Rotary Engine Displacement (L/rev)
 

I have been toying around with the idea of putting a Roots Blower (preferably Eaton) on my 8, but I am trying to figure out what sized blower it would require. Eaton has basically 4 sizes, M45, M62, M90, M112. I'm sure It couldn't use the M90 or M112, but i cannot figure out whether I should want the M45 or M62. I haven't done a lot of calculations, but from what I've found in the specs, the Eaton blowers are designed for up to 14000 RPM, which would put the optimum blower/crank ration at 1.5. I am having trouble figuring out just how much volume the RENESIS pumps though. I know there are some tricks to the 1.3L number, but I wanted to make sure that the actual drawn volume is actually 1.3L/rev. I know people have had discussions about it being equivalent to a 2.6L boinger, but this is simply because of the boingers take 2 revs to complete a cycle, correct?

If this is the case, it looks like the M62 is the proper blower:

- having about 455cfm (12885.6 L/min) at 14,000RPM @ 10psi boost
- RENESIS displaces 11700 L/min at 9000RPM

therefore this should be enough, am I correct? of course fine tuning would be required by adjusting the Blower's pulley size to actually alter the boost, but I'm looking at ballpark figures right now, just to determing the correct blower to use.

A standard supercharger draws like 15% power of the engine to give power back to it. I would definetly think using a clutch type IHI supercharger (AMG are using them in their Kompressors Benz) with water cooler will do better without compromising at the low rpms (900-2500rpm).

The hard part would be tunning the supercharger in when to engage during engine load. The whole idea it's to gain power when you need it & when you don't need it (city driving/rush hour driving).

..if your ONLY choice it's to use the eaton supercharger series w/o intercooler, use the M90 & see if you can adapt a clutch type pulley with an rpm switch, or some sort of ingenious activation depending on throttle load. Offcourse, you will also need to compensate for fuel as well when you need it. (And, you can't go cheap by just installing raising fuel preassure alone... I hate those BS add about using fuel regulators to compensate for fuel!!!)

I don't think the extra cost of the IHI blower would outweigh the slight performance drag that the newer Gen IV Eatons have when in bypass. As for watercooling, I plan on using a water intercooler, but this is dependant mostly on space considerations. There doesn't seem to be a whole lot of room in the RX-8's engine compartment for even the blower.
This is another reason why I chose the Eaton. Simply engaged / disengaged by vaccuum. Not as customizable as the clutch type blowers, but a whole lot simpler to design in.
Why would you suggest baseing the activation on RPM? The goal of using the blower woud be to provide some more torque at the low RPM's, to seriously improve drivability, and not have to flip-flop through the gears as much. This is why Eaton uses vacuum to activate the blower. Also, why do you suggest the M90? it seems that it is way to large for the 1.3L Renesis. The M62 provides more than enough, at 10psi boost with about a 1.5:1 ratio.

Well, I suggested the M90 since it runs a much cooler & denser charge as well, & also, the Renesis runs a much higher compression. M62 IMHO would work as well, but then you have to go for low compression rotors.

Also, Rx-8 uses a MAF (Mass air flow) vs. MAP sensor. Some complications you'll have in tunning as well.

A MAF sensor does not complicate tuning. The MAF is placed upstream of the supercharger.

An M90 is not more efficient than an M62. They have the same architecture, just different sizes.

Crazzyfuzzy,

An M45 is too small for the Renesis. The M45 is good for about 200 hp. An M62 for about 275 hp and an M90 for about 400 hp. I think you will have to go with the M90.

Something else to consider is you cannot turn the blower to 14,000 rpm if the engine cannot handle the volume from the supercharger. Maybe with the Renesis and an M90 you can only turn the blower at 10,000 rpm when the engine is at 9,000 rpm. I have not done the calcs to check this. Just speculating.

The tough part with the Renesis is the wide rev range. You have to make the natural blower output flow rate and natural engine intake flow rate somewhat follow the same curve. If not then the engine will be peaky compared to the naturally aspirated engine. The peakiness comes from the high variablility in boost pressure throughout the rev range because of the differential pumping rates of the engine and blower.

With this variability I think it will be damn near impossible to make an aftermarket supercharged Renesis mimmick the characteristics of the naturally aspirated Renesis. Now if one put a little CVT between the blower and engine then we could have a Renesis that had some true bottom end grunt and the sweet top end. Of course I think the extra cost and weight of the supercharger, aftercooler and gearbox would better be spent on a larger normally aspirated Renesis.

well, you mean something like a compressor which you could limit the speed of it's rotation somehow?? *cough*turbo*cough* :)

i appreciate that there is some amount of lag, but with a correctly sized (and pricey enough) turbo, this can really be well countered, as i'm sure you know. :)

I don't see why the M90 would be necessary. It will draw more at 14,000 than the RENESIS will at 9,000. This difference in flow is what actually produces the boost. I need to do some true gas calculations to verify this, but it seems like ideally, you'd want the smallest blower that can be geared (pully'd actually) to produce the correct boost.

The only advantage I can see for putting the larger blower on there, is you can use diffent ratio pully's to allow the blower to spin considerably slower, possibly increasing its life, but that's all I can see...

Just did some ideal gas calculations, and these are the results:

Max RPM range for the blower would be achieved with a 1.47:1 pulley ratio, allowing the blower to spin at its 14,000RPM max with the engine at 9,500RPM.

At this ratio:
The M62, would be able to push from about 7.5psi of boost with no intercooling, to about 1.9 psi of boost with perfect intercooling (removing all the heat of compression). This would move the mass air flow of about 1.5L NA engine.

The M90 would be able to push from about 18.6psi of boost with no intercooling, to about 10.2 psi with perfect intercooling (removing all the heat of compression). This would give it an effective displacement of a 2.2L NA engine.

So, it looks like if you are planning on running without an intercoller, the M62 would give you all you would want. Much more boost than this, and you'll be looking at too much heat, considering the 10:1 compression ratio.

Why are you using psi in your calculations instead of lb/hr? PSI is not a measurement of airflow, and it will change drastically depending on ambient temps, IC efficiency, displacement, flow rates of the intake and exhaust systems, etc. etc. etc.

---jps

I realize that PSI is not airflow, all my calculations were done using ideal gas laws. I then converted the results to PSI, so the average car-guy can understand it. When dealing with two positive displacement pumps (in this case, the Roots SC, and the RENESIS engine) it is easy to calculate the flow-rates, and, using the Delta-T graph from Eaton's Spec sheet for the blowers, I was able to convert this to psig in the intake plenum, which is what is read on a boost gage. As for intercooling efficiency, I didn't feel I wanted to try to tackle that, so I posted min/max (no intercooling/perfect intercooling)

EDIT: I also assumed an ambient temperature of 77F

Yup. Like I said earlier I still would prefer a larger displacement Renesis.

well, that's the beauty of the flat torque curve of the Renesis, combines with a pos. disp. pump like a roots blower, the flat curve stays the same, which would be virtually the same as a larger NA engine, except with a supercharger, with a bypass, you can basically lower the engine size when cruising. I'd be willing to bet that a 1.3L renesis with a medium sized, bypassable SC, would offer a very similar, if not higher, torque curve as a larger displacement renesis, but have better mileage while cruising.

You know you are correct. Since the Renesis has such consistent torque through the rev range it would mean that the volumetric efficiency is nearly constant through the rev range. Coupled with a positive displacement blower and the normally aspirated characteristics can be maintained.

i hate to start another war over turbo vs. supercharger, but wouldnt a turbocharger be a better option for most applications? Im not pissing on superchargers, i love all forms of induction. If I thought blowing in a hose in the cabin would give me some extra boost id do it even it looks like im screwing the car.

So..... how bout them turbos!!

So there has not been an update on this has anyone actually done this or what... If so would like to contact them. I have a M90 that I woukd love to try and get on my 8..

Pettit and Hymee both used a Positive Displacement S/C http://shop.pettitracing.com/pettit_...t/g-62947.aspx