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Intake plenum benefit

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gt6racer2

Automotive
Oct 10, 2005
97
Engine - 400cc Single cylinder 4 stroke on motorcycle.
Question. What is the design purpose of an intake plenum with "snorkel" inlet ?
My feelings :
a) Noise reduction. ( It is noisy with no airbox - not a suck noise, more like the exhaust sound )
b) Possible Tuning, smoother running ?
I'm not concerned about noise. If so should I remove the airbox for better running? ( I will rejet ( Mikuni CV carb ) to suit the increased air flow I'd expect.)

Thx.


 
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Any engine will make the most horsepower by allowing it to intake the most volume of air, given that the proper fuel/air ratio is maintained for proper combustion by rejetting the carb or remapping the fuel injection.

Unless the air box and snorkel are designed to provide a ram effect at speed like some of the newer bikes, removing the air box will eliminate restriction before the carb and potentially give more power.

There are other factors to consider. Many times the intake tract at the entry to the carb is rough and a better transition for the air is benneficial. Because of this removing the air box will sometimes net less airflow. A velocity stack is a good solution to solve this problem. Another thing to consider is an air filter. Unless this is clean enviroment (?) racing engine or an experiment you need air filtration. There are all kinds of "cone" filters out there to solve that issue.
 
Thanks for the post. I agree and understand your points. My bike has a cone filter inside the plenum and there's no ram effect. ( BTW I run a Triumph in SCCA - I built a ram air system for that with a big airbox and stacks on the carb inside the box ). However, I also know that sometimes less is more - ie a smaller exhaust pipe can be more effective than a big one for a given flow need ( "smooth/laminar" V "erratic" flow). The snorkel length could I assume smooth out the pulsed intake, and with correct design actually improve air flow ( like stack/runner length ).
I'm really wondering why Suzuki went to the expense of this system. Is it needed for smooth running or is it just a noise thing ?
 
I'm thinking that noise reduction would be a big factor when the OEM designed the stock airbox and likely a main reason for the long snorkel entry.
Also, as you say, it could smooth out the pressure pulses through the carb and keep a more steady flow for better low speed metering. (less violent vacuum signal variations). I don't think this will be much of a factor at higher RPM's but may make a difference at low RPM's especially on a single cylinder engine.

That being said, to get the maximum HP from that engine the same rules apply - more air, more HP. For normal use you are probably pretty good with the cone filter in the air box. Still, if you can do it without chopping up good parts, you might try removing the snorkel and opening the hole into the air box to remove any restriction there. Having the filter inside the box won't hurt you as long as you eliminate any restriction ahead of the filter. In other words, too big of hole is just right........... :)
 
Tuned length and a bell mouthed entry into the duct, or stack or trumpet or velocity stack or ram tube or whatever you want to call it, are very good at promoting airflow between the plenum and the valve, but tuned length into the plenum is in my opinion is a waste of time.

Snorkels in my opinion are for sound dampening and maybe cold air induction only. Reducing the temperature of the air being drawn into the motor most certainly is an effective way to increase power.

The shape of the filter is not significant. The pore size and surface area are what counts.

Regards

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Great guesses guys but Low end throttle response is the short and sweet of why you put those on the intake.

Single cylinder creates "dead air" and having more intake area before and after the carb reduces the restriction when the valve opens and allows the air to accelerate faster, this problem is amplified by sudden drops in vaccum(opening throttle).

Some bike manufactures actually take this into account and design their bikes to run with the air box cover off for best performance. Bikes like susuki tend not to even like to run with the cover off but will increase power with a few holes in the cover. But all seem to work well with more volume between the filter and the carb(model dependant) and the carb and the valves(not model dependant).

 
Thanks for the responses. First, before i start I should, say that the bike from the factory runs lean at low throttle openings - and had a slight surge/tightness at low throttle.
I've now modified the air box to remove the snorkel and open the hole size. First and most noticeable was the noise increase - to the rider the intake is now louder than the exhaust ( but at least that masks some of the bikes mechanical thrashings ). Throttle response became less crisp, and the surge is worse at very low throttles - suggesting I'm more lean ( and hence air flow has improved even at low rpm.). I've rejetted the carb ( pilot, main and needle ) and with more gas the bike is better responding than it was new - and less prone to stall. So my conclusion is we're all right- the snorkel does reduce noise, but it also smooths the air which I believe allow the factory to run closer/leaner carburation at low throttle (and hence meet emmissions needs).
 
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