Tyre size vs Torque
My 240GD has a final drive ratio 5.33 with the 617 300D engine with 4 speed gearbox.
Currently I have Bridgestone Duelers A/T 235/85/R16's.
I noticed that my wagen struggles up long hills and tend to get very warm. Off-road I also tend to have to use high revs to have enough power.
It was suggested to me to go to a smaller tyre size. According to mbnz.org you can loose up to 17.5% power per 1" you go bigger from factory standard size.
The question is what size will be the best and what will my torque increase be if any?
Any other cons for smaller tyres? (groud clearance, fuel consumption, top speed)
Thanks
Johann
Johann,
I would not shorten the gearing of your truck.... bear in mind that standard 300 GDs came with 4,8 diffs and on top of this many with the 5-speed gearbox with overdrive in 5th.... Bottom line is that the long gearing provided by your current tyres is only partially offsetting the short gearing provided by your gearbox/diffs combination.
To better picture the situation, my older 300 GD had the 4.8 diffs and 5-speed gearbox and it was fitted with 31x10.5R15 tyres (787 mm diameter). Yours has the 5.3 diffs, 4-speed gearbox (with 1:1 fourth) and 235/85R16 tyres (806 mm diameter). My older G was obviously longer geared then yours, which helped to keep the revs down on the motorway and still had sufficient punch off-road. I can't remember using 1st low when driving uphill (only used downhill).
Yes, you have to sometimes rev the engine because the G is a very heavy truck with a comparably small engine and you sometimes need to build up momemtum. But this becomes part of your off-road driving technique.
All 300 GDs tend to become hot when driving up long hills on the motorway, specially flat-out, but if it doesn't reach the red zone it is not a problem. And if it does very often you might have cooling problems, not related to your overall gearing. Recore your radiator if it's very old, fill the cooling system with a Mercedes-approved cooling agent and replace the thermostat to one recommended for the climate you live in.
Botom line is that if you shorten the gearing you'll probably going to put even more stress in your truck's driveline in all but 10% of the truck's usage.
Regards,
Johann,
Take this from an (ex) fellow south african - the easiest way to increase off-road prowess is to go bigger tyres, so stick with what you've got. By going smaller you'll reduce the contact patch of your rubber. You'll gain power/torque but will only result in more wheelspin. Unimogs have 20in wheels and the first ones had only about 45bhp. Same reasoning for big tyres on the back of tractors and dragsters...
The cooling problem is quite likely sediment in the radiator or a viscous coupling on the way out. Fair enough and good on you if you want to keep everything Merc, but if your budget doesn't stretch, here are some possibilities:
Take out the complete viscous fan but put the mounting bolts back onto the pulley. Then drill two 6mm holes directly opposite each other directly through the bodyof the viscous unit. Let the oil drain out and then lock it all together by fitting two 6mm bolts with nyloc nuts through these holes. Put this fan under the seat for an emergency backup. Now go to a scrap-yard and ask for a new-era skyline 6cyl(pref. 3.0SGLi) electric radiator fan. Should be about R50 and should increase the airflow by about 50%. It is a nice flat-bodied one that will fit in nicely behind the grille. Remeber it is two-speed so get the correct sender unit from someone like midas. The rest should be self-explanatory, but ask if you get stuck.
For a cheap new radiator, get in touch with denver tractor parts. From memory i think a 165 massey or similar one should do the job, but do check first - they've got loads of different ones and cost peanuts compared to anyone else's. And they're brand new intended for harsh environments. I've had good results with them - quite strong and durable and much higher capacity than what you have at the moment.
You'll notice with this set-up that there is noticeable more power from the engine on the open road and during heavy off-road work in 48 deg Celsius, you'll have a nice constant teperature. If the electric fan fails in the middle of nowhere(never happened to me), you can still get out the old one from underneath the seat to get you home...
Let us know how you get on...
I think you're going in the right direction - that fixed fanblade is a load of rubbish on the open road - estimate to lose about 3-8bhp at high-ish revs.
I had my Injector pump calibrated last week and good heavens what a differance it made! My G is pulling like a train. Much more power.
Just broke my second crankshaft pulley bolt this weekend thus the engine etc is in for balancing and to find the root cause.
Johann
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Thanks for all the advice. So far most people said I should stay with the bigger tyres.
Gear ratio's too short, rpms too high, far lower top speed, traction lost, groud clearance etc.
Just funny that mercedes recommends far smaller tyres?
We have our 25th G celebration next week and I'll try some of the other G's smaller tyres to get the seat of my pants experience.
I think some of my overheating problems is not that the cooling system is not working it is just not enough for the 300D. As my G is originaly a 240D it doesn't have an oil cooler and only the single 5 blade metal fan. There is a plastic fan from Merc without the viscous with 9 blades which should be more affective but at a cost of R650.
I think the skyline and massey route is not a bad route to take especially for the pocket.
Always nice to hear from an expat :D
I'll give some feedback in 2 weeks time.
Johann