Hello UK Locost friends,

I am currently rebuilding a 1998 Jetta engine and in the process I am tossing the stock EFI and fitting instead a set of brand new Keihin motorcycle carburetors. These are 4 carbs designed for large bikes (1100 cc +). I am looking for opinions as to the length of the intake manif runners that will be needed.

On My list (Locostusa) some have suggested that I fabricate long runners because nowadays the trend is to long runners which favour low to mid range rpm performance. I am not sure about that with carbs.

For one, my new engine is going to receive a performance camshaft cut for for mid to high rpm range performance. And the other question I have is : long runners may work well with EFI port injection and not so well with carbs because of what might happen to the fuel in a longer tube . Am I right or wrong? Your advice will be most welcome as I am about to cut aluminium to make the new manifold.

Philippe
Canada

Most manifolds I make are 80-100mm long, but that tends to be dictated by space, required angles etc.

Some info here;
Jenvey

Which engine is it? I seem to remember the 8v's have the inlet ports arranged in pairs, so you'll want it reasonably long to avoid the runners bending through steep angles.

Essentially longer is better, the ideal length for the average road engine is much longer than people ever actually manage (it's about 20" for something like the VW 8v's). Space limitations, especially in a locost, tend to mean that you can't get anywhere close to that.

I've been doing lots of research into this recently and have lots built into a spreadsheet. Based on the info i've got, you need an inlet that is in total either ....

47.73802383 cm
41.45 cm
56.38656312 cm

This is measured from the valve face to the end of the inlet. The reason you've got three values is ........ different people have different views. Now in theory (if i've got this right) you can do a multiple of this, so if you half it you'll get the same effect but not quite as strong.

The big question is - which length is right ........... no idea!

(edited to correct one of the numbers - copied the wrong thing from the spreadsheet).

[Edited on 28/11/10 by MikeR]

Thank you all for your guidance. Indeed the shape and length of an intake manifold is not a simple matter. I was reading yesterday about the sets of 4 carburetters on motorbikes and intrigued at the short length of the runners. Hardly any runner at all, the carbs are mated to a symbolic manifold via some sort of rubber sleeve. Why is it that on car engines (even carburetted ones) we see longer runners?

Philippe

Bike engines rev higher. The shorter the runners the higher the rpm that the resonances occur at. Modern bikes also have quite well designed airboxes that will also provide resonant airflow at rpms tuned to help out with dips in the torque curve of the engine.

[Edited on 30/11/10 by martyn_16v]