UK Monster Owners Club Forum » .: Technical :. » Engines, Clutch, Gears » Pistal piston comp ratio?

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Old 17-09-2019, 04:35 PM   #1
Bitza
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Pistal piston comp ratio?

Hi all, I have a S2R 1000ds engine overbored to 98mm with Pistal "street" pistons (sometimes referred to as 11.5:1), has anyone any experience of this particular combination and an idea of what kind of compression ratio you would expect? Is there any difference between the 1000 heads and those on the 1100 engines? Thanks Bitza.
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Old 17-09-2019, 07:06 PM   #2
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I had my S2R 1000 bored out to 1100 too and it was a great mod. I had all the work carried out by Moto Rapido but I'm afraid I don't know what the exact capacity or compression ratio was or, indeed, what pistons they used. I had a Power Commander III, got a custom map to suit the mods and it ran like a dream, (until I fitted lumpier cams and lost a load of mid-range torque for very little top-end gain - but that's another story!).

Although the 1000 and 1100DS motors have identical valve sizes, lift, duration and timing, the part numbers for the heads are different. (The 1100 Evo differs again with completely different valves and timing.)

I know that the design of the DS heads changed at some point, with the later models having a thinner journal at the left hand camshaft end and, because of this, they also have different end caps. (The aftermarket MVS caps I fitted came in two different inner step lengths to cater for both head types.)

It may be that the change happened when the 1000 became an 1100. (I believe that these thinner journals may have contributed to the oft-reported grinding noise on the Evo if the belts are too tight http://www.ukmonster.co.uk/monster/s...ad.php?t=48249).

So, while some of the individual parts in the heads are interchangeable, (valves for example), the heads themselves are probably different, (but may well fit nevertheless).

Hope that helps.
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Old 18-09-2019, 08:43 AM   #3
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My very hot 839cc was restricted to 11:1 because that's about what the engine can handle long term. (builder Kämna's words)
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Old 18-09-2019, 09:11 AM   #4
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On old Carb monster 900 I was running about 12 had to push it down as bike was very hard to start and was burning starter motors for breakfast when cold.

Cold starts were basically rock the engine till you feel it is at low compression stroke, then start so starter motor can build up momentum :')

Fine on weekend bike only. There was no reliability issue during 18k miles I have done but service on valves was done every 6-7k miles max and valves were always out by then.
Also valve seats were gone at 8k miles then 7k miles after.
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Old 18-09-2019, 10:19 AM   #5
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This is probably too modified for reasonable comparison:

I have a 1000ds bored to 1100 with big valves, sport cams, head-work, modified crank and rods.
Sorting it properly is on my never ending to-do list...

I'd estimate around 110bhp, it was still lifting the front wheel going into 4th at around 110mph
It was running a FIM ECU and I never got round to dyno setup although it appeared to be running quite rich.

It ran like a dream for 1500 miles running in on the road and 8 trackdays, half way through the 9th it started spitting oil through the airbox due to pressurising the crancase. Also judging from the surface of the combustion chambers it runs too hot for long term use. I also encountered an issue with the valve seats on one head which I can't see being related to the crankcase pressurisation issue.

1000ds (992cc) 94 bore x 71.5 stroke, claimed 10:1 compression ratio

992/2 = 496, stock compression ratio of 10:1 gives a combustion chamber volume of 49.6cc

increasing bore to 98mm

1100 (1078cc) = 539 /cylinder

539 / 49.6 = 10.9:1 compression ratio just by virtue of bore increase, any further increase is down to piston crown shape/ head

heads before


heads after



bearing in mind the 41/47mm valves in the after shots (stock is 40/45mm)

piston comparison


Kaemna reckoned this was 11.8:1 although I didn't measure it while putting the motor together

I documented the stupidity here http://ukmonster.co.uk/monster/showthread.php?t=46630
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Old 18-09-2019, 02:32 PM   #6
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Slob, what do you think caused the crankcase to pressurize and spit out the oil?
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Old 18-09-2019, 03:05 PM   #7
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Nothing visually or measurably obvious, although I didn’t have the tools to do a leakdown test. I’m assuming it has to be a ring sealing issue. I’ve got a new set of rings and I’m planning a much larger, corse style breather as mitigation.

Not that I’ll be riding anything for several months at least :-(
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Old 18-09-2019, 06:11 PM   #8
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With a higher than standard compression ratio you probably need to adjust the ignition timing a few degrees (cannot remember which way - advance or retard - probably retard). The extra compression would make it run hotter anyway but if the spark timing is sub optimal that will definitely make it run hotter still. Extra heat > lubrication breakdown > ring and bore wear > excessive pressure in crankcase - maybe?
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Old 18-09-2019, 06:24 PM   #9
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I’m not trying to post mortem my issues at the moment, just trying to give mike the info he’s after, although it does occur to me i have a knackered 1000ds rear head and a hyper 1100evo one somewhere in the garage, if I can find them both I’ll take a side-by-side photo. (still looking for a front evo head)
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Old 19-09-2019, 08:42 AM   #10
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Quote:
Originally Posted by 350TSS View Post
With a higher than standard compression ratio you probably need to adjust the ignition timing a few degrees (cannot remember which way - advance or retard - probably retard). The extra compression would make it run hotter anyway but if the spark timing is sub optimal that will definitely make it run hotter still. Extra heat > lubrication breakdown > ring and bore wear > excessive pressure in crankcase - maybe?
On the 888 we're up at 13.5:1, maybe a bit more as we have set the squish at 1mm as well. Yes the ignition timing is altered (retarded) and the cams are timed to suit, we also run 105 octane fuel to keep the burn as cool as possible with the higher comp. All things to consider.
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