its not april 1st
ive now got a flow bench and it cost me £24,
in its simplest terms al a flow bench is doing is meassuring how much a head is restricting the flow of air through it, so if you fit a head to a box with a hole in it that lines up with the combustion chamber, apply a vacuum to the box and meassure how strong that vacuum is you can work meassure the pressure drop and therefore flow restriction of the head, i wont know how much the heads are flowing but i will be able to work outs which ones flow the most
the starting point a wooden box, put together with plenty of glue to seal the joint and some gaffer tape over the exterior joints to make sure its air tight.
because an engines cylinder walls effects airflow through the head you need a hole in the top of the machine the same diameter as the cylinder bore and obviousl it also has to line up correctly, here you can see i used an old head gasket and some cut down valve guides which were the same diameter as the bolt holes in my head
location plate screwed into pace, i used some silicon to get a seal between the plate and box, which should fairly easily seperate later if needed
to get a seal between the head and the bench i used self adhesive foam strip
drilled a couple of holes to attach a couple of vacuum cleaners, one i bought especially for teh job, its a tescos value one cost me the princely sum of £12, to make it suck a bit harder i removed its bags and the filters, the other cleaner is our everyday dyson, which incidentally now know doesnt suck as hard as the cheapo tesco one.
first test with a head on the bench, i wasnt sure i would have enough vacuum to get proper readings with the inlet valve fully open, so a plonked a head on, one with no spark plugs or valves in it at all, reasoning if i could read a pressure drop one that head i was never going to put one on it that followed any more air.
the next pic you can see my DIY manometer, just some piping in a u shape filled with fluid, one end open to air the other connected to the flow bench
power off, fluid levels level
power on and we have a prssure drop, magic, just meassure the height difference between the two levels and you have a meassure of air flow, as i said bfore i wont know how much air, but the bigger the difference between the levels the greater the flow restriction.
in fact bunging up teh ports with a few fingers showed me i needed a taller manometer for flow meassuring, but the first one still has a use, put a tin ube in in and if you probe the ports you can work out where the most airflow takes place
i tried several bmw heads and they all had the majority of flow on the floor of the port, it wasnt proper flow testing as non of them had any inlet valves inthem but i stil saw some interesting results, namely, one head had a std port, a "conventionally" modified port, one with a lowered port floor, one with a raised port roof, teh std one flowed least, as youd expect, the conventionally modded one came next, the one with the lower floor which has a really sharp short turn came nex which was a suprise conventional wisdom says it should of been the worst head of the lot, best was the raised port, making me wonder if the best was to combine the raise and the lowered designs, stuff you would be really hard to work out without a flow bench,
as it happens a pinto has the same head bolt spacing as the bmw m10, so i popped the linford head on the flow bench without an inlet valve so to was in the same condition as the bmw heads the poor pinto head flowed less air than even the std bmw ones, although the testing i did was only playing and not valid until i repeat it only this time properly with valves in the heads and taking actual and proper meassurements
Reply With Quote
Originally Posted by graham bahr 
) at such low lift a tiny fraction more or less lift makes a huge difference to the flow fig
Bookmarks