16 replies to this topic

[Wiggy]

Just out of interest really.

Is there any reason why you can't destroke a +.060 1330 down to a 79mm stroke and therefore creating a 1293cc?

Then you've got a rebore left in the block to create a 1340cc at a later date.

I suppose that it wouldn't as 'square' and therefore a free to rev as a 1340cc but it seems like a good idea in my head.

Just wondered if there was any reason why it's a) pointless B) stupid c) possible etc. Heh.

Cheers.

[miniboo]

why not just bore to 1293 or +0.20 in the first place?

I cant see that you would be able to get a crank down that much to be honest

[Wiggy]

why not just bore to 1293 or +0.20 in the first place?

For the same reason people destroke a 73.5mm bore 1380cc.

[miniboo]

sorry i dont understand

what do people destroke a 1380 to?

[Wiggy]

A 1340cc generally.

[miniboo]

right sorry got ya now.

i can't see there is that much benefit and i dont think that is a common thing to do in fact i have never heard of doing it. So i would say that means expensive!!

[Sprocket]

De stroked cranks are available from virtualy all the big name tuners and are not that expensive either, but cooper S rods will be required. 79mm stroke is achieveble by offset grinding the big end journals to that of the cooper S rods, but the same can be done by stroking to 84mm, increasing capacity.

Destroking is quite common place in motorsport as it enables the engine to stay within certain race classes. The added bonus is that with a bigger bore and shorter stroke for the same CC the engine can produce more power and rev higher too. It does how ever mean that the engine has to be built for it and can never be taken back to standard stroke due to the excess metal that has to be machined off the top of the block.

[Dog]

Bit of a daft Q... but how much extra power does a 1340 make over a 1330?

[Jammy]

Hows longs a peice of string?


I was talking to GW about all this destroking malarky the othe day. It was then that I got the idea of using an 850 crank (68.25 stroke) and using it in a bored 998 block. When we worked it out it made for quite a square engine. One that would be good for supercharging...

[mambaman]

de-stroking is quiet popular at the moment, my own build is using a 73.5mm bore (1380) and a 76mm stroke giving a final capacity of around 1289cc. The idea is to let the engine rev a little more freely.

the 850 crank in a 998 bore is something thats been done a lot historically. Infact it was very popular thing to do in racers - using an oversized 998 piston in the later 850 block (thick cylinder walls - i think). Capacity of such an engine can be upto around 940cc (850stroke on a +60 998 piston) and can be increased again to nearly 960cc with the new +80 minispares 998 pistons. Some people opt to use an 998 block when doing this as they are available in A+ variety, but are a bit more involved to fit the crank.

[Pavel]

What exactly is needed to fit a 850 crank to a 998 block? Or for that matter to fit 998 pistons into a 850 engine. I've got access to a 850 engine and a 998 engine which i could easily experiment with and this sounds lke an interesting idea, especially if i manage to get one of them BINI s/c on it:)

And while we're on the subject of engines... how do you get a 1275 engine and its derivatives (overbores) to rev higher other than simply short stroking it... (no reason to this question, just interesting to know!)

[pikey7]

usually the limiting factor in rev limits is valve bounce. Once you get past a certain limit, the springs can't keep up, and things start to part company. not good.....

You could fit a higher rated spring, but you need to be careful that the rockers and cam can take it.

As usual though, its not quite as simple as that.......

[Jammy]

What we need is an in depth thread explaining hi-lift rockers....

[Sprocket]

Piston speed is the limiting factor, not the valves. Valves can be springed acordingly. 9,000 and 11,000 have been achieved but this would have been a short stroke big bore such as the cooper 970S or the Cooper 1071S. 850s if built right are good as well And very much a racing engine.

Like i said its to do with the speed of the piston. lets take a standard big bore block with 70.61mm bore, using a short stroke crank of 76mm, the piston acceleration and decceleration speeds would be less than they would be on a 81mm crank, IE the piston has to travel a lesser distance in the same time, Distance over Time = Velocity or Speed.

So, why is piston speed important. There comes a point that components fail due to the forces generated by the mass of the piston being for want of a better word, thrown backwards and forwards, stretching and compressing the connecting rod and its bolts and also causing crank flexure. The higher the piston speed the greater the forces generated on the reciprocating and rotating components and failure may be the out come. Shorten the stroke reducing the piston speed and as a result you lower the force enabling the components to survive the higher revs

Short stroke 6000RPM low piston speeds and forces, long stroke high piston speeds and high forces.

[Pavel]

So if you got harder springs for your bod standard metro 1275, enough to cause no valve bounce at 10k rpm, would the conrods let go at a certain point? Just want to find out what damage actually happens without tryin it out lol