I was just wondering given everything else equal in an na build how much horsepower would you be leaving on the table not running 9.7 comp rotors?

 

Not a whole lot in a 2-rotor, about 8-12whp. But it all adds up.

 

I'd say about 5hp or so and about 1000rpm.

I've built s4 stock motors, s5 stock motors, s4 motors with s5 rotors, and s5 motors with s4 rotors. You wanna know my favorite setup? The s5 motor with s4 rotors was the easiest to drive and had the most response. Seems that the computer mapping and the s5 intake combined with the slightly heavier/torquier s4 rotors made for a nice combo. More willing to take off from a stop than the regular s5 rotors, had a great responsive midrange, and still a good top end (though not quite what a stock s5 would have been).

Bottom line, if you have access to an s5 rotating assy. for a reasonable amount, I guess I'd go ahead and do it. I wouldnt put 3 or 400 dollars into it like most of these guys though...the difference in power is negligible. The difference in driving feel is more pronounced...the s5 rotors have a better top end but not much grunt down low, while the s4's have a stronger pullout on the low end but you can tell they don't want to spin quite as well at high rpms.

 

GSL-SE rotors are 3mm where as the S4 and S5 rotors are 2mm and have to be machined to take 3mm seals. From what I understand 3mm are better but that is another big discussion.

Also I believe GSL-SE rotors are heavier than S4 or S5 rotors.

 

the SE rotors are even heavier than the s4...they have a nicer feel down low, but IMO you give up too much top end for an NA application. I have built hi comp turbo motors with them and that was a good application for them since they are very strong and already set up for 3mm. Also the SE rotors have thicker side seals.

 

On an NA 2mm seals are reccomended, 3mm is really only if you're trying to salvage a rotor with a worn apex seal groove.

3mm is for high boost turbo cars.

 

i argee, i built a gsl-se engine to put in my gsl-se, with the 89+ rotating assembly, and it was nice under 2500rpms, but over that the stock s4 motor that came out was better....

the 89 rotors are nice, but id have no problem using the s4's...

gsl-se rotors are 12lbs each, s4 rotors are nearly free....

 

the S5 rotors are nice to have and i'd use them for my own car, but like i've said many times before to people that ask this question, they are not worth losing sleep over. the only people who the difference would matter to are all-out racers that NEED to have any edge they can get within set limitations. for the average joe on the street it matters very little. so if you have S4 rotors already, then leave them be.

 

High Compression is good just a little more fragile to tune. Lighter and higher compression is a good thing for an NA motor, not a bad thing.

Unless you have access to RX8 13B Renesis parts, that would be my personal preference because those parts are clearanced from the factory with 10:1 compression.

 

I am planning within the next year to build up an N/A. S4 or S5 depending on what I find.
I was wondering which one I really should go with.
I have been thinking about the differences between S4 and S5. I presume that an S5 is better.
I was thinking If I go with an S4, what I could do to it to make it better, like turn it into an S5. I ghess its possible, just have to ask, Is it? Anyone done it?
But I was thinking about the whole rotor differences between heavier/lighter, higher/lower compression.
Now Thanks to "rotary resurrection's" post, I see that the S4 rotors have a little more torque, but lack a little at the top end.
But the plan I have for my motor is to bridge-port it, So I ghess if I go with an S4, the BP will take care of that top end.
Anyway.
I saw this thread and thought, "this is exactly what Ive been thinking about the past few days"
So thats just my 0.02 cents.
Later.

 

Heavier rotors DO NOT give more torque, the reason they might feel like it is because they'll have a greater flywheel effect when leaving a stop light, so the car will be easier to get off the line because there's more torque stored in the engine, but an engine with a lighter rotating assembly will feel better from a roll, because it'll actually transmit more torque to the road, because it takes less torque to accelerate them up to speed, so more gets to where it's needed.

The physics of it are undeniable.

 

You are correct, on paper the lighter setup will always win all across the board.

For all intents and purposes, especially for most of the readership of this forum, talking theory and physics is one thing, but talking real world application and experience, and feel is quite another. The heavy rotors produce the feel of a lot better low end pull, which is basically what torque does too, so it can be said that they feel more torquey. IT helps driveability a lot, but hurts top end pull.

OF course we are talking small amounts here...even the early 13b rotors won't make your rx-7 pull stumps or anything. Still, any average joe can jump into an 86-88 rx-7 and then into an 89-91 and tell you there's a significant difference in how they drive in traffic from a stop, or pulling out on a hill. IT was one of my first observations when I started meeting other 7 owners when I had my first.