16 replies to this topic

[germini30]

Hi, i currently have a drilled standard airbox with a k and n element fitted inside.
Ive recently fitted a twin box rc40 instead of a single box stainless rc40 in a bid to quiten down the car inside.
I was out last Sunday on a car run and the roar from the induction side is very loud.
I have just acquired a standard airbox with no mods to it.
Will this be better and then fit the k and n inside it?
Im not out for all out power and the engine is a 1275cc mg metro unit with spi unleaded head fited.
Thanks all.

[absx2]

The standard airbox is always going to be the most quiet option.

 

Any chance of a picture to see where your holes are drilled as the best place to keep the noise down is on the front underside closest to the rocker cover.

I can`t really notice any more induction noise with the smaller holes on the front underside.

 

Did the twin box reduce the " boom " on overrun ?

[alpder]

I have a twin-box RC40, and a drilled airbox with K&N filter (on HIF38/1275). Intake noise is negligible. Have you got pictures of your airbox?

 

No 'boom' on over-run. I did have to extend the exhaust tip a few inches behind the valence on my van, which hugely reduced interior noise.

Edited by alpder, 21 June 2026 - 09:44 AM.

[germini30]

Can't seem to post pics on here for some reason. There are several holes on the underside front of the airbox.
I could just swap airboxes and see what and if theres any difference

[germini30]

Also what oil is best for the dashpot??? Was using 3 in 1

[alpder]

Here's my airbox. I've cut off the inlet pipe and drilled two large holes in the side - didn't want to weaken the 'floor' of it, nor risk the holes being partly inside the filter. I've also screwed it closed with #6 self-tappers, because it won't stay clipped shut so air (and noise) leaks around the top/bottom of the filter without them.

.

 160IMG_20260331_134811000cr.jpg   71.58K   4 downloads

[germini30]

Here we go, my current airbox. The dashpot had 3 in 1 in it but now it seems to be gone and theres hardly any resistance when I push the plunger up and down.

Attached Files

•  Messenger_creation_AED75DC8-2BDF-4D9D-96EA-ABECDA4ED1BB.jpeg   39.62K   1 downloads

[germini30]

Im leaning towards standard airbox with the k and n element then flexible aluminium duct to the grille for cold air. I was always told cold air is best for intake and always wondered why the airbox is drilled directly above the exhaust manifold

[alpder]

Taking air from outside directly might cause carb icing - not all the time, but in the right humid conditions and especially at part-throttle. The modest potential power benefit doesn't seem worth the pain of adding ducting. The faster you're going the more quickly the air around the airbox is being flushed with fresh anyway, just by the force of it coming in the front.

[andyapanel]

Keith Calver has done lots of research on this, which I think you can find on line.

It is persuasive.

I have never suffered carb icing as a result of modifying the box.

In winter I run a hot air intake into the end of the nozzle.

[alpder]

I have never suffered carb icing as a result of modifying the box.

Nor I... but OP is thinking of running an intake duct to the front grille.

[absx2]

Here we go, my current airbox. The dashpot had 3 in 1 in it but now it seems to be gone and theres hardly any resistance when I push the plunger up and down.

 

That one shouldn`t have a notable difference in induction noise over a completely standard airbox.

 

As for oil it depends entirely on your individual state of tune but I would top it up with 3 in 1 and go from there.  

[Ethel]

Metro filter housings are a bit better & were used on some Mini models.

All carbs will have left the factory with engine oil in their dampers. Thinner oil will allow the piston to rise faster & give a weaker mixture under acceleration. Performance tuning modifications typically require the opposite.

[Spider]

The secondary and seldom mentioned function of the Air Filter assembly is a Silencer. Here in Australia it's a registration (like an MOT) check.

 

For all out power, cool air is what is needed, however for economy and smoothness, warm air is better.

Carb Ice forms not just from temperature but also humidity, though if cold enough it will form.

The correct Grade of oil for the Carb Dashpot is 30 weight (monograde), however, the factory recommended using normal engine oil, that's all I've ever used.

[germini30]

Here's my airbox. I've cut off the inlet pipe and drilled two large holes in the side - didn't want to weaken the 'floor' of it, nor risk the holes being partly inside the filter. I've also screwed it closed with #6 self-tappers, because it won't stay clipped shut so air (and noise) leaks around the top/bottom of the filter without them.
.
160IMG_20260331_134811000cr.jpg

Thats a great idea. I'd imagine less noise through a large hole as opposed to 6 smaller ones