Topic: Thinking about slash cut short pipes, what are the pluses/minuses?

[deafcaferacer]

I rode my friend's bike the other day and it had on those slash cut pipes that didn't go much further than the pegs, but I loved the feel and the roar of those pipes (pretty loud) and am thinking about getting those for my '78 cb750. The pipes end just past the footpegs, so not sure if that's good for back pressure, etc.

Questions- will this affect performance to the point where I'd have to rejet or at least have to play with the mixture?

I have a RC exhaust on right now.

Also concerned about back pressure in terms of efficiently pulling out exhaust gas.

Would like some opinions, thank you.

[MoMo]

They'll probably help you get closer to your avatar title

[Tempast]

You'll have almost 0 backpressure and will most likely have to rejet. Lower end (under around 5k) will probably feel quite sluggish and you'll constantly be pulling on the throttle. If you like the pipes, throw a baffle in them and see how it feels.

[deafcaferacer]

They'll probably help you get closer to your avatar title

Not worried about that part as I am 100% already there lol.

[deafcaferacer]

You'll have almost 0 backpressure and will most likely have to rejet. Lower end (under around 5k) will probably feel quite sluggish and you'll constantly be pulling on the throttle. If you like the pipes, throw a baffle in them and see how it feels.

Rejet to bigger mains or smaller?

I have my eyes on some slash cut pipes on ebay and they appear to come with baffles but not sure, will ask seller that question.

[MoMo]

Baffled would make it easier for you to rejet.  Most likely have to go up on the mains one or two sizes if pipes are open.   In PA open pipes will not pass inspection...Larry

[deafcaferacer]

Thanks Larry.

[Stoli]

I did a lot of searching on this a few months back because I plan on using drag pipes on my 3rd build. Probably won't have that up and running for another month, so I can't relay any first hand experience but according to what I found, if I remember correctly, there will be a sag in the midrange power if you leave the pipes open. I found a good video of some guys with a bike on a dyno (can't remember what web site offhand)  and they showed that by introducing a little back pressure, the problem could be improved a bit. They used a thumbscrew a few inches in from the end of each pipe so the "thumb" part was inside the pipe. Back in the day, they called them lollipops. I believe that guys would turn the thumbscrews perpendicular to the pipes when they were riding on the street (to increase back pressure), then when they wanted to maximize the top end power, they turned them parallel to the pipes to reduce back pressure.

[vames]

Just my $0.02. You should baffle it not just to get some back pressure but to keep it from being obnoxious.

[Stoli]

This is a really good video. It's a different engine, different carbs, different pipes, but it is a good explanation of the theory:

https://www.youtube.com/watch?v=KjKUKhHQLHg

Maybe it doesn't apply to a CB750 K8 but I found it interesting that in their particular example, the midrange sag was caused by an over rich mixture. The length and diameter of the pipes are factors in that the sag is primarily caused by exhaust pulse reversion. A baffle or lollipop helps to dampen that reversion and thereby addresses the problem somewhat. I'd be very interested to know how they work out if you pull the trigger.

[deafcaferacer]

Thanks for the input Stoli. Very interesting vid- "changing carb setup is like putting your arm in a sling to cure a headache..." good one.

Looks like this is one of those "not so fast" things.. I probably shouldn't be too eager to pull the trigger on this just yet until I do a little more reading. The thumbscrews are a good idea, I suppose they go in place of the baffle and so are a matter of personal preference?

The pipes I have my eyes on do have removable baffles.

[Parkwood60]

Seeing as how Russ Collins was a wizard with these motors, and you have a RC pipe on there now, I really think you would lose a good chunk of power with shorty open pipes. A good 4-1 is about the best there is as far as making power.

Besides having a personal dislike for loud pipes, and loud bikes and the inconsiderate #$%*s who used to ride them near my apartment when I live in Venice Beach.

[KeithB]

That is an excellent video and should be a "must watch" for anyone modifying bikes.
It would be interesting to see tests like that done with stock vs pod intake options.
I think we would see similar evidence that stock is better, overall, for street use.

[deafcaferacer]

Been away on vacation.. passed through the nice town of Brattleboro, Vermont where there's a shop "Vintage Steele" that specializes in classic bikes. Stopped by there for a quick look- mostly Yamaha 650's a few 750's and one 550 around.

Anyway, back to the subject-

The thing that keeps coming back to me is the risk of loss of mid-range power on these short pipes.

Not sure I'm willing to lose that or face the possibility of having to rejet or adjust the mixture. This one is going on the back burner for now.

[jaguar]

Can't think of a single upside to a short open exhaust.
Well not a functional one.

[Stoli]

Hey DCR. This is my current project that has pipes similar to the slashcut MACs that you are considering. They are actually headers from a '78K (#2 & #3 tweaked a bit) with the mufflers cut off and some stainless ends added. I plan on wrapping the headers and leaving 10" of the stainless exposed. I do have baffles for them. My goal is to roll this project out of the basement by the end of the month and fire it up. Optimistic but might happen. I suspect it will take a while for me to dial in the carbs. I'll let you know how it goes.

[Stoli]

Reporting back as promised -

I put the cheap 4" baffles that you can find on ebay and all the Harley shops in these pipes. Never did try it without the baffles but it is really loud with them so without them would be extra really loud I suppose. I'm all for pissing off the neighbors from time to time but they are a bit over the top. My wife knows when I'll be home 10 minutes before I get there. So there is a plus right there - her boyfriend is always gone by the time I get home. They do have a sweet sound though. Just wish I could tone it down a few more decibels.

From a performance standpoint, I can't notice any degradation in any throttle regime. I'm sure a dyno would show differently but I don't care about that. Not racing the bike or anything. There is a bit of popping on decel. I know that can be indicative of a lean situation but the plugs show otherwise so I think it is due to the pipes. I have configured the carbs (76K) with 120 main jets, 44 slows and the needle raised one notch from center. Stock was 105/40/center notch. Idles smooth and goes like hell.

All things considered, I don't think I would replace the exhaust on a well tuned bike with the drag pipes. I like them on this particular build but I chose to go this route because the bike is lowered a bit and they offer more ground clearance than most of the 4 to 1s out there. They also fit the look I was going for.