[ChappiesZook]

Hey guys I’m sure that this has been covered a million times, but I would just like some help with my particular situation.
I have built a 1989 wide track zook. It’s running a stock G13B DOHC out of a GTI, paired with a Microtech ecu (currently untuned) and is sitting on a set of 30” tires. I use it as a daily run around car, but go 4wd every weekend. I often go on the highway and struggle to sit on 100kmh. I would love to be able to easily sit on 100kmh, even when going steep uphill. The engine is in good condition, and has clean plugs etc. what should I do? I really want it to be able to go on the highway and even be able to speed up and overtake (150kmh would be nice) whilst being able to 4wd every weekend. I am in desperate need of advice and I appreciate any help.



Many thanks,
Chappy

[Eddy]

Start by pumping up your tyres.

[Gwagensteve]

You have an untuned ECU, an engine that needs to rev to make torque and you’re running bigger tyres which move cruise rpm down.

Get your ecu tuned, then see if you need to correct the gearing.

A 1.0litre Sierra can hold 100km/h on the highway with 36kw. You have 75kw, theoretically.

[ChappiesZook]

Thanks Steve, I knew that I needed to do something just not sure exactly what. I’ll put it into the shop and get it tuned ASAP. I’m assuming by “correct the gearing” you are talking about something like a transfer reduction or something? If you were to run said reduction gears what ratio would you run? Cos from what I’ve seen zooks run 27-28” stock, meaning that it was a pretty big increase from stock. No wonder it’s struggling on stock gearing XD

[Brenno]

4.9s would be perfect

[Gwagensteve]

A Sierra is 26" stock.

Even though you need gearing correction, a GTi should be able to pull the car along at 100km/h - a stock carby G13 can manage that, even with 30" tyres. (just) in 5th gear, and easily in 4th.

I suspect your ECU is running with heavily retarded timing and super rich will be enough to drive the car to the dyno without hurting the engine, but no more.

The tallest transfer gearset (4.1) will correct your gearing to pretty close to standard on the highway, but being as you have a GTi, it would really be happier spinning a bit harder than standard (which is ~3550rpm @ 100km/h)

How deep you want to in the transfer will depend on what your driving. If you drive mud and aren't into super technical driving, 4.1 will be fine. If you drive very steep/technical terrain, 6.4-6.5 will feel like you have loads on control with 30" tyres.

[ChappiesZook]

Sorry it took so long to respond, been out of signal for work. Thanks again everyone for the advice! If I was to go 6.4:1 (I plan on doing mud and sand as that’s my thing, but eventually some more technical stuff) would it have any negative effects towards its drivability around town and on the highway? As I said I’m hella new to this technical stuff and have very little understanding of how it works

[Gwagensteve]

You're not alone in having trouble understanding this. It comes up frequently - the most common being "what will my highway top speed be with reduction gears" or " I don't want 6.4's because I don't want to to be revving too high on the highway" - Both of which are myths.

First question, what size tyre are you running/planning to run with the 6.4's?

6.4's lower high range by 16%. That means if you put 16% bigger tyres on, the car will drive like it has standard tyres on it.

26" + 16% = 30.16"

So, with 30-31" tyres, your car will feel like it has standard tyres on it on road. That means it will accelerate about the same, you'll be in the same revs at the same road speed as with standard tyres, and 5th gear will be usable on the highway.

Two things to consider - a 30-31" tyre is considerably heavier, has higher rolling resistance, and is less aerodynamic than the standard tyres, so you will use a little more fuel and the car will feel a little bit down on power, but it's not a huge issue. It'll be pretty good considering how much bigger than standard the tyres are.

The other thing is the speedo won't read properly. the speedo is calibrated for axle ratio and tyre size. The speedo pickup is AFTER the transfer case, so even though the revs are correct, the speedo will be wrong. (reading ~84km/h at 100kmh)

I will add this though - 6.4 is very low with a 30-31" tyre, especially in mud and sand, and importantly, it results in a huge gap between high and low. In a normal car, low 4th gear is somewhere between high 1st and high second. With 6.4:1 gears, low 4th is still lower than high first. That can be an issue if the car can't quite hold high range on slippery mud or sand - drop into low range and you have no speed at all. (like 30km/h maximum)

6.4's are for steep terrain and very technical driving.

[tanshi]

slight correction, 6.4s are 17% in high, 4.9s are 16% in high.

4.9s will be a much better compromise for on road gearing vs offroad gearing for mud and sand.

[vet 180]

I think 4.9's would be more suitable for you being mud and sand is what you drive mostly. If that is strickly what you drive with no intention of ever driving technical and low range isnt your biggest issue only high range then vitara 5.12 diff gears could work for you. However if you want to have a crack at more technical driving 4.12 -4.90 transfer gears might be more suitable.

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