Rear dif?? can this change top speed?

[ims101]

Hi
Please could some wise mionr master help?
Does changing the rear dif alter speed/aceleration? is it worth changing and what is the best ratio?
I have a 1959 convertible with the original 948cc block it has a 1098 head a biger carb (H4 I think?) and a KN air filter it has always ran well with a easy 80mph top speed but is quite slow getting there (especialy with modern traffic). Or is there a better way to inprove your 0-60 + may be top speed??

thanks for reading

Ian Smith

[bmcecosse]

Unless it's a 4 wheel drive conversion - it will only have a rear diff ! Yours will be the original lower ratio (4.55) (unless someone has changed it) and so this will give you good acceleration - but the engine will be revving quite hard at high speed. The later 1098 cars had 4.22 diff - this almost completely negated the advantage of the more powerful 1098 engine as far as acceleration was concened but does give slightly more restful cruising - although by modern standards it's by no means high geared ! If your head is froma 1098 - has it been skimmed to raise the compression ratio ? If not - you will be on a hiding to nothing - there will be no extra power over the standard head. First move really is to check the general engine condition - get a compression check done to see if everything is in good order. If this shows less than 160 psi on the bores then there's the problem. Really good engine should be 180 psi. Bigger carb is all very well - but was the inlet manifold modified to take the bigger carb ? To get the best from a 948 engine you really need to change the camshft - either the one from the 1098 engine (AEA 630) or perhaps one of the aftermarket cams - SW5 from Swiftune seems to be popular - and the MG Metro cam is excellent too. This with a good head - either the 1098 head (12G202) or better still the MG 1100 head (12G295) will boost the power - both need skimming before use on a standard 948 (or 1098) engine. If the bigger 1.5" carb is also fitted on a well modified inlet manifold then ~ 45/50 bhp can then be expected (wilder mods can squeeze 80/90 bhp from the 948 engine in racing tune!!) - but this (45/50 bhp) is pretty much what the standard 1098 gives anyway. So - realistically - the best upgrade is the go for a standard 1098 engine - or better still - the 1275 from the Spridget or Marina/Ital. And keep your lower diff for good acceleration - but you will soon get tired of the high revs in the cruise! Some get round this by fitting a Sierra 5 speed gearbox - but now it's starting to get expensive.

[ims101]

When I fitted the carb I also changed the inlet manifold the 1098 head was skimmed befor fitting but I'm not sure how much was removed?? What is the recomened amount to be skimmed and why? Do you think I might need to have it reskimmed or should I go for a new head that has been adapted for lead free petrol? if I do that should I go for a 948 and change the cam or a 1098 one? I dont really want to change the whole block as I would like to keep her fairly original but still want to be able to drive happly in moderen traffic without causing a ten mile que behind me javascript:emoticon(':lol:'). The five speed gear box sounds a nice upgrade especialy when the one on her now gives up.

[bmcecosse]

Really - the 1098 engine complete is a lot better (and you could 'cheat' by transferring the old engine number to the new engine) - and with some modifications can easily give 60/70 bhp without being difficult. The 1098 head has 26.4cc chambers - the original 948 head had 24.6 - and a lowish CR anyway. So normally one would skim 60 thou (1.5mm) off the face to hopfully get somewhere between 9 and 9.5 CR. The 630 cam is a lot better - and the same timing and lift was used by Rover to the end, however they realised that if it was timed 4 degrees advanced it gave better 'torque' and economy. So if fitting that cam it's worth getting an offset key (Minisport etc) and using that to get the bet from the engine. I still say a complete 1098 engine is the way to go. My 1098 traveller certainly has no trail of traffic following - quite the reverse - I'm usually on the wrong side of the road squeezing my way past them ! It's a 1098 fitted with the later 12G940 head from a 1275 engine - with a bigger carb on the MG Metro alloy inlet manifold. I reckon it's putting out about 65 bhp - and loads of torque low down.