Bigger carbs vs. bottom end performance

[AKheathen]

Yeah Jeff and i think it was Kevin Herr that told me the same thing.

 

It just doesnt make sense that 2 venturis of the same size (33mm at 10% and 28mm at 18%) could give you different results if they were passing the same amount of air.

 

People talk about velocity ect which is pertinent in a four stroke application because the flow goes straight from carb to cylinder but in this instance the transfer phase is a separate ordeal from the intake. A smaller carb just creates more pumping losses at higher rpm's

........no.......

[SLORYDER]

........no.......

 

Well then explain to me how velocity through the intake affects the power on a banshee.

[SLORYDER]

I know it might seem like I'm being hard headed, but just saying that the velocity is too low on a bigger carb isn't enough for me. I would actually like to get a grasp on what's going on here, but I can't do so unless I can actually visualize what the mechanism is that is affecting the power. Maybe I'm wrong, but I would be willing to bet that if a bigger carb does cause a loss of bottom end, it has nothing to do with the velocity of the air fuel mixture into the cylinder, like most people are inclined to believe.

 

If velocity did play a role in this, to me it would be insufficient velocity through the carb bore to pull the correct amount of fuel out of the slow jet. If this were the culprit I believe that it could be corrected with jetting.

[hercalmighty]

Pretty much any time you change how air enters or exits the motor its going to effect how the motor performs. 2 strokes are extremely senstive to airflow through them. That's how they work. 4 strokes use mechincal devices to adjust how air flowa through them. 2 strokes use strictly air to perform and to get air in and out of them. The shape, length, diameter, even texture will change how the air flows. The bigger the pipe the less force is created. So, since the carb piping is bigger it loses some of that force. So the motor needs to create more vacum to pull the same amount of air at the same speed. This is where your porting, intakes and exhaust come into play. If your not matching up your parts right the motor isn't going to perform as well. If everything is matched up right the bigger carbs will perform just as well down low as up high, but not matched up and you will lose that power.

Go read the thread for sniper pipes. They really explain well how all the parts work together and that matching everything up will really raise the effeciantce of the motor. Also check out The Two Stroke Shop. They do a lot of things to make great motors and explain a lot of the velocity and flow for motors. They have been doing a lot with GP style bikes.

[SLORYDER]

Pretty much any time you change how air enters or exits the motor its going to effect how the motor performs. 2 strokes are extremely senstive to airflow through them. That's how they work. 4 strokes use mechincal devices to adjust how air flowa through them. 2 strokes use strictly air to perform and to get air in and out of them. The shape, length, diameter, even texture will change how the air flows. The bigger the pipe the less force is created. So, since the carb piping is bigger it loses some of that force. So the motor needs to create more vacum to pull the same amount of air at the same speed. This is where your porting, intakes and exhaust come into play. If your not matching up your parts right the motor isn't going to perform as well. If everything is matched up right the bigger carbs will perform just as well down low as up high, but not matched up and you will lose that power.

Go read the thread for sniper pipes. They really explain well how all the parts work together and that matching everything up will really raise the effeciantce of the motor. Also check out The Two Stroke Shop. They do a lot of things to make great motors and explain a lot of the velocity and flow for motors. They have been doing a lot with GP style bikes.

 

 

But what you aren't taking into consideration is the fact that at lower RPM's, there is not a difference in the size of the "piping" due to the fact that the slide is restricting the throat of the carb.

 

Doesn't matter if it's a 10mm carb wide open or a 35mm carb barely cracked open, you are still getting the same amount of flow through there.

 

Perhaps the problem lies when putting down the trail, and you suddenly crack the throttle wide open, I could see where the large bore of the carb may have trouble "keeping up" with the relatively weak vacuum of the cylinder.....

IMO this condition could be remedied by not going full throttle from a near standstill..

Learning how to ride with the new carbs.

 

Is this making sense to anyone?

Edited June 8, 2010 by SLORYDER

[RZBansheeMan]

You say that going from 26mm carbs to 33mm carbs will cause you to lose bottom end performance.

 

Please explain why, so I will know

 

Thank you.

 

I think that this subject has a multitude of answers / theories / explanations....

 

I am no expert to Banshee's by any means, but I would think that since stocker carbs will produce upwards of 70 hp, that most builds would not require a bigger carb other then just to say you have bigger carbs....

 

Now as for the bigger carb vs. bottom end loss, I think the loss usually happens because people may be putting bigger carbs on a engine that doesn't require a bigger carb. I believe that bottom end can be retained or gained with the use of bigger carbs with proper porting, proper intakes, proper pipes, proper air filter, proper compression / squish, and proper ignition timing.

 

I don't think that just putting bigger carbs on and jetting them down is the right idea for trying to retain bottom end, unless maybe you have the throttle adjusted so the slides don't open all the way....

 

Just my thoughts

 

Mike

[Erwin8r]

Slo, I think you answered your own question. Think about how your ride your Shee, and the analogy you propose. At constant thorttle, the size of the carb orifice is almost immaterial. Add the need to accelerate, however, and things change dramatically. It is well known that the larger carbs do not provide optimum charging when called to accelerate from a low RPM in a low-vauum scenario. The smaller ones do better in that respect. It is also well known that the larger carbs do a much better job of filling the cylinders at higher RPMs'--but you've already wsaid all of this...

 

There. LOL!

[SLORYDER]

So learn not to floor it and you're good!?

Haha I don't know. The velocity thing didn't make sense to me and my OCD would not let me have peace.

 

I wish I could ride my shee

 

I'm in Colorado for another 2 months and my baby is in Louisiana. I have, however bought "the wife" a blaster and the boy an lt80 since I've been here to get a little fix here and there.

 

I do, however have her jugs and head and by the time I get home she will have a serious port job going.

Edited June 8, 2010 by SLORYDER

[hercalmighty]

Like RZ said, it all comes down to setup. Bad setup=bad performance. As afr as the "piping" goes you have to look at not just the opening of the slide but the size of the hole before and after the slide. Ok, think about this, if you have two water hoses, one is 1" and one is 2" with a valve only opened to 1" and the water pressure is the same to both, which is going to flow more evenly? The water would be flowing to the valve, stopping and being squeezed down then open back up to a larger hose again.

[Pasi S.]

In my opinion the 2" pipe will flow more because there is close to twise the area comparing to that 1" pipe and you said that the pressure was the same in both pipes. But thats just water not air.

 

1" pipe fully open = 5.064 sq.cm

2" pipe 1/2 open = 10.129 sq.cm (if I didn´t calculated it wrong) so same pressure = more flow.

 

-Pasi S.

[shanYE west]

With the bigger carbs.. the lack of intake velocity doesn't allow the incoming air to pick up a good fuel charge.. therefore creating a lean condition in lower RPMs. The smaller carbs keep velocity higher at lower RPMS.. wich lets them pick up the fuel charge better.

 

 

[hercalmighty]

I was just using that as an example to think about it this in a different sense. I wasn't really trying to show the amount of flow but the way it flows. The smaller hose is going to have a more even flow to it then the bigger hose with a half opened valve.

[SLORYDER]

With the bigger carbs.. the lack of intake velocity doesn't allow the incoming air to pick up a good fuel charge.. therefore creating a lean condition in lower RPMs. The smaller carbs keep velocity higher at lower RPMS.. wich lets them pick up the fuel charge better.

 

Yes, I can fully understand this.

Would it be possible to alleviate this problem by going up on your slow jet and maybe on your needle?

[AKheathen]

the "no" is that a 2-stroke carb isn't as much unlike a 4-stroke carb as far a sizing, as you think. it's always been a rule that you calculate total cfm, and run the largest size carb below that, or mabey just a little bigger, if you really want, but the point is to get the full signal strength in as many throttle positions as possible, or you will likely not get it tuned near as good as you can. several factors here- i already outlined the ratio of tuning increments breifly, so next look at the shape..... in your example of a 26@ 10% opening, the fuel is fed more strategically placed, and shaped for good dispurtion into a more close-bordered stream. basically- the opening will be closer to round, with the fuel in the middle, not having so far to spread to mix. on the 35@ 4%, the shape of the opening is more flat and long, with the fuel coming from less percentage of the area, having to spread farther, and the delivery, being intended for higher flow, doesn't come out as broken to a fine mist as it should. what's more important/effected, is the enrichment effect, as already mentioned. while on a 4 stroke carb, you have the accellerator pump to inject fuel at the begining of the airstream , since there is a delay in the venturri on sudden opening. on a 2-stroke, you don't have the full recipricating pulse because of the reeds, but you get a little high velocity "spit" back through the middle of the carb, and it pulls fuel back out, then gets sucked in, so you have a double-enrichment effect, which is stronger, when compared to incoming flow, than at higher establoshed flow. with a larger carb, there is more area that is away from the jet for it to travel and not pickup fuel, so there is allot less enrichment. another thing to think about, is how the carb is tuned as a total- the main is set for 3/4- wot, which is a larger range on a big carb, and,of course, proably more than the smaller carb will flow, so, tha leaves you with just the needle and pilot vs. all 3 circuits on a smaller carb. so, when you compare large vs. small, the smaller has more fine tuning in the same range. the pilot circuit is pretty much for idle and just putting, not really ridding, but on an oversized carb, you are using it for more than it's designed. now, all this aside, nothing is impossible, and the thing i always suggest to anyone who really feels they need the igger carb, is practice using roll-on throttle, and if you get really good at tuning and understanding just exactly how every little thing works, like airjet, needle jet, cutout, etc, then you can tune an oversized carb to run as good, or better than most tune an appropriate size carb. basically, you are not just gonna throw a huge carb on and be happy with a few adjustments from a starting point, unless you have alreay put in an obscene amount of time/experience, like a builder would.