13B-MSP question
 

Apologies ahead of time to the admins: I know this isn't really even a RX-7 question, but since the information will be applied to a future frankenrotor project for my FB, i figured I'd post it here:

I know we've established what makes the top end of the Renesis engine more powerful, but what is the cause for it's crappy low end torqe in relation to the rest of it's power band? What was it about the engineering of the engine that did this? I know the SE has a pretty good torque band to it, but why not the Renesis (as far as N/A's go...)? Is it something that was sacrificed for the higher output towards the redline?

I tried searching on the forums for a definitive answer, but nothing I came up with was very clear and most of the threads were about the horsepower not really the torque... thanks ahead of time!

Rotary with torque?

Believe it or not, the torque curve, or lack of it, depending on how one looks at it is what makes a rotary. Rotary engines have a very flat torque curve compared to the peaky piston curves. The total area under the curve, along with the generally higher redline than most pistons engine nets more usable total torque. Rotaries will never be a good grunty truck engine, but the flatter torque curve lends itself very well to autocrossing and road racing.

As far as the Renesis engine, can't help there.

crappy torque? have you ever driven an Rx-8?

Yeah, I have driven one, and I kinda phrased that badly. ^_^ I didn't mean that the torque itself was crappy. The car's actually got GREAT torque compared to my FB (sad but true), but what I meant was why there is such a big torque curve compared to the SE? The MSP has a more legitimate curve versus the SE which is more of a gradual line that starts at around 2700rpm. Its lower, but more constant. Why is this?

yeah, i didn't understand because that was perhaps the first thing i noticed when i bought my Rx-8. that thing pulls from down low in any gear (given appropriate speeds, of course).

anyway, as to your question ... my first guesses would be a combination of the zero overlap design (side exhausts) and the intake manifold design. it's a phenomenal thing really, at least in my opinion. just wish the Rx-8 was as reliable as the FB ... :)

It's because of the Rx-8's pipe organ, also known as the intake manifold. Multipule tuned runner lengths combined with multipule port timing can create an engine with a much broader tq curve.

What he said. ^

The SE also gets it's torque from the 6 port actuators, which also
tune the intake.

i'm not sure i understand the question, but the gsl-se is 133lbsft vs the rx8's 159.

so the 8's got 26 lbs/ft more

Okay, so the reason the torque output band on the MSP is so much larger from 0 to redline is mainly because of the design of the intake manifold and the porting?

And just to feed my imagination, what would have to be changed in the design of the MSP to give it a more constant torque curve like the SE? (Imagine you can build all the parts from scratch, redesigning the original MSP parts, not doing a frankenrotor job). Would it involve eliminating the peripheral ports and the multi-ports internally? Would it require a manifold designed more closely to the earlier 13B's?

I'm really just curious as to what makes it tick, if you know what i mean. Sorry if I wasn't too clear. I'm not too familiar with the engineering of rotary engines right now, but i'm trying to work on that... :biggrin:

Sorry j9, didn't mean to be confusing (I've got to stop doing posts late at night), i didn't mean to compare the peak torque, but the torque curve. If you put the two next to each other, the SE starts low, but then jumps up to peak torque between 2500 and 3K (2750?), as opposed the the RX-8 which curves up and up and up until it reaches its peak much higher in the RPMs (5500?).

What do you mean? Something on mine AWAYS seems broken or needing tweaking...:lol:

There is no reason you would want to make the 8's tq curve more like the se's. the 8 should make more through out the entire RPM band. The reason they look different is because where the SE starts to fall on it's ass the 8 keeps on climbing.

just having a really hard time adjusting to the concept of ignition coils being a "maintenance item" in a relatively stock setting. won't even go into all the other shit they reap when they fail. in 20+ years of driving, owning and maintaining FBs (that i bought at 8+ years old) and driving for years and years. they rarely failed unless i broke something. at 68,000 miles, i guess i just expected more on the reliability front with the 8.

* * *

as far as your comparison between the old 6-port and the Renesis 6-port, the SE doesn't compare simply because of time. even if you ignore the advantages of the new exhaust ports, you're still dealing with (1) lighter rotating assembly, (2) higher compression, (3) better intake manifold design, and (4) more precise engine management [that run a better ignition system (compared to a distrbutor - personally, i like the Gen II CAS systems best) and 6 smaller injectors (as opposed to 2 relatively large ones)]. i think you could get a significant bump in the SE if you were to get a more modern ignition AND true engine management system running things. i might also mention that i think the Rx-8 gearing helps to hide any dips in torque the engine may have.

for rotaries, there's no denying that the SE's motor represented progess, if nothing else. however, the Renesis sort of represents the same thing - adjusted for 20 years later.

It's not that this is something that I want to do, I'm just kinda trying to reverse-engineer the development of the rotoary engine to get a better feel for how they are designed and what makes them tick, as well as see what they improved on over the years that they obviously didn't think of back in 1984 or didn't have the technology to make.

It's not so much that I'm trying to compare the two directly, since I know they are two completely different animals, but as I see it, the 13B-RESI is like the great-grandfather of the 13B-MSP and I'm kind of trying to see who the sons and daughters of the SE married to end up with the Renesis. Because I think I understand the changes that were made between the S3 and S8 and I was just trying to see how the RX8 engine came about.

...

Back to the torque curve, the main reason i brought that comparison up was because when comparing the performance of the two, it was the biggest difference in terms of something relating to engine design that i could find.

And it wasn't that I wanted to try and imagine how to make the MSP's torqe low and constant like the SE, but more like if the engine were to be re-engineered, what would it take to reach the peak torque (159) earlier in the rpms then keep it there till redline like the SE?

I'm really only familiar with the first gen engines while only knowing the generalities of the FCs and FDs (wasn't origially too interested in forced-induction), and little or nothing about the SE3P's engine. And so to better understand my engine, I'm trying to get a better understanding of other NA rotary engines, particularly newer ones. (Thanks again!)