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lecroy
Guest
I had a T120 and a TR6. The TR6 was a 650cc and had a single Amal carb that I doubt was over 30mm. My Harley is 1000cc and came with a 42mm but now has a 36mm Mikuni and runs pretty good with it.
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kyle.quamme
Guest
But the issue has been brought up about the CFM of a Harley vs the CFM of an inline 4, and consensus is that the inline 4 will pull a lot more CFM. Maybe start with a 38mm Mikuni and go from there?
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Commodus
Guest
No, that seems like the wrong way to do it. Instead, there must be a way of simply measuring the CFM your engine is using. I bet there is someone on this site that could tell us.
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kyle.quamme
Guest
On Vintage-Suzuki.com, they have a VM 38 for a 460cc I believe, and a TM 38 that they say should work on a 500, so i dunno for sure, but I'd think something in the 38-40 range VM style would work, the thing would be getting it to work perfect.
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Commodus
Guest
Yeah, but as drhermanstein (I think that's right) says, displacement isn't really the issue. And carbs are expensive, so you probably wanna get it right the first time.
AND I'm really interested in the answer, heh :lol:
What size is your engine? Is it a GS550 four?
Have you considered using a single twin choke, side draught carb like a Dellorto or Weber.
The advantages of using such a setup are:
1. You get a strong scavenging pulse between 1&2 and 3&4 inlets, as the inlet timing is always 180 degs seperated at each branch.
2. It is easier to fabricate a manifold that keeps the inlet tracks at the same tuned length, than for a single choke carb setup.
3. From your CFM calculations, you can further calculate the choke diameters required to deliver optimum performance for your peak HP. By using a Dellorto, you have a range of chokes available for any given carb model, ie, with a 40 Dellorto you can run a range of chokes from 25-32mm diameter. A 45 Dellorto's choke size range is 28-36mm.
4. These carbs are fitted with an accelerator pump and are relatively easy to tune.
5. Changing jets on these carbs is a breeze. No need to remove the carb from the bike.
6. Once tuned, they stay in tune with no need to do individual balancing, as the butterflies are fitted to a common shaft.
7. Velocity stacks are available that can be enclosed in a purpose built airbox.
The dis-advantages are:
1. Space would be tight around the rear of the frame because of the length of these carbs.
2. Economy is slightly compromised by the use of a pump jet circuit.
Just another option for you to consider.
The advantages of using such a setup are:
1. You get a strong scavenging pulse between 1&2 and 3&4 inlets, as the inlet timing is always 180 degs seperated at each branch.
2. It is easier to fabricate a manifold that keeps the inlet tracks at the same tuned length, than for a single choke carb setup.
3. From your CFM calculations, you can further calculate the choke diameters required to deliver optimum performance for your peak HP. By using a Dellorto, you have a range of chokes available for any given carb model, ie, with a 40 Dellorto you can run a range of chokes from 25-32mm diameter. A 45 Dellorto's choke size range is 28-36mm.
4. These carbs are fitted with an accelerator pump and are relatively easy to tune.
5. Changing jets on these carbs is a breeze. No need to remove the carb from the bike.
6. Once tuned, they stay in tune with no need to do individual balancing, as the butterflies are fitted to a common shaft.
7. Velocity stacks are available that can be enclosed in a purpose built airbox.
The dis-advantages are:
1. Space would be tight around the rear of the frame because of the length of these carbs.
2. Economy is slightly compromised by the use of a pump jet circuit.
Just another option for you to consider.
D
drhach
Guest
Cfm
Cfm
CFM is not hard to calculate. You need to do some conversion, but it's not that hard. What does CFM stand for? Cubic Feet per Minute. That's volume per minute. Well, your volume is the volume of the bike (750cc, 1000cc, etc), and your per minute is the RPM of the engine, right? Your volume is constant, in other words no matter how fast or slow you are going, the engine is the same size. What changes is the RPM's. So you you need to find you CFM at the max RPM of the bike. Call it 10000.
One cc is 1centimeter X 1 Centimeter X 1Centimeter. 1 centimeter is equal to .033 feet so 1 cubic centimemeter is equal to .033 feet X .033 feet X .033 feet. Or .0000359 cubic feet. Multiply that by the size of the bike. In my case 1000. So 1000 X .0000359 is .0359 cubic feet. Mulitply that by the maximum RPM (10000), .0359 cubic feet X 10000 RPM and you get 359 CFM.
Now there's one other thing to consider. These engines are four stroke engines (intake, comperssion, power, exhaust). That means that for two turn of the crank, they are only drawing air/fuel once. So, you take your previous figure and divide by two. So, theoretically, you need about 180 CFM for a GS1000 to function at 10000 RPM.
Hopefully I didn't bore too many people.
Cfm
CFM is not hard to calculate. You need to do some conversion, but it's not that hard. What does CFM stand for? Cubic Feet per Minute. That's volume per minute. Well, your volume is the volume of the bike (750cc, 1000cc, etc), and your per minute is the RPM of the engine, right? Your volume is constant, in other words no matter how fast or slow you are going, the engine is the same size. What changes is the RPM's. So you you need to find you CFM at the max RPM of the bike. Call it 10000.
One cc is 1centimeter X 1 Centimeter X 1Centimeter. 1 centimeter is equal to .033 feet so 1 cubic centimemeter is equal to .033 feet X .033 feet X .033 feet. Or .0000359 cubic feet. Multiply that by the size of the bike. In my case 1000. So 1000 X .0000359 is .0359 cubic feet. Mulitply that by the maximum RPM (10000), .0359 cubic feet X 10000 RPM and you get 359 CFM.
Now there's one other thing to consider. These engines are four stroke engines (intake, comperssion, power, exhaust). That means that for two turn of the crank, they are only drawing air/fuel once. So, you take your previous figure and divide by two. So, theoretically, you need about 180 CFM for a GS1000 to function at 10000 RPM.
Hopefully I didn't bore too many people.
L
lecroy
Guest
"But the issue has been brought up about the CFM of a Harley vs the CFM of an inline 4, and consensus is that the inline 4 will pull a lot more CFM. Maybe start with a 38mm Mikuni and go from there?"
I'm just tossing out examples of things I have seen myself. Let's run the same numbers for my bike and we get in the 250CFM range. Add 12 PSIG of boost (where I stopped last year) and you get around 380 CFM at the compressors inlet. All with one little 42mm carb. At 30 PSIG it would be around 450CFM.
BTW, this carb is only recommended for applications in the 210 CFM range. I guess my point of this is that if your not looking for an optimum solution, you can get away with a lot.
Post pictures of your manifold once you get it done.
I'm just tossing out examples of things I have seen myself. Let's run the same numbers for my bike and we get in the 250CFM range. Add 12 PSIG of boost (where I stopped last year) and you get around 380 CFM at the compressors inlet. All with one little 42mm carb. At 30 PSIG it would be around 450CFM.
BTW, this carb is only recommended for applications in the 210 CFM range. I guess my point of this is that if your not looking for an optimum solution, you can get away with a lot.
Post pictures of your manifold once you get it done.
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Commodus
Guest
Yay! I knew somebody would know! :-D
Nerobro
Forum Mentor
While that is "true" that's also assuming zero intake vacuum, and 100% VE.
Most carbs are rated at a certian number of inches of vacuum. While a 180cfm carb may be enough, you'll be looking at signifigant vacuum at 10,000rpm. Vacuum means less horsepower. So you'll want a carb that's much larger than that. dont' ask me for hard numbers, becuase I came up a blank when I last started trying to figure out what it would take to put a single carb on a GS.
Keep in mind, that in comparison to "real" engines, motorcycle engines are almost always disturbingly overcarburated. My little 49hp bike has a 4bbl carb with 1.25" venturis. Though the bike really only breathes through one carb at a time. So if you had a manifold on it.. in theroy a single carb would do it. Still it's a 1.25" carb.
Most carbs are rated at a certian number of inches of vacuum. While a 180cfm carb may be enough, you'll be looking at signifigant vacuum at 10,000rpm. Vacuum means less horsepower. So you'll want a carb that's much larger than that. dont' ask me for hard numbers, becuase I came up a blank when I last started trying to figure out what it would take to put a single carb on a GS.
Keep in mind, that in comparison to "real" engines, motorcycle engines are almost always disturbingly overcarburated. My little 49hp bike has a 4bbl carb with 1.25" venturis. Though the bike really only breathes through one carb at a time. So if you had a manifold on it.. in theroy a single carb would do it. Still it's a 1.25" carb.
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kyle.quamme
Guest
Thank you for all of the information on here. Since I only have a 550 I'm thinking something that can flow 180CFM will be plenty. I am pretty sure I am going to go ahead with this project. My father owns all of the tools that I will need to make my manifold and I know that once I get it tuned in, I am going to be really happy with the dependability.
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Dan Ruddock
Guest
You won't like it!! Will idle like a over camed V8. Dan
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kyle.quamme
Guest
Well I guess I'll just have to figure that out for myself now, won't I?
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drhach
Guest
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kyle.quamme
Guest
My father is insisting that I make a header with tubes as close to equal length as possible, so he won't help me fabricate my original design, but from what I've gathered, it won't really make a difference. :? I'll try to make it happen but he seems dead set on making me stay with the setup I have, and well, I'll have to unless I have access to his tools.
Your father is right. You should try to get the inlet tracks as close as possible to equal length. If you don't, you will compromise performance.
The timimg of sound resonance is almost a beneficial on the intake as it is on the exhaust. All purpose built performance 4-1/extractors, are designed with equal length headers. The correct tuned length, back pressure and sound resonance is the key to their performance. If you lengthen the intake tracks, you will reduce the rpms where your maximum HP is produced.
Some performance 2 strokes, fitted with expansion chamber exhausts, produce over 1/2 a bar (7 lbs per square inch) of vacuum at the port window/crankcase by cleverly utilising the effects of soundwaves.
Nerobro
Forum Mentor
There's a lot more to it than just that.
Equal length is HIGHLY important.
Tuning for resonance is much more difficult. As diameter changes the frequency it responds to. So does length. As does temprature, and vacuum level.
Now, you get to throw in the fact that you're running a wet manifold. So length is almost more important than tuning. Having crap mixture reach your combustion chambers is worse than not having good resonant tuning.
This is a situation where trying to copy a good wet 4cylinder manifold would be your best bet. Problem is.. there aren't a lot of good 4 cylinder manifolds.
Equal length is HIGHLY important.
Tuning for resonance is much more difficult. As diameter changes the frequency it responds to. So does length. As does temprature, and vacuum level.
Now, you get to throw in the fact that you're running a wet manifold. So length is almost more important than tuning. Having crap mixture reach your combustion chambers is worse than not having good resonant tuning.
This is a situation where trying to copy a good wet 4cylinder manifold would be your best bet.
Problem is.. there aren't a lot of good 4 cylinder manifolds.
There's a lot more to it than just that.
Equal length is HIGHLY important.
This is a situation where trying to copy a good wet 4cylinder manifold would be your best bet.
Another 2 good reasons to seriously look at a sidedraught twin choke setup.
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drhach
Guest
Your father is absolutely right about the equal length intake runners. There is a formula that you can use that will optimize the length according to your chosen inlet diameter and which RPM you would like to boost. Unfortunately equal length runners only give you an increase for one area of you RPM range. That's not a bad thing though. You might want to also consider adding a plenum or an air box. That would actually give you something to mount the carb to is if you want a single and you could still have equal length runners. You could either do a side inlet or a top mount. I think if you did it right you could stuff it all under the frame and it would look pretty good. You may need to experiment a little but, the right size airbox will get you some pretty good throttle response.
