[oneworden]

Okay i am ready to get beat up on this one.

i am just trying to put everything together on my exhaust for my CB750 DOHC. i found a company that is selling 1 5/8th OD glass pack baffles. my idea is to cut my stock headers (4 into 4) just before the mufflers and insert these baffles about 3 inches then weld the crap out of them. what i don't know is my ID on the stock headers. from a glance 1 5/8ths doesn't look to far off and i can't imagine them being much different from the sohc's diameter.

to put a picture in your head i my plan is to have the baffles stick out enough to stretch a set of turn downs past the stock pegs. after all the smoke has cleared i will be header wrapping the crap out of them.

if anyone has there pipes apart or has done something like this i could use some courage. the liquid kind wasn't enough.

 

[catboy]

so if i read you right, you're talking about welding these things INTO your pipes in place of any muffler then sticking on some sorta cosmetic tips beyond 'em?

 

[oneworden]

yeah...my goal is to eliminate the huge 4 into 4 megaphone set up, and to do some thing other than a 4 into 1 out the side of the bike.

 

[catboy]

...sounds like a restrictive abortion of major back pressure issues! those engines, in stock form, have VERY little tolerance for even the slightest exhaust modifications and what you're describing wouldn't fit anywhere near any definition of minor. i hope you're not planning on any performance gain with that set-up! and if it's purely cosmetic, then get used to lookin' at it alright 'cause it won't be runniin' for chit!

 

[oneworden]

hmm...well my major reasoning is that i like "under the bike" exhaust, like the buells and newer r6's. performance isn't my major concern, i like to be able to turn more than i like to hit 120mph. i was going to slip in the baffles to create back pressure and to keep the sound a tad lower than ear busting. you really don't think i will be able to get it running right?

 

[HackAsaw]

sure you can make it run right if you know how to tune........

but it will run stronger and more efficiently with a well designed muffler that facilitates the expansion of the exhaust slug


church keys packed in glass and stuffed into the ends of the header pipes will reduce the sound level and performance too

 

[kenessex]

Joe,
Everything is tunable, and with enough effort you could get it to run cleanly. With the CV carbs you would be into shimming needles, changing jets and drilling slide holes. Catboy is probably on target that you would not gain much performance wise and probably a net loss. I agree with the concept of going for something different and might look into some sort of under engine muffler, ala Buell. Look at what the new R-6 and R-1 yamahas use. You may even be able to adapt a stock R-1 box to fit.I think some e-bay research may provide some useful info.

Ken

 

[KeninIowa]

So you're thinking of using baffles that look something like these guy's CC inserts?

http://www.carchemistry.com/

Two thoughts;

Don't weld them in

Make them yourself

 

[bmartin]

I've seen guys cut the exhaust just like your talking about, but they add an extension megaphone 4X4, 4X2 or 4X1 with the exhaust stuffing inserted in the megaphone section. This type of operation is usually done because the back part of the stock exhaust is either rotted away or is smacked up. I've even seen people just cut the exhaust and let them rip - What ever the case, I don't think it's very good for performance and/or a typical BYM (back-yard mechanic like myself) has the skills or equipment to make this kind of mod work very well. Unlike HD's - open pipes on hondas just wears out the engine and pisses off everyone in the neighborhood.

You'd be better off trying to pick up something off ebay or your local salvage yard (like a 4X1) or a set of slip-ones to finish off the length of your cut pipes.

Of course if you have a friend who knows how to weld - you might get something together that looks pretty neat - it may not work well - but it might look neat.

 

[oneworden]

I really appriciate the responces. as you all saw in my original post i knew i was going to get beat'en up for this one, but i was really hoping that everyone would agree that it was a good idea.

i have looked at a few motorcycle salvage yards who think that since the 4 into 1 exhaust fits a CB750 DOHC that it is worth its weight in gold. the fact of the matter is that they are covered in rust and who knows how thinned out the metal is after 26 years on a bike.

then Flea Bay...the short story is that i keep getting killed my those snipers.

 

[catboy]

dunno if "worth their weight in gold" would necessarily apply to the DOHCs. from what i've read, a lot of dealerships would swap to 4 to 1 exhaust all the time cause it would increase sales. that said, i've seen SO many, i'd be surpised if they were even slightly difficult to find...and cheap.

my '79 Ltd even got 4 to 1 and i'm positive they never left the factory that way.

geeto would know way more about it.

 

[catboy]

the kinda gold that turns your finger green maybe...

http://cgi.ebay.com/ebaymotors/Parts-Accessories_Motorcycle-Parts-Accessories__1979-CB750-DOHC-Header_W0QQitemZ180275302351QQddnZPartsQ20Q26Q20AccessoriesQQadnZMotorcycleQ20PartsQ20Q26Q20AccessoriesQQddiZ2811QQadiZ2863QQcmdZViewItem?hash=item180275302351&_trkparms=39:1|65:1|240:1318&_trksid=p4506.c0.m245&timeout=1218639790994

 

[oneworden]

oh that is perfect!! damn local pick up only

 

[bmartin]

Check the ad again - this guy will ship to you, he just prefers a local pickup. Looks like it's a Kerker and it's missing the rear cone section - but that can be purchased with no problem for Dennis Kirk. Here's your chance to get something decent - I'd jump all over this one.

 

[oneworden]

quote:Originally posted by bmartin

Check the ad again

yeah for some reason i always pay half attention to everything...lol.

 

[catboy]

what year is your DOHC?

 

[oneworden]

quote:Originally posted by catboy

what year is your DOHC?

It is an 81' cb750k

 

[Geeto67]

ok, I have heard of new DOHC 4 into 1 pipes being expensive (those ones the F group imports from germany are $700) but if you scrounge you can find a deal. I just bought a rare as hell bassani 4 into 1 for k and F bikes for $150. The paint was shit but there were not holes or major rot.

The stock DOHC pipe for an F (4 into 2) is kinda expensive because they all rotted out so those will be pricey. Same goes for the 4 into 4 DOHC headers, so if you have a nice set I recommend against cutting it up, you should be able to sell them to get a nice aftermarket header.

Vance and Hines still sells a new 4 into 1 for the DOHC bikes. cost is $375 but that is for a brandy new stem to stern exhaust. it is for an F so you will need a plate adapter for the rear mount but it is basically a piece of angle iron cut to length.
http://cgi.ebay.com/ebaymotors/VANCE-HINES-PRO-PIPE-EXHAUST-HONDA-CB1100F-900F-750F_W0QQitemZ110275089923QQihZ001QQcategoryZ35573QQrdZ1QQssPageNameZWD1VQQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247

Mac also makes a full exhaust for the cb750F and K bikes for about $250. This is also a brand new pipe.

you should be able to find both these pipes cheaper used if you look around hard enough.

here is a cheapie ebay jobbie:
http://cgi.ebay.com/ebaymotors/1979-CB750-DOHC-Header_W0QQitemZ180275302351QQihZ008QQcategoryZ35596QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

now as to your idea of an under bike exhaust there are a few issues.

#1) mounting. you will need to fab an under bike mount since it is not good to have these bikes hang by the headers. also your muffler idea will be heavy and you will need to support that.

#2) flow and distance. On the two bikes you mentioned both use a chambered muffler (similar to a car muffler). The idea is to have the exhaust gasses travel the same distance as they would in a full length exhaust but for it to all happen under the bike. These exaust systems are also engineered for torque, on the buell so it can out accelerate the sportster with the buell high flow heads (which give it more top end charge), on the r6 it is so the bike can actually get out of its own way under the powerband (the r6 also has an in exhaust valve to help build back pressure as well). you can use an automotive chambered muffler (I think dynomax has a few) but it will be big an bulky and you will need adaptors.

If you would like to know how I would do it - I would get a 4 into 1 header similar to the one in the ebay auction above (without the muffler it ends under the bike), and then I would go look for a FZR exup valve and see if I can adpat that to the header. Then I would use a resonator tip for a car because resonator tips have baffles in them and you can sometimes pack them with glass as well. It will still be loud but slightly less so. older fzr exup valves are cable/servo activated.

 

[bmartin]

While the Vance pipe is a great pipe, I believe that pipe hangs a little low on the side of the bike and may give you a problem with lean ground clearance. I just took another look at the set listed on ebay and there is still no bids on it - you might pick up a real deal on this ebay listing. As Hackasaw recommended - you could add a sport bike muffler (with small mods) to the pipe that's listed on ebay. You can find tons of sport bike mufflers either new or from a salvage yard.

If you really feel like spending money on new - as recommended - you could go with a mac system or a new kerker system from Dennis Kirk. The kerker is about 600 dollars new. There is one japanese guy that has a kick ass 4-1 setup for about 700 dollars. He advertises on ebay once in a while.

Check out the 78 KZ650 in the project section and look at his pipe setup.

 

[kenessex]

Joe,
It doesn't need to be for a 750DOHC for it to work if you can make brackets and such. I don't know what what the pipe on my Sec550 race bike is really for. I didn't use the muffler section that came with it and made the header section fit. I got it for $25.00 on E-bay.

Ken