Jump to content
SAU Community

Recommended Posts

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?

  • Like 1

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.

@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.  

 

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.

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.

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?

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 by hjtrbo
update

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).

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?

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. 

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.
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.
  • Like 1

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.

  • Like 1
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 by hjtrbo
spelling
  • Like 1
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]

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • @Kapr Haha yeah thats the one. I missed that you had a built up engine, I wouldn't want to run it on there either then. It was good in my situation just to replace the original turbo on a stock engine. @MBS206Yep definitely not a replacement for anything name brand
    • You are selling this? I have never bought something from marketplace...i dont know if i trust that enough. And the price is little bit "too" good...
    • https://www.facebook.com/share/19kSVAc4tc/?mibextid=wwXIfr
    • It would be well worth deciding where you want to go and what you care about. Reliability of everything in a 34 drops MASSIVELY above the 300kw mark. Keeping everything going great at beyond that value will cost ten times the $. Clutches become shit, gearboxes (and engines/bottom ends) become consumable, traction becomes crap. The good news is looking legalish/actually being legal is slighly under the 300kw mark. I would make the assumption you want to ditch the stock plenum too and want to go a front facing unit of some description due to the cross flow. Do the bends on a return flow hurt? Not really. A couple of bends do make a difference but not nearly as much in a forced induction situation. Add 1psi of boost to overcome it. Nobody has ever gone and done a track session monitoring IAT then done a different session on a different intercooler and monitored IAT to see the difference here. All of the benefits here are likely in the "My engine is a forged consumable that I drive once a year because it needs a rebuild every year which takes 9 months of the year to complete" territory. It would be well worth deciding where you want to go and what you care about with this car.
    • By "reverse flow", do you mean "return flow"? Being the IC having a return pipe back behind the bumper reo, or similar? If so... I am currently making ~250 rwkW on a Neo at ~17-18 psi. With a return flow. There's nothing to indicate that it is costing me a lot of power at this level, and I would be surprised if I could not push it harder. True, I have not measured pressure drop across it or IAT changes, but the car does not seem upset about it in any way. I won't be bothering to look into it unless it starts giving trouble or doesn't respond to boost increases when I next put it on the dyno. FWIW, it was tuned with the boost controller off, so achieving ~15-16 psi on the wastegate spring alone, and it is noticeably quicker with the boost controller on and yielding a couple of extra pounds. Hence why I think it is doing OK. So, no, I would not arbitrarily say that return flows are restrictive. Yes, they are certainly restrictive if you're aiming for higher power levels. But I also think that the happy place for a street car is <300 rwkW anyway, so I'm not going to be aiming for power levels that would require me to change the inlet pipework. My car looks very stock, even though everything is different. The turbo and inlet pipes all look stock and run in the stock locations, The airbox looks stock (apart from the inlet being opened up). The turbo looks stock, because it's in the stock location, is the stock housings and can't really be seen anyway. It makes enough power to be good to drive, but won't raise eyebrows if I ever f**k up enough for the cops to lift the bonnet.
×
×
  • Create New...