Mazdarian

iTrader: (1)

I don't know my sh*t, was just wondering;

- What's the highest theoretical compression figure obtainable by mixing parts/years?

- Could you surpass that figure by using custom internals? if so; how?

- Would a N/A rotary benefit of that high of a compression?

- What would be the drawbacks? Would it be driveable?

Nicked this pic off an old thread from '07, but they were talking about diesel rotaries..

peejay

Highest theoretical compression IIRC is either 12:1 or 15:1, if there was no ditch at all.

Practically speaking, 9.7 is about the upper limit for power. There are minimal gains versus the 9.4 rotors, and people haven't found power gains (and in many cases, found nothing but losses) with 10:1 RX-8 rotors. This is for engines running at sane RPM.

For race engines, there is power to be made with LOWER compression.

The rotary is kind of like a flathead in that airflow has to go through a pinch during the engine's cycling, higher compression ratio hurts that airflow and causes power losses at high RPM.

Kenku

I'm of the opinion that one of the big loss areas with higher compression is on possible benefits from overlap, but I can't prove it yet.

Mazdarian

iTrader: (1)

So what you're saying is 'unless you plan on running 10k+ rpm, don't bother'?

Mazdarian

iTrader: (1)

@ Kenku,

Pardon my french, but 'potential benefits from overlap'? I thought overlap was a bad thing?

j9fd3s

iTrader: (3)

we're saying that 9.7:1 is where most people get maximum power, going higher people tend to loose power.

a piston engine is the same, except they would use a larger number, like 12.

its basically the geometry of the engine, its different than a modern piston engine, and is playing by the same rules in a different scale

http://foxed.ca/rx7manual/manuals/RE...amoto-1981.pdf

Kenku

Overlap is like anything else - it's good or bad depending. At low throttle openings, for example, it means you're diluting the intake charge by drawing exhaust in. At high RPM and full throttle, the exhaust inertia can help pull air and fuel into the combustion chamber - even if some of it is going out the exhaust port, it's getting inertia moving in the intake tract and helping.

What peejay is talking about is motion of the burning air/fuel charge. In piston engines, look at domed pistons. On something like the old school Hemi, to have high compression you have to have a big piston dome that (among other things) gets in the way of the burning mixture - so more modern engines have shallower lower volume combustion chambers so that they can have high compression without interfering with things moving around. Well, on rotaries, if you take things to the extreme with no dish in the rotor whatsoever, at top dead center, the rotor is dividing the combustion chamber in half.

peejay

if I was building an all out N/A engine, I'd start with 8.5 rotors.

peejay

Maybe, maybe not. Any chamber flow blocking effects during the overlap period might serve to moderate the "pipeyness" of a high overlap engine, and broaden the powerband. Or at least make large holes in the torque curve less likely.

So when's your dyno going to be up and running?

Kenku

Hey, broadening the powerband means making the peak smaller; narrow powerbands are just something to use close ratios to deal with.

Seriously, ok maybe? It's a hypothesis; I have a gut feeling that gains from having exhaust scavenging help the intake flow will be more help than hindrance.

Dyno got sidelined by various bad RL stuff last year, in short, but I'm starting to get back on track. And now I've got a kid due in... 2 weeks-ish. I'm sure that won't affect how much time I have to work on car stuff, right? I'm actually stepping back from the idea of doing a GT-3 car and going back to an EProd project my dad started a while back - ok, some of the suspension/hub solutions I'll have to do are stupid but that's racing.

Mazdarian

iTrader: (1)

[QUOTE=Kenku;12340822] Seriously, ok maybe? It's a hypothesis; I have a gut feeling that gains from having exhaust scavenging help the intake flow will be more help than hindrance.

Dyno got sidelined ...[/QOUTE]

This whole thread got sidelined the minute you two ran into eachother yet again 😂 Next thing I know we'll be talking about ketchup on hotdogs y/n

On a serious note though; could said exhaust scavenging along with port overlap be tuned provide maximum charging benefits at a certain rpm / rpm range, REALISTICALLY? Sounds to me like there's a heck of a lot of variables involved spanning all the way from the inlet in the bumper back to the collector/megaphone/whatever you run back there that you decided will make you the most horsepower/torque.

peejay

That is how bridge and peripheral ports work so well. They can be tuned to make a LOT of power in a narrow band, or they can be set up to have a broader power band with less of a peak.

The Mazda Factory Race (or MFR) peripheral ports had a nice compromise. Peak torque was decently far from peak power for a moderate transmission to work well.

peejay

Digging this up... when I bought my RX-3 it came with a number of vintage Popular Mechanics type magazines that had various Wankel articles in them. I only just recently started reading them. These books are from the early to mid 1960s and are in remarkably good condition. I wish I could scan these mages in without destroying the books! Reading an article about the latest in Wankel technology that also has an article that has a sneak preview about the 1963 Mustang II show car that will be turned into a production car, is rather interesting.

Anyway, the internal airflow issue has been known all the way back then. An article about Curtiss-Wright's evolution of design spent a good bit of time discussing these transfer losses.

Rotaryboss2144

Hmm I believe the bath shape may also have lots to do with it. According to Edgar Perez he’s made 16% more HP with he’s 11:1 comp billet rotors, the bath design overall looks smaller but the recess is still somewhat got decent depth.

pknoutrx2

Why not start with 9.0 rotors, they are lighter. This would be best balance of wieght, better seals and comp. If you use FD rotating assembly you also get the best factory shaft/gear combo.