Hi guys,

Studying forced induction solutions for months, and now in process of tuning my TTS Rotrex kit, I still don't have completely understood how to read a centrifugal compressor map, and I do want to

Here is the example of my setup:
- AP1, F20C stock engine
- decat pipe + HKS dual exhaust
- rotrex C30-94 TTS supersport kit: it pushes about 9,5 psi on my car with 125/105 mm pulley combo, which means it is maxed out (SC speed is maxed out at 8800 engine rpm)

Here is the documentation and compressor map about my rotrex:



I tryed to do some math, but I must be wrong somewhere...
- Engine displacement (each 2rpm): 2000cc = 122 ci = 0.07 cubic feet
- max RPM: 8800 (my tuning limit)
- volumetric efficiency: 1 (random choice)
- max Flow (NA) = 0.07/2*8800*1 = 308 CFM = 0.178 kg/s

Now, with a pressure ratio of 1,65 (equals 9,5 psi boost):
- max Flow (1.65 pressure ratio) = 0.178 * 1.65 = 0.293 kg/s (at 8800rpm, supercharger at 100k rpm)

Now, when I look at the compressor map:
- the intersection of 0,30 kg/s and 100k rpm speed is at 2,5 pressure ratio ! (I know there is some pressure lost between the rotrex outlet and my intake manifold, but this much is impossible of course)
- the intersection of 0,30 kg/s and 1,65 PR is at about 82k rpm ! (which is impossible with my pulley combo)
- the intersection of 1,65 PR and 100k rpm is out of the map ! (which I don't know what to think of )

Does anybody know what I am doing wrong, and how to read that map correctly ?

Thanks for your help.

 

In one sense, its kinda moot. The intent is to use this math to determine what size blower you should use, and you already have a blower.

You can already see what power levels and performance envelope others have achieved on this platform with that blower by searching for threads that used that blower size (albeit limited examples. The C38 range is much more popular on this platform).

C30-94 is definitely a viable size if you are on a budget and your power goals are reasonable. It was the size used for both original KW & TTS kits. If you are looking for a power increase that will be thrilling on the street, while retaining the cars high rpm power delivery, you've made an excellent choice. Its also a great choice if you're not looking to go down a rabbit hole of upgrading one system after another to handle loads they weren't designed for.

The next step up would be the C38-81. Most who go FI either start out or end up becoming power junkies and want more and more. Hence your modest choice not having as much appeal to most here. It also doesn't have a very good upgrade path if you become hooked on boost.


But you still want to do the math to know more about the blower you have, don't you? I know I would!

The simple version. Determine your hp goal, and follow the simple formulas in the Rotrex Handbook.

Rotrex Technical Handbook


The more complex math. Draw a line on the compressor map that represents your motors flow at each rpm point, to see if your requirements align with the more efficient areas on the map for that compressor.

The excellent link below details that for the popular fast and furious Toyota 2gr platform, but the author wrote it so its easy to substitute numbers from a different platform and recreate the results. Have it!

Please post what you find. I've been too lazy/busy lately to do this myself, so you'll be doing me a favor!

Toyota Forum, excellent blower sizing maths

 

You are absolutely right!
I already own this blower and I know what it is capable of. I read a lot of threads here about the "old high-boost KW kit" using this same blower. I also have the dyno graph of the kit that I bought used, when it was installed on the previous AP1: it made 376bhp. I also have begun using my car with this kit, and running virtual dyno: it says about 330whp...

I definetely am on a "budget" (but not cheap) build: what I mean is a build that I will track and that won't cost me twice the installation price when breaking diffs
I also bought this kit because the FI used sales are rare here in France, and I had a good opportunity.

To be honest, now that I run the car, I could also think "damn I wan't more !" when driving straight. But I wan't to handle it on track and twisty roads, and I think this will be fun enough

Right again ! I do wan't to understand what I bought.

Using this simple version, following the example in page 11 of the handbook, I have results that are very near from what I calculated above.
- Flow = HPsupercharged/1200 = 380bhp/1200 = 0,317 kg/s (I previously estimated 0,293 kg/s)
- PR = HPsupercharged/HPna * 1,15 = 380/240*1,15 = 1,82 (I previously estimated 0,293 kg/s)
What I still don't understand is that this point (intersection of 0,32kg/s and 1,82 PR) is absolutely not on the 100k rpm line on the C30-94 map...

Thank you for that link. I read it completely, and understood most of it. I will try to get a moment to really do that exercice.

I also found a great information in the following document:
Engine_Hydrolock_Mitigation

It shows the volumetric efficiency of the F20C:

This could be used to do the maths...

 

This Society of Automotive Engineers paper on tbe F20C is supposed to include the Volumetric Efficiency. But it'll cost $30 for tbe document.

F20C SAE paper

But tbe VE of F22 is here.

I'm sure the F20C VE is also >1 at some rpms.

 

My work is in progress. I am doing the maths, following exactly the procedure given here :
https://www.mr2oc.com/threads/2gr-fe...ter-v6.671953/

Here is where I am now (and I have to stop, will continue later):

 

Awesome work!

I'm sure you're as surprised as I am to see it not align better with blower efficiency range.

Still, on that blower the center area is already more efficient than even many other Rotrex. So skirting the next level down is probably close to skirting the center range on a different blower.

I'm guessing you're gonna do the C38-81 next! : ) Just to see how the other half lives!

 

Thanks! I like your help and analysis.

I still have some work to finish my worksheet, and make it look better. Then I will be able to apply this to c38-81, F22C or else...
I just hope I made no mistake in the sheet...

Do you understand why my results are in contradiction with the real pressure ratio/boost that this c30-94 produces in real life?? (9,5-10psi max, at 100k rpm)??
Because I still don't!

 

Hi guys, here is my daily report !
I have now finished the whole process of sizing/maths, follow the procedure explained on MR2OC link.
Here are my results:





I still have no answer to the following question:
Why are my theorical results so far from the reality ???
For comparison, here is a virtual dyno graph of my car, with MAP sensor information
Scale in mBar, sorry... can be easily compared to Pressure Ratio; 166mBar equals to 10 psi of boost which is my max boost at 8800rpm.
My maths gives me a theorical boost of 18psi at 8800rpm which is 8 psi higher than what I really get on the car ! I know there might be around 3 psi pressure drop between my blower and my intake manifold (because of intercooler, piping etc.), but 8 psi is a huge difference and I can't understand that....

 

Just an interesting observation of this graph, it looks to be identical to a stock torque curve of the engine off a dyno sheet. Really seems to be a direct correlation between volume efficiency of this engine and what the engine puts out on paper and to the wheels.

 

Go to sci-hub.tw and pop in the doi URL from Car Analogy's link above.