the outlet, and centrifugal force depends on speed, and lots of it; at low speeds the supercharger is hardly moving any air at all, but the amount of air moved goes up almost exponentially with speed. Centrifugal superchargers always make peak boost right at redline and there isn't much you can do about that, since that is just how they work.

The way to make boost earlier is by increasing the drive ratio of the supercharger (I.E. Pullying it down). It will spin faster when the engine is at lower RPM and make more boost. But then you have to worry about making too much boost up top; ECS uses a restrictor plate to prevent that. Another issue with pullying down too much is that you are increasing the drive power into the blower by spinning it faster, but reducing the surface area available for belt contact. Pully down too much, and you start needing wider pulleys like the 8 rib system...

Hope that made sense

 

Wow PowerLabs, that is great information. This will make my next choice of supercharger a lot easier.

 

Well, it doesn't tell the whole story... Remember the supercharger is just one component in an FI kit I'm still waiting for someone to come up with a good twin screw kit; hopefully it'll happen by next year when I get an LS3 C6...

 

Crystal Clear. GREAT Explanation.

 

Ill add my knowledge to this question.

Lightening the flywheel does not increase horsepower per say but it does decrease drive line weight therefore decreasing rotational mass and translating more power to the wheels. Also a lightweight flywheel allows the engine to rev faster (again be decreasing rotational mass) in neutral allowing easier rev matching (primarily for down shifting).

A new clutch essentially helps the tranny "grip" the power from the transmission. I replaced my clutch on my last car (350z) and it allowed me to chirp the 2nd to 3rd shift (not able to before) because it slipped less which is caused by an increase in clamping force and a softer compound in the clutch itself (think softer tires/more grip).

Downsides of both of these mods are decrease in driveability and decreased life of these parts. The flywheels weight is engineered from GM for maximum performance while still allowing easy starts. Decreasing the mass of the flywheel makes it harder to get the car going smoothly (not sure on the science on that one). And a grippier clutch will not slip as smoothly on starts and will be a bit jumpier. Also as with softer tires, a softer compound clutch will wear out faster.

Please correct me if im wrong on anything above.

 

Thanks for the lesson on boost Sam, that was very informative.

So to put it into simple terms, I'd need a two speed pulley system if I wanted to generate boost at lower rpms and not blow up my stock bottom with my set up then correct?

The explanation of lightening the flywheel was perfect as well.


Thanks again fellas, you guys are the best!

 

You'd need a smaller pulley and a restrictor plate. That's all

 

I feel stupid. I should have known that! Sam - you are the man. My shop told me that "in a round about way" not more than a week ago.

I know compression ratio effects the boost in some fashion. Can you explain?

 

Why not use a variable restrictor plate and a smaller pulley so the boost can be altered without having to change the pulley?

Seems like a variable plate could be fairly easily constructed and controlled electrically from inside the car!!

Perhaps the issue is the tune???

 

This is the approach Taylor at DP was working on. He released info a couple months ago but I haven't seen anything about it since.

 

Taylor

Can you be as helpful on this item as you were on the Z06 fuel pump question? Hope so and thanks by the way.

Might even be worth transporting the car to DP to get the FI install done?

 

http://www.youtube.com/watch?v=nAG1enBPwso

lot's of testing going on as we speak

 

Great, answers another thread question I had on boost by gear. I am an A6 user so I don't think the micro-switch approach will work for me.

I am wondering about the same servo concept but simpler and only used to select a maximum boost level from the cabin.

I am sure you would sell a bunch (at $350-400) if you had an A6 version with user programmable boost levels per gear and a selection of programs (daily driver/save the stuff, drags, drags with money on the line, etc.) perhaps 4 program scenarios.

What effect does the restrictor plate size have on the tune? Do engine parameters need to change to match the boost curve or do they track the boost using the MAF?

 

Compression ratio doesn't affect boost very appreciably; it will change the volumetric efficiency of the engine some (the engine's ability to take in more or less of its theoretical capacity), and that will have an effect (more volumetric efficiency = less boost at any given airflow number. That is why higher lift / longer duration camshafts drop your boost level even if there is no valve overlap) but the main effect of changing the compression ratio is that it will change how much boost you can run on any given octane fuel; the higher the compression ratio, the less boost you can run, specially on pump gas. On the plus side, the higher compression ratio engine will be more responsive, more efficient and make more power and torque both on and off boost. It sounds great, but high compression severely handicaps maximum power: since all gases heat up when compressed, you can only compress air and gasoline so much before it spontaneously ignites, but, because air compressed by the supercharger is intercooled before being fed to the engine, you can reach much higher charge densities by dropping the compression ratio and upping the boost, and thus make a lot more power

 

Gentlemen I am new to all of this but I just add a 3000 torque converter this help me get in to boost much quicker. The torque converter has 3 ballon plates and it able to be loc up at wot,plus it program to work with my trans

 

Is there a question in there somewhere?

 

I am only stating that if you want to go in boost much quicker at a 3000 stall

 

Don't get me wrong Dan, I have zero issues with the performance of the car, it's as fast as I can handle. I was just curious as to why the two types of blowers made boost at different rpms.

So Sam, by dropping the CR, does that make the stock motor more capable of handling higher boost levels without blowing up?