R&D platform


User avatar
bobcat
Forum Regular
Forum Regular
Posts: 164
Joined: Mon Aug 22, 2011 6:35 pm
Location: Dallas Texas
Location: Dallas Texas

Re: R&D platform

Post by bobcat »

Yeah Rick, the old single cam 750s were fun to modify. They were so mildly
tuned and under stressed that you could do some extensive mods to them
without harming reliability and get a lot more power out of them. I had a
76 750F I bought because it had a 900 kit (before I knew much about working
on them). I found out that the guy who built it had milled the head .060" to
get the compression up because the pistons were flat top. This retarded the
cam considerably since he didn't degree it. The bike was all low end grunt and
quit making power at about 6000rpm. It would have made a lot more power
everywhere if the cam timing had been restored. Better yet a stage 1 cam would
have really made it scream. I have several more SOHC stories but I'll save for later
as they are relevant to the topic of this thread....which I will continue in a while.
I've got to get out of this chair for a while....
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

User avatar
bobcat
Forum Regular
Forum Regular
Posts: 164
Joined: Mon Aug 22, 2011 6:35 pm
Location: Dallas Texas
Location: Dallas Texas

Re: R&D platform

Post by bobcat »

....Next up, the 81B and 82C. The motors being identical have the LCs, going
in the direction of this thread. The in. cam is advanced 2.5 degrees to 105.
The exh. are back to 107.5 like the 79Z. Rather than increasing power by raising
the rpm which would challenge the already insufficient cooling, they chose to
improve the low and midrange power by increasing the valve overlap phase^ at
the end of the exh. stroke by opening the in. valve sooner (advanced in. cam).

More later, I am not concentrating well enough (pain) to go into the next subject;
Valve overlap^ and cam chain stretch*. I will get to how I came up with the LC numbers
I use soon I promise....
You do not have the required permissions to view the files attached to this post.
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

User avatar
bobcat
Forum Regular
Forum Regular
Posts: 164
Joined: Mon Aug 22, 2011 6:35 pm
Location: Dallas Texas
Location: Dallas Texas

Re: R&D platform

Post by bobcat »

After reading and posting in another thread, I question whether or not
I should continue with this thread.
I'm trying to be clear and thorough in explaining how all these functions
add up to what I propose. In the process I may cover things that have already
written about or posted but not necessarily all tied together. If I am in error
or you disagree correct me or explain. I am never through learning.

If this is taken as arrogance or "craving attention", I will cease and desist.
It is not my desire to step on toes or offend anyone, just share information.
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

User avatar
bobcat
Forum Regular
Forum Regular
Posts: 164
Joined: Mon Aug 22, 2011 6:35 pm
Location: Dallas Texas
Location: Dallas Texas

Re: R&D platform

Post by bobcat »

Issue resolved and thanks for your commentary wyly. I will proceed with this
thread in a while when I can concentrate and focus a little better.

Note, there's a couple of posts deleted at the end of that thread but it's a good thing.
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

User avatar
bobcat
Forum Regular
Forum Regular
Posts: 164
Joined: Mon Aug 22, 2011 6:35 pm
Location: Dallas Texas
Location: Dallas Texas

Re: R&D platform

Post by bobcat »

In the discussion of valve overlap^ we run into subject overlap such as
port velocity, cylinder filling, etc. and how the overlap affects it. This
is where cam timing comes into play and why it is important.

Valve overlap is when, near the end of the exh. stroke, the IN. valve(s)
and EXH. valves are open at the same time, IN. opening as EXH. is closing
and the rising piston is nearing TDC. The EXH. gas velocity leaving the cyl.
is enough to initiate the IN. port flow, pulling the fresh fuel/air charge into
the cyl. even though the piston is rising.
What is critical is the timing of the overlap in relation to the crank/piston
position. If it happens too late, some of the EXH. gas and fresh charge get
pushed back into the IN. port interrupting the flow into the cyl.. Eveidence
of this can be seen as the burned coloring of the EXH. Just inside the IN. port
around the valve seat area.
If the overlap happens too soon, excess incoming charge is pushed out the
EXH. port, wasted. The greater the overlap, the more of each condition happens
and cyl. cranking pressure (compression) is lost..... Break time from this chair.. :(
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

User avatar
bobcat
Forum Regular
Forum Regular
Posts: 164
Joined: Mon Aug 22, 2011 6:35 pm
Location: Dallas Texas
Location: Dallas Texas

Re: R&D platform

Post by bobcat »

....However with optimum timing settings the cylinders fill more completely
which is the goal. Even though there is a slight loss in cranking pressure
resulting in a few peak HP loss, the low end and mid range power (torque)
is improved considerably. That is where the stock CBX suffers due to the relatively
small displacement of each cyl, and the high stock lobe center numbers.
AS said in the Review thread "you have to get these things really wound up
but they really go when you do" (not exact quote, wording). ....break...
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

Rick Pope
ICOA Rally Director
ICOA Rally Director
Posts: 2177
Joined: Wed Sep 03, 2003 3:16 pm
Location: Lawrencburg, IN
Location: Lawrenceburg, Indiana

Re: R&D platform

Post by Rick Pope »

Bob, thanks for this re-fresher course. It's almost like reading an old Gordon Jennings column from way back. I'm looking forward to when you tie it all together with head pipe diameters and lengths, and collector designs.

BTW, that old SOHC 750 wears a small diameter Hooker header.....
Rick Pope
Either garage is too small or we have too many bikes. Or Momma's car needs to go outside.

User avatar
bobcat
Forum Regular
Forum Regular
Posts: 164
Joined: Mon Aug 22, 2011 6:35 pm
Location: Dallas Texas
Location: Dallas Texas

Re: R&D platform

Post by bobcat »

HaHa Rick, Gordon Jennings was likely one of my early teachers as I started
reading and studying this stuff a long time ago even before I got into road racing**.
Mike Nixon was a huge help in understanding the "whys" after I had learned much
of the "whats" and "how tos". He could write this much better than I but I'm trying.

I intend to get into exhaust systems later as a separate discussion in the EXH. topic
category. For now I'm just trying to focus on cam timing without wandering off into
minute (boring) detail.

** I only got to race once, very unexpectedly. The rest was building motors and bikes
for others.
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

User avatar
bobcat
Forum Regular
Forum Regular
Posts: 164
Joined: Mon Aug 22, 2011 6:35 pm
Location: Dallas Texas
Location: Dallas Texas

Re: R&D platform

Post by bobcat »

...So the questions are what are the optimum LC numbers and how is that determined ?
How did Honda arrive at the numbers that were used ? (my evaluation).

When the CBX was first introduced, the focus was on peak HP and top speed In the power
race, to be king of the hill and it was briefly. Then the HP limits were introduced in 1980
and Honda had to dial it back to comply. When Honda decided to make the CBX a "sport
touring bike" they needed to improve the low and mid range power more suited for that
category of rider and probably to pull the extra weight. This was accomplished by advancing
the IN. cam, increasing valve overlap. They were going in the right direction but were likely
limited by emissions rules, fuel mileage considerations and smooth (tame) idle.

The optimum LC numbers are lower yet. ....continued...
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

User avatar
Syscrush
ICOA Member
ICOA Member
Posts: 1590
Joined: Fri Sep 14, 2012 2:29 pm
Location: Toronto, ON
Location: Toronto, ON

Re: R&D platform

Post by Syscrush »

Reading with great interest. Thanks for putting the effort into this.
Phil in Toronto
A cool guy deserves a cool bike, a dork needs a cool bike...
Pics of Perry, my '79.

Post Reply

Return to “ENGINES: Maintenance, Performance, Parts Sources”