13 replies to this topic

[clubby crazy]

Hi,
been reading through a lot of topics but cant find anything on this so here it goes.

Im thinking of building a 1390 engine using a 1275 block bored by 60 thousands of an inch and increasing the stroke to 84.3 mm. my question is do you have to have a different length connecting rod for a longer stroke? it seems reading several topics and articles that there is a difference in length between the cooper s and standard 1275 conrod length wise also the stroke being slightly different? also due to the length of the stroke changing would the pistons need to be shorter so as not to go smashing into the head?

Please correct me if I am wrong on that but that is just from memory and the amount of material I am reading numbers are starting to fry my brain.

any help would be really appreciated.

[mini13]

normally the top of the piston is skimmed to allow for the extra stroke, there are also some short height pistons that will give room for the stroke.

the S rods are used as they have a smaller big end diameter allowing the non S crank to be offset ground to obtain the change in stroke.

the rods from the shorter stroke S motor are slightly longer than normal S and 1775 rods.

[Guess-Works.com]

1275, cooper S 1275 and 1071 rods are all the same length, only the 970 rods are different in length...

I struggling to see the benefits of using a stroked crank with an S rod and a plus 60 bore over a std crank using std rods and a 73.5mm bore... other than emptying your wallet...

Can you enlighten me as to what you expect to get from such an engine ?

Edited by Guess-Works.com, 08 January 2010 - 08:24 PM.

[bmcecosse]

Yes - does seem a strange and v expensive way to go!

[clubby crazy]

I do like to read into subject and probably over read some of them.

From various sources it seemed that you could get slightly more torque from a longer stroke although there really is only 2mm difference. I wasn't sure about boring out to 73.5 mm due to wall thickness of the cylinder bores, i just figured that going to 60 thou max would be best bet but only gives 1330 so having a longer stroke would give slightly more power.

Though it was just a thought, money isn't a problem luckily so i wasn't looking so much at prices just how certain things work with each other.

I was planning on using the car for leisure use as well as some track days since she isn't my main car and more of just a fun learning curve. So wasn't a hundred percent sure about going for a 1380 just through a 73.5 mm bore my thoughts where a longer stroke would allow me to rebore if i needed to.

[mini13]

dont worry about the bore size, there are quite a few 1380 turbo's that dont have bore issues...

but you do have a point about leaving room for reboring.

[clubby crazy]

Well i need to get the car ready for an engine first before i even look to building one. It could be fun to build one just to be different though I am still unsure about the gaining 10cc to loosing cylinder wall thickness just in case there was a problem.

But thanks for answering the original question about stroked cranks and using s con rods i might even be silly and take the bore to 73.5 with a stroked crank, doesn't that take it to 1430?

[Guess-Works.com]

Yes all the 'big' engines are 73.5 or even 74mm bores with stroked cranks, there have been known instances of 1500 - 1600 engines... however there is a common phrase that engine building is a compromise between three things, performance, reliability and cheap(cost)... you can't have all three.

The 73.5mm bore is a tried and tested solution, and proven to be reliable if built correctly, and should last as long as any other rebore/engine.

In terms of stroke vs bore, the mini engine is already considered a long stroke engine which provides good torque characteristics, compare a 1380 mini engine which has a 73.5mm bore and an 81.28mm stroke.... an equivalent more modern engine like the 1.4 K series has a 75mm bore and a 79mm stroke and the 1.4 Zetec is even shorter with a 76mm bore and 76.5mm stroke

[clubby crazy]

so is it possible to have a reliable preformance large bore engine such as a 1430 without looking at cost for an ocasional driver?

[Guess-Works.com]

No as that's all three... If you want performance and reliability, it'll cost. The more performance you want the more the cost...

You can get a reasonably performing 1380 with reliability at a reasonable cost.. example being look at a built engine from someone like MED.

[clubby crazy]

I was hoping to assemble the engine myself, I have built a few 998s without any problem, I did look at the kits you can buy for the 1380s which seem a good idea will have to keep them in mind.

Though what gearbox options are there? Can a standard 1275 box take the extra power or would it have to be uprated to straight cut gears? I did look at your website and played with the ratio calculator which is brilliant. I was wandering if for example a 1380 engine would cope with a close ratio straight cut gear box with a 3.1 final drive and 0.9 drop gear set on a 185/70 R13 wheel? I understand that every engine will have slightly different torque and power outputs so it is difficult to tell until building an engine is built and run but there must be a way to calculate if it theoretically possible using an average numbers?

Sorry for all the slightly off topic questions, but I would rather learn this way than the more expensive way. Thanks for all the info so far though its giving me a lot to think about.

[Sleepy Stu]

If the gearbox isnt in immaculate condition a standard rebuild will be required. As a minimum you should be looking at fitting a cross pin diff and a centre oil pickup during the rebuild.

Depending on the power output some people also go for straight cut drop gears as well!

Edited by Stewart_GT, 09 January 2010 - 03:53 PM.

[bmcecosse]

Make the engine as large capacity as you wish - but they all still have to breathe through the same cylinder head - and that's the main power limiting area - not the capacity. Unless of course you 'blow' it! Or fit an 8 port head. That's really the way forward - NOT just extra capacity with built-in unreliability.

[clubby crazy]

I was looking at the 7 port heads though it seems there are good and bad things said about them. cant seem to find that of information on the 8 ports.

Is there much need for a quaife or Salisbury diff compared to a x pin diff in terms of putting down power onto the road i understand the principles of both but wasn't sure if a lsd would be a good option due to picking the right setting for it.