## Calculating intake length

So, ive been reading that you can calculate what the ideal length of an intake is. So, is there a formula of some kind? Help a n00b tuner out.

## Re: Calculating intake length

look in the wiki under the performance tuning section, there is a link to gordon jennings two stroke tuner's handbook, it's in there.

## Re: Calculating intake length

i hear the closer to the cylinder the better but i could be wrong

## Re: Calculating intake length

ive been reading that book, and its got a ton of info that im defiantly going to be using. but, i cant seem to find anything about calculating intake length.

bumpity

## Re: Calculating intake length

it's not so simple that someone can tell you in a post. it is explained in a simplified manner in the book, and that takes half the 'crankcase pumping' chapter, mostly under 'resonance effects' and 'carb location' sections, with some useful references in the expansion chambers chapter as well too.

## Re: Calculating intake length

I've read that chapter in Jennings' book and it never really gets specific on how to calculate a length with a formula. I would say that for the size and HP of the engines we're running, your best bet would be to keep it as short as you can and don't create any sharp bends. Also, you want it to flow as smoothly as possible into your intake port on your cylinder.

## Re: Calculating intake length

shorter = more responsive throttle. longer = less responsive throttle. there is the simple version. i didn't know that it really mattered :S

## Re: Calculating intake length

This is for a 2 stroke. A 4 stroke the constant would be 84,000 instead of 42,000 This will get you real close to your target rpm without getting into temperatures and such.

N x L = 42,000, where N represents the desired engine rpm to tune for and L is the length in inches from the opening of the ram tube to the reed valve or end of the intake port. "Let's say you're running an engine that develops peak horsepower at 8,400 rpm and want to tune for maximum ram effect at the horsepower peak. Then, L should equal 5 inches, as in 8,400

## Calculating intake length

I've been told the longer the intake the more torque you'll gain. It would probably be a hair less responsive too, but not any big deal.

## Re: Calculating intake length

the formula is right at the top of the

## Re: Calculating intake length

here is another, but you'll have to figure out how to apply it yourself:

Helmholtz Resonance. Formula for designing a tuned length intake tract:

http://www.phys.unsw.edu.au/~jw/Helmholtz.html

## Re: Calculating intake length

long equation short, it doesn't matter on mopeds.

I've never felt or witnessed bikes that performed better or worse with longer or shorter intakes.

and if you go by joeho's rough formula, the difference was less than an inch between 8,500 rpm tuning and 5,000 rpm tuning.

my rule of thumb has always been keep it long enough to clear all the fins and obstructions around the motor, don't sacrifice cooling for a closer carb.

## Re: Calculating intake length

EZ way to is for 50cc ENG = 50cc reed to jet /center of carb , 65cc ENG = 65cc reed or piston valve port to jet /center of carb . Is wear torque starts More CCs in manifold = more torque less snappy RPMs

## Re: Calculating intake length

how about how the short stock intake is preferred over the taller one on maxi's?

even though i had to jam it in on a 2-speed everyone always said keep it short.

## Re: Calculating intake length

the hemholz resonance is the easiest calculation that is actually worth anything. even then, it is pretty iffy. Any wave-based calculation that doesn't take into account displacement, intake bore, port size, etc, is absolutely meaningless.

Longer intake will generally promote torque, shorter, generally promote horsepower. The throttle response will be more dependant on intake volume than anything else, sizing the intake properly will give you the best throttle response, not making it shorter or longer.

The hemholz equations work out to about 6 inches in length from the mouth of the carb inlet, to the inner edge of the intake port, for a 50cc engine, with a 17 mm intake tract.

Modeling this with any real degree of accuracy requires way more understanding than anyone on this forum is capable of, me included, my verdict after playing with this on the bench for years is that you should make the intake fit and make it capable of handling vibrations, and not worry about 'tuning' it, because the gains will be minimal at best.

## Re: Calculating intake length

graham! havent heard from your in a while! so what happens if i do make a LONG intake on my bike?

## Re: Calculating intake length

this is pretty interesting stuff guys. i guess i wont really worry about it. ill just make it long enough to keep the carb off the motor and make sure it is nice and freeflowing.

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