Cb750 Dual Carb Intake Design

[ncologerojr]

You didn't say you wanted to downgrade performance. You ask for design help for a design that downgrades performance.

One can only deduce that you don't mind a performance downgrade for looks.

Seriously?

So, because you guys don't believe the claims of performance gains you assume that I don't either?

I didn't know that it downgraded performance when I started this thread. Right in the first post I said that I got the idea after seeing cycle-x and murry's kits get very good feedback. If you search the kits there are tons of claims of performance gains. If you ask both manufactures they will each boast performance gains.

One of you even said "Just like ones you can buy that have been proven to work well..."

If I was so knowledgeable about this concept why would I ask?

What happened here is I posted a bad idea. You guys assumed I knew it was a bad idea, and ran with it. Is this form only for experts posting ideas so other experts can know it's been accomplished? I've already explained that I'm not an expert, so how did I sneak in here?

One of you guys explain how any of this makes sense.

 

[AlphaDogChoppers]

Oh! Fer fuck sake!

http://youtu.be/unkIVvjZc9Y

 

[irk miller]

You just happened to touch on one of the subjects that guys get nasty about and have really strong opinions. You've entered a debate that's gone on for years. Sometimes, it's a matter of how you introduce the subject, sometimes it's something you said along in the thread. Don't take it to heart. Take the opinions of those of the same ilk, and dismiss those that aren't. It's pretty simple. Test and test and test some more. When you find what you're looking for, let us know the results. Funny thing about this whole argument is Honda engineers did it too. The 1984 GL1500's had two 36mm CV Keihins supplying a flat-six engine. Honda's goals were "smoothness, quietness and enormous power." So, if you can make it work...

 

[ncologerojr]

You just happened to touch on one of the subjects that guys get nasty about and have really strong opinions. You've entered a debate that's gone on for years. Sometimes, it's a matter of how you introduce the subject, sometimes it's something you said along in the thread. Don't take it to heart. Take the opinions of those of the same ilk, and dismiss those that aren't. It's pretty simple. Test and test and test some more. When you find what you're looking for, let us know the results. Funny thing about this whole argument is Honda engineers did it too. The 1984 GL1500's had two 36mm CV Keihins supplying a flat-six engine. Honda's goals were "smoothness, quietness and enormous power." So, if you can make it work...

You nailed it. Thanks for actually reading my posts.

Trust me, I dont take it to heart. Where I work everyday you need thick skin. I guess the problem is that I try not to get involved in that debate, and didn't see it coming. I could care less what other people do to their bikes, it's not my bike.

I had plans to build, test and show results. I was just looking for some input to help me get started. I didn't anticipate people making it about themselves and acting like teenage girls.

Maybe they should man the fuck up.

 

[cxman]

nick sent you a few emails

 

[xb33bsa]

You just happened to touch on one of the subjects that guys get nasty about and have really strong opinions. You've entered a debate that's gone on for years. Sometimes, it's a matter of how you introduce the subject, sometimes it's something you said along in the thread. Don't take it to heart. Take the opinions of those of the same ilk, and dismiss those that aren't. It's pretty simple. Test and test and test some more. When you find what you're looking for, let us know the results. Funny thing about this whole argument is Honda engineers did it too. The 1984 GL1500's had two 36mm CV Keihins supplying a flat-six engine. Honda's goals were "smoothness, quietness and enormous power." So, if you can make it work...

: gl1500 is cammed for a flat torque curve and does not rev very high
it is a large disp it can afford to be all mid low power
it is all about the cam dude cb750 dohc is a hot rod motor compared to a wing

 

[mydlyfkryzis]

You just happened to touch on one of the subjects that guys get nasty about and have really strong opinions. You've entered a debate that's gone on for years. Sometimes, it's a matter of how you introduce the subject, sometimes it's something you said along in the thread. Don't take it to heart. Take the opinions of those of the same ilk, and dismiss those that aren't. It's pretty simple. Test and test and test some more. When you find what you're looking for, let us know the results. Funny thing about this whole argument is Honda engineers did it too. The 1984 GL1500's had two 36mm CV Keihins supplying a flat-six engine. Honda's goals were "smoothness, quietness and enormous power." So, if you can make it work...

GL1500 : 100 hp (74.6 kW) @ 5200 rpm
DOHC CB750: 72 hp @ 9000 rpm

Gl1500: 66.7 HP/liter
DOHC 750: 96 HP per liter

If you do the math, a 750 version of the GL1500 would be about 50HP.

Anyone have any dyno readings of a 2 carb CB750 to compare?

n engine is an air pump. The more efficiently it moves the air, the better. Valve sizes, port sizes, carburetor sizes, all contribute or detract from this efficency?

Why are dual cams usually more powerful than single cams? Dual cams allow better intake port/valve angles for more efficient flow....

HIGH performance car engines, using carbs, never use dual or single barrel carbs. A HIGH performance V* will use (2) 4 barrel carbs. At full throttle, each cylinder essentially has it's own carburetor.

It is much cheaper and easier to use less carburetors on a bike, but the reason it is not done is performance. Even fuel injected bikes use multiple throttle bodies.

So just say you don't mind a performance hit for looks. It's ok to be artistic, and want the looks, but the argument you can get the looks and performance with your setup is just hilarious.

You aren't building a flow bench are you? Are you really designing a intake that is better? Are you using fluid dynamic program to see what flows best?

You worry what other people think, you can stop...Just do it, who really cares? Not me...but I am not doing this to troll, just to present the idea that some of your premises are faulty.

Good luck

 

[irk miller]

GL1500 : 100 hp (74.6 kW) @ 5200 rpm
DOHC CB750: 72 hp @ 9000 rpm

Gl1500: 66.7 HP/liter
DOHC 750: 96 HP per liter

If you do the math, a 750 version of the GL1500 would be about 50HP.

Anyone have any dyno readings of a 2 carb CB750 to compare?

n engine is an air pump. The more efficiently it moves the air, the better. Valve sizes, port sizes, carburetor sizes, all contribute or detract from this efficency?

Why are dual cams usually more powerful than single cams? Dual cams allow better intake port/valve angles for more efficient flow....

HIGH performance car engines, using carbs, never use dual or single barrel carbs. A HIGH performance V* will use (2) 4 barrel carbs. At full throttle, each cylinder essentially has it's own carburetor.

It is much cheaper and easier to use less carburetors on a bike, but the reason it is not done is performance. Even fuel injected bikes use multiple throttle bodies.

So just say you don't mind a performance hit for looks. It's ok to be artistic, and want the looks, but the argument you can get the looks and performance with your setup is just hilarious.

You aren't building a flow bench are you? Are you really designing a intake that is better? Are you using fluid dynamic program to see what flows best?

You worry what other people think, you can stop...Just do it, who really cares? Not me...but I am not doing this to troll, just to present the idea that some of your premises are faulty.

Good luck

You quoted me, of which this project is not mine. My only point, is there are other reasons to go with the a 2 carb setup than HIGH PERFORMANCE- such as mid range torque or simply better mid range around-the-town driving or fuel mileage, etc. It's not all about getting the most hp out the motor. There can be many factors involved- cams for example. And for the record, we went with a 2bbl over a 4bbl on one of our 350 Chevy pickups for a host of reasons that didn't include drag racing. One of the reasons was the style of manifold. Something that is being left out of the conversation is what range is most of the use of the bike being used in. As in the 350, it's a work truck. Never will it be used above 3500 rpm's. So with that truck, the second two barrels rarely open and only maybe 10hp to have a 4bbl isn't worth it or necessary.

 

[xb33bsa]

You quoted me, of which this project is not mine. My only point, is there are other reasons to go with the a 2 carb setup than HIGH PERFORMANCE- such as mid range torque or simply better mid range around-the-town driving or fuel mileage, etc. It's not all about getting the most hp out the motor. There can be many factors involved- cams for example. And for the record, we went with a 2bbl over a 4bbl on one of our 350 Chevy pickups for a host of reasons that didn't include drag racing. One of the reasons was the style of manifold. Something that is being left out of the conversation is what range is most of the use of the bike being used in. As in the 350, it's a work truck. Never will it be used above 3500 rpm's. So with that truck, the second two barrels rarely open and only maybe 10hp to have a 4bbl isn't worth it or necessary.

yes it is all about the CAM spec,that is my whole point you aint gonna take a 100hp to the liter motor with valve overlap and tun it into a midrange torquer without a different cam spec
you might be able to just change the lobe centers but you are going to need to do serious testing

 

[mydlyfkryzis]

You quoted me, of which this project is not mine. My only point, is there are other reasons to go with the a 2 carb setup than HIGH PERFORMANCE- such as mid range torque or simply better mid range around-the-town driving or fuel mileage, etc. It's not all about getting the most hp out the motor. There can be many factors involved- cams for example. And for the record, we went with a 2bbl over a 4bbl on one of our 350 Chevy pickups for a host of reasons that didn't include drag racing. One of the reasons was the style of manifold. Something that is being left out of the conversation is what range is most of the use of the bike being used in. As in the 350, it's a work truck. Never will it be used above 3500 rpm's. So with that truck, the second two barrels rarely open and only maybe 10hp to have a 4bbl isn't worth it or necessary.

You were the one to mention the GL1500 has 2 carbs for power....

Compression ratio, Timing both ignition and cams, stroke, all have something to do with power.

But the OP cammed up for HP, then is carbureting down...Cross purposes.

That's the point of my comments....If he wanted torque, he might of said so, but he said "looks" .....

He can do what he wants, but if he is making a public point, he will get public comment...

 

[ncologerojr]

You were the one to mention the GL1500 has 2 carbs for power....

Compression ratio, Timing both ignition and cams, stroke, all have something to do with power.

But the OP cammed up for HP, then is carbureting down...Cross purposes.

That's the point of my comments....If he wanted torque, he might of said so, but he said "looks" .....

He can do what he wants, but if he is making a public point, he will get public comment...

Did you not get the point that this thread was started because I wanted input on an idea? I addressed you directly saying that I didn't know it was going to kill performance. I didn't know I was "carbureting down". In simple terms, replace 4 carbs with two bigger carbs makes sense. I said way back that I'm no expert like yourself. Sometimes we have bad ideas. I said that I ditched the stock air box for "looks", I also said that I've been searching for a good alternative.

Maybe the problem is your reading comprehension.

 

[ncologerojr]

Some quotes form my fist few posts for everyone who skimmed over them before posting. Did anything I said indicate that I knew it would result in less power, but didn't care?

"I think it would be cool to make manifolds, if they work."

"I've been thinking over making a dual VM34 carb set for my dohc cb750."

"I know the basic principals of how carburetors work. However I don't know very much about manifolds, which is why I posted."

"Sometimes it takes me hours to build something and it doesn't work, but I probably learned a bit by making it. I'm not a mechanic, engine guru, carb genius etc. nor am I in a rush to get things done."

I welcome public comments, but understand what your commenting on first. I never claimed to possess great knowledge of engines.

 

[Rich Ard]

Sometimes the only way to know a horse is dead is to beat it

 

[ncologerojr]

I also said way back that I was going to ditch the idea now that I've learned a little. For anyone who skipped over that...

For those who haven't checked my build thread, I tried making an air box to help the performance of my cv's. I got bashed for not being an expert on that either, but I did get some good input too. I've got a few ideas to remove the pods and improve the design.

 

[ncologerojr]

Sometimes the only way to know a horse is dead is to beat it

Absolutely. It's how I enjoy my morning coffee.

 

[mydlyfkryzis]

The later posts were not really directed at you. Deviant made comments that continued the discussion. There are a few dead horses requiring a beating. It is an interesting subject. More of a thread jack than a horse beating ceremony.


Sent from planet Earth using mysterious electronic devices and Tapatalk

 

[ncologerojr]

The later posts were not really directed at you. Deviant made comments that continued the discussion. There are a few dead horses requiring a beating. It is an interesting subject. More of a thread jack than a horse beating ceremony.

Agreed.

Now I'd really like some input on my aluminum air box idea. I'll put it out there on this destroyed thread before starting another thread to be hijacked.

Right from the start I will say that I want to build it in an effort of preserving engine performance while improving appearance. I started this discussion in my build thread. I hate the look of the ugly plastic stock air box. Contrary to where this thread went, I still want my bike to perform as well as possible. I really don't want to shell out $800+ to buy and tune high end carbs.

In my first attempt I think I made some mistakes.

1. Keeping the pods as filters. I miss understood the downfalls of using cheap pods. I now know.

2. The air box may be to small to hold sufficient air volume for the engine to breathe freely.

3. I think I need a larger intake area further from the carb intakes.

4. Smoother transition into the carbs.


* Personally, I would rather have a handmade, polished, tig welded, aluminum copy of the stock air box on my bike than the original if need be to maintain reliable performance. I have shaved enough weight by switching to the lith-ion battery alone to make up for the weight difference in the air box.

Would anyone be interested in offering technical input for this design?

 

[surffly]

How much of the air of do you really see?
Most is hidden behind the side covers.
On the smaller bikes guys try for the "open triangle" look all the time.
On the larger bikes that's harder because there is just more stuff in that area.

You have a picture of your set up now?
Some types of carbs will run okay with pods or stacks, but like we were talking about before that comes with give and take.

While I have never used them, I would assume some of the later flat slides or other oem carbs would work better. But you aren't running stock carbs now right?

 

[ncologerojr]

How much of the air of do you really see?
Most is hidden behind the side covers.
On the smaller bikes guys try for the "open triangle" look all the time.
On the larger bikes that's harder because there is just more stuff in that area.

You have a picture of your set up now?
Some types of carbs will run okay with pods or stacks, but like we were talking about before that comes with give and take.

While I have never used them, I would assume some of the later flat slides or other oem carbs would work better. But you aren't running stock carbs now right?

I strive for the open triangle look, no side covers or anything. I like bikes that look as minimalist as possible. Just my taste.

I currently run gsx750 flat slide carbs with pods. I've invested a lot of time in tuning them. They are a bit big for the 750, but I've read countless threads on other forums saying they work. They are much better than stock carbs with pods, but far from perfect IMO.

For this air box I would go back to the stock cv's.

My current set up, and my 2hr air box attempt:

 

[mydlyfkryzis]

These engines benefit from a slightly longer intake length...The problem with the pods is not only the blocking of ports on the CV carbs, but the intake is just too short....

I like your air box, on a level, better then the 2 carb manifold, IMHO.

I think a foam type (UNI) filter would be better. The aluminum box you made a little larger. The Box you made is good. Without it, the center carbs receive hotter air then the outboard carbs, and the air pressure in the center is different then the outboad. this then requires different jetting inboard and outboard. Your box equalizes pressure and temperature more, much like the stock box.

I think you are on a better track with this.