how much mpg would the 26b get with just stock parts? ive always thought that more rotors mean more gas consumption. but recently ive came across a thread that said that it actually might be more efficient than a 20b or something of that sort

 

welcome to the board.

your "stock parts" comment makes me a little nervous. generally speaking, more rotors DOES mean more fuel consumption. and yes, a 26B will consume more than a 20B. i'm not familiar with what you read, but it was probably a comment geared specifically toward the R26B and was more of a relative thing than a general statement.

I will try to get you a number on the R26B, but keep in mind that the number will probably be specific to that engine - meaning it won't necessarily apply to other 26B engines that various people/companies have been since then.

 

First, let's make sure you have your engines sorted out...

R26B Race Engine - It has racing peripheral porting, no turbos, a racing slide throttle, and a racing dry oil sump. In this picture it has variable intakes, but they were only used for one race and the rest of the time it had fixed a intake similar to the 20B Race Engine. There are 3 spark plugs per rotor in order to improve fuel consumption. This engine was only available from Mazda Motorsports and its sponsored race teams, and was never installed in a Mazda production street car.


20B Race Engine - It has a slide-throttle intake, peripheral porting, and dry oil sump similar to the R26B. This engine was also only available from Mazda Motorsports, and was never installed in a Mazda production street car.


20B-REW Engine - From the Mazda JC Cosmo luxury car. This is a street-legal engine with twin turbos, stock ports, wet sump, butterfly throttle, and other parts typical of a street car. In most cases this is what people put in their RX-7.


MPG is affected by the engine's fuel consumption rate, as well as the car's gearing, tire size and grip, weight, and aerodynamics. Other factors like road surface and incline, wind, temperature, humidity, ambient pressure, and gravity will also affect MPG, but evaluators try to factor them out as much as possible.

If you look at the engine fuel consumption rate as a function of engine rpm, then with everything else being equal an engine with a larger displacement will consume more fuel than an engine with a smaller displacement. For example, if you have a car with a 3-Liter engine that goes 150mph at 6000rpm, and you replace the engine with a 4-Liter engine of the same efficiency but you do not change the gearing, then the car will still go 150mph at 6000rpm, but the larger engine will consume 33% more fuel because it is 33% larger.

Therefore, we usually look at engines based on something called "brake specific fuel consumption" (BSFC), which is a measure of how much power the engine produces for a given amount of fuel. Let's now look at a similar example using BSFC. You have the same car with a 3-Liter engine, and the designers of this car have decided that the power required to go 150mph is 200hp, the 3-Liter engine develops 200hp at 6000rpm, and therefore an appropriate gear ratio is chosen for the car. However, the 4-Liter engine of the same efficiency may produce 200hp at only 4500rpm. Therefore, if the gear ratio is changed, then the car can go 150mph at 4500rpm. If both engines have the same BSFC across the rpm range, then the fuel consumption rate would be the same at 200hp, so in this case the car would have the same fuel consumption rate regardless of which engine/gear combo is used. However, if both engines have a better BSFC at 4500rpm than they do at 6000rpm, then the larger engine would actually have a lower fuel consumption rate at 150mph in this case.

So, to answer your question, the R26B does have a better BSFC than a 20B Race Engine due to the spark plug configuration, but it will burn more fuel at a given RPM.

More about the R26B engine here:
http://www.rotaryeng.net/Mazda_R26B_US.pdf

 

thanks. so far im enjoying the great wealth of information available here.

my bad. i am not specifically asking about the r26b. i know that there are some people running 4 rotor conversions that are custom and derived from either the 13b or 12a sort of like this guy.
4 Rotor Build - NoPistons -Mazda Rx7 & Rx8 Rotary Forum
i also understand that they are probably made for racing purposes so they do not care much about mpg.

 

I mean most people with turbo or 3 rotor n/a and up even dont care abkut gas mileage, better to just get a dail and have the sports car..

 

Those are custom projects, and the BSFC will depend on how the engine is built and tuned. There are literally billions of possible combinations, so it is futile to speculate on the performance.

Cars have MPG, engines have BSFC. An engine by itself is not capable of traveling, and therefore will travel no miles for any given gallons of fuel.

BSFC is important for endurance racing because fuel stops cut into your track time. This is why Mazda took the time to add the extra spark plugs to the R26B engine. BSFC is not very important for drag racing because the engine only needs to run at peak performance for a few seconds.

 

^ in that vein, this thread did prompt me to look up a number on the R26B. I got 51.881 liters per 100 km at 213.58 kph. Suprecars.net was my source.

 

Really? So how did the engine travel 100 km without a car? Maybe somebody rolled it down a mountain?

 

wow. tough crowd today .... I mis-spoke. for the record, I wasn't challenging anything you said (I was actually quite impressed and learned the actual context for BSFC because of your first post). I was simply supplementing what you said while doing what I told the OP I was going to do.

that number was clearly while attached to (and powering) the 787B. I will try to dig around a little more simply because I like having more than one source, but for a race car, it seems reasonable enough, I suppose.

 

Hehehe, I'm just busting your chops because I went through all that trouble to explain that MPG does not apply to the engine.

It works out to about 4 1/2 MPG, which sounds reasonable. However, I would imagine that the MPG would change with different tracks, drivers, and weather conditions, so don't kill yourself trying to find an accurate number.

 

indeed. it did.

 

hearsay: I remember people talking about how bad their 'hypermiling' mileage was in comparison to their track days with the split being minimal. Is that just a function of how low the mileage is to begin with, or is there some truth in the claims that the wankel just likes to rev (in this context)?