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2006 Ducati ST3 S ABS
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Discussion Starter · #1 ·
What magic does Ducati pull off to make it possible to use the CRs they do? At the seemingly-typical 11.5/1 ratio I would expect detonation to be a problem, especially as temperatures rise. Do they use knock detection sensors and software to retard timing? And if so, is the degradation of performance noticeable when it is doing its thing?

Pilgrim
 

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It's probably in the combustion chamber design.
My 853 runs somewhere near 13:1 on 98ron fuel with no worries but does go harder with octane booster.
 

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2006 Ducati ST3 S ABS
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Discussion Starter · #3 ·
It's probably in the combustion chamber design.
My 853 runs somewhere near 13:1 on 98ron fuel with no worries but does go harder with octane booster.
Thanks for the thought. I'd like to think it's the chamber; I admire thoughtful engineering.
I've built or rebuilt a couple of Harley engines with 10+ CRs and, with the help of a local head genius, gotten them to run well on the 91 octane stuff that's the best I can get here in Washington State.
My long exposure to gunfire has deafened me to the point that I cannot hear detonation, so although I prefer good chamber design I do like know the electrons are there to back me up.
 

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Overlap has a lot to do with it as well, the compression ratio is measured static but in operation it's considerably different.

The ECU manages ignition timing and fuel on seperate maps, not sure if there are knock sensors (don't think so....) but it's a lot more advanced than the old vacuum advance on a distributor, they can manipulate the timing to be whatever they need it to be at and given RPM and throttle opening anywhere in the map to eliminate any damaging detonation
 

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Proper design with no sharp edges and well set up chamber flame path and proper squish leads to less detonation all of which enable higher ratios to be used without detonation and as loony says decent fuel and ignition maps all help the cause.
 

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I think the 14:1 compression ratio for the V4 is due to two major things.
a. The Ducati engines have a rather large bore/stroke ratio and hence the combustion chambers have quite bad area/volume ratio. This is actually good as it gives more cooling losses that reduce temperature and hence knock probability. Not great for efficiency though.
b. Motorcycle engines tuned for good performance at high engine speed have quite poor torque at low speed due to poor gas exchange. That in turn gives a low effective compression ratio at low speed where the risk of knock is higher.

You can compare a 600 cc single cylinder engine with a inline four of the same displacement. The former would need to reduce compression ratio to well below 10 as the latter can use 13-14.
 

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I run a 4 valve 650cc single cylinder motor with 10.5:1 compression no worries on 98ron.
It has a 100mm bore, 25/55 cam with .380" lift and pulls like a train from 3000 to 7000rpm with no dips... go figure.

V4's and the like have knock sensors to retard timing at high throttle low revs to keep detonation at bay and so lose out at low revs but they have so much spare power it don't get noticed anyway.
 

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OK, perhaps the right phrase is "about 10:1" and not "well below 10:1" but in any case it is far less than the 13-14:1 we see with short stroke small displacement cylinders. Is there any bikes without kick control these days, it became standard by 1980:s for cars.
 

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ultimately depends firstly how crap the combustion chamber is. the older 2v engines will detonate well at 11.5 with a chamber that's probably as bad as a harley, whereas that's low comp in any of the 4v. then how big the cams are, which determines effective comp ratio.
 

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2006 Ducati ST3 S ABS
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Discussion Starter · #11 ·
ultimately depends firstly how crap the combustion chamber is. the older 2v engines will detonate well at 11.5 with a chamber that's probably as bad as a harley, whereas that's low comp in any of the 4v. then how big the cams are, which determines effective comp ratio.
To Belter and before . . . all good stuff; thank you.
Does anyone out there have the numbers (lift, duration, etc) on a 992 (2006 ST3) cam?

Pilgrim
 

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2006 Ducati ST3 S ABS
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Discussion Starter · #13 ·

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2006 Ducati ST3 S ABS
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Discussion Starter · #14 ·
Thanks, Floppyjohn. I've got a Dobeck controller waiting for delivery of the bike (this week, I hope!) Disfortunately, I'll be out of town from mid-November until January. Once back, I'll ride it to get a baseline feel and then start fooling around with the engine when the Seattle winter rains set in for real.

Pilgrim
Oops! Sent too soon. I still have a ??? to all.
Bikeboy mentions re-valving and head work along with cam adjustments. There must be a shop in the US that does such work. Suggestions?
 

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Its static vs dynamic compression - the cylinder cant build pressure until the intake valve closes.

Some of the GM cams back in the 1960's were well over 300* advertised duration. Closing the intake late bleeds off cylinder pressure down low where detonation is a problem.
 
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