DaleClark

iTrader: (55)

Hey guys -

One thing that's been kicking around in my head is what are the limiting factors in exceeding 8000 RPM in an FD?

As is, the FD starts petering out power-wise at 8000 RPM anyhow, going much past 8k isn't really worth it as power is dropping off.

So, questions I have -

- What is the restriction in power at that RPM? Intake manifold? Spark not fast/accurate enough? Ports in the engine? Throttle body?

- What is needed for the engine to be happy at higher RPM? The RX-8 can run to 9k so it's definitely possible with the 13b. I know traditionally it's been a case of under driving accessories so they are happy with spinning at high RPM. I also remember there can be some sort of harmonic loosening with the tension bolts?

Anyhow, my thought experiment for the week. Seems that if the RX-8 can rev to 9k, the FD could as well.

Dale

OG BBF

it's an interesting subject which has always peaked my interest, I know the rx8 rotors are lighter and along with that the counterweight is lighter too which contributes to the higher rpm capability. When we talk about upping the rev capabilities of a 13bRew some of the things that comes to mind is simply getting the rotors lightened, also clearanced. A lighter flywheeel in turn will assist in upping those revs.

ive read a lot also about how the stationary gears are often overlooked when trying to achieve a higher rpm engine. I know they can deform in high horsepower applications so I would imagine an increase in rpm for a prolonged amount of time could have the same effect.

I would also suspect you are right in mentioning the tension bolts. Again with regards to high horsepower engines the block can twist underload so I would imagine any change in engine geometry when pushing past 9-10k rpms would be catastrophic (the clearencing of rotor tips will limit the rotor impacting the iron to a degree). I would imagine a studded block would be a must.

Im in the same boat as you wondering, researching. I would love to see what rpm someone could get a 13b up to. I know the 787b is a 4 rotor but you have to marvel at the revving capability of that machine.....and for f1 engines also. With piston engines it's a bit more logical if you want to increase revs, with a rotating mass opposed to a lateral mass it's far more challenging. I hope an expert can chime into this very interesting discussion.

j9fd3s

iTrader: (3)

the race engines from the 80's were revved to 9k, and the FD engine was basically built to the same specs, but its actually better because its newer, or even the Rx8, they changed the apex seal height and shape, and added a little side clearance on the rotors and that was basically it.

the reason the FD doesn't make power up that high is basically that it isn't tuned that way. the turbos are restrictive that high, the intake is too long, port timing starts to be conservative.

IRPerformance

iTrader: (41)

With the stock porting and turbos there isn't much of a point reving that high as the flow of the system can't keep up. Add porting, bigger turbo, free flowing intake/and exhaust, fuel, and proper tuning and the power band will shift up. Still, I don't recommend going higher than 8100 on the stock rotating assembly. For build that will see this rpm on a regular basis, I recommend a rx8 eshaft (which is half a lb lighter) balancing, and side clearancing the rotors. I also add 1-2 thousandths of seal clearance, depending on the power goals. The renesis motors have bigger ports than any factory rotary. The rotating assembly is lighter. The shorter height apex seals also have less mass, and the clearancing is looser, allowing for more rpm.

Red94fd

iTrader: (5)

About 6 years ago, a guy down in miami, experimented on my rx8 with lighter rotors and reved passed 9k, but it was not tested on the track. It melted the cat and overheated frequently,.. most likely needed tunning. The rx8 e shaft is lighter too, as you know. I think is the waight of the rotating assembly that makes the difference. I am not an engine builder or tuner, so deeper technical details, i don't know. Just sharing what happened.

billyboy

Seems somewhere around 7.2 is power plateau that holds to 8ish without going to bridge or beyond.

Somewhat REW based circuit engines down here, can see 11k, but that's centre bearing and tear downs every couple of meetings.

DaleClark

iTrader: (55)

Thanks guys! This all makes a lot of sense.

I think it would be really cool to build a 9k engine with a fat power band. I've been thinking of what I would like with a "dream engine", at some point I'd like to go back through the engine in my car and make it super sweet.

Of course the current engine runs solid, is super reliable and makes great power - time to mess with it .

Dale

arghx

iTrader: (3)

Put the rotating assembly aside for a moment. The stock REW makes peak power at 6500 rpm. Even if you kept a rev limiter at about 8200, you can shift the peak power point. You need later intake port closing timing first. It's the same on a piston engine with variable valve timing or the n/a rotaries with auxiliary ports. They retard the closing of the intake event. You also need a larger turbine housing A/R , and of course the compressor side needs to flow enough air.

if you look at say BNR turbos, they are going to struggle to make high rpm power due to exhaust side restriction. And then a big single on stock ports is going to be limited by intake port closing timing.

Intake manifold runner length is more important on a non turbo.

silentblu

iTrader: (1)

No on hand experience, but I have had conversations with a few people that say oiling mods are one of the minor often forgotten factor.
The jets in the E-shaft for bearings, the Oil pressure regulator in the rear iron, and I would say balancing of the motor.