Bike Carbs - Tuned Inlet Length

[mattbenselin 0]

Hi everyone,

 

I have recently purchased a set of Kawasaki 40mm Keihin ZX-9R C carbs and I have a suitable manifold for my 8 valve GTI.

 

My question relates to the ideal length of the manifold, carbs and velocity stacks together for optimum power or torque?

To break it down, the manifold length can be designed to have shorter or longer runners, the carb length, that is from inlet to outlet is a constant and I don't know but I would think most bike carbs are very similar in this value, and finally the velocity stack, ideal for smooth air entry, can be variable in length and inlet radius.

 

I think that changing this total length has an effect on engine torque and power, but how does one calculate the ideal length of any one component or total length to optimise power and torque for a given engine? I don't know the measurements of the Bogg Brothers manifold but they have a very good reputation so if anyone has one and can shed light on their design that would be great? The manifold I have has 95mm runners with a 5mm thick mounting plate. Does anyone know the optimum length these runners should be? or...

 

...If we have a given length for inlet manifold, we are left to tune the total length by way of velocity stacks. I have a set of 4 Pipercross velocity stacks which measure 60mm in length, 90mm total outside diameter and 40mm inside diameter.

 

My current design means these will sit 10mm from the inlet on the carbs making a total air passage length of about 275mm (5mm plate+95mm runner+15mm silicone hose+90mm carb+10mm v.stack mount+60mm v.stack). The stacks will be mounted inside a sealed air-box which will sit behind the grill and will be fed by a large panel filter facing the oncoming stream of air (with the radiator lowered).

 

Does this sound like a practical and worthwhile design? Can more power/torque be made (or equally is any being lost) by changing the design? Any thoughts would be grealty appreciated. Sorry for the long question but at least the whole issue is on the table.

 

Thanks, Matt

Edited October 21, 2008 by mattbenselin

[Cameron 16]

Yeah, it's a bit of an exact science and to be honest I don't think many (or any) people on here would be able to give you a formula to reliably calculate the ideal length. There's just too many variables! For instance, cylinder capacity, valve size, valve lift, duration and cam profile etc etc etc. I think the majority of tuners go by experience and trial and error, the only people to actually calculate it would have been snapped up by race teams. Even those people would be using CFD to calculate it.

Just as a general rule though, a shorter inlet tract will move peak torque up the rev range, hence creating more peak power. A longer tract will move peak torque lower down, reducing the 'peaky' nature and increasing midrange grunt. All depends what you're after to be honest.

Edited October 21, 2008 by Cameron

[welshpug 1,670]

not sure on exact figures, and whether they'd be suitable but I've seen Sandy's inlets being over the 300mm length IIRC.

[mattbenselin 0]

Thanks for the info, I know people change the length of the manifold on Mi16 conversions to change torque values (as well as squeeze it in!). So I'm glad to hear the same goes for this set-up, but a rough minimum or maximum value would be great so I know I am heading in the right direction. Also as I am using my car for 99% road use more midrange power is preferable over top end. Does my approx. length of 275mm put me in the range of mid or upper power?

[mattbenselin 0]

Hi just to add, received a message from danpug, big thanks, that the Bogg Brothers manifold has a flange thickness of 6mm and the runners are 100mm in length with a bend approx. 3/4 of the way down so the carbs sit under the bonnet.

[Sandy 191]

If you want strong mid range I would aim to start with around 350mm from trumpet mouth to valve. I run up to 430mm on some race engines. The right set up will bring power up just about everywhere rather than see-sawing mid range and peak, but I don't believe it's as calculable as some people claim. Experimentation is the ideal way.

[mattbenselin 0]

That's great info Sandy cheers.

 

As I stated in the first post, does it matter too much which component I alter to change the length, I mean I can't change the carbs and don't want to start messing with the manifold if I can help it, so that leaves the velocity stacks. I was planning on using some 40mm ID / 42mm OD exhaust tube (which is perfect size) to run from inner carb mouth to the stack, originally only 10mm to clear the carbs themselves. Do you think if I cut some other lengths and tried a few to see which gave best curves? Lastly do you know the approx. measurement for an 8 valve XU from valve to head inlet?

 

Thanks again

[Cameron 16]

Well, I guess it's the same sort of principle as injector position. With the carbs closer to the valve you will get better low rev and low temp tractability and with it further away you will get slightly better peak power. Much easier to change the trumpet length though, so I'd just stick with them on the mani and change the trumpets.

[Sandy 191]

Yep, or space it between the carbs and trumpets. Use solid section though, you'll not get as strong results with flexible hoses.

 

Valve to port face on the 8 valve is about 95mm mean.

[Cameron 16]

Yeah, that's a good idea. If you get some short trumpets then you can add different length tube between the trumpet and carb to change the length and see what results you get. Far cheaper than getting hold of loads of different length trumpets.

[mattbenselin 0]

The trumpets I have are 60mm long, making the minimum total length about 360mm from valve to trumpet mouth. Is this too long to start with, should I really be trying to start shorter, 340/350? I'm going to use a slightly modified head with 275 cam if that helps. Also the manifold I have has circular runners, and as we all know the head has square inlets. Should I mark up, enlarge and smooth the inlets to match the manifold as best I can, or leave it as it is? (Make it look more like a XU10 head) The internal diameter of the manifold runners is about 45mm.

 

Cheers guys, Matt

[welshpug 1,670]

done well to fit that length in I would imagine, I would try it and see how it goes, you can always shorten the trumpets.