Breva 850-report card

[mg877]

Here's a topic for those whose grey matter tends to get caught up in these sorts of thought threads............

I've owned a few Guzzis in my time and have owned the breva 850 for a couple of months now.
First off, great bike! 'Sexy' is not doing it justice, handles well, sounds great and is plenty powerful-for me, anyway.
But the way it makes its power is not what I'd hoped for, I have to say. It almost needs to be ridden like a 2-stroke before those 70 horses are available i.e. 5,000-7500 rpm. In fact it occurs to me that in real world riding the breva 750 probably delivers the same hp i.e. between 2000-4500rpm.
I suspect that the engine has been tuned to provide impressive hp numbers rather than rideability, hence the almost identical rpm numbers for torque and hp (7000 & 7500).
I think that what has happened is this; MG use essentially the same engine/chassis for the various incarnations of the big block norge/breva and, even tho' the 850 has a shorter stroke it still uses the same cycle and engine parts as its brevona. Hence we find the same cylinder head, valve sizes/ throttle bore on the 850/1100 and 1200 machines. This means that they all share 46mm valves and 45mm throttles. Inevitably, I think that this means that the 850 valve/throttle bore is a bit big for its displacement - hence the high torque peak. Interestingly, the 1100 and 1200 big blocks (with same cams, valves, throttles) have progressively lower rpm torque peaks but almost identical rpm HP peaks. I seem to remember that this was also true of smaller BMW twins such as the 650 and 850 twins (using the same heads as the 1000 engines) back in the 70's, 80's.
I'm over the squiddley side of riding the back roads and really get off on the long-legged performance of my previous Guzzis however, I also really like this 850 Breva. So, the thoughts that I am having are to perhaps squeeze down the intake ports (just before the valve guide) to perhaps 85% of the current bore in the hope of lowering the torque peak to perhaps 5500/6000, along with changing the intake/exhaust lengths to match this rpm level.
Any thoughts? Pete Roper? Anyone?
Or should I just go out and get an 850T?

 

[GTM®]

Ah-HA! One has finally surfaced. Welcome mg877.

I'm off to bed, but I'll be anxiously waiting to see what transpires here tomorrow, and will add my $.02 then.

 

[guzzibob]

An 850T won't get you anywhere near 70 Hp at any rpm. Why not get a big Breva/Norge 1100/1200. I have an 850T3FB, 1000S & Norge. As you might expect, engine & tranny wise the Norge is infinitely superior-at any RPM. Not that I didn't have a great time riding my T3 up on Angeles Crest today......

 

[Brian UK]

I must admit, until I just looked up the teck spec for the 850, I didn't believe that they would fit the same 40mm injector bodies. Some Italian logic here I suppose, but you will never get a wide torque range with bodies that big. Or is it just cheaper for them not to have a smaller set made up?

 

[draidt]

Brian UK wrote:

I must admit, until I just looked up the teck spec for the 850, I didn't believe that they would fit the same 40mm injector bodies. Some Italian logic here I suppose, but you will never get a wide torque range with bodies that big. Or is it just cheaper for them not to have a smaller set made up?

Bingo !

 

[sbaker]

Hello;

Isn't the smaller displacement about Insurance, Road Taxes, Rider's Licence, etc in some countries in the EU?

 

[RJVB]

Insurance, yes, license possibly, taxes probably not. But I'd hope that's not the only thing. An engine that revs more easily and has its torque a bit higher up in the revs should be better suited to the sort of congested urban traffic we tend to get here.

This has just come up in the thread about a new small block in the MG-24 chat section.

 

[john zibell]

Yes, the throttle bore reduction will lower the max efficiency RPM for the engine. But I think the best way is to use smaller diameter throttle bodies with the smaller butterfly valve. I'll attempt to post an Excel spreadsheet I use to calculate efficiency based on Carb size (throttle body), bore, and stroke. The best "rideability" is 1000 rpm lower and higher than the number calculated.

The idea is to select the throttle body that will give the maximum air velocity (best fuel mixing) at the RPM you want to ride. Of course there are other variables such as cam profile, valve size and so on, but the formula for air velocity should help.

BMW has done a similar trick on the R1100 RS vs. the R1100GS. Th inlet tube is smaller on the GS, lowering the max torque RPM, but also reducing HP readings. Doable since all other parameters are the same. So if you hat to "hop up" a GS for street use, you install the RS inlet tubes.

I've included an Excel spread sheet that may be helpful in determining throat/carb size. Enter teh bore, stroke and Carb/throttle bore size to calculate optimum RPM.