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What Final Drive?

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#1 critchie2006

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Posted 19 March 2013 - 06:04 PM

I'm building a 1380 engine, lightened, balanced and cross drilled crank. Lightened flywheel. Centre main strap. Kent 286 cam, centre oil pick up pipe, turbo high capacity oil pump, duplex timing chain and vernier, 1.5 roller tip rockers, Swiftune adjustable distributor, webcon 7 port head and a set of twin 40 weber. I've also bought a 4 pin diff, I like quick acceleration but don't want the car screaming at 70mph so what final drive am I best going for? I have the engine out doing the work on it but can you change the diff and final drive gears easy enough with the engine in place? For future reference if I'm not happy with the final drive I choose. Please as much advice as possible. Thanks

#2 mini*mad

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Posted 19 March 2013 - 06:31 PM

higher the number ie. 3.44 the better for acceleration
lower the number ie. 2.7 better for top speed

as itll be torque'y i'd for for something in the middle like 3.1
and itll be VERY tricky to change it in the car so would make the decision now based on what you want/use it for

http://www.guess-wor.../Tech/ratio.htm

very useful calculator for working out RPM and speed with your wheel and tyre choice
then flick through the diffs and see what looks good!

#3 blacktulip

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Posted 19 March 2013 - 06:36 PM

i would say 3.1 too.

#4 critchie2006

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Posted 19 March 2013 - 06:46 PM

I'll be running on 10" wheels too :)

Edited by critchie2006, 19 March 2013 - 06:47 PM.


#5 Orange-Phantom

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Posted 19 March 2013 - 06:59 PM


You've got a very similar spec to what I've got.

Definitely use the Guessworks guide to give you a good idea as you haven't specified what gearbox/drop gears and size wheels you are using?

I'm running a 3.1 with 10 inch wheels and 1:1 drop gears. In 4th it would be fine on a motorway (about 4000 rpm at 70). I would prefer a 3.4 in it being honest as 1st gear is too long for my liking and with the cam (I have a SW-10) you don't have an awful amount of useable power below 3000. Which means you have to slip the clutch and rev it to get a good getaway on any decent incline. Flat roads are fine. I suppose it depends where you are going to be driving it as well.

On an aside have you bought all the bits for that engine yet?

#6 Cooperman

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Posted 19 March 2013 - 07:00 PM

First of all the wheel diameter does not have much effect as the rolling radius of the 145/80 x 10, 167/70 x 10, 145/70 x 12, 165/60 x 12 and 175/50 x 13 is very similar. Now, you have chosen a 286 cam, which gives its peak power at 6400 rpm, maybe a hundred revs lower in a 1380, but in any event it means you will need to really rev it to make it go well. The 1.5:1 rockers will also move the bottom end of the rev range up a bit. Personally for a road 1380 I would have used a 266 cam to maximise torque in the mid-range. You have not mentioned what gearbox you are using. With the 286 cam you do need the gears to be fairly close together to get the best from the engine. What you don't want is to change up at, say, 6500 rpm only to find that in the next gear you are out of the best power range because the revs are too low. If you are using a close-ratio straight cut box, then a 3.76:1 FDR might be best, although that will only give just over 15 mph/1000 rpm in top. With a standard box a 3.44:1 might be best, although the gear ratios will still be a bit wide for that cam. It is necessary to take the engine out to change the final drive ratio.

#7 mike.

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Posted 19 March 2013 - 07:03 PM

Good points above - You should match the final drive to your cam and style of driving.

I've got a 3.44 in mine which is good for acceleration and revs to around 4000 at 70.

#8 KernowCooper

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Posted 19 March 2013 - 07:09 PM

Whats the cars intended use ? and with a 286 cam and 1.5 roller rockers you dont want to relaxed a gearing otherwise you'll find your self outside the cams power band and changing gears all the time, and kind of try and turn the 286 into a cruiser which won't work to well. I think a 3.44 diff would be better and at 70mph in 4th would be 4250rpm

You could go lower but at the expense of rpm/1000.

Road Speeds per 1000 rpm On 10" dia wheels

differential ratio MPH per 1000 rpm in 4th gear
3.44:1 16.47
3.647 15.0
3.765 14.7

#9 Cooperman

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Posted 19 March 2013 - 08:50 PM

In my 1310 cc 'S' with a 286 I run a SC CR gearbox and a 3.9:1 FDR. That is about 14 mph/1000 rpm in top, but 0 - 60 is 7.4 seconds using just under 7000 rpm in 2nd. The power does not come in until about 3200 rpm when its hot.

#10 Orange-Phantom

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Posted 19 March 2013 - 08:55 PM

The only reason why I'm asking if you have all the bits for the Engine is if you haven't purchased your cam yet and are set on a 286, have a look at Swiftune's SW-10. The SW10, is basically a real hot road cam like a 286 but with better emissions and suited to road use where wear characteristics are more important than outright performance.

If your wanting a more sedate drive take Cooperman's advice or have a look at the Swiftune SW-05 or even ACDodd's own cams http://www.mlmotorsp...id=86&Itemid=30

Also from the very knowledgeable ACDodd..
http://www.mlmotorsp....php?topic=11.0

#11 KernowCooper

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Posted 19 March 2013 - 09:15 PM

Or even Kents 276 on your 1,5 rockers

#12 Orange-Phantom

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Posted 19 March 2013 - 09:18 PM

Very true!

Or even Kents 276 on your 1,5 rockers



#13 KernowCooper

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Posted 19 March 2013 - 09:22 PM

Have one in my 1330 and its got good road manners without having to rev it to pull away in normal driving and does town driving acceptible, comes on strong at 2500rpm to 6000rpm, and you wont have to pull the engine down for rebuilds no where near as often as the 286

Edited by KernowCooper, 19 March 2013 - 09:23 PM.


#14 critchie2006

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Posted 19 March 2013 - 10:02 PM

Hi, thanks for all the input. I have purchased all parts I have listed for the engine, the gearbox was the only part I've not purchased anything for. Just wasn't sure what to go for, there is so much to choose from. The car will be my daily drive when finished with maybe a track day every now and then. My first plan was to just fit the 4 pin diff and change the final drive but by the sound of things I should really go for straight cut gears. Are the straight cut gears easy enough to fit? My dad who is a mechanic will be helping me but I need as much info as I can get before I give him the good news. Thanks again

#15 welshdan

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Posted 19 March 2013 - 11:30 PM

I really like close ratio gears. Use tran x clubman set in my turbo, use mk3 s helical close ratio gears in my mpi. It really flies and makes a difference, if i was to fit a high lift cam as you propose i would not do so without close ratio gears. As said they will help to keep the engine 'on cam' at gearchanges instead of the power band 'dropping off' if compared to wider ratio gears





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