Well boys & girls, this weeks lesson has come about for a number of reasons, one of which being an attempt to help you all understand the age old saying --> """Bigger isn`t always Better"""
Now some people are going to agree with "some" of what i say here... & some are not
big deal, whoop doop, whatever,,, Talk to the hand
if you know what i know , & if you`ve done what i`ve done , if you`ve experienced what i`ve experienced, if you`ve seen what i`ve seen, if you`ve heard what i`ve heard, & if you were as outspoken as me,,, then you`d be typing this exact same stuff right here , right now too
remember there are a "FEW" reasons for me posting this stuff...
How big do "YOU" need your head???
if you have a little donk,,, you only need a little head
if you`ve got a big donk,,, then you can certainly use a bigger head than that little donk... GET IT????
Now that statement could very well be the end of the lesson right there
but remember,,, there are a "FEW" reasons for this topic being here
sooo,,, lets get on with it shall we?
IF,,, for example, we are building a mild road motor, with some poke for some club race stuff,,, like a "To-work-&-back" mini that you want to have some fun at the race track on the weekend with too... yeah?
ok, lets start with a fairly fresh, un-mucked-with cyl head & start porting out bigger right?
well,,, not really,,, if it were me, i`d leave the main sections of the ports quite standard,,, That is to say that if you looked into the mouth of the ports they would look pretty stock std & "Most" people would "assume" (or could easilly be lead to believe) that it is a totally stock std head with no porting done
""""BECAUSE""" i have only worked on the area around the valve seats , the area around the guide bosses , & the combustion chamber would be de-shrouded, shaped & polished
I would have fitted 1st oversize valves, & made the holes that they fit into a little bigger & shaped the entry into the chamber & removed & shaped the guide bosses & trimmed & bullet-nosed the new valve guides
Now this particular head wouldn`t look like it had anything done to it if it were looked at by the average Joe/Jane Doe... such little work could quite easilly be seen to be "NONE"
But, the fact is that this particular head would be VERY WELL SUITED to the engine i described above,,, say a nice little 1330 or 1360 road engine with occasional track use,,, the cam shaft selected for this "Particular" engine would be a fairly smooth road mild cam like a Wade 240 or an Ivan Tighe 104 or a GR Re13 or such similar grind,,, mild & smooth with very little lumpyness,,, just a roady with some poke right?,,, why would anyone in their right mind fit a "RACE" cam to a road motor right??? A RACE cam would need a Big race head & big high compression to allow it to work, it would also need a crose ratio gear-box to help keep the engine within the power curve through the gear changes , & obviously hi-octane race fuel would be needed etc etc etc
In "THAT" case a bigger head would be warranted yeah?
Getting the picture???
Horses for courses right???
BUT,,,, if you were not very experienced with mini cyl heads,,, if you really hadn`t looked at a whole heap of them,,, if you weren`t "FULLY ON THE PROGRAM" with all this stuff then that mild roady head we just described above could quite easilly be seen to be stock standard with absolutely nothing done to it right? someone could tell you that & you`d Believe them yeah?
funny that hey---> cause hours & hours of hard work, die-grinding,,,shaping, de-shrouding, shaping & pollishing & cc-ing chambers, modifying the whole thing as a package to suit this little road engine etc etc etc has gone into it
Lets now look at the sizes shall we (seeing as it`s "How-BIG" is the point to the topic)
ok,,, the stock 1275 inlet valve is about 33m right,,, take away 7mm for stem thickness,,, now take away a few mm (say 3mm for arguments sake as that would be a pretty narrow seat at 1.5mm per side) ,,, ok now we have only 23mm of avaliable hole left
Now i know this should all be done using the "AREA" of the """Avaliable-Hole""" but i`m doing it this way for ease of understanding without having to baffell anyone with complicated maths,,, & it`s pretty damn obvious this way isn`t it???
ok so we have approx 23mm of avaliable """USAB:LE""" Inlet hole right
well the inlet port "mouth" has 26mm sideways (horizontally) & it`s got even more than that up & down (Vertically) so our stock std inlet mouth """"avaliable hole size""" is WAY WAY bigger than the std 23mm """Avaliable inlet valve hole size"""" at the inlet valve seat
It`s pretty damn obvious that the inlet port mouth is simply """NOT""" the restriction here
Got it???
The main restriction is quite obviously the valve & the valve seat area
ok,,, so lets work the head,,, lets""""Port it out"""""
well,,,, "THIS" is the point where you have to ask yourself a serious question---> are you going to race it full on?,,, is it a road motor or a race motor???
because i`m sure you will find very little gain (if not even loose performance) doing anything major to the mouth of the port for a road engine
lets now show you why--->
if we now carve out the valve seat area, (the hole that the inlet valve shuts on) lets whack a bigger valve in there yeah?,,, lets make it 35mm-ish like a big MK1 "S" valve,,, no,,, i`ll tell you what,,, lets make it even bigger --> a 36mm ok? (that`s pretty big inlet valve for a road motor)
so,,, now we have 36mm inlet, take away our 7mm valve stem,,, then take away our (thin) valve seat at 3mm (total) & we are now left with 26mm
funny that,,, we now have a 26mm """avaliable"" hole size at the valve seat (where it`s the most restricted,,, & yet we still have bigger than 26mm at the port mouth because remember that the inlet port mouth is bigger (taller) than 26mm isn`t it
so even with a big 36mm inlet valve we "STLL" don`t really need to open the port mouth do we??? the port mouth """""STILL ISN`T a RESTRICTION"""",,, is it?
Opening up the port mouth isn`t going to do diddly squat,,, is it?
In fact,,, if (for example) we opened this part of the port up, then we`d get a "LOwer" air speed through there & that`s not what we want to do,,, we want as much air speed as possible & we actually want that air speed as evenly distributed across the entire port area as possible,,, we don`t want a slow area & a fast area & an even faster area somewhere else (sort of) ,,, we want it the as much the same right across the whole port (particularly hard to achieve i might add)
so,,, If &/or whenever someone shows you a head,,, & they try to tell you &/or you may "THINK" it`s untouched & still totally stock std,,, just ask yourself a question---> has this head been worked on """"""""inside""""""" you know---> where it actually matters!!!!!!!!!!!---> ,,, at the valve seat , the guide bosses,,, Have the chambers been de-shrouded ???? etc etc etc
It`s not that hard to see if the head is off the car,,, & especially if the valves have been taken out
if you`re un-sure, then get a """known to be stock std"""" head to compare it to
without this type of knowledge & experience, you could well be taken for a ride
you could quite easilly be relieved of some hard earned money to "PORT" the head which has already been done quite well
Ok,,,,
so now lets get back to the start with the old saying about "Bigger isn`t always better"
"""IF""" you had a really big engine (like a 1380cc or 1426cc or 1510cc or bigger) then simply because the bore is bigger in diam then you can actually fit bigger inlet valves in there without too much shrouding of those valves by the bore
remember that we like to "de-shroud" the combustion chamber in an attrempt to let "some" inlet gasses out through the sides of the valves (beside the chamber walls) to get the best chamber fill we can,,,, well the same "shrouding" problem can occur from the cyl walls if the bore is too small or the valves are too big for that bore size
Lets look at a 74.7mm bore (1426cc or bigger with stroked crank added) as opposed to a 73mm bore (1360cc or bigger with stroker crank added) ---> now we have another 1.7mm to play with right?
wrong... we only have half that,,, we only have ,85mm to play with but we can actually use it by fitting bigger inlet valve, but the head must be de-shrouded to allow the gasses to escape through the side of the cyl/valve,,, so we "Match" the head to the block & shape the chamber to suit
we can`t go much bigger really can we??? we`re not going to gain much more are we???
so,,, there`s not much point fitting a super dooper huge valve in there if it doesn`t actually allow any air/gas through the side,,, we`d in fact be "Loosing" some because that super dooper huge valve is actually shrouded by the chamber walls &/or the cyl walls (if you had big enough cam/valve-lift such as a full race cam/head combo
You`ve probably read what GR has typed previously about ports being tooo big & how bad that is by it creating less torque,,, less air speed etc,,, now i`m not going to quote GR cause i`m not going to bother going back through the relevent threads to find them to quote them,,, but he`s right,,, he knows his heads & i agree
i don`t bother opening up the mouth of the port much,,, more of a clean-up & more importantly an """angle them up better""" job than an "opening then up" type thing.
However,,, if you looked through my own personal full race Head you`d probably ask why i opened them right up motha F#$%n huge??? if what i`m saying here is right
well,,, ok then,,, this is where i go back to saying """"Horses-For-Course"""
we`re talking about "road" engines with occasional club race use here,,, with race heads (& i know GR will diss-agree with me here but still) RACE heads need to be as big as possible,,, we`re using huge cyls , huge cams, huge valve lift & huge revs to keep our air speed high
road (& weekend club race) engines need as much "LOW_VALVE_LIFT" inlet air speed as we can possibly achieve,,, """"THAT`S""" what makes a good road/club engine work well,,, combined with all the other necessary "MATCHED" components & clearances to suit... """"BECAUSE"""" they only have "LOW_VALVE LIFT""" compared to a race engine
Soooo..... if anyone trys to tell you that a particular cyl head is standard,,,, make sure you have the head off the car,,, take the valves out,,, clean it up getting all the soot off it & then compare it to a "known" standard cyl head,,, then & only then you can see "really" what has been done to it
looking at the mouth of the inlet port doesn`t tell you diddly if you`re unsure of what you`re looking at in the first place
end of rant
edit for spelling mistakes
