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View Full Version : sheet metal intakes...runner length



lvrpool32
01-18-2008, 12:49 PM
I have the chance to buy a sheet metal intake kit at a very good price, and other than having to make mounting plates for the del'lorto's, its pretty much all there. Now the intake runners are over length, I cut them to size, so I was wondering, what are the primary effects of runner length on an intake....longer more top end? what length gives you better mid range?

The reason I am going this route is that most of the quad del'lorto/weber manifolds have been around since the 60's, so the port size is very small and you have to add material to these old intakes for them to cover the larger ports found on newer heads (such as my AFR's)
So something custom is appealing, was wondering if anyone had any information on intake design , looking for any helpful technical feedback.

Nick

GetMore
01-18-2008, 07:24 PM
Longer runners tend to give better bottom end power, while short runners are good for top end.
The length is supposed to be long enough to pull from the carb and then have the moving column of air ram in a little more after the piston stops moving downward. The length required for this varies based on engine speed.

DLinson
01-19-2008, 07:31 PM
In college for my Senior design project I designed and constructed a carbon fiber intake manifold for our Formula SAE car. Fun project, I considered designing one for my car but had enough custom stuff on it.

The runner length is inversly proportional to the engine speed, higher the engine speed, the shorter the runner length. There is an euation to calculate resonant tuning speed for a naturally asperated engine:

Nt(rev/min)=(955/K)*a*(A/lVeff)1/2

Nt = engine speed at resonant tunning
K = a constant equal to about 2 for most engines
a = the sound speed (m/s)
A = effective cross-sectional area of the inet system(cm2)
l = effective runner length from inlet to valve (cm)
Veff = Vd(rc+1)/[2(rc-1)] (cm3)
Vd = volume of one cylinder (cm3)
rc = compression ratio

The above is for a single cylinder engine but should be pretty close. If you want to get crazy, do a search on the web for Helmholtz theory for multicylinder engines which takes into acout the intake runners of the other cylinders.

Another important thing to know about intake runners is that they should tapper down to a smaller area as they reach the valve. 2 to 3 degrees of tapper per wall is a good start. This will keep the up the mommentum of the charge as it travels to the cylinder.

There's more but I can't remember them all right now.

Good luck.
Dennis