fuel economy 101

Place for discussions about the RX-8
User avatar
warpc0il
Spin Doctor
Spin Doctor
Posts: 22287
Joined: Sat Sep 06, 2008 10:56 pm
RX-8: 231
Colour: Lightning Yellow
Location: Groomsport, Co Down, NI
Has thanked: 304 times
Been thanked: 1180 times
Contact:

Re: fuel economy 101

Post by warpc0il » Wed Oct 16, 2019 9:51 am

Hyper-mileing is a very artificial driving mode and, taken to extremes would also involve things like increasing tyre pressures to maximum to reduce rolling resistance. Even just holding 70 in top for long periods is fairly unusual compared with real-world day-to-day driving*.

I'm sure when you were monitoring fuel flows you would have also seen that, when going up hill, even on a motorway, it was more efficient to change down and run with a lighter throttle in a lower gear than staying in top.

Yes, gear ratios are a compromise, based on a whole variety of factors which people encounter in all sorts of driving conditions.
Fuel economy would have been one factor but not the top of the list for a sports car, when choosing ratios for the 8.
I still suggest that, for the average reasonably mechanically-aware driver, in real-world road conditions, not trying to hyper-mile, that the original S1 or R3 gearing would provide the best results.

*this driving scenario would almost guarantee a "Cat failure" CEL on any 8 with a decat, as it's just about the only time that the PCM gets to see if the cat is working. However, how few people with decats ever see that CEL, without bothering to have it mapped-out. This shows how rarely people drive (their 8) like that.
Dave
The Spin Doctor ™
uǝǝɹɔs ɹnoʎ ʇɹǝʌuı ǝsɐǝld :ɹoɹɹǝ

User avatar
Dr. FrankenRex
Former Member
Former Member
Posts: 4515
Joined: Thu Jan 08, 2015 1:30 pm
RX-8: R3
Colour: Velocity Red
Location: Cotswolds
Has thanked: 230 times
Been thanked: 488 times
Contact:

Re: fuel economy 101

Post by Dr. FrankenRex » Wed Oct 16, 2019 10:13 am

Yes, hypermiling is a different driving condition, but the point I was trying to make is that lowering the rpm at a given speed by a small amount compared to the Mazda design is likely to increase efficiency as they designed a sports car rather than a diesel city-box. You would, obviously, be making other sacrifices such as acceleration, but it would increase efficiency and has nothing to do with whether the gearbox and diff are 'matched'

User avatar
PeteH
Committee Member
Committee Member
Posts: 5841
Joined: Mon Apr 11, 2016 1:26 pm
RX-8: 231
Colour: Custom
Location: West Sussex
Has thanked: 178 times
Been thanked: 825 times

Re: fuel economy 101

Post by PeteH » Wed Oct 16, 2019 10:37 am

This kind of discussion always makes me smile. No-one knows the optimum gearing for an RX-8. Mazda are the only people with enough information to calculate it, and they almost certainly didn't bother. They definitely had a free choice in gear ratios, so they put in exactly what they wanted, and there first priority will have been to ensure that it could reach top speed in fifth gear. Second, they will have ensured that 6th was a sensible step (not a large gap). Then they would have arranged the other ratios in "geometric progression" (that's almost always done). Finally, they would have made sure that first gear was low enough to do a fully loaded hill start on a 1 in 3 hill (or some similar criteria).

Gearing for economy was only just starting to be a "thing" in the late 90s when the 8 was designed, and it wouldn't have been applied to a sports car like the 8. It's massively popular now, of course, and car that do 1500rpm on the motorway are not uncommon. But it should be said that modern turbo engines are designed to pull that gearing, and they have full boost and huge torque available at those low revs. My 335D would do 1500rpm at 70mph in 8th gear. But as soon as you breathed on the throttle it would change down to 6th. And in normal driving it would change gear every couple of seconds. I wouldn't fancy this strategy with a manual gearbox!

It should also be pointed out that the 8 is actually very highly geared. It is geared for a theoretical top speed of 185mph in 6th. It just doesn't feel high geared because of the huge revs it can reach.

Back to the beginning. To work out the optimum gearing for minimum fuel use you would need to know the bsfc (brake specific fuel consumption) at every rev point, and every throttle point. You would then need to know the power requirement at every road speed. You would then need to do a simulation with a range of gear ratios, and run an optimiser to identify the optimum. We don't have all that information. And, until we do, everyone is speculating.

(Having said that, I would like to try a "economy optimised" RX-8. Lightened, with aero tweaked for minimum drag, air con removed, low rolling resistance narrow tyres, and high gear ratios. Just for a giggle, and to see what's possible with a rotary. It would be a horrible car to drive though....)
These users thanked the author PeteH for the post (total 2):
Rostyghost (Wed Oct 16, 2019 10:46 am) • ChrisHolmes (Wed Oct 16, 2019 5:04 pm)
RX-8 in Time Attack 2016 (2nd in class) & 2017 (Championship Winner)
http://www.rx8ownersclub.co.uk/forum/vi ... 04&t=69554

User avatar
warpc0il
Spin Doctor
Spin Doctor
Posts: 22287
Joined: Sat Sep 06, 2008 10:56 pm
RX-8: 231
Colour: Lightning Yellow
Location: Groomsport, Co Down, NI
Has thanked: 304 times
Been thanked: 1180 times
Contact:

Re: fuel economy 101

Post by warpc0il » Wed Oct 16, 2019 10:55 am

Let's see if we can agree on this..

For any given road speed and load (gradient, headwind, etc) there is a range of engine revs that could been appropriate to drive.
The best fuel economy is going to be in the lower part of the range and the best acceleration is going to be nearer the top.
Below that range the engine is labouring.
Above that range and you're risking engine damage (in a piston engine) or just hitting the beep in an 8.

Defining the range gets complicated by the relative shapes of the power and torque curves, among other things, but it's clear that the numbers are higher for the Renesis than for the average piston engine.

Above 1st and until you reach top gear, you have the option of staying in the gear you're in, or dropping down a gear to move the revs up in the range, or short-shift into a higher gear to move those revs lower in the range, at that same road speed. So you have the choice between fuel economy and performance.

Unless you're driving most of the time in top gear, the overall gearing makes no difference to that choice.

However, gearboxes don't have exactly even steps between gears, there may be a larger step between 2nd-3rd than between 3rd-4th, while more gears ( e.g. 5 or 6) gives you finer control on where you can position the revs within that range in those conditions.

Here's the S1 6-speed ratios in action
6 speed 231.JPG
6 speed 231.JPG (62.18 KiB) Viewed 158 times

The choice of the 5-speed for the 192 was partially cost, partially marketing (to differentiate from the 231 6-speed) but also in recognition of the different power characteristics of the 4-port engine, which actually makes slightly more torque in the mid-range than the 6-port, and the ratios are therefore a better match. The 5-speed not just being a 6-speed without the 6th gear.

The choice of the gearbox for the R3 was almost entirely based on cost, as it was only intended to be a limited production run and they thought they could just re-use the 6-speed from the later MX-5. The lower revs of the MX-5 engine had two impacts, the first of which being obvious and meant that the R3 needed the change of diff ratio to use the MX-5 'box. The second wasn't apparent until much later when the R3 synchros started to chew themselves up - but that's another topic...

Update: I wrote this while PeteH was also writing his post above, not in direct response to it, though I don't disagree.
These users thanked the author warpc0il for the post (total 2):
v-rex (Wed Oct 16, 2019 1:35 pm) • ChrisHolmes (Wed Oct 16, 2019 5:06 pm)
Dave
The Spin Doctor ™
uǝǝɹɔs ɹnoʎ ʇɹǝʌuı ǝsɐǝld :ɹoɹɹǝ

User avatar
Juey
Club Member
Club Member
Posts: 3005
Joined: Sat May 29, 2004 1:49 am
RX-8: Import
Colour: Black Cherry
Location: Peterborough
Has thanked: 20 times
Been thanked: 94 times

Re: fuel economy 101

Post by Juey » Wed Oct 16, 2019 10:57 am

When my 8 had it's auto box, 4th gear (top) was very long indeed. Long M'way trips yielded good mpg (best tank of 354 without trying to)....and that was with the 20% drain on resources that the gearbox was compared to the manual. So big gearing would make a difference at M'way cruise speeds.
3000rpm gave 75mph - which is about 60mph in a manual.

Introducing The Overdrive Switch off of the 70's !
Last edited by Juey on Wed Oct 16, 2019 1:14 pm, edited 1 time in total.
These users thanked the author Juey for the post:
bigpete8 (Wed Oct 16, 2019 12:06 pm)
Black Cherry Mica
JDM Evolve. 192 6 spd manual from auto

User avatar
bigpete8
Club Member
Club Member
Posts: 884
Joined: Wed May 17, 2017 12:27 pm
RX-8: 231
Colour: Sunlight Silver
Location: Tyne & Wear
Has thanked: 74 times
Been thanked: 80 times

Re: fuel economy 101

Post by bigpete8 » Wed Oct 16, 2019 12:12 pm

Used to love the overdrive on my MGB GT
Took a bit of practice* but could get a half shift from 3rd at full tilt (about 20 mph :) ) adding the overdrive then onto 4th whilst disabling the overdrive.

*pratice especially as mine had the switch at the RHS of the dash not on the gearstick - but on the plus side it wasn't disabled by the vacuum pressure so could do it at full throttle unlike later models.
Sunlight Silver 231
Trying run on a budget to keep the noise level down - from the missus!

User avatar
warpc0il
Spin Doctor
Spin Doctor
Posts: 22287
Joined: Sat Sep 06, 2008 10:56 pm
RX-8: 231
Colour: Lightning Yellow
Location: Groomsport, Co Down, NI
Has thanked: 304 times
Been thanked: 1180 times
Contact:

Re: fuel economy 101

Post by warpc0il » Wed Oct 16, 2019 12:20 pm

I used to drive an original shape Sunbeam Rapier, with similar (same?) transmission, overdrive on both 3rd and 4th, which allowed for clutchless changes. Great for getting just that bit more speed when exiting a fast bend or accelerating up a motorway sliproad.

The guy that owned the car messed about with the wiring to allow him to get overdrive in 2nd, which was pointless, and then somehow selected reverse with the overdrive unit still engaged - who knew it would go bang in such a spectacular way :shock:
Dave
The Spin Doctor ™
uǝǝɹɔs ɹnoʎ ʇɹǝʌuı ǝsɐǝld :ɹoɹɹǝ

User avatar
bigpete8
Club Member
Club Member
Posts: 884
Joined: Wed May 17, 2017 12:27 pm
RX-8: 231
Colour: Sunlight Silver
Location: Tyne & Wear
Has thanked: 74 times
Been thanked: 80 times

Re: fuel economy 101

Post by bigpete8 » Wed Oct 16, 2019 1:08 pm

Ermmm..... Was the guy who owned it the same as the one who used to drive it by any chance?

For the popular MGB 3.5 v8 (sd1) engine conversion you had to disable overdrive on 3rd as well otherwise the torque would destroy it!
Sunlight Silver 231
Trying run on a budget to keep the noise level down - from the missus!

User avatar
Dr. FrankenRex
Former Member
Former Member
Posts: 4515
Joined: Thu Jan 08, 2015 1:30 pm
RX-8: R3
Colour: Velocity Red
Location: Cotswolds
Has thanked: 230 times
Been thanked: 488 times
Contact:

Re: fuel economy 101

Post by Dr. FrankenRex » Wed Oct 16, 2019 1:22 pm

warpc0il wrote:
Wed Oct 16, 2019 10:55 am
The choice of the 5-speed for the 192 was partially cost, partially marketing (to differentiate from the 231 6-speed) but also in recognition of the different power characteristics of the 4-port engine, which actually makes slightly more torque in the mid-range than the 6-port, and the ratios are therefore a better match. The 5-speed not just being a 6-speed without the 6th gear.
The ratios of the 5 speed are definitely not a better match to the engine... Anyone who has driven a 192 in anger with a 5 speed 'box will tell you that the rev drop between 3rd and 4th is hideous. I understand the torque point, but it drops the revs down way too low and the car 'bogs down' for want of a better phrase. Against a 231 in a drag race (have done this myself) the 231 pulls away quite well there - not down to the increased power, but mainly because it's closer to being 'on the boil' - another example that Mazda didn't get it right.
PeteH wrote:
Wed Oct 16, 2019 10:37 am
This kind of discussion always makes me smile. No-one knows the optimum gearing for an RX-8. Mazda are the only people with enough information to calculate it, and they almost certainly didn't bother.
As you say, Pete, nobody knows, but what annoys me is people saying Mazda got it perfect when they didn't - not in the scenario of efficiency of the gearing at least. It A) wasn't designed for efficiency and B) wasn't designed with any kind of efficiency logic applied at all.

I get your point about turbocharged and diesel cars being efficiency tuned, my wife's A4 manual had a stonkingly long 6th gear (theoretically capable of 215mph!) which kept the RPM down to about 1400 at 70mph. For a 2.0tdi estate it frequently managed 75mpg on a long run. However the RX8 is not 'most efficient' when run at 3,500rpm, which is what I am talking about in this example. If that was the case people would be doing 30mph in 2nd gear all the time in the 8 (yes, I know people do, hence my example, because it is definitely NOT the most efficient way of doing it).
Juey wrote:
Wed Oct 16, 2019 10:57 am
When my 8 had it's auto box, 4th gear (top) was very long indeed. Long M'way trips yielded good mpg (best tank of 354 without trying to)....and that was with the 20% drain on resources that the gearbox was compared to the manual. So big gearing would make a difference at M'way cruise speeds.
3000rpm gave 75mph - which is about 60mph in a manual.
This proves my point. Whilst I know the response will be "the 192 has more torque", it is utter balderdash of an argument. The 192 has a LITTLE bit more torque, but the 231 at that kind of rpm and speed - without the parasitic loss of the auto slusher - would certainly be just as if not more efficient.

User avatar
v-rex
Club Member
Club Member
Posts: 256
Joined: Wed Jan 31, 2018 9:24 pm
RX-8: PZ
Colour: Brilliant Black
Location: Pinner, NW London
Has thanked: 97 times
Been thanked: 44 times

Re: fuel economy 101

Post by v-rex » Wed Oct 16, 2019 1:27 pm

Dr. FrankenRex wrote:
Wed Oct 16, 2019 1:22 pm
If that was the case people would be doing 30mph in 2nd gear all the time in the 8 (yes, I know people do, hence my example, because it is definitely NOT the most efficient way of doing it).
I am one of those people. I always just thought 2nd for 30pmh, 3rd at 40mph, 4th at 50 mph and so on. Keeps it at around 4K-ish where the engine seemed happy. I will continue reading and find out why I've been doing it wrong all these years .... :o

User avatar
New Duke
Club Member
Club Member
Posts: 1313
Joined: Sat Dec 03, 2016 8:38 pm
RX-8: R3
Colour: Sparkling Black
Location: Colchester, Essex
Has thanked: 438 times
Been thanked: 353 times

Re: fuel economy 101

Post by New Duke » Wed Oct 16, 2019 1:59 pm

I should have been clearer in my language that I was specifically referring to motorway/dual carriageway cruising at 70mph. Maybe not as dedicated as hypermiling. But definitely assuming relatively flat motorways and not much traffic allowing a relatively constant 70mph. Which is usually the case for my commute into London as I travel outside of peak hours.

I don't have the specific knowledge of differences between the models or the relative ratios that you folks have, so was being very sweeping and general in my assumptions. For example:

In the R3 with R3 gearbox and diff 70mph was pretty much an even 4000rpm
Now with an S1 gearbox 70mph is an even 4500rpm
So my very basic assumption was that an S1 with an R3 gearbox and S1 diff 70mph would drop to about 3500rpm. Which I assumed would be more fuel efficient assuming the conditions above.

But that was a very basic (perhaps wrong) assumption on my part and I'm always happy to be educated/set straight here.

The modification to fit an R3 box into an S1 described on the US forum is exactly the same as the mod to fit an S1 box into an R3. It just means swapping a couple of cables on the loom. I watched bigpete do it in all of 2 mins :thumleft:

User avatar
kopite72
Club Member
Club Member
Posts: 10568
Joined: Wed Dec 28, 2011 8:30 am
RX-8: R3
Colour: Diamond Grey
Location: Cork Ireland
Has thanked: 298 times
Been thanked: 504 times

Re: fuel economy 101

Post by kopite72 » Wed Oct 16, 2019 2:03 pm

I can tell you exactly because I have a S1 diff fitted to my R3 that cruising at 70 is indeed 3500 RPM or close enough...
PLAN...RR HB OR FB PLATINUM REBUILD
FULL RESPRAY

DONE...
FULL MZ CUSTOMS KIT
MILLTEK CATBACK
JAPSPEED DROPLINKS
FULL ALLOY REFURB
RR D585 COILs
6000K HID KIT
TINTS
20MIL SPACERS
PROJECT THREAD....http://www.rx8ownersclub.co.uk/forum/vi ... 04&t=59971

LOOKED AFTER BY THE FANTASTIC CREW AT ROTARY REVS
LOVING DORITOS SINCE 1991

User avatar
PeteH
Committee Member
Committee Member
Posts: 5841
Joined: Mon Apr 11, 2016 1:26 pm
RX-8: 231
Colour: Custom
Location: West Sussex
Has thanked: 178 times
Been thanked: 825 times

Re: fuel economy 101

Post by PeteH » Wed Oct 16, 2019 2:18 pm

New Duke. Fully agree. Your question was valid, and your assumptions are reasonable. Raising the gearing might help. Nobody knows, and it would be interesting to find out. I'm not sure about your gear ratio calcs though, so I'm going to do a couple of sums...

Dan. Jeez, you like to prod the controversy tree.... I'll just pick up a single point. Just because 3500rpm might be the most economical revs at 70mph, that doesn't mean 3500rpm will be the most economical revs at 30mph. I thought my response below made that clear... The optimum revs is dependant on road speed, throttle, gearing, and bsfc. Your assumption is based on linear thinking, and linear extrapolation. The analysis of optimum efficiency is just about the least linear problem you could ever contemplate. Let me just say, in simple terms, the optimum revs at low road speed are low, and the optimum revs at high road speed are high. How low, and how high, need complex analysis, but that is the basic trend.
RX-8 in Time Attack 2016 (2nd in class) & 2017 (Championship Winner)
http://www.rx8ownersclub.co.uk/forum/vi ... 04&t=69554

User avatar
Dr. FrankenRex
Former Member
Former Member
Posts: 4515
Joined: Thu Jan 08, 2015 1:30 pm
RX-8: R3
Colour: Velocity Red
Location: Cotswolds
Has thanked: 230 times
Been thanked: 488 times
Contact:

Re: fuel economy 101

Post by Dr. FrankenRex » Wed Oct 16, 2019 2:53 pm

I'm not intending to poke to controversy tree, I'm getting annoyed with the inherent trend of people on the forum stating Mazda is best and then continuing on a condescending trend towards making the other person seem like they're being the bullish one.

In this scenario it has been pointed out by multiple people that Mazda did no such thing with regards to the design or selection of the gearbox and differential ratios, and yet the message coming from the forum is to simply discredit my reasoning because I'm not accepting what is being said. You can run sums, you can tell me I'm making assumptions of linearity (I'm not). The simple point I'm trying to get across is that the theory supports that a longer ratio gearbox and differential combination would be more efficient at motorway speeds - as originally posed.

This has been backed up by Juey's findings, and supported further by mine whilst hyper-miling. If an engine is more efficient at 3,000rpm at 75mph it should be even more efficient doing 3,000rpm at 70mph as there is less resistance from all parts of the system - assuming all components are the same. Remove a chunk of the parasitic loss from the auto box and it should be amplified.

By all means do a couple of sums, but what I'm saying is that maybe - just maybe - Mazda didn't design something perfect for a small seemingly inconsequential group of owners who use their cars on the motorway...
v-rex wrote:
Wed Oct 16, 2019 1:27 pm
I am one of those people. I always just thought 2nd for 30pmh, 3rd at 40mph, 4th at 50 mph and so on. Keeps it at around 4K-ish where the engine seemed happy. I will continue reading and find out why I've been doing it wrong all these years .... :o
It isn't going to do anything *wrong* at all, and the renesis engine will usually feel happier at a higher-than-normal rpm, but from driving and monitoring fuel consumption data, fuel flow data etc. it is more efficient (on a flat gradient) to be in 3rd or even 4th gear at 30mph for a constant speed/time. If you need to accelerate then it won't be most efficient anymore, but I'm talking about maintaining a speed on a relatively flat stretch of road. The RX8 will happily do 40th in 6th - assuming a 231.

Worth noting, though, that it provides fairly minimal gains - around half an mpg.

User avatar
PeteH
Committee Member
Committee Member
Posts: 5841
Joined: Mon Apr 11, 2016 1:26 pm
RX-8: 231
Colour: Custom
Location: West Sussex
Has thanked: 178 times
Been thanked: 825 times

Re: fuel economy 101

Post by PeteH » Wed Oct 16, 2019 2:59 pm

Right. At a genuine 70mph the various version of the RX-8 will be doing the following revs:

s1 192: 3150rpm
s1 231: 3400rpm
s1 auto: 2700rpm
s2 192: 3250rpm
s2 231 (and R3): 3450rpm
s2 auto: 2300rpm ( :shock: )

If you put an R3 gearbox in an s1 231 your revs will be 3200rpm
If you put an s1 231 diff in an R3 your revs will be 3200rpm

If you put an s1 231 gearbox in an R3 your revs will be 3650rpm
If you put an R3 diff in an s1 231 your revs will be 3650rpm

The highest gearing you can get (with proper Mazda bits) is to put an RX7 Auto diff into an s2 auto. Your revs will be 2050rpm.
The lowest gearing you can get (with proper Mazda bits) is to combine the R3 diff and the s1 6 speed box, as described above (3650rpm)

Footnote. I had no idea the s2 auto (the 6 speed six port one) was so massively high geared. I guess that's the way the 'Mercans wanted it.... FYI, if it could hit the red line in top it would be doing 305mph. :lol: :lol: :lol:

Before everyone jumps up and down saying "my car revs higher / lower than this". This a genuine 70mph, not a speedo 70mph. This is with stock tyres. This makes some assumptions about the tyre being in good condition, having full tread, and not "slipping" much. It is also genuine rpm, not the rev counter rpm. In other words: "Your results may vary in the real world"......
These users thanked the author PeteH for the post:
ChrisHolmes (Wed Oct 16, 2019 5:11 pm)
RX-8 in Time Attack 2016 (2nd in class) & 2017 (Championship Winner)
http://www.rx8ownersclub.co.uk/forum/vi ... 04&t=69554

User avatar
PeteH
Committee Member
Committee Member
Posts: 5841
Joined: Mon Apr 11, 2016 1:26 pm
RX-8: 231
Colour: Custom
Location: West Sussex
Has thanked: 178 times
Been thanked: 825 times

Re: fuel economy 101

Post by PeteH » Wed Oct 16, 2019 3:05 pm

Dr. FrankenRex wrote:
Wed Oct 16, 2019 2:53 pm
If an engine is more efficient at 3,000rpm at 75mph it should be even more efficient doing 3,000rpm at 70mph
You do know what you've done there, don't you....? :D
These users thanked the author PeteH for the post:
Dr. FrankenRex (Wed Oct 16, 2019 3:20 pm)
RX-8 in Time Attack 2016 (2nd in class) & 2017 (Championship Winner)
http://www.rx8ownersclub.co.uk/forum/vi ... 04&t=69554

User avatar
Phil Bate
Spin Doctor
Spin Doctor
Posts: 9193
Joined: Thu Mar 22, 2012 12:49 am
RX-8: 231
Colour: Sunlight Silver
Location: South Wales
Has thanked: 34 times
Been thanked: 161 times

Re: fuel economy 101

Post by Phil Bate » Wed Oct 16, 2019 3:08 pm

Dr. FrankenRex wrote:
Wed Oct 16, 2019 2:53 pm
...If an engine is more efficient at 3,000rpm at 75mph it should be even more efficient doing 3,000rpm at 70mph as there is less resistance from all parts of the system...
I am now confused :-k


edit: snap!
These users thanked the author Phil Bate for the post:
Dr. FrankenRex (Wed Oct 16, 2019 3:20 pm)
Phil's subtle sunlight silver - stronger, faster, with added red bits and stuff engine donor!
Operation Mint - Kuro preservation project
The pimped out Glacier White XE - quintessential British motoring from Jaguar

User avatar
v-rex
Club Member
Club Member
Posts: 256
Joined: Wed Jan 31, 2018 9:24 pm
RX-8: PZ
Colour: Brilliant Black
Location: Pinner, NW London
Has thanked: 97 times
Been thanked: 44 times

Re: fuel economy 101

Post by v-rex » Wed Oct 16, 2019 3:12 pm

Cheers, Dr FrankenRex, I will experiment with 3rd and 4th at 30mph, which is the speed around town I mostly do. I've always pretty much driven this way, I think they mentioned staying in 2nd at 30pmh on the Prodrive day and I never questioned it subsequently...

User avatar
Dr. FrankenRex
Former Member
Former Member
Posts: 4515
Joined: Thu Jan 08, 2015 1:30 pm
RX-8: R3
Colour: Velocity Red
Location: Cotswolds
Has thanked: 230 times
Been thanked: 488 times
Contact:

Re: fuel economy 101

Post by Dr. FrankenRex » Wed Oct 16, 2019 3:28 pm

Touche on that single point both, it wasn't phrased well - I understand that the particular example references a shorter gear ratio than a longer one, but it forms part of the larger discussion.

What I meant, but didn't communicate effectively, was (arbitrary numbers for example's sake):

If an auto does 75mph at 3,000rpm and runs at 30mpg (lol), the exact same car doing 3,000rpm at 70mph (with a different gear ratio) would be more efficient on the same road, say 31mpg - in the same conditions - due to having less resistance from tyres, hubs, wind etc. - agreed?

Thus if you had a car with less parasitic loss, running at the same RPM at the same speed, which is already defined as being more efficient than running it at 4,000rpm, then the gains would be greater still.

Whilst not explained very well in that single post, what I'm trying to reiterate is that if you could alter the final drive or top gear of a 231 to run 70mph at 500rpm less it would result in an efficiency gain. Something that has been argued against a few times above for reasons I'm not entirely sure on. Yes, there would almost certainly be performance drawbacks (unless you were simply able to swap out 6th gear for a longer one), but the question here is around efficiency, not performance.

EDIT: That is nuts on the S2 Auto box btw... Can only presume it's because they just sit at 55mph on freeways all the time...

Nerdstrike
Former Member
Former Member
Posts: 2616
Joined: Fri Feb 11, 2011 7:47 pm
RX-8: 231
Colour: Velocity Red
Location: Cambridgeshire
Has thanked: 7 times
Been thanked: 30 times

Re: fuel economy 101

Post by Nerdstrike » Wed Oct 16, 2019 3:42 pm

Wow, the s2 auto must have been a complete sluggard on the highway at 2300rpm.

Apart from the heresy and RPM range, you'd do pretty well with a CVT and a rotary. Much more like aircraft operating conditions!

There is a utility function whereby the car retains enough torque to handle reasonable highway slopes and a bit of acceleration so it can change speed without changing gear. However, that utility function will vary market to market depending on the speed limit and typical driving conditions. Here in the UK it might be sensible to optimise for 80mph efficiency, or 50mph for that matter depending on where you live.

As we know the RX8 will do acceptably if trudged along at 50mph in top, but if you drop the revs for 70mph, you might not be able to use top at 50mph any more and you lose efficiency for most regimes except for the mythical empty road.

User avatar
PeteH
Committee Member
Committee Member
Posts: 5841
Joined: Mon Apr 11, 2016 1:26 pm
RX-8: 231
Colour: Custom
Location: West Sussex
Has thanked: 178 times
Been thanked: 825 times

Re: fuel economy 101

Post by PeteH » Wed Oct 16, 2019 3:50 pm

Dan. The problem here is that I'm saying "nobody knows", and you are saying "I know. I know it's more economical". I'm not saying you are wrong. I'm saying you might be wrong. I'm trying to dig into why you are so certain, so that I can explain why it's not so simple, but I'm not clear on why you are so certain.

Let me try this. If you have a car doing 4000rpm at 70mph, and it's doing 30mpg. Suppose you then think, "I'll make it more economical by raising the gearing", because that always works. So you fit a longer diff ratio, and now your car does 3500rpm at 70mph. Will it be more economical? You can't be sure it will be. Why not? Because when the car had short gearing you may have been using 15% throttle to maintain 70mph, but when you raised the gearing you may have moved the engine into a lower part of its torque curve, so you now need 25% throttle to maintain 70mph.

So. Is you engine more economical at 4000rpm and 15% throttle, or at 3500rpm and 25% throttle? Nobody knows. You state that you have two "data points" (Juey's auto, and your hypermiling) which suggest that the higher gearing is more economical, and that may be true, but I'd rather have much more info than that before I started stating that I knew "for certain" that something was definitely always true.

I reiterate. I'm not saying you are wrong. I am saying that you don't have enough facts to support your assertion. And I have said all along that it would be fascinating to build a car to get those facts.
RX-8 in Time Attack 2016 (2nd in class) & 2017 (Championship Winner)
http://www.rx8ownersclub.co.uk/forum/vi ... 04&t=69554

User avatar
Dr. FrankenRex
Former Member
Former Member
Posts: 4515
Joined: Thu Jan 08, 2015 1:30 pm
RX-8: R3
Colour: Velocity Red
Location: Cotswolds
Has thanked: 230 times
Been thanked: 488 times
Contact:

Re: fuel economy 101

Post by Dr. FrankenRex » Wed Oct 16, 2019 4:22 pm

If I'm being frank, Pete, it wasn't your replies I was arguing against ;)

I get what you're saying and I understand that there isn't enough data to prove conclusively one way or the other.

User avatar
warpc0il
Spin Doctor
Spin Doctor
Posts: 22287
Joined: Sat Sep 06, 2008 10:56 pm
RX-8: 231
Colour: Lightning Yellow
Location: Groomsport, Co Down, NI
Has thanked: 304 times
Been thanked: 1180 times
Contact:

Re: fuel economy 101

Post by warpc0il » Wed Oct 16, 2019 4:48 pm

This hypothetical empty road is the problem, not only empty but also totally flat.

As has been stated, some of these clever high-efficiency auto transmissions run a really high gear on the overrun or light throttle, but do anything with that throttle to cope with an incline or to match the traffic in the outside lane, and it'll change down a bunch of gears.

Similarly you can drive an 8 on this mythical highway at 40 in top, and it will be more efficient, until reality kicks in.

How much throttle does it take to accelerate from 40 to 50 in top, compared to 5th, or 4th, or 3rd..?

Just to throw in another factor here. From the day I fitted the ram air intake I can drive in most conditions in a gear higher than before and it still takes less throttle action to accelerate smoothly.

Weird ha...

Sent from my SM-G800F using Tapatalk

Dave
The Spin Doctor ™
uǝǝɹɔs ɹnoʎ ʇɹǝʌuı ǝsɐǝld :ɹoɹɹǝ

User avatar
New Duke
Club Member
Club Member
Posts: 1313
Joined: Sat Dec 03, 2016 8:38 pm
RX-8: R3
Colour: Sparkling Black
Location: Colchester, Essex
Has thanked: 438 times
Been thanked: 353 times

Re: fuel economy 101

Post by New Duke » Wed Oct 16, 2019 5:05 pm

PeteH wrote:
Wed Oct 16, 2019 2:59 pm
Before everyone jumps up and down saying "my car revs higher / lower than this". This a genuine 70mph, not a speedo 70mph. This is with stock tyres. This makes some assumptions about the tyre being in good condition, having full tread, and not "slipping" much. It is also genuine rpm, not the rev counter rpm. In other words: "Your results may vary in the real world"......
Thanks for doing those calcs on the various RPMs at 70mph Peter. I knew you'd see this comment coming! ;)

There's a much bigger variation in genuine vs speedo rpm than I imagined. Based on my own rev counter at least (with all the variables you mentioned accounted for of course) there's hundreds of rpm variance between the two. That's huge. I had no idea rev counters were so inaccurate. I assume that's the same for rev data collected over ODB too.
warpc0il wrote:
Wed Oct 16, 2019 4:48 pm
Similarly you can drive an 8 on this mythical highway at 40 in top, and it will be more efficient, until reality kicks in.
Totally agree. It's not relevant for most commuting I'd imagine. I'm very lucky that most of my motorway driving is in flat southern areas very late at night. I'm blessed that sometimes I can do 100 miles and not touch the brake.

On an unrelated note since fitting the same air ram scoop I suspect that it's causing a bit more drag and hurting fuel efficiency a bit. But that's just another total assumption on my part! ;)

User avatar
ChrisHolmes
Events Team Member
Events Team Member
Posts: 15894
Joined: Sun Jan 29, 2006 10:04 pm
RX-8: 231
Colour: Custom
Location: Cheltenham
Has thanked: 3992 times
Been thanked: 1595 times

Re: fuel economy 101

Post by ChrisHolmes » Wed Oct 16, 2019 5:18 pm

So how about when you supercharge a Renesis? What happens to the economy then? I cannot check mine as the OBD function is dead due to having the Adaptronic ECU fitted. On the recent trip to Wales, which included numerous hills and the odd little sprint I calculated 17.7mpg. Also prior to going I logged two 25mph to ??mph runs on the sand fill up.
Supercharged
Racing Beat Alloy flywheel
Exedy Stg 1 Clutch
White Line ARBs
Tein Monosport Coilovers
Speedline diamond cut alloy wheels
Racing Brake 4 pot calipers, slotted discs & alloy bells
Racing Beat Custom front bumper, grills & Side Skirts
Mazdaspeed Rear Spoiler (Customised)
Rear Diffuser
Vented Carbon Bonnet
SOHN Adaptor
Custom Leather interior & Boot trim
Adaptronic ECU

Post Reply

Return to “RX-8 Discussion”