R45 v R65 Carbs on a 650cc - Any idea on HP?

[jonathanrowley]

So I finally upgraded the Carbs to a set of 1983 spec R65 32mm BINGS.

Full Spec
BMW R65 mono 1986 22,502 miles Carburettors 32mm
Bing 64/32/359 and 360

The Bike was originally an R45 but I upgraded to bigger pots and pistons shortly after it bust a Valve on the originals.

It ran fine but you could feel it was a little restricted on full throttle - the engine asking for more fuel please!

It ran well yesterday after an initial twiddle and carb balance - post timing. I could feel more top end for sure.

Anyone care to guess what sort of BHP difference that would unleash.
Specs say R65 pulls 50 BHP and R45 35 BHP.

Interested in your views.

[nhmaf]

If you've now got post-1981 R65 heads, cylinders, and appropriately sized carbs, you've probably got the 50 BHP, or near enough available, thought the HP peak is up over 6K, IIRC.

The monolever R65 bikes' engines were retuned for more torque and less peak HP, I think largely through cams, exhaust/chamber, and possibly different sized valves.   These shared the same monolever frame as the R80 / R100 of that period (1986-88) I think..

[Tony Smith]

The monolever R65 bikes' engines were retuned for more torque and less peak HP, I think largely through cams, exhaust/chamber, and possibly different sized valves.  

Purely on the basis that there are no alternate parts listed I'd be confident that the pistons and cams were the same from 1981 to end of days. The heads are different, but the valves are the same size 1981 on - the difference is the change in rockers in (IIRC) 1987. The post 1987 heads are desirable in any airhead because they have the last word in unleaded fuel valve seats and also the superior rocker gear.

Interestingly you can convert older heads to the later rocker gear simply by knocking out the steel "pads" or "Button" that the rocker gear mounts to. I suspect that any change in performance between engines that do or do not have the later rockers would be down to the very small difference in effective ratio due to a few thou difference in the installed height of the rocker shaft and a very minor change in the rocker ratio. I would be prepared to bet that this was unintended by BMW who after realizing what happened did a corporate "MEH" and moved on. However the fact remains, there are two "sets" or No.1 and No.2 rockers, either of which will fit 1974 to 1995, the second and more expensive (GBP62) are the later design and the earlier (GBP45) ones will interchange on eiother the new or old rocker mounts, as will the shaft.

Having said that, the later rocker gear is just so good in terms of the ability to "tune out" the "clickerty-clicks" that characterise the earlier rocker gear it would be almost (but not quite at north of GBP200) worth doing to an earlier bike for that alone. It irritates me that the wife's R80 engine is absolutely silent in the rocker department.

I'm tempted to spend the small amount of money required to convert to the modern rocker mount, but retain the old rockers just to be able to shim out the clickerty click.

[Barry]

I don't know how the mono engine compares with US model R65's because I've never seen published power and torque figures for the low compression engines.

For Euro models the only specific I've seen in print about the mono engine is that the compression was reduced from 9.2:1  to  8.7:1  but I doubt that's the full picture given the torque characteristics.

It makes 48HP @ 7250  and 35.3 lbft @ 3500

Compare that with an 81-84 Euro model R65 with 9.2:1 compression

50hp @ 7250 and 38.6 lbft @ 6500

So for Euro models the mono makes less power and less torque which is to be expected from the lower compression. But that doesn't explain the torque peak being vastly lower down the rev range. If they didn't change the cam then they must have done something clever as the torque curve looks  flatter and dips after 3500 before rising again.  That's characteristic of some sort of charge boost in a particular rev range due to induction/exhaust tuning.

[wilcom]

By looking at the chart you wouldn't believe it was the same motor!!

[nhmaf]

The mono's do/did have that large 'collector box' just aft of the gearbox that functioned as a very large crossover.   I know my 88 R100 made less peak HP than earlier model R100 models, but it definitely felt torquier at lower revs, and was one of the smoother airhead engines I've experienced.    I have to dig out my Boxer Bible book to look up specs....

[Motu]

The US R65 Mono was detuned, the Euro R65 was retuned - I guess seeing as it was the same as the R80, they would give it the same feel.  Much lower torque peak, and it pulls a taller gear ratio.  Having ridden an LS to compare to my Mono, the Mono definetly comes out of corners stronger, and short shifting used rather than holding between corners (we have corners in NZ).  The fat mid range is very noticeable in comparison.

Is it the exhaust system? Possibly, but I seldom use the stock system, normally I have a 2 into 1 with a Supertrapp.  Last year I repacked  the muffler and painted the exhaust...and while it was off blocked the balance pipe.  Now I have a big burst in power at 6,500rpm, and goes all the  way to 8,500 and probably beyond.   Suspicious that the LS has max torque at 6,500, and at max torque an engine usually changes character.  Was it the repack, or the blanking of  the balance pipe?  I've changed to another muffler, and will test this weekend, then remove the blanking plug and have another test.  I'm happy with how it runs, but it's a change, and I want to know why.

[Barry]

Even if the max torque is slightly lower at least it's in a rev range that's used every day. I think I'd like the low/midrange torque of the mono model.