whats a reasonable length for the runners?

engine is 2litre 16v vauxhall x20xev
carbs are zx9r
manifold is my own concoction.

the 'working prototype' is already on version 3, and could do with being modified again to allow the alernator belt a bit more room.

My carbs [ZX6R on Zetec] are pretty much bolted straight to the head. I'm sure this isn't ideal, but they work really well.

Stu

Look on http://www.jenvey.co.uk at the FAQ, there's a lot of information about this.

Grtz Bram

[Edited on 9/6/09 by bram boekestein]

If I remember correctly the intake runner length can actually affect the performance of the engine in several ways (thus some cars using variable length intakes to change the characteristics of the engine as revs change).

It does affect the performance, by up to around 15%.

What rpm do you want it tuned for?

It moves changes the torque curve and therefore the point at which you develop peak power in the rev range.

Shorter runners generally give a peaker power and torque curve.

Longer runners generally give a nore spread out torque curve.

Depends what you want from the engine.

David

hmm, what rpm do i want it tuned for, good question. not even sure what the max rpm of the engine is.
i guess it will sit at 70 at about 3000rpm, so power just above that like my BMWs always seem to have been?

most imporantly at the moment, i need it a little shorter to clear the alternator. had loads of problems with this, then it was all sorted, and then became a problem again
what i don't want obviously, is to shorten it to suit the alternator, only to find out it makes the engine terrible to drive. if its going to make it worse i'll look at finding an alternator with a shorter body or smaller diameter body, but its only very very slightly too big


[Edited on 9/6/09 by blakep82]

What length is it currently?
It won't hacve too muh of an effect, at least not that noticable. On a dyno it can though, but it also deoends on where the base line comes. It's esy enough to fit a few sets of trumpets when on the dyno or the road, and see what they do. alternativley, have 2 pairs of differnt lengths, giving a wider torque curve, as having a torque boost at 3000rpm on a performance engine isn't too handy.

from valve to airbox end of the carb is about 24cm i think. if i go a little shorter, it gives a little more clearance at the top of the carb, a little more angle to the carb, and clearance to the bottom to allow the alternator

That's getting a little too short for the first harmonic, which'll be around 9000 at a guess, and the second at 4500. If you could get it to about 30-35cm you'll be at a good starting point.

that's all assuming it's not running an air box or Helmholtz.

35cm will start to get into problems with not having space for air filters or an airbox.
what sort of length would i be looking at with an airbox as i may well go for it?

bout 30 ish, not much difference like, depending on the volume of the air box.

if it makes you feel any better, on my current car, i had to buy a new altrnator, remove all my power steering, and fit a different alt mount to get the bodies to fit, and they still hit the bonnet with straight trumpets, so all my trumpet/airbox gubbins has to curve.

thinking about it, if 30-35 is sort of ideal, reading through you said its maybe not going to be noticable. 22-24 isn't going to be terrible is it? stu said his are pretty much right at the head. if i just do it without driving the car first, i'll never notice any worse performance lol

Just to point out, you won't be beaten of the line by auto mini's if you don't have the right length, it's just nice to try and make the most of it.

Just using the 2nd 3rd and 4th harmonics will give some gains, about 7% for the 2nd. In which case 24cm will give around 4500rpm peak, but it's only for about 250 rpm either side, so no use to peak power really, and the longer the runner, regardless of tuned length, will give better inertial charging, but a slight, bearly noticeably reduction in throttle responce.

What kind of engine are you after? A peaky, 8000rpm limit with a narrow power band, or something a bit saner.

i'm after something sane as its a road car, but i don't want the car to look like a race car, and be embarassed off the line by decent cars

Well, really, as it's a road car practicallity is key. You need a filter, and an air box is the best method as such. So, rather than worry too much and make too much work, just make it as long as possible, and be done. As nice as it'd be to get ti perfect, to notice a diffeerence you'd have to be doing 1/4 miles.

As for the airbox, try and make it as big as poss, at least 3-4 litres.

[Edited on 9/6/09 by brianthemagical]

excellent, cheers brian!

i'll get some of pics to show you what i'm up against. theres not a great deal of room in there you see

Not a problem, i know how it feels trying to find useable info.

Here's some photos of whati'm up against. a bit tight for space on belts and stuff!