Re:Re:Lacking power

wrote:

Why will lowering the gearing make it a monster on the motorway, that's not how lowering the gearing works.

Lowering the gearing brings the engine and gearbox revs back to where they were before going for larger tyres.

I know because I have done this on both my Jimny and SJ. You lower the gearing appropriate to the increase in tyre size.

Yes of course, you are correct, l never thought about the fact that it would simply compensate for the larger wheels!
 

Lambert wrote: A little while ago I looked into doing what the Australian contingent know as a GVM upgrade, essentially to increase carrying and towing capacity on my gen3. I stopped almost as soon as I started. Disregarding the physical modifications needed forum a minute as they are simple, the legal requirements are an absolute mine field, I'd sooner cut my leg off with a rusty spoon. Then there's also the costs of getting the appropriate approval. And then you would have it to insure. The problem is though that if you don't do it properly then get found out the whole world will become full of pain and stress. What I'm getting at is its not possible to just keep bolting more and more stuff to your vehicle, there are all sorts of consequences.

since Lambert mentioned about GVM upgrade i have seen an add for ironman GVM upgrade especially designed for gen4 jimny, they must be listening to you Lambert

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Re fuel economy and performance, bigger tyres will have put your speedo out. Your tyres are 7% bigger diameter, so your speedo and odometer are both reading 7% lower than standard. This means you will probably be driving 7% faster than you realise, and clocking up 7% fewer miles whilst you do it. These factors combined mean you'r calculated fuel economy will appear to be massively lower than it actually is.
Regarding performance, the cheapest option is always to use all the performance you already have. Small petrol engines make their power at high rpms. Changing gears close to the redline will massively improve the acceleration. Inevitably this will use more fuel- accelerating faster always does, but the cost of fuel will be nothing compared to the cost of fitting a turbo, let alone the economy hit that will cause.

wrote:

Hi everyone,

I recently bought a MK4 Jimny that had been heavily customised by Buzz Special Vehicles. It's had front and rear recovery points mounted, underside protection plates as well as an ARB recovery bumper and winch. Unfortunately I'm finding the car to lack any sort of pulling power especially in 4th & 5th gear (I assume due to the addition of heavy extras). At 60MPH I also only average around 19MPG which is less than ideal. 

The car has had so many extras that I have no issue paying any amount of money in order to try and increase the pulling power and MPG. What would be your recommendations? I've been informed that a turbo with boost control or a supercharger would be the best bet. Also, with all the additional weight; bigger alloys and tyres, OME lift, ARB diff locks, recovery mounts, recovery bumper, protective plates and sills. Would you say there's extensive strain on the factory transmission and clutch?

We want to add a roof rack tent and various camping equipment to the rear so I imagine it's only going to get worse. 

Any help would be appreciated guys! 


 

Hi, interesting thread. Quite a lot of replies already.


Not sure I can quite fathom the £20k's worth of mods. Lots of them aren't needed and I'm sure wouldn't really cost that much. Not knocking, your choice obviously. But ultimately you have a mild lift, off road tyres, bumper and some trim bits. Which I suspect is pretty standard on many Jimnys.

In terms of performance and mpg. Have you driven a standard Jimny as a point of reference? How does yours compare? I ask this, as the Jimny is not a speedy vehicle, although they should feel peppy. But being a small displacement naturally aspirated petrol engine, they do their best work at higher rpm. At low rpm the engine should be smooth and willing, but will not be shoving you in the back. One the level mine will pull 30mph in 5th no problem. But on hills you will need to work the gears and likely keep it over 3000/3500rpm to allow for meaningful acceleration.

19mpg however sounds horrendous or a miscalculation. How are you working this out? And is it over an entire tank or just a short city/town trip? What sort of terrain/roads and what is your driving style?

For reference, I'm also running some mods and currently get 31-34mpg. I think claims of well over 40mpg are unrealistic. Even when 100% standard the best I got over a tank was 39.8mpg. You'd have to drive very very gentle to see consistent 40mpg+ IMO. But 19mpg is not right at all.


Weight shouldn't really be an issue here. The Jimny has 4 seats and can carry 4 adults. Doing so should not half your mpg. Off road tyres and to some extent a lift will have a bigger effect. But if I'm reading this right, you are only running a 235/70R16, which is about 1.5" bigger than stock. Big, but not massive. Have you compared your speedo reading to GPS to see what difference it has made?

Re-gearing would be an option, but probably more for low 1st crawl speed. I'm not sure how impacted your cruising speeds are. Is yours an automatic or a manual?

Mine is a manual and pulls approx 20mph per 1000rpm in top. So 70mph would be circa 3500rpm in 5th gear.

As for adding more power. You can tune n/a engines, but as a rule it will be about moving the torque curve higher up the rev range, as this will give more power. But make for a more revvy experience. There are also limits to what a 1.5 n/a petrol will be able to produce.

Forced induction (turbo/supercharing) is the next option. Not overly difficult in a base sense. But unless someone has done the R&D and leg work, you will be at ground zero. In either setup you can get PEAK power gains, but for drive-ability it is the mid range torque increases that forced inductions offers, this will give the biggest improvement on the road.

The engine internals are the biggest question mark here. Can they handle boost and if so, how much? If they can't, then it is an engine rebuild job, which could double the cost of such a conversion. And any forced induction offering will be pricey to begin with.

Making more power/torque may also result in other drivetrain related issues too. Which is somewhat unknown at the moment. Best guesses would be to look at how the 3rd Gen copes.

The only other option would be to look at engine swaps. But again would need R&D at this stage and have several potential pitfalls. Likely to be very costly, but if you have the money could be an option still.



Probably less of a popular opinion on here. But have you considered a Jeep Wrangler? The 3 door is still pretty compact, but can be had with a lot more power and just as capable or more so than a Jimny off road. The Rubicon version even comes with off road tyres and lockers as standard.

300 bhpton.

I agree with your comments except fuel consumption, I regularly get 44mpg and engine swaps. Yes it's possible to shoehorn a modern engine, petrol or diesel, under that bonnet but 200bhp ( for sake of argument ) is likely to play havoc with the drive train and the road holding will become a bit awry which begs the question, why am I doing this?
Which puts credence into your last comment 

Roger

wrote:

I think my point has been lost in the side tracking. My issue is overtaking at speeds over 55mph. Now in most cases this wasn't an issue but yesterday for example on the M6 there was a quick lane closure with no warning which meant I had to merge into my right lane. I indicated and moved but the car in the lane I was moving into was forced to break and I was unable to accelerate to keep the flow of traffic. It's only in circumstances like that & inclines at speed which cause an issue. The gears you'd use for off roading are absolutely fine and it is great but 4th and 5th feel underpowered.

Thanks for your replies

Based on your experience, your problem would appear to be the gearing, which as has been discussed ad nauseum is affected by the increase in tire size.  It's not that the power isn't there, but, the "multiplication factor" provided by the transmission has been altered.

The simplest way to deal with this is to switch to a smaller tire, another option is to "re-gear" - it might be possible, if you have a manual transmission vehicle to swap in the transfer case from an automatic as that has a 1.32:1 reduction in high range, whilst the manual has none.  Similarly the axles from the automatic can be used as that has lower gearing (4.3:1) than the manual (4.09:1).

I believe another post in the discussion  mentions that the increase in tire size is around a 7% change, based on my experience with larger tires, anything over a 5% change will be noticeable - for what it's worth, this experience is on a 2.0 liter Grand Vitara, with no added weight, just swapping the tires for larger ones and repeating the process.