camshaft timing

I'm thinking of upgrading a CB250RS-engine with a big-bore and a different
camshaft. The big-bore was no problem, but the camshaft turns out to be a
real pain. There are several different parts readily available, all with
different timings and promises on torque, top-end, mid-end etcetera. For
now, I've been comparing the timing-numbers, but I can't find any logic in
that. Since I have the choice in having the stock one reground or a new one
delivered for about the same price, I would like to do some 'calculating'
myself if that's possible. Is there anyone who can help me in the search for
a clear understanding of, or maybe a calculation applet or formula?

For the record, the standard profile reads 10/40 and 40/10. I've found
similar timing on a XL350, and the most stated are:
22/48 and 50/22 for a better mid- and top-end.
33/63 and 60/33 for maximum top-end.

I want to gain mid- and top-end since it's going to be a street and racebike
(50-50), but I do not want to suffer too much reliability.

Any pointers please? Many thanks!

Peter

'00 Ducati M600 - '82 Kawasaki KDX 175 - '81 Honda CB250RS

 

To be honest, I'd junk the idea of boring out the 250 lump and simply
go out and buy a 500 engine from an XL500, FT500 and the like.

They're damn nearly a drop-in fitment, and will give more power than a
tweaked 250 ever will. I've seen a 500-ised CB250RS: you really had to
look twice to appreciate it for what it was.

 

Well as you've probably figured out:

10/40 = 10deg BTDC inlet opens / 40deg ABDC inlet closes

and

40/10 = 40deg BBDC exhaust opens / 10deg ATDC exhaust closes

thus giving 20deg of overlap at the top of the stroke.

The 22/48 and 50/22 simply means that the valves open earlier and
close later giving more overlap (44deg), and so on.

33/63 and 60/33 are even more extreme.

In each succeeding case, the valves are open for longer and overlap
more. This will *tend* to improve breathing at higher RPM and hence
more power because:
a) the valves are open wider for longer and
b) the increased overlap will allow any exhaust resonance to pull
through more mixture

The down side: Wider cam timings will probably stuff lower & midrange
torque. Jetting will probably have to be changed, possibly across the
full range, and the exhaust & inlet may need modifying to make full
use of the altered timing.

As TOG says, better to just get a bigger bike.

 

It was somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember TOG@Toil,

AARRRRGGGGHHHH!!!!!!!!!! Not the dreaded FT! Itym XBR500.

--

Dave

GS 850 x2 / SE 6a
SbS#6 DIAABTCOD#16 APOSTLE#6 FUB#3
FUB KotL OSOS#12? UKRMMA#19 COSOC#10

 

Exactly my reaction.

When the XBR500 was launched, testers of the time praised it as being "like
a CB500RS".

 

Well, the FT is OK as long as you convert it to kickstart, throw away
the exhaust (which you'll have to anyway) and fit a less restrictive air
filter.

 

Can you get replacement exhausts, or do you just use the Big Collector Box?

 

There ain't no substitute for cc's.

 

Someone must do them. If not, it's not hard to source a custom-made
jobbie.

 

Not that I'm planning to get one. Actually, I was having a close look at a
v. clean VN1500 nomad/Classic Tourer thing in the local emporium yesterday:
99T reg, spotless, green, added sissy bar and spotlights, 9K miles (on a
5-year-old bike ffs), £5895. Which I think may be on the high side, but
these are pretty rare, so it's hard to establish a frame of reference.

 

AOL to that

 

My dealer's got one which sounds very like that, i his floor, too. Can't
recall the price. It's the only thing to do wiuth a big Jap cruiser,
IMHO - give it a screen, panniers, and maybe a decent seat, and turn it
into a sort of budget tourer.

But 6k sounds like silly money, really, for a tart's boudoir with
panniers. 5k, better still 4.5k, and it makes sense.

 

That's sort of what I was thinking. I was mesmerised by the bigness and
shiny-glitteriness of it, but in reality a Cali would be 1-1.5K cheaper, be
more powerful, economical, and have seriously better tank range.

 

Nail - head - BANG!

Plus it would handle and stop much better. Only the seat would be the
issue - the Cali has a seat that is utter total crap.

Nothing that can't be (relatively) cheaply rectified, mind.

I really, really liked the injected Cali I tested in 1997 (shit! - seven
years ago).

 

A Cali would cope with luggage and a pillion, and do the v-twin, cruiser
thang, so replace both the RT and the Drifter. One thing the Trophy did was
focus my mind on what I want and need from a bike.

What I'd save on the purchase would leave me plenty for something from
Corbin or the like.

I'm torn between the looks of the T3-style Cali and the later Cali
III/1100/LAPD styles. I suppose the injected motor would be the clincher.

Hmm...

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=43121&item=2495869396&rd=1
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=43121&item=2496716998&rd=1
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=43121&item=2496464841&rd=1

 

Yeah, yeah Max - the blower ... I know.

You take a 750 motor and turbo it - then hog it out to 1100 and turbo
that. See my point?

 

The original Cali, the T3 850cc engined one, is the best looker.

The injected one is the best for riding.

The carbed 1000s are almost all shite.

 

That was my perception. Roll on the NEC, then.

 

Remember that Yamaha was developing an XS1100 Turbo before sanity
prevailed (or possibly, after they'd killed all their development
riders) and they decided to turbo the XJ650 instead.

 

(The Older Gentleman) struggled
to ejaculate:

Pah. Wusses.

Supercharged V-Max, that's what they should have been developing. Now
*that* would be worthy of Champ's plate logo.