[craig641]

Folks,

Does anyone have some formulas for header length and ID of the header itself? I have a copy of Graham Bell's Four Stroke Tuning book and the formula he has for ID is:
(sq root of (cylinder volume, CI /((primary length +3) *25) ) * 2.1

On my engine that would give .29 inches. Either I'm doing something wrong or my pipe diameter is less than 1/3".

Craig

 

[jbranson]

Consider yourself lucky...I gotta find some 0.199" tubing.

I've yet to find an exhaust pipe OD formula that seems to be anywhere close to real world dimensions for small engines.

A lot of the reading I've done on the subject...usually ends up saying that the best results are often achieved by trial and error using a dyno. Even the Honda SAE paper on Megaphones sums it all (after pages of formulas) by saying that no one really understands completely how they work. They just know, shorter does this, longer does that, taper does one thing, reverse cone does another.

As for the formula above...usually rpm is included somewhere in the formula.
JohnnyB

PS. I just tried working the formula a couple of different ways...still no where close. I'm guessing that for my bike it should come out somewhere in the area of .75 - 1.00"...at least it should if the formula is to produce results that usefull in any, way, shape or form. Cause these are the OD's that would actually be ideal.

 

[craig641]

Well crap. That's not what I wanted to hear. So, what size dies do you have for your tubing bender???

Craig

 

[HackAsaw]

well there are many engine scholars that will, under no uncertain terms, that when that overly simplistic formula actually works out on the dyno as paper indicates it should...... it's nothing more than a coincidence

several reasons and the phenomena of "unsteady viscous flow" isn't the least of them

of course none of more complete formulas ...... which take into account the valve events and rpm........ are regarded any differently by many of these same credible people

decent baseline but without taking all the factors into account, it's still a hand grenade toss and a reasonable guideline at best

 

[jbranson]

Craig,
Biggest I have is 1 1/4". I'd want 1 3/8" on a 350. I run 1 1/8 on my 175. Your motor is pretty hot, could probably get away with 1 1/2", but that would lean towards top end power rather than mid range.

I thought you were all set for pipes. Didn't you have something tricky made up?
JohnnyB

 

[jbranson]

HackAsaw,
I wrote up a piece of software to calculate intake track length based on the Helmholtz Resonance formula. So far on the dyno it's been within 5mm of ideal length (of course length varies with what rpm you tune for), for several bikes I've used it on.

But...intake track simulation is way easier than exhaust. Heated gases being pushed behave a lot more complex than ambient being pulled. I wouldn't even attempt to try to write any software for exhaust, I just stick with what I know is ball park and zero it in on the dyno. And my ball park knowledge only covers road race single and twin four strokes up to about 350cc.
JohnnyB

 

[parks61]

with 4 strokes.
-parks
ps. the 2 stroke calculations can get even more stupid.
pps. trial and error isn't always formulaic but it's always rooted in logic if not intuition.

 

[peteN]

this is what we got at MMI:

primary pipe length call P

P=(850*(exhaust opening angle+180 degrees))/RPM
result will be in inches
ID=[sq root of(cc's/(P*25))]*2.1

example: 125cc single to operate @ 8000 rpm, exhaust opens at 35 degrees

P= (850*215)/8000 or 182,750/8000 or 22.844 inches
ID= root(125/(22.844*25))*2.1; end result being ID=.982 inches

I don't know if you can reverse the formula to solve for a 1.0" pipe, though i don't think it works quite that way. I also don't know why you use cc's in the formula to solve for inches(maybe i missed something that day).

hope this helps out some. oh, BTW, length formula is for header only, whatever comes after is noise or back pressure control. measurement for length is from the exhaust valve itself, not the cyl head.

 

[jbranson]

Pete...you posted that once before didn't you?

Forgot about that, wanted to write a quick program using that and see how it worked out. I know I saved it somewhere. Thanks for posting it again.
JohnnyB

 

[HackAsaw]

yeah I do like zero reversion intake tracts tuned to the second, third, and fourth pulse

I have two bikes tuned that way and they work very well

quote:Originally posted by jbranson

HackAsaw,
I wrote up a piece of software to calculate intake track length based on the Helmholtz Resonance formula. So far on the dyno it's been within 5mm of ideal length (of course length varies with what rpm you tune for), for several bikes I've used it on.

But...intake track simulation is way easier than exhaust. Heated gases being pushed behave a lot more complex than ambient being pulled. I wouldn't even attempt to try to write any software for exhaust, I just stick with what I know is ball park and zero it in on the dyno. And my ball park knowledge only covers road race single and twin four strokes up to about 350cc.
JohnnyB

 

[craig641]

peteN,

That's the same formula that's in Graham Bell's book, except he subtracts 3 from P.

I also was confused to why you used cc when you were solving for inches so I converted to cubic inches when I used his formula for pipe ID. If I leave it as ccs I get a pipe ID of 1.25". That's still a bit smaller than the conventional 1.5" for a 350 motor.

Maybe it's time to invest some money and see what works.

Thanks,
Craig

 

[jbranson]

Craig,
1.25" doesn't sound far off. Mike Rich suggested 1" OD for my bike. If this formula calculates ID then a typical pipe would end up about 1 3/8 OD for your bike. And about 1" OD for mine (formula said .89" ID)

Seems like I suggested 1 3/8" OD to you in the past sometime. The problem I've seen when you get to these ODs is that often the exhaust port is larger than that. You really don't want an abrupt step down in size from the port to the pipe. These formulas usually recommend these size pipes to keep exhaust gas speed up...if the first three inches of the exhaust (meaning the port) are oversize kind of negates the advantage of going to a smaller pipe.
To really take advantage of the smaller OD you'd probably need to raise the floor of the exhaust port and then offset the header up a bit to match it. I've seen this done on very tricky engines I've had here at the shop.

I had a 230cc (175) twin here at the shop built by Craig Hansen that had 1" OD pipes and the port treatment described above. Although it had never been started and I found several gross errors in the engine build....so I'm not sure how much stock I'd place in it.

Next time you have the pipes off, measure the exhaust port diameter. For the most part there's going to be no reason to make the header larger or smaller than that.
JohnnyB

 

[craig641]

JohnnyB,

I just went out into the garage to measure things up. The pipes I used last year did have an 1 3/8" OD. So maybe I'm closer to being correct than I thought I was.

The exhaust port diameter is close to 1.1" My crappy calipers aren't long enough to get all the way in to the head. And 1.1" isn't a lot different than stock. But anyhow the pipes are larger than the exhaust port.

Craig

 

[HackAsaw]

the worlds fastest 750 cc triumph twin used 1 3/8 pipes and turned in the neighborhood of 8000 rpm

I have very strong doubts that a 350 will be optimum with the same diameter

 

[jbranson]

Bear in mind that Craigs 350 turns about 11,000 rpm, close to 40% more than 8,000 rpm. Is the triumph a 360 degree crank with 2-1 exhaust? The 350 is a 180 degree crank with 2-2 pipes.
Was that triumph a speed record type bike? If so they were probably tuning for a very specific rpm, very specific cam profile, very specific flow characteristics in the head. A road race bike has to be tuned with a bit more range of operation.

Craig...do you know what size pipes Todd Henning used on his 350's? I'd guess that he did a couple hundred hours of dyno testing...although his heads were very heavily massaged so I'm not sure how applicable it would be.
JohnnyB

 

[craig641]

JohnnyB,
I think that Monkey has a set of Henning pipes on his bike. If he would be kind enough to measure them I'd appreciate it.
Craig

 

[HackAsaw]

well yes but it will never flow as much intake or exhaust as that LSR pushrod 45 incher did

40% more rpm....... ok

50% + less displacement

btw........ are all the critical dims for a CB350 posted anywhere?

intake and exhaust port diameters and volumes


intake and exhaust valve diameters

chamber volumes

piston weight

cam timing

bore, stroke, rod length

just curious, I don't have any of those engines left nor do I think I have a shop manual on my book shelf

I'm not trying to argue in the least, I've zero hands on hot rodding one of these, just trying to learn is all

 

[HackAsaw]

has anyone ever 360 cranked or 270 cranked a CB?

Now I'm just about bent of just skipping and going naked

and then finding me a hot rodded 305

 

[joe c]

stan lippert told me the type1s were all 360's i know my type1 cp77 is. im pretty sure the closest id for todds bikes were 1 3/8. i remember chris saying the actual id was something freakish and impossible to get. so 1 3/8" it was. todd built some fast shit, but he wasnt stupid chasing impossible things to get. plus people seem to forget how great of a rider her was. not only winning alot of vintage races, but also quite a few modern races back in the day. having kc do alot of work with you helps too!

hack, if you want alof of super tech answers, i can give you contact info for all of the best cb350 motor builders in the country. the people we send our stuff to. chris m oversaw the construction of my last motor, stan lippert did my head, and i was never fortunate enough to have todd do anything for me. but would have loved it. chris has his own pistons made, but even admits the science of it escapes him. his pistons are todd copies. if you could get the run number from one of those guys you could call wiseco and ask. generally, if youre running stock pistons, youre not allowed to really do anything to them, so they get tossed in the motor. bing bang done.

bore stroke and rod length should be in any decent book or paper, or easy enough to figure out with a few measurements. valve sizes i cant remember. but they are easy to find also. the big motors all have variable cam timing depending on comp. alot of guys run different head gaskets. so clearance becomes important. you basically have to have a slotted cam to keep things from exploding instantly. jb knows all this crap.

id like to know why no one makes stepped headers for these things??

jc

 

[monkey]

craig

mine are 1 1/2" outer diameter headers.. with jemco pipes to match. I went with buffs setup... coulda stuck with what i previously had.... tim tighlman has em now. i reckon i may fab a new set with a certain megaphone builder here... to make something different at some point... my guess is to pony up and have mike rich do some head work when I hit the lotto.

I would pull them off the bike... but I have way too much stiff apart at this point.... maybe a at the track we can compare and contrast...

matt