SV 650 Big Bore kits?

[northwind]

Originally Posted by Knaapie

Mmmm, this creates perspectives. I thought it was only advisable to do with an "out of the crate" engine, like was done with yours.

Nah... Obviously if the engine's tired it's not going to help, mind. And if it's suffered any loss of power or efficiency such as worn cam chains or damaged valve seats, then tuning it for power doesn't make an awful lot of sense unless you give it a full refresh at the same time (which a reputable tuner would, presumably). The SV engine's good for pretty huge mileages, assuming something nasty doesn't happen to it first, they don't really wear out in general use.

But really, the heavier pistons and higher power and heat stresses don't do much to everyday wear and tear, and the crank and rods don't get weaker in real terms over their lifespan, so a well used engine won't be significantly more vulnerable to the potential problems of a big bore than a brand new engine is.

Thanks Northwind. Decent and understandable story.

Trying to get my head around this.. What does the basic extra 50cc give you?(assuming that the rest of the engine remains the same).

[northwind]

(huge post warning!)

More power all over... From that point of view it's ace, no replacement for displacement as they say. It's also fairly cheap to do, for what it is- compare the gains with £1000 worth of exhaust, induction and remapping and it's a no-brainer. A 700cc SV's a very nice thing indeed, lovely to ride slow or fast. You can get as much power with high comp, but not the midrange grunt. SO in terms of benefits, it's huge.

But there's no such thing as a free lunch- the pistons are heavier, and the explosions are bigger, which all creates more stress on the bottom end. Also the liners are thinner, which increases the risk of thermal stress failures. It's a matter of opinion how bad this is- SV650Racer's opinion is that it's safe, or near as, based on her experience. The opinion of people who've snapped a rod or crank after doing it tends to be that it's suicidal, and that you have to be an idiot to do it. When you get conflicting informed opinions, it's sensible to look in the middle

But the trouble is, there's just not good facts out there to say exactly how great that risk is- some people will get tens of thousands of trouble free miles, others have the engine destroy itself beyond realistic repair within a few thousand. And the real downer is, that can happen to a totally standard engine, so even when you get a case like that you never know whether's it's because of the bigbore, or whether it's a manufacturing or mechanical defect, or poor maintenance, or just plain bad luck- engines break. SV650Racer had a shell fail, if memory serves, which they put down to a defect but which another person might have blamed on overstress, so even there there's room for interpretation.

Certainly, absolutely nobody's benchtested an SV engine with the Wiseco or JE bigbores to the extent that you need to, in order to really know the long term implications. Even the few people out there who've done this modification a large number times can't really say for sure- everyone develops their own impression which usually varies. Bike tuning's a maddeningly inexact science if you want solid answers

Personally, I think it's nonsense to imply there's no risk, or that every failure's caused by some unconnected fault or flaw- there's just too many cases of bigbored SVs spitting their guts, and too many experienced tuners have seen it happen- sometimes to their own bikes. But then, it's equally nonsense to write it off as building a time bomb, since you do get some that'll run and run. It's all just about offsetting risk against reward, using educated guesses.

I decided against it- if I was racing I'd do it, and I might well do it for my spare engine if I ever rebuild it, but when you need reliability under road conditions, for tens of thousands of miles, I don't personally think it's a good option. I'm probably going to go with higher comp and lighter pistons, bit of give and take there.

Racers tend to have the conceit that race use is hardest on an engine, and that if a particular tune can survive extended race use then it can survive anything... That's where the highest short-term stresses are but a road bike can spend more time at peak power on a single long motorway trip than most race bikes will in their lifetime- the individual stresses are lower, but they go on for hours. (as anyone who's ridden up the M6 can testify!) It's like the difference between a knife would and a papercut, one knife wound's worse than one papercut but 1000 papercuts is generally worse than one knife wound The stresses aren't really more or less for either, they're just different. Honda learned the error of high-stress testing engines with the VF750, it'd withstand anything for short durations but road use killed it.

You can build an SV bottom end to be extremely strong- Something like a Falicon crank and rods to suit. It'll cost you more than another SV though. There comes a point when, for most people, tuning becomes a mental illness.

I should just cut-n-paste my Big Bore Ramble to save time

Northwind really has hit the nail squarely on the head.

I personally have only experience of 700cc kits being raced and not used on the road(abeilt for short tripes) so cant comment on long term stresses I have seen sarahs bike be, in terms of things breaking, 100% reliable(and yes i'm aware the definition of breaking can be subjective) all the way to another bik ethat managed to be run in on the dyno and then spat in's guts on the first time it hit the trakc and then my bike in the middle which spat it's guts around the time it was due a refresh anyway.

One thing worth mentioning though is thats there are as many cases of minitwin engines spitting there guts as tuned SV engines going pop so it's hard to conclusively say yes the 700cc is bad.

There is no denying though that i have never heard of any 700 kits snapping cranks only rods(mine did this) and one holed a piston due to cheap nasty fuel and because of this mine has now been built with carrillo rods just to try and stop it happenieng again. Yes I nklow you could argue that what i'm now doing it transfering the weakness somewhere else in the engine but fingers crossed it survives and it will manage half of next year aswell before it needs a refresh.

I guess though there is no denying that anytuning will stess an engine more than an untuned one but thats true for any aspecxt and not just big bores. Even just fitting an exhaust will in theory add more stress to the eingine as it produces more HP than before. The only thing to stop you is how mcuh you are prepared to pay and wheather you consider it worth it. At the end of the day to get the best out of a 700cc kit you need the whole hog done and by the time you assume machine work and a decent 'zorst system such as the M4 or arrow you are probably looking at £2000 minimum. would it not be cheaper to just get an SV1000 and be done with it even assuming the £2000 helps with extra running costs and insurance.

Anyway. If you want the 700cc kit then go for it but remember. you will spend about £2000 getting done and your bike will probably be worth about £100 more because of it. If your okay with that then fine. if not dont bother and just get a bigger bike.

Thanks for the good reply Northwind.

I rode my standard naked SV K1 from Porthcawl to 440 miles into France in p*ssing rain on Aug 23 and back again on 28th. Coming back in the sun I was in the 80's/90's/100's all the way. The bike was rock sold except for one hissyfit on the way down when the front pot was misfiring with the wet and no Fen ext. I wouldn't feel at all happy about doing that if I had just increased the bore without ensuring that the rest of the engine could cope with the extra power.

ps I'll be posting about the the trip and where we stayed etc again.

[northwind]

Originally Posted by chunkytfg

There is no denying though that i have never heard of any 700 kits snapping cranks only rods(mine did this)

Best SV photo ever:



Not one of mine obviously, I only have 2 toasted cranks in my collection This is a selection that Twin Works Factory pulled out of their scrap bin. The one that halved itself was bigbored, so was one of the ones with the break by the flywheel, though that's more a power stress than a weight stress. The rest are boring, spun bearings. One of 'ems not SV, no idea what it is, TL maybe.

Originally Posted by Tildo

Save your money and get one of these




http://www.telegraph.co.uk/motoring/...t/mfbike21.xml

nice 95 bhp sounds usefull

Originally Posted by northwind

Best SV photo ever:



Not one of mine obviously, I only have 2 toasted cranks in my collection This is a selection that Twin Works Factory pulled out of their scrap bin. The one that halved itself was bigbored, so was one of the ones with the break by the flywheel, though that's more a power stress than a weight stress. The rest are boring, spun bearings. One of 'ems not SV, no idea what it is, TL maybe.

Fair enough. Now i have heard of a snapped crank in a big bore. Be interesting to know if it is from a K3 though as wasnt it those that had the weak cranks?

[northwind]

Not sure... Couldn't tell you how to tell the different to be honest, I've never handled an injected engine below the pistons.