Using FD2R 5.062 Final Drive?

[fr4n]

^not always

[ChargeR]

To clarify my point above regarding fuel consumption, here's a greatly simplified explanation. At any given speed on flat ground a car needs a certain amount of power to continue at that same speed, rolling resistance, driveline drag and aerodynamic drag need to be overcome along with engine accessory power consumption. So for example driving along at 100km/h your car's engine might need to be outputting 15kW to maintain speed .This number is just something I picked at random, I'm not sure what a realistic number for a passenger car is.)

But there's many different conditions under which the engine can generate this power. A K20A2 for example probably develops at least 15kW from not far above idle all the way to the rev limiter, with only the throttle position needing to be varied to deliver that amount of power. So you could theoretically drive along at say 1500 RPM at 100km/h if your gearing was modified to suit, with the engine providing the required 15kW to maintain speed. However at this lower RPM the engine would obviously need a wider throttle opening to develop this power output. And it might use more fuel than at the original higher RPM, with a smaller throttle opening.

The most common way to quantify this difference is with a BSFC map, like the one below, which is a 3 dimensional graph dependent on RPM and power output. BSFC stands for brake specific fuel consumption. Which is basically how much fuel an engine uses at a given power level. So a lower number is better. Looking at the figure below, you can see that the best BSFC for that engine is around 2500 RPM, but at a power output of 120ish kW. Which is pretty useless for just cruising at part throttle, which is what we're discussing.

For the purposes of working out what RPM is best for highway cruising you would mark a horizontal line at the approximate power level you need to maintain speed, and see what RPM gives the lowest BSFC. In the case of this engine it looks to be around 3500RPM. So you can see from looking at this BSFC map, that lower RPM is not always better. There's a sweet spot for all engines where you'll get the power you need, for the least fuel consumed.




Note that this is a gross simplification, most often these BSFC maps are made on an engine dyno and thus don't consider driveline drag or real world driving conditions. There's a lot of stuff going on in an engine that affects efficiency and thus BSFC that I don't understand or haven't touched on here.

[paulcurr]

Ah thanks charger. Kinda understand he logic behind it but cant really read the graph? Where is the desired speeda indicated? Stpid questiob maybe but its out there now

[ChargeR]

Ah thanks charger. Kinda understand he logic behind it but cant really read the graph? Where is the desired speeda indicated? Stpid questiob maybe but its out there now

The BSFC graph is kind of like a topographical map (The type that shows the elevation of terrain) as in it's representing a 3 dimensional quantity on a 2D bit of paper or computer screen. Each of the marked lines or contours, like the central one in the above picture marked "250", represent the points where the fuel consumption is 250 grams per kW the engine develops for an hour, in the same way that contours on a topographical map show the elevation of the terrain. So any combination of RPM (The X axis) and torque (the Y axis) that puts you on that contour will give that fuel consumption level. So if you know you need "Z" amount of torque to maintain a given speed then you can look along the graph at that torque level to see at what RPM the engine uses the least fuel to deliver that torque, like the example lines I've put on the graph below. When the engine's developing 46.8Nm of torque it uses the least fuel at around 3500 RPM as marked with the orange X, as that point corresponds with the lowest BSFC "contour" the horizontal line matching that torque value intersects, a value of about 300 g/kWh.

Unfortunately this example doesn't quite work though, because to change the RPM at a particular speed you need to change the gearing, thus also changing the torque to the wheels. So BSFC maps plotted in the way the one above is have limited uses.

I mistakenly said above that the Y axis was power, I didn't look closely enough at the graph and I have seen BSFC maps plotted as power vs. RPM. I'll edit that post now.


Saturn_99_1.9l_dohc_bsfc1.jpg

[butter]

I will be running a FD5.08 and TSX 6th gear soon on my K24.
So I shall report back on how much of a mistake this was.

My expectations is that I will have made my 1st gear and 2nd gear redundant.
But I never plan on launching the car anyway.

Reporting back after as its a quite day at work...

nothing stopping you using 5.1 with a k24 but i'd say with the extra torque launching/taking off, first gear would become a bit of an issue, or a bit useless

Spot on gunpoons.
First gear acceleration will have traction before VTEC / peak torque, but will slip the wheels dramatically when VTEC engages @ 4,500 rpm / peak torque. (RPM needle shoots straight to limiter as soon as the wheels slip)
50% throttle in 1st gear VTEC is still enough to spin the wheels. If I intend on accelerating in 1st gear VTEC, I'd have to feather it.
I'm now finding myself shifting as soon as the wheels spin at 4,500rpm making the majority of power in 1st gear an issue. Too much power so straight line traction is now the limiting factor.

k20 head? yeah I never would drema of runnig the 5.1FD on the k24 lol, I would imagine 1st would be useless but keen to hear your thoughts on 2nd gear..

K24 head with rev limiter set @ 7,600RPM.

2nd gear WOT in VTEC will put me on the limit of traction in a straight line. Any steering input whilst in VTEC will have me chripping / squeeling the tyres.

(I only use 3rd/4th/5th gear at Winton.)

Keen to see how this would affect launch on k24

I have launch control set at 4,000rpm which is 500rpm shy of peak torque of 260nM @ the hubs.

Never tried launching as I'm worried the OEM drive shafts would just chew up the hub splines. Or maybe snap the drive shafts? Not going to find out

However, as I mentioned, launching the car was the very least of my concerns. If getting the car off the line quickly, than a shortened final drive is not for you.

Note: Tyres are Federal 595 RSR 225/45/17 with -1* camber.

[Timmy_B]

Who needs 1st and 2nd LOL
I do think its a bit of a high final drive for a k24.
Im still running the DC5 5 speed box and 1st gear (Same ratio as DC5R box) is just wheel spin if you try and put the power down.
Due to the long 2nd gear it will pick up and put the power down.
I remember talking to Butter at WTAC about the gearing. He did say 2nd gear was a little on the edge of power and wheel spin.

I would like to see what a stock DC5R box would do with the better ratios and FD and then look into changing gears or FD.

[butter]

Having a higher ratio final drive, such as 5.1FD on the DC5R/S gearboxes should be purely a track orientated modification.

I'd rather have the stock ratios for street driving, however my decision to change my final drive was based solely on matching the gearing at Winton. I would strongly suggest that if you choose to get a get a 5.08FD with a K24, that you get the TSX/CL9 6th gear. However I'm not sure how usable the 5.1FD + TSX 6th gear is with a K20. (more below)

DC5R with 5.08 FD + TSX 6th gear:
5th gear @ 100km/h is at 4,400 RPM
6th gear @ 100km/h is at 2,800 RPM

5th gear @ 60km/h is at 2,500 RPM
6th gear @ 60km/h is at 1,700 RPM

I believe 6th gear would 'bog' down too much without the torque of a K24.
(Note 6th gear RPM at 60km/h for me is fine, however power delivery @ 1,700 RPM on a K20 would struggle)

Also, Top speed of 5th gear is 178km/h @ 7,600 RPM limiter
Would need to shift into 6th gear at Phillip Island's main straight and (if im fast enough) at the end of the back straight at Winton approaching the S bend.
Matt and Napo are hitting 210km/h+ with their stock DC5 / EP3 gearboxes, so I'm curious to know where shifting into 6th gear at 180km/h lands in with my car and if there's any acceleration with the longer 6th gear in comparison.

[plAythiNG]

I was going to post last week, but thought I leave it till today after I got to drive my car with the 5.1FD and TSX 6th,

110km/h @ 2800rpm for me.

110km/h @ 2600rpm for me.

80km/h @ 1800rpm from memory

60km/h @ 1500rpm from memory


This is all in 6th gear. as Butter mentioned and as I thought a couple years ago when planning this mod, for street driving (or up to 70km/h) 6th gear isnt my ideal gear of choice as it is a bit too low in the rev range for my liking.

Im using the aforementioned final drive with an OEM JDM Flywheel and acceleration increase is great. I feel the car just climbs really nice and IMO feels faster than stock gearing with tuned ECU complete with I/H/E. I know I have been driving around in a stock car modded purely with just an Injen CAI for the last year but I also mentioned this same feeling when I drove Shak's car with the same drivetrain mods when I still had my I/H/E/KPRO a couple years ago.