Exhaust diameter suggestions???

J

jammer415

Been Around the Block
Regarding a 1972 CB350F... The stock exhaust is a 4 into 4 exhaust at 1.25" diameter until each pipe hits an individual muffler.

I'm putting a new exhaust on and would like to maximize low/mid end torque to be able to tackle traffic on the steep San Francisco hills.

Right now the pipes are cut off right where the center kickstand is located. I was thinking to throw on a 4 into 2 exhaust by welding on two Y-pipes on each side, then running reverse cone mufflers from the single outlets that remain... but what diameter should the outlets of the Y-pipe be??

I'm thinking 1.75"??
 
So apparently the 4 into 1 exhausts that come on CB400F have an outlet diameter of 1.5" out of the collector? (Is that because the exhaust gasses have time to cool/contract by that point?).

So considering the smaller displacement of the CB350F and the lack of collector chamber, should the outlets on my Y-pipes for my 4-2 exhaust be 1.5"??
 
Furthermore, in order to maximize low/mid end torque, would it help if the two Y-pipe outlets merged into an x-pipe before they split again into individual pipes/mufflers on each side of the bike?

So I guess the result would be a 4-2-x-2 exhaust?

Any thoughts? Advice?
 
there will be a best pipe combo mimicking the stock length pretty much
but you wont gain any noticeable low or mid power all you can hope for is good tune and clean running
it is a rev happy motor ;) if you want torque its
the wrong bike
 
xb33bsa said:
there will be a best pipe combo mimicking the stock length pretty much
but you wont gain any noticeable low or mid power all you can hope for is good tune and clean running
it is a rev happy motor ;) if you want torque its
the wrong bike
Click to expand...

+1
Time for a sprocket change.


.
 
Are you right with your dimensions? The total diameter for the CB400F exhaust mounting is 1.5" and that requires a collar on the pipe so anything over 1.25" simply won't fit into the head.

Also I have a spreadsheet here which works out header length and diameter. That spreadsheet works out the ideal header diameter for the 400F with a 466 big bore kit as 1". I would assume the 350F is a fair bit less.
 
400F pipes bolt on to 350Fs, no problems other than peg clearance. Mimicking them would be fine (or even using 400F pipes). Not a lot of low end torque on these things, but rev it up and you'll have no problem with the SF hills. I've posted a few times outlining what you need to do to make the swap work if you're interested, search for it and you'll find it.
 
Thank you for all the advice.

I considered bolting on CB400F headers/exhaust, but would prefer to keep the cost and labor down (if I can) and just use the original headers (especially if the CB400F headers will not increase torque). Plus I love the exhaust on each side.

So since the juiciest torque will be in the higher RPM range... Should I be installing a larger diameter pipe to make it more free flowing at higher RPMs?

... I understand that this isn't the torqiest bike to begin with, but I'm trying to maximize the torque of what I have to work with.
 
And yes I would like to mimic the exhaust closest to original, but since the original was a 4 into 4 (with 4 individual mufflers) and I'm putting a 4 into 2 (with 2 mufflers)... I'm asking about what diameter to install to compensate for two headers flowing through one pipe on each side? Which size will mimic the closest to original?

The mufflers will be free flowing reverse cone mufflers, but what diameter pipe at the point where the two headers merge into one and flow into a muffler in each side?
 
Not trying to be unfair or a dick, but it sounds like you've got some reading up to do. Remember that although the stock pipes for a CB350F were 4-4, each pipe was significantly baffled and nowhere near to "free flowing." If simply increasing the diameter of the pipe led to HP increase, people would be riding around with ridiculously large diameter exhaust. Using a 4-1 system or a 4-2 system won't really replicate the 4-4 system; the question is what is going to make your bike run best. What's the diameter of the mufflers you have? That's going to have to be the diameter of the output after your Y. And there aren't any guarantees that it's going to be ideal, but you're going to have to go by trial and error, including likely rejetting your carbs.
 
Thank you for your expertise. No offense taken.

The only reason I figured of increasing the diameter of the pipes was to compensate for exhaust gases from two headers flowing into one pipe/exhaust (like the Mac Exhaust system available for CB750F). Ideally I would like to mimic stock backpressure and wondering if anyone on here knew which size pipe and core diameter of muffler I should use to accomplish this on a 4-2 exhaust.

And yes, you are right, I do have a lot of readying to do... which is why I am on this forum trying to determine what readying I should be doing :)
 
First step is to determine the actual bore of the pipes and work from there. Next the collector outlet on a 2 into 1 should be 130% to 150% of teh pipe area of the primaries if it's a 4 into 2. If it's a 4-2-1 pipe use 115-120% for the secondary.

If the pipes were really 1.25, the secondary would be 1.375 to 1.5" 1" would go to 1.125" etc.
 
Thanks!! That's the type of raw data I'm looking for!

Is there a way to measure the backpressure on these older cafe bikes to determine what backpressure I should be striving for with comparison to a custom exhaust?

Also, would it help to scavenge the exhaust gasses by welding in an x-pipe within a 4-2 setup after the two Y-pipe mergers?
 
That IS some good info Teazer.

Jammer, I don't know much about the 350F motor. But you'll want to get the inside diameter of the headers first (since they are probably double walled). There's a lot of info about the science of exhaust gases around the internet.....and not very much is conclusive. So you will pretty much be experimenting no matter what you do.
 
This is the spreadsheet I have for my 466 build. I kept all the same data except changed the motor size to 350. Ideal header diameter including bends was less than 1", although someone else would ned to confirm valve timing etc.
 

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Wow that spreadsheet is epic. Thanks for converting it over to a 350

I measured the ID of my pipes and looks like that is 1.125", so will be recalculating the diameters for the mergers.

I also found this exhaust calculator which has some good info on it:

http://www.mez.co.uk/mezporting/exhaust_length.html

...it states that if I'm going to be merging headers/pipes, i need to merge cylinder #1 with #4 and #2 with #3, any thoughts on this?
 

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That's what I'm doing on my 466. 4-2-1 merge collectors tying in the top two pipes and bottom two pipes:

 
Hey Teazer,

When you say 115%-120% for the secondary collector on a 4-2-1... Do you mean 120% of the original header size or 120% of the outlet on the first collector?
 
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