Intake Modifications

Andrew /

Nov 16, 2007 07:13PM

Hi All,

I've been doing a lot of reading on the exhaust side of things and have that sorted out pretty well but now I am turning my attention to the intake side....I know all about the need for good filtration and proper air/fuel mix but my question has to to with tuning the intake runners for optimal length....I tried a utility , I can't remember exactly where but my engine has a rev limit of 6,000 RPM (bike motor) and the program called for a tuned length of 25 inches....do you think this means from carb to the piston of 25 inches or can it be split up say...carb to piston 12.5 inches and a velocity stack between the carb and the air filter of 12.5 inches....I know most carbs are very close to the cylinder head but are there real gains to be had by moving the carb further out for more "ram" effect? or is the real ultimate setup a modifies reed induction system?....Any ideas on the best intake?....By the way, I tries putting the carb 25 inches from the piston and the idle speed went way up, low end torque seemend great but mid and upper RPM went away....before my max was 34.4 MPH (gearing limited)....after mod speed was 25 MPH....I was using a very small diameter pipe to the carb hoping to keep gas velocity high but will try larger pipe to see if that helps. FWIW the carb is 14 mm and the engine is 70 CC.

Thanks in advance for any and all input.

Andrew

Re: Intake Modifications

maciek /

macelius

Nov 16, 2007 07:33PM

24" sounds too long for an intake..have you read up on it in the tuner's handbook? shorter intakes give better response whilst longer give a wider power band.

Re: Intake Modifications

B Brickner /

xmopedracer

Nov 16, 2007 07:37PM

The tuned length of 25inches is way off. Do a search of old posts. I've talked about it before.

You can do an internet search to find Chrysler Ram tuning. And that should bring it up maybe. Just remember that you have a 2 stroke motor and it fires every revolution where as a 4 stroke fires every other.

As far as keeping velocity up. Keep the intake tract the same size throughout. A 4 foot door leading to a 2 foot crawl space. Is still just a 2 foot crawl space. Remember that and you should be good.

Re: Intake Modifications

B Brickner /

xmopedracer

Nov 16, 2007 07:43PM

Here it is. http://www.mopedarmy.com/forums/discuss/read.php?f=7&i=32652&t=32590

Re: Intake Modifications

Andrew /

andyinchville1 OP

Nov 16, 2007 08:00PM

HI,

Thanks for the replies so far....I just wanted to mention where I got my earlier idea that I needed an intake of 24+ inches long...I found the calculator on http://www.bgsoflex.com/intakeln.html.....

My bike engine revs to about 6000 max before she sounds like she's gonna blow....I do like the more practical length your formula offers but will probably have to experiment in either case?.....I also have a boost bottle that I haven't hooked up yet but I figure I would optimize one thing at a time.

Thanks again for the link to the other thread too.

Andrew

Re: Intake Modifications

Warisesi /

Nov 16, 2007 08:32PM

That calculator is for a 4 stroke not a two stroke. best is keep it short straight and same cross section area as the outlet side of the carb. downsize to maintain velocity does not work because with a two stroke you are filling the crank case not directly into the cylinder like a 4 stroke. the rapid stop start stop start keeps things suspended nicely. you do want to add a bell mouth to the front of the carb and also make sure you have your filter or screen far enough away from the mouth of the carb so it doesnt get all wet from the fuel fog.

Re: Intake Modifications

Andrew /

andyinchville1 OP

Nov 16, 2007 08:48PM

HI,

Thanks for that advice...I did build the 24" that it suggested but if that is for a 4 stroke should I simply cut it in half and see how she runs?....I did locate another site and got some formulas for intake length and according to them it came out to 14.1 inches but I think that was for a 4 stroke since it was an auto site....so maybe divide by 2 or multiply by 2 since we are only 2 stroke? Rather than reving my engine to death I used 5800 as max RPM this time.

Thanks

Andrew

Re: Intake Modifications

Warisesi /

Nov 16, 2007 09:03PM

keep everyone informed hot it turns out, obviiusly if you start wif a long intake you can shorten it easier then trying to make one longer. keep it straight as possible.

also post the links cause lots of people are car gear heads also not just moped people so they might need the calculators.

MOPED ARMY

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