I've got a Vortech V3 Si on a Subaru EJ253 (with cams) and I'd like to get more power out of it. I'm maxed out the Vortech at 52k impeller speed.
We only make about 320whp at 15.5psi @ 6200rpm.
I'm thinking about putting a Rotrex C38-71 or -81 on it and spinning it to Rotrex redline of 90k with a 70mm pulley (crank pulley is small: 5.22" so 70mm Rotrex with 7.5 step up ratio will get us around 90k impeller @ 6300rpm).
I am wondering if anyone can offer any insight? The C38-71 should put us right in the middle of their compressor map... unless of course the Rotrex is so much more efficient that I should size up the Rotrex, expecting to make more power at a given Pressure ratio. Even on the C38-81 we'd still likely be inside of the white "island" on the compressor map.
My goal is basically to make as much power as possible without having to rev out the engine much more or more at all (i.e. I don't want to rev it out to 7k) I've also looked at ATI's procharger CS-1 which has an 8.3:1 step up ratio. If I am going through the effort of starting my build over from scratch I want a significant increase over 320whp we're getting now.
Here is a chart of my current power, estimated lb/minute and kg/sec (which is what the Rotrex go by) - Note everything beyond 52k rpm is a calculation AND these are all power levels at 6200rpm (i.e. 6psi @ 6200, 8psi @ 6200, 12 psi @ 6200.rpm). (Sorry my lb. /minute columns should be divided by 10, but you can also look at this as ~flywheel HP).
Rotrex compressor maps are here (note there's like 4 different C38 models) - I believe the 71 or 81 are best fit. A note on the compressor maps which is slightly deceiving. The white island on the 71 is 75% efficient. The white island on the 81 is 72% efficient. So.... even operating in the 2nd best ring of efficiency on the 71 is just as good as the best ring on the 81. Which would make more towards the C38-71.
Big question is whether the Rotrex or a different smaller SC will make significantly more HP, (even if that means more boost). within the RPM parameters it can operate at. The Vortech V3 Si is really not the best choice for this little 2.5l motor, because it's so large, but when I started I wasn't aware of better options like Rotrex or ATI Procharger CS-1.
Wouldnt it be easier to upgrade to a t-trim blower for more power? Should atleast keep you happy for a while I would think and would be as easy as an oil line and reuse of your existing brackets?
My problem is not really with the compressor flow because it is way oversized already. It will actually flow 700-800hp at 14.7psi. I need to flow, something like 450hp-500 (crank) at 18psi. I believe what I need is something smaller that is more suited to this motor. Here is where I'm at on the Si trim compressor map, and where I might be on a smaller C38-71 Rotrex.
The big question is WHERE in the impeller range of the Rotrex will I make 14psi and 400hp.... that will determine how much head room I have to make MORE POWER!
I'm going to email Kraftwerk (USA distributor for Rotrex) to see if they have any ideas.
My current Vortech V3 Si... I'm almost off the surge line of the compressor map!
Rotrex--right in the middle... probably nearly 10% more efficient just by being more appropriately sized:
One other thought............. ATI makes a procharger for small motors (although no compressor map is available). With a huge step up ratio (8.3), meaning we can max out their supercharger at 80k rpms with a nice big 3.4" drive pulley.
The procharger CS-1 uses a typical straight cut gear internals ,which I'd venture to be slightly less efficient but more robust than Rotrex. You sure do read a lot about Rotrex's breaking. And they are considered "non serviceable" according to Rotrex. The ATI Procharger CS-1 is the middle ground, bonus that you do not have to plumb any oil lines for it.
Again my goal is going from ~300 to 350whp , by getting a SC that will deliver more boost - say 14 to 18psi. The unknown is how much boost any given SC will deliver at max impeller speed at a constant 6200rpm (redline).
I don't think it will necessarily help. The The Si that I have is no where near maxed out in terms of air flow. I don't need nearly as much as it can deliver. At 14.7 psi, the Si is most efficient at around 600 crank hp. I am so far off from that (about 400hp),
I believe I need a compressor that is smaller and most efficient at 14.7(2.0 pressure ratio) around 400ish crank hp but also has some head room for another 4psi (18-19psi).
What I think is that at a given impeller speed it will only create so much boost (regardless of engine size). A larger or more efficient impeller may make a little more boost, but spinning faster is what will create a lot more boost.
As an example... if I put the same SC on a much bigger engine, it may still make 14.7 PSI ... but flow 600hp at the same impeller speed.
I can get a kit that'll allow me to spin the V3 Si to 60k rpm, but it is expensive and complicated to spin it faster, because the step up ratio is low and my crank pulley is small.
You need a smaller compressor that spins faster. You being maxed out on impeller rpm AND right next to the surge line basically means you have a big impeller spinning too slowly to function at the higher PR you need to make more power at the current engine rpm. I don't know much about the various centrifugals and rotrexs available but it just seems like you need a smaller one that you can spin faster.
You need a smaller compressor that spins faster. You being maxed out on impeller rpm AND right next to the surge line basically means you have a big impeller spinning too slowly to function at the higher PR you need to make more power at the current engine rpm. I don't know much about the various centrifugals and rotrexs available but it just seems like you need a smaller one that you can spin faster.
Thanks, that is what I think... I just want to make sure I don't get a SC that is so much smaller that I have to spin it to redline and don't end up making any more power or boost.
I've some some research and it seems like a C38-81 Rotrex makes 15psi on various vehicles, a C38-81 makes 16psi.
Maybe a C38-91 makes 17psi.
I know PSI is not PSI... but it seems that given a constant impeller speed. a particular SC will not make any more than roughly X psi .
I believe I can make 17psi on the Vortech , with a billet impeller, but I bet the Rotrex is more efficient. It is a lot of work to start from scratch though. (brackets, pulleys, ugh!)
I am still mulling over whether a smaller SC would do better here. As I mentioned I am riding up the left side of the compressor map. Some mild head work would get me a little further towards the center.
I am also tempted to try a procharger CS-1. It is a smaller SC (max 550hp) that rev's to 80k rpm. It has a LARGE step up ratio which allows the use of a 3.4" SC pulley to get to impeller redline @ 6300rpm on my motor. (I currently use a 2.4" SC pulley and am not yet at the Vortech's impeller redline... but won't go any smaller on the SC pulley due to slip concerns).
What I would see as a win is to get 17psi out of a SC at 6200-6300rpm instead of the 14psi I get now. I can get to 17psi on the Vortech by adding a billet impeller + larger crank pulley (total $1000).
Beyond that if I want to get bigger with the Vortech I need to rev out the motor (and/or do some serious head work). I believe I can easily hit 400+whp @ 7200rpm with the Vortech with the billet impeller + larger crank pulley (total $1000) plus ceramic bearing upgrade (Another $600), not including motor work.
So basically what I am looking at:
Smaller SC:
+Possibly more efficient
-A lot of work to refit , would need to sell Vortech to get funds to swap so no easily going back! (I could sell Vortech but keep my bracket....)
+Possibly more power/boost @ 6200-6300rpm with larger SC pulley because we can rev it out to 80krpm vs Vortech's 52krpm
-If it made less than the Vortech I'd be bummed!
Vortech:
+I already have it and it fits well
+Can support 400+ WHP no problem if I spin out the SC to 60k rpm and spin the motor to 7200rpm (cost +$1600 in supporting mods just for the SC, not including motor mods) - this would be about 21psi.
-Limited to about 17psi @ 6200-6300rpm due to pulley configuration.
-Small step up ratio = small SC pulley
-Big SC is likely less efficient than a smaller one--I am at the left side of compressor map
More power is my goal - however, I am not so sure about revving this thing out so much. I mostly drive around town and not too much up to 5500+rpm let alone higher. But I bet it would be fun!!
