Stock to pod filter - Page 3 - Honda Rebel Forum
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post #21 of 26 Old 02-28-2017, 02:23 PM
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Welcome to the form kobe.

Depends. Maybe and maybe not. If you get the jetting right, and tune the exhaust backpressure correctly, it can be an improvement, but it may be so subtle that you don't even notice, even though you want to.

On the other hand if you get it wrong it can be detrimental, but again you may not even notice, or even believe it improved because you want to.

Gas mileage will suffer with the bigger jets in either case whether you get improved performance or not.

Reliability will definitely suffer since you removed the crankcase breather/vacuum.

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post #22 of 26 Old 03-01-2017, 08:19 AM
 
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forgive the Mr. Wizard hat if I may beg your pardon for a moment, not taking sides just adding a little engineering to the tribal knowledge....


The Throttle Plate at the down-stream end of the carburetor's throat controls the engine speed by choking off the air getting into the engine, making it work harder to suck air in.
The job of a carburetor (or fuel injector) is to make sure the just right amount of fuel, 1 part gasoline to 14.7 parts air, is added so that each cylinder full of air has the same optimum fuel/air mixture. The more air going through the engine, the more gasoline has to be added to maintain that optimum 14.7 to 1 ratio.


Gutting the exhaust system and letting it free-flow means the exhaust stroke takes less energy, so the engine has more umph left over to suck in air.
Freeing the air intake system also means the intake stroke of the piston takes less energy, so the engine has more umph to suck in air.
In either/both cases, the engine pistons are working less to suck in air or push out exhaust, so it runs at a higher rpm for the same throttle plate position.

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"The bravest thing for me to do is admit when I am wrong" - unknown
HRF Answer #1 You should take the MSF Rider Course
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post #23 of 26 Old 03-01-2017, 10:07 AM
 
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Quote:
Originally Posted by kryton View Post
Gutting the exhaust system and letting it free-flow means the exhaust stroke takes less energy, so the engine has more umph left over to suck in air.
Freeing the air intake system also means the intake stroke of the piston takes less energy, so the engine has more umph to suck in air.
In either/both cases, the engine pistons are working less to suck in air or push out exhaust, so it runs at a higher rpm for the same throttle plate position.
I've said many times that airflow into an engine is very complex, and I don't profess to understand it completely.
To begin with, it is not a smooth stream of air, but a pulsating flow. 3/4 of the time there is no air going into the engine cylinder, and then there's a quick gulp through the carb. The other cylinder on a Rebel takes a gulp on the next engine revolution so that air enters or exhaust leaves the engine about half the time in quick bursts.

The stop and start nature of the flow is subject both to the static restrictions in the gas paths and also to dynamic wave action reflected in the intake and exhaust tracts. Ideally, a reflected low pressure wave arrives at the exhaust port just as the exhaust stroke is finishing, resulting in a thorough scavenging of the cylinder making the maximum room available for the intake stroke to refill the cylinder with fuel and air.

The cylinder scavenging and filling efficiency is also very complex and depends a lot on the dynamic reflected exhaust waves from the previous exhaust explosion helping or hindering instantaneous flow at various RPMS. This is why cutting exhausts and modifying intakes has significant operational effects that may not all be positive.

The pistons are the driving force for this flow, but their effect is constant, based only on engine RPMS . The intake/exhaust resistance to flow, both static and dynamic, governs the net flow, not the "umph" of the engine.
The fact is, the more fuel/air mix that can be stuffed into the cylinder at a given rpm the more "umph" or power output will be available at the crankshaft. Typically a given setup will result in optimum flow at specific RPM ranges, and engines can be tuned to work best at mid range RPMS (like the Rebel) or at high RPMS (like sportbikes).

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post #24 of 26 Old 03-01-2017, 12:35 PM
 
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[QUOTE=kobe.anafarrior;1114610]Will it decrease performance even if you rejet the carb after putting pod air filter on[/

Some of the folks here on the site seem to speak from experience that it will be very hard to jet. That very well could be the case. I built a Yamaha TTR125/YZ85 conversion dirt bike once and I tried to use a POD filter because it worked better with the layout. I tried every jetting combo I could and could not keep it from stumbling even though it ran great on top. I ended up redesigning a intake hose and epoxying the stock air box. I was able to jet the bike to run perfectly.

I have a small Honda pit bike that we big bored/ big valve head/ oversized carb/ etc. I have a pod on it and I have it jetted perfectly.

It all dependss on the application, and from what I have read here, the rebel may be tough to get right with a pod filter.
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post #25 of 26 Old 03-01-2017, 06:13 PM
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I don't think anyone is saying that it will be hard to jet MX429. It is just that it will likely take trial and error to get it close. Then it is still debateable as to whether you actually gained anything, or just made your fuel mileage worse, and reliability less. It has been done a bunch, but there are no great and wonderful success stories of drastically improved performance. Usually at best it is reported that it seems about the same, or maybe it feels a little better, but I'm not sure. To be truthful though, I don't know of any stories of disastrous performance or engine failure either, but not sure we would hear those, or if they would ever be attributed to the mods.

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post #26 of 26 Old 09-17-2018, 07:23 PM
 
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Kryton, just finished reading this thread and your explaination on how making intake and exhaust mods effect the engine is truly amazing
Props to you sir
STV
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