centri's - boost vs rpm

[Alter Ego Trip]

I've been wondering about this lately. Twins have the advantage of building boost much earlier as opposed to a centri. Centri's build on a linear curve, which is dictated by RPM (dumbed down).

So, what would be the disadvantage of dropping extra pulley sizes and then limiting total RPM. For example: a 3.33 pulley at 6500 RPM hits 11lbs. Let's say at 4000 RPM it's putting out 6lbs, whereas a twin might be putting down it's full 11. So by dropping a couple extra sizes we'd build boost earlier, say instead of 6 lbs at 4000 RPM we're at 7 or 8, but then limit RPM at the point it reaches 11lbs.

Would this not result in more power down low, with the same boost up top, while gaining the extra advantage of not putting so much stress on the rotating assembly?

I know there's something wrong with this reasoning, I just can't figure out what.

 

[07TwinScrewdGT]

well there are a few reasons why but a good one would be lets say you do that and lower your pulley and you make your 11psi at 5K instead of at 6500 so before when you pulled 2nd gear to 6500 and shifted to 3rd you would of been lets just say as an example around 4000 rpm but if you are shifting at 5K in 2nd it will effectively drop the rpms when you shift to 3rd to lets say 2500 instead of 4000 because you shifted 1500 rpms earlier (this is also known as short shifting) so now you are lower in your power band in 3rd gear making less power AND torque than you wouldve at higher rpm and it will take longer to pull that gear so you will end up slower... not sure if im wording that right... u see my point? also a twin screw makes max boost down low like you said but a twin screw at 11psi only pulling to 5K on a dyno will make alot less power then it would if you let it pull to 6500 at the same 11psi... So in the end you might make more mid range power and torque with your centrifugal but shifting early you are going to make less power OVERALL at 11 psi at 5K versus using a bigger pulley and making the 11 psi at 6500 even though it takes longer to make the boost... could be wrong but thats my .02

 

[Alter Ego Trip]

I understand your explanation and it makes sense. I wasn't thinking of quite so drastic of a change; maybe a drop from say 6500 to 6000 RPM.

And short shifting isn't an option for me, lol.

 

[bigray327]

Consider this: unless you're rolling around on ET Streets all the time (which some guys are), do you wanna be making big boost down low? I don't. On the street, I have enough loss of traction only making a few psi down low, so it's good that the big boost doesn't come in until traction isn't necessarily an issue.

 

[Alter Ego Trip]

Consider this: unless you're rolling around on ET Streets all the time (which some guys are), do you wanna be making big boost down low? I don't. On the street, I have enough loss of traction only making a few psi down low, so it's good that the big boost doesn't come in until traction isn't necessarily an issue.

Does a 555R count?

 

[8306gt]

I understand your explanation and it makes sense. I wasn't thinking of quite so drastic of a change; maybe a drop from say 6500 to 6000 RPM.

And short shifting isn't an option for me, lol.

This is exactly how mine is set up, I am running a 10% over crank pulley and a 3.48 blower pulley and have the trans set up to shift at 6000 rpm. My car made 13.1 lbs at 6200 so I don't really need to spin it any higher however if I were feeling brave there is another 60 or so horsepower by spinning it to 6600.

 

[eng943]

The outcome of doing that is going to vary based on what type of centri blower you have, but you are correct in theory.

Frankly, and as others have pointed out, it's a bit of an excerise in futility if you ask me. Any car when being raced is going to be between redline and about 1,500 rpm below, which is where a centri is more efficient. Chopping it's ability to spin up for the sake of more low end is counter intuitive.

A better solution for low end would be gears or T/C if you have not already done so.

 

[lito]

You could pulley it down and install a wastegate that is boost controlled so you could flat the boost around where you want.

I'll have a F1R car on the dyno with this in thursday. Car is making 22psi up top and want to leave it at 13-14 so it can be used on pump.

In theory it works, and customer commented me that it does but I have to see it first.

 

[bigmachano]

You could pulley it down and install a wastegate that is boost controlled so you could flat the boost around where you want.

I'll have a F1R car on the dyno with this in thursday. Car is making 22psi up top and want to leave it at 13-14 so it can be used on pump.

In theory it works, and customer commented me that it does but I have to see it first.

that would be really interesting. wouldn't that be inefficient as hell? not to mention the car would sound like a vacuum cleaner! but i love the concept. please update us on that one

 

[lito]

that would be really interesting. wouldn't that be inefficient as hell? not to mention the car would sound like a vacuum cleaner! but i love the concept. please update us on that one

The car already sounds like a huge sewage vac truck combined with a garbage truck.

F1s with the red race bov are just horrible on the street, everybody turns around as you pass.

Inefficient yes, but gives a chance to use the car on the street on pump and race on the strip without changing pulleys around.

Will publish data when it is done.

 

[05yellowgt]

F1 with race valve loud?? No way! AHAHAHAHAHAhAhAHAHA!!!


Doing what lito suggests does work, at the cost o generating a bit of extra heat since you are spinning the blower harder. In the end, what the OP is asking to be able to do just makes more sense to do with a turbo setup.

 

[JeremyH]

Im interested just to see the details of making this setup work. Its gonna have to be blow through since the wastgate would be going on the discharge side of the blower, And would you want the bov upstream or downstream of the wastgate, assuming the best place for the wastegate is going to be closer to the blower.

Its seems like centri-turbo envy to me though lol

 

[Germeezy3]

That is how a lot of OEM setups control boost, with a bypass valve to vent boost to maintain a certain level through redline once the max boost threshold is reached.

 

[JeremyH]

That is how a lot of OEM setups control boost, with a bypass valve to vent boost to maintain a certain level through redline once the max boost threshold is reached.

What are you talking about?

We are talking about centri blowers...

 

[james05gt]

^ I think he meant wastegate? I don't know.
Might be going off topic but I remember reading an article on Muscle Mustangs about it, centri with a wastegate that is.

 

[Germeezy3]

I was mainly referring to the strategy of using a wastegate to limit max boost. I was just over looking at a friends S351 with a Vortech and wastegate setup.

 

[Marc s]

I understand your explanation and it makes sense. I wasn't thinking of quite so drastic of a change; maybe a drop from say 6500 to 6000 RPM.

And short shifting isn't an option for me, lol.

Your post is timely, I have planned to test this at the next TNT. My car with the 4.38" pulley makes 14lbs of boost at 6,600. The HP and torque are 549/452. With the 4.13" pulley my car makes 14lbs of boost at 6,000 and the HP and torque are 549/490. I currently run a 122-123 MPH and cross the finish line at 6,100 RPMs. I believe the smaller pulley and shifting at 6,000 instead of 6,600 will decrease my ET's.

 

[Alter Ego Trip]

Your post is timely, I have planned to test this at the next TNT. My car with the 4.38" pulley makes 14lbs of boost at 6,600. The HP and torque are 549/452. With the 4.13" pulley my car makes 14lbs of boost at 6,000 and the HP and torque are 549/490. I currently run a 122-123 MPH and cross the finish line at 6,100 RPMs. I believe the smaller pulley and shifting at 6,000 instead of 6,600 will decrease my ET's.

Yes, but I'm running an auto so I don't have to worry about that as much. My boost will drop a little, but not enough to be that detrimental. I am very curious about the significant increase in torque with the smaller pulley, with no loss of HP. I'd love to see the graphs side by side to see how the curve is shifted between the two.

 

[SteveP]

An option you could go with is getting the Innovater's West 10% Crank pulley. With that and a 3" pulley on the blower, I make up to 7lbs of boost right away And the cams help me with the top end. So I'm building boost all the time. And what's great is if I'm just cruisen around, i can still stay out of boost (long as I shift under 2500rpms)

 

[Marc s]

Yes, but I'm running an auto so I don't have to worry about that as much. My boost will drop a little, but not enough to be that detrimental. I am very curious about the significant increase in torque with the smaller pulley, with no loss of HP. I'd love to see the graphs side by side to see how the curve is shifted between the two.

4.38" pulley vs 4.13"

With the 4.13" pulley I had 39.5lb injectors. It started going lean so we shut it off at 6,200. We installed 60lb injectors and the 4.38" pulley to remove some HP. However, the HP and torque under the peak is much stronger with the smaller pulley. That's why I am going to swap back to the 4.13" and shift at 6,000.