Mercedes Hymer 660 Automatic

big fello

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Hi All,

I have a 1991 mercedes hymer 660 4 speed automatic 2.9 5 cyclinder. I am wondering is there anybody out there that have changed the diff ratio to increase the speed on the open road without over reving.

I would be grateful for any help.

Thanks

Big fello
 
I have an 87 660 with the same problem....sounds like it is going to explode at 55 !, i have read around on the web and am looking for a set of 16" wheels as this seems the simplest way of addressing the problem,, i just hope they fit under the arches!
 
have you had any luck

hi, i am also interested in changing the ratio of a four speed auto to enable it to cruise at 70. have you had any luck?
 
The 410D came in 3500kg and 4600kg GVW both on twinwheel axles. The large Hymers as far as I'm aware were always based on the 4600kg chassis. It might be that the 3500kg used a higher axle ratio (perhaps just a change of the crownwheel) but retaining the same axle, if so changing to this crownwheel may be a possibility. It was certainly possible to do so on some of the Type 2 vans (709D 711D etc)
Upping the wheel size 1" will only decrease the engine revs by about 4% for a given speed and could be an expensive option eg wheels, tyres and recalibration of speedo.
 
We're not talking of a Warp Factor 3 increase in speed, just a means of decreasing the engine revolutions for any given speed specifically in top gear. You would need to increase the power to noticeably increase the top speed, unless of course the 410d in standard form is already "overpowered".... I think not!
 
I was browsing through the MB parts catalogue the other day and the T1 rear axle ratios seem to fairly constant across all the models with some weird ratio of 9:44/3 (whatever that means?? looks more like a Bible reference!!).

However some early G wagon rear axles also fit the T1s which may provide some higher ratios - but only the single rear wheel models - 200 and 300 series.

As for putting 16" wheels on, it wont make that much difference unless you can get 100% aspect ration tyres to fit them - the standard tyres being 205R14 - my 309D has a tyre placard that states 235R14 can also be fitted but I get "never even seen that size" when asking. You can fiddle with tyre and wheel sizes and see what it will do to your top speed at Tyre sizes

Just have to bear in mind with the T1s is that they were designed primarily for the commercial vehicle market (not campervans) where gradeability in lower gears when fully loaded was more important, and high top speeds were not a priority for a vehicle that was designed for urban distribution work. I think the ambulance versions were fitted with petrol engines to give them better acceleration.

The other way to get a bit more top end would be to fit a 5 speed manual gearbox, 70mph is possible but sounds like the vehicle is coming apart - but I think the gearbox linkages would be a nightmare to try and retro fit on a Hymer.

You could also do some research on whether or not the gearbox from the 300 series station wagons would give better results - the 300D are capable of 90mph but whether they'd be capable of starting a 4 tonne vehicle is another matter.
 
Putting 16" rims on will give more than 1% difference.

I did this years ago when I got a set of 13" rims and fitted them to an Escort van (12").

If you measure the circumference of a tyre on a 15" rim and then measure the same on a 16" tyre, you can calculate the % difference between the two.

If you are looking to change the diff ratio, it has to either be a complete diff or to change the crownwheel and pinion. Stripping and assembling a diff is a specialised business as you need to clock it in various places for backlash etc. Even then, you need to find out if the half shafts in the axle will be compatible with the new diff as they locate in the bevel gears in the diff.

Many years ago, I was a Quality Control Inspector for Eaton, Yale and Towne who made axles, diffs and special purpose drives for vehicles. Like other things, it looks a simple bit of kit but is deceiving. :)
 
You don't necessarily have to change the pinion... just the crownwheel. The crownwheel teeth remain the same size but differ in number to give the required circumference, ie ratio.
Every inch in wheel diameter will give you 3.142" extra circumference (pi). The % differs for every diameter but c. 4% for a standard van wheel.
 
Byronic said:
You don't necessarily have to change the pinion... just the crownwheel. The crownwheel teeth remain the same size but differ in number to give the required circumference, ie ratio.
Every inch in wheel diameter will give you 3.142" extra circumference (pi). The % differs for every diameter but c. 4% for a standard van wheel.
Click to expand...

Sorry Byronic but you have got that wrong. Crownwheels and Pinions always come as a set. If you change the number of teeth on a crownwheel, it will not mesh with the pinion as the tooth pitch will be different.

Also, these are Hypoid gears and the tooth angle of the pinion will not match the angle of the crownwheel.

If it were possible to do so, altering the pinion to one with a larger number of teeth would have more effect than altering the crownwheel. Sadly, neither is practicable.
 
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What you write makes perfect sense, and your being in the business gives you the advantage over me. A Mercedes crownwheel was listed on ebay, apparently it had been replaced with a higher ratio version, and I enquired if the pinion was for sale, I was informed that it hadn't been replaced, I then assumed that some setups didn't necessarily need to be replaced as a matched set, and that the crownwheel teeth just mated further along the pinion cone; wrong!
 
Hi Byronic,

That is why I am very reluctant to buy anything off Ebay. You have to take too much on trust with people who just sell the occasional item.

C/wheels and Pinions are paired up as they are 'lapped' to ensure a perfect contact pattern. If they were not, they would be noisy from new. Eventually they would bed together a bit but then the clearance would be too much and drastically shorten the life of the teeth as they are under load for most of the time.

We used to do MOD stuff including the parts used in Challenger tanks. A Tank turret turns by using 4 sets of C/wheel and Pinions but it is 2 C/wheels to each Pinion. They were machined to the highest spec then lapped in before heat treatment and then lapped again afterwards until perfect (however long it took). This was because the Turret had to turn silently. Moving many tons of steel silently is not easy.
 
Hola Maingate,
Just looking at my MB Manual (nothing better to do)and it states that the "crownwheel" and pinion i.d. numbers must match", and gives 2 differing diameters for the crownwheel but only by c.4%. presumably both able to fit the same spec. pinion.
There's even mention of Prussian Blue to mark the teeth contact areas. As you state the setting up of a diff. requires special skills/tools, eg the pinion fastener on mine requires a torque of 1050Nm (770 ft/lbs)!!
 
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16" Wheels

Hi,

I fitted a set of 16" Merc wheels to my S660 a couple of years ago. They give you much more stability and a better cruising speed. Roughly, you knock the speddo out by about 4-5 miles mph at about 60mph; ie, the speedo showing 60 but actually doing 64-65mph.

With the wheels you end up with about 12" more rubber on the road. You need to make sure the tyres are no more wider 205 or they will rub the back springs. If they do you can either scrape of the rust and grease where the springs rub or, failing that, get a set of merc washers and fit them to the drum before you fit the wheels.

The only difference that i have found is the vehicle takes a litter longer to get started when on steep slopes due to the lack of power and the bigger wheels taking longer to rotate. Other than that its all good with no problems.

I was insured with Comfort and now the Camping and Caravan Club and informed both about the wheel size. Neither changed the policy nor made any complaint. I think the van also looks better and wonder why 14" where fitted as standard.

I thought they made such a difference that when I found another set of 16" wheels at the scrappy I bought them as well and intend to fit them on the next Hymer I buy -- which will be in the next couple of years when my wife thinks out 1986 model needs a "modern feel" and we hopefully go full time -- although I will be sorry to see mine go as I have spent so much time messing about and fixing things on ours you end up loving them, albeit
skint!!!

Regards, Tgee
 
Just a quick advice about changing wheel sizes - if you increase a wheel diameter and can still get a tyre with the same aspect ratio (height of tyre to width of tyre as a percentage) you will get an increase in ground clearance and top speed AND a reduction in 1st gear gradeability. eg
Tyre size Tyre Ht Radius Diam Circum revs/mile % increase
205/100R14 205 mm 383 mm 766 mm 2405 mm 416
205/100R16 205 mm 408 mm 816 mm 2565 mm 390 6.6%

In the example of a 205/100R14 tyre if you fit tyres with an aspect ratio below 100% then you get

205/90R16 184 mm 388 mm 775 mm 2436 mm 411 1.3%
205/80R16 164 mm 367 mm 734 mm 2307 mm 433 -4.1%

And it gets worse as you decrease the aspect ratio into the common light truck/van sizes of 205/65 and 205/75. The tyre height is the major factor which now influences the overall diameter of the wheel/tyre combination. To improve handling lower aspect ratio tyres are now common on cars, trucks and vans (the tyres dont "roll" so much", with an increase in wheel diameter to maintain ground clearance.

Reading the tyre size info eg;
205 = tyre height in mm
next number /75 = the aspect ratio, except when the aspect ratio is 100% when it is ommitted.
next number R14 = tyre radius in inches
 
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