So as to not hijack the TB hose mod thread (see below), I started this thread. While I find the little 650 mill to be very spirited, I dislike the strong engine braking effect and find it often makes for rougher shifting unless being either aggressive (i.e. higher revs), or shifting below 4K in the lower gears. With previous bikes (last street bike was a Triumph Speed Triple), I could leisurely and smoothly up and down shift w/o any clunkyness and allow a long coast off throttle in an urban setting awaiting the light to change.

Engine braking is indeed a function of the motor, but some FI systems keep a small bit of fuel squirting on deceleration to avoid a sudden lean mixture in the catalytic converter. To quote the Sewer Urchin, "That's baad." It used to be done with dashpots on carburetors, now it is done with CPUs.


Engine braking occurs when off (or partly off) throttle, so it is dependent on the amount of air admitted at the throttle bodies. With the tube coupling both TB's at fittings past the throttle plate, some intake air is admitted from the other TB as the vaccum is sequentially balanced between the two via the tube...


So would removing the sub throttles and PAIR tubing as suggested with the airbox mod then cause an decrease in engine braking, but a corresponding increase in backfiring and popping on decel?

With a conventional carb system on my 1996 Triumph Speed Triple, replacing the stock cans with aftermarket slip-ons did increase backfiring until I rejetted richer, so adding fuel with FI at decel makes sense to reduce backfiring. What I do not understand it why there is so much more engine braking with the V than the Speed Triple. RPM and compression ratio (10:01 vs 10.6:01) does not account for all of the difference. My experience with engine brakes (i.e. Jacobs Brake, found on diesel engines) is that the air flow is restricted at decel, braking the engine with back pressure developed with the restricted flow.

If my thinking is not faulty, then there must be something restricting the free flow of air upon decel, has anyone that has done the airbox and sub-throttle mod found a decrease in the engine braking effect on the stock Versys?

Cheers,

My limited understanding of engine braking is a Compression Ratio and Flywheel weight. Higher compression caused more engine braking and lower flywheel weight (inertia) causes more engine braking. For the life of me, I cannot figure out why the Versys would have more engine braking.

It's lighter....

True 460 lbs to the Versys wgt. of 400 lbs., but does that really explain it?

The flywheel wgt. may be more of an issue though. Hacktracker what's the engine braking like on your GSX?

Personally I like the engine braking. When I am coming into a turn I just loose some throttle let the engine slow me down some and then get back on the throttle coming out. No brakes is faster. :D

Could it something to do with an affect on the sub-throttle valves from pairing/balancing the vacuum of the two intakes?

I don't know if by removing the sub-throttle assembly, if air flow through the TB's would be less restricted. From what I see the sub-throttle butterflies swing loose within the velocity stacks. When the throttle is closed, do these valves close and restrict air flow resulting in increased engine breaking?

Is there anyone that has done the sub-throttle and airbox mod that can comment on their experience? Or someone more experienced with the FI system on the V?

Cheers,

Sealing off the PAIR actually reduces exhaust popping experienced with low restriction exhaust silencer. It doesn't affect engine operation or engine braking as the PAIR only adds air in the exhaust ports past the exhaust valves, upon engine braking. Coupling both TB's with a tube at the vaccum synch fittings is the only mod that was commonly reported to reduce engine braking somewhat. Removing sub-throttle plates has the reported effect of reducing idle speed a bit, which is then readjusted at the throttle stop idle adjustment screw. This is perhaps caused by increased air velocity at the smaller opening at sub-throttle plate before it enters the main throttle plate opening.
I've never found the engine braking to be excessive or to cause any problems... Make sure your idle is up to at least the recommended 1300 rpm.

I'm with Baddone - I like the engine braking. 'makes the corners easier and a lot less braking when in traffic.

I guess I just have to get used to the lighter bike, My Speed Triple was just so much smoother and responsive w/o seeming jerky. It also shifted much better and you could easily snick up and down w/o the clunky feeling of the Versys.

I will probably look at doing the airbox and TB mod with a PCIII later in the year.

Cheers,

I did the TB mod a few days ago, everything else is stock. It smooths out the jerkiness pretty good. I am happy with it. It's a very easy & cheap mod. Give it a try. If you don't like it, it's very easy to reverse, no harm no foul...