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General Category >> Rubber Side Down! >> Loss of Torque with pod Filter?
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Message started by Drifter on 08/03/11 at 07:28:37

Title: Loss of Torque with pod Filter?
Post by Drifter on 08/03/11 at 07:28:37

From past experience with several different smaller CC japanese bikes removing the stock airbox causes running problems usually in the low rpm range, a GPZ 305 wont run hardly at all with pods. I have seen a few S40s with pod filters so my question is this do you notice a loss of lower rpm torque compared to the stock set up or a flat spot as the RPMs increase?
A long intake tract produces torque a short one is better for high rpm horse power. I understand the carb to head distance does not change but the airfolw to the carb has to be considerably different. I did notice a loss of torque on my Triumph when i went to pods.

I have come to understand the reason for this is it disrupts the airflow across the jets and fuel is not drawn into the engine as the carb was designed causing poor atomization of the fuel or not enough fuel at all causing the bad running condition or a loss of mileage.

I also understand jetting changes are required on most engines when the air filter and or exhaust is modified to get closer to the 12.5 to 14.4 AF ratio that most engines want. :)

Title: Re: Loss of Torque with pod Filter?
Post by MotoBuddha on 08/03/11 at 08:14:00

It's primarily a jetting issue. Too much air with each gulp. Think of it this way -- if a water faucet is turned on only part way, there's still only so much flow you could suck out of it. So regardless of how much more vacuum the additional air flow might be causing across the fuel nozzles, it can't suck out more than the jet can flow. You experience this at low RPM because, first off, the additional air isn't moving very fast and it's not creating much additional suction and, secondly, you're using a different jet circuit than at higher rpm.

Title: Re: Loss of Torque with pod Filter?
Post by Drifter on 08/04/11 at 06:35:45

I agree that the air flow speed with a pod should be slower than the factory set up and that usually causes the above problems on alot of bikes. So back to my question does this happen on this engine with the pod filters? Anyone that has the stock set up and switched to a pod should be able to answer this question seat of the pants dyno..

Better yet has anyone dynoed one with a stock filter and another run with a pod. The torque curve should change depending on which set up this engine likes. :)

I am going to run taller chain drive gearing so torque is what i need not HP.

Title: Re: Loss of Torque with pod Filter?
Post by Gyrobob on 08/04/11 at 07:19:22

497F646B79687F0D0 wrote:

If you want low rpm torque, you need a long skinny runner going the the intake valve. Long so that the pressure waves come together at low rpm, and skinny so that the velocity stays high at low rpm.

In our bikes, the carb has to mount right where it does -- on the cylinder head, so,.. we can't do much about the length and diameter of the runner from the carb to the intake valve.

What we can modify is the stuff upstream of the carb. If you put a big pod filter on the carb, you are using the shortest and widest possible intake runner, therefore reducing low rpm torque a LOT.

It would be an interesting experiment, but I'd like to see what kind of low rpm torque improvement we'd get if we stuck a filter on the end of a 2' long 1" diameter hose or pipe, and attached the other end to the upstream side of the carb. At full throttle, you'd have a really long runner hooked up to the intake valve,.. theoretically creating some really monstrous pressure waves at the intake valve at some value of low rpm, an effect not unlike a supercharger.

Title: Re: Loss of Torque with pod Filter?
Post by Serowbot on 08/04/11 at 10:09:50

I have a K/N filer in the stock filter box... it did fine...
... but, when I removed the box access cover, the bottom fell out of my torque curve...

Too much, is too much... :-?..

Title: Re: Loss of Torque with pod Filter?
Post by ralfyguy on 08/04/11 at 10:18:29

I have the stock filter and removed the lid and the snorkel. Did not seem to change anything low end torque wise, BUT the high rpm improved quiet a bit, especially with a 150 main versus the stock 145. I am happy with the setup.

Title: Re: Loss of Torque with pod Filter?
Post by dasch on 08/04/11 at 11:10:11

I have a cone K&N and unbaffled exhaust. With some slight rejetting - think I didn't lose anything. I could swear I didn't.

Title: Re: Loss of Torque with pod Filter?
Post by SoE on 08/04/11 at 19:15:35

Put a cone filter on mine and opened up the air fuel screw some. Still jumps quicker than sh#t off a stick...