Otto Maddox
Otto Maddox Dork
12/23/11 11:22 a.m.

I love RX-7s. I get bored with people talking about their terrible MPG and incredible power for their displacement.

From RX7.com

"Rotary Displacement

As rotary engine enthusiasts, we all know the obvious, intoxicating charms of Wankel wonder. Unique, unusual, lightweight, compact, high revving, what’s not to like? One of the most commonly deliberated subjects is “what is the rotary engine’s displacement”? An excellent question. Here’s our best to make the case for the engine’s displacement.

Conventional reciprocating engines are the standard that most relate to. We all know that these engines are measured by measuring the area of the bore, multiply by the stroke and then multiply by the number of cylinders. Simple as that.

For simplicity, we’ll use the 13B engine as the standard we’re calculating. You can use these ideas and apply them to a 12A or 20B yourself. The rotary engine is obviously unique. The engine has two rotors shaped roughly like a triangle. This makes for three combustion faces per rotor and a total of six for a standard two-rotor engine. Each face has a “swept volume” or displacement of 40ci(654cc) and there are a total of six faces. With this known, the engine displacement should be 40ci(654cc) times six to equal 240ci(3.9L), right? In a way, yes, but that would not be a comparable displacement to the 4-cycle engine.

The key for comparing the displacement between the 4-cycle engine and the rotary engine is in studying the degrees of rotation for a thermodynamic cycle to occur. For a 4-cycle engine to complete every thermodynamic cycle, the engine must rotate 720° or two complete revolutions of the crankshaft. The rotary engine is different. The engine rotor rotates at 1/3 the speed of the crankshaft. On two rotor engines, front and rear rotors are 180° offset from each other. Each rotation of the engine (360°) will bring two faces through the combustion cycle (the torque input to the eccentric shaft). This said, it takes 1080° or three complete revolutions of the crankshaft to complete the entire thermodynamic cycle. Obviously, we have a disparity. How can we get a relatable number to compare to a 4-stroke engine? The best way is to study 720° of rotation of the two-rotor engine. Every 360° of rotation, two faces of the engine complete a combustion cycle. 720° will have a total of four faces completing their cycle. 40ci(654cc) per face times four faces equals 160ci or 2.6L. That’s a well-reasoned number and now gives us something to be able to compare to other engines. In addition, since four faces passed by in the comparison, it’s like a four cylinder engine.

Now we know, the 13B compare well to a 2.6L 4-cylinder 4-cycle engine."

Curmudgeon
Curmudgeon SuperDork
12/23/11 11:32 a.m.

That's why SCCA classifies them as the same displacement as a 2.6L engine.

Rufledt
Rufledt HalfDork
12/23/11 12:09 p.m.

Speaking of the terrible mileage, I sure wish my RX8 got the mileage of a 2.6L 4-cylinder 4-cycle engine! Not enough to do anything about it, though. In fact, I wish it had a turbo...

ReverendDexter
ReverendDexter SuperDork
12/23/11 12:17 p.m.

A'ight, I'll bite... so where does 1.3L come from?

sobe_death
sobe_death Reader
12/23/11 12:30 p.m.

654cc + 654cc=1308cc? Two swept faces. I didn't think the 13 in 13B ever really meant 1.3L, did it?

aussiesmg
aussiesmg SuperDork
12/23/11 12:40 p.m.

the width of the housing is 130mm wide, in a 12a it is 120mm. Hence 13B and 12A

z31maniac
z31maniac SuperDork
12/23/11 1:10 p.m.

Lightweight?

Isn't everyone doing LS1 swaps in RX-x's because the weight is so close?

Brett_Murphy
Brett_Murphy GRM+ Memberand Dork
12/23/11 1:25 p.m.

I used to be able to pick up a 13B by myself- just the engine, no accessories. I probably wouldn't want to now that I'm not young and stupid any more (and haven't been hitting the gym like I used to, either), but the point stands.

I've never tried to pick up an LSx.

ReverendDexter
ReverendDexter SuperDork
12/23/11 3:03 p.m.
sobe_death wrote: 654cc + 654cc=1308cc? Two swept faces. I didn't think the 13 in 13B ever really meant 1.3L, did it?

Rotaries are always advertised as 1.2L for a 12A and 1.3L as for the 13B and Renesis.

iceracer
iceracer SuperDork
12/23/11 5:33 p.m.

For our ice racing, we just double the advertised displacement, thus- 2.6 L. We done that for years. Not that we have many rotaries racing.

1988RedT2
1988RedT2 SuperDork
12/23/11 6:33 p.m.
ReverendDexter wrote:
sobe_death wrote: 654cc + 654cc=1308cc? Two swept faces. I didn't think the 13 in 13B ever really meant 1.3L, did it?
Rotaries are always advertised as 1.2L for a 12A and 1.3L as for the 13B and Renesis.

Closer to 1.1 actually--1146cc. But I've never paid much attention to the comparison with a boinger.

RexSeven
RexSeven SuperDork
12/23/11 7:37 p.m.
z31maniac wrote: Lightweight? Isn't everyone doing LS1 swaps in RX-x's because the weight is so close?

I don't have any figures, but I think the LS1 slots in between the N/A 13B and 13BTT (93-95 RX-7) in terms of weight, and is about the same weight or slightly lighter than a 13BT (87-91 RX-7 Turbo).

I just bought two 13BTs, one fully dressed and one partially dressed (missing TMIC, A/C pump, etc.). PO and I were able to shimmy the partially-dressed engine on its oil pan (no damage aside to paint), but could not lift it. There was NO way we were moving the fully-dressed engine without a cherry picker. I also got three manual transmissions and I can lift one of those myself.

Canute
Canute Reader
12/23/11 9:26 p.m.

Power of an '80s two-something liter four and the fuel economy of an '80s five something liter V8 with the oil use of a two stroke lawnmower ? I just weighed my 13B with five speed and got 385. No air or power steering. How much does the five speed weigh? My Mustang WC T5 weighs ~75 without the bellhousing. That I can pick up easy. I'd be shocked if an LS1 came anywhere near the 300-330 that 13B weighs. Maybe add 100-150 pounds.

RexSeven
RexSeven SuperDork
12/23/11 9:59 p.m.

The Ultimate Engine Weight Chart, last updated in 2005:

http://fixrambler.com/engineweightchart.txt

A 13B-REW (FD) tranny weighs about 112lb according to RX-7club.com (http://www.rx7club.com/showthread.php?t=250486). I'd say a 13BT (FC) tranny is about the same. A short-block 13B isn't very heavy (~155lb) but the ancillaries like the turbo manifold, A/C, etc, add weight quickly. With my 13BTs I'm planning to add to my FC I will be paying close attention to weight.

Osterkraut
Osterkraut SuperDork
12/23/11 11:07 p.m.
aussiesmg wrote: the width of the housing is 130mm wide, in a 12a it is 120mm. Hence 13B and 12A

Cool story, if it were the least bit true. Rotor housing width is ~80mm for the 13B

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