Diff ratio opinion on driveability at 110km/h

[hjtrbo]

Diff ratio opinion on driveability at 110km/h

Hi all,

In the middle of a custom RB25DET build and have yet to drive a Skyline. Its diff ratio selection time, the options I have are the standard Ford 9" ratios. 

The engine has not been on the dyno yet, but has forged bottom end, hi flow turbo, injectors, mild cams and a programmable ECU so I do not know what it is like down low.

I need to know if 3.89:1 will be ok on the highway in the 100 - 120km/h range. More to the point, are these engines completely hopeless at around 2400 - 2800rpm? If so, then I can go 4.11 which increases the RPM range to 2500 - 3000rpm. 

Anyone running a ratio different from factory please may I have your opinions on how you find the day to day driveability on the open road?

[Steve85]

What are you aiming for in terms of the car/diffs? Are you looking to have a car that can sprint, or are you looking for relaxed driving on the freeway and slightly better fuel economy?

One way to overcome long gears it to add more power (which brings its own challenges). 

Quote

I need to know if 3.89:1 will be ok on the highway in the 100 - 120km/h range. More to the point, are these engines completely hopeless at around 2400 - 2800rpm? If so, then I can go 4.11 which increases the RPM range to 2500 - 3000rpm. 

My car has 4.375s in front and rear diffs and it makes 550hp atw. This makes it super fast in a straight line, but has resulted in it pulling around 3100 ish RPM at 100-110 (I can't be exact because the change in ratio isn't known to the speedo). It also means I am generally one gear higher than other cars on the track everywhere (until I run out of gears).

If you have a car with more power than standard you should be ok, i would assume. If it were me, I would dyno the car to find out what power you make at 110kph and then you can decide for yourself if it is the right mix of cruising vs pickup.

[hjtrbo]

@Steve85

I'm looking for compromise (illusive). This car will see track time, just as much as it will see street time and club events.

What do you think about the revs your are pulling when cruising at 110 on your car, I mean, do you think they are too high and / or the car lets you know it, or is it fine? Look at the idling speed of a F1 engine 4000+ rpm lol. I'm just trying to get a feel for how these engines are on the cruise. And yes, dyno is the best option to gauge, however, it's a bit hard without a diff lol (access to chassis dyno only).

Just trying to see if I can actually get a car part right the first time.  

 

[Steve85]

In my car (and my opinion of course) the diffs are way too short. It's one of the reasons I had the power reduced, the car just sprints too fast for my mind to keep up.

I'm actually budgeting to have them changed to a lower ratio. The problem is, it's expensive (I will have to pay someone) and I barely drive the car.

If I were in your position knowing what i know, I would stick with the 3.89. It seems like it would be ok, and if you've just worked your engine, i'm sure the car will pull fine.

[KiwiRS4T]

Running an RB30/25DET on stock 4.1 ratio is too busy for my taste. I would prefer R34 3.545 diff so in my opinion 3.89 should be fine. With an RB30 it would be near perfect but with an RB25 with about 280RWKW it should be great.

[hjtrbo]

well written opinions lads, thanking you

[admS15]

Stock 4.11 with rb25 5speed will sit at close to 3000 rpm at 110 km/h. If you're going to be doing a lot of street driving , I would stay with the 4.11, if drag racing , a higher number, for highway cruiser go lower . Do you already have the 3.89 and are you using rb25 gear bag?

[hjtrbo]

Hey @adms15

Yeah I got the RB25 box, haven't purchased diff gears yet. The ball is rolling on the rest of the diff. Most bits are locked in, i.e Eaton true trac centre, housing width, suspension pick up points, brakes, bearings, axles etc etc.

Only thing left to work out is the ratio (which I'm gonna think about for a week)

Again, just trying to chase a compromise between the highway cruiser and track. That said, when the car hits 5th on the straight at Phillip Island, I don't want it to die in the arse. This car will never ever be racing for sheep stations.

No one is saying NOT the 3.89 so thats good so far...

Edited June 14, 2017 by hjtrbo
update

[Steve85]

My car tops out around the 250kph mark. It was doing this speed about three quarters of the way down the sandown straights. I believe I have a standard GTR box (at least the box ratios).

[admS15]

@hjtrbo what car is all this in and how much is it expected to weigh?

What exactly is the highflow turbo? Hypergear?

 

[hjtrbo]

@admS15 it's in a HJ Holden Belmont Ute. Weight is approx 1450kg + driver. Turbo is a hybrid setup, OEM housing stripped / enlarged and Biago Turbo parts put back in. Happy at around 18PSI the builder tells me. Not sure on flow rate. 

[admS15]

Ok at 1450 you are around the same weight as an r33 gtst. As a compromise ratio for street and track I would recommend 4.11 will be good at everything but great at nothing, lol. My own car runs a r33 highflow turbo, full boost by 3500rpm, 4.11 gears, weighs 1420kg, I daily drive it during the week and circuit race it whenever I can. On the street it's almost perfect. On the circuit, I wish it had 4.3 so I could hold 3rd gear around slower corners, I've thought about doing it but have decided to leave the 411 as a compromise. Top speed is probably somewhere around 260. I've had it up to 230 at sandown at roughly 6.5k. You should probably go for a ride in someone's r33 and see what stock gearing feels like. Where are you located?

[hjtrbo]

I'm in Trafalgar VIC. I don't know anyone with a Skyline. All the boys love diesels down these ways lol. Yes, if I could understand how this engine goes with stock gearing is that would be perfect. My background of car ownership is a long line of Holden and Ford V8's and XR6T's, so i'm used to 1800 to 2000 cruising revs. 

4.11's aye, back to the spreadsheet again. And your comment similar to "a jack of all and a master of none" is exactly what I am chasing here! Well said. 

[Beyond Blue R33]

Diff ratio opinion on driveability at 110km/h Hi all,
In the middle of a custom RB25DET build and have yet to drive a Skyline. Its diff ratio selection time, the options I have are the standard Ford 9" ratios. 
The engine has not been on the dyno yet, but has forged bottom end, hi flow turbo, injectors, mild cams and a programmable ECU so I do not know what it is like down low.
I need to know if 3.89:1 will be ok on the highway in the 100 - 120km/h range. More to the point, are these engines completely hopeless at around 2400 - 2800rpm? If so, then I can go 4.11 which increases the RPM range to 2500 - 3000rpm. 
Anyone running a ratio different from factory please may I have your opinions on how you find the day to day driveability on the open road?

I'm at 4:1.1 went to Albany from Perth and back recently about 420km each way. Was 3000rpm at 115 kph in 5th gear. Sounds high but I got 415 km's out of 3/4 of a tank. Boost was wound down to 14 psi on blitz controller. From what you've said I would go the 3:89:1 if your looking at economy/cruising. The car lights up 1st and 2nd really quick so the longer ratio would be better in my opinion with that power. (I'm 340 rwhp at 18psi) If you've got those mods the longer ratio will help with traction in 1/2 aswell as better economy. I think. I ran some flat 13's on the 1/4 at about 6500rpm in 4th at 177 kph terminal over the line. You might have to hit 4th up a big hill cruising but so what. I wish I had the longer ratio. I'm Not in a daily driver so just my opinion.

[admS15]

I'm in Trafalgar VIC. I don't know anyone with a Skyline. All the boys love diesels down these ways lol. Yes, if I could understand how this engine goes with stock gearing is that would be perfect. My background of car ownership is a long line of Holden and Ford V8's and XR6T's, so i'm used to 1800 to 2000 cruising revs. 
4.11's aye, back to the spreadsheet again. And your comment similar to "a jack of all and a master of none" is exactly what I am chasing here! Well said. 

If your ever around dandenong , I can happily take you for a spin. Yeah Xr6t's and v8's cruise nicely at those rpm but with an rb25 the closest you'll get to that will be a z33 6spd box, will need bulk $ to do that. Could always just put an ls1 with t56 in it, lol.

[hjtrbo]

Review time

4.62____ track day

4.3_____border line on to short for cruising at 115km/h

4.11____maybe

3.89____likely

3.7_____To tall for the track

3.55____Getting into the Falcon ratios

Staying with 3.89 so far. If I hate it at the track, I will have no choice but to buy a 2nd centre which would likely be a 4.62 (266 km/h @ 7500rpm on 245/45/17's). Downside is spending Friday nights before Super Sprints draining diff oil and swapping out centres and then getting to reverse the process on Sunday night. 

Thanks for all your input guys. It has helped me narrow down the list.

[hjtrbo]

6 minutes ago, admS15 said:

If your ever around dandenong

Could always just put an ls1 with t56 in it, lol.

Thanks for the offer, i'll keep it in mind

It was tempting, but so common now. I think a lot of people will be surprised when they hear an HJ boosting past with that unique Skyline note. 

Edited June 14, 2017 by hjtrbo
spelling

[admS15]

Thanks for the offer, i'll keep it in mind
It was tempting, but so common now. I think alot of people will be suprised when the hear an HJ boosting past with that unique Skyline note. 

Will leave quite a few with a wtf expression on their face[emoji106]

[Bumblebee]

Ive got 4.11s in the 33.. makes 265rwkw and i feel something taller would be nice for highway.. having it sit around 2500 would be better than almost 3k..