FCB tech ?'s

I run 15/38 at NHIS...can go 12k rpm if I have too.

Ok...here's the deal on gearing...and yes I'll claim to know what I'm talking about on this topic.

You ALWAYS...at any track, using fixed gearbox ratios....want to be geared so you just kiss your rpm redline at the fastest part of the track.
You NEVER want to NOT use top gear at ANY track. If you are not using top gear you are effectively giving away a gear to your competitors, you have just changed your five speed into a four speed.

You'll note that the fast guys have a selection of sprokets hung on their trailer wall, and notes on which gearing to run at what track.

NHIS is a "short" track, a "slow" track. It's the shortest gearing I run anywhere. I'll go to 16/38 at Mosport and Beaver Run, NHIS gearing at Shannonville short track.

At Loudon you want to be close to red line at 2/3 the way down the front straight, you want to just kiss top gear coming into T3, and you might touch top gear after you come down the hill in back and approach t10...depends on how good your line is that lap.

15/36 sounds right for a 350...they pull a couple k rpm less than my 175. Craig...what's that about 10,000 rpm?

NEVER GIVE AWAY ONE OF YOUR GEARS.
JohnnyB

PS. And be prepared to change that gearing as you get more skilled. Come out of t12 harder you'll get to redline sooner. If all of a sudden you find yourself topping out 5th 1/2 way down the straight...or feeling like you are running out of gears coming into three....hey you are getting faster, drop a tooth on the back sprocket.


Edited by - jbranson on Apr 03 2007 9:41:15 PM

Joe...if you are just touching 5th and feeling like another gear on the straight at NHIS then that's about perfect. Same thing on my bike...if I gear for t3 then I'm too short for the straight...if I gear for the straight I'm a bit too tall for t3.

Even Chris isn't going to win at NHIS with Mosport gearing...and no one will win Mosport with NHIS gearing. Depends on what tracks you race...I can pretty much use 15/38 on any technical track, 16/38 on any long (fast) track.

Some people can live with being geared too tall...if they are light and on a bike with some balls they can get away with ignoring 5th gear...but they are still giving away speed. There's a big reason race bike builders go to the trouble to install 6 speeds and close ratios.
If you are geared too short you are screwed...doesn't matter if you are Rossi, and engine only turns so many RPM before it blows up...or if it doesn't blow up it will just sit there giving away 10mph to the bikes around it and floating the valves.

I ran NHIS gears at Beaver Run one year....led the first three laps running the engine up to about 13k...then it blew up. Wondered why it was so easy to stay ahead of everyone....that is until it blew up.
JohnnyB



Edited by - jbranson on Apr 03 2007 10:21:16 PM

There is a ton of mis-information about trap disks and 2-1 systems. If you are running rich with 10 four inch disks then the problem lies elsewhere...that's enough for a V8 to breath out of.
For instance...I run 6-7 3" disks on a 2-1 209cc bike that I turn 12,000 rpm..and it makes about 25hp.

Secondly...you don't need more disks on a 2-1 system than you do on a 2-2 system. I've almost given up trying to explain this. Both cylinders DO NOT breath through the exhaust at the same time...so you do NOT need twice the disks. Unless two exhaust pulse enter the collector at the same time...you effectively have only once cylinder exhausting through the megaphone at one time.

There is no convincing people that big loud exhaust doesn't mean more power.

TT, you JET for weather changes, you don't change the exhaust (yes I know you know that). Change two disks out of 12 will make absolutely no difference in rich/lean except in your mind ( Don't take that in a mean way). Sounds like your bike is altogether too rich...just like most 350's I see on the track. You have to get jetting close before you can begin to do the very subtle tuning of the exhaust.
If your exhaust is ever wet sooty in ANY conditions then it's too rich by quite a bit. Trapps will make them look richer just because most of the soot hangs around rather than disappearing out an open meg.
If you dropped a couple of jet sizes you'd probably see more of a power increase than anything you can do with the exhaust. On race fuel the tailpipe should be a medium gray...might be light gray on a cold day....darker on a hot day.
You dyno tuned it to get the jets dailed in right?
JohnnyB

PS. I started my exhaust system at 1.250" OD...now I'm going to 1.125 OD...yes smaller..the same system used on Mary's 175...you know..the one Zack rode and did times that would pretty much trash the whole FCB field. And I'm even going to build and try a 1" OD system...which by the way is what Mike Rich recommended....and I don't know what to tell anyone that doesn't know what a tremendous tuner he is.
Last big bore 175 I had in the shop (240cc)...built by Craig Hanson out west...guess what...had 1" OD head pipes.

It's about hp, not size, not noise.

This is a biggie folks...a bunch of you could be going faster. Get to a dyno. I trust nothing but a lap timer and a dyno.

PSS. I'll edit yet again...don't want you guys to think I'm being nasty here. I just feel for you guys...a lot of you giving up performance. A lot of you guys are getting fast enough that it's time to give up the wive's tales and guesses and do some research and development. Think about it...what do all the fast guys have in common...they know their bikes extremely well, or have them tuned by experts. Seek out the advice of the guys that are going fast, or the guys that build their bikes. Attention to detail is everything.



Edited by - jbranson on Apr 04 2007 02:26:55 AM

Edited by - jbranson on Apr 04 2007 02:38:59 AM

Wow, big differences in gearing between the LWSV gearing and FCB or stockish bikes.... you guys aren't short shifting those things are you. I guess you can tell when they run out of steam better than I can from my seat in front of the computer. Craig's bike is done by Mike Rich...so I know it can turn some rpm.

Probably the simplist way to jet a four stroke that seems to work is jetting it down till it falls on it's face...then go up about two sizes. And by "fall on it's face" I mean you put it in 2nd gear, crank it wide open for a few seconds...if the main jet is too small it will literally stop running momentarily....but it will be real snappy up to that point. For good power, just like a 2 smoke, you want to be as lean as you can be without damaging the engine...and still be able to run WOT at red line without starvation.

Intake is important...manifold must be reasonably well matched to the port, no gaskets hanging in the air stream, no leaks of course. A small "step" or obstruction in the intake can totally screw up air flow at WOT.

Guys like Pete T. can go through a bike and find all the little errors that add up to about 20 percent lost power. If timing just a bit off, jetting is just a bit off, exhaust a bit off, float level a little wrong, carbs not perfectly synced, plugs a little old, wires a little worn, cam chain a little loose...before you know it you've given up a big chunk of your hp.
JohnnyB

Ideally you want to run the largest jet you can and still be correct. If you can run a larger jet and still have the right mixture it means more AIR is going into the engine...which of course requires more fuel.
Using my Helmholtz frequency software you can come real close to the ideal intake track length for a given engine and rpm range. Intake track length is measured from where air enters the carb...so a velocity stack is part of the intake length...that's why they make them in different lengths..they are a tuning tool.
If you remove stacks and all of a sudden you are very rich...it means you have messed up the timing of the intake charge...less air...which means less fuel is needed. If you add stacks and go lean...that means you are getting to the correct length and intake charge pressure waves are timed properly....meaning more air...requiring more fuel.

Jetting is supposed to be kind of the last step in the build...when you already know you are using the correct parts and design.

Ken...actually OEM CB350 head pipes are way too big...even too big for a hot engine. I'd be running 1 3/8 OD if I had a hot 350 engine. Probably 1 1/4" for a stocker. (Of course most people's minds are deranged by a concept like this). But for the bike build...unless someone plans on making custom head pipes...the stock ones will have to do. Often going to something like 1 1/4" from 1 1/2" will also require matching the exhaust port to the pipe size...you don't want an abrupt step down in size. That's why most really hot engine have reshaped exhaust ports...often going down in cross section to increase exhaust gas velocity.

Once again I'll use our screaming, fast as mofo 72cc Honda single as an example...it has a 3/4" OD head pipe, makes tons of power and turns 13,000 rpm.
A 1 1/2" OD head pipe has 4 TIMES the cross section of a 3/4" pipe...which means it would be suitable for a single cylinder of 288cc...which would be much larger than one cylinder of a CB350. If you do all the numbers...the equivilant pipe size for one cylinder of a 325cc CB350 would be 1 1/8" OD. Or rounded to 29mm ...how big is the exhaust valve on a stock 350? I'd be a tad leery of 1 1/8", but 1 1/4" would be great for a stocker. (that's 25% larger in cross section than a 1 1/8" pipe). Cross section area goes up exponentially with OD...very easy to get too big too quick. Smaller cross sections keep exhaust gas velocity high with helps extract gasses....of course there is a limit to that.
JohnnyB



Edited by - jbranson on Apr 04 2007 8:59:46 PM

Yeah, someone told me once that OEM pipes were double layered. I'd figure two layers of maybe 16ga? That's Down to around 1.25" ID.
So yeah baby...that would work.

I typically build .049 wall headers...been using .060 lately. That's a 1/8th off the OD. Typically when a tuner quotes you an OD, they are assuming .049 (18ga) to .060 (16ga).

But yeah...by all means, stock head pipes if they are double thick.

I will differ with you on the heat issue...takes more heat to heat more metal..that heat must come from the exhaust gases. So during off throttle time, the pipe cools, not as much because it's thicker, but also takes longer to heat up when you go WOT...so it's typically a wash. (although an air gap would help a lot) The super trick setup is very thin walled head pipes with wrap...heats up very fast because it's thin, stays very hot due to the wrap...best of both worlds....except for fact you'll trash a set of head pipes in one or two seasons due to acclerated corrosion. I guess that's what you are suggesting with cutting off the outer layer and wrapping....yeah, another good idea.

Mike when it comes to the specifics of a 350 I'll have to defer to you, you have way more experience with them than I do.
JohnnyB



Edited by - jbranson on Apr 04 2007 10:52:23 PM

200gp will be fun this year...probably picking up another 3-4 bikes in the class. Probably see average grids of around 12 bikes. I've watched the class slowly pull closer and closer to me each year. Some due to more skill, some due to sorting their bikes. Frank C. is the wild card...any given day he can challenge for the win, or stay back and play with other riders. Last year...I think it was an ULWSV race....lap 7, I'm starting tilt it into turn one and someone on a 350gp bike maybe blows by underneath me...I check up for a moment and start to go back on line....sure enough there's Aaron drafting this guy right underneath me to take the lead, took me half a lap or so to get back around him.
Many times during mid race I could literally reach out and touch other 200gp guys as we exit T3 or the chicane. And if Frank C. is anywhere close to you, you need rear view mirrors because he'll find a hole inside or outside at the strangest times. And Doug Donelan if his bike is on, has no problem running inches away from you if need be. I'm real comfortable with the guys in the class, lots of skill and control with all the guys up front. They race hard, might even lean on you sometimes, but never do anything nasty.
But...sometimes it's a little of a pain, you gotta have your game face on or you are going to get beat. Every time I've ever pulled up a bit thinking I had things wrapped up, someone would be right on my ass before I knew it.
JohnnyB