As we all know, the renesis is a fast spinning engine. What not all of us might know is that the drive train is probably stealing a good chunk of power. Say WHAT? Yes, thats right, the drive train (transmission in specific) it probably gobbling power. First of all, here's what points me to the conclusion that the Rx-8 drive train is taking up more power than most piston engine transmissions: the renesis rotary is rated, from the factory, at ~138hp. However, in many dynomometer tests, the peak hp number is somewhere around 180hp, depending on engine condition and age. That's almost a quarter of you total power down the drain. Now, some of you might be saying 'well, thats just the hp number, not what you're actually using'. That's true, but this drop in power is coming from a drop in torque. This leads me to the reason why the transmission is taking up so much power: all gear systems have some form of inefficiency. In other words, the transmission is taking up precious power. Why I believe the Rx-8 transmission is taking of so much more power than a transmission from a piston engine (~25% compared to ~15%) is that the rotary engine spins so much faster than a piston engine. Why this translates into such a large power loss is that the genesis makes not as much torque, at high rpms, which is a double slammer: It has little actual torque to begin with, and the high transmission speeds equate to high resistance. The solution that is most obvious to me is to reduce the primary reduction in the engine, effectively slowing down the transmission and allowing for a higher efficiency. pic:
This picture shows a 'normal' transmission, one without a large primary reduction.
pic:
This picture shows how the primary reduction could be modified to allow slower transmission speeds (and higher torque) so as to increase drive train efficiency. This change would also allow for a lower final drive ratio, further increasing efficiency.

I know I could be totally off my rocker, so correct me if I am, but I believe that this actually has merit. Comment as you will.

 

Anybody?

 

Your grammar makes it somewhat difficult to decipher, but wouldn't reducing the ration make more drivetrain loss? From my incredibly limited understanding, reducing the primary reduction ration would lose torque. I'm probably totally wrong though

 

Even mazda had trobble building a trans. to hold up to the rotarys high rpm output...

 

It's not rpms that kill transmissions. It's torque. In other words if the same transmission had 238 hp going into it with an input speed of 3000 rpm, it's much more stress than 238 hp going into it with an input speed of 9000 rpm. Your idea would actually result in greater transmission stress not strength.

If the engine does make 238 hp and a quarter of the power of the engine was lost in the transmission, that would mean that the tranny would be robbing about 59 hp. The engine loses about 19% or so of its energy in the form of heat through the coolant and it needs a cooler (radiator) to be dissipated. Where is all of that hp going in the form of heat in the transmission? If we lost that much, we absolutely NEED to have a transmission cooler and we don't. We see the need for them on very high hp cars but even then if a 600 hp engine lost only 10% of it's power to the transmission, it would still be 60 hp.

The truth is that we don't lose that much. It's actually right at 30 hp for the ENTIRE drivetrain which means the transmission loses a little less. We only lose a couple of a percent in the differential and since the total loss is about 12%, that means the transmission only loses about 10%.

We have a torque converter being installed on a large mud pump package that I'm designing at work right now. A torque converter is much more parasitic than a simple transmission due to losses through a fluid medium. It robs us of 15% of our input horsepower. We do have a large oil to water cooler on it but with an input of 1050 hp, 15% is alot of energy to get rid of.

There are some people that have dyno'd at 170 rwhp and a few others that have topped 200 rwhp including at least one at 220 rwhp. The engine itself is actually capable of 230 rwhp (260 hp). The variances that we see have more to do with other things such as tuning, gas, and the effects of proper or improper lubrication to the center of the apex seals. It is not power being lost to the drivetrain. The sad fact is that most of them just DON'T have 238 hp! You aren't using what you didn't have in the first place.

 

Not true. The issue was having a torque converter on automatic transmissions that could live at high rpms. It had nothing to do with transmission strength.

 

I stand corrected.

 

So, you're saying that the reason most Rx-8's don't put that much power to the ground is because of poor engine tune? Wow, that's pretty sucky considering that the engine would only make about 211hp (right on par with what the automatic's rated at) assuming total drive-train loss of 15%. Are you sure?

 

you'd have to probably build at least 6 complete replacement gear/shaft sets and sell them for 3 - 4 thousand dollars each just to break even on the concept, good luck ...

 

I have been doing some number crunching on the 8 vs the s2k since both are rated around the same hp/tq. The huge loss in hp/tq is mostly due to leverage. If you plug in (so to speak) a rotary into the s2k and use its gearing and tire size, you will get the same numbers to the wheels as the s2k's piston engine. The only reason the s2k is faster and has more whp/wtq is due to leverage. The math is simple and getting 220 whp to the 8 NA is very simple actually.

correct me if I am wrong but, the s2k has a 4.1 rear end but, they neglect to tell you that the s2k has a secondary reduction gear as well that makes the 'effective' rear end much higher than the 4.1 you see advertised. The secondary gear is a 1.16 for the ap1 and a 1.208 for the ap2 resulting in a 4.756 for the ap1 and a 4.9528 for the ap2. Secondly, the wheel/tire diameter for both ap1 and ap2 are 24.9 and 24.7 respectively and, the 8 is a 26. Given the leverage difference, the 8 is going to be slower as less whp is transfered due to leverage. If you reverse the math, the 8 actually has slightly more tq and about the same hp as the s2k. This is just my thoughts on it and the math supports it but, maybe I am missing something. Please let me know and I have a thread asking about a rear end swap too if you or someone else knows the answer to that question as well.

 

You apparently missed the rest of his post. He is saying that improved tuning can free up some more ponies (which is proven), but most of the engines are not actually putting out 228/232/238/240 (Whatever) in the first place, due to engine health issues such as apex seal lubrication/wear. (And ignition problems, MAF problems, etc...) Most RX-8s don't have healthy engines. Most are sick, too large a percentage are terminally sick.

Fixing those issues will help you far far more than trying to design a new transmission.

 

Ahhhh... Thanks for pointing me in the right direction.

 

 

It calculates out easier on paper than in reality, Einstein.

 

I will put it to the test on a dyno then. I really dont know what is going to happen or what the numbers are but, I happen to have a set of 17 inch rims with some different diameter tires than what is on the 8 stock. When I get a chance, I am going to put a new set of plugs, coils, wires, and make a run with the stock tire/wheel combo, then I will make a run just after than with the other set of wheel/tire combo and let you know what happens so that it will be a good test. The first run will be with the tune up, as my car needs one about now and, the second run will be right after the wheel/tire change so that conditions will be the same. I may be able to do this in a week or two. What do you think is going to happen TeamRX8? Has anyone ever done this? You know alot more about this car than I do. I am just learning everything I can about it. I understand what you mean about the paper and reality as I have to deal with that every day at work with engineers that dont listen