26 replies to this topic

[Oldskoolbaby]

Im just about to drop a 140bhp, 1400cc, a seires and sccr box in to my thirty but dont no what diff to put in it. I want blistering acceleration more than any thing but with a top speed of around 105-110 mph tops. Its on ten inchers with yoko 032rs so grips not a problem;) I was thinkin of a 3.7 to 1

[Madmax]

hmm, the fav is of course the 3.44:1, which maxes out in most minis at around the speed your looking at

what does the engine rev to? im assuming quite a bit considering its putting out 140bhp? is that at the Fly or wheels?

[Bluemini]

Good acceleration would come from a 3.9:1 or higher, but good top end speed would come from something like a 2.9:1. Altho you have a big engine so all that will kinda go out the window to a point! Personaly I would go with a fairly standard 3.4:1, with those sort of power figures you'll have no problem accelerating hard, but still have a fairly good top end. Another thing to considder too is, if you go for a ratio that is too high, your going to have grip issues when you put your foot down.

[Oldskoolbaby]

Ive built the engine to take about 7500-8000rpm safely. Ive built the head specifically for nitrous with a low compresion and HUGE valves(37/31 11stud Cooper S) Ive also put a launch control system on it (deemon tweeks). Its got next to no weight on the back end with the battery up front with me and its fairly stripped out. Obviosly given it hasnt been on the road yet the power is all quoted but ive seen many race angines with similar specs to mine. It has a nos kit on it as well so if it dont make 140 bhp ill bloody make it;)

[Jammy]

With it being bored out so much, are the cylinder walls going to be able to take NO?!

[dklawson]

I don't believe you are going to get both blistering acceleration and a high top end with a close ratio gearbox and a tall final drive. The combination is all but mutually exclusive. You said this is a car on 10" wheels. If you choose 3.44 for the FD you'll be running 4k RPM at 65 MPH. Even on a well balanced engine that will get old very fast. A taller FD will be even worse. In theory you could double the 4000 RPM at 65 MPH (on 10" with a 3.44) and predict a top speed of 130 MPH at 8k RPM. However, with the aerodynamics of a toaster, you'll need a lot of that 140 hp to get even close.

[Madmax]

lol dynamics of a toaster

minis are worse than that, at least a microwave!

entirely correct you are however

[Jammy]

A microwave?! Have you not seen the new slimline microwaves they are producing nowadays?! I reckon the drag coefficent of a double kithcen sink with the little bit in the middle for rinsing stuff is more like it!!

With my 1425 I think I would be lucky to get 130 mph, and certainly wouldn't want to be that high in the rev range for any amount of time, that last 10-20mph just wouldn't be worth a engine rebuild.

[Madmax]

actually, i think a whole kitchen would be more aerodynamic...

anyway, i think the general consensus (now theres a big word) is that you should go with the 3.44 diff

[Oldskoolbaby]

The first gear in the box i built up is relativly long with the next 3 gears being very close. Its basicly a street racer so any one who knows Northampton knows in ent going to do much over 30 on a quiet day with the old speed cameras. But when the cats away the mice will play. Itll ocasionaly travel to MiltonKeynes for the sunday cruise. Ive got a tool chest in the garage that will level peg the mini in aerodnamics

[Guess-Works.com]

Oooh... interested how you built the gearbox, as it must be a bespoke build... and would like to add it to my FD calculator...

Gearbox ratios

This also may be useful for you to help define you Final drive requirements.

[Oldskoolbaby]

The gear set i assembled was from mini sport. Im only an apprentice plumber so a Jack Knite box will have to wait untill the next rebuild. The gear ratios are 1st 2.25(29 teeth) 2nd 1.73(25 teeth) 3rd 1.25(22 teth) and 4th 1:1(19 teth). Its the fist time ive ever attempted a gear box but im sure the last gear is the same ratio as the helical cut type. They should be just closer together. Shouldnt they?

[Guess-Works.com]

They sound like Abingdon ratios... and yes your final ratio in any 4spd box is 1:1 (aka direct drive) which is why it's important to get the right drop gears and Diff ratio to get your desired output.

[Oldskoolbaby]

Im gunna run standard 1300 A+ drop gears for now. The straight cut drops are just a tadge too loud for even me. I love the A series growl more than enything so ill stick with standard ones for now. I do really want extreme acceleration. Ive heard that miglias run extremly high final drives and they do 60 in under 4 secs on slicks. And they do about 110 tops depending on the cuircuit. Bill Sollis' knocks out about 140bhp, about what i want with nos.

[Jammy]

I'm not sure, but will standard ones be able to cope with that amount of power?!