Morris Minor Owners Club

Hi, me again!

I'm planning to make a ram pipe/stub stack for my HIF44 carb to go inside my K&N filter. I've found the ideal radii (1/8th minimum) and draft angles (7 degrees) in the vizard book, but i know i've read somewhere (but can't find it now!) the minimum distance between ram pipe mouth and air filter casing. there is a minimum distance at which flow will not be impeded. i imagine it's dependant on throat size, but is it as simple as '1 3/4 carb = 1 3/4 gap' or can you go less, as if you followed that rule the 'curtain area' would be much greater than the cross sectional area of the carb.

i want to find this value to know how deep to make my ram pipe/stub stack. i am aware that increase in length reduces power but increases torque (and varies how low down peak torque occurs), but as i've only got an 83mm deep filter, i won't be going that long anyway!

thanks in advance!

Hello Chris,

do a simple calculation to give the area of the carburettor choke. Then do a calculation using the outer diameter of your ram pipe and factor in a depth that gives the same area. If possible add say 25% extra clearance or more to allow for friction loss. This should give you an idea if your plan is feasible?
An alternative is to make an air box and install a filter canister down near the radiator. Also, if possible, duct in cool air to the air filter.

Alec

so you're suggesting curtain area (based on diameter of ram pipe) = 1.25 x CSA of carb throat?

ok i'm making an excel spreadsheet to work out lengths and angles and all sorts!

not sure what to use for choke area. should i use throttle disc size, or carb mouth (which is slightly bigger) or should i take into account the intrusion into the bridge and take off the 'chord' it creates?

taking 44 as the carb diameter (as it's a HIF44 carb) my spreadsheet gives me a ram pipe with a 69mm diameter mouth, 74.23mm long (leaving 8.77mm gap). those dimensions give me a draft angle of 7.02 degrees (which is ideal apparently).

8.77 doesn't sound like a very big gap, but that is the figure i get based on equalling carb CSA to ram pipe curtain area, including the 25% safety.

Hello Chris,

basically it sounds as if you have plenty of room. I would use all available distance, too much is better than just enough?

Alec

I was only really trying to recall the size vizard had determined. i've re-read the whole ram pipe champter and not found any mention!

thanks for your input. i may just make a nice thick stub stack, about 15-20mm thick. if i find it lacking in lower down torque i could make a longer pipe when it comes to it!

just thought i'd show you what i've come up with:
[frame][/frame]
this is just a CAD rendering at the mo, once i find a nice big block of stainless i'll machine it up!

this is 50mm overall, leaving me 33mm air gap which should be plenty!

Well the K & N ones if you can get hold of one these nowday's are of quite a short length and believe it or not with an as cast finish rather than a polished one like the traditional ones made by the tuning companies years ago and I don't suppose it makes any difference if its made from alluminium or stainless.

[frame][/frame]

i would have thought 'as cast' would offer a bit of turbulence. i notice that one is quite (very) short, but likely due to the installation issues with minis.

i was thinking stainless would be preferable as i could get a smoother finish. ally is likely to get surface corrosion and affect flow possibly. i supposed i could paint or laquer the ally.

length is key to where in the rev range you get the most advantage. the longer it is the lower down the range you get the torque boost (although i can't see it turning it into an F1 car )

The oracle, Vizard, is at pains to point out the a mirror finish isn't needed or even very desirable.

except for reflecting heat from inlet manifolds!

ok ally it is! it's easier to machine anyway!

You only need to radius off the entrance to the carb - there is no point in a 'ram' pipe since it would only (maybe) work at ONE narrow rev band - and because you have 4 pistons all pulling through the one carb, and they in two pulses (2-1 then 3-4) from each branch of the manifold - the pulses are all over the place and very uneven........ The radius is to minimise the vena contracta and works well if done properly. You should have a minimum of 1.5 X the diameter of the hole away from any obstruction........ So say after radiusing the HIF44 mouth is ~ 50mm, then the top face of the filter casing should be at least 75mm away.

duly noted. i could radius the carb mouth, but then i'd have to adapt the filter baseplate to match. i'll shorten my design down to a stack then.

I made up my own rampipe. You can get bare spinning's from ITG & I welded one to a baseplate I made. tried with and without last time it was on a rolling road & there was a notable difference to the power curve

a good difference?

i guess it would increase peak power, but did it climb earlier?

was only a few horses in reality but it did smooth out a power dip it had at lower rpm
It will likely have an effect on the fuelling so the needle may need changing. mine is a custom one now.

hence my interest in a rolling road tune-up!

thanks for your help