falcon

iTrader: (13)

Yeah I need to find a turbo with a choker line past 400 cfm that will still spool quick. If I can get 8 to 10 psi by3200 rpm id be happy. With my built motor and the headwork and cams it should help.

vehicular

iTrader: (3)

The choke line on a compressor map is an indicator of how much air the compressor can move on it's own without the Rotrex cramming air down it's throat. I just can't say for certain what the effect's will be when you try to do that.

You can pretty easily calculate the choke flow rate of the area of the GT2252 inlet, minus the area of the leading face of the wheel (nut/ wheel hub plus area of the blades), and I'm pretty sure you're still a ways under that (estimating, without having a wheel in front of me to measure for the calculation), but I don't know what other factors to accommodate for.

triple88a

iTrader: (2)

Also wouldnt over spinning the smaller turbo become an issue at redline?

vehicular

iTrader: (3)

The wastegate bypassing the turbine should keep overspin under control, as long as you don't try to run the guts out of the setup. As the Rotrex spins up, demand on the turbo to keep up will drop off significantly.

triple88a

iTrader: (2)

In other words you'll need a very big wastegate and a very big bypass valve.

falcon

iTrader: (13)

No. not with an appropriate wastegate. I think I would run the wastegate on absolute press at the manifold for target boost. Ie you want 18psia fast as possible and hold all the way to redline.. In the low rpms the turbo would boost higher and as tge rotrex starts taking over the turbo would actually push les air because the wastegate is compensating.

vehicular

iTrader: (3)

Yes on the wastegate, no on the bypass valve. As demand on the turbo drops off (as the Rotrex spins up), you'll be routing more and more exhaust gas around the turbine housing. You'll need a wastegate biased exhaust manifold, i.e. a header with the wastegate at the end of the collector, and the turbine inlet off to the side. If you treat wastegate placement like it's just a pressure tap, you're going to severely limit the effectiveness of the setup as you force the exhaust gas to go through the turbine, which really isn't needed much at the point that the wastegate will open up.

No on the bypass valve. A bypass/ bov is totally flow dependant, unless you're trying to cut a blower out of the loop, which you wouldn't be here. I'd use the new Synchronic pypass valve vented to atmo.

falcon

iTrader: (13)

^^ you seem to get the idea.

I think a GT2560R would be a good turbo for this plan. Quick enough to spool and large enough compressor that it shouldn't have much issues allowing 300CFM to flow through it.

I think what I may do is instead of running 14PSI on the Rotrex and 5-6PSI on a turbo, run 8-10PSI on both. Keeping both power adders in their efficiency zone which would in turn = lower intake temps and less stress on both the blower and the turbo.

vehicular

iTrader: (3)

You would so little benefit from this that you might as well toss the Rotrex and just run the 2560.

I asked about this on another board, and a guy who has done some compound turbo stuff suggested that you divide the mass flow of the system by the PR of the large compressor to get an idea of where in the turbo's compressor map you really are. This puts you all up in the efficiency range on a 2252...

sixshooter

iTrader: (12)

I agree the 2560 will be too big to do what you want.

I would also agree that you will want a large external wastegate to control boost. With the Rotrex alone you are mechanically limited by its spinning speed from overboosting. But just like the super big gulp of air you are giving the compressor side of the little turbo by force feeding it with the Rotrex, you are going to be seeing far more than it's normal capacity coming into the turbine side from the engine. Much, much more exhaust will be generated than it would be capable of generating itself without the help from the Rotrex. You will need a big EWG to control boost, but with it this setup should work very well and give you solid boost control.

falcon

iTrader: (13)

So right now I am pushing a 2.0PR and 14PSI. So if I want to have a max of say 20PSI then I devide 2.36 (20PSI PR) by 2.0PR of the Rotrex to get 1.18PR. So I need to find a turbo that is efficient in teh 1.18PR area on the map? But at what CFM?

sixshooter

iTrader: (12)

With the external WG, you can have the little turbo go all the way to 20psi as soon as it can and then have the Rotrex supplement its volume as the Rotrex spools up and maintains 20psi all the way up the rpm range. Your boost pressure should be flat from when the little turbo gets spooled to target all the way to max rpms. You need a little turbo that is capable of the PR of your target if possible or as close as is practical.

Remember, the turbo compressor's efficiency will improve as the Rotrex continues to build flow and pressure. Lets say at 3000rpm the little turbo by itself could generate 16psi out of your 20psi target. Add to it what capabilities are being added to it by the Rotrex at 3000 rpm and you very well might have your 20 psi. You will get no more than 20 psi because the EWG will cap the boost. But you will get 20psi early and will hold it throughout the rpm range.

I would think that any little turbo should be able to get you the last 6psi from what is coming out of your Rotrex. Spooling up quickly to a good PR will be the important part.

falcon

iTrader: (13)

Yeah you and I have similar thinking. That's the idea that brought me to this in the first place. Have a little turbo spool like a madman off the bat and then as the Rotrex takes over the overall efficiency increases and the little turbo can spool down a bit.

Now is the question. Can a GT2252 spool to 15PSI and not be completely out of it's efficiency blowing a **** ton of hot air?

I need to play around with some compressor maps. First off I need to find where I am on my Rotrex map from 1000RPM to 8000RPM.

sixshooter

iTrader: (12)

Buncha different compressor maps:
http://www.squirrelpf.com/turbocalc/map.php



Looks good to me.

falcon

iTrader: (13)

So here's a question. When the turbo is spooling say 6PSI while the rotrex is pushing 14PSI through it. Does that mean it's still sitting at a 1.4 PR on the map but waaay over to the right on the lb/min scale? Would that not push it past it's choke line? Or when you said earlier that you devide by 2, then I take whatever CFM the Rotrex is pushing and devide that by two to get what the turbo is actually doing at that 1.4PR?

giblets

I would have thought the the turbo would need to be able to flow the lb/min entering the engine, but that's just conjecture.

If the turbo is 'sucking' from the rotrex, is that going to effect the level of boost immediately after the rotrex? i.e, the turbo makes the 1.6 behave like a larger engine, I would assume that the rotrex on a larger engine would see more flow and less boost for any given rpm?

JasonC SBB

Correct and thus the reason for my post, which falcon blew off.

JasonC SBB

Correct. An s/c between the turbo and the engine just makes the engine look bigger. An s/c before the turbo just makes it look like the engine is running below sea level. The PR is still the PR of the absolute pressures at the compressor inlet and outlet. The mass flow rate is still the mass flow rate.

JasonC SBB

Has anyone pointed out yet that a centri s/c and a turbo aren't exactly complementary?

giblets

well falcon already said he's doing it because he likes messing with the car,

I say buy two turbos and see which one you like more

are you going to have two boost gauges?